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ISBN print: 978-3-0364-0983-2
ISBN eBook: 978-3-0364-1983-1
DOI book: 10.4028/b-2kmCij
© 2026 Trans Tech Publications Ltd, All Rights Reserved

Sustainability on Material Forming

Edited by

Giuseppe Ingarao

Sustainability on Material Forming

Special topic volume with invited peer-reviewed papers only

Edited by
Giuseppe Ingarao

Copyright © 2026 Trans Tech Publications Ltd, Switzerland

All rights reserved. No part of the contents of this publication may be reproduced, processed or transmitted in any form or by any means without the written permission of the publisher.

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Volume 1051 of
Key Engineering Materials
ISSN print 1013-9826
ISSN web 1662-9795

Full text available online at https://www.scientific.net/book/sustainability-on-material-forming/978-3-0364-1983-1

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp -1-0

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Accepted:
2026-04-14

Online:
2026-04-17

Preface

The original version of this paper is available on https://www.scientific.net/KEM.1051.-1.pdf

Reducing anthropogenic environmental impact is an urgent issue to deal with. Technological innovation is needed in all the economic sectors in order to replace current practices with environmentally friendly alternatives. Scientists have to make a research effort to decouple the economic growth from the resource consumption. Reducing the environmental impact of Manufacturing and more in general putting in place the strategies of the Circular Economy (CE) paradigm are some of the most important targets to deal with. Manufacturing scientists are called to innovate the manufacturing processes as well as to find new processes suitable for End-of-Life components reuse. The issue is a platform for stimulating the scientific debate about environmental sustainability of forming processes. The issue welcomed papers dealing with both the environmental impact analysis of forming processes as well as their role as CE enablers.

Energy and resource efficiency of forming processes, LCA analyses of formed component/products, forming processes as Recycling/Remanufacturing/ Reuses strategies, Industrial Symbiosis practices involving forming processes, are some of the topics that are discussed within the issue.

Editor

The original version of this paper is available on https://www.scientific.net/KEM.1051.-1.pdf

Giuseppe Ingarao
University of Palermo

Co-editors

The original version of this paper is available on https://www.scientific.net/KEM.1051.-1.pdf

Daniel Cooper
University of Michigan
Rosa Di Lorenzo
University of Palermo

Table of Contents

Preface

Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Luigi Benvenuto, Ersilia Cozzolino, Agnese Testa, Chiara Mandolfino, Alessandro Simeone, Enrico Lertora, Luca Settineri, Antonello Astarita

Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

Pooya Hosseini, Joost R. Duflou

Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

Dorna Peymani, Matteo Strano

Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

Iacopo Bianchi, Massimo Di Pietro, Archimede Forcellese, Chiara Mignanelli, Michela Simoncini, Tommaso Verdini

Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Usman Aziz, Uceu F.H. Suhuddin, Lars Rath, Léon F. Schröder, Volker Schmidt, Benjamin Klusemann

Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Ana Lyvia Tabosa da Silva, Angela Daniela La Rosa, Geir Ringen, Xiang Ma

Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

Francesco Borda, Vito Basile, Francesco Gagliardi, Luigino Filice

Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Iacopo Bianchi, Pietro Forcellese, Gioacchino Fratini, Thomas Lamberti, Tommaso Mancia, Michela Simoncini

Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

Dorothea Czempas, Junhe Lian, David Bailly

Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Pierpaolo Carlone, Mario Emanuele Di Nardo, Roberto Mele, Francesco Borda, Francesco Gagliardi, Francesco Napolitano, Pietro Russo

A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Chunyi Gao, Hao Wu, Bamber Blackman, Nan Li

On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Ersilia Cozzolino, Dario de Fazio, Paolo De Sio, Antonello Astarita

Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

Yohannis Solomon, Davar Hemyari, Sotirios A. Grammatikos

Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Antonio Piccininni, Giuseppe Ingarao, Letizia Barcellona, Riccardo Puleo, Angela Cusanno, Fabrizio Micari, Gianfranco Palumbo

Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Steffen Gabsa, Werner Homberg

Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

Mauro Carta, Pasquale Buonadonna, Sergio D’Angelo, Paolo de Berardinis, Gabriele Derosas, Mohamad El Mehtedi

Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Peter Frohn-Sörensen, Apostolos Aslanidis, Michael Geueke, Bernd Engel

Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

David Bailly, Moritz Gouverneur, Junhe Lian

Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Agata Sposato, Giovanni de Martino, Gianni Stigliano, Claudio Cignali, Michele Dassisti, Domenico Umbrello

Keywords Index

Authors Index

The original version of this eBook is available on https://www.scientific.net/book/sustainability-on-material-forming/978-3-0364-1983-1

Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 1-8

doi: 10.4028/p-ZK3upW

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-19

Revised:
2026-02-16

Accepted:
2026-03-07

Online:
2026-04-17

Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Luigi Benvenuto 1,a*, Ersilia Cozzolino 2,b, Agnese Testa 3,c, Chiara Mandolfino 1, d, Alessandro Simeone 3,e, Enrico Lertora 1,f, Luca Settineri 3,9, Antonello Astarita 2,h

1Department of Mechanical Engineering, Polytechnic School of University of Genoa, Via Opera Pia 15, 16145 Genoa, Italy

2 Department of Chemical, Materials and Industrial Production Engineering, University of Naples Federico II, Piazzale V. Tecchio 80, 80125 Naples, Italy

3 Department of Management and Production Engineering, Polytechnic of Turin, Corso Duca Degli Abruzzi 24, 10129 Turin, Italy

a*luigi.benvenuto@edu.unige.it, b ersilia.cozzolino@unina.it, c agnese.testa@polito.it, d chiara.mandolfino@unige.it, ealessandro.simeone@polito.it, f enrico.lertora@unige.it, gluca.settineri@polito.it, h  antonello.astarita@unina.it

Keywords: electroplasticity, tensile testing, energy consumption, titanium alloy.

Abstract

Modern manufacturing increasingly demands energy- and resource-efficient solutions. Conventional metal forming often requires high temperatures to reduce flow stress, resulting in high energy consumption, especially for low-formability alloys. Electrically-Assisted Manufacturing (EAM) has emerged as a promising alternative, leveraging the electroplastic effect, i.e. electricity's direct influence on plastic deformation. Documented benefits include reduced forming forces, improved ductility, and altered fracture modes. Indeed, integrating electroplasticity into manufacturing aligns with Industry 4.0 and decarbonization goals, enabling lower energy consumption, extended tool life, and greater compatibility with renewable energy sources. This study compares conventional tensile testing and electro-assisted tensile testing (EAM) of Ti6Al4V, evaluating both mechanical results and the energy consumption of the testing machine under different conditions. The comparison results highlight the potential of pulsed current to improve material formability while reducing energy consumption, offering a more sustainable approach to manufacturing.

Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

Titanium alloys, particularly Ti-6Al-4V (Grade 5), are extensively employed in the aerospace and automotive industries due to their high specific strength, high corrosion resistance and excellent mechanical properties at elevated temperatures [1]. Despite these advantages, their processing remains a significant challenge; Ti-6Al-4V exhibits low ductility and high flow stress at room temperature, often leading to premature failure and high tool wear [2]. Traditionally, these limitations are overcome through warm or hot forming processes. However, conventional heating methods are inherently inefficient, involving significant energy losses, surface oxidation and long cycle times, which conflict with the modern manufacturing goals of decarbonisation and resource efficiency [34].

Electrically-Assisted Manufacturing (EAM) has emerged as a promising solution to these challenges. This technology relies on the Electroplastic Effect (EPE), a phenomenon first systematically observed by Troitskii in 1984 [5], where the application of a high-density electric current during deformation leads to a reduction in flow stress and an increase in elongation [6]. While the macroscopic benefits are well-documented, the scientific community has long debated the dominant mechanisms of EPE, broadly categorising them into thermal and athermal effects. Joule heating is the primary thermal contributor, causing thermal softening; however, experimental evidence often shows stress drops that exceed those predicted by temperature increase alone [7].

Magargee et al. [7-8] demonstrated that while Joule heating is a significant factor in Commercially Pure (CP) Titanium, the sensitivity to current density suggests complex thermally activated behaviours.

Further investigations have revealed that electricity can induce microstructural changes comparable to traditional heat treatments but at lower global temperatures. Kim et al. [10] and Hong et al. [11] proved that pulsed current can induce "electrically-induced annealing" and recovery of dislocation density, effectively softening the material during forging or tensile deformation. More recently, groundbreaking research by Zhao et al. [12] in Ti-Al alloys has shown that electroplasticity originates from defect-level reconfigurations, such as enhanced cross-slip and twinning, which prevent the localization of planar slip bands, a mechanism that cannot be rationalized by simple Joule heating.

Despite these advancements in material science, a critical gap remains regarding the industrial sustainability of EAM. Most literature focuses on the metallurgical benefits, while the global energy balance is often overlooked. As highlighted by Ingarao et al. [13] and Liu et al. [14], the transition to a circular economy requires manufacturing processes to act as enablers of energy efficiency. EAM offers a localized heating approach that eliminates the need for large industrial furnaces, potentially reducing the carbon footprint of the forming stage [15].

This study addresses this gap by providing a comparison between conventional tensile testing and Electrically-Assisted (EA) testing of Ti-6Al-4V. The novelty of this work lies in the dual-track analysis: alongside the characterisation of mechanical properties (yield stress, tensile strength, and ductility), a rigorous energy absorption assessment is conducted for both configurations, but also, from a sustainable perspective. Energy consumption was measured during the tensile test of Ti-6Al4 V , either with or without the application of pulsed current, to assess the energy consumption associated with reducing the applied loads while improving material formability. By applying electroassisted techniques, the required loads for deformation were lowered, enhancing the material's formability. The results offer valuable insights into the energy efficiency of this approach, demonstrating how reducing mechanical forces needed for deformation can lead to significant energy savings while maintaining or even improving material performance.

Materials and Methods

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

Material Characterisation and Specimen Design.

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

The material investigated in this study is the Titanium alloy Ti-6Al-4V (Grade 5), supplied in sheet form, 1 mm thick. Two tensile samples were machined from the as-received sheets via milling operations to minimise the alteration of the microstructural properties of the edges. The geometry of the samples was designed in accordance with the EN ISO 6892-1 standard [16]. A rectangular crosssection of 16 mm×1 mm was selected for the gauge length. A proportionality factor k=9 was adopted to determine the initial gauge length (L0) based on the relationship L0=kS0, where S0 is the initial cross-sectional area fixed equal to 16 mm2.

A tensile test was conducted with a pulsed current, alongside a reference test performed without it. To accommodate the specific requirements of the electrically-assisted (EA) test, the standard geometry was modified by extending the grip sections. This elongation was necessary to ensure sufficient space for both the mechanical clamping of the testing machine and the aluminium electrodes required for current injection. To ensure the safety of the equipment and the accuracy of the electrical measurements, an ad hoc experimental setup was developed, taking into account the need for electrical insulation between the specimen and the metallic grips of the testing machine. In particular, this was achieved by bonding glass-fibre-reinforced epoxy composite tabs onto the specimen heads using a rigid structural adhesive. This configuration ensured that the electrical current flowed exclusively through the gauge length of the specimen, preventing leakage into the testing frame.

Experimental Setup and Electrical Parameters.

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

Uniaxial tensile tests were conducted using a Galdabini Sun 5 universal testing machine equipped with a 50 kN load cell and a dedicated software interface for data acquisition. All tests were performed under controlled environmental conditions, with an ambient temperature of Tr=23±2∘C and a relative humidity of RH=50±5%. The crosshead speed was set to a constant value of v=5 mm/min. This velocity was selected as a trade-off to maintain a quasi-static strain rate while keeping the total test duration under 5 minutes, thereby mitigating excessive uncontrolled Joule heating during the electrical tests.

A high-power programmable DC power supply (Itech IT-M3900D) was used to apply pulsed current to specimens during the tensile tests.

The IT9000 software controlled the waveform, which in this work was imposed with a maximum current density Jtheoretical  of 20 A/mm2, calculated as the ratio between the imposed current at the top time and the initial cross-section S0. Fig. 1 shows the trapezoidal waveform, while Table 1 shows the values for the duration of each stage. The pulsed mode was chosen over continuous current to exploit the athermal effects of electroplasticity while limiting the thermal softening component, consistent with recent literature findings on electropulsing-induced plasticity in metals [17]. Based on the duration of each stage of the pulse current, a frequency of 0.17 Hz was fixed for a total number of pulses derived from the end of the tensile test.

[picture]

Fig. 1. Schematic representation of the single trapezoidal current pulse waveform applied during the EA tensile tests.

Fig. 1. Schematic representation of the single trapezoidal current pulse waveform applied during the EA tensile tests.

Table 1. Temporal parameters defining the trapezoidal pulse cycle employed in the experimental campaign.

Phase Description | Symbol | Duration [s]
Start time (delay) | Δtstart | 5.00
Ramp-up time | Δtup | 0.03
Peak holding time (Top) | Δttop | 0.25
Ramp-down time | Δtdown | 0.03
End time (delay) | Δtend | 0.50
Total pulse period | T | 0.31

Most experimental studies reported in the literature present the applied electric current density without accounting for electrical losses, presenting the results considering the theoretical current density and not the effective one. This work aims to fill this gap of knowledge and, to this purpose, the DEWESoft SIRIUS modular data acquisition (DAQ) system, equipped with a DEWESoft DS-CLAMP-150DC current sensor, was employed to measure the actual current flowing through the specimens during the EA tensile test and discuss the results in relation to the effective current density.

Energy Consumption Measurement Framework.

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

The energy consumption of the universal testing machine was monitored during the tensile test of both samples. To this purpose, a Fluke 435 Series II Power Quality and Energy Analyzer was connected to the power input of the testing machine and the auxiliary equipment. The instrument acquired the instantaneous values of phase voltage, neutral voltage, and ground voltage, as well as the current intensity on both the phase and neutral lines throughout the entire duration of the tests. This setup allowed for the calculation of the specific energy absorption for both the baseline (mechanical work only) and the EA configuration (mechanical work plus electrical energy supplied by the generator), providing the dataset necessary for the comparative energy assessment.

Fig. 2 shows the general overview of the experimental arrangement, showing the universal testing machine interfaced with the high-power programmable DC supply and the energy analyzer (a) and a close-up of the setup during an EA test (b), showing the custom aluminium clamps used for current injection onto the specimen gauge length. The electric current causes visible thermal reddening along the surface of the metal.

[picture]

Fig. 2. Experimental setup for Electrically-Assisted (EA) tensile test (a) and detail of the test piece during current passage and consequent temperature increase (b).

Fig. 2. Experimental setup for Electrically-Assisted (EA) tensile test (a) and detail of the test piece during current passage and consequent temperature increase (b).

Results and Discussions

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

Mechanical Behaviour and Flow Stress Analysis.

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

As mentioned in the previous section, two tensile samples were tested: one in a traditional way, as a reference, and another one with the application of a pulsed current. The theoretical current imposed by the current generator is 320 A , thus resulting in a theoretical current density of 20 A/mm2. The current measurements of the effective current flowing in the specimen during the test, considering the electrical losses, showed an effective current of 290 A , thus resulting in an effective current density of 18 A/mm2. A total of 35 pulses were applied until the break of the EA test.

Fig. 3 shows the post-fracture view of the samples tested in the reference configuration (a) and with EA (b). The latter highlights the fibreglass-reinforced epoxy resin tabs used for electrical insulation at the grip points. Significant surface oxidation (distinct blue/purple colouring) is evident, particularly in the necking area, providing qualitative evidence of highly localised Joule heating prior to fracture.

[picture]

Fig. 3. Post-fracture view of Reference-B and EA-C tested specimens.

Fig. 3. Post-fracture view of Reference-B and EA-C tested specimens.

The engineering stress-strain curves (Fig. 4) for the reference condition and the electricallyassisted configuration were analysed to quantify the Electroplastic Effect (EPE), while the mechanical properties of the most representative test specimens from the two samples are summarised in Table 2.

[picture]

Fig. 4. Stress-Strain curves for Reference-B and EA-B.
Table 2. Summary of mechanical properties for Reference and EA conditions.

Fig. 4. Stress-Strain curves for Reference-B and EA-B. Table 2. Summary of mechanical properties for Reference and EA conditions.

Sample ID | Strain @Rp0.2 [-] | Yield strength Rp0.2 [MPa] | Strain @Rm [-] | Tensile strength Rm [MPa] | Strain at break [-]
Reference | 0.053 | 326 | 0.208 | 419 | 0.424
FA | 0.061 | 187 | 0.083 | 201 | 0.299
Variation | +14.4% | -42.6% | -60.2% | -52.0% | -29.5%

The application of an effective pulsed current density of 18 A/mm2 resulted in a substantial "softening" of the material. As shown in Table 2, the yield strength dropped by approximately 42.6%, while the Ultimate Tensile Strength (UTS) decreased by 52.0%. This behaviour is consistent with the classical theory of electroplasticity described by Conrad [6], where the reduction in flow stress is attributed to the combined action of Joule heating (thermal softening) and the electron wind force aiding dislocation motion.

It is noteworthy that the ratio between Yield Strength and Tensile Strength ( Rp0.2/Rm ) increased from 0.77 in the reference state to 0.93 in the EA state. This flattening of the plastic region suggests that the work-hardening capability of the Ti-6Al-4V alloy is significantly inhibited by the electric
current. According to Magargee et al. [8], this suppression of strain hardening is typical in Titanium alloys under electric flow, as the rate of dynamic recovery is accelerated by the energy input.

Ductility and Failure Mechanisms.

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

Contrary to the expectation that EAM enhances formability, the experimental results indicate a 29.5% reduction in elongation to failure for the EA samples compared to the baseline. It is hypothesised that this premature failure is driven by localised Joule heating at the necking region. As the cross-section decreases during necking, the local electrical resistance increases, leading to a spike in current density and temperature [2]. This causes a thermal instability that accelerates failure, overriding the athermal benefits of defect reconfiguration described by Zhao et al. [12].

Comparative Energy Assessment.

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

The sustainability of the Electrically-Assisted (EA) process was evaluated by comparing the total energy demand of the experimental cell in both reference and EA configurations. The data, summarized in Table 3, account for the electrical energy absorbed by the universal testing machine.

Table 3. Average energy consumption of the testing machine for Reference and EA tests.

Sample ID | Duration [s] | Energy Consumption [Wh] | Energy Consumption [kJ]
Reference | 233.7 ± 1.9 | 1030 ± 7 | 286.0 ± 2.1
EA | 174.7 ± 7.2 | 307 ± 1 | 85.1 ± 0.3
Variation | -25.3% | -70.2% | -70.2%

As shown in Table 3, the application of pulsed current resulted in a remarkable 70.2% reduction in mechanical work. This reduction is significant from a machinery perspective: lower forming loads imply reduced tonnage requirements, less elastic deflection of the machine frame, and potentially reduced wear on tooling, extending the service life of the equipment. This reduction is directly linked to the electroplastic softening effect: since the material flow stress is lower, the machine's electric motors perform less mechanical work to maintain the constant crosshead speed of 5 mm/min.

It is important to note that the total energy demand of a universal testing machine includes constant auxiliary loads (control electronics, cooling, sensors). Therefore, the saving on the total machine absorption suggests a much higher percentage of saving on the net mechanical work performed. When scaling this technology to industrial presses, where the forming work represents a larger share of the total energy consumption, the benefits of EA-induced softening would be even more pronounced, leading to significant reductions in the factory's carbon footprint and operational costs.

Conclusion

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

The present study investigated the electro-mechanical response and the energy efficiency of Ti-6Al4 V titanium alloy under pulsed Electrically-Assisted Manufacturing (EAM) conditions. By integrating mechanical testing with a dual-source energy assessment (machine tool grid absorption and generator pulse energy), the following conclusions can be drawn:

Mechanical Performance and Softening: the application of a theoretical 20 A/mm2 pulsed current induced a significant reduction in flow stress, with a decrease in Yield Strength and Tensile Strength of approximately 42.6% and 52.0%, respectively. This confirms the effectiveness of the electroplastic effect in reducing the force required for deformation.

Ductility and Localized Effects: a reduction in elongation to failure (-29.5%) was observed in EA specimens. This is attributed to localized Joule heating at the necking region, where the reduced cross-section causes a current density spike, leading to thermal instability and premature fracture.

Machine Energy Efficiency: the analysis of the testing machine's grid absorption revealed a direct sustainability benefit. The EA configuration required 70.2% less electrical energy (approx. 200 kJ saving) compared to the reference cold tests. This proves that the reduction
in material flow stress translates into a measurable decrease in the energy demand of the machine's drive system.

Total Energy Balance: while the process requires additional electrical input for pulse generation, the low duty cycle (approx. 5.3%) ensures that this consumption remains a small fraction of the total energy balance. The energy saved by the machine tool, combined with the potential for shorter process chains and the elimination of furnace pre-heating, positions EAM as a highly sustainable alternative to conventional hot forming for low-formability alloys.
Future work will focus on optimizing the pulse parameters and the duty cycle to mitigate localized overheating, aiming to maximize ductility without compromising the significant energy savings achieved.

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.1.pdf

X. Li, B. Ji, Q. Zhou, J. Chen, and P. Gao, 'Influence of Grain Size on Electrically Assisted Tensile Behavior of Ti-6Al-4V Alloy', J Mater Eng Perform, vol. 25, no. 10, pp. 4514-4520, Oct. 2016, doi: 10.1007/s11665-016-2268-0.

B. Kinsey, G. Cullen, A. Jordan, and S. Mates, 'Investigation of electroplastic effect at high deformation rates for 304SS and Ti-6Al-4V', CIRP Ann Manuf Technol, vol. 62, no. 1, pp. 279-282, 2013, doi: 10.1016/j.cirp.2013.03.058.

T. A. Perkins, T. J. Kronenberger, and J. T. Roth, 'Metallic forging using electrical flow as an alternative to warm/hot working', J Manuf Sci Eng, vol. 129, no. 1, pp. 84-94, 2007, doi: 10.1115/1.2386164.

V. Stolyarov and A. Misochenko, 'A Pulsed Current Application to the Deformation Processing of Materials', Sep. 01, 2023, Multidisciplinary Digital Publishing Institute (MDPI). doi: .

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H. Conrad, 'Electroplasticity in metals and ceramics', Materials Science and Engineering, vol. 287, 2000.

B. J. Ruszkiewicz, T. Grimm, I. Ragai, L. Mears, and J. T. Roth, 'A Review of ElectricallyAssisted Manufacturing with Emphasis on Modeling and Understanding of the Electroplastic Effect', Journal of Manufacturing Science and Engineering, Transactions of the ASME, vol. 139, no. 11, Nov. 2017, doi: 10.1115/1.4036716.

J. Magargee, R. Fan, and J. Cao, 'Analysis and observations of current density sensitivity and thermally activated mechanical behavior in electrically-assisted deformation', J Manuf Sci Eng, vol. 135, no. 6, 2013, doi: 10.1115/1.4025882.

J. Magargee, F. Morestin, and J. Cao, 'Characterization of flow stress for commercially pure titanium subjected to electrically assisted deformation', J Eng Mater Technol, vol. 135, no. 4, 2013, doi: 10.1115/1.4024394.

M. J. Kim et al., 'Electric current-induced annealing during uniaxial tension of aluminum alloy', Scr Mater, vol. 75, pp. 58-61, Mar. 2014, doi: 10.1016/j.scriptamat.2013.11.019.

S. T. Hong, Y. H. Jeong, M. N. Chowdhury, D. M. Chun, M. J. Kim, and H. N. Han, 'Feasibility of electrically assisted progressive forging of aluminum 6061-T6 alloy', CIRP Ann Manuf Technol, vol. 64, no. 1, pp. 277-280, 2015, doi: 10.1016/j.cirp.2015.04.084.

S. Zhao et al., 'Defect reconfiguration in a Ti-Al alloy via electroplasticity', Nat Mater, vol. 20, no. 4, pp. 468-472, Apr. 2021, doi: 10.1038/s41563-020-00817-z.

G. Ingarao, O. Zaheer, and L. Fratini, 'Manufacturing processes as material and energy efficiency strategies enablers: The case of Single Point Incremental Forming to reshape end-oflife metal components', CIRP J Manuf Sci Technol, vol. 32, pp. 145-153, Jan. 2021, doi: 10.1016/j.cirpj.2020.12.003.

F. Liu, Y. Li, S. Ghafoor, Z. Cheng, F. Li, and J. Li, 'Sustainability assessment of incremental sheet forming: a review', Mar. 01, 2022, Springer Science and Business Media Deutschland GmbH . doi: 10.1007/s00170-021-08368-6.

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The original version of this eBook is available on https://www.scientific.net/book/sustainability-on-material-forming/978-3-0364-1983-1

Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 9-20

doi: 10.4028/p-SCsG8B

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2025-12-11

Revised:
2026-02-11

Accepted:
2026-02-17

Online:
2026-04-17

Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

Pooya Hosseini 1,2,a*, Joost R. Duflou 1,2, b

1 KU Leuven, Mechanical Engineering Department, Celestijnenlaan 300, Heverlee 3001, Belgium

2 Member of Flanders Make, Gaston Geenslaan 8, 3001 Leuven, Belgium

a* seyedpooya.hosseini@kuleuven.be, bjoost.duflou@kuleuven.be

*corresponding author: seyedpooya.hosseini@kuleuven.be

Keywords: titanium machining chips, solid-state recycling, environmental impact assessment.

Abstract

Titanium alloys combine strength, low weight, and corrosion resistance, making them vital in high-performance industries; yet machining generates substantial chips that is difficult to recycle via conventional remelting due to contamination and high energy requirements, reducing material sustainability. Solid-state recycling methods, like Shear Assisted Processing and Extrusion (ShAPE), provide a promising alternative by consolidating chips below the melting point while preserving alloy chemistry. This study assesses the environmental performance of ShAPE across a system boundary spanning degreasing through consolidation and extrusion. Impacts were quantified using Cumulative Energy Demand (CED), Global Warming Potential, Environmental Footprint, Average Dissipation Rate (ADR), and Lost Potential Value (LPV), with ADR and LPV applied for the first time to solidstate recycling of scrap from discrete manufacturing. Scenario analyses addressed variations in torque, argon consumption, and electricity mix. Energy demand and CO2-eq for the ShAPE process were estimated at 279.51−567.75MJ and 17.22−32.35 kg per kg of wire, respectively, with sensitivity analysis showing that variations in torque constitute the dominant determinant of these environmental outcomes. While energy demand is comparable to, or moderately lower than, that of traditional wire fabrication only under low- and baseline-torque conditions, ShAPE substantially reduces the resource dissipation and lost material values, with its overall environmental impacts further decreasing by 45.45% when powered with greener electricity. These results highlight ShAPE as a viable route for circular titanium production, preserving material value & reducing dependence on primary extraction.

Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

Titanium and its alloys, particularly Ti6Al4V, play a central role in high-performance engineering sectors due to their exceptional strength-to-weight ratio, corrosion resistance, and biocompatibility. These properties make the alloy indispensable in aerospace structures, medical implants, and energy applications, where material performance directly influences system efficiency and safety [1], [2]. However, the strategic importance of Ti6Al4V is accompanied by a substantial environmental burden. Primary titanium production remains one of the most energy-intensive and carbon-intensive metallurgical processes in industrial use today, largely because it relies on the multistage Kroll process, high-temperature chlorination, and extensive vacuum metallurgy [3]. As global demand for titanium continues to rise, driven by lightweighting policies, expansion of aerospace manufacturing, and increasing adoption of additively manufactured components, the pressure on titanium supply chains and associated environmental impacts intensify accordingly. These trends highlight the urgency of improving circularity and developing efficient recycling strategies capable of reducing reliance on primary titanium while maintaining the alloy's stringent performance requirements. Machining operations used to fabricate Ti6Al4V components generate substantial quantities of chips, often representing 50-90% of the original billet mass depending on part geometry and manufacturing route [4]. Despite their high intrinsic material value, these chips constitute a challenging scrap stream due to their heterogeneous morphology, high surface-area-to-volume ratio, and susceptibility to oxidation and contamination during machining and handling. Conventional recycling practices for titanium, primarily based on remelting technologies such as vacuum induction melting and vacuum
arc remelting, are poorly suited to this form of scrap. Fine chips exhibit elevated reactivity at high temperatures, leading to burning losses, increased formation of interstitial contaminants, and compromised melt quality. As a result, chips are commonly downgraded to lower-grade feedstocks or blended with clean bulk scrap to dilute impurities, reducing their overall economic and environmental utility [5]. Moreover, remelting routes require substantial thermal energy input and complex vacuum processing to ensure alloy integrity, further diminishing the sustainability benefits typically associated with recycling. These limitations underscore the need for alternative recycling pathways tailored specifically to the traits and constraints of machining chips. In recent years, solidstate recycling technologies have emerged as promising alternatives to conventional remelting for the valorization of machining chips. These processes consolidate metallic scrap below the melting point, hence mitigating oxidation-driven losses and preserving valuable alloy chemistry. Unlike melt-based processes, solid-state methods do not remove pre-existing oxides or surface contaminants. The chemical composition of the incoming scrap therefore plays a critical role in determining the feasibility and final quality of the recycled product. In practice, this necessitates careful selection or segregation of scrap to ensure that oxygen and other interstitial elements remain within specification limits required by the target application. Approaches such as hot pressing, powder metallurgy, and severe plastic deformation have demonstrated the potential to convert discontinuous chips feedstocks into fully dense, structurally sound products [6], [7]. Among the emerging techniques, Shear Assisted Processing and Extrusion (ShAPE), developed at the Pacific Northwest National Laboratory (PNNL), offers a promising route for the conversion of Ti6Al4V machining chips into wire feedstock, which can subsequently be used in downstream processes such as wire-based additive manufacturing [8].

Although recent advances have demonstrated the technical feasibility of solid-state consolidation techniques, their environmental burden remains largely unexplored. The existing research on ShAPE, for instance, has predominantly focused on process mechanics, microstructural evolution, and mechanical performance. Comparable attention has not been given to quantifying the energy requirements, emissions, and resource savings associated with this type of melt-free recycling routes. Life cycle assessment (LCA) studies on titanium recycling have historically emphasized conventional remelting pathways, with limited consideration of chip-specific valorization strategies or emerging solid-state techniques. As a result, there is little evidence-based understanding of how processes like ShAPE perform in relation to established recycling routes, nor of the conditions under which they offer environmental advantages. This gap in the literature hinders informed decision-making regarding the integration of solid-state recycling technologies into titanium supply chains and limits the potential for data-driven evaluation of their role in circular material strategies.

The present study aims to address the knowledge gap in the environmental evaluation of solidstate recycling of Ti6Al4V machining chips via the ShAPE process developed at PNNL, USA. The study further incorporates a comparative assessment with conventional remelting-based recycling routes, allowing quantification of the relative environmental performance of the solid-state approach. By explicitly linking process characteristics, scrap selection, and operational parameters to environmental outcomes, this work provides a systematic and evidence-based evaluation of chipbased valorization strategies, offering insights into the sustainability potential and process design considerations for emerging solid-state recycling technologies in titanium supply chains.

Materials and Methods

The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

The environmental performance of Ti6Al4V chips recycling via the ShAPE process was evaluated in accordance with ISO 14040/44 standards. The LCA was performed using the SimaPro v10.1 software environment, and background data were sourced from the Ecoinvent v3.9 database (cut-off system model). All foreground processes were modelled using primary experimental data from this study. The goal of the study was to quantify energy consumption, material use, and environmental impact associated with converting machining chips into wire feedstock, and to compare the performance of the ShAPE process with conventional remelting-based recycling routes. The functional unit was defined as 1 kg of consolidated Ti6Al4V wire, providing a consistent basis for comparison across recycling pathways, with the system boundary encompassing washing, drying,
and consolidation/extrusion in the ShAPE process, as illustrated in Figure 1. Sorting of chips was not considered, based on the assumption that chips selection procedures are equivalent to those applied in traditional recycling. In the current industrial practice (Reference), titanium chip are briquetted and incorporated in limited proportions into a blend with titanium sponge and master alloys. This feed is assembled into a mechanically stable consumable electrode by welding the sponge pieces under argon shielding, with chips briquettes embedded within the structure. The electrode is then remelted in vacuum arc remelting to produce an ingot, which is subsequently processed via conventional thermomechanical steps into wire. A broad range of LCA methods was applied to capture the environmental performance of the ShAPE process from multiple perspectives, including the Environmental Footprint 3.1, Cumulative Energy Demand (CED), IPCC Global Warming Potential (GWP, 20-year horizon), and Mineral Resource Dissipation (MRD). To the best of the authors' knowledge, MRD has not previously been applied to any solid-state recycling techniques, providing an additional perspective on resource use in titanium scrap valorization. Due to the lack of publicly available industrial-scale life cycle inventory (LCI) data, the analysis relies on laboratory-scale process parameters and operational conditions reported by the process developers in literature [8], [9], which are used to estimate energy consumption, material inputs, and waste generation associated with chips consolidation and extrusion, as detailed in the following section.

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Fig. 1. Material and process flow of the ShAPE solid-state recycling route.

Fig. 1. Material and process flow of the ShAPE solid-state recycling route.

Analytical basis for Specific Energy Consumption (SEC) estimation

The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

The Specific Energy Consumption (SEC) for the lab scale Friction Stir Extrusion (FSE) was determined through an empirical analysis of the total electrical power input relative to the mass flow rate (m˙) using the process parameters reported in [8]. The total specific energy consumption, SEC total,  is defined by the energy balance that includes the specific mechanical energy input (SECme), the equipment's electrical efficiency ( ηeq ), and the auxiliary power specific consumption ( SECaux ). Using the billet diameter ( Dbillet =0.019 m ), ram velocity ( Vram=0.02 m/min≈3.333×10−4 m/s ), and the density of Ti6Al4V, the billet cross-sectional area ( Abillet ≈2.835×10−4 m2 ) allowed for the calculation:

m˙=Abillet ·Vram ·ρ≈4.19×10−4 kg/s(1)

The total mechanical power input (Ptotal ) delivered to the material is the sum of the rotational power ( Ptorque  ) and the axial thrust power ( Pram  ). The axial power, Pram =Fram Vram =90×103 N.3.333×10−4 m/s≈30 W, was calculated but noted as negligible compared to the rotational power. The rotational power, Ptorque =M.ω, utilizes the tool's angular velocity ( ω=2π N/60, with N=236 RPM, yielding ω≈24.71rad/s ). It is worth mentioning that the experimental torque ( M ) was unavailable and was therefore estimated to define a robust sensitivity range reflecting uncertainty in the effective
friction coefficient. Upper and lower bounds of 400 Nm and 100 Nm , with a baseline of 200 Nm , were selected as reasonable limits based on values in the literature for friction-based processing [9],[10].

SECME=Ptotal m˙(2)

The lower bound for the SECme was calculated to be 5.90MJ/kg, derived from the minimum estimated total mechanical power input of Ptotal, low =2.47 kW. Conversely, the upper bound for the SEC SEM  was determined by the maximum estimated power input, Ptotal , high =9.91 kW, resulting in a value of 23.68MJ/kg. The SEC total  integrates the SEC me  range with the uncertainty in ηeq  and SEC aux . The SEC aux  accounts for the constant power draw of the lab setup's ancillary systems ( Paux ∈[550, 1250]W ), and is calculated as SECaux=Paux/m, yielding a range of 1.31 to 2.99MJ/kg.

SECtotal=(SECMEηeq)+SECaux(3)

The sensitivity analysis provided the bounding SEC total values: 8.26MJ/kg (using SECme, low, ηeq, high =0.85, and SECaux, low  ) to 34.6MJ/kg(SECmE, high, ηeq, low =0.75, and SECaux, high ). To provide a thermodynamic context, the Specific Energy Retained (SECretained) in the material was quantified as the minimum theoretical energy required to heat the Ti6Al4V sample from ambient temperature to the extrusion temperature ( Text ≈980∘C ). Utilizing an estimated average specific heat capacity of Ti6Al4V ( Cp―≈600 J/kg∘K ) over the temperature rise ( ΔT=955∘K ):

SECretained =Cp―·ΔT≈0.57MJ/kg(4)

The thermal efficiency (ηth ), defined as the ratio of retained energy to input mechanical energy (SECretained/SECme), was calculated to be between 1.7% and 4.8%. This result emphasizes that the large SECme is necessary not for the net heating of the titanium, but primarily to provide the high mechanical power needed to overcome the temperature-dependent flow stress (28−35MPa at 1000∘C, and 52−59MPa at 950∘C at the strain rate of 0.1/s [11]) and to compensate for massive continuous heat loss (Ploss ≈Ptotal −Pretained ) to the surrounding tooling and machine structure, which is typical for FSE on materials with high processing temperatures [12]. The specific argon consumption was estimated to quantify the inert-gas auxiliary load required for processing reactive titanium alloys at elevated temperatures. In the absence of measured flow data, literature values for inert-gas shielding in high-temperature solid-state titanium processing were used. A conservative argon flow rate of 4 1/ min was assumed to maintain a protective atmosphere around the friction tool and heated workpiece. The total argon consumption was determined by multiplying this flow rate with the total estimated run time, accounting for both the active extrusion and the necessary gas shielding during tool plunge, dwell, and initial cooling ( 7.5 minutes total). The total volume of 301 was then normalized by the mass of the material extruded ( 0.188 kg ) to yield the specific consumption ( 0.16Nm3/kg−Ti ).

The energy and material consumption for the prior Friction Stir Consolidation (FSC) step was estimated for the billet. Since the process description was derived from a lab-scale, multi-charge batch method [9], the Specific Energy Consumption (SEC FSC ) is governed by 3 primary factors: mechanical work ( Eme ), fixed auxiliary overhead ( Eaux  ), and pre-heating ( Epreheat  ). The total electrical energy required for the consolidation of 1 billet was normalized by the final consolidated billet mass ( mbillet  ).

SECFSC=1mbillet [EMEηeq +Eaux +Epreheat ](5)

The billet mass was determined from the process parameters: mbillet =3 charges ×0.016 kg/ charge =0.048 kg. The total time the machine and auxiliary systems were running was calculated to be 600 seconds, including pre-compaction, 3 consolidation cycles (at 60 s each), setup, and a final cooling period. The mechanical energy applied is derived from the rotating tool's power integrated over the active consolidation time ( tactive =180 s ). In the absence of direct torque measurements, a sensitivity analysis was performed based on comparable friction stir processes using a ∅19 mm tool & 450 RPM :

EME=(M·ω)·tactive (6)

Where M is the rotational torque and ω is the angular velocity ( 47.12rad/s ). The torque sensitivity analysis utilized a range of 100 to 400 Nm for the rotational torque, which consequently translated to a mechanical power range of 4.71 to 18.85 kW . The axial ramming power was included but deemed negligible ( ≈1.1 W ). The final mechanical energy value was corrected by an assumed machine efficiency ηeq∈[0.75,0.85]. The auxiliary energy, which covers the fixed power draw of the machine's control system, cooling unit, and motors over the entire 600 s run time, was calculated by assuming a standard auxiliary power draw of 1000 W according to [13], resulting in a total Eaux 600 kJ; this yields a significant fixed specific energy overhead of 12.5MJ/kg that noticeably influences the final SECFSC value due to the disproportionately small billet mass ( 0.048 kg ). The energy required to pre-heat the chips to 200∘C was calculated based on the billet mass, the specific heat capacity of titanium, and a heater electrical efficiency (ηoven ≈0.6[14]) : Epreheat =9.24 kJ. Thus, the calculated SECFSC based on the sensitivity range for torque is [33.49,106.95]MJ/kg. During the FSC, argon gas was supplied locally to the chip feed region and tool-material interaction zone via an external nozzle positioned adjacent to the rotating tool. The argon flow was applied continuously throughout the FSC to minimize chips oxidation at high temperatures. The process was conducted in ambient atmosphere; no sealed or fully enclosed chamber was used. The argon consumption was estimated using a flow rate of 41/min over the total 10 min machine run time, yielding a total volume 401 , which, when normalized by the billet mass of 0.048 kg results in a specific argon consumption of 0.833Nm3/kg.

It is worth noting that a unified torque sensitivity range of 100−400Nm was assumed for both FSE and FSC to reflect the balance between their distinct mechanical demands. In FSE, high torque is required to overcome the flow stress of a dense billet and maintain continuous material flow under substantial extrusion pressure. In contrast, although FSC involves lower initial resistance due to the loose packing of chips, this is offset by two factors: (i) a 90% higher rotational speed ( 450 vs. 236 RPM), which proportionally rises the power demand, and (ii) the scroll-patterned tool face geometry, designed to enhance material transport and induce intense shear deformation. The combined effect of increased rotational speed and tool-induced shear compensates for the lower chip packing resistance, thus justifying the adoption of a similar torque range for both processes. A sensitivity analysis was performed to evaluate how variations in inert-gas usage influence the environmental performance of the ShAPE. Since the process developer reported that higher argon flow rates during the FSC and FSE stages help reduce oxidation and improve wire quality, the total argon consumption in these steps was scaled using a dimensionless Specific Argon Consumption Coefficient (SACC). The actual argon demand for consolidation and extrusion was multiplied by SACC to reflect different operating conditions and quality targets. The adjusted argon consumption is calculated as:

Arsensitivity =SACC.Aractual ,1≤SACC≤2(7)

Although FSE is often performed directly on cold-briquetted chips, an intermediate FSC step was intentionally introduced in this work. FSC enables enhanced inter-chip bonding, oxide layer disruption, and densification compared to conventional briquetting, which is particularly critical for Ti6Al4V chips. This pre-consolidation results in a more homogeneous feedstock, improving process stability during FSE and reducing the risk of defects such as incomplete bonding. While FSC introduces an additional energy input, it contributes to improved material integrity and can reduce extrusion forces and process variability during FSE, supporting a system-level optimization rather than minimizing energy at a single process step. The LCI of the ShAPE recycling process is summarized in Table 1.

Table 1. Life Cycle Inventory of the ShAPE process (values in the 'Quantity' column are reported per 1kg of material processed).

Step | Input | Quantity | Remark
CNC Turning | Electricity & auxiliary | [15] | Turning was modeled using the Ecoinvent process 'Chromium steel removed by turning, primarily roughing', adjusted for the specific cutting energy of Ti6Al4V (0.89-1.27 J/mm³ [16], [17]).
Degreasing & Drying | Electricity | 1.70 kWh | The washing step was modelled using the Ecoinvent process 'Degreasing, metal part in alkaline bath'. Drying was based on [18], assuming a mid vacuum oven (53l) with 40-70% efficiency [19]. No solvent and heat recovery were assumed.
Auxiliary | [15]
Briquetting | Electricity | 0.40 kWh | The parameter was adopted from the specifications of an industrial briquetting machine, based on data reported in [20].
Friction Stir Consolidatio | Electricity | 9.30 kWh (low torque) 15.07 kWh (baseline) 29.70 kWh (high torque) | The range of reported electricity consumption reflects the dependence of energy requirements on the different torque values applied during the process calculations..
Argon | 0.83 Nm³ |
Friction Stir Extrusion | Electricity | 2.29 kWh (low torque) 4.25 kWh (baseline) 9.61 kWh (high torque) | The range of reported electricity consumption reflects the dependence of energy requirements on the different torque values applied during the process calculations.
Argon | 0.16 Nm³ |
Waste Trearine | - | 5-10% | Ti6Al4V waste generated during the ShAPE is assumed unsuitable for reuse and is instead sent to downstream ferrotitanium production, for which environmental credits are included.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

Figure 2 presents the normalized impacts of the ShAPE process for the low- and high-torque boundary conditions, which span the energy demand estimates described earlier. Under all torque conditions, the ShAPE route exhibits substantially lower impacts than conventional Ti6Al4V wire manufacturing when the latter is modeled without scrap incorporation. However, this comparative advantage diminishes once a more realistic Reference scenario with 60% scrap integration [21] is considered. In this case, the ShAPE process achieves environmental performance comparable to traditional wire fabrication only under the low- and baseline-torque conditions; at the high-torque extreme, its relative environmental performance substantially degrades, exhibiting a noticeable increase in some impact categories. The impacts reported for ShAPE in Figure 2 assume a SACC of 1. Since higher SACC values reduce oxidation during FSC and FSE, as experimentally demonstrated in [8], but require proportionally more argon, rising SACC is expected to worsen the overall impacts.

The high normalized impacts for the Reference at a scrap-to-sponge (SS) ratio of 0% primarily arise from the titanium sponge production via the Kroll process. This route is dominated by energyintensive chlorination, magnesio-thermic reduction, distillation, and Mg recovery steps, collectively resulting in great environmental burdens. One exception is the slightly higher impact of the ShAPE in the 'Ionising radiation' category relative to the Reference. This difference is attributable to the contrasting electricity mix profiles between the region where the ShAPE was developed and the regions supplying titanium sponge in the Reference. The LCA assumes sponge in the Reference to be sourced from China ( 42.5% ) and Japan ( 24.5% ), with the remaining 33% attributed to regions with impact profiles similar to Chinese production. According to the International Energy Agency, the contribution of nuclear power to the electricity supply in the United States, China, and Japan is 17.8%, 4.6%, and 9.5%, respectively. The higher nuclear share in the U.S. grid, where ShAPE recycling route was developed, thereby explains the comparatively higher contribution to the 'Ionising radiation'.

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Fig. 2. Normalized impact results of the ShAPE process (for both torque extremes) compared to the Reference for producing 1 kg of wire, assessed using the Environmental Footprint method. The impacts shown for the ShAPE, at all torque conditions, correspond to the actual argon consumption with the lowest specific argon consumption coefficient. The values labeled as ' SS ' in the Reference scenario represent the fraction of titanium scrap incorporated into the consumable electrode used in conventional vacuum arc remelting.

Fig. 2. Normalized impact results of the ShAPE process (for both torque extremes) compared to the Reference for producing 1 kg of wire, assessed using the Environmental Footprint method. The impacts shown for the ShAPE, at all torque conditions, correspond to the actual argon consumption with the lowest specific argon consumption coefficient. The values labeled as ' SS ' in the Reference scenario represent the fraction of titanium scrap incorporated into the consumable electrode used in conventional vacuum arc remelting.

The CED and CO2-eq emissions associated with the ShAPE process per intermediate product for producing 1 kg of Ti6Al4V wire are presented in Figure 3, including the torque-based sensitivity ranges derived from the calculations described earlier for both the FSC and FSE stages. Under the SACC=1, the process was found to require 279.51−567.75MJ of cumulative energy and to emit 17.22−32.35 kgCO2-eq per kg of wire, depending on the torque demand during consolidation and extrusion. When compared to traditional wire fabrication, assuming a 60% scrap input typical of U.S. vacuum arc remelting [21], the ShAPE route exhibits a variation of −25.34% to +51.64% in CED and −47.69% to −1.75% in CO2-eq relative to the Reference. These findings indicate that, under the discussed process parameter combinations, the ShAPE route can become highly energy intensive, with energy requirements that surpass those of traditional remelting-based wire production [22]. It is important to note, however, that the present comparison is based on laboratory-scale operational characteristics, including batch-mode processing. Noticeable impact improvements are expected as the process transitions to pilot scale, where higher throughput and improved thermal management can reduce per-unit impacts. The sensitivity analysis further reveals that variations in the torque needed to overcome materials flow at chips interfaces have a great influence on both consolidation and
extrusion performance. Among all process stages, FSC contributes the largest share to both CED and CO2-eq, followed by the FSE step, reflecting their dominant roles in the process energy balance.

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Fig. 3. Cumulative energy demand and CO2-eq emissions of the ShAPE route, respecting the material/process flow shown in Figure 1. The values reported assume that both the consolidation and extrusion operate at the lowest specific argon consumption coefficient.

Fig. 3. Cumulative energy demand and C O 2 -eq emissions of the ShAPE route, respecting the material/process flow shown in Figure 1. The values reported assume that both the consolidation and extrusion operate at the lowest specific argon consumption coefficient.

Figure 4 illustrates the relative variation in CED and CO2-eq for the ShAPE process as a function of the SACC across the two torque extremes. The results clearly show that torque variation exerts a substantially greater influence on the impacts than changes in SACC. This is particularly evident in cases where doubling the torque requirement (Baseline → High) produces a considerably larger impact shift than increasing SACC from 1 to 2 . Under the examined process conditions, the ShAPE provides only marginal CED advantages over traditional wire fabrication, and only when operating at the lower torque boundary when considering the average value of the SACC. In the ShAPE route, electricity is the sole energy carrier across all process steps, whereas the Reference scenario includes both electrical and thermal energy inputs at various stages of the titanium value chain. This difference in energy mix explains the divergence between the CED and CO2-eq observed decreasing trends.

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Fig. 4. Cumulative energy demand and CO2-eq variation of the ShAPE process for producing 1 kg of Ti6Al4V wire relative to the traditional wire fabrication (Reference), shown as a function of torque change and the specific argon consumption coefficient (SACC).

Fig. 4. Cumulative energy demand and C O 2 -eq variation of the ShAPE process for producing 1 kg of Ti6Al4V wire relative to the traditional wire fabrication (Reference), shown as a function of torque change and the specific argon consumption coefficient (SACC).

Although the ShAPE recycling process does not substantially reduce energy demand compared to conventional wire fabrication, its inherent characteristic of eliminating primary titanium sponge consumption leads to significant improvements in other critical environmental indicators, namely the Average Dissipation Rate (ADR) and the Lost Potential Value (LPV). The ADR quantifies the rate at which a material's inherent utility or economic potential is irreversibly dissipated over its lifecycle, reflecting the depletion of critical resources due to consumption or dispersion. The LPV measures the unrealized value of a material when it is not effectively recovered or recycled, representing the potential economic or functional utility loss when a material exits the circular supply chain [23]. The ShAPE process enables reductions of up to approximately 12 kg Fe-eq in ADR and 69 USD in LPV per 1 kg of wire produced, relative to traditional remelting-based fabrication ( SS=60% ) as shown in Figure 5. These results highlight the process's ability to preserve material value and resource potential, providing measurable benefits for resource efficiency and circularity in titanium production. In a complementary scenario analysis, the ShAPE was assessed under an alternative electricity supply configuration with a more balanced distribution of fossil and non-fossil energy sources, represented by the hypothetical 'Region B' defined in Table 2. Under the low-torque condition and the highest SACC value, situating the ShAPE process in 'Region B', whose electricity mix contains 43% more green energy than that of the United States (average mix), reduces the overall impact by around 45.45%, underscoring the substantial influence of regional electricity composition on the process's environmental performance. Compared to conventional wire fabrication, the adoption of a greener electricity mix yields an overall impact reduction of 55.9% (Figure 6).

Specific Argon Consumption Coefficient (SACC)

Fig. 5. Reduction in Average Dissipation Rate (ADR) and Lost Potential Value (LPV), relative to traditional wire fabrication ( SS=60% ), as a function of torque extremes and SACC, evaluated using the methodology described in [23].

Table 2. Electricity supply profile of the United States, where the ShAPE process was developed, compared to 'Region B', a hypothetical region with a higher share of renewable electricity in the energy mix.

Region | Non-renewable (%) | Renewable (%)
| Fossil | Nuclear | Wind, Solar, Geothermal | Water | Biomass
United States (average mix) | 63 | 25 | 7 | 3 | 2
Total | 88 | 12
B | 32 | 13 | 25 | 13 | 17
Total | 45 | 55

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Fig. 6. Comparison of the overall environmental impact of the ShAPE process operated in two regions with different electricity mixes versus traditional manufacturing (Reference scenario, SS=60% ) for the production of 1 kg of Ti6Al4V wire.

Fig. 6. Comparison of the overall environmental impact of the ShAPE process operated in two regions with different electricity mixes versus traditional manufacturing (Reference scenario, S S = 60 % ) for the production of 1 kg of Ti6Al4V wire.

Conclusion

The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

This study presents an environmental impact assessment of solid-state recycling of Ti6Al4V machining chips via Shear Assisted Processing and Extrusion (ShAPE), using 1 kg of wire as the functional unit and a system boundary including washing and drying, and consolidation/extrusion. Impacts were evaluated with multiple metrics; Environmental Footprint, Cumulative Energy Demand (CED), Global Warming Potential, Average Dissipation Rate (ADR), and Lost Potential Value (LPV), under varying torque conditions, argon consumption, and alternative electricity mixes. The ShAPE requires 279.51−567.75MJ of CED and emits 17.22−32.35 kgCO2-eq per kg of wire, with torque needs in consolidation/extrusion governing energy demand and emissions. Low- and baselinetorque conditions yield environmental performance comparable to the Reference, while high-torque extremes substantially elevates some impact categories. Sensitivity analysis indicates torque has a greater influence than argon use. Although energy demand is not consistently reduced, ShAPE improves resource efficiency by eliminating primary titanium sponge input, lowering ADR by approximately 12 kg Fe-eq and LPV by 69 USD per kg of wire. A greener electricity mix further reduces overall impacts by 45.45%, or 55.9% relative to traditional wire fabrication. Limitations to the study include reliance on a lab-scale experimental setup-based life cycle inventory and batchmode operation, which may overestimate per-unit impacts. Overall, ShAPE recycling offers a promising route for circular titanium manufacturing, preserving material value, reducing primary extraction, and supporting sustainable production. Optimizing process parameters and scaling up, particularly with low-carbon electricity, could further enhance its environmental performance.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.9.pdf

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G. Mustafaraj, J. Cosgrove, M. J. Rivas-Duarte, F. Hardiman, and J. Harrington, "A methodology for determining auxiliary and value-added electricity in manufacturing machines," Int. J. Prod. Res., vol. 53, no. 17, pp. 5265-5277, Sep. 2015, doi: 10.1080/00207543.2015.1026615;JOURNAL:JOURNAL:TPRS20;WGROUP:STRING:PUB LICATION.

M. E. Ertem, S. Şen, G. Akar, C. Pamukcu, and S. Gurgen, "Energy balance analysis and energy saving opportunities for erdemir slab furnace #3," Energy Sources, Part A: Recovery, Utilization and Environmental Effects, vol. 32, no. 11, pp. 979-994, Jan. 2010, doi: 10.1080/15567030903436863;WGROUP:STRING:PUBLICATION.

"Database - ecoinvent." Accessed: Jul. 24, 2025. [Online]. Available: .

M. Younas, S. H. I. Jaffery, A. Khan, and M. Khan, "Development and analysis of tool wear and energy consumption maps for turning of titanium alloy (Ti6Al4V)," J. Manuf. Process., vol. 62, pp. 613-622, Feb. 2021, doi: 10.1016/J.JMAPRO.2020.12.060.

M. Ali Khan et al., "Statistical analysis of energy consumption, tool wear and surface roughness in machining of Titanium alloy (Ti-6Al-4V) under dry, wet and cryogenic conditions," Mechanical Sciences, vol. 10, no. 2, pp. 561-573, Dec. 2019, doi: .

Z. Wang, Y. Bao, and M. Wang, "The effect of different cleaning methods for titanium chips on the carbon content of ferrotitanium after remelting," Journal of Materials Research and Technology, vol. 18, pp. 2208-2216, May 2022, doi: 10.1016/J.JMRT.2022.03.115.

"Lab1st 1.9 cu ft Vacuum Drying Oven 53L-1ab1st-eshop." Accessed: Jul. 28, 2025. [Online]. Available: .

"Arno®Brik - ATM Recyclingsystems." Accessed: Nov. 04, 2025. [Online]. Available: .

O. Takeda and T. H. Okabe, "Recycling of Ti," Extractive Metallurgy of Titanium: Conventional and Recent Advances in Extraction and Production of Titanium Metal, pp. 363387, Jan. 2020, doi: 10.1016/B978-0-12-817200-1.00016-8.

B. Hendrickx et al., "Industrial symbiosis in aluminium parts manufacturing: Towards material efficiency by combining friction stir extrusion and molten metal deposition," Sustainable Materials and Technologies, vol. 46, p. e01755, Dec. 2025, doi: 10.1016/J.SUSMAT.2025.E01755.

A. Charpentier Poncelet et al., "Midpoint and endpoint characterization factors for mineral resource dissipation: methods and application to 6000 data sets," The International Journal of Life Cycle Assessment 2022 27:9, vol. 27, no. 9, pp. 1180-1198, Sep. 2022, doi: 10.1007/S11367-022-02093-2.

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 21-27

doi: 10.4028/p-Jy7jtU

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2025-12-18

Revised:
2026-02-09

Accepted:
2026-02-17

Online:
2026-04-17

Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

Peymani Dorna 1,a* and Strano Matteo 2, b

Politecnico di Milano, Dipartimento di Meccanica Via Previati 1/c, Lecco, Italy

a*Dorna.peymani@polimi.it, b  matteo.strano@polimi.it

*Corresponding author

Keywords: polypropylene (PP), recycled polypropylene (rPP), pellet-based additive manufacturing (AM), sustainable manufacturing.

Abstract

This study evaluates the processability of recycled polypropylene (rPP) in pellet-based material extrusion (MEX) to support more sustainable additive manufacturing. Virgin polypropylene (PP) and post-consumer rPP obtained from end-of-life woven builder bags were processed in neat form and as PP/rPP blends with increasing recycled content. Melt-flow behavior was characterized using a Technological Melt Flow Index (TMFI), a process-specific metric reflecting the combined effects of temperature and screw rotation. Disk-shaped specimens were printed to assess deposition behavior through the build-up rate (BUR), which integrates shear flow in the extruder and elongational deformation during deposition. TMFI results show that rPP exhibits markedly higher flowability than virgin PP below 200∘C, indicating potential for lower-temperature, energy-efficient processing. In contrast, printing experiments reveal that BUR systematically decreases with increasing rPP content. This trend indicates a transition to an elongation-dominated deposition regime, where rPP displays higher resistance to extensional deformation during deposition, resulting in narrower roads and reduced spreading. Regression analysis confirms that BUR is governed primarily by flow-rate setting ( F% ) and nozzle speed ( Sp% ), whereas nozzle temperature Te(∘C) has only a minor influence within the investigated window. Overall, the results demonstrate the competing rheological effects introduced by recycling and highlight the need for tailored parameter optimization to enable higher rPP incorporation in pellet-based 3D printing.

Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

The widespread use of plastics has grown rapidly in recent decades, driven by their low cost, durability, and versatility across nearly all industrial sectors. However, the accumulation of plastic waste, combined with inefficient collection and disposal practices, has created significant environmental concerns. These challenges have intensified the need for effective recycling strategies that can reduce the environmental impact of plastics while conserving raw materials and energy resources [1]. According to the data, the most often used plastics are polypropylene (PP), low density polyethylene (LDPE), poly (vinyl chloride) (PVC), and high-density polyethylene (HDPE) [2].
Mechanical recycling is commonly carried out for thermoplastic polymers such as PP, poly(ethylene terephthalate) (PET), polyethylene (PE), polystyrene (PS), and PVC [3]. This process first sorts the plastic waste, followed by shredding. Shredding consists of breaking down the part into small pellets, granules, or powder forms. The recycled material is processed to produce the final component through material melting and extrusion. The processing techniques primarily involve extrusion and injection molding [4].
Material extrusion (MEX) is an AM process in which a polymer is selectively dispensed through a heated nozzle and deposited layer by layer to form a three-dimensional object [5]. Recycling thermoplastic waste for MEX may lead to reductions in shear viscosity, molecular weight, and thermal or mechanical properties after one or more cycles of processing [6]; however, some of these effects can be mitigated by appropriately adjusting the process parameters [7]. Degradation during recycling also affects the compatibility of polymer blends [8]. Various combinations of process parameters in MEX lead to different fabrication qualities and mechanical properties. Fused Filament

Fabrication (FFF) and Fused Granular Fabrication (FGF) are among the most common additive manufacturing methods based on material extrusion [9]. Unlike traditional filament-based printing, FGF directly processes polymer pellets, enabling the in-line blending of different materials during the printing operation. This capability allows the production of parts that combine multiple functional properties, such as enhanced strength and improved thermal stability, within a single manufacturing process [10].
Recycled polypropylene (rPP) exhibits distinct processing behavior compared to virgin PP, particularly in its Melt Flow Index (MFI), a widely used indicator of material processability [11]. MFI reflects how polypropylene flows during melting and shaping, directly influencing processing stability and final part quality. Recycled PP typically shows higher MFI than virgin PP due to structural degradation during service life and reprocessing, including reductions in molecular weight and melt viscosity, as well as changes in crystallization and mechanical response. The dominant consequence of recycling is chain scission, which lowers melt viscosity and increases MFI, thereby affecting both processing performance and the properties of printed components. Although shear viscosity ( η ) and MFI are standard metrics for characterizing polymer melts, the MEX 3D-printing process can be influenced by elongational viscosity [12], a relationship for which limited literature evidence is currently available. Given the higher MFI of rPP compared to virgin PP, one might expect that materials with high recycled content could be processed at lower nozzle temperatures. In practice, however, the printability of rPP is often poor. While insufficient interlayer adhesion contributes to this issue, a more significant factor may be the elevated elongational viscosity of recycled polymers [13].
The present paper aims to demonstrate these effects to advance understanding of the underlying extrusion mechanisms and to support future improvements in the energy efficiency of 3D printing with recycled materials. The paper therefore investigates the flow behavior and processing response of recycled polypropylene (rPP) in comparison with virgin PP under material-extrusion conditions. To this end, the flow behavior of both materials was examined under controlled variations of nozzle temperature and screw speed, and a series of PP/rPP blends was prepared to explore the potential for increasing recycled content while maintaining stable printing performance.

Materials and Equipment

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In this work, a virgin random-copolymer polypropylene (PPR 3260, TotalEnergies) was used in pellet form, with a Melt Flow Index (MFI) of 1.8 g/10 min(230∘C/2.16 kg, ISO 1133) and a density of 0.902 g/cm3, representative of a standard extrusion-grade material. The recycled random polypropylene (rPP) was supplied by Gamma Meccanica (Bibbiano, Italy) and produced from postconsumer end-of-life woven builder bags. The rPP is available in three different colors: brown, pine (dark green), and light green. This material exhibits a substantially higher MFI, although the specific value was not measured by the supplier. The materials were processed in their neat forms and in controlled blends to assess how increasing rPP content influences melt rheology and printability under MEX conditions.
Both virgin and recycled polypropylene were supplied in pellet form and required drying prior to extrusion, as PP can absorb small amounts of moisture that cause voids, bubbles, surface defects, and reduced mechanical performance. To ensure consistent material quality, all pellets were dried using a rotating-drum dehumidifying dryer (3devo Precision Pellet Dryer), whose continuous stirring ensures uniform heating. Drying was conducted at 80∘C for three hours, effectively removing moisture while avoiding thermal degradation.
All tests were performed using a pellet-extrusion printer (Direct3D F30 Pro), equipped with a dedicated granular screw extruder and a 0.8 mm nozzle. The system employs a Cartesian frame with linear guide rails, enabling stable deposition and precise control of extrusion dynamics. Printing parameters were controlled through the open-source slicing software Super Slicer.
In the slicer software, all G-code files were generated with a nominal transverse printing speed of 20 mm/s. However, this value can be overridden at the start of each print using the Sp% parameter, which increases or decreases the programmed feed rate. The machine firmware (Marlin) then
determines the corresponding screw rotational speed (RPM) required to achieve the commanded nozzle speed. The printer software also allows independent adjustment of the screw's reference rotational speed through the flow-rate percentage parameter F%, which proportionally modifies the targeted material throughput. In practice, neither Sp% nor F% can be increased indefinitely, as the extruder is subject to motor torque and power limitations; therefore, the relationship between these parameters and the actual flow-rate is not linear and may reach saturation at high settings. Moreover, at elevated screw speeds, wall slip may occur, meaning the true material flow-rate does not necessarily continue to increase with higher Sp% or F%.

Melt Flow Characterization

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

The flow behavior of the polypropylene feedstocks was experimentally characterized under processing conditions representative of FGF.
In this study, the term Technological Melt Flow Index (TMFI) denotes a process-specific flow metric obtained directly from the pellet extruder, with the print head held stationary and positioned far above the build plate. Extrusion tests were thus performed by vertically extruding material into air. Unlike the standard MFI defined by ISO 1133 or ASTM D1238-which measures polymer flow under lowshear, fixed-temperature conditions that differ significantly from those in pellet-based extrusion, this metric reflects melt output under the actual thermo-mechanical conditions of the process. The TMFI is calculated by normalizing the extruded mass to the measured extrusion time and reporting the result in g/10 min to facilitate comparison across processing conditions.
The TMFI was computed as:

TMFI( g/10 min)= Extruded Mass M( g) Extrusion time t( s)×600(1)

Four polypropylene feedstocks were examined: one virgin PP and three recycled variants distinguished by color (brown, pine, and green). Each material was extruded at three temperatures Te ( 170,200 , and 230∘C ) and two screw-rotation speeds RPM ( 30 and 60 rpm ) summarized in (Table 1). For consistency across trials, each extrusion run comprised exactly 50 screw revolutions. Five replicates were produced for every material-temperature-screw-speed combination, resulting in 120 samples in total. Parameter combinations were alternated throughout the testing sequence to minimize systematic effects associated with temperature drift or residence-time variation within the extruder. After cooling, each extruded strand was weighed, and its TMFI was calculated. The resulting dataset, comprising material type, Te, RPM speed, and flow response TMFI, was analyzed using the statistical software JMP. A standard least squares regression model was built to evaluate the main and interaction effects governing melt-flow behavior.

Table 1. Processing parameters for melt flow characterization of polypropylene feedstocks.

Material | Nozzle temperature (°C) | screw-rotation speeds (rpm)
Virgin PP | 170 / 200 / 230 | 30 / 60
Recycled PP (brown, pine, green) | 170 / 200 / 230 | 30 / 60

3D Printing of specimens with mixtures

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

To experimentally evaluate the printability of recycled polypropylene (rPP), a series of PP/rPP blends with increasing recycled content were prepared. Starting from a 100% virgin PP baseline, additional mixtures were produced, in which 25%,35%, and 50% of the total material weight was replaced by recycled PP . The aim was to explore whether higher fractions of recycled material can be incorporated in a sustainable manner while maintaining stable material-extrusion performance. All mixtures were processed into small disk-shaped specimens with nominal dimensions of 28.2×28.2×0.6 mm (Fig. 1), which were selected as a representative test geometry for systematically probing extrusion behavior.

For each formulation, three printing parameters were varied: nozzle temperature Te(∘C), Printing speed factor Sp%, and the imposed flow-rate percentage F%.
The disk-shaped specimens were printed under the conditions reported in Table 2.

Table 2. Printing parameters for disk-shaped specimens.

Parameter | Value
Printing method | FGF
Materials | Virgin PP / rPP blends (100/0, 75/25, 65/35, 50/50)
Nozzle diameter | 0.8 mm
Layer thickness | 0.3 mm
Nozzle temperature (°C) | 164–225
Printing speed factor, Sp% | 25–200
Flow-rate percentage, F% | 2500–9000

These settings were selected to investigate how thermal input and deposition rate influence the flow behavior and extrusion stability of the blends. During each print, the mass flow-rate (MFR) was recorded as the primary output variable, serving as an operational analogue to melt-flow index under processing-relevant conditions, while the resulting deposition behavior was subsequently quantified through the build-up rate (BUR).
The MFR data were analyzed to quantify the sensitivity of flow behavior to changes in temperature, flow-rate setting, speed and blend composition.

[picture]

Fig. 1. Disk-shaped specimen geometry used in the 3D-printing experiments.

Fig. 1. Disk-shaped specimen geometry used in the 3D-printing experiments.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

Thermo-Mechanical Effects on Melt Flow Behavior.

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

A clear distinction is observed between virgin PP and the rPP feedstocks across the investigated processing window (Fig. 2). At low nozzle temperatures, particularly at 170∘C, virgin PP exhibits markedly reduced flowability, resulting in substantially lower TMFI values compared to all recycled materials. In contrast, the rPP streams maintain moderate mass flow under the same conditions, reflecting their intrinsically lower melt viscosity. This behavior is consistent with the reduction in molecular weight typically associated with mechanically recycled polypropylene, arising from chain scission during service life and reprocessing.
As nozzle temperature and screw rotational speed increase, the flow response of the materials diverges significantly. At low shear rates, polymer chains remain in a highly entangled configuration, leading to elevated viscosity and limited flow. With increasing thermal and mechanical input, the
polymer chains experience enhanced deformation, promoting disentanglement and alignment along the flow direction, which in turn reduces the effective viscosity of the melt.
Under aggressive processing conditions ( 230∘C and 60 RPM ), virgin PP undergoes a pronounced increase in TMFI, ultimately exceeding the flow-rates of all recycled variants. This crossover behavior indicates a stronger sensitivity of virgin PP to the combined effects of temperature and shear, suggesting a dominant shear-thinning response at elevated shear rates. In contrast, the recycled materials exhibit a more gradual and near-linear increase in TMFI with increasing temperature and rotational speed, pointing to a more constrained rheological response.
The statistical analysis of the TMFI dataset confirms a strong and systematic dependence of melt flow behavior on material type, nozzle temperature, and screw rotational speed. The regression model demonstrates an excellent overall fit ( R2=0.91 ), indicating that the selected processing parameters successfully capture the dominant mechanisms governing mass flow during pellet-based extrusion. Diagnostic evaluation confirmed the robustness of the model, with no evidence of significant outliers or systematic deviations.

[picture]

Fig. 2. TMFI response of virgin and recycled polypropylene as a function of Te(∘C) and RPM.

Fig. 2. TMFI response of virgin and recycled polypropylene as a function of T e ( ∘ C ) and RPM.

The regression analysis further highlights the significant interaction between Te ( ∘C ) and RPM. Temperature increments exert a substantially greater influence on TMFI when the extruder operates at higher rotational speeds, whereas at lower speeds the same temperature increase produces a comparatively modest effect. This interaction reflects the coupling between thermal softening and mechanically induced shear, which together governs the effective viscosity of the melt during extrusion.
While quantitative differences in TMFI are observed among the brown, pine, and green recycled PP streams, their overall flow responses follow similar trends, indicating that processing conditions exert a stronger influence on melt flow behavior than feedstock-specific variations within the investigated range.
At higher screw speed of 60 RPM, this ordering is no longer systematically preserved across the investigated temperature range. Under increased shear input, virgin polypropylene exhibits a more pronounced shear-thinning behavior, resulting in a rapid decrease in melt viscosity and a corresponding increase in TMFI. Consequently, the blend does not necessarily maintain an intermediate melt flow response at elevated screw speeds, highlighting the strong process dependence of melt flow behavior in polypropylene systems.
In conclusion, according to the TMFI data, the rPP should allow significantly higher flow-rate if printing below 200∘C, potentially allowing energy efficiency improvements over the virgin counterpart.

Results of 3D Printed Specimens with Mixtures

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

The fabrication of disk-shaped specimens enabled evaluation of melt-flow behavior under depositiondominated MEX conditions. In this regime, the polymer flow response is determined not only by shear deformation within the extruder but also by the elongational stretching and spreading of the
melt during deposition. Accordingly, the measured BUR represents an effective mass-flow metric that integrates the melt's rheological characteristics with the deformation imposed during printing. Regression analysis of the experimental dataset shows that BUR is primarily controlled by the imposed flow-rate percentage F% and the nozzle travel speed Sp%, while the influence of nozzle temperature Te(∘C) is comparatively weak within the investigated processing window.
A key observation is the systematic decrease in BUR as the recycled polypropylene content increases (Fig. 3). This trend contrasts with the shear-dominated behavior seen in TMFI measurements and indicates a shift toward an elongation-dominated flow regime during deposition. As the melt exits the nozzle, it undergoes significant elongational deformation driven by nozzle motion. Despite its lower shear viscosity, recycled polypropylene appears to resist elongational flow more strongly, producing narrower extruded roads, reduced lateral spreading, and consequently a lower mass build-up rate.

[picture]

Fig. 3. Effect of recycled polypropylene content and processing parameters on the BUR.

Fig. 3. Effect of recycled polypropylene content and processing parameters on the BUR.

Across the examined temperature range, Te(∘C) exerts only a minor influence on BUR, confirming that all materials were processed under fully molten conditions in which temperature is not the dominant governing variable. Instead, extrusion behavior is dictated by the combined effects of deposition kinematics and the material's elongational rheology.

Summary

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

This work investigated the thermo-rheological behavior and 3D-printing performance of virgin and recycled polypropylene under pellet-based material extrusion. TMFI measurements showed that recycled polypropylene (rPP) exhibits substantially lower shear viscosity and higher mass-flow capability than virgin PP at temperatures below 200∘C, indicating potential for reducing nozzle temperature and associated energy consumption. Under high thermal and shear inputs, however, virgin PP displays a stronger shear-thinning response, ultimately exceeding the flowability of all recycled variants.
In contrast, 3D-printing experiments revealed an opposing trend: increasing recycled content leads to a systematic decrease in build-up rate (BUR). This behavior demonstrates that deposition dynamics in material extrusion are not governed solely by shear viscosity, as reflected by TMFI, but are strongly influenced by elongational rheology during deposition. Despite its higher melt-flow index, recycled polypropylene exhibits greater resistance to elongational deformation, resulting in reduced lateral spreading, narrower extruded roads, and consequently a lower effective BUR.
Across the investigated processing window, nozzle temperature was found to have only a minor influence on BUR, confirming that all materials were processed under fully molten conditions and that deposition kinematics, together with elongational flow behavior, dominate the printing response. Overall, the results indicate that:

Recycled polypropylene can enhance shear-driven flow behavior and enable lower-temperature processing in pellet-based extrusion.

The same recycled content can hinder deposition performance by increasing elongational resistance, reducing build-up rate, and narrowing the stable processing window.

Sustainable integration of rPP in material extrusion requires balancing these competing effects through optimized control of screw speed, flow-rate setting, and printing kinematics.

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.21.pdf

B. Mrowiec, "Plastics in the circular economy (CE)," Environmental Protection and Natural Resources, vol. 29, no. 4, pp. 16-19, 2018, doi: 10.2478/oszn-2018-0017.

K. Ragaert, L. Delva, and K. Van Geem, "Mechanical and chemical recycling of solid plastic waste," Waste Management, vol. 69, pp. 24-58, 2017/11/01/ 2017, doi: .

M. Tuo et al., "Synergistic mechanism for greenhouse gases reduction, pollution control, and economic development in the plastic packaging industry: A coupled analysis of material, value, and environmental flows," Resources, Conservation and Recycling, vol. 227, p. 108712, 2026/03/01/ 2026, doi: .

I. Tsuchimoto and Y. Kajikawa, "Recycling of Plastic Waste: A Systematic Review Using Bibliometric Analysis," Sustainability, vol. 14, no. 24, p. 16340, 2022. [Online]. Available: .

M. Spoerk, C. Holzer, and J. Gonzalez-Gutierrez, "Material extrusion-based additive manufacturing of polypropylene: A review on how to improve dimensional inaccuracy and warpage," Journal of Applied Polymer Science, vol. 137, no. 12, p. 48545, 2020, doi: . 48545.

J. A. Rodríguez-Liébana et al., "Morpho-structural and thermo-mechanical characterization of recycled polypropylene and polystyrene from mixed post-consumer plastic waste," Journal of Environmental Chemical Engineering, vol. 10, no. 5, p. 108332, 2022/10/01/ 2022, doi: .

H. Wu, H. Mehrabi, P. Karagiannidis, and N. Naveed, "Additive manufacturing of recycled plastics: Strategies towards a more sustainable future," Journal of Cleaner Production, vol. 335, p. 130236, 2022/02/10/ 2022, doi: .

S. Saikrishnan, D. Jubinville, C. Tzoganakis, and T. H. Mekonnen, "Thermo-mechanical degradation of polypropylene (PP) and low-density polyethylene (LDPE) blends exposed to simulated recycling," Polymer Degradation and Stability, vol. 182, p. 109390, 2020/12/01/ 2020, doi: .

J. Marchewka and J. Laska, "Processing of poly-1-lactide and poly(l-lactide-co-trimethylene carbonate) blends by fused filament fabrication and fused granulate fabrication using RepRap 3D printer," The International Journal of Advanced Manufacturing Technology, vol. 106, no. 11, pp. 4933-4944, 2020/02/01 2020, doi: 10.1007/s00170-020-04981-z.

G. D. Goh et al., "Exploring PLA/TPU blends in pellet-based printing for multifunctional applications: Blending and interfacial properties," Materials Science and Engineering: R: Reports, vol. 164, p. 100981, 2025/06/01/ 2025, doi: .

T. Zhiltsova and M. S. A. Oliveira, "Sustainable Polypropylene Blends: Balancing Recycled Content with Processability and Performance," Polymers, vol. 17, no. 11, p. 1556, 2025. [Online]. Available: .

J. Aho, J. P. Boetker, S. Baldursdottir, and J. Rantanen, "Rheology as a tool for evaluation of melt processability of innovative dosage forms," International Journal of Pharmaceutics, vol. 494, no. 2, pp. 623-642, 2015/10/30/ 2015, doi: .

A. Kassab, D. Al Nabhani, P. Mohanty, C. Pannier, and G. Y. Ayoub, "Advancing plastic recycling: Challenges and opportunities in the integration of 3D printing and distributed recycling for a circular economy," Polymers, vol. 15, no. 19, p. 3881, 2023.

This ePub file was created using machine-generated methods and automated software to convert the source text into an ePub format and may contain formatting inconsistencies from the original version. Formatting may vary depending on the device used.

The original version of this eBook is available on https://www.scientific.net/book/sustainability-on-material-forming/978-3-0364-1983-1

Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 29-39

doi: 10.4028/p-zb6Lcl

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-12

Revised:
2026-02-16

Accepted:
2026-02-17

Online:
2026-04-17

Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

lacopo Bianchi 1,a*, Massimo Di Pietro 1, b, Archimede Forcellese 1,c, Chiara Mignanelli 1, d, Michela Simoncini 1,e and Tommaso Verdini 1,f

1 Università Politecnica delle Marche, Dipartimento di Ingegneria Industriale e Scienze Matematiche, Via Brecce Bianche 12, 60123 Ancona, Italy

a* i.bianchi@staff.univpm.it, b m.dipietro@pm.univpm.it, c a.forcellese@staff.univpm.it,

d  c.mignanelli@pm.univpm.it, em.simoncini@staff.univpm.it, ft.verdini@pm.univpm.it

Keywords: composite materials, 3D printing, sustainability.

Abstract

The use of composite materials and specifically of Fiber Reinforced Polymers (FRP) is continuously increasing in structural applications due to their high strength-to-weight ratio. From an environmental perspective, composites still face relevant challenges due to impactful petroleumbased matrices and large amounts of waste generated during manufacturing processes. This study proposes the reuse of FRP machining waste as filler in Masked Stereolithography (M-SLA) 3D printing. Scraps from FRP laminates, obtained during drilling operations, were incorporated into a photocurable resin and used to print tensile and flexural specimens with increasing filler contents ( 0−5wt% ) and mechanical characterization tests were carried out. A cradle-to-grave Life Cycle Assessment (LCA) was performed to quantify the potential environmental benefits associated with the reduced use of virgin resin. Results show that the use of recycled FRP waste leads to a loss of tensile strength and stiffness (up to 61% and 21% respectively) but it also provides a reduction in Global Warming Potential (about 2% at 5wt% filler). This demonstrates that the proposed strategy can improve the sustainability of 3D-printed components, especially for non-structural applications.

1. Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

The use of Fiber Reinforced Polymers (FRP) in the market has significantly grown in the last decades due to their high mechanical properties, lightness and tailorable physical properties. These characteristics make composites an ideal solution for high performance sectors such as aerospace, automotive, maritime and energy [1]. However, their widespread use presents some sustainability challenges, in particular because of the impacts related to raw materials extraction, waste produced during the manufacturing processes and the difficult End of Life (EoL) management. In particular, this latter aspect is especially critical for thermoset polymer matrix composites, where recycling is particularly challenging. Many recycling technologies have been developed, and they are based on mechanical, thermal or chemical processes [2]; many of these methods remain energy-intensive and expensive, with limited adoption in industry. In addition, the recovered materials have typically degraded properties with respect to the virgin composites.
Filament Winding (FW) is a highly automated manufacturing processes for FRP used for axisymmetric components such as pipes and pressure vessels. FW is based on the deposition of impregnated fibers tows on a rotating mandrel via a computer-controlled machine, allowing to produce high performance structures. Despite its advantages, FW can be associated with relatively high energy consumption and may still generate composite waste, in particular during post processing operations such as drilling and trimming (i.e. of outer regions of the manufactured components).
Simultaneously, 3D printing is a promising manufacturing technology due to the possibility of producing complex geometries, reducing material use and waste. These technologies allow to produce components layer by layer, positioning material only where needed according to precise CAD models. Amongst the possible 3D printing technologies, Stereolitography (SLA) is one of the most commonly used; it is based on the use of photopolimeric resin that harden due to the effect of a light source (e.g. laser, LCD monitor...). Also in the case of 3D printing, despite the possible advantages in terms of
weight reduction and optimized processes, some issues related to environmental sustainability still arise. In fact, most of the virgin material used for these processes (resin, filaments...) have a synthetic origin (i.e. petroleum based) and are therefore associated with relevant environmental impacts. Hence, material use reduction and parts weight optimization is a crucial aspect in 3D printing to limit the final components environmental impacts.
In this context, photopolymer resins are often added with particulate or fiber fillers to enhance specific mechanical or physical properties or to reduce overall resin consumption. Scientific literature provides numerous examples of these solutions. Mineral and ceramic such as silica, alumina and talk were added to the resin with possible benefits in terms of increase in strength, modulus and wear resistance [3]. Metals oxides (e.g. titania, zinc oxide...) were also added to the virgin resin to improve mechanical and antibacterial properties [4]. Other examples are represented by natural fillers such as wood flour, cotton and flax fibers and other agricultural waste [5-7]. Moreover, other studies focused on the use of recovered plastic waste and their reuse in 3D printing [8,9]. These studies showed how the use of filler can bring both structural and environmental benefits. Despite the strong interest, literature still lacks of studies concerning effective ways to reuse CFRP manufacturing waste and ways to further improve SLA production process sustainability.
The present study aims at addressing these issues related to 3D printing and composite materials waste management by investigating the feasibility of using FRP scraps as fillers in photopolymer resin used in Masked Stereolitography (M-SLA) 3D printing. To the authors knowledge, this work represents a novelty in scientific literature, as it investigates the direct reuse of FRP machining scraps in a 3D printing process, combining experimental mechanical characterization with a cradle-to-grave environmental assessment.
Composite waste in the form of powder obtained from drilling operations was added at different weight percentages to neat photopolymer resin. Hence, mechanical characterization tests and sustainability analysis were carried out to determine whether this approach can provide a sustainable alternative to traditional 3D printing materials.

2. Materials and Methods

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2.1. Materials and 3D printing technology

The main material used in the present study is the Elegoo translucent standard commercial resin. It is a LCD photocurable polymer that can be used in Masked stereolithography processes, and it mainly consists of epoxy acrylate resin and monomers and 3−5% by weight of photoinitiators, required to trigger the polymerization process. A translucent resin was selected to enhance contrast with the filler to better analyze its dispersion and interfacial interaction.
In order to improve the sustainability of the raw materials and reduce the consumption of neat resin, the resin was filled with fiber-reinforced scraps obtained from chip-removal manufacturing processes. Specifically, epoxy resin composites reinforced with basalt fiber were used. Tubular components were produced via a filament winding process starting from composite towpreg with a fiber and resin mass fraction equal to 72% and 28%, respectively. After the winding and oven curing process, the components were subjected to a drilling process as in common industrial practices for assembly or installations purposes. Figure 1 shows the drilling process of the FRP tubular component. This results in waste material in the form of powder and short fibers that was employed as a filler in the present study.
Starting from the resin and the filler, several material configurations were prepared to evaluate the effect of varying filler content within the resin on the mechanical properties of the resulting components. More specifically, 0% (neat resin), 0.5%,1%,2%,2.5% and 5% filler content in weight were added to the resin. For each configuration, raw resin and FRP powder were weighed to obtain the target filler/resin weight and mixed together by means of mechanical stirrer. Hence, the so obtained composite material was poured within the tank of a Stereolitography (SLA) machine for the specimens production. Hence, 1 hour was waited before starting the printing process to allow the resin to set and let possible gas bubbles escape.

The Elegoo Saturn 4 Ultra LCD 3D printer was used for the present study. The machine features a Masked Stereolitography (M-SLA) that exploits a UV LED lights produced via a LCD screen to cure the photopolymer and produce the component layer by layer. The light is used to selectively harden a layer of the polymer according to the information provided by a digital instruction file, in turn based on a STL model of components to be produced. As each layer is completed, the building platform rises slightly to allow liquid material to get in contact with the LCD and to harden the next layer. The process continues until the final component is completed. 5 specimens at the time (either flexural or tensile specimens) were produced via M-SLA. Layer height equal to 0.05 mm , bottom exposure of 30 s , normal exposure of 8 s were set as process parameters and were kept the same for all the material configurations.
After the printing process, two postprocessing steps were carried out. At first, the produced parts were washed in isopropyl alcohol for 5 minutes to remove excess uncured resin. After that, the parts were subjected to UV lights for 30 minutes to ensure complete curing of the resin. The Elegoo X bundle, consisting of a washing and curing machine, was used for the postprocessing.

[picture]

Fig. 1. Drilling process and scraps production for the tubular FRP composites.

Fig. 1. Drilling process and scraps production for the tubular FRP composites.

2.2. Mechanical characterization

Tensile and flexural tests were carried out to evaluate the mechanical properties of the resin with different filler content. Neat resin samples were also produced to evaluate the raw materials properties and provide a baseline for the study. This was used as a reference to evaluate the effect of the FRP filler addition.
Type IV dog bone specimens were produced according to the ASTM D638-22 standard for tensile properties of plastics. The specimens have thickness, gauge length, and width equal to 3.2 mm,33 mm and 6 mm , respectively. 5 specimens for each material configuration were printed and tested using a MTS810 universal testing machine. A crosshead speed equal to 5 mm/min was used for the test in line with the standard recommendation. An extensometer and a load cell were used to register data and create stress-strain curves for each specimen. Hence, a spreadsheet was used to evaluate the most relevant mechanical characteristics such as Ultimate Tensile Strength (UTS), Elastic modulus (E) and maximum strain.

Figure 2 presents the tensile specimens right after the printing process on the building platform (Figure 2a) and a detail of the dog bone specimens with recovered composite filler (Figure 2b).
Similarly, the ASTM D790-17 standard was followed to assess the materials flexural properties. As for the tensile tests, 5 specimens for each condition were produced and tested with a speed equal to 2 mm/min. Specimens with thickness, width and length equal to 3.2 mm,13 mm and 127 mm respectively were produced.
Fracture surfaces of the specimens were observed by means of the Leica M205 C Stereomicroscope to investigate fracture mechanisms and interaction between the filler and the matrix resin.

[picture]

Fig. 2. 3D printing of tensile specimens (a) and detail of a printed specimen.

Fig. 2. 3D printing of tensile specimens (a) and detail of a printed specimen.

2.3. Life Cycle Assessment analysis

The environmental sustainability of the 3D printing process and the reuse of composite waste as filler was evaluated by means of the Life Cycle Assessment (LCA) methodology. The framework proposed by the ISO 14040-14044 standards was followed, and the four iterative phases were carried out: Goal and Scope Definition, Life Cycle Inventory, Life Cycle Impact Assessment (LCIA), and Results Discussion. These phases are detailed in the following paragraphs.

Goal and scope Definition

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

The present LCA analysis aims at evaluating and comparing the environmental impacts associated with the production of components produced via M-SLA using raw materials constituted by variable percentages of raw resin and recovered filler material. In particular, the analysis aims at understanding the environmental implications of using recovered FRP waste obtained from machining processes in 3D printing processes.
To do so, the Functional Unit (FU) is defined as the production of a tensile specimen with dimension defined by the ASTM D638 standard by means of a M-SLA process. Initially, the FU only considers geometric requirements, and it is suitable to compare components that do not have strict structural requirements (e.g. aesthetic components or prototypes).
A "Cradle to Grave" approach was followed and the following phases were included within the study: raw materials (resin and filler) production and transport, packaging materials, 3D printing phase, post processing (including washing and post curing) and End of Life (EoL). No impacts associated with the use phase of the components were considered as they would have been negligible for selected FU; this also contributes to give generalizability to the results as specific use phase impacts would strongly change depending on the application of the produced components.

In addition to the initial investigation, a subsequent LCA analysis was conducted to assess the environmental performance of components characterized by different filler content with defined structural requirements. Hence, an additional LCA and FU were defined as a tensile specimen that exhibits a strain equal to 0.6% when subjected to a load of 363 N . These values were based on the tensile test results of the neat resin specimens and were considered a reference for all scenarios.
Since the materials with different filler weight percentage have different mechanical properties (as reported in the results section), the FU should have different thickness depending on the material considered. The cross-section area of the different specimens was hence calculated considering simple mechanical relationship (i.e. Hooke's law, stress definition) and the elastic moduli of the different materials. This results in parts with bigger cross sections as the elastic modulus decreases. Since the neat resin scenario was selected as reference, the cross section of the part in this scenario is the same considered for the previous LCA (i.e. for nonstructural components).
This approach allowed to compare components with the same stiffness across different scenarios. In this way, it was possible to evaluate the environmental behavior of the FRP scraps reuse under different functional conditions (i.e. for aesthetic components or structural ones) [10]. The same scenarios previously described were considered in both analyses.

Life Cycle Inventory

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

Laboratory measurements were carried out to gather primary data related to the different produced specimens. The dimensions and weight of parts with different filler weight content were measured using a caliber and precision scale. Hence, the weight of each constituent (resin and filler) was calculated considering their nominal weight in percentage. The Ecoinvent 3.8 database was employed to model the resin production. For what concern the FRP filler, a cut off approach was considered and no burden related to its production were considered. This material is today typically disposed of in landfill or incineration facilities, so no impacts were considered associated with its production. Similarly, resin packaging (HDPE bottle, cardboard box, plastic wrap) was weighed and modelled according to the commercial database. The impacts of packaging were allocated to the functional unit considering the size of the resin bottle ( 0.51 ) and the weight of resin used for each material configuration.
Transport impacts were evaluated considering the material supplier, their distance with the production facility (considered in Center Italy) and Ecoinvent datasets related to road and air transport.
Energy consumption for the printing and post processing phase was directly measured using a power meter; these phases are considered the same for all the scenarios. Hence, the Ecoinvent dataset for electric energy (low voltage energy mix, Italy) was used. Isopropanol use was evaluated considering the washing tank capacity and the number of washing cycles possible before solvent substitution is required.
For all materials configurations EoL was modelled as landfill disposal modelled according to the Ecoinvent database.
Table 1 summarizes the main LCI data.

Life Cycle Impact Assessment

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

The LCA dedicated software SimaPro was employed to model the scenarios with different composite materials. The software is equipped by default with the Ecoinvent database.
The Global Warming Potential impact category, calculated according to the IPCC (International panel on Climate Change) methodology with a 100 years time horizon was considered for the analysis. This is one of the most commonly used impact categories in literature for LCA analysis concerning composite materials and 3D printing.
After the software modelling phase, the results were exported to a spreadsheet for further analysis and graphical representation.

Table 1. Main LCI data.

Filler weight in percentage | Part weight (g) | Filler Weight (g) | Resin weight (g)
0.0% | 6.082 | 0.000 | 6.082
0.5% | 6.034 | 0.030 | 6.064
1.0% | 6.025 | 0.061 | 6.045
2.0% | 6.010 | 0.123 | 6.008
2.5% | 6.072 | 0.154 | 5.989
5.0% | 6.021 | 0.310 | 5.895
Energy consumption (kWh) |  | Resin transport (km)
Printing machine | 0.03 | 13000 plane
Washing | 0.001 | 430 truck
Curing | 0.018 |  |
Packaging (g) for SLA resin (0.5 kg
primary packaging (HDPE bottle) | 82
secondary packaging | 58.56
Plastic wrap | 7.1

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

Mechanical characterization

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

Figure 3 presents the results of the tensile tests for the materials characterized by increasing recovered filler weight content. Figure 3a reports typical stress-strain curves for the different specimens. It can be noted that the neat resin ( 0% ) exhibits the highest tensile strength and strain a break amongst the presented curves; in line with the typical behavior of polymeric resins, the curve exhibits an initial linear elastic region, followed by a plastic deformation stage and finally failure.
The use of the recovered filler substantially reduces the UTS and strain at break of the resin.

[picture]

Fig. 3. Results of the tensile test for the specimens with increasing recovered filler content: a) typical stress-strain curves for the different materials; b) Elastic modulus as a function of the filler content; c) Tensile strength as a function of the filler content.

Fig. 3. Results of the tensile test for the specimens with increasing recovered filler content: a) typical stress-strain curves for the different materials; b) Elastic modulus as a function of the filler content; c) Tensile strength as a function of the filler content.

The initial slope of the curve (initial elastic region) is similar to the different alternatives. This can be also seen in Figure 3b, in which the elastic modulus is plotted as a function of the filler weight content. The Elastic modulus is reduced as the filler content increases, ranging from 3.16 GPa (Neat resin) to 2.47 Gpa ( 5% ), showing a maximum reduction equal to 21%. This decrease is more pronounced at low filler contents, with a reduction of approximately 16% between 0% and 1% filler, while at higher filler loadings ( 2.5−5% ) the modulus values tend to stabilize, showing negligible further variation.
On the other hand, the UTS strongly decreases with increasing recovered filler percentage. A reduction equal to 61% in UTS is observed between the neat resin and the 5% filler percentage resin (i.e. from 54.95 Mpa to 21.42 MPa ). As for the elastic modulus, the reduction in mechanical properties is mainly observed when moving from the neat resin to low filler contents, with a decrease of about 58% between the neat resin and the 1% filler content. Beyond this point, the values tend to stabilize, remaining nearly constant for filler contents between 2.5% and 5%. The reduction in UTS can also be associated with the loss of strain at break with increasing filler content; this indicates embrittlement of the printed parts and therefore an early formation of cracks and their propagation [11-13].
The loss of mechanical properties with increasing filler content suggests that the FRP waste does not act as a reinforcement or a stiffening agent, but rather it lowers the load bearing capacity of the polymer. This behavior is expected when rigid particulate/short fibre is added to a polymer and poor bonding with the matrix is obtained. This, along with the possible presence of voids or trapped air introduced during filler/resin mixing is the probable cause of loss in mechanical properties in the recovered composite parts.
The behavior is also confirmed by the microscope image analysis as reported in Figure 4. Figure 4a shows the fracture surface of the neat resin where no clear defects of inclusion can be seen. On the other hand, Figure 4 b reports the fracture surface of a tensile specimen with 5% in weight of filler. In this case, several discontinuities can be observed, directly associated with the presence of the filler. Some fibers appear to be well embedded within the matrix, while numerous white spots, corresponding to residual matrix fragments from the recycled filler, appear poorly bonded to the photocured resin. These regions likely act as stress concentrators and weak interfaces, representing the main cause of the observed reduction in mechanical performance.

[picture]

Fig. 4. Surface fracture of specimens with 0% (a) and 5% (b) of recovered FRP filler.

Fig. 4. Surface fracture of specimens with 0 % (a) and 5 % (b) of recovered FRP filler.

These regions likely act as stress concentrators and weak interfaces, representing the main cause of the observed reduction in mechanical performance.
Table 2 presents the results of the tensile and flexural tests in terms of max stress, strain, and elastic modulus. Overall, the same trend observed for the tensile properties can be seen for the flexural properties, with a decrease in load bearing capacity, ductility, and stiffness as the filler percentage increases. The max stress decreases by 63% from the neat resin to the 5% material configuration. On the other hand, the flexural modulus does not decrease significantly even for the highest filler percentage, with maximum reduction with respect to the neat resin equal to 5%.

Table 2. Tensile and flexural tests results.

Filler content | Tensile properties | Flexural properties
| Max Stress [MPa] | Max Strain [mm/mm] | Elastic Modulus [MPa] | Max Stress [MPa] | Max Strain [mm/mm] | Elastic Modulus [MPa]
0% | 54.95 (±3.70) | 0.032 (±0.007) | 3155.78 (±644.48) | 94.71 (±6.02) | 0.053 (±0.004) | 2685.94 (±370.18)
0.5% | 33.23 (±3.23) | 0.014 (±0.004) | 2775.10 (±172.97) | 57.62 (±5.13) | 0.023 (±0.005) | 2846.78 (±129.67)
1% | 22.86 (±2.42) | 0.009 (±0.002) | 2666.26 (±319.24) | 67.50 (±3.66) | 0.030 (±0.002) | 2682.85 (±193.47)
2% | 21.20 (±3.82) | 0.008 (±0.001) | 2640.82 (±237.15) | 32.94 (±4.17) | 0.014 (±0.002) | 2584.40 (±167.51)
2.5% | 19.50 (±3.55) | 0.009 (±0.002) | 2471.52 (±250.03) | 44.49 (±6.00) | 0.019 (±0.003) | 2479.25 (±267.27)
5% | 21.42 (±2.34) | 0.009 (±0.001) | 2471.00 (±213.57) | 34.87 (±4.57) | 0.020 (±0.002) | 2540.78 (±233.23)

Sustainability assessment results

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

Figure 5 shows the results of the LCIA in terms of Global Warming Potential for the production of the tensile specimens constituted by resin with different filler percentage. The results show that the use of recovered composite material as a filler within the resin can provide environmental benefits. In fact, impacts reduction equal to 2% is obtained as the neat resin scenario is compared with the 5% filler one. Although the filler content is relatively low, this reduction is still significant, as the substitution directly affects the main contributors to the overall environmental impact, namely the production and transport of virgin resin. In fact, resin production and transport can account for up to 35% and 25% respectively of the total impacts of the scenarios. This is due to the synthetic origin of the resin (i.e. petroleum based) and the long transport distance required from the manufacturer (in China) and the final part production site. Hence, the reduction of the amount of virgin resin used in the process represents a good strategy to improve the sustainability of 3D printed components. On the other hand, the recovered filler has no impact, and its reuse also contributes to reducing the amount of material sent to landfill or incineration facilities. The printing and post-processing phases remain unchanged irrespective of the filler content in the resin, contributing approximately 15% and 20% of the total environmental impacts, respectively. These phases impacts are mainly related to the electric energy use of the machines (printers, washing and curing stations) and can be reduced if more sustainable energy sources (e.g. based on renewable energy) are used.
Although the environmental sustainability benefits increase with higher filler content, some technological limitations were observed. Increasing the filler percentage beyond 5% results in a more viscous resin, which becomes increasingly difficult to print successfully. As a result, the printed parts become more fragile, often showing poor surface quality and incomplete polymerization. Preliminary tests with 10% filler showed these issues, as printing could not be successfully completed. Therefore, further improvements are needed to mitigate the mechanical property reduction from a structural standpoint and to enhance the processability of resin with higher filler contents.

[picture]

Fig. 5. LCIA results for parts constituted by different filler percentages with defined geometry or with defined structural requirements.

Fig. 5. LCIA results for parts constituted by different filler percentages with defined geometry or with defined structural requirements.

Regarding the structural components LCA analysis, the graph reports the total life-cycle impacts for the configurations ensuring the same stiffness ("Total Same Stiffness").
Although this second LCA analysis for structural components involves a different functional unit, the total impacts of the structural components are displayed close together in the graph. This is to clearly show how impacts can vary depending on the structural requirements of the components used in different scenarios. This approach was necessary to provide a better understanding of the proposed recovery solution from both structural and environmental perspectives. This comparison is also possible since the two analyses consider the filler percentages.
A significant increase in environmental impacts is observed with the rise in filler content. This trend is explained by the reduction of the elastic modulus as the filler percentage increases, which requires larger cross-sections and therefore a higher resin consumption to achieve the same mechanical performance. Consequently, the impacts associated with resin production, material transportation, and the 3D printing process increase. Overall, an increase of approximately 18% in total impacts is observed when moving from the neat resin to the 5% filler configuration. However, the increase in impacts tends to stabilize beyond 2.5% filler, with even a slight reduction. This behavior can be attributed to the fact that the mechanical properties no longer decrease after that percentage, while the percentage of recycled filler increases, leading to a lower overall resin consumption.

Conclusions and Future Developments

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

This paper investigates the reuse of Basalt Fiber Reinforced Polymer powdered waste as a filler in a photocurable resin for M-SLA 3D printing processes. Resin with increasing filler content ( 0,0.5,1, 2.5 and 5wt% ) was used to produce tensile and flexural specimens to perform a mechanical characterization of the materials. In addition, a "from cradle to grave" LCA analysis was performed to assess the environmental implications associated with the composite waste reuse.
The main outcome can be summarized as follows:

The ultimate tensile strength strongly decreases very as small amounts of filler is added to the resin: a 58% reduction between neat resin and 1wt% filler (from 54.95 MPa to 22.86 MPa ), and overall, a 61% reduction at 5wt% is observed.

The elastic modulus decreases from 3.16 GPa (neat resin) to 2.47 GPa ( 5wt% filler) ( 21% total reduction). The largest loss is observed between 0% and 1wt% filler ( 16% decrease), while Elastic modulus values stabilize between 2.5−5wt%

If nonstructural components are considered, the use of the recovered filler provides a reduction in environmental impacts. Global Warming Potential is reduced by 2% with respect to the neat resin scenario as a 5% filler material is used. Despite the low filler content, benefits are registered as the use of filler reduces the impacts of the main life cycle contributors, namely resin production and transport.

As structural components are considered, the use of recycled filler increases the GWP by up to 18%; this is due to the reduced stiffness of the materials with increasing filler content.
Overall, the reuse of FRP waste can provide environmental benefits, but some aspects of the process should be further investigated. Future work will focus on ways to improve the interface between the recovered material and the photopolimeric resin (e.g. via sieving of the waste powder) in order to improve the mechanical properties of the recovered materials. In addition, solutions to increase the filler content without reducing the processability of the resin will be investigated. These solutions, along with a continuous monitoring of the environmental impacts of the reuse process, can provide great sustainability benefits in the composites and 3D printing sectors.

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.29.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 41-48

doi: 10.4028/p-Xv47nM

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-14

Revised:
2026-02-12

Accepted:
2026-02-17

Online:
2026-04-17

Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

The original version of this paper is available on https://www.scientific.net/KEM.1051.41.pdf

Usman Aziz 1,a*, Uceu F. H. Suhuddin 2,b, Lars Rath 2,c, Léon F. Schröder 3,d, Volker Schmidt 3,e, Benjamin Klusemann 1,2,f

1 Institute for Production Technology and Systems, Leuphana University Lüneburg, Universitätsallee 1, Lüneburg, 21335, Germany

2 Solid State Materials Processing, Institute of Material and Process Design, Helmholtz-Zentrum Hereon, Max-Planck-Straße 1, Geesthacht, 21502, Germany

3 Institute of Stochastics, Ulm University, 89069 Ulm, Germany

a* usman.aziz@leuphana.de, b uceu.suhuddin@hereon.de, clars.rath@hereon.de, d leon.schroeder@uni-ulm.de, e volker.schmidt@uni-ulm.de, f benjamin.klusemann@leuphana.de

Keywords: friction extrusion, friction consolidation, recycling, AA6082, bonding and microstructure.

Abstract

Recycling aluminum chips remains a major challenge in aluminum manufacturing because it is difficult to retain the original quality alloy properties while reducing the carbon footprint and ensuring a sustainable process. This work investigates the microstructural evolution and bonding quality of compacted AA6082 chips processed through friction extrusion/consolidation. The residual material left inside the extrusion container after processing at a high extrusion ratio was analyzed using SEM, EDS, and EBSD to understand bonding mechanisms and microstructure evolution in front of the die. The SEM results show that voids are still present between the chips in the initial compacted material which already shows bonding, while these voids are reducing towards the die interface, particularly related to the present severe plastic deformation. EDS analysis confirms the presence of Al(Fe,Mn)Si intermetallic particles, which break and disperse in the matrix because of shear deformation due to die rotation. EBSD analysis reveals that grains are coarser near the base material, and subdivisions of grains near the die interface are significant because of continuous dynamic recrystallization.

1. Introduction

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In general, recycling of aluminum (Al) chips can be done in two main ways: conventional recycling and direct conversion. In the conventional route, the chips are melted, cast into billets, and then processed further, for example by hot extrusion, to produce rods, wires, or similar products [1]. In contrast, direct conversion avoids melting and instead consolidates the chips into a solid part through solid-state methods. Additionally, direct conversion methods offer significant advantages over traditional remelting-based recycling. Remelting often leads to problems such as oxidation, buildup of impurities, and a loss of material due to the formation of brittle intermetallic phases [1]. These limitations become more severe for machining chips, whose irregular geometries and high specific surface areas promote rapid native oxide formation, thereby diminishing remelting efficiency [1,2]. To address these limitations associated with machining chips, solid-state recycling approaches such as friction extrusion/consolidation (FE/FC) avoid melting and instead rely on frictional heating and severe plastic deformation (SPD), enabling microstructural refinement while minimizing oxidation and energy consumption [3-5]. Recent studies have shown that FE can successfully produce functional aluminum products with reduced environmental impact, supporting its integration into sustainable manufacturing pathways. Furthermore, FE has been extensively applied to recycle Al alloy scrap through solid-state consolidation into extruded products such as wires, wide components (sheets) [6] and rods with improved mechanical properties thoroughly reported [4, 7-9].
Regarding Al scrap consolidation and bonding behavior investigations, Baffari et al. [5] reported that sheet-metal scrap processed by FC resulted in a compacted billet exhibiting variations in bonding quality along the billet height. Whereas material near the die interface (top region) was severely
deformed and fully consolidated, showing complete bonding due to intense stirring and recrystallization, the intermediate region exhibited partial consolidation through solid-state welding between the chips. The bottom region was mainly compacted with limited metallurgical bonding and reduced mechanical integrity [5]. Additionally, Tang and Reynolds [10] obtained dense, well-bonded disks via FC of Al chips. The strongest bonding occurred in the recrystallized core region, while a recurring weakly bonded ring appeared near the bottom outer edge where deformation and heating were insufficient [10]. Similarly, Poy Ignacio et al. [11] demonstrated that FE at high extrusion ratio enables effective solid-state consolidation of Al chips, producing wires with good metallurgical bonding; however, excessive temperature buildup during processing promotes defect formation such as voids and cracks, which degrade mechanical properties along the wire length. On the other side, Mejbel et al. [12] investigated the consolidation of Al chips using FC with particular emphasis on chip-to-chip bonding mechanisms. The results revealed spatial variations in consolidation and bonding quality across the billet cross-section, governed by differences in heat generation, process parameters, plastic deformation, and material flow. Puleo et al. [13] analyzed variations in consolidation and bonding quality by studying the formation of chips-chips bonds during the frictionbased solid-state process. It was found that conventional bonding criteria from processes like sintering or SPD were not suitable for chips bonding, so a new friction-specific bonding criterion was developed based on experiments and simulations. This model enabled prediction of bonding occurrence throughout the billet, addressing issues of incomplete consolidation in regions distant from the tool interface. The developed criterion accurately predicts consolidation and bonding success from parameters such as tool rotation speed, applied force, and consolidation time, making FC process design more reliable. In this regard, multi-step FC strategies were introduced, where multiple passes of stirring and consolidation improved bonding uniformity and microstructural integrity [14].
Although the consolidation and bonding behavior of aluminum chips have been extensively studied in the production of consolidated billets, comparatively little attention has been paid to the bonding characteristics of the residual material remaining inside the die during the FE process at high extrusion ratios. In particular, the evolution of chip-chip interfaces and their influence on consolidation during FE remain insufficiently understood. Despite the growing interest in FE, the microstructural development from the base material region, i.e. compacted chips, towards the die interface, which in the end determined the final properties of the extrudate, has not been systematically investigated. Therefore, a detailed examination of this residual consolidated zone is essential to clarify the underlying deformation and bonding mechanisms. Accordingly, this study systematically examines the residual AA6082 material from chips at multiple locations in front of the die to elucidate their bonding behavior and microstructural evolution.

2. Materials and Methodology

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The FE machine FE 100 (BOND Technologies), shown in Fig. 1, was employed. A total of 300 g of AA6082-T6 alloy chips were processed. The chips were produced during a turning operation on a lathe machine on clean AA6082 rod material, without the application of coolant or lubricant. The chips were initially compacted inside the extrusion container through several compression steps performed without rotation. This procedure resulted in approximately 62% compaction, corresponding to a bulk density of about 1.67 g/cm.

[picture]

Fig. 1. Illustration of Friction Extrusion Machine FE100 (Bond Technologies) and process principles [15].

Fig. 1. Illustration of Friction Extrusion Machine FE100 (Bond Technologies) and process principles [15].

The friction extrusion process was operated at a target force of 400 kN , with a spindle speed of 300 RPM. A featureless die with a planar face and a two-step inflow angle of 7.5∘ and 15∘, leading to a 1.5 mm die bore or extrusion ratio (ER) of 1111, was used. Within this study, the focus is on the residual material, i.e. the material in front of the die. Therefore, due to the high extrusion ratio, this process can also be considered as friction consolidation for the residual material. The residual material was cut, ground, and polished in colloidal silica according to standard metallographic procedures. The prepared sample was analyzed at 20 kV through an FEI Quanta 650 field emission gun (FEG) scanning electron microscope (SEM) equipped with a TSL OIM electron backscatter diffraction (EBSD) system at step size of 0.2μ m and an EDAX energy dispersive X-ray spectroscopy (EDS) system.

3. Results and Discussions

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a. SEM Analysis

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The macrograph from the base material to the die interface is presented in Fig. 2, highlighting also the specific locations where microstructural analysis was conducted. The evolution of the macrostructure is divided into three distinct zones. The first one is feedstock/compaction zone, where the alignment of chips changes during compaction and the feedstock material becomes vertically aligned due to compressive forces; in this zone, chip boundaries remain clearly visible, indicating the absence of metallurgical bonding. The second zone is thermo-mechanically affected zone (TMAZ) 2, an intermediate zone in which the chips are drawn in the direction of material flow towards the die-feedstock interface, and the chip boundaries gradually fade, suggesting the initiation of diffusion and partial metallurgical bonding. The third zone is TMAZ 1, the zone closest to the die interface, where chip boundaries completely disappear, and a fine-grained microstructure is formed because of severe plastic deformation, indicating full metallurgical bonding. The material from TMAZ 1 serves as primary source of the extruded material, where the extruded wire is not focused on this study.

[picture]

Fig. 2. Macrograph of the residual material after friction extrusion of Al chips at distances of 15, 10, 5 , and 0 mm from the die-material interface, where 15 mm corresponds to the compacted chip region and 0 mm is adjacent to the die interface.

Fig. 2. Macrograph of the residual material after friction extrusion of Al chips at distances of 15, 10, 5 , and 0 mm from the die-material interface, where 15 mm corresponds to the compacted chip region and 0 mm is adjacent to the die interface.

In Fig. 3, SEM micrographs obtained at distances of 15, 10, 5, and 0 mm from the die-feedstock interface are presented. The micrographs reveal distinct microstructural variations across the residual material, where the region at 15 mm is representative for the compacted chips, while the 0 mm position is adjacent to the die, representative of the extruded material.

[picture]

Fig. 3. Micrographs taken from the base material interface towards die-feedstock interface at 15 mm,10 mm,5 mm, and 0 mm , see Fig. 2.

Fig. 3. Micrographs taken from the base material interface towards die-feedstock interface at 15 m m , 10 m m , 5 m m , and 0 mm , see Fig. 2.

Chemical composition analysis by EDS reveals that the bright particles in the microstructures are Al(Fe,Mn)Si intermetallic particles, which were originally formed during the alloy's casting/solidification. Fe and Mn have low solubility in Al, therefore, excess Fe and Mn segregate, forming the Al(Fe,Mn)Si intermetallic. During FE/FC, the particles do not dissolve, instead the particles are getting smaller and dispersed in the Al matrix towards the die-feedstock interface. Shear deformation induced by the tool rotation broke the large particles into smaller sizes and distributed them randomly in the Al matrix [16].
The bonding behaviour between base material and die interface can be summarized as follows: At 15 mm (compacted chips region), there are voids between the chips indicating very less deformation by the die. At 10 mm , most of the voids between the chips are closed, but smaller discontinuous voids still exist, indicating compressive deformation from the die. Additionally, there are indications of significant metallurgical bonding between the chips (highlighted by red arrows in Fig. 3) pointing out that the process temperature and compressive force are getting high enough to induce metallurgical bonding. This progression in metallurgical bonding indicates that bonding quality increases steadily due to compression induced by the die's axial force, followed by shearing promoted by die rotation and a rise in temperature as the material approaches the die interface. The transformation of the chips boundary from feedstock material to the 10 mm area suggests that a sintering-like process occurred during FE/FC, leading to voids as voids are initially larger at the early stage; however, during sintering they tend to shrink to a minimum size. In some regions the voids may disappear completely, while in others they remain [17]. Heat and deformation bring chip surfaces into intimate contact, initiating diffusion and plastic flow. These mechanisms cause neck formation at particle contacts, followed by neck growth through surface and lattice diffusion, leading to particle coalescence and densification. However, these mechanisms progress unevenly; some regions even fail to fully bond, leaving voids. Moving further, at 5 mm , metallurgical bonding improves, and the extent of bonding gaps decreases, with the remaining unbonded zones forming a wavy and discontinued path indicating shear deformation, i.e., induced by die rotation. At the die interface, there are no visible voids or chips boundaries anymore; therefore, the material is characterized as consolidated. The seen pores in the micrographs, randomly distributed, are most likely formed during metallographic sample preparation due to particle drop-off.

b. EBSD Analysis

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The microstructures obtained at distances of 15, 10, 5, and 0 mm are shown in Fig. 4. In the inverse pole figure maps (upper maps), grain boundaries are categorized according to the misorientation angle between adjacent grains, where misorientation angles between 2∘ and 15∘ are classified as low-angle grain boundaries (LAGBs) and misorientation angles greater than 15∘ are defined as high-angle grain boundaries (HAGBs), as depicted as white and black lines, respectively.

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Fig. 4. EBSD maps and SEM grain boundary contrast images were obtained at 15,10,5, and 0 mm positions. The arrows in the maps show chips boundaries, while highlighted ellipse show elongated grains because of high strain resulting from friction and compression.

Fig. 4. EBSD maps and SEM grain boundary contrast images were obtained at 15 , 10 , 5 , and 0 mm positions. The arrows in the maps show chips boundaries, while highlighted ellipse show elongated grains because of high strain resulting from friction and compression.

The grain size distributions at distances of 15, 10, 5, and 0 mm demonstrate a clear location-dependent microstructural refinement. At 15 and 10 mm , the grain size histograms (Fig. 5) are broad and irregular, indicating chip-to-chip variability and heterogeneous deformation, with several larger grains contributing to higher mean diameters ( ~26.5 and 40.3μ m, respectively). In contrast, the distributions at 5 and 0 mm shift toward smaller sizes and become narrower, with lower mean grain sizes (~17.6 and 16.6μ m), showing a consistent reduction in average grain diameter. This overall decrease in grain size toward the position 0 mm indicates increasing microstructural refinement with decreasing distance.

[picture]

Fig. 5. Grain size distributions and mean grain diameter at distances of 15,10,5, and 0 mm .

Fig. 5. Grain size distributions and mean grain diameter at distances of 15 , 10 , 5 , and 0 mm .

As the distance decreases towards the die-feedstock interface, the microstructure evolves from inhomogeneous chips to a homogeneous microstructure with fine equiaxed grains. The region at 15 mm consists of chips of varying sizes and exhibits an inhomogeneous grain structure. At 10 mm , there is no significant change in the microstructure other than the voids between the chips being closed as mentioned earlier. Nevertheless, it is still possible to observe the chip boundaries, as highlighted by the black and red arrows in the images. At the 5 mm region, the chips and grain structure are reoriented due to the geometrical strain imposed by the die rotation. Some LAGB segments transform into HAGBs due to progressive dislocation accumulation. Some elongated grains become thinner, and LAGBs form within the grains, subdividing the elongated grains into smaller subgrains.
Along with these elongated grains, misorientation angle measurements are presented in Fig. 6. An increase in the misorientation angle in Fig. 5a indicates that grain orientations evolve continuously due to subgrain rotation until exceeding 15∘ or transforming into HAGBs. The LAGB-to-HAGB transformation and subgrain rotation result in the formation of new grains, which is characteristic of the continuous dynamic recrystallization (CDRX) mechanism. In contrast, the subgrains inside elongated grains in Fig. 5b have similar grain orientation. With increasing strain, the elongated grains are geometrically pinched off into fine grains, indicating geometric dynamic recrystallization (GDRX) [3].

Further, as the material approaches the die interface ( 0 mm ), plastic deformation leads to a gradual increase in boundary misorientation, refined grains, and promotes the transformation of LAGBs into HAGBs, resulting again in CDRX [18, 19].

[picture]

Fig. 6. Misorientation along the arrows in the EBSD maps, indicated by (a) red ellipse and (b) black ellipse in region 5 mm in Fig. 4.

Fig. 6. Misorientation along the arrows in the EBSD maps, indicated by (a) red ellipse and (b) black ellipse in region 5 mm in Fig. 4.

Supplement to that, the grain boundary misorientation results (shown in Fig. 7) at 15, 10, 5, and 0 mm do not follow a strict monotonic variation in the fractions of LAGBs and HAGBs. Nevertheless, the regions at 5 and 0 mm , which exhibit finer mean grain sizes, show a comparatively higher proportion of HAGBs, whereas the 15 and 10 mm locations contain relatively more LAGBs. This behavior is consistent with enhanced recrystallization and boundary transformation accompanying grain refinement during deformation.

[picture]

Fig. 7. Grain boundaries misorientation angles distribution at distances of 15,10,5, and 0 mm , showing the relative fractions of LAGBs and HAGBs.

Fig. 7. Grain boundaries misorientation angles distribution at distances of 15 , 10 , 5 , and 0 mm , showing the relative fractions of LAGBs and HAGBs.

c. Mechanism of chip bonding

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To summarize the mechanism of chip bonding, Fig. 8, provides an overview of how bonding formation progresses through the different stages during FC. Initially, the voids exist among the compacted chips, the intermetallic particles are large, and the microstructure is inhomogeneous. Next, the microstructure and intermetallic particle size remain the same; however, the voids are partially closed, indicating compression. Afterwards, chip boundaries are consolidated, and mostly intermetallic particles in the matrix are broken into smaller particles and dispersed in the matrix, induced by severe plastic deformation as well as elevated temperatures. In the region close to the die, the material experiences even more severe plastic deformation and is exposed to high temperatures, leading to most particles being broken into smaller particles, the disappearance of chip boundaries, and grain refinement due to CDRX/GDRX, resulting in a homogeneous microstructure.

[picture]

Fig. 8. Progressive compression and bonding of irregular chips during the FE/FC process, evolving from weak initial interfacial contacts to full consolidation near the die interface.

Fig. 8. Progressive compression and bonding of irregular chips during the FE/FC process, evolving from weak initial interfacial contacts to full consolidation near the die interface.

4. Conclusion

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This study shows that the residue of the compacted AA6082 chips formed during friction extrusion/consolidation in front of the die exhibits a pronounced gradient in bonding quality and microstructural development from the compacted chips base material towards the die interface. SEM and EBSD analyses reveal that, near the base material, the microstructure is inhomogeneous and contains a high density of LAGBs. As the material approaches the die interface, increasing strain promotes progressive subgrain rotation, leading to a gradual increase in boundary misorientation and the transformation of LAGBs into HAGBs causing CDRX. The chip boundaries contain voids near the base material, however, their presence is significantly reduced towards the die interface. Additionally, intermetallic particles are broken apart and dispersed within the matrix as a result of severe plastic deformation near the die interface. Overall, bonding quality changes noticeably within the residual compacted chips and becomes stronger from the base material toward the die. Consistent with this gradient, the measured grain size decreases toward the die interface, together with a relatively higher fraction of HAGBs in the refined regions, supporting progressive recrystallization and microstructural homogenization under increasing deformation.

5. Acknowledgement

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This work was supported by German Research Foundation (DFG) within Priority Program SPP 2489 DaMic - subproject P8, project number 562138004.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.41.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 49-59

doi: 10.4028/p-GG3de0

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-15

Revised:
2026-02-12

Accepted:
2026-02-17

Online:
2026-04-17

Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

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Ana Lyvia Tabosa da Silva 1,a*, Angela Daniela La Rosa 2,b, Geir Ringen 1,c, Xiang Ma 3, d

1Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway.

2 Department of Manufacturing and Civil Engineering, Norwegian University of Science and Technology, 2815 Gjøvik, Norway

3 SINTEF Industry, 0314 Oslo, Norway

a  ana.l.t.d.silva@ntnu.no, b  angela.d.l.rosa@ntnu.no, c  geir.ringen@ntnu.no, d  xiang.ma@sintef.no

Keywords: Climate change, LCA, Aluminium, reinforced concrete, innovation

Abstract

The most common structural material used in the construction sector is steel-reinforced concrete. However, concrete cracking and reinforcement corrosion demand constant monitoring as well as timely and costly maintenance activities. Furthermore, concrete has substantial environmental impacts, being responsible for about 7% of total CO2 emissions worldwide. Innovative materials in construction engineering have been studied with the goal of improving the sector's environmental performance, mostly by reducing cement content in concrete. In this context, assessing the environmental profile of such innovations is essential to avoid shifting environmental burdens elsewhere. This study evaluates the climate change impact of a novel reinforced concrete that incorporates calcined blue clay as a supplementary cementitious material and Aluminium (Al) as reinforcement. Using a cradle-to-gate Life Cycle Assessment (LCA), the climate change impact of this innovative material is compared with that of conventional steel-reinforced concrete. The result show that the climate change impact of the innovative concrete is 46% less than that of the incumbent solution. Acknowledging the early development stage of the new concrete and the limitation regarding data robustness, this work contributes to the problem-solution space and provides direction to further explore possibilities for fully unlocking the new material's potential, so it can outperform the incumbent one in terms of greenhouse gas emissions in the future.

Introduction

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Concrete is one of the most widely used materials in the construction sector. However, it has substantial environmental impacts, mainly due to its cement content. Cement production alone is responsible for around 7% of global CO2 emissions [1]. Demand for cement continues to grow: its production has increased by a factor of 34 over the last six decades, while the world population has grown by only a factor of 3 [2]. In the context of today's sustainability goals, innovation in concrete is therefore essential.

Innovation in concrete technology includes the development of new mix designs, materials, and application techniques, driven by environmental regulations, rising performance requirements, and resource scarcity. Many recent studies focus on cement/clinker substitution using supplementary cementitious materials (SCMs), aiming to improve both environmental and mechanical performance [3]. However, Pushkar and Verbitsky (2016) [4] highlight that higher levels of clinker substitution do not necessarily lead to lower environmental impacts. Consequently, other research has turned to optimized mix designs that reduce cement content through the incorporation of recycled materials and industrial by-products [3].

The environmental burdens of the construction sector are closely linked to its intensive consumption of natural resources and energy, high waste generation, and significant greenhouse gas emissions. To address these challenges, the sector is pursuing strategies such as improving process energy efficiency, reducing waste and emissions, and optimizing designs through material and mass
reduction [5,6]. Focusing on concrete, in building structures, it is often used in combination with steel reinforcement [7]. Steel reinforcement requires relatively thick concrete cover layers to protect the steel from early corrosion and structural deterioration. This requirement increases material consumption and associated impacts. As a result, there is an urgent need for alternatives to conventional steel-reinforced concrete that can support a more climate-friendly construction sector. One such innovation is Aluminium reinforced concrete, which combines corrosion-resistant Aluminium with a specially designed, low- pH concrete mix. This novel concrete system is the focus of the present study.

Achieving low greenhouse gas emissions in construction demands the redesign of both materials and processes. Comparing the climate change impact of such innovations is crucial to ensure that the burden is mitigated rather than shifted elsewhere in the life cycle. Life Cycle Assessment (LCA), as defined in the ISO 14000 series, provides a framework to identify environmental hotspots across a product's life cycle [8].

The objective of this study is to compare the climate change impacts of traditional steel reinforced concrete with those of innovative Aluminium reinforced concrete using LCA. The results will help identify climate change hotspots and support the adaptation and further development of a novel, climate friendly construction material, with the aim of offering a solution that addresses both current needs and future challenges.

State-of-the-Art in Concrete Innovation

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Concrete has been used for long in the construction industry due to its versatility, strength, durability, and affordability [5]. However, cement content in concrete is of environmental concern due to the energy-intensive processes associated with its production [5]. Alternative to traditional concrete, therefore, has been sought with potential to diminish these impacts. Furthermore, alternative production processes and supportive policies are imperative to assure emission reductions in the sector [5].

Literature has explored means to make concrete more sustainable, methods ranged from developing alternative binders, incorporating waste or by-products as concrete mix ingredients, improving material performance and durability, and reducing impact from production and application processes [9]. In this section, the state-of-the-art literature about concrete innovations and environmental performance will be reported.

Kouvara at al. (2023) [10] evaluated a polymer concrete incorporating recycled PET. Although resin production caused high impacts per kilogram, the material became environmentally advantageous at the component level (building) due to its low density and high strength, enabling up to 80% material savings.

Several low-energy and high-performance reinforced concrete were analyzed in Zingg et al. (2016) [11]. Hybrid wood-concrete slabs can reduce energy use by 50% and emissions by 70%. High performance concrete prestressed with carbon fibre reinforced polymer achieved ~60% energy and 70% emission savings per meter of beam compared to conventional reinforced concrete, while ultrahigh performance fibre reinforced concrete with reduced clinker and synthetic fibres achieved over 50% savings per cubic meter.

Bio-inspired designs have also emerged. Horn et al. (2018) [6] demonstrated that a graded concrete optimized for bending loads could reduce climate impacts by 7−19%. Furthermore, biological approaches, such as self-healing bacterial concrete [5], promise up to 60% emission reductions by lowering maintenance needs and using bacteria grown on organic waste, though evidence remains largely qualitative. Bio-based materials like hemp concrete [12] show reduced life-cycle impacts due to improved building energy performance and CO2 sequestration, despite burdens from lime binders in the composition of the hemp concrete.

The results of the research on recycled aggregates and supplementary cementitious materials of Daci and E. Mester-szab (2024) [9] indicates that system boundaries significantly influence LCA outcomes and that optimizing both binder and aggregate use is essential to avoid burden shifting. The
service life and engineering context of the innovative concrete also plays a role in the LCA results, as explored in the work of Jiang et al. (2025) [13] with high-performance glass pervious concrete. Other studies explored carbon-reducing mixes, including net-zero designs using carbonated recycled aggregates [14], mine tailings as sand replacements achieving modest 3% footprint reductions [15]. Finally, Kamath et al. (2025) [16] showed that carbonated cementitious materials initially have higher impacts at laboratory scale but may outperform conventional concrete when produced industrially under optimized energy and material conditions.

Alternative binders such as shea nutshell ash [17] can improve climate performance at ≥15% replacement rates. Waste-plastic concretes [18] showed that binder-based plastic mixes outperform plastic-filler mixes, with renewable energy offering up to 83% further emission reduction.

In construction technologies, 3D-printed concrete provides similar manufacturing impacts to traditional construction but reduces on-site emissions, lowering total carbon footprints [14].

Finally, Alfocea-Roig et al. (2024) [19] compare non-structural precast magnesium phosphate cement (MPC) made with magnesium from primary production and from steelmaking waste against ordinary Portland cement. MPC based on steelmaking residues shows markedly lower climate impacts (up to −42% vs. Portland cement and −56% vs. primary-magnesium MPC), while primary-magnesium MPC performs worse than Portland cement. Here lies the importance of carrying LCA, innovative clinker substitutes may not always yield environmental savings.

Aluminium Reinforced Concrete

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Literature agrees that the use of blended cement can reduce the impact of climate change per kg of cement by 42% between 2020 and 2025 [20]. The binder used in the concrete mix developed for this work is a blended cement with reduced clinker content. Besides cement, the concrete mix has seven other components, including supplementary cementitious materials, in proportions designed to hold Aluminium reinforcement, which cannot be disclosed here due to confidentiality aspects. However, characteristics and properties of the innovative concrete will be discussed shortly in this section.

The primary innovation of this concrete mix concerns its pH . Conventional concrete is highly alkaline, which leads to hydrogen evolution and degradation of Aluminium reinforcement. In contrast, the new mix is formulated with an acidic pH that prevents these reactions and preserves Aluminium integrity. This is achieved by replacing 55% of the cement with calcined blue clay, combined with a water-cement ratio of approximately 0.43 . The Aluminium reinforcement itself consists of about 92% Aluminium scrap.

The innovative concrete offers several potential benefits: reduced concrete cover requirements for the reinforcement, no maintenance associated with carbonation- or chloride-induced corrosion, a higher water-to-binder ratio enabling easier casting and lower plasticizer demand, and accelerated carbonation, which enhances CO2 uptake and thus reduces the overall carbon footprint.

The new Aluminium-reinforced concrete can be engineered to meet specific strength requirements and is primarily intended for structural road applications. However, ongoing development work is still determining which structural types and use cases are most suitable for this innovation.

Life Cycle Assessment

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Life Cycle Assessment (LCA) is a systematic and holistic approach to analyze a product or service's environmental effects, also enabling pinpointing environmental hotspots. Through it, LCA supports well-informed ecological decisions. The LCA methodology application is guided by the International Organization for Standardization (ISO) norms 14040 and 14044 [8], followed in this study, comprising four phases: goal and scope definition; life cycle inventory; life cycle impact assessment; interpretation.

The main goal of this study's LCA is to compare the climate change impact of two different reinforced concrete materials for structural applications with similar workability (water/ cement ratio) and mechanical properties. Consequently, the study aims to assess and compare the climate change
impact of steel-reinforced concrete and Aluminium-reinforced concrete, identifying environmental hotspots.

The functional unit of this assessment is 1 slab of reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure. The innovative Aluminium-reinforced concrete slab was engineered to satisfy the same load-bearing and stiffness requirements as the steel-reinforced slab. Because steel relies on standard rebar while Aluminium uses an optimized hollow extruded profile, the two slabs differ in geometry.

Structural equivalence between the two solutions was verified using mechanical performance data provided by Alugreen project partners. The Aluminium-reinforced slab requires a smaller concrete volume ( 2.16 m3 compared with 6 m3 for the steel-reinforced version), due to both the material characteristics (a low- pH concrete mix that improves stress distribution) and the reduced concrete cover made possible by Aluminium's corrosion resistance. The slab volumes were defined to ensure equal structural capacity (ultimate bending moment), identical serviceability performance (deflection), and consistency with reinforcement-to-concrete interaction behavior observed in laboratory tests.

The geographical context of the study is Norway. This attributional LCA study uses the database Ecoinvent v. 3.11 cut-off prioritizing process on the geographical order Norway>Europe>Global.

The boundary of the studied system is from cradle to gate, and it considers all flows of materials, energy, and emissions from raw materials extraction until manufacturing. The use and end-of-life phases are excluded from the scope. Furthermore, in this study, transport is not considered because all materials are deemed to be manufactured in the same location.
The system boundary is reported in the flowchart of Fig. 1. Foreground processes are represented; background processes are not depicted to avoid cluttering of the illustration. Multifunctionality is addressed by cut-off, remarks go to the post and pre-consumer scrap flows (recycled Aluminum): for the first, burdens are accounted upstream the studied system, meaning that it enters the studied system burden free; for the latter, it is modeled as closed-loop recycling, meaning that it contains the embodied impacts of the initial material. The latter modeling concretizes using a co-product allocation approach. This conservative approach is adopted and recommended by the Aluminum producer partner involved in the case study because pre-consumer scrap is also sourced in the market. Current, the market for pre-consumer scrap is competitive, with prices as high as primary Aluminium's price. In addition, the fact that 40% of Aluminium's worldwide production ends up as pre-consumer scrap, strengthens the adoption of the co-product allocation approach, assuring that burdens are really accounted, avoiding greenwashing.

The inventory data used in this study originates majorly from a Norwegian Aluminium producer and a Norwegian research institution involved in developing the innovative concrete under the Alugreen project, a major Norwegian research and innovation initiative focused on circular Aluminium solutions. Due to a confidentiality agreement, the full inventory data cannot be publicly disclosed, but it can be highlighted that 92% of the reinforcement alloy consists of Aluminium scrap and the rest 8% of primary Aluminium, which is added in the alloy mix due to mechanical property requirements. An exception is the data source for the inventory of the extrusion process, which is adapted from the report of European Aluminium (2025) [21].

[picture]

Fig. 1. System boundaries and foreground processes for Aluminium and steel reinforced concrete (functional unit: 1 slab of Aluminium-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure and 1 slab of steel-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure). Source: author's own.

Fig. 1. System boundaries and foreground processes for Aluminium and steel reinforced concrete (functional unit: 1 slab of Aluminium-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure and 1 slab of steel-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure). Source: author's own.

The impact category chosen in the impact assessment phase is climate change, with the indicator global warming potential GWP 100a, which quantifies the global warming potential over a timeframe of 100 years in terms of kg of CO2 eq. The GWP 100a method is part of the Environmental Footprint method family, which is a recommended impact assessment method by the European Union [22]. The single focus on the climate change impact category is justified by assuming that if the new reinforced concrete does not have favorable characteristics in terms of greenhouse gas emissions, its future market viability is compromised due to the low emission direction envisaged in the construction sector. Therefore, the climate change impact category can be seen as a feasibility starting point for materials innovations in the construction industry [19].

The quantitative interpretation phase of the LCA will follow in the Results and Discussion sections.

Results

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Aluminium reinforced concrete

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Fig. 2 shows the climate change impact network for the innovative Aluminium reinforced concrete. The contribution of unit processes towards the total impact of the studied functional unit is indicated by the width of the connecting arrows. On the flow chart, "1p" corresponds to 1 slab. The node cut-off is set to 5%.

[picture]

Fig. 2. Climate change contribution analysis for 1 slab of Aluminium-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure. Source: Simapro network.

Fig. 2. Climate change contribution analysis for 1 slab of Aluminium-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure. Source: Simapro network.

The contribution analysis of the innovative reinforced concrete highlights a successful effort in reducing the influence of cement on the impact of a construction material. However, the Aluminium reinforcement becomes the largest contributor, accounting for 80.2%. The innovative concrete, besides not being the highest impact contributor towards the innovative reinforced concrete, still reveals the role of clinker as a major influencer in climate change, accounting for 12.5% out of the 13.1% of Portland cement's contribution.

Steel reinforced concrete

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Fig. 3 shows the climate change impact network for the incumbent steel reinforced concrete. The contribution of unit processes towards the total impact of the studied functional unit is indicated by the width of the connecting arrows. On the flow chart, "1p" corresponds to 1 slab . The node cut-off is set to 9.5%.

[picture]

Fig. 3. Climate change contribution analysis for 1 slab of steel-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure. Source: Simapro network

Fig. 3. Climate change contribution analysis for 1 slab of steel-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure. Source: Simapro network

The contribution analysis of the traditional steel reinforced concrete reveals that the steel reinforcement contributes the most, with 61.3%, followed by the concrete, which has clinker accounting for about 80% of its contribution. In steel production, pig iron production has a considerable contribution, mainly with iron sinter production, which accounts for about 25% of pig iron's contribution, indicating high emissions during ore preparation.

Table 1 compares the climate change indicator results between the functional units of the two reinforced concrete:

Table 1. Climate change profile of the compared alternatives. FU: 1 slab of Aluminium-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure and 1 slab of steel-reinforced concrete with a nominal axle load of 3.3 ton intended for structural applications in road infrastructure. Source: author's own.

Climate change impact [kg CO₂eq]
Aluminium reinforced concrete | 2740
Steel reinforced concrete | 5060

The innovative concrete is about 46% less emissive than the incumbent alternative for the same functional unit. It is important to note here that the dimensional volume of 1 slab of Aluminium reinforced concrete is 2.16 m3, and that of 1 slab of steel reinforced concrete with the same structural properties is 6 m3.

Discussion

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The LCA methodology is mainly used for the environmental assessment of innovative construction materials, as it allows identification of environmental bottlenecks and suggestions for potential improvements.

The climate change impact of two structural construction materials is compared in this work, being one of the materials an innovation in the field without approved applications, as it is found for the incumbent material. The results show that a focus on the reduction of cement's greenhouse gas emissions can increase the emissions contribution of the reinforcement by 33%, shifting environmental concerns from concrete to the reinforcement. Furthermore, the results underscore the importance of evaluating recycled materials' environmental performance: recycled Aluminium does not have less climate change impact than primary steel in the analyzed context, leading to higher overall impact from the reinforcement. The use of heating in Aluminium's extrusion is also highlighted, suggesting a sensitivity analysis on the combustion material for heating to further enhance the climate change performance of the innovative reinforced concrete.

One must be attentive that the mass of concrete and reinforcement in the two analyzed reinforced concretes is not the same. The common structural properties and safety in the new material are achieved with a decrease of 64% in concrete mass and of 11% in reinforcement mass due to the material properties provided by the innovative concrete mix and Aluminium. On this account, considering the application of the innovative concrete in an infrastructure leads to design innovation that can yield significant material reduction in construction, and so greenhouse gas emissions reduction. Zingg et al. (2016) [11] support the latter, affirming that looking at structural levels containing the innovative material, and the whole life cycle of the structure, can access potential environmental savings. This is of relevance for Aluminium, because it is a naturally corrosion free material, which can save maintenance related emissions in infrastructure.

Conclusion and Suggestion for Future Work

The original version of this paper is available on https://www.scientific.net/KEM.1051.49.pdf

The achievement of CO2 emission reduction in the construction sector can only progress further with the implementation of innovative technologies and materials together with different policy frameworks and regulations [22,23]. This work evaluated the climate change impact of an innovative construction material, Aluminium reinforced concrete, through a LCA. The LCA results show a favorable result for the innovative material when compared to the incumbent solution, steel reinforced concrete. A reduction of 46% in greenhouse gas emissions is achieved by comparing the production of 1 slab of each material.

This study's climate change impact results motivate to broaden the scope of the LCA to include the utilization of the innovative concrete in a practical application, in a cradle-to-grave approach. Literature suggests that looking into structural levels containing the innovative concrete can reveal deeper potential environmental savings. This is particularly relevant for Aluminium due to its corrosion resistance and almost infinite recycling properties. Furthermore, acknowledging that the
innovation is at an early development stage, another suggestion is to integrate future socio-economic scenarios, based on projections from Integrated Assessment Models (IAMs), modifying background process inventories and projecting them into the future when industrial scale is reached [24,25,26]. This prospective LCA can enhance the robustness of the analysis by identifying improvements needed on background supply chains that can make the innovative concrete outperform the incumbent one [28].

In addition, conducting a supplementary analysis that includes recycled steel within the scope can provide valuable insights by enabling an early comparison of climate-change performance, should recycled steel become the incumbent solution. Furthermore, including recycled steel in the analysis enhances the completeness for the assessment of the innovative solution.

Limitations and Contributions of the Work

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This study is empirical and relies on data from both laboratory and industrial scale processes to assess the climate change impact of an innovative reinforced concrete slab through an LCA. While part of the life cycle inventory benefits from having real industrial production data from one of the project partners, the other part of the inventory relies on less robust data from laboratory experiments, since the innovation is not fully mature yet, thus, not representing optimized industrial conditions. The laboratory-scale data were scaled up to the best of the authors' knowledge to enable a comprehensive comparison and full-scale environmental assessment. Therefore, assumptions have been made regarding its viability as a commercial product and the energy requirements during its manufacturing, so one should acknowledge the existence of uncertainties in the results.

Besides the data limitation, the LCA of this work contributes to the problem-solution space, suggesting anticipated meaning without waiting for all the information to be available, thus providing strategic questions, engagement, and direction to forward-looking research activities, including the improvement of the innovative concrete design to understand the implications of different choices on the climate change impact performance of the material.

Acknowledgment

The original version of this paper is available on https://www.scientific.net/KEM.1051.49.pdf

The authors would like to express their gratitude to the Research Council of Norway for supporting the project number 328843, and, particularly, La Rosa for project number 336488.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.49.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 61-71

doi: 10.4028/p-9JSPCw

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-15

Revised:
2026-02-10

Accepted:
2026-02-17

Online:
2026-04-17

Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

The original version of this paper is available on https://www.scientific.net/KEM.1051.61.pdf

Francesco Borda 1,a*, Vito Basile 2,b, Francesco Gagliardi 1,c and Luigino Filice 1, d

1 Department of Mechanical, Energetic and Management Engineering, cube 45 C, Ponte P.Bucci, University of Calabria, Rende (CS) 87036 Italy

2 CNR-STIIMA, Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing, National Research Council, Via P. Lembo 38F, Bari 70124 Italy

afrancesco.borda@unical.it, b  vito.basile@stiima.cnr.it, c  francesco.gagliardi@unical.it  dluigino.filice@unical.it, (*corresponding author)

Keywords: LCA, LCC, Topological Optimization, Multi-criteria Tool, Sustainability.

Abstract

The growing demand for high-performance, sustainable micro-moulded components requires integrated approaches to material and process selection. The study presents a Life Cycle Engineering (LCE) framework for the integrated selection of materials and manufacturing technologies for micro-injection molds, combining Life Cycle Assessment (LCA), Life Cycle Costing (LCC), and multi-criteria decision models. The methodology implements multicriteria cost impact maps and ternary LCA-LCC-technical performance model, allowing for result normalization and sensitivity analysis with respect to criterion weighting. The framework is applied to molds fabricated from steel, aluminium alloy, polyether-ether-ketone (PEEK), and high-temperature resin, using both subtractive and additive processes, with topological optimization. Mass reductions of up to 22% achieved through optimization translate into cost and environmental impact savings of 30−45% during production and use phases, although with potential service life reductions of up to 50% for polymeric materials. LCA and LCC analyses highlight production and use as the dominant life cycle phases, with end-of-life (EoL) impacts being comparatively minor. Sensitivity analysis shows that: (i) cost-prioritized scenarios select optimized steel molds; (ii) scenarios prioritizing lightweight design and environmental performance select advanced polymers and additive manufacturing; (iii) balanced scenarios identify PEEK as the optimal solution. The proposed framework enables the concurrent selection of material and technology aligned with design objectives and geometric optimization, providing quali-quantitative support for sustainability-oriented industrial decisionmaking across the life cycle.

Introduction

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Life Cycle Engineering (LCE) is a comprehensive, sustainability-driven methodology used in product development to balance technical performance, costs, and environmental impacts across a product's entire lifespan. LCE encompasses activities over one or more life cycles and requires rigorous analysis to quantify sustainability and set precise targets for reducing environmental impact. By integrating complementary technologies and decision-making frameworks, LCE allows environmental objectives to be achieved by ensuring products remain functional and economically sustainable from raw material extraction to disposal [1]. The implementation of LCA within the Sustainability Development Goal (SDG) framework transforms high-level policies into actionable engineering requirements. This integration ensures that every phase of a product's life, from resource extraction to end-of-life recovery, supports the broader mission of social and environmental equity. With the tightening of international regulations, the ability to quantify and act on life cycle data becomes the primary mechanism for achieving the 2030 goals [2]. Indeed, the sustainable development concept was identified as meeting the needs of the present without compromising the
ability of future generations to meet their own needs [3]. This approach is crucial since design decisions made early in the design process can have multiple impacts on life-cycle metrics such as time, cost and quality. The motivation for LCE is the environment, economy, regulations and standards. Design for the environment (DFE) is an element of LCE. The primary goals of DFE are to efficiently manage renewable resources, reduce the use of non-renewable resources, and minimize toxic release into the environment [4]. According to LCE principles as life-cycle design, is a decisionmaking methodology that considers environmental, performance and cost requirements throughout the duration of a facility. According to LCE principles, the life cycle design of a product or service is a decision-making methodology that takes into account environmental, performance and cost requirements throughout the entire life of an asset. Numerous existing methods analyse the cases studies in different dimensions, namely in terms of costs [5], environmental impacts [6], social impacts [7] or/and products technical performance [8]. Considering the aforementioned works, different sustainable models were applied by the authors, some already published [9, 10, 11]. In this work an innovative approach is proposed to provide designers and practitioners to choose the most sustainable way to develop engineering design alternatives for a specific problem.

Design Alternatives from a Life Cycle Engineering Perspective

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To evaluate the impact of different design alternatives in the entire life cycle of a product, three pillars are considered, namely economic, environmental and technical or functional performance. Considering these three sustainability pillars, a ternary model can be used. This type of model allows the aggregation and mapping of the best choice according to the boundary conditions and a system of importance weights is given to each sustainability pillar. On the other hand, the map model provides a basis for analysing situations where alternatives are technically or functionally equivalent, or where technical aspects are highly correlated with cost and environmental performance analysis. Indeed, the map model offer better support in choosing between different design alternatives with different technical/functional performances and when the importance attributed to the various parameters involved in the use life is not a relevant issue. According to both proposed models, the selected case study is represented by a microinjection molding process. Specifically, the design of molds used in the microinjection process is a highly complex task, as it must comply with numerous technical constraints, including material selection, mechanical loads, molds lifespan, production rate, surface quality of the final parts, processing time, and characteristics of the injected materials. The investigated case study is detailed by authors in [10, 11]. Figure 1 reports the proposed decisionmaking support framework of the LCE model considering both models, i.e. map and ternary model.

The flowchart provides an overview of the proposed sustainability approach, starting from the definition of boundary conditions up to the obtained outcomes for the representation of the solution. The proposed model provides all the necessary information that needs to be available during the design phase in a single decision-making tool that integrates the three main pillars of economic and environmental sustainability and technical performance of the output. Considering the economic perspective and environmental sustainability the impacts are obtained through Life Cycle Costing (LCC) and Life Cycle Assessment (LCA) methodologies. These are obtained using the Process Based Approach (PBA) applied to model both costs aliquots and environmental resources consumption with the relative emissions. The detailed methodology regarding PBA can be analyzed in [10,11].

[picture]

Fig. 1. Flowchart and boundary conditions of the LCE proposed model.

Fig. 1. Flowchart and boundary conditions of the LCE proposed model.

Map model

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The proposed map model aims to compare different design alternatives in the microinjection molding process. Firstly, integration of costs related to the environmental impact during the different product life cycle. Secondly, the separation between cost assessment and environmental impact evaluation often induces decision-makers to select design alternatives based on two sustainability criteria thereby creating a trade-off with mechanical performance considerations.

The first step to implement the map model to compare the design material alternatives involves standardising the costs and environmental emission developed in each phase of the life cycle. The normalized values are subsequently aggregated to evaluate production flow (i.e. material and product manufacturing) and use flow (i.e. mold use and EoL phases). Specifically, for each design material alternative the Score ( S1,…,Sk ) is accounted according to Eq. (1):

Sk=(nPEk+ε·nUEk)·γ+nPCk+ε·nUCk(1)

where, nPEk and nUEk represent the normalized production and use environmental emissions for each design alternative, respectively; nPCk and nCUk represent the normalized production and use flow costs for each design alternative, respectively. Looking at the terms ε and γ, both terms represent the importance of the environmental costs and emissions of the product's life cycle. For instance, when production costs have an importance of 100%, this means that latter are extremely important for any decision-maker involved in the supply chain. Finally, the decision domain ( ε,γ ) represents the best choice considering costs and environmental emissions.

Ternary Diagram model

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The ternary graph model provides a space of solutions when a Technical performance Assessment (TA) has to be included in the model, in this case the integrated performance assessment can be added to the model via ternary graph firstly introduced by [12]. In the proposed model, each axis represents the pillar that has to be assessed, in this model economic, environmental and technical performance of the investigated mold system are incorporated in a multi-criteria decision tool. Thereby, for each investigated solution (1,…,k), the normalized LCC (nLCC), normalized LCA (nLCA) and normalized technical performance assessment ( nTA ) are accounted by using the Eq. (2):

Sk=w1·nLCC+w2·nLCA+w3·nTA(2)

where, w1,…,3 represents the weight of each pillar, i.e. economic, environmental and technical, respectively. By applying Eq. (2) the outcome of the model allows a global assessment of the investigated case study. On the other hand, using this approach the importance of each weight is a critical task because for each specific case study the weight may change and consequently the best application domain.

Life Cycle Engineering model

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To evaluate the best choice that meets the correct trade-off between the economic, environmental and technical performance of the investigated molds, two problems were analysed. Firstly, the mold material selection problem, in this case a set of different materials was selected within the design constraints. Looking at the selected materials, the mold design was adapted to guarantee the similar technical performance due to the expertise of the designers. The other part of the investigated case study involves the technology selection problem strictly linked to the material choice; specifically, different injection mold configurations were selected considering different manufacturing processes that perform differently in the manufacturing phase.

Mold components material selection

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To demonstrate the applicability of the proposed model to material selection, the present study investigates a case study focused on molds used in the microinjection molding process, which are usually manufactured from steel. The model aims to evaluate the feasibility of using alternative materials with different properties, such as aluminium alloys and high-performance polymers, and to assess their potential to enhance mold performance. A preliminary analysis of the mold's static and dynamic behavior was carried out to identify candidate materials for the next phases of the proposed model. The selected materials and their respective properties, summarized in Table 1, are steel (AISI H13), aluminium alloy (AA 7075-T6), polyether-ether-ketone (PEEK), and high-temperature resin Formlabs (HT V02).

Table 1. Properties of set of selected materials.

Properties | Set of materials
| Steel | AA | Peek | HT V02
Yield Strength (a) [MPa] | 1000 − 1200 | 480 − 520 | 90 − 110 | 75 − 90
Young's Modulus (b) [GPa] | 205 − 215 | 70 − 72 | 3.5 − 4.0 | 2.7 − 3.2
Density (c) [kg/m³] | 7800 | 2900 | 1320 | 1200
Ductility (d)[kN·m/kg] | 30 − 40 | 50 − 60 | 20 − 25 | 10 − 15
Strain Hardening exponent (e) [N·m/kg] | 0.10 − 0.15 | 0.06 − 0.10 | 0 | 0
Corrosion Resistance (f) [0/0.5/1] | 0.5 | 1 | 1 | 1
Hardness (g) [HV] | 480 − 520 | 160 − 180 | 25 − 30 | 20 − 25
Ferromagnetism (h) [0/1] | 1 | 0 | 0 | 0
Coefficient of Anisotropy (i) [-] | 1.6 | 0.7 | 1.2 − 1.5 | 1.3 − 1.6

In mold design practice, material selection is typically driven by the fulfillment of minimum safety requirements to ensure the successful execution of the process. This approach often overlooks higherperformance materials that may increase manufacturing costs in the production phase although could provide increased performance throughout the entire mold lifecycle. Such improvements may include extended service life, reduced mold weight, and, consequently, optimized efficiency during the use phase. Another relevant aspect of the present case study concerns the temporal scope of the analysis and the different stakeholders involved. Costs and environmental impacts generated during the manufacturing phase are attributed to the mold producer, whereas those arising during the use phase are attributed to the molding operator. Moreover, impacts generated during the use phase may extend beyond company boundaries, as environmental emissions can vary significantly depending on the energy mix of the country in which the mold is operated. Consequently, benefits such as energy savings achieved through mold weight reduction may be valued differently by the mold manufacturer compared to the operator, particularly when considering both environmental impacts and cost implications related to the material selection in the manufacturing phase. The investigated case study highlights the importance of using a model separating the manufacturer from operator in the product's life cycle.

Boundary Conditions

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To perform the proposed analysis considering the overall life cycle and material selection for microinjection molds, boundary conditions have to be defined. Specifically, the analysed phases and both environmental and costs flows have to be detailed, as reported in Figure 2.

[picture]

Fig. 2. System boundaries for the analysed molds.

Fig. 2. System boundaries for the analysed molds.

Economic and Environmental Assessment

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After defining the system boundaries and considering the mold lifecycle phases, both environmental and cost impacts were evaluated using LCA and LCC methodologies, as previously described. The LCC and LCA results for the selected materials show that raw material-related costs (materials, energy, and processing) have a major influence, while the End-of-Life phase contributes negligibly. The use phase is more significant than the End-of-Life phase. For a production volume of approximately 1000 parts, the LCC results indicate that the total cost-effectiveness corresponds to the sum of all lifecycle phases, as reported in Table 2. By considering the expected production volumes for molds manufactured from PEEK and HT V02 resin and taking into account the microinjection molding process conditions reported in [10,11], the experimental validation allowed the estimation of an average safe service life of approximately 1000 and 500 injection cycles, respectively, as reported in [12]. This assumption was adopted within the LCC framework.

Table 2. LCC assessment of molds for the analysed materials considering a production volume of about 1000 pieces ( €/pc ).

Molds lifecycle phases | Analysed Materials
| Steel | AA | Peek | Formlabs
Materials production | 7.21 | 8.51 | 7.25 | 11.97
Mold manufacturing | 4.90 | 5.62 | 5.47 | 7.84
Mold use phase | 0.39 | 0.24 | 0.30 | 0.40
EoL of mold | 0.11 | 0.06 | 0.13 | 0.16
Overall LCC | 12.61 | 14.43 | 13.15 | 20.32

The Ecoinvent v3.10 data provided by SimaPro 9.6.0.1 software and information extracted by experimental setup and literature were combined as well as data modelled and calculated for the investigated processes and materials, e.g., detailed values of energy and masses. The energy consumption considered in the LCA analysis was modelled based on the electricity market in Italy.

To assess the impact of mold used in the microinjection molding process IMPACT World+ methods v1.04 was used, available in the SimaPro 9.0.6.1 software. The outcomes were reported in Table 3.

Table 3. LCA assessment of molds for the analysed materials ( kgCO2eq/pc ).

Molds lifecycle phases | Analysed Materials
| Steel | AA | Peek | HT V02
Materials production | 4.13 | 10.27 | 4.49 | 4.82
Mold manufacturing | 18.60 | 15.01 | 6.25 | 4.06
Mold use phase | 0.81 | 0.28 | 0.15 | 0.13
EoL of mold | -11.47 | -4.41 | -2.13 | -2.12
Overall LCA | 12.07 | 21.15 | 8.76 | 6.89

The environmental impact due to microinjection process is almost related to the material production and manufacturing processes, strictly linked to energy consumption during the manufacturing step, i.e. machining, roughing and finishing, tempering, quenching for metal and post processing for the polymeric material. Considering the use phase, the environmental impact is due to an injection of one piece, this value is calculated according to [10]. Looking at EoL phase, the material recycling energy is accounted for considering a percentage recycling rate of 98% and 90% for steel chip and mold scraps [10]. Whereas the aluminium recycling rate for chip and scraps are estimated by literature [13]. Polymeric materials EoL recycling impact, i.e. PEEK and Formlabs resin were assumed mechanically recycled and incinerated with heat recovery according to [14, 15], respectively. Considering PEEK, the recycling rate was extracted by literature [16].

Discussion of Results

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This section examined the results of the assessments calculated in the previous phases. The decision space is divided according to the importance levels due to production and use flows. In detail, the decision space is realised considering Eq. (1). The most suitable materials from both environmental and economic perspective are graphically presented in Figure 3. In detail, the ( ε,γ ) decision domain represents respectively the level of importance given to both costs and environmental impacts considering the use (utilisation and EoL phases) flows. In the model, the production flows were assumed to be significant for any stakeholder involved in the supply chain.

[picture]

Fig. 3. Map model applied to material selection for molds manufacturing.

Fig. 3. Map model applied to material selection for molds manufacturing.

Technology Selection

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The second part of the analysis focuses on mold material and manufacturing technology selection based on technical properties. Considering the weight of each requirement that the molds have to be satisfy, the proper manufacturing process has to be selected. To select the properly material for mold's manufacturing the weight of each requirement directly associated with the material properties was defined in Table 4.

Table 4. Weight used for the material properties characterization.

Mold requirements | Weight | (a) | (b) | (c) | (d) | (e) | (f) | (g) | (h) | (i)
Resistance | 0.25 | 10.00 | 5.00 |  |  |  |  |  |  |
Lightweight | 0.15 | 4.00 | 4.00 | 7.00 |  |  |  |  |  |
Corrosion resistance | 0.05 |  |  |  |  |  | 15.00 |  |  |
Stiffness | 0.20 |  | 10.00 | 5.00 |  |  |  |  |  |
Formability | 0.15 |  |  |  | 5.00 | 5.00 |  |  | 8.00 | 5.00
Easy to handle | 0.10 |  | 2.00 | 5.00 |  |  |  | 8.00 |  |
Easy to recycle | 0.10 |  |  | 7.00 |  |  |  |  | 8.00 |
Tot. | 1.00 | 3.10 | 4.05 | 3.25 | 0.75 | 0.75 | 0.75 | 0.80 | 2.00 | 0.75
Weighting [%] |  | 19.14 | 25.00 | 20.06 | 4.63 | 4.63 | 4.63 | 4.94 | 12.35 | 4.63

To calculate the score for ( TA ), the weights associated with each material property listed in Table 4 were used. Specifically, the score ( Sk ) for each material analysed was calculated by normalizing the material characteristics listed in Table 1 and weighing them using the weight calculated in Table 4. Therefore, the score ( Sk ) for each material was obtained by summing each weighted material characteristic for each mold requirement. The overall score for the selected material that respects the mold requirements results in 0.76,0.43,0.29,0.30 for Steel, AA, PEEK and HT V02 resin, respectively. Based on the obtained results, and therefore of the material that proves to be the most suitable, it is possible to select the type of technology that optimizes the overall sustainability assessment. The proposed decision-support tool allows the definition of an optimal decision space consistent with the strategic priorities of the different stakeholders. For instance, if the objective is cost minimization without considering environmental impact, and mold robustness must be guaranteed, the selected material is likely to be steel. Consequently, since no topological optimization process is applied, the selected manufacturing technology is expected to be conventional machining. Conversely, if mold lightness is a key requirement, the material chosen is likely to be AA, and if the mold requirement specifies conformal cooling channels, the manufacturing technology would be additive manufacturing.

Global assessment

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In this section, an integrated and comprehensive assessment can be made based on the results obtained for economic, environmental and technical performance. As described in the previous sections, the outcome values for each dimension were normalized and converted into dimensionless quantities, thereby allowing the consistent allocation of weights to each assessed factor and to each dimension. Specifically, the sum of the weights of the three dimensions must be equal to 100%. In detail, when representing different combinations of weights, it is difficult to assign weights in a single way. Indeed, the choice of weights generally reflects the company's strategy, which can design to assign different levels of importance according to the expected life cycle of the product. To overcome this issue, the best choice for representing the decision space is a ternary graph, where for each axis a dimension of analysis is represented. Specifically, the ternary model not only identifies the best material for a given set of weights but also indicates the range of weight combinations that should be used to obtain a specific output. For instance, if the objective of the company is to minimize only the economic dimension (1) with an importance of 90% and a TA relatively low around 20%, the sustainable tool set as the best choice steel material, as shown in Figure 4. On the other hand, if the company's scenario is more balanced with an importance of more 50% for the TA, 30% for the LCC and 10% for the LCA the outcomes of the model shown the point ( 2 ) in the graph with the AA as selected material; otherwise, if the scenario is more than 50% for the LCA, around 40% for the economic perspective and 10% for the TA, the point in the graph is ( 3 ) and the best material remain AA.

[picture]

Fig. 4. Ternary graph applied to material selection according to different analysed scenarios.

Fig. 4. Ternary graph applied to material selection according to different analysed scenarios.

Analysis of the ternary diagram and scenarios enables the selection of the most suitable material. If the objective is cost minimization with no consideration of environmental impact, while ensuring a minimum technical performance of the mold, scenario (1) is obtained. Conversely, if a more balanced scenario is considered, in which the cost constraint is relaxed and both environmental impact and service life of the mold are regarded as important from a technical perspective, then scenario (3) applies, with AA as the selected material.

Sensitivity analysis

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Additional parameters, including reliability and design complexity with mass reduction, were evaluated to broaden the decision space. Based on previous studies [10,11], the environmental and economic impacts of optimized molds were estimated by comparing the material required for optimized and conventional components. Reduced material use provides cost benefits, while optimization introduces extra energy consumption costs that depend on the manufacturing technology used. Conversely, the optimization process results in additional energy consumption costs varying with the selected manufacturing technology. The global assessment, considering the sensitivity analysis performed, led to some changes with respect to the scenario shown in Figure 5. Indeed, with
the reduction in the amount of material used, the environmental performance of AA decreased and, consequently, the model predicted the use of two other materials that promote mold lightening and a resulting reduction in environmental impact over the entire life cycle. Therefore, analysing the results, if a weight greater than 85% is assigned to costs, the optimized steel mold represents the best option (1). In contrast, the mold manufactured in AA appears in a region where greater importance is attributed to the environmental dimension and technical performance is weighted between 30−45%. This outcome can be ascribed to the better performance-weight ratio of AA compared with steel. Conversely, when the production scenario is shifted with a focus on lightness at the expense of technical performance, the model converges toward a resin solution (4), which enables improved environmental performance, mainly due to a less energy-intensive use phase. On the other hand, when a more balanced scenario is considered, in which the three dimensions analysed are assigned approximately the same importance (interval between 40−60% ), the optimized mold made of PEEK (3) represents the most suitable solution. Finally, the model not only provides a decision-support tool capable of identifying the best material choice but also determines the most appropriate manufacturing technology. Indeed, the environmental and cost assessment accounts for both topological optimization and the associated production route. Consequently, for solutions (1) and (2) and for the corresponding percentage of material removed, conventional machining is the most advantageous process, whereas for solutions ( 3−4 ) additive manufacturing offers environmental benefits. In contrast, this outcome is not universally valid, as highlighted in other studies [13, 17], where the two technologies are examined in greater depth: the choice of technology varies according to the percentage of material that must be removed. Indeed, if the optimization process is pushed further at the expense of component service life, the most advantageous technology may no longer be the conventional one.

[picture]

Fig. 5. Ternary graph applied to the selection of mold alternatives.

Fig. 5. Ternary graph applied to the selection of mold alternatives.

Conclusions

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The work proposes an integrated evaluation framework based on LCE to support material and technology selection for mold design and manufacturing, with a focus on economic, environmental, and performance sustainability. The approach combines LCC and LCA with decision-support and multi-criteria models, allowing the evaluation of alternative scenarios and identification of trade-offs among the considered dimensions. The results show that the reduction of mold mass, achieved through topological optimization, leads to significant environmental and economic benefits, at the expense of a potential impact on the service life of the mold. The sensitivity analysis further revealed that optimal choices strongly depend on the relative importance assigned to cost, environmental impact, and technical performance criteria. When extreme scenarios are considered, such as cost minimization, optimized steel molds prove to be the preferred solution, whereas scenarios oriented toward lightness and environmental efficiency tend towards advanced polymeric materials and the use of additive manufacturing technologies. Conversely, when a more balanced scenario is adopted,
high-performance materials such as PEEK emerge as competitive alternatives, capable of combining adequate mechanical properties with reduced impacts over the mold life cycle. The proposed model therefore does not merely identify the most suitable material but also allows to determine the production technology most consistent with the design objectives and with the desired level of geometric optimization, showing how conventional subtractive machining and additive manufacturing assume different roles as requirements change. The model represents an adaptable decision-support tool, capable of integrating the needs of different stakeholders and guiding manufacturing companies toward solutions that are more sustainable from technical, economic, and environmental perspectives. Future work aims to minimize weighting subjectivity through Analytic Hierarchy Process (AHP) and extend the framework to a Life Cycle Sustainability Assessment (LCSA) incorporating social sustainability aspects [18, 19]. Finally, future research activities may extend the model by including additional production scenarios, different production volumes with different types of runners, and further sustainability metrics for materials, as well as experimental validation on real industrial case studies, with the aim of further consolidating the effectiveness of the proposed framework and promoting its large-scale application.

Acknowledgments

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This work was supported by the European Union under the Italian National Recovery and Resilience Plan (NRRP) of NextGenerationEU, partnership on "Next generation of sustainable and highly efficient molding processes" Italian Projects of Significant National Interest (PRIN) PNRR 2022 grant n. P2022ZE23N.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.61.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 73-81

doi: 10.4028/p-Y0QEyo

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-16

Revised:
2026-02-12

Accepted:
2026-02-17

Online:
2026-04-17

Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

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lacopo Bianchi 1,a, Pietro Forcellese 2, b*, Gioacchino Fratini 3,c,

Thomas Lamberti 4, d, Tommaso Mancia 2,e and Michela Simoncini 1,f

1 Università Politecnica delle Marche, Dipartimento di Ingegneria Industriale e Scienze Matematiche, Via Brecce Bianche 12, 60123 Ancona, Italy

2 Università degli Studi eCampus, Dipartimento di Scienze Teoriche e Applicate, Via Isimbardi 10, 22060 Novedrate, Italy

3 CITIGAS Società Cooperativa S.p.A., Zona Ind.le Colleranesco, 64021, Giulianova, Italy

4 Bluenergy Revolution Società Cooperativa, via Greto di Cornigliano 6, 16152, Genova, Italy

ai.bianchi@staff.univpm.it, b  pietro.forcellese@uniecampus.it, c  ufficiotecnicogas@citigas.it, d tlamberti@bluenrev.com, e tommaso.mancia@uniecampus.it, f m.simoncini@staff.univpm.it

Keywords: Hydrogen storage, LCA, sustainability, Aluminum Casting

Abstract

Aluminum components production is associated with significant greenhouse gas emissions due to both raw material extraction and energy-intensive manufacturing processes. In particular, the melting phase required high thermal energy and conventional energy sources (e.g. fossil fuels, national grids...) can result in relevant environmental impacts. This study evaluates the environmental sustainability of four different energy supply systems for aluminum die casting through a comparative Life Cycle Assessment (LCA). Four scenarios were analyzed: natural gas, national grid electricity, photovoltaic (PV) electricity with battery storage, and PV-powered hydrogen production with metalhydride storage. A cradle-to-gate approach was adopted, including energy production, storage, raw materials extraction, tool manufacturing, casting operations and finishing. The environmental impacts were modelled using SimaPro, and Global Warming Potential (GWP) was calculated according to the Intergovernmental Panel on Climate Change (IPCC) methodology. The results show that renewable-based solutions represent the most sustainable alternatives, with impact reductions up to 62% compared with traditional approaches. PV electricity with battery storage achieves the lowest unitary impacts ( 0.15 kgCO2 eq/kWh). Hydrogen produced from PV electricity also provides significant reductions relative to natural gas and grid electricity and offers high operational flexibility. The metal-hydride storage system shows slightly lower impacts than battery storage, due to its long service life and minimal hydrogen losses. These results highlight the potential of renewable energy and green hydrogen as alternative energy carriers for industrial production.

Introduction

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Sustainability has become a key aspect of industrial manufacturing and enhancing the environmental performances of energy-intensive processes is crucial to support decarbonization and a sustainable development of many industrial sectors [1]. This trend is also driven by increasingly stringent European and international regulations, making sustainability a fundamental requirement for companies.
Aluminum components are characterized by low weight and good mechanical properties and are used for several applications in transport, packaging, aerospace, and automotive sectors. However, aluminum components production chain is associated with relevant greenhouse gas emissions; this is related to both raw materials extraction and energy intensive production processes. As a matter of fact, it is estimated that more than 3% of world energy is used for aluminum melting [2]. Within the aluminum market, the casting process is widely used to produce near net shape components. In this process, aluminum ingots and scraps are melted at about 700∘C and then injected into a steel mold at high pressure. This step is one of the most critical aspects in the aluminum economy as it is associated
with high energy use [3]. The energy demand, as long as the emissions of traditional energy carriers, results in significant environmental impacts. In fact, thermal energy required for melting is typically supplied by natural gas furnaces or by electric resistance furnaces powered from the national grid. In turn, national energy mixes still rely largely on fossil fuels, while the use of renewable energy sources is often limited [4]. Consequently, even when a significant share of secondary aluminum is used, new products are associated with relevant greenhouse gas (GHG) emissions.
This issue highlights the need to evaluate the use of alternative energy routes from renewable sources. Among these, solar photovoltaic (PV) systems have spread in recent years also thanks to the reduction in installation cost and the possibility to integrate modules on industrial building roofs [5]. To effectively use PV energy in the manufacturing process, it is necessary to introduce energy storage solutions in the system to have flexibility and decouple generation from use. A possible solution is represented by battery storage, typically based on lithium-ion technology [6]. Batteries are characterized by high round trip efficiency, modularity, and capacity. This makes batteries optimal solutions in combination with electric resistance furnaces. However, production and End of Life of batteries is a critical aspect as extraction and disposal of constituent materials have relevant environmental impacts [7]. A possible alternative to battery storage is represented by green hydrogen. This gas can be produced starting from PV electric energy and using electrolyzes [8]. H2 can be stored and combusted to provide thermal energy in gas-powered furnaces, and this makes it possible to use this gas in aluminum industrial production. Hydrogen can be stored in high-pressure vessels, but this approach presents several critical issues related to the management of high operating pressures, as well as safety risks and potential leakage [9]. An innovative approach is represented by Metal Hydride storage in which hydrogen is stored in solid state form thanks to reversible chemical bonds created between the gas and a metal alloy. Solid state storage avoids typical risks associated with high pressure vessels and is characterized by a high volumetric storage capacity, minimal hydrogen losses, and long cycle life [10].
Despite the growing focus on sustainability, literature lacks a systematic assessment of the environmental impacts of different energy carriers for industrial production, particularly in comparisons between traditional and innovative solutions. In this context, this work presents a Life Cycle Assessment (LCA) analysis of aluminum die cast products manufactured using different energy sources for metal melting. The objective is to determine whether innovative solutions can effectively improve the environmental performance of industrial production and to identify potential improvement opportunities across different systems.

Methods: Life Cycle Assessment Analysis

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The goal of the study is to evaluate the environmental sustainability of different energy supply systems for industrial applications. This assessment makes it possible to determine whether innovative solutions can deliver environmental benefits and identify potential areas for future improvement. To this end, a case study of aluminum casting was selected, and different energy sources for the metal melting process were considered.
To quantify and compare the environmental impacts of different energy supply in industrial applications, a Life Cycle Assessment (LCA) analysis was conducted. The analysis follows the methodology described by the ISO 14040-44 standards and it is constituted by four iterative phases:

Goal and scope definition: the goal of the analysis, the Functional Unit (FU) and the system boundaries (i.e. the phases considered within the study) are defined.

Life Cycle Inventory (LCI): the input and output data related to the phases considered within the system boundaries are gathered.

Life Cycle Impact Assessment (LCIA): the inventory data are translated into possible environmental impacts according to defined impact categories (e.g. Global Warming Potential, Ozone Depletion, Cumulative Energy Demand...).

Results discussion: the results are critically discussed to draw conclusions and identify criticalities and improvement possibilities.

The following phases are detailed in the next sections.

Goal and Scope Definition

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The goal of this study is to compare the environmental impacts associated with innovative and conventional energy sources and carriers used for aluminum melting in casting processes. Hence, the Functional Unit is defined as the production of an aluminum component with a weight of 10 kg via die casting. Given the focus on energy sources, a "Cradle to gate" approach was selected to include all impacts associated with part production. More specifically energy production, storage and distribution, raw materials extraction, tools production, part casting, auxiliaries, and part finishing were included within the system boundaries. The part of service life and End of Life (EoL) were not included in the study as they would have been the same in the different scenarios and would not have changed the comparative analysis.
The investigated process consists of a conventional aluminum casting route. At first, a mix of aluminum ingots and scraps are melted in a furnace to approximately 700∘C. Gas or electricity powered furnaces were considered depending on the specific scenario (and the energy source). Impurities are removed, and the molten metal is transferred to the holding furnace and injected at high pressure into a steel mould. In turn, the mold is produced by a supplier from steel block through CNC machining (e.g. milling). The part hence cools down and solidifies before being ejected from the mold. A cooling system (e.g. water cooling) can be used to speed up the process. After ejection, runners and overflows are removed and finishing operations are carried out. Four different scenarios were considered in the analysis:

Scenario 1: natural gas. In this scenario, the melting and holding furnaces are powered by natural gas (gas furnace).

Scenario 2: national grid electricity. Aluminum is molten in a resistance furnace powered with electric energy from the national grid. Italian energy mix is considered.

Scenario 3: Photovoltaic (PV) system with battery energy storage. Electric energy is provided by a roof installed PV system. To provide a stable power supply, the system includes a battery storage unit that acts as an energy buffer. A resistance furnace is considered also in this scenario.

Scenario 4: PV powered hydrogen system with metal-hydride storage. PV electric energy is converted into hydrogen ( H2 ) through the Proton Exchange Membrane (PEM) electrolyzer operating at 30 bar. The gas is hence directed to a solid-state storage system based on metal hydrides which operates at the same pressure. This system is based on a reversible chemical reaction in which a metal alloy absorbs the gas, resulting in a stable metal hydride. During charging, hydrogen binds the metal structure and releases heat. As hydrogen is required by the user, heat is supplied to the system, leading to a release of the gas. Hence, H2 can be used in a gas furnace similar to traditional natural gas.

In all scenarios, auxiliary electric energy demands are supplied by the national grid. Hence, the main difference between the scenarios relies on the metal melting process. Figure 1 shows the system boundaries for the considered scenarios.

[picture]

Fig. 1 System boundaries of the four scenarios considered in the present study

Fig. 1. System boundaries of the four scenarios considered in the present study

Life Cycle Inventory

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Both primary and secondary data were gathered in the LCI phase.
The main input data related to aluminum casting was retrieved from relevant literature sources related to metal parts LCA [11,12]. Starting from the FU weight ( 10 kg ) and the constituent percentages (virgin ingot and aluminum scraps, 40% and 60% respectively), the weight of each input material was calculated. Considering the FU weight a parametric approach was followed to estimate the energy consumption for the melting and holding furnaces, auxiliaries and mold weight.
Steel tool was considered for the casting process; raw steel input and milling process for mold production were considered. Both raw materials and milling impacts were retrieved from the Ecoinvent database. In addition, transport from the supplier to the final components manufacturer was considered. The impacts of tools production were allocated to the functional unit considering the number of parts that can be produced using one mold during its service life (up to 100′000 units according to). Impacts related to the casting process were considered the same for all scenarios with the exception of energy production and use.
The energy consumption required by the gas-powered melting and holding furnaces (Scenarios 1 and 4) was calculated considering the unitary energy requirements ( 0.67kWh/kg and 0.13kWh/kg respectively [11]) and the amount of material used for the production of the FU. The same approach was followed for the scenarios with the resistance furnace [13]. In this case, a higher efficiency can be obtained, and the unitary energy consumption is as low as 0.5kWh/kg for metal melting.
Energy production and supply were different amongst the four scenarios. Scenario 1 considers natural gas and its use was modelled using the Ecoinvent database. Scenario 2 considers electric energy use form the national grid; Italian energy mix was assumed and, as for the previous case, energy production was modelled using the commercial database. In Scenario 3, PV energy from modules installed on the roof of the manufacturer building was considered (Ecoinvent); Li-Ion battery with a capacity of 300 kWh was assumed for energy storage and its impacts were calculated considering the weight of the battery pack and the impacts per kg provided by Ecoinvent. A 90% round trip efficiency was considered for the battery [14]. The impacts of the storage system were allocated to the functional unit based on the total number of kWh managed by the battery over its lifetime ( 3000 chargedischarge cycles were assumed). In Scenario 4, the same energy production system as Scenario 3
(roof PV modules) was considered. Hence, electric energy was converted into gas hydrogen via PEM electrolyzer. For this Scenario, primary data were provided by the company involved in the analysis. An energy consumption equal to 7 kW was measured as well as the electrolizers productivity equal to 124 g/h. An efficiency of 59% was considered for the electrolyzer and it was calculated considering the electrolyzers productivity ( kgH2/h ), their energy use and the gas calorific value ( 33.3kWh/kg ). Impacts associated with electrolyzers production were not considered within the system boundaries as would have determined almost negligible impacts ( 1−5% of energy production impacts) according to recent literature [15]. Hydrogen storage via metal hydride system was considered and modelled according to literature [16]; an allocation procedure analogous to the one applied to the electric battery was employed considering the storage system service life and the total weight of stored hydrogen. A 5% gas loss was assumed for storage.

Life Cycle Inventory

Table 1 Main LCI data

Life Cycle Inventory
FU part weight | 10 kg
Virgin material | 50%
Die manufacturing
Service life | 100000 p
Steel mold | 63.68192 kg
steel removed | 74.00871 kg
Mold transport | 200 km
Metal casting
Melting furnace | 0.673497 kWh/kg
Holding furnace | 0.134699 kWh/kg
Electric tot energy | 0.5 kWh/kg
Other Electricity Input
Cooling water system | 0.180556 kWh/kg
Finishing | 0.333333 kWh/kg
Battery energy storage
Battery capacity | 300 kWh
Battery weight | 3000 kg
Depth of Discharge | 0.8
Charge–discharge cycles | 3000 p
Tot kWh lifetime | 720000 kWh
Efficiency | 90%
Metal hydrides storage
Tot hydrogen stored in lifetime | 80000 kg
Tot energy stored used by user | 2664000 kWh

The LCA dedicated software SimaPro was employed to model the defined scenarios and to conduct the LCIA phase. Global Warming Potential was hence calculated according to the methodology proposed by the International Panel on Climate Change (IPCC) and impacts evaluated in terms of kg CO2 eq. This methodology was selected as it is one of the most relevant and most used in literature analyses for the evaluation of the environmental impacts of industrial production [9].

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.73.pdf

Figure 1 presents the impacts per kWh delivered to the final user according to the four energy carriers investigated within the study: natural gas, electric energy from the national grid, electric energy from PV modules with battery storage and hydrogen produced starting from PV electric energy. This comparison shows the influence of the selection of energy systems over the environment performances of the aluminum casting process. The comparison is expressed per kWh of useful energy delivered to the melting process and this allows comparability across the different energy carriers, regardless of whether the energy is supplied as electricity (Scenario 2 and 3) or as thermal energy (Scenario 1 and 4). Among conventional solutions (Scenario 1 and 2), electricity from national grid has the highest impacts ( 0.28 kgCO2 eq vs 0.38 kgCO2 eq for natural gas and grid electricity respectively); this is mainly due to the composition of the Italian electricity mix (for which fossil based contribution is still significant) and distribution efficiency.
Solutions based on renewable sources (Scenario 3 and 4) represent the most sustainable alternatives, with impacts reduction equal to up 62% with respect to the traditional approaches. Photovoltaic electric energy with battery storage is the solution with the lowest unitary impacts ( 0.15 kgCO2 eq/kWh); 53% of these impacts are associated with PV energy production while the remaining 47% is due to the battery storage system. To calculate the unitary impacts, the battery production footprint is allocated over the total amount of energy the system delivers throughout its service life ( 720,000 kWh ). Despite this, battery production still represents a significant contribution; extending the battery service life would further reduce these impacts, leading to the very low unitary energy impacts mainly related to PV modules use.
The PV and hydrogen system, which can supply heat from gas combustion, shows higher impacts with respect to the PV and battery configuration ( 26% higher) but still remains significantly more sustainable than the two traditional solutions (i.e. 32%−52% reduction). Two main contributors can be identified also in this case: energy production and storage systems. The metal hydrides storage system has impacted slightly lower than those of the battery storage; similar allocation procedures were carried out, leading to impacts equal to 0.06 kgCO2 eq (whilst the battery 0.07 were obtained for the battery). In fact, despite the relevance of the alloys used in the system, the long service life makes this storage solution sustainable [16]. In addition, hydrogen storage has high efficiency with only 5% losses. The main factor that contributes to increasing the impacts of the hydrogen system with respect to the battery one is represented by the efficiency of the electrolysis process in which electric energy is used to produce hydrogen (i.e. about 60% ). Increasing the efficiency of the conversion is crucial to further improve the sustainability of this system. Despite that, this result confirms the advantages of renewable hydrogen that combines sustainability with flexibility. The highest efficiency is obtained as hydrogen is directly used in gas-based furnaces; in the case of reconversion of the energy carrier in electricity, higher impacts would be obtained due to the low efficiency of the reconversion stage.

[picture]

Fig. 2 Impacts per kWh available to the final user for the four considered energy systems

Fig. 2. Impacts per kWh available to the final user for the four considered energy systems

Figure 2 and Table 1 report the results of the LCA analysis for the casting process. As described above, the main difference between the four scenarios relies on the "Metal melting and maintaining" phase that considers different energy carriers. The other phases (raw materials, tools, accessories, and finishing) remain the same across different scenarios. The main contributor is represented by raw materials, accounting for up to 97% of total impacts (i.e. 93.937 kgCO2 eq). This is due to the high unitary impacts of virgin aluminum (i.e. 22.4 kg of CO2 eq). Lower emissions are expected as higher scrap aluminum percentage is employed; however, this aspect depends on the required quality of the final products and the specific industrial sector. For graphical clarity, the production of raw materials is not reported in Figure 2.

[picture]

Fig. 3 LCIA results in terms of GWP and contributions of the production phases

Fig. 3. LCIA results in terms of GWP and contributions of the production phases

Among the other phases, melting is the most relevant one and determines the advantages of one scenario over the others. As in Figure 1, the renewable energy solutions provide relevant impacts reduction with respect to the natural gas (Scenario 1) and national grid (Scenario 2) alternatives. However, in the case of aluminum melting, electricity from the national grid results in lower emissions than natural gas, even though the impact per kWh is lower for natural gas. This is because electric furnaces operate with higher efficiency, thereby reducing overall energy consumption and emissions. For the same reason, Scenario 3 offers even greater benefits than the other alternatives: it relies on electricity for melting (thus achieving high furnace efficiency) while also sourcing that electricity from renewable low-impact energy systems. Hydrogen still represents a promising solution; however, further improvements to the melting system may be required to enhance its overall efficiency.
Tools production determines negligible impacts; despite the relevance of the mold manufacturing process (accounting for 600.1 kgCO2 eq), its impacts are allocated to many components, resulting in low carbon footprint for the considered FU. Accessories (i.e. the cooling system) and finishing operations are mainly related to electric energy use from national grid and account for about 2% of total impacts. Also for this energy consumption, more sustainable energy systems can be employed to further improve the sustainability of the four scenarios.

Table 2 Impacts contributions for the four scenarios in terms of GWP

| Tot | Raw materials | Tools | Metal melting | Accessories | Finishing
Scenario 1 | 98.181 | 93.937 | 0.006 | 2.241 | 0.702 | 1.295
Scenario 2 | 97.883 | 93.937 | 0.006 | 1.943 | 0.702 | 1.295
Scenario 3 | 96.677 | 93.937 | 0.006 | 0.737 | 0.702 | 1.295
Scenario 4 | 97.444 | 93.937 | 0.006 | 1.504 | 0.702 | 1.295

Conclusions and Future Development

The original version of this paper is available on https://www.scientific.net/KEM.1051.73.pdf

This study investigated the environmental impacts of an industrial aluminum die casting process using different energy sources and carriers for the metal melting phase. Four scenarios were considered to
represent traditional energy sources and renewable solutions: natural gas, electric energy from the national grid, PV energy with battery storage, and PV powdered hydrogen production. A cradle to gate approach was followed to focus on the manufacturing stages and energy use. The main results are summarized as follows:

PV electricity with battery storage is the most sustainable option, reducing impacts per kWh by up to 62% compared with conventional solutions.

Hydrogen produced from PV energy provides lower impacts with respect to traditional solutions and high flexibility. The metal hydride storage system is efficient and has low impacts; however, the overall system performance is sensitive to electrolizer efficiency.

Electric furnaces are characterized by higher efficiency with respect to gas-powered ones. Hence, even if natural gas determines lower impacts per kWh with respect to grid electricity, the latter can be the most sustainable for metal melting.

Raw materials account for up to 97% of GWP due to high unitary footprint of primary aluminum production. A higher recycled material rate is a valuable solution to reduce final products impacts.
Future studies could focus on improving the efficiency of conversion and storage technologies, continuously monitoring the sustainability of innovative scenarios. In particular, specific focus on hydrogen systems could further enhance their competitiveness relative to battery-based solutions. These aspects, along with process efficiency improvements, could contribute to a sustainable development of die casting.

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.73.pdf

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The original version of this eBook is available on https://www.scientific.net/book/sustainability-on-material-forming/978-3-0364-1983-1

Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 83-92

doi: 10.4028/p-Y6ds7q

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-16

Revised:
2026-02-13

Accepted:
2026-02-17

Online:
2026-04-17

Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

The original version of this paper is available on https://www.scientific.net/KEM.1051.83.pdf

Dorothea Czempas 1,a*, Junhe Lian 1, b and David Bailly 1,c

1 Institute of Metal Forming (IBF), RWTH Aachen University, 52072 Aachen, Germany

a  dorothea.czempas@ibf.rwth-aachen.de, bjunhe.lian@ibf.rwth-aachen.de, c  david.bailly@ibf.rwth-aachen.de (*corresponding author)

Keywords: Twin-roll casting, scrap recycling, hot shortness, tramp elements

Abstract

Increasing scrap usage in steelmaking is vital for resource efficiency and CO2 reduction, but elevated residual copper limits adoption due to hot shortness during hot forming. Conventional continuous casting promotes Cu segregation in interdendritic regions, and subsequent slab reheating accelerates oxidation-driven Cu enrichment at the steel-scale interface, where liquid Cu penetrates grain boundaries and weakens cohesion. Twin-roll casting (TRC) offers a promising alternative, as its high solidification rates suppress Cu segregation and its near-net strip production eliminates slab reheating and minimizes oxidation. In this work, the hot-shortness resistance of a 0.75wt.%Cu construction steel processed by TRC is evaluated and directly compared with a conventionally cast and reheated counterpart. The comparison reveals that TRC effectively mitigates copper-related damage mechanisms. Cu remains primarily in the thin scale without penetrating the substrate, enabling hot rolling and downstream processing without cracking. In contrast, the conventional route forms a thick, brittle, Cu-rich scale that promotes grain-boundary penetration and severe hot shortness. Overall, TRC expands the allowable copper content in flat steel production and broadens alloy design opportunities for scrap-based steelmaking.

Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.83.pdf

The increasing use of scrap in steelmaking is widely recognized as a key pathway toward improved resource efficiency and reduced CO2 emissions. However, the presence of residual elements imposes significant constraints on the applicability of scrap-based routes for flat steel production. Among these elements, copper is particularly critical, as it strongly promotes hot shortness during hightemperature forming operations and therefore limits allowable copper contents in industrial practice [1].
The susceptibility of copper-containing steels to hot shortness is governed by a combination of metallurgical and process-related factors. During solidification in conventional continuous casting, copper segregates into interdendritic regions due to its limited solubility in ferrite. Subsequent slab reheating prior to hot rolling accelerates oxidation of iron. During this process, iron oxidizes preferentially and at a significantly higher rate than copper, while the inward diffusion of copper remains limited, leading to progressive copper enrichment at the steel-oxide interface. When local copper concentrations increase and temperatures exceed the melting point of copper-rich phases, liquid copper can form and penetrate along grain boundaries. The presence of liquid copper substantially reduces grain boundary cohesion, and under mechanical loading during hot deformation these weakened boundaries are prone to rupture, giving rise to surface cracking and, in severe cases, liquid metal embrittlement. As a consequence, conventional slab casting routes impose strict upper limits on the permissible copper content in flat steel products, thereby limiting the use of coppercontaining scrap [1].
A promising approach to enable the use of scrap containing elevated levels of copper and tin is to fundamentally modify the process conditions that give rise to hot shortness. In flat steel production, TRC directly addresses the key drivers of copper-induced embrittlement by simultaneously limiting oxidation, segregation, and high-temperature deformation. In contrast to conventional slab-based routes, TRC replaces slab casting and subsequent slab reheating by producing near-net-shape strip
directly from the melt, thereby substantially reducing thermal exposure, oxidation, and overall process complexity while improving energy efficiency and material yield [2]. The fundamental differences between the conventional slab-based route and the twin-roll casting process, as well as their implications for copper behavior, are schematically illustrated in Fig. 1.

[picture]

Fig. 1 Schematic comparison of a) conventional slab-based processing and b) twin-roll casting highlighting differences in c) copper behavior and hot-shortness-relevant mechanisms in flat steel production based on [7].

Fig. 1. Schematic comparison of a) conventional slab-based processing and b) twin-roll casting highlighting differences in c) copper behavior and hot-shortness-relevant mechanisms in flat steel production based on [7].

In the twin-roll casting process, molten steel solidifies between two counter-rotating, water-cooled copper rolls, forming a continuous strip with typical thicknesses of approximately 1 to 3 mm . Rapid heat extraction at the roll surfaces leads to the formation of thin solid shells that are joined in the roll gap, while ceramic side dams ensure a stable melt pool. As strip is produced directly from the melt, slab reheating is avoided, and further thickness reduction can be achieved by inline hot rolling using the residual casting heat.
By avoiding slab reheating and extensive high-temperature exposure, TRC substantially reduces oxidation and surface scale formation, thereby suppressing oxidation-driven copper enrichment at the steel-scale interface, which is a critical prerequisite for liquid copper formation and grain boundary weakening. In addition, the high solidification rates inherent to TRC, reaching up to approximately 1000 K/s, reduce copper segregation during solidification and contribute to maintaining grain boundary integrity. Similar effects have been reported for rapidly and sub-rapidly solidified copperbearing steels, where increased cooling rates were shown to significantly improve resistance to hot shortness by suppressing copper segregation and grain boundary wetting [3].
Beyond these metallurgical effects, TRC also offers significant process and energy advantages. Compared to conventional hot strip production routes involving continuous casting, slab reheating, and multiple rolling stages, the energy demand can be reduced from approximately 3.5 GJ per ton of steel, or around 2.1 GJ per ton for thin slab casting, to values as low as 0.5 GJ per ton due to the elimination of reheating and extensive hot deformation [4].
The metallurgical and process-related advantages of twin-roll casting are not only of academic relevance but have also been demonstrated under industrial conditions. Ultra-thin strip casting technologies based on the twin-roll principle have reached a high level of technological maturity, enabling stable near-net-shape production of flat steel products at industrial scale [5]. In particular, industrial experience gained with the CASTRIP® process has shown that strip casting routes tolerate higher levels of residual elements than conventional slab-based processing, including steels with elevated copper and tin contents [6]. Earlier investigations on near-net-shape cast strip produced from
scrap have similarly demonstrated improved processability and property profiles for Cu - and Sn containing steels compared to conventional routes [7].
In addition to copper-containing construction steels, twin-roll casting has been successfully applied to compositionally demanding alloys such as high-silicon electrical steels. Previous studies have demonstrated that the high cooling rates inherent to TRC strongly influence microstructure evolution, texture development, and downstream processability, thereby expanding accessible alloy design windows beyond conventional limits [8,9].
While current industrial practice demonstrates the feasibility of processing copper-containing steels via strip casting, the copper levels applied remain deliberately conservative, primarily to ensure process robustness and product consistency. Building on these industrial achievements, further exploration is required to assess the true limits of copper tolerance achievable through twin-roll casting and to develop a deeper understanding of the governing metallurgical mechanisms.
The aim of the present study is to improve the understanding of how twin-roll casting affects copperrelated hot-shortness behavior in comparison to conventional processing. Particular emphasis is placed on oxide scale formation and copper enrichment at the surface as key indicators of hotshortness susceptibility. To this end, a hot-shortness-sensitive construction steel containing 0.75wt.% Cu was produced by twin-roll casting and characterized in the as-cast condition as well as after selected downstream processing steps.
To approximate conventional slab-based processing, the same alloy was subjected to a reheating treatment representative of industrial slab reheating conditions. The behavior of the twin-roll-cast material is directly compared with this reheated condition in order to isolate the influence of solidification rate and thermal exposure on copper-related damage mechanisms. The following sections describe the materials, processing routes, and experimental methods employed in this study.

Materials and Methods

The original version of this paper is available on https://www.scientific.net/KEM.1051.83.pdf

Strip casting, reheating simulation and further processing To demonstrate the robustness of twinroll casting with respect to elevated copper contents, a hot-shortness-sensitive construction steel alloy containing 0.75wt.%Cu was investigated. The strip casting experiments were conducted on the laboratory-scale twin-roll caster at the Institute of Metal Forming (IBF), RWTH Aachen University. For this purpose, approximately 150 kg of the alloy were melted in an induction furnace and transferred via a refractory channel system and a tundish into the casting gap between two counterrotating, water-cooled copper rolls with a width of 150 mm . To suppress oxidation of the melt surface and to control the heat transfer conditions between the casting rolls and the solidifying strip shells, the melt pool was shrouded with argon gas. Solidification occurred directly at the roll surfaces due to rapid heat extraction, leading to the formation of thin solid shells that were joined in the roll gap to produce a continuous steel strip. A more detailed description of the twin-roll casting facility and process conditions is provided by Daamen et al. [10]. The as-cast strip had a nominal thickness of approximately 1.7 mm . Samples were taken from the strip for subsequent characterization and further processing.
To approximate the thermal exposure associated with conventional slab-based flat steel production, a reheating simulation was performed on samples extracted from the twin-roll-cast strip. The material was held at 1100∘C for 3 h under conditions representative of industrial slab reheating in a pusher furnace. This treatment was intended to reproduce oxidation behavior and copper redistribution comparable to those occurring during conventional slab reheating prior to hot rolling, while maintaining identical alloy composition.
In order to investigate the influence of downstream processing on microstructure and mechanical properties, selected samples of the twin-roll-cast strip were further processed via two different routes. For the hot rolling (HR) route, specimens were heated for 5 min in a furnace under an argon atmosphere and subsequently rolled in a single pass from the as-cast thickness of 1.7 mm to a final thickness of 1.2 mm . This processing step was intended to approximate an inline hot rolling pass following strip casting.

For the cold rolling route, the as-cast strip was cold rolled from an initial thickness of 1.7 mm to a final thickness of 0.5 mm in three rolling passes. The cold-rolled material was subsequently subjected to recrystallization annealing at 710∘C for 5 h under an argon atmosphere in order to obtain a fully recrystallized microstructure. The experimental process chain used to compare twin-roll casting, rolling states and reheating conditions is illustrated in Fig. 2.

[picture]

Fig. 2 Experimental process chain of the twin-roll-cast material.

Fig. 2. Experimental process chain of the twin-roll-cast material.

Microstructural characterization, chemical analysis and mechanical testing Microstructural investigations were carried out on metallographically prepared cross-sections using light optical microscopy (LOM). The analyses were performed using a VHX-7000 digital microscope (Keyence). In addition, scanning electron microscope (SEM) images were acquired using a ThermoFisher Helios 5 Hydra UX. Particular emphasis was placed on the characterization of oxide scale formation, copper distribution at the steel-scale interface, and potential copper penetration along grain boundaries. Microstructural features resulting from the different processing routes were documented and compared.
To verify the chemical composition of the cast material and to account for potential deviations or impurities introduced during processing, chemical analyses were performed using spark emission spectroscopy. The analyses were carried out on ground samples that were cleaned with ethanol prior to measurement, using a BELEC Variolab spectrometer.
Mechanical properties were evaluated by uniaxial tensile testing at room temperature. A50 tensile specimens were tested on a Z100 universal testing machine (ZwickRoell) at a constant quasistatic strain rate of 1×10−3 s−1. Yield strength, ultimate tensile strength, and total elongation were determined in order to assess the influence of processing route and microstructure on mechanical performance.
The susceptibility of the investigated alloy to hot shortness was assessed based on metallographic observations of surface condition, oxide scale formation, and copper distribution after processing. In addition, the behavior of the material during hot rolling was evaluated qualitatively with respect to surface integrity and the occurrence of cracking. The twin-roll-cast material was directly compared with the reheated condition representing conventional slab-based processing.
For industrial benchmarking, two commercially produced CASTRIP strip materials with copper contents of 0.35wt.% and 0.40wt.% were acquired and characterized. These materials were selected to represent industrial strip-cast steels with elevated residual copper levels. Chemical composition and mechanical testing were performed under the same conditions as applied to the twin-roll-cast material investigated in this study.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.83.pdf

Castability, chemical composition and surface condition after twin-roll casting The chemical composition of the cast strip, determined by spark emission spectroscopy, is summarized in Table 1. The alloy is based on a DC03 deep-drawing steel with low carbon and manganese contents and deliberately excludes alloying elements such as Ni and Si that are known to mitigate copper-induced hot shortness. With a measured copper content of 0.75wt.%, the investigated material represents a composition that exceeds typical limits for conventionally processed flat steels.

Table 1 Measured chemical composition of the twin-roll-cast strip alloy used in this study with comparison of the two Castrip alloys, determined by spark emission spectroscopy (wt.%).

| IBF Twin-roll-cast strip | CASTRIP Twin-roll-cast strip t=1.88 mm | CASTRIP Twin-roll-cast strip t=0.7 mm
Fe | Equ. | Equ. | Equ.
C | 0.092 | 0.014 | 0.002
Si | 0.02 | 0.13 | 0.15
Mn | 0.42 | 0.47 | 0.44
P | 0.01 | 0.007 | 0.011
S | - | 0.002 | 0.002
Cr | 0.03 | 0.10 | 0.14
Ni | 0.02 | 0.11 | 0.12
Mo | - | 0.02 | 0.03
Cu | 0.75 | 0.35 | 0.40
Al | 0.002 | 0.002 | 0.002
Sn | 0.001 | 0.013 | 0.015

Despite this composition, strip material could be successfully produced by twin-roll casting. The casting trial yielded a continuous strip segment of sufficient length for subsequent characterization and processing, demonstrating that elevated copper contents do not adversely affect solidification and strip formation during twin-roll casting. The strip casting step itself does not promote copper-related damage. Instead, the solidification conditions established during casting define the initial copper distribution, which governs its behavior during subsequent high-temperature exposure and deformation.

[picture]

Fig. 3 Ferritic-pearlitic microstructure of the as-cast twin-roll-cast strip and localized copper-rich features along grain boundaries

Fig. 3. Ferritic-pearlitic microstructure of the as-cast twin-roll-cast strip and localized copper-rich features along grain boundaries

Microstructural examination of the as-cast strip revealed a ferritic-pearlitic microstructure characteristic of low-carbon steels processed under high cooling rates. In addition to the matrix microstructure, isolated copper-rich features were locally observed along grain boundaries as can be
seen in Fig. 3. These copper enrichments remain limited in extent and are not associated with surface cracking or other indications of hot-shortness-related damage in the as-cast condition.
Metallographic examination further revealed the formation of a very thin oxide scale on the strip surface. Copper was predominantly incorporated within this scale layer, as illustrated in Fig. 4, while no penetration of copper into the steel substrate was observed. This behavior indicates that the limited thermal exposure and reduced oxidation inherent to the TRC process effectively suppress the conditions required for copper enrichment at the steel-scale interface and subsequent grain boundary wetting. In contrast to conventional slab-based routes, the absence of prolonged high-temperature exposure prevents the formation of liquid copper films during processing.

[picture]

Fig. 4 Oxide scale on the as-cast twin-roll-cast strip with copper-rich inclusions within the scale.

Fig. 4. Oxide scale on the as-cast twin-roll-cast strip with copper-rich inclusions within the scale.

Effect of reheating simulation on copper redistribution and hot-shortness susceptibility To evaluate the behavior of the investigated alloy under conditions representative of conventional slabbased processing, a reheating simulation was applied to the twin-roll-cast strip. The objective of this experiment was to isolate the effect of prolonged high-temperature exposure in an oxidizing atmosphere, corresponding to the slab-reheating step. Reheating at 1100∘C for 3 h resulted in a pronounced change in surface condition and copper distribution compared to the as-cast state. Metallographic analysis after reheating revealed the formation of an approximately 1.43 mm thick, brittle oxide scale accompanied by pronounced copper enrichment beneath the scale layer. In addition, copper penetration along grain boundaries into the steel matrix was observed (Fig. 5). This redistribution of copper establishes critical conditions for hot shortness, as grain boundary cohesion is locally reduced once copper-rich regions form during prolonged high-temperature exposure.

[picture]

Fig. 5 Oxide scale and copper redistribution after reheating simulation, showing a thick scale layer (left) and copper enrichment beneath the scale and along grain boundaries (right).

Fig. 5. Oxide scale and copper redistribution after reheating simulation, showing a thick scale layer (left) and copper enrichment beneath the scale and along grain boundaries (right).

In contrast to the comparatively homogeneous copper distribution in the as-cast TRC condition, the reheating treatment promotes pronounced subsurface enrichment driven by diffusion and oxidation
kinetics. The results demonstrate that copper-related degradation is not primarily governed by the solidification stage but can be activated by extended thermal exposure under oxidizing conditions. Since conventional slab casting typically involves slower cooling and increased segregation tendencies, local copper accumulation during reheating is expected to be at least comparable, if not more pronounced.
These findings highlight the decisive role of slab reheating in conventional processing routes and help explain why elevated copper contents remain challenging in slab-based flat steel production.

Influence of downstream processing on microstructure and mechanical properties To assess the processability of the twin-roll-cast material and the range of achievable microstructures, different downstream processing routes were applied, including hot rolling with a low thickness reduction and cold rolling followed by recrystallization annealing.
Hot rolling of the as-cast strip resulted in a fine-grained ferritic-pearlitic microstructure with predominantly equiaxed grains, as shown in Fig. 6 a). This microstructure is characteristic of thermomechanical processing involving moderate deformation and subsequent phase transformation under controlled cooling conditions. The absence of surface damage during hot rolling confirms that copper-related degradation mechanisms remain suppressed in the twin-roll-cast condition.
In contrast, cold rolling followed by recrystallization annealing produced a homogeneous, pancakelike grain structure, as illustrated in Fig. 6 b). This microstructural state reflects complete recrystallization after substantial cold deformation and represents a distinctly different grain morphology compared to the hot-rolled condition.

[picture]

Fig. 6 Comparison of microstructures after different downstream processing routes: twin-roll-cast material investigated in this study after a) hot rolling with low reduction and after b) cold rolling followed by recrystallization annealing and industrial CASTRIP reference material after c) low inline hot rolling and after d) high rolling reduction ( t=0.7 mm ).

Fig. 6. Comparison of microstructures after different downstream processing routes: twin-roll-cast material investigated in this study after a) hot rolling with low reduction and after b) cold rolling followed by recrystallization annealing and industrial CASTRIP reference material after c) low inline hot rolling and after d) high rolling reduction ( t = 0.7 m m ).

For industrial benchmarking, these microstructures are compared with those of two industrial CASTRIP reference materials, shown in Fig. 6 c) and d). While both materials are produced via strip casting, they differ in alloy design. The alloy investigated in this study is based on a DC03-type lowcarbon steel with a deliberately increased copper content of 0.75wt.% and without additions of Ni and Si. In contrast, the CASTRIP reference materials are ultra-low-carbon steels containing 0.35-0.40 wt.% Cu in combination with small additions of Mn,Si, Ni and Cr .

The CASTRIP material processed with a low degree of inline hot rolling exhibits a pronounced Widmanstätten-type ferritic microstructure. In contrast, the hot-rolled twin-roll-cast material shows
a predominantly equiaxed ferritic-pearlitic grain structure without pronounced acicular features. Conversely, the CASTRIP material subjected to a higher degree of inline rolling reduction shows a homogeneous, pancake-like grain structure comparable to that obtained after cold rolling and recrystallization annealing in the present study. Despite these differences in alloy composition and copper content, the close qualitative agreement highlights that similar microstructural states can be achieved in laboratory-scale and industrial strip casting routes.

Mechanical properties and benchmarking The different microstructures obtained through these processing routes are reflected in the mechanical properties. A quantitative comparison of yield strength, ultimate tensile strength and elongation at fracture is summarized in Table 2. The hot-rolled twin-roll-cast material exhibits a comparatively high yield strength combined with moderate elongation, consistent with its fine-grained ferritic-pearlitic microstructure and the associated grainboundary strengthening formed under conditions of limited hot deformation. In contrast, the coldrolled and recrystallized condition shows lower yield strength but increased ductility, reflecting the homogeneous pancake-like grain structure obtained after complete recrystallization and the reduced dislocation density after annealing. These trends follow established structure-property relationships for low-carbon steels.

Table 2 Overview of processing routes and mechanical properties of the investigated twin-roll-cast alloy compared with industrial CASTRIP reference materials and standardized low-carbon steels [11,12].

Material | Composition | Process route | Cu (wt.%) | YS (MPa) | UTS (MPa) | A50 (%)
This work- HR (t = 1.2 mm) | DC03-type low-C steel, no Ni/Si | TRC; reheating; hot rolling | 0.75 | 365 | 420 | 21
This work- CR+RX (t = 0.7 mm) | DC03-type low-C steel, no Ni/Si | TRC; cold rolling; RX (710 °C, 5 h) | 0.75 | 275 | 341 | 22
CASTRIP (t = 1.88 mm) | Ultra-low-C strip steel | Industrial TRC; inline HR | 0.35 | 235 | 403 | 18
CASTRIP (t = 0.7 mm) | Ultra-low-C strip steel | Industrial TRC; high HR reduction; CR | 0.4 | 340 | 325 | 35
ASTM A1039 CS Type B | Commercial low-C sheet steel | Slab / thin slab; HR & CR | ≤ 0.20 | 170-240 | 300-380 | 25-35
DC03 (EN 10130) | Low-C deep-drawing steel | Slab; CR; RX | < 0.10 | 140-200 | 270-330 | 34-42

For industrial benchmarking, the mechanical properties of two CASTRIP reference materials were determined under identical testing conditions as applied to the material investigated in this study. The CASTRIP material processed with a low degree of inline hot rolling exhibits a strength-ductility balance comparable to that of the hot-rolled twin-roll-cast material, whereas the CASTRIP material subjected to a higher rolling reduction shows mechanical behavior similar to that of the cold-rolled and recrystallized condition. This correspondence is consistent with the qualitative microstructural similarities discussed in Section 3.3.
Quantitative differences in strength levels between the investigated alloy and the CASTRIP reference materials can be attributed primarily to differences in chemical composition. The twin-roll-cast material produced at IBF exhibits consistently higher yield strength, which is associated with its higher carbon content and deliberately increased copper level of 0.75wt.% compared to the ultra-low-carbon CASTRIP steels containing 0.35−0.40wt.%Cu. Despite these differences, comparable elongation values are achieved, indicating that elevated copper contents do not inherently
compromise ductility when copper-related damage mechanisms are suppressed by the strip casting route.
In comparison, conventionally produced low-carbon sheet steels such as DC03 and standardized grades according to ASTM A1039 CS Type B exhibit similar mechanical property windows only at significantly lower copper contents [11,12], as summarized in Table 2. This reflects the limitations imposed by slab-based processing routes, in which prolonged reheating and extensive hot deformation restrict the allowable copper content. The present results therefore demonstrate that twinroll casting enables mechanical property levels comparable to industrial and standardized low-carbon steels while accommodating substantially higher residual copper contents.

Summary

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This study demonstrates that twin-roll casting fundamentally shifts the processing window for copper-containing steels. A construction steel alloy with 0.75wt.%Cu, exceeding typical limits for conventionally processed flat steels, was successfully produced as strip and further processed without hot-shortness-related damage.
Microstructural analysis reveals that copper-related degradation mechanisms are not activated during strip casting itself but are governed by thermal exposure and oxidation during subsequent processing. While the twin-roll-cast material exhibits only localized copper enrichments and a thin oxide scale, reheating under conditions representative of conventional slab-based processing leads to extensive oxidation, pronounced copper enrichment beneath the scale, and grain-boundary penetration.
By applying different downstream processing routes, the twin-roll-cast strip can be transformed into distinct microstructural states with corresponding mechanical property profiles. The resulting strength-ductility combinations are comparable to those of industrial strip-cast reference materials and conventionally produced low-carbon sheet steels, despite the substantially higher copper content. These findings demonstrate that elevated copper levels do not inherently limit mechanical performance when copper-related damage mechanisms are effectively suppressed.
Overall, the results identify twin-roll casting as a key enabling technology for increased scrap utilization and circular flat steel production. By tolerating higher residual copper levels, the process appears well suited to act as a sink for copper-bearing scrap streams and thus offers a viable pathway to accommodate the increasing availability of recycled steel in future flat steel production.

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.83.pdf

D. A. Melford, The influence of residual and trace elements on hot shortness and high temperature embrittlement, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 295 (1980) 89-103. https://doi.org/10.1098/rsta. 1980.0078

M. Ferry, Direct strip casting of metals and alloys: Processing, microstructure and properties, 1st ed., Woodhead Publishing in Materials, CRC Press, Cambridge / Boca Raton (2006).

C. Lu, W. Wang, J. Zeng, C. Zhu, Sub-rapid solidified high copper-bearing steel with excellent resistance to hot shortness, Scripta Materialia 239 (2024) 115800.

S. Ge, M. Isac, R. I. L. Guthrie, Progress in strip casting technologies for steel: Technical developments, ISIJ International 53 (2013) 729-742.

T. Matsushita, K. Nakayama, H. Fukase, S. Osada, Development and commercialization of twin roll strip caster, IHI Engineering Review (2009).

D. G. Edelman, P. C. Campbell, C. R. Killmore, K. Carpenter, H. R. Kaul, J. G. Williams et al., The development and manufacture of ultra-thin cast strip steel products with high residual element levels via the CASTRIP® process, Conference contribution, University of Wollongong (2011).

A. R. Büchner, S. Pötschke, J. Eliasson, R. H. Gronebaum, Improving of the properties of near net shape cast strip containing copper and tin from scrap, Report (2006), eDoc: 290761, ISBN 9279020765.

M. Müller, D. Czempas, D. Bailly, G. Hirt, Twin roll casting and secondary cooling of silicon steel, Metals 10 (2021) 1508.

M. Müller, D. Bailly, G. Hirt, Microstructure evolution and magnetic properties of a silicon steel produced by twin-roll casting, Steel Research International 93 (2022) 2100506.

M. Daamen, B. Wietbrock, S. Richter, G. Hirt, Strip casting of a high-manganese steel (FeMn22C0.6) compared with a process chain consisting of ingot casting and hot forming, steel research international 82 (2011) 70-75. . 201000267

ASTM A1039/A1039M, Standard Specification for Steel, Sheet, Hot-Rolled, Carbon, Structural, and High-Strength Low-Alloy, ASTM International, West Conshohocken, PA (latest edition).

EN 10130, Cold rolled low carbon steel flat products for cold forming, European Committee for Standardization (CEN) (latest edition).

This ePub file was created using machine-generated methods and automated software to convert the source text into an ePub format and may contain formatting inconsistencies from the original version. Formatting may vary depending on the device used.

The original version of this eBook is available on https://www.scientific.net/book/sustainability-on-material-forming/978-3-0364-1983-1

Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 93-103

doi: 10.4028/p-sjE2MS

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-18

Revised:
2026-02-12

Accepted:
2026-02-17

Online:
2026-04-17

Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

The original version of this paper is available on https://www.scientific.net/KEM.1051.93.pdf

Pierpaolo Carlone 1,a*, Mario Emanuele Di Nardo 1, b, Roberto Mele 1,c, Francesco Borda 2, d, Francesco Gagliardi 2,e, Francesco Napolitano 3,f and Pietro Russo 3,g

¹Department of Industrial Engineering, University of Salerno, 132 Via Giovanni Paolo II, 84084 Fisciano (SA), Italy

2 Department of Mechanical Energy and Management Engineering, University of Calabria - Ponte P Bucci 87036 Arcavacata di Rende (CS) Italy

3 Institute of Polymers, Composites and Biomaterials, National Research Council, Via Campi Flegrei 34, 80078 Pozzuoli, Italy

a*pcarlone@unisa.it, b  mdinardo@unisa.it, crmele  @unisa.it, d  francesco.borda@unical.it efrancesco.gagliardi@unical.it, ffrancesconapolitano@cnr.it, g  pietro.russo@cnr.it

Keywords: circular economy, recycling, additive manufacturing, fused filament fabrication, polymer composites.

Abstract

Polymer processing in modern industry is often not efficient from the point of view of energy and material consumption. This production system must be revised to pursue a circularity of products and materials. Recent developments in additive manufacturing technologies for polymers and polymer-based composites are enabling changes of production paradigm from "Design for Manufacturing" to "Manufacturing for Design", providing new intriguing lightweight solutions. This paper summarizes the lesson learned from the RELIVE project "REcycling of pLastic wastes integrating extrusion and additIVE manufacturing techniques", in terms of methodology and results.

Introduction

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The environmental challenges are global and systematic. To achieve the long-term sustainable targets determined by the European Union, the core system of society needs to be dramatically modified. The production paradigm in modern industry is linear and oriented to the consolidated extraction-transformation-usage-disposal chain. This way of thinking for the production system is not feasible any longer and must be changed. In recent years, the development of innovative processes set new approaches in production, introducing the concept of "Manufacturing for Design" as an alternative to the consolidated "Design for Manufacturing". Modern manufacturing is often not efficient from the point of view of energy consumption and efforts to reduce environmental impact by lifecycle management initiatives in manufacturing have been presented recently [1]. In this framework, process digitalization and automation are increasingly adopted to improve resource efficiency, as widely explored in advanced composite manufacturing routes where defect reduction and automated programming strategies are pursued to minimize material waste and rework [2,3]. In the traditional economic system, materials are mostly extracted from fossil sources and the products, obtained from the transformation of raw materials by consuming energy, at the end of their life cycle, are thrown away as waste. This production system must be stopped to pursue a circularity of products and materials [4]. Circular economy and sustainable manufacturing represent promising strategies to reduce the impact of human activities on climate changes. Nowadays, the mainstream for advanced and mass production sectors must be the development of new strategies based on total life-cycle approaches and on the 6R concept (Reduce-Reuse-Redesign-Remanufacture-Recycle-Recover). In particular, lifecycle-based metrics are becoming central also for lightweight structures, as shown by recent LCA studies on composite components for e-mobility applications [5], reinforcing the need for production strategies that couple performance with measurable sustainability benefits. As far as polymeric materials are concerned, the continuous growing market and usage in different fields
together with the constant increase of their waste highlight the relevance of a proper recycling strategy both for economic and social reasons. Furthermore, the residual value of this waste has been stimulating the development of different recycling routes based on mechanical, thermal or chemical approaches to maximize recoverable energy in more sustainable and economically effective perspectives. Currently, the plastic economy is highly linear, but the transition to a circular economy, retaining plastics at their highest value [6], is always more essential considering their massive consumption and the adverse impacts on the environment if their wastes are not processed properly [7]. In the past decades, several industries assumed lightweight polymer-based materials as a driving trend at the design stage [8]. The wider usage of polymer-based materials already provides some environmental benefits considering their enhanced manufacturability and lightness. Nevertheless, the concept itself of producing polymers, often not renewable materials on an acceptable time scale, releasing them again in the environment as wastes, is far from any idea of sustainability [9,10]. In this historical period, the implementation of agile integrated manufacturing approaches based on the principle of reusing waste is mandatory to reduce the environmental impact in production [11].
Fused filament fabrication (FFF) technology has been addressed as a key manufacturing approach able to reduce the consumption of material [12]. FFF is a 3D printing process that allows parts production by a "layer-by-layer" approach and, nowadays, it is a consolidated method exploited to manufacture complex and performant end-use products. Additive manufacturing in general has been defined by the European Community as an advanced technology that can "offer a range of opportunities that will enable European industry to expand its leadership in the emerging markets for the products and services of the future" [13]. Fused filament fabrication (FFF) is a consolidated technology within the panorama of additive manufacturing technologies, but it is mostly employed to process near-amorphous low-performance thermoplastics. Recently, the introduction of reinforcing (long or short) fibers inside the filament has further spread the usage of FFF to obtain advanced products. Reusing end-of-life thermoplastics for making filaments employed in FFF can be a relevant step to circular economy production. This manufacturing strategy is not rigorously a new idea [14]. Nevertheless, the recovery and treatment of these wastes often imply a reduction in mechanical and physical properties [15]. Consequently, regenerated filaments are poorly appealing and are marginally used only for secondary production. Therefore, the property control of parts produced from recycled or reprocessed thermoplastics is strictly required [16]. Enhancing the polymeric properties by inclusion of opportune fillers and reinforcing fibers can be pursued in filament extrusion, also to counter warp and shrink phenomena, which the processed semi-crystalline polymers can be subjected to [17]. Recent experimental evidence demonstrated the potential of recycled plastics such as polypropylene and polyethylene terephthalate as new FFF feedstock materials. Anyway, the absence of in-depth process-property correlations as well as the scarce availability of theoretical models lead to insecurity of potential customers [18]. Similar needs for robust process understanding, traceability and defect mitigation are well-known in multiple composite manufacturing processes, where impregnation/permeability phenomena and the control of preform processing conditions are key to part quality [19,20]. In addition, also energy-efficient solutions have been investigated to enhance process control and potentially reduce cycle times and energy consumption [21,22]. In literature, some results concerning the temperature and velocity fields of the thermoplastics and their relationship with the formation of the beads and the layering of the extruded materials are reported [23]. Other researchers conducted simulations aiming to assess the influence of the nozzle geometries on the flow behavior of the thermoplastics [24]. Strict assumptions have been accepted in the development of these models, which involve for example the simplification of the interaction between the fibers, the hypothesis of very low fiber volume fraction or of a Newtonian fluid behavior for the thermoplastic melt [25]. New models and tools to predict flow, fiber orientation, neck and bond formation and chain diffusion, for instance, are desirable, assuming early approaches, derived from sintering and thermoplastics welding [25] and accounting for transient thermal and molecular diffusion [26]. Similarly, the cooling down of the deposited filament and its solidification are critical factors influencing the products' shape and stability [27].

The RELIVE project, "REcycling of pLastic wastes integrating extrusion and additIVE manufacturing techniques", funded by the Italian Ministry of University and Research under the PRIN program (Prot. N. 2022FBB37P), explored the feasibility of integrating additive manufacturing of reinforced polymers by FFF with the mechanical recycling of thermoplastic wastes, proposing a direct strategy able to bring plastic waste to new life and addressing major economic and technological restraints. More specifically, shredded thermoplastic waste was characterized according to material typology, purity and processability by thermal and rheological analyses. Subsequently, the ground materials were extruded, as they are or with the addition of reinforcing fibers, into wires to be fed directly to a nozzle for the final transformation using the FFF technique. Experimental analysis, numerical modelling and simulation activities were conducted to elucidate advantages and limits of the proposed solution. The implementation of the RELIVE project has been articulated in five work packages (WPs), as depicted in Figure 1. The first (WP0) and one (WP4) have been devoted to management and dissemination initiatives. Technical WPs, namely WP1, WP2, and WP3, aimed to:

Identification, at least at a qualitative level, of the composition of the secondary raw materials through thermal and molecular investigations;

Homogenization of recycled plastics by extrusion under suitable conditions and analysis of the rheological behavior of extruded materials;

Systematic investigation of the extruded filaments and subsequent deposition by FFF using innovative tooling and machines;

Assessment of the influence of the integrated process parameters on the ultimate performance of additively manufactured products;

Numerical analysis and optimization of the proposed manufacturing route looking also at the efficiency of the proposed recycling strategy.

This paper summarizes lessons learned from the RELIVE project in terms of methodology and main results. More details could be found in focused papers published by the authors elsewhere.

[picture]

Fig. 1. Schematic of the RELIVE project.

Fig. 1. Schematic of the RELIVE project.

Materials and Methods

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Feedstock selection

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The rationale behind the selection of waste polymeric materials to be recycled as a reinforced filament for additive manufacturing by FFF was based on materials processability by extrusion and on potential industrial applications. Given the considerable variety of polymers used for industrial applications and, therefore, of potentially acquirable waste for this research, special interest has been addressed to two mass sectors: food packaging and automotive. In the first area, given the relatively short average lifespan of packaging and the intrinsic properties of the polymeric materials typically used, polyolefins and increasingly bio-based plastics, the recycling strategy explored in this research could alleviate the environmental impacts associated with both waste and the end-of-life of these products, which are still largely disposed of in landfills. The second sector addressed, the automotive industry, in addition to using significant quantities of polypropylene for various components, accounts for nearly 40% of the total demand for polyamide-based engineering plastics (PA). Here too, despite the greater durability of the components, recycling, both industrially and for end-of-life products, remains challenging, given that these polymers can undergo thermomechanical degradation if not properly reprocessed, with ultimate performance further impaired by the aging processes inevitably experienced by the components during their life. That said, the research focused on polylactic acid (PLA) and polyamide 6 (PA6) mechanically recycled by extrusion and reprocessed to obtain filaments, even filled with long glass fibers, using FFF technology. In detail, filament production was performed using an innovative rotating impregnation die, previously patented by some of the authors [29], specifically developed for the manufacturing of fiber-reinforced polymer filaments suitable for FFF . The die enhances the interaction between the polymer matrix and the fibers. By inducing a swirling flow within the melt, the rotation improves fiber wetting and promotes partial fiber interlacing. The custom impregnation die consists of a static section coupled to the extruder, which supplies the polymer melt. Following a 90∘ redirection, the melt stream meets the reinforcing fibres, which are impregnated and guided coaxially toward the die exit. The fibres are fed from the top and preheated via an auxiliary heating band; this prevents thermal gradients in the polymer and inhibits backflow. A thermocouple is employed to monitor and regulate the preheating temperature. Subsequently, the polymer-fibre bundle enters a rotating die section that induces a swirling flow to enhance fibre wetting. This rotation is driven by a 24 V gear motor through a chain-and-pinion transmission. Before discharge, a fixed nozzle suppresses the swirling motion and realigns the flow, ensuring thorough impregnation and dimensional stability of the final composite filament. The subsequent stage involves the deposition of the composite filaments utilizing a custom FFF system. The material is fed into an extrusion head and deposited onto a heated build plate maintained at 55∘C. The composite filament is driven through a 1.2 mm nozzle and extruded in the molten state at a temperature of 180∘C. Both the co-extrusion and deposition processes, including the resulting filament outputs, are illustrated in Figure 2. The recycled materials were systematically characterized and analyzed with experimental tests and theoretical analyses.

Experimental tests

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Recycled extruded materials were characterized in terms of thermal properties and rheological behavior, while the filaments, including those reinforced with long glass fibers were analyzed through quasi-static tensile tests and morphological-structural aspects. Calorimetric measurements were carried out using a TA Instruments DSC Q2000 equipped with a refrigerated cooling system (RCS) and a nitrogen purge flow of 50 mL/min [30]. The standard protocol consisted of a first heating scan from room temperature (RT) to 200∘C to erase the previous thermomechanical history, followed by a short isothermal step ( 1 min ) at 200∘C, a controlled cooling from 200∘C to RT, and a second heating scan over the same temperature range to identify the intrinsic thermal transitions of the material. In addition, a dedicated set of experiments was performed by varying the cooling rate during the cooling step in order to assess the material response under both uniform and non-uniform cooling conditions, and to evaluate the influence of cooling history on the subsequent thermal behavior.

Melt rheological behavior was investigated under controlled thermal and atmospheric conditions by means of rotational rheometry using a modular compact rheometer (MCR 302, Anton Paar GmbH). Small-amplitude oscillatory shear measurements were used to determine the complex viscosity, and the data were converted into an equivalent shear viscosity through the Cox-Merz rule, allowing the reconstruction of the dynamic viscosity surface η(T,γ˙) over shear-rate and temperature ranges representative of filament manufacturing conditions. The resulting viscosity data were fitted using the Cross-WLF constitutive model, expressed as:

η(T,γ˙)=η0(T)1+(η0(T)γ˙τ*)1−n(1)

where η(T,γ˙) is the shear viscosity ( Pa·s ), γ˙ is the shear rate ( s−1 ), and η0(T) is the zero-shear viscosity, which accounts for the temperature dependence of the melt viscosity. The parameters τ*(Pa) and n(−) are, respectively, a critical stress controlling the onset of shear-thinning and the powerlaw index describing the extent of shear-thinning ( n<1 ). Furthermore, filaments were preliminary tested using an Instron model 5564 dynamometer on 8 cm -long specimens, pre-conditioned at room temperature and 50% relative humidity for 24 hours, setting the crosshead speed to 2 mm/min and a gauge length of 30 mm . Successive characterization was conducted at filament as well as sample level, by means of standard microscopic analysis, XCT analysis and mechanical testing. XCT scans were performed using uniTOM HR X-ray tomography system, operating at tube voltage of approximately 110 kV and current value of about 50μ A, providing an isotropic voxel resolution of 6 μm. Data processing, namely segmentation and distribution and orientation analysis, was performed in the MATLAB environment included, using a dedicated MATLAB script specifically developed to identify the regions of interest corresponding to the reinforcing fibers and the matrix. Afterward relevant microstructural descriptors (fiber volume fraction, spatial distribution, and voids) were calculated through a structure tensor-based approach. The resulting information provides insight into the quality of the filament production process and supports the interpretation of the mechanical behavior of additively manufactured composites based on recycled thermoplastic materials. Tensile and flexural characterizations were performed using a universal testing machine (MTS Criterion ® Electromechanical Test System) equipped with a 1 kN load cell. For the tensile tests, ten specimens per filament type were evaluated at a constant crosshead speed of 1 mm/min. To prevent gripping damage, individual filaments were mounted on rigid supports prior to testing. The tensile modulus was derived from the linear slope of the stress-strain curves. Flexural tests were conducted on composite samples with a support span of 51 mm and a displacement rate of 1.35 mm/min. Tensile and flexural testing protocols followed the ISO 11566 and ASTM D790-15 standards, respectively.

Numerical analysis

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The deposition was investigated through transient CFD simulations under the assumptions of laminar, incompressible, non-isothermal flow of a highly viscous non Newtonian polymer melt. The process was modelled, using the ANSYS-FLUENT package, as a two-phase system using a finite volume formulation, where the interface is captured by a Volume of Fluid (VOF) approach based on the transport of a volume-fraction field. This framework enables tracking the free surface evolution of the extrudate during its exit from the nozzle, contact with the substrate, and progressive cooling and, consequently, solidification. The governing physics included mass and momentum conservation coupled with the energy equation to account for heat transfer between the molten polymer, the surrounding air and the build plate. The melt rheology was introduced through a temperature and shear-rate dependent viscosity law, calibrated on experimental data as previously explained. In particular, the same Cross-WLF constitutive formulation used for the characterization was adopted to reproduce both shear-thinning behavior and the strong temperature sensitivity typical of recycled thermoplastics during extrusion and deposition. For numerical robustness and to represent the transition toward solid like behavior, viscosity bounds were imposed consistently with the experimentally observed processing window. In addition, phase change was accounted for through
an enthalpy-porosity formulation, enabling the progressive transition from melt to solid to be represented within the energy equation by introducing a liquid fraction field and a permeability like damping term in the momentum balance. The enthalpy-temperature relationship required by this approach, including the latent-heat contribution and the relevant transition range, was defined using the DSC data previously obtained, thereby ensuring that the onset and evolution of solidification in the simulations were consistent with the experimentally measured thermal behavior. The computational domain reproduced the nozzle-substrate interaction, allowing an efficient yet physically meaningful description of bead formation and thermal gradients. Boundary conditions were defined to mimic realistic manufacturing conditions, the nozzle walls were treated as no-slip boundaries kept at the extrusion temperature, whereas the substrate was modelled as a moving wall with prescribed speed and controlled temperature to reproduce deposition kinematics and heat extraction. Pressure outlet conditions were applied at the open boundaries to emulate an ambient environment. Additional modelling ingredients included surface tension at the polymer/air interface and thermal coupling among phases. The simulations provided useful qualitative and quantitative insights into the deposition mechanisms, highlighting the role of temperature driven viscosity evolution, interface stability, and cooling conditions on bead morphology. In the present CFD model, fiber effects were not explicitly included, in fact the melt was treated as an equivalent single-phase non-Newtonian matrix. Nevertheless, complementary modelling strategies are available for fiberreinforced material extrusion. In particular, in a recent work a FEM based framework coupled with orientation tensors and an orientation-averaging homogenization scheme was used to incorporate process-induced anisotropy, stemming from mesostructure orientation, into component-scale predictions of deformation/warpage [28].

Results and Discussion

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The RELIVE project explored the opportunities that can be derived by using plastic wastes to shape new products in a straight manufacturing route that starts from. In this case, the main technological challenges are represented by both the lower performance of plastic materials at the end of their life compared to raw materials, due to inevitable aging phenomena suffered by them during their useful life, and by the usual heterogeneity of considered wastes. The philosophy behind the proposed approach is quite simple and involves, after the section of the waste materials, the identification of the manufacturing process to be used for reshaping, the characterization of the materials properties, and the definition of strategies for properties enhancement. In the framework of the project, the investigated case study was given by the mechanical properties' enhancement coming from combination by co-extrusion of fibers and plastic waste integrated online with the additive manufacturing step. In Figure 2, the plastic waste, the recycled filaments and the deposited sample, processed during the RELIVE project, are shown.

The considered plastic wastes, namely PLA and PA6, were characterized by specific rheological and thermal properties, which essentially depend on their molecular and structural characteristics. It should be noted that in this investigation the starting material was already sorted, nevertheless it is not excluded to extend the approach to mixtures of different polymers by introducing a preliminary sorting operation. Themo-mechanical characterization pointed out first the alteration of the properties of the materials and, consequently, of the processing window.

[picture]

Fig. 2. (a) Schematic representation of the process; (b) Configuration of the customized FFF platform.

Fig. 2. (a) Schematic representation of the process; (b) Configuration of the customized FFF platform.

An integral part of the procedure was the redefinition of the processing window, replacing trial and error procedures and relying on appropriate numerical modelling and simulation. Reasonably, this procedure could be further improved in the future, using opportune AI tools, however, at the moment the trusted data available on recycled materials are not enough to build a robust database. In Figure 3, some results provided by the thermal characterization and the numerical simulation of the deposition are depicted.

[picture]

Fig. 3. DSC integrated data into temperature and viscosity fields evolution on the free surface and symmetry planes of the deposited bead.

Fig. 3. DSC integrated data into temperature and viscosity fields evolution on the free surface and symmetry planes of the deposited bead.

Numerical simulations showed that the coupled thermal and rheological fields govern both flow stability inside the nozzle and shape retention after extrusion. In particular, the DSC curve in Figure 3 was used to define the melting-related temperature window relevant for extrusion, to extract the latent heat implemented in the enthalpy based formulation and to identify the solidus and liquidus temperatures adopted to model the phase change interval. Consistently with these thermal inputs, the predicted temperature distribution highlights a steep axial gradient along the nozzle and an even faster cooling of the extrudate immediately after deposition. As a direct consequence, the dynamic viscosity contour plots reveal a pronounced viscosity stratification: within the nozzle, high shear rates near the walls activate shear-thinning and maintain a low-viscosity "lubricating" layer, whereas the lowershear core remains comparatively more viscous, thus giving insight on temperature process window
to optimize processability. Furthermore, during deposition, the rapid temperature decay causes a sharp increase in viscosity, progressively suppressing flow and enabling the filament to transition from a deformable melt to a stable bead. Overall, the simulations indicate that maintaining the melt above the phase change interval up to the nozzle exit is essential to ensure continuous extrusion, while a controlled post extrusion cooling rate is critical to achieve dimensional stability and reliable interlayer bonding for coalescence purpose.

Figure 4 provides a qualitative comparison of the representative results obtained from tensile and flexural testing of the filament specimens. Two distinct filament configurations, linear and braided, were evaluated under various loading conditions. As visible in Figure 4 (a), the tensile stress-strain curves of the glass fiber reinforced plastic (GFRP) filaments exhibit an initial linear elastic response for both configurations, followed by brittle fracture without significant plastic deformation, a behavior characteristic of fiber-reinforced polymer composites. The braided filaments achieve higher ultimate strain and tensile strength compared to the linear filaments, suggesting enhanced load transfer efficiency between the fibers and the matrix and improved resistance to localized defects. Conversely, the linear filaments fail at a lower strain, consistent with a highly aligned but less damage-tolerant structure. Specifically, the developed composites exhibited tensile strengths of approximately 50 MPa and 70 MPa for the linear and braided filaments, respectively. This represents an increase of roughly 60% and 115% compared to typical values for virgin PLA filaments of comparable diameter (ranging from 30−40MPa ). When compared with the literature on glass-fiber reinforced PLA, the braided filaments demonstrate excellent properties, matching the performance of optimized commercial composite filaments (typically ~60MPa[31] ) and reaching values comparable to recent scientific studies on glass-reinforced extrusion ( ~70MPa [32]). The introduction of continuous fiber reinforcement effectively counteracted the thermomechanical degradation typical of the recycled matrix. This yielded stiffness values of 3.225 GPa and 3.256 GPa for linear and braided composites, respectively. A significant improvement over virgin PLA ( 0.813 GPa ) aligning with the 3−3.5GPa range reported for industrial composites [31, 32]. Regarding Fig. 4(b), the loaddisplacement curves from the three-point bending tests indicate that the braided configuration provides higher load-bearing capacity and flexural stiffness. These specimens sustain higher peak forces and demonstrate an extended quasi-linear regime, suggesting effective stress redistribution facilitated by the interlaced fiber architecture. In contrast, the linear specimens reach failure at lower displacements and peak loads, indicating a reduced capacity to delay crack initiation and propagation. The limited scatter in both tensile and flexural data (standard deviation of about 5% ) confirms high repeatability and consistent manufacturing and testing conditions. Finally, the outcomes demonstrate that the innovative braiding technology improves both tensile and flexural performance, increasing strength and structural reliability. The observed differences can be attributed to the effect of fiber orientation, constraint of matrix voids, and the improvement of stress concentrations to the braided configuration.

[picture]

Fig. 4. Representative data from (a) tensile and (b) flexural tests.

Fig. 4. Representative data from (a) tensile and (b) flexural tests.

The idea behind the RELIVE project well reflected the concept of modeling the role of the circular economy for climate change mitigation, widely recalled in Horizon Europe Framework Programme (Horizon Europe) with the aims of developing novel, safe, environment friendly and commercially attractive methods of recycling a wide range of composite materials proposing tools that will enable
us to demonstrate the circularity and the environmental benefits of the solutions tested (HORIZON-CL4-2021-RESILIENCE-01). In the same direction, the HORIZON-CL4-2021-RESILIENCE-01-26 has been focused on enhancing efficiency and contributing to less waste and emissions while improving material/product/process quality all along the lifecycle of a service system. In a similar fashion, the project goals moved in the direction of contributing to decreasing dependency of Europe on imported raw materials.

Conclusion

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The RELIVE project explored the feasibility of integrating additive manufacturing of reinforced polymers by FFF with the mechanical recycling of thermoplastic wastes, proposing a direct strategy able to bring plastic waste to new life. The implemented activities and achieved outcomes confirmed the initial hypothesis concerning the reference paradigm. In particular, the definition of innovative "from waste-to-product" integrated additive approach, provides the following tangible impacts:

extending the life and/or the applicability of polymeric materials even for high-end application by introducing specific additive and/or reinforcements (short and long fibers) in their recycling phase, overcoming the limits in the recycling of end life plastics derived by the inevitable degradation of the properties of the pristine materials;

application of numerical approaches, eventually using advanced tools derived from the Industry 4.0 paradigm, in both the manufacturing and testing stages allows engineers to virtually test novel strategies reducing the usage of materials and resources.

Additionally, the investigated paradigm or approach reasonably provides a reduction of environmental burden in comparison with landfill and combustion, combined with greater value recovering from the thermoplastic wastes at the end-of-life of products. This aspect deserves further investigation and quantitative analysis using, for instance, LCA tools.

Acknowledgment

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This work was supported by the Ministero dell'Istruzione, dell'Università e della Ricerca, PRIN2022: Recycling of plastic wastes integrating extrusion and additive manufacturing techniques (RELIVE), code 2022FBB37P, CUP: D53D23003520006.

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 105-111

doi: 10.4028/p-817agE

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-18

Revised:
2026-02-17

Accepted:
2026-02-17

Online:
2026-04-17

A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

The original version of this paper is available on https://www.scientific.net/KEM.1051.105.pdf

Chunyi Gao 1,a, Hao Wu 1, b, Bamber Blackman 2,c and Nan Li1, d*

¹Dyson School of Design Engineering, Imperial College London, London, UK

2 Department of Mechanical Engineering, Imperial College London, London, UK

a  chunyi.gao21@imperial.ac.uk, b  h.wu@imperial.ac.uk, c  b.blackman@imperial.ac.uk, d*n.li09 @imperial.ac.uk

Keywords: fiber metal laminates, thermoplastic composites, one-shot forming.

Abstract

Fibre metal laminates (FMLs), combining metal alloy sheets with fibre-reinforced polymers (FRPs), offer high specific strength and good fatigue resistance for lightweight structural applications. However, conventional manufacturing routes for thermoplastic FMLs rely on separately forming and bonding or hot pressing, which involve multi-stage forming routes, long heating cycles, high energy consumption and limited industrial scalability. To address these limitations, a novel nonisothermal one-shot forming route integrating hot form quench (HFQ) with FRP stamp forming is proposed in this study. In this process, separately heated metal and FRP blanks are stamped together in cold tools, enabling simultaneous forming and adhesive-free bonding within a single operation. Ubending forming trials were conducted using AA6082 aluminium alloy sheets and carbon fibrereinforced polyamide 6 (CF/PA6) laminates. The influence of FRP temperature state and aluminium surface condition on forming quality and interfacial bonding performance was systematically examined. Solid-state FRP forming limited excessive polymer flow, resulting in stable bonding but a higher intra-ply void content, whereas molten-state forming promoted polymer redistribution and reduced void content at the expense of bonding performance, leading to local debonding in highly deformed regions. In addition, chromic acid etching of the aluminium surface improved bonding and mitigated debonding after forming and post-form T6 artificial ageing. These results highlight the importance of balancing polymer flow behaviour and aluminium surface condition in non-isothermal one-shot forming, providing a practical and energy-efficient route for manufacturing thermoplastic FML components.

Introduction

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The growing demand for higher energy efficiency and lower emissions has increased the need for lightweight structures in transportation. Fibre metal laminates (FMLs), consisting of alternating metal sheets and fibre-reinforced polymer (FRP) layers, represent a hybrid material system that combines a high strength-to-weight ratio with good fatigue and impact resistance [1]. By integrating the loadbearing capacity of metals with the stiffness and damage tolerance of FRPs, FMLs have demonstrated reliable performance in aerospace structures where weight reduction and structural integrity are critical [2]. These attributes have also stimulated interest in extending the application of FMLs to the automotive sector, where high production volumes and strict cost requirements must be satisfied [3].

Existing manufacturing routes for FMLs can be broadly classified into two categories: separately forming and bonding, and one-shot forming. In separately forming routes, the metal and FRP layers are formed independently and subsequently joined through a bonding step. This approach offers greater design flexibility and avoids limiting the formability of the metal by the FRP layer. Typical examples include metal forming followed by adhesive bonding or thermoplastic welding. However, these routes require multiple tools and a multi-stage workflow, which increases tooling cost, energy consumption and overall cycle time [2]. One-shot forming aims to deform the metal and FRP layers simultaneously in a single operation. Hot pressing is a representative example, in which the laminate stack is heated to soften the polymer prior to forming. Nevertheless, heating and cooling the entire laminate and tool set lead to long thermal cycles, high energy demand and reduced tool life, making
such processes difficult to scale for high-volume production [4-6]. These limitations underline the challenges in developing cost-effective, energy-efficient and industrially scalable forming routes for FML components.

To address the limitations of conventional manufacturing forming routes of FMLs, a novel nonisothermal one-shot forming strategy was proposed by Li et al [7]. In this adhesive-free process, separately heated metal and FRP sheets are stamped together within a short cycle using cold tools with in-die quenching, which reduces overall cycle time and energy consumption. The material system comprising aluminium alloy AA6082 and carbon fibre-reinforced polyamide 6 (CF/PA6) provides a suitable platform for implementing this forming approach. This concept combines the hot form quench (HFQ) process for AA6082 with the stamp forming process for CF/PA6. In the HFQ process, the AA6082 blank material is heated to its solution heat treatment temperature of approximately 540∘C to achieve sufficient formability for forming. The heated blank is then rapidly transferred to a cold tool for quenching and stamping, enabling good formability and the production of complex geometries [8]. The stamp forming of CF/PA6 follows a similar sequence, involving preheating of the laminate and rapid transfer prior to forming. The similarity between the two forming routes, particularly in terms of preheating, rapid transfer and cold tool forming, facilitates their integration into a single operation. An important factor enabling this integration is the temperature difference between the processing windows of the two materials. The melting temperature of PA6 is well below the HFQ forming temperature of AA6082. When the separately heated blanks are stacked prior to stamping, the hot aluminium surface can locally melt the PA6 at the interface, allowing the polymer to flow and wet the metal surface, thereby forming an adhesive-free bond. As only the metal and FRP layers require heating while the tools remain cold, the process reduces energy demand, shortens the cycle time and provides a feasible route for large-scale manufacturing of thermoplastic FMLs.

Although this novel non-isothermal one-shot forming concept offers a potential route for producing thermoplastic FMLs with reduced energy consumption and shorter cycle time, the deformation and bonding mechanisms involved in such processes remain insufficiently understood. In particular, the influence of the FRP temperature state and aluminium surface condition on forming quality and interfacial bonding performance has not been systematically investigated. To address these gaps, controlled forming trials were conducted using AA6082 and CF/PA6, with forming quality and bonding evaluated through optical microscopy observations under different nonisothermal forming conditions.

Materials and Methods

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Materials.

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In the forming experiments, the materials included 2 mm thick carbon fibre reinforced PA6 FRP panels in woven and unidirectional (UD) forms, and 1.2 mm thick aluminium alloy AA6082-T6 sheets. The nominal dimensions of both materials are listed in Table 1. To accommodate thermal expansion and ensure proper fitting within the blank holder fixture, approximately 2 mm was trimmed from the blank length prior to forming. The aluminium sheets were tested under both untreated and acid-etched surface conditions. In addition, a lubricant was applied to the blank holder to reduce surface friction between the steel tools and the blanks during forming.

Table 1. Material parameters.

Material | Material grade | Material Details | Length | Width | Thickness
Aluminium | AA6082 | T6 temper | 120 mm | 40 mm | 1.2 mm
FRP | CF/PA6 | UD and woven Twill 2/2 | 120 mm | 40 mm | 2 mm

Forming Set-up.

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Non-isothermal forming trials were carried out using a 250 kN hydraulic press equipped with two independent furnaces for separately heating the metal and FRP blanks. Fig. 1 (a) presents an overview
of the experimental setup, including the metal furnace, the FRP furnace, and the forming rig mounted on the press. The forming tool consisted of a punch, a matching die and a guided blank holder, as shown in Fig. 1 (b) and (c). The forming configuration corresponded to a U-bending configuration. All tools were maintained at room temperature throughout the experiments.

[picture]

Fig. 1. Experimental forming setup.

Fig. 1. Experimental forming setup.

The forming process consisted of three stages: separate heating, transfer and stacking, and stamping, as illustrated in Fig. 2. Each heated blank was transferred over a short distance and stacked in the tool immediately prior to forming. The die was driven downward at a constant speed to achieve the U-shaped deformation, followed by a short holding period to ensure quenching of the aluminium sheet and effective interfacial bonding between metal and FRP layers.

No additional adhesive was used; interfacial bonding was achieved through thermally induced viscosity reduction of the PA6 matrix at the metal-FRP interface, followed by quenching that led to re-consolidation of the interface.

After forming, the specimens were subjected to artificial ageing to regain the T6 temper of AA6082. The formed parts were subsequently examined by visual inspection and optical microscopy of cross-sectional samples to evaluate forming quality and bonding performance.

[picture]

Fig. 2. Forming sequence showing separate heating of the blanks, transfer and stacking, stamping, and the final formed part.

Fig. 2. Forming sequence showing separate heating of the blanks, transfer and stacking, stamping, and the final formed part.

Test Conditions.

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The forming trials were designed to examine the influence of key process variables under nonisothermal forming conditions. Both solid-state and molten-state forming of the FRP layer were considered. Solid-state forming was conducted at 215∘C, below the PA6 melting point of 221∘C, whereas molten-state forming was carried out at 235∘C. In this work, the term "solid-state forming" refers to the temperature state of the FRP layer, where the overall temperature across the FRP thickness remains below the polymer melting point before stacking. It is noted that, despite the solidstate classification of the FRP layer, local and transient melting of the PA6 matrix is expected to occur at the metal-FRP interface due to heat transfer from the hot aluminium sheet during stacking and forming.

For all forming conditions, the aluminium alloy was subjected to a solution heat treatment at approximately 535∘C prior to forming, followed by forming at an aluminium temperature of around 420∘C. In addition, the effect of aluminium surface preparation was investigated by comparing untreated and acid-etched AA6082 sheets at a fixed FRP forming temperature of 225∘C, while all other process parameters were kept constant.

Results

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Effect of FRP temperature.

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The forming results revealed clear differences in forming quality and bonding performance between solid-state and molten-state forming of the CF/PA6 layer. When the CF/PA6 FRP layer was formed in the solid state, the formed parts exhibited good bonding performance. Visual inspection showed no evidence of interfacial debonding between the aluminium and FRP layers. Optical microscopy of cross-sectional samples confirmed that the metal-FRP interface remained continuous, as shown in Fig. 3. In addition, optical micrographs revealed the presence of voids within the FRP layer, particularly in regions associated with polymer flow bands.

[picture]

Fig. 3. Formed FML specimen under solid-state FRP forming conditions, with optical micrographs of selected cross-sections.

Fig. 3. Formed FML specimen under solid-state FRP forming conditions, with optical micrographs of selected cross-sections.

In contrast, when the CF/PA6 FRP layer was formed in the molten state, a reduction in bonding performance was observed. Local interfacial separation occurred predominantly in the corner regions of the formed parts, as shown in Fig. 4. These regions exhibited distinct separation between the aluminium layer and the adjacent FRP layer. Optical microscopy further showed that the FRP layer contained fewer voids compared with the solid-state forming condition. The polymer matrix penetrated more effectively into the fibre bundles, resulting in a more compact internal structure. Despite the improved intra-ply consolidation, the overall bonding performance of the FML was reduced under molten-state forming conditions.

[picture]

Fig. 4. Formed FML specimen under molten-state FRP forming conditions, with optical micrographs of selected cross-sections.

Fig. 4. Formed FML specimen under molten-state FRP forming conditions, with optical micrographs of selected cross-sections.

Effect of aluminium surface condition.

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Clear differences in bonding performance were observed between specimens formed using untreated and acid-etched AA6082 sheets, as shown in Fig. 5. For specimens formed with chromic acid etched aluminium, the metal-FRP interface remained largely intact after forming and subsequent artificial ageing. Minor interfacial separation was observed primarily near the corners and flange regions. Despite these local defects, the interface appeared predominantly continuous across the formed parts.

In contrast, specimens formed using untreated aluminium exhibited pronounced interfacial debonding and cracking after artificial ageing. These defects indicate insufficient interfacial bonding strength and a reduced ability of the interface to accommodate the shape mismatch arising from residual stresses introduced during forming and artificial ageing.

Additional insight into the mechanical integrity of the metal-FRP interface was obtained from post-forming double cantilever beam (DCB) tests conducted on FML specimens produced using the same one-shot forming route. All test conditions, including material configuration and processing parameters, were kept identical, with aluminium surface condition being the only variable. The results showed that interfaces formed with chromic acid etched aluminium exhibited a markedly higher interlaminar fracture toughness (GIC≈350 J m−2) compared with untreated aluminium interfaces (GIC≈40 J m−2 ). In addition, etched specimens displayed stable crack propagation behaviour, whereas untreated interfaces were prone to unstable crack growth and premature delamination. These observations are consistent with the macroscopic interfacial degradation observed after artificial ageing and confirm the critical role of aluminium surface condition in achieving mechanically robust interfacial bonding.

[picture]

Fig. 5. Macroscopic comparison of formed and aged FML parts produced using chromic acid etched and untreated AA6082 sheets.

Fig. 5. Macroscopic comparison of formed and aged FML parts produced using chromic acid etched and untreated AA6082 sheets.

Discussion

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Influence of FRP temperature.

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Under solid-state forming conditions, the viscosity of the PA6 matrix remains high across most of the FRP thickness. Apart from a thin interfacial layer that is locally melted by contact with the hot aluminium surface, the polymer largely remains in a solid or highly viscous state. The high viscosity through the thickness restricts polymer movement in the through-thickness direction and limits redistribution of interfacial PA6 into the FRP interior. As a result, polymer loss from the metal-FRP interface is mainly confined to local squeeze-out at the edges, while a sufficient amount of PA6 is retained at the interface to maintain good bonding performance.

In contrast, under molten-state forming conditions, the PA6 matrix exhibits low viscosity throughout the entire FRP thickness. In this case, the interfacial polymer is not only displaced laterally by squeeze flow but can also migrate into the FRP interior along the thickness direction. The
combined in-plane and through-thickness polymer flow leads to depletion of PA6 at the metal-FRP interface, particularly in regions subjected to high pressure and curvature, such as corners and flanges. This depletion reduces the bonding performance.

Optical microscopy observations provide additional insight into this mechanism. The higher void content observed within the FRP layer under solid-state forming conditions indicates limited polymer mobility and restricted through-thickness flow. In contrast, the reduced void content under moltenstate forming conditions reflects enhanced polymer redistribution into the fibre architecture. These observations indicate a trade-off between FRP layer consolidation and metal-FRP interfacial bonding governed by the extent of through-thickness polymer flow during non-isothermal forming.

Influence of aluminium surface condition.

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The improved bonding performance observed for chromic acid etched aluminium can be attributed to the modified surface condition induced by the etching process. Chromic acid etching thins the native oxide layer and surface contaminants while generating a chemically active and micro-textured aluminium surface [9]. This modified surface promotes improved wetting of the molten PA6 at the interface during stacking and forming, which facilitates more intimate interfacial contact between the polymer and aluminium. As a result, the interface exhibits improved interfacial bonding and is better able to accommodate the deformation imposed during forming, as well as the thermal and residual stresses introduced during artificial ageing.

In contrast, untreated aluminium surfaces provide less favourable conditions for polymer wetting and interfacial conformity. The reduced interfacial bonding strength is insufficient to accommodate the shape mismatch generated during forming and ageing, leading to interfacial cracking and debonding. These observations highlight the critical role of aluminium surface preparation in achieving reliable bonding performance during non-isothermal one-shot forming of thermoplastic FMLs.

Conclusion

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A novel non-isothermal one-shot forming route for thermoplastic FMLs based on AA6082 aluminium alloy and CF/PA6 was investigated. The results show that both the temperature state of the FRP layer and the surface condition of the aluminium strongly influence forming quality and interfacial bonding performance.

Solid-state forming of the CF/PA6 layer resulted in improved interfacial bonding, whereas moltenstate forming increased the risk of debonding despite enhanced intra-ply consolidation. In addition, chromic acid etching of the aluminium surface significantly improved bonding performance and mitigated interfacial degradation after forming and artificial ageing.

These findings emphasise the importance of controlling polymer flow and aluminium surface condition in non-isothermal one-shot forming. Further optimisation of process control is expected to support the transition towards high-volume industrial manufacturing of thermoplastic FML components.

Acknowledgement

The original version of this paper is available on https://www.scientific.net/KEM.1051.105.pdf

Chunyi Gao acknowledge the PhD Scholarship from Imperial College London. Nan Li and Bamber Blackman would like to thank UKRI Impact Acceleration Accounts within Imperial College London (EPSRC - EP/X52556X/1). For the purpose of open access, the authors have applied a Creative Commons Attribution (CC BY) license to any Author Accepted Manuscript version arising.

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 113-123

doi: 10.4028/p-6nRYxY

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-18

Revised:
2026-02-15

Accepted:
2026-02-17

Online:
2026-04-17

On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

Ersilia Cozzolino 1,a*, Dario De Fazio 1, b, Paolo De Sio 1,c* and Antonello Astarita 1, d

¹Dept. Of Chemical, Materials and Production Engineering, University of Naples Federico II, P.le Tecchio 80, 80125, Naples, Italy

ª*ersilia.cozzolino@unina.it, b dario.defazio@unina.it, c paolo.desio@unina.it, c antonello.astarita@unina.it

(*corresponding author)

Keywords: sustainability, sheet metal forming, electroplasticity, titanium alloys, reshaping

Abstract

Electroplasticity in sheet metal forming is a relatively recent method that involves applying an electric current to metal sheets during or before the forming process. Existing research on ElectroAssisted (EA) forming primarily focused on material characterization; few studies have investigated the effect of electropulsing on loads, power, and energy consumption during sheet metal forming, and no studies have explored the reshaping of previously formed titanium sheets after the Electro-pulsed treatment (EPT). This research aims to bridge some of these gaps of knowledge by applying two different electropulsing treatments, varying in current density, to square Ti6Al4V specimens prior to shaping and reshaping. performed using dies and counter dies having different geometries. Load, power, and energy consumption data were measured to assess the benefits of EPT compared to an untreated specimen serving as a reference. The findings suggest that EPT can significantly reduce the energy consumption and forces required for both shaping and reshaping of titanium components, extending their useful life and reducing the need for remelting. The study highlights the potential of EPT as a sustainable solution for reducing the environmental impact of titanium sheet disposal and recycling, improving material efficiency, and optimizing industrial forming processes.

Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

In the pursuit of net-zero emissions by 2050, a growing focus is being placed on the reshaping and forming of metal alloys like titanium, which is widely used in aerospace, automotive, medical, and industrial sectors due to its exceptional properties, including a high strength-to-weight ratio, corrosion resistance, and biocompatibility. Among the various objectives is achieving carbon neutrality by 2050, with a key focus on reducing the environmental impact of the end-of-life of components made from stamped sheets. This goal emphasizes the need for sustainable practices throughout the lifecycle of these materials, from production to disposal and recycling. Reshaping is increasingly seen as a method to reduce the environmental impact at the end of life of these components, offering a way to extend their usefulness and reduce waste [1].

From a sustainability perspective, recycling and forging titanium are energy-intensive processes, although they are more environmentally friendly than producing virgin titanium. The traditional recycling method for titanium involves melting scrap material and casting it into ingots or reusable forms. Due to titanium's high melting point of 1668∘C, this process requires substantial energy, approximately 361 megajoules (MJ) per kilogram. While conventional recycling produces highpurity recycled titanium, it is energy-intensive and has high environmental costs [2][3].

Moreover, traditional sheet metal forming methods as well as heat treatment performed by using traditional furnaces, are very energy-intensive, thus highlighting the need for more sustainable, energy-efficient manufacturing practices [4][5][6]. As a result, there is an increasing demand to reevaluate and redesign conventional processes to reduce their environmental impact [7].

Electroplasticity in sheet metal forming is a relatively recent method that involves applying an electric current to metal sheets during or before the forming process to reduce the yield strength and
make them easier to shape with lower loads [8]. This process enhances material formability, reducing energy consumption, forces required, and tool wear. It allows for easier deformation at lower temperatures, improving the efficiency and quality of processes like stamping, deep drawing, and bending [9]. Thus, electropulsing has been demonstrated to alter the grain structure, reduce internal stresses, and facilitate a more uniform material response during forming processes, offering a potential alternative to traditional pre-treatment methods [10] [8].

Despite existing studies in the literature on Electro-Assisted (EA) tensile tests, which primarily focus on material characterization, there is a lack of research investigating the effect of electropulsing before sheet metal forming of Ti6Al4V on loads, power, and energy consumption. Additionally, no studies have explored the reshaping of sheets previously formed after undergoing Electro-Pulsed Treatment (EPT). This highlights the need for further investigation into the potential benefits of EPT in these processes.

This study aims to address existing gaps in understanding the effects of EPT on the shaping and reshaping of Ti6Al4V alloy. Specifically, two different EPTs, differing in current density, were applied to square specimens ( 0.6 mm thick) prior to shaping and subsequent reshaping. To evaluate the impact of EPT on both processes, dies and counter-dies with various geometries were used, ensuring that the study focused on the effects of EPT independently of the final shape. Loads, power, and energy consumption were measured during the forming process to quantitatively assess the benefits of EPT. A specimen was also formed through shaping and reshaping without treatment, serving as a reference.

The main objective of this research is to explore the impact of EPT on material performance during stamping, providing valuable insights into how such treatments can optimize industrial forming processes. The main findings of the work demonstrate that EPT can serve as a solution to the end-oflife of titanium sheets, improving their environmental impact by extending their useful life, as well as that of the tools, for the production of other components, rather than resorting to their end-of-life after initial use or remelting. This approach aims to enhance sustainability by enabling the reshaping of titanium components, thus reducing material waste and energy consumption associated with their disposal and recycling.

Materials and Methods

The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

In this work, the effectiveness of the EPT process on the formability of titanium sheets was tested. To this purpose, the response in terms of load, power, and energy consumption of stamping twice, using different shape dies and counter dies, was investigated and discussed.

Fig. 1 illustrates the experimental setup adopted to perform the EPT (Step I) and the consequent stamping process (Step II), either including shaping and subsequent reshaping of sheets. In particular, firstly the EPT was performed on two 50×50 mm2 cold-rolled Ti6Al4V sheets (Fig. 1c), having a thickness of 0.6 mm , by means of the current generator Itech IT-M3900D (Fig. 1a). The IT9000 software was used to set the operational parameters of the generator to perform the EPTs of this work: for all of them, the pulsed mode was chosen over continuous current as the pulsed current is more effective than continuous current in enhancing formability, according to recent literature findings on electropulsing-induced plasticity in metals [11]. Aluminum clamps (Fig. 1b), machined from an AA2024 block, were used to hold the specimens during the EPT. Most experimental studies reported in the literature present the applied electric current density without accounting for losses. To address this limitation, a DEWESoft DS-CLAMP-150DC current sensor (Fig. 1d), connected to a DEWESoft SIRIUS modular data acquisition (DAQ) system, was employed to consider the effective current flowing through the specimens during the EPT in the analysis of the results. Temperature was also monitored during all the EPTs over time by means of an Optris IR infrared thermal camera.

[picture]

Fig. 1. Experimental setup adopted to perform the Electro-pulsed treatment (EPT) and the consequent double stamping process (shaping and reshaping)

Fig. 1. Experimental setup adopted to perform the Electro-pulsed treatment (EPT) and the consequent double stamping process (shaping and reshaping)

Table 1 contains the main inputs of the process parameters imposed by the current generator to perform two EPTs, indicated as EPT1 and EPT2, differing in the theoretical current intensity, Itheor , and the theoretical current density, Jtheor . The theoretical current density is given as follows:

Jtheoretical =Itheoretical /S(1)

where S is the cross-section of the specimens, which is equal to 30 mm2.
A specimen indicated as "base" was not treated by the EPT but formed by stamping as a reference for the EPT samples. For each test of the experimental plan (Table 1), three samples were treated and processed in the same way, ensuring repeatability and consistency of the results.

Table 1. Experimental plan of the EPT

Test
mathematical formula
mathematical formula

TT

Base | 30 | - | - | - | - | - | -
EPT1 | 30 | 300 | 10 | 40 | 0.18 | 0.03 | 0.5
EPT2 | 30 | 600 | 20 | 40 | 0.18 | 0.03 | 0.5

As mentioned above, a pulsed current was applied to the EPT samples, selecting a total of 40 pulses, indicated in Table 1 as N. Fig. 2 is a schematic representation of the applied pulse, showing either the theoretical and effective current flowing through the specimens. It can also be observed that each pulse consists of the following main time stages:

Start time (SS): the ramp-up period to reach the maximum current value.

Total time (TT): the duration for which the maximum current is maintained.

End slew (ES): the ramp-down period from the maximum current value to zero.

In this work, the pulse frequency f was fixed at 0.18 Hz for both the EPTs.

[picture]

Fig. 2. Schematization of the time stages of each electric pulse

Fig. 2. Schematization of the time stages of each electric pulse

At the bottom of Fig. 1, the setup adopted to perform the stamping of the titanium sheets, either those treated by EPT and the reference one - the base material - is presented. Two different dies and counter dies, having different shapes, have been printed in ABS by Fused Filament Fabrication (FFF) and used during the stamping to test the response in terms of loads, power and energy consumption in imposing the first shape to the material, indicated as shaping (Fig. 1i), and the second one, indicated as reshaping (Fig. 1j). The main dimensions of both the shaping and reshaping dies are the following:

The total width and depth of each die are 50.00 mm .

For the shaping die, the central feature measures 25.00 mm , with a height of 42.79 mm , including an additional top section of 10.00 mm . The radii and specific contours within the die are designed to optimize the shaping process.

The reshaping die has a central feature of 35.57 mm in height, with an overall height of 49.50 mm and an additional top section of 10.00 mm . The radii at the bottom of the reshaping die are 27.87 mm , facilitating the reshaping process. These precise dimensions are crucial for ensuring the proper fit and function of the dies in the electro-assisted forming and reshaping processes.

The PCE Instruments PCE-MTS500 stand (Fig. 1g) was adopted to perform the stamping process, with the die and counter-die securely fixed in place. The Futek 400 Button Load Cell was used, placed under the lower die in pre-compression (Fig. 1f), to measure the force during the stamping process. Moreover, the high-speed DS CAM 1100 M (Fig. 1h) was utilized to capture frame-by-frame images throughout the stamping process, which were then correlated with the force measurements for a comprehensive analysis of the results. Finally, the power of either shaping and reshaping of each sample was measured by means of the DEWESoft SIRIUS modular data acquisition (DAQ) system (Fig. 1e), equipped with a current sensor and two tension cables, which were connected to the monophase with neutral electrical system of the stand.

A sampling period Δt equal to 0.02s was selected and, based on the active power, pactive , measurements, the total energy consumption was calculated as the sum of energy consumption for shaping, Eshap , and that for reshaping, Ereshap , the same titanium sheet, as follows:

ETOT =Eshap +Ereshap =∫0Tshaping pactive (t)·dt+∫0Treshaping pactive (t)·dt≈(2)

∑i=0.02Tshaping Pactive ,i·Δt+∑i=0.02Treshaping Pactive ,i·Δt
Tshaping  and Treshaping  are the duration of the shaping and reshaping processes performed at the same speed, fixed at 1.8 mm/s. To standardize all tests, the stamping process was defined as starting when the counter-die first contacts the sheet, with the sheet already positioned on the die.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

In this section, the main results obtained by the EPT and the consequent stamping process are presented and discussed. As mentioned in the previous section, during the EPT, the current effectively flowing into the specimens was measured by using a current sensor connected to the current generator cables connected to the clamps holding the samples. Table 2 contains the effective current intensity Ieff  and the corresponding current density Jeff  applied to the specimens.

In this regard, a current loss of approximately 8% was observed when imposing both a theoretical current intensity of 300 A and 600 A . As a result, a 10% reduction in the effective current density was identified.

Table 2. Main results of the EPT monitoring

Test
superscript number
mathematical formula

EPT1 | 30 | 300 | 10 | 275 | 9
EPT2 | 30 | 600 | 20 | 550 | 18

As mentioned in the previous section, the temperature was monitored during the EPTs and then analyzed by means of the Optis PIX Connect software. The maximum temperature reached during the EPT1 was around 100∘C, while the maximum temperature reached during the EPT2 was around 250∘C. Fig. 3 and Fig. 4 show the load trend over time during the shaping and reshaping, respectively, of all the investigated sheets.

The base material - stamped without any prior EPT - showed a steadily increasing load over time during both the shaping and reshaping processes. This trend indicates that the material experienced significant resistance to deformation, requiring progressively higher forces to continue deforming as
the process progresses. The force increased relatively sharply, particularly in the later stages of both the shaping and reshaping processes, reflecting the greater resistance of the material to plastic deformation without the assistance of pulsed current prior to the forming process.

For EPT1 ( Jeff=9 A/mm2 ), the trend showed a moderate reduction in force compared to the base material. The load increased more gradually over time, indicating that the material is more easily deformable due to the electroplastic effect at this lower current density. Thus, the load required to achieve deformation was lower than that of the base material, and the rate of increase in force slowed down. This suggests that the pulsed current reduced the material's flow stress and improved formability, although to a lesser degree than in EPT2.

The EPT2 (Jeff =18 A/mm2) showed the most pronounced reduction in force among the three conditions. The load curve for EPT2 rose more slowly compared to both the base material and EPT1, indicating the most significant reduction in material resistance due to the higher current density. The force required to continue deforming the material is much lower throughout the process, with a notably slower increase in load. These findings suggest that the higher current density provides a stronger electroplastic effect, making the material more easily deformable and requiring less force to achieve the desired shape.

Shaping

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[picture]

Fig. 3. Load over time during the shaping of titanium sheets

Fig. 3. Load over time during the shaping of titanium sheets

Reshaping

The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

[picture]

Fig. 4. Load over time during the reshaping of titanium sheets

Fig. 4. Load over time during the reshaping of titanium sheets

As mentioned in the previous section, a high-speed camera was used during either shaping and reshaping of all the investigated sheets. The most representative frames, corresponding to specific stage of material deformation, are highlighted in Fig. 3 and Fig. 4. These stages are labeled with numbers and correspond to specific moments in the load vs. time curves.

In particular, for the shaping process (Fig. 3), the following three points were labeled:

I; at this stage, the base material (blue curve) required a higher load to start deforming, with the camera showing localized resistance to deformation. For EPT1 ( Jeff =9 A/mm2 ), the load resulted in lower values, indicating reduced resistance, and the camera shows more uniform deformation. EPT2 ( Jeff =18 A/mm2 ) required the lowest load and displayed the most uniform and smooth deformation, reflecting the most effective reduction in resistance due to the higher current density.

II; as deformation progresses, the base material required increasingly higher loads, with the camera showing localized resistance. In EPT1, the load increases more gradually, with the camera revealing more uniform deformation and reduced localized resistance due to the electroplastic effect. EPT2 required the lowest load, with a slower increase in force and more even deformation, as the higher current density facilitates smoother plastic flow with minimal resistance.

III; at this stage, the base material required generally the highest load, with the camera showing significant localized plasticity as it reached its yield point. EPT1 required less force, with the camera showing more uniform deformation and fewer areas of strain concentration. EPT2 showed the lowest load and the most uniform deformation, with minimal strain localization, indicating that the higher current density facilitates substantial deformation with minimal force.
After the shaping, the sheets were reshaped using another couple of FFFed die-counter dies to perform the reshaping process. Four labeled points were considered for the EPT samples, and five for the base material (Fig. 4). In particular:

I; at this stage, the base material required significantly higher force, with the camera showing localized strain, indicating resistance to reshaping. For EPT1, the force is lower, and the camera shows more uniform deformation with reduced strain, suggesting the electroplastic effect is easing reshaping. EPT2 required the lowest force, with the camera showing the most uniform reshaping and minimal resistance, reflecting the most effective reduction in material resistance.

II; as reshaping continues, the base material required a significantly higher force, with localized strain visible in the camera frame. EPT1 showed a more gradual increase in load, with the material deforming more evenly and reduced strain localization. EPT2 required the least force, with the camera showing uniform reshaping and no strain localization, indicating that the higher current density greatly reduces material resistance during reshaping.

III; the base material required significantly higher forces to continue reshaping, with localized strain visible in the camera frames, indicating resistance to deformation. EPT1 requiref less force with more uniform deformation, showing reduced strain localization due to the electroplastic effect. EPT2 required the lowest force, with uniform reshaping and minimal resistance, reflecting the most effective reduction in material resistance due to the higher current density.

IV; this stage is the final one for the EPT samples, while the base material reached its maximum deformation and highest force.

V; this stage is the final one for the base material, and it can be observed that the material required more force to maintain its shape, with visible strain localization.

Table 3 contains the main results of the load and power monitoring during the stamping process. In particular, Fmax, shap  and Fmax, reshap  are the maximum forces occurring during shaping and reshaping, respectively, as a result of the load monitoring performed by using the button load cell described in the previous section. Similarly, Pmax , shap, and Pmax , reshap are the maximum power measured during shaping and reshaping, respectively, as a result of the power monitoring.
Etot  is the total energy consumption calculated as the sum of energy for shaping and for reshaping, as indicated by Eq. 2. For all these response variables, the standard deviation was calculated, as shown in Table 3. Fig. 5 depicts the main outputs of the load measurements (Fig. 5a) and the energy consumption calculations (Fig. 5b).

Table 3. Main results of the monitoring of the stamping process

Test | Fmax, shap (N) | Fmax, shap, dev std | Fmax, reshap (N) | Fmax, reshap, dev std | Pmax, shap (W) | Pmax, shap, dev std | Pmax, reshap (W) | Pmax, reshap, dev std | Eshap (J) | Eshap, dev std | Ereshap (J) | Ereshap, dev std | Etot (J) | Etot, dev std
Base | 6756 | 285 | 7581 | 256 | 333 | 8 | 405 | 24 | 967 | 30 | 1540 | 47 | 2507 | 56
EPT1 | 6703 | 223 | 7037 | 246 | 255 | 19 | 342 | 32 | 643 | 50 | 1340 | 52 | 1983 | 72
EPT2 | 5009 | 200 | 6570 | 231 | 256 | 15 | 307 | 29 | 656 | 34 | 1279 | 54 | 1935 | 64

[picture]

Fig. 5. Main results of the a) load measurements and b) energy consumption calculation of shaping and reshaping of titanium sheets investigated in this work

Fig. 5. Main results of the a) load measurements and b) energy consumption calculation of shaping and reshaping of titanium sheets investigated in this work

Concerning the force analysis, the base material required the highest final force at both the shaping and reshaping stages, indicating significant resistance to deformation throughout the entire process. EPT2 (Jeff =18 A/mm2) showed the largest decrease in maximum force ( −25.86% ) in the shaping phase, and a significant decrease in the reshaping phase as well ( −13.34% ), indicating that higher current density leads to the greatest reduction in material resistance, making reshaping easier and requiring less force at the final stage of both shaping and reshaping processes.

Concerning the power measurement, the base material required the highest power to maintain the deformation throughout the process, with noticeable decreases in power in the EPT conditions. In EPT1 and EPT2, both the shaping and reshaping phases showed significant reductions in power compared to the base material (about 24% less).

Regarding the energy consumption calculations, in both shaping and reshaping phases, EPT1 and EPT2 required less energy than the base material. In particular, EPT1 showed a 33% reduction in energy consumption for shaping, while EPT2 showed a 32% reduction, reflecting a more efficient deformation. These findings indicate that the short EPTs help to reduce the energy required to perform the subsequent forming process, likely due to lower resistance to deformation, even without achieving high temperatures over extended periods, as supported by recent literature, as confirmed by the recent literature [10].

During reshaping, EPT1 and EPT2 resulted in 12.9% and 16.9% reductions in energy consumption, respectively, confirming that higher current densities applied during the EPT significantly improve energy efficiency of the consequent forming process. The total energy consumption required for the entire process, considering shaping and reshaping, is also lower in both EPT1 and EPT2, with EPT2 using the least energy overall, confirming that higher current densities lead to more energy-efficient stamping processes.

Moreover, the standard deviations associated with both maximum forces, maximum power and total energy consumption (Table 3 ) are consistently limited across all investigated conditions, indicating good repeatability and stability of the experimental procedure. The relative scatter remains low compared to the corresponding mean values, and no significant increase in variability is observed between shaping and reshaping stages. Moreover, the differences measured among Base, EPT1 and EPT2 conditions are generally larger than the respective standard deviations, suggesting that the observed trends are representative of actual process and material effects rather than experimental noise.

Finally, it is paramount to underline that, for the sake of brevity, only the frames of one EPT specimen and the base material specimen have been presented for both shaping and reshaping processes (Fig. 3 and Fig. 4). This was possible because the curves for the specimens subjected to EPT followed the same trend, though with different force values due to the varying current densities, and were distinct from the base material. Additionally, it is important to highlight that the different
geometry of the die and counter die used for reshaping likely influenced the load curve's shape, as it had to account for the initial sheet position, which was already formed and not flat as in the shaping process.

A limitation of this study is the use of small, simple geometries (50×50 mm2), which may limit the direct applicability of the results to larger, more complex geometries typically encountered in industrial settings. While the choice of small samples was necessary to control experimental variables and focus on the fundamental effects of EPT on Ti6Al4V sheets, it is important to recognize that realworld applications often involve larger, more intricate geometries. Future work will seek to address this limitation by extending the investigations to larger-scale specimens, which will more accurately represent industrial conditions. Additionally, simulations and modeling studies will be undertaken to predict the performance of EPT under a broader range of geometries and operational conditions. These efforts will help bridge the gap between fundamental research and practical industrial applications, improving the transferability of the findings from this work, particularly in alignment with current industrial priorities focused on energy efficiency and the sustainable reuse of high-value alloys.

Conclusions

The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

This work investigates the effects of electropulsing on cold-rolled Ti6Al4V specimens performed prior to the stamping process, either shaping and reshaping, on load response, power consumption, and energy consumption, which were experimentally monitored. The main conclusions that can be drawn are the following:

The base material showed the highest load values and a steep increase in force, indicating significant resistance to deformation compared to the material electrically treated by EPT prior the stamping process.

Selecting a higher current density during the EPT, the lowest load values and the slowest increase in force were reached during either shaping and reshaping, indicating the most significant reduction in resistance, improving material formability the most.

Applying a higher current density during the EPT resulted in the most significant energy savings and lower force requirements, making it beneficial for more efficient forming processes.

Thus, based on the findings of this work, using EPT with varying current densities can optimize the subsequent forming processes, obtaining improvements in material formability and energy efficiency for both shaping and reshaping stages. These results contribute to ongoing research focused on reducing environmental impact, as they suggest the potential for reshaping materials that have already been formed into different shapes. This approach could play a key role in minimizing the environmental footprint of components made from stamped sheets at the end of their lifecycle, aligning with goals for reusability and remanufacturing in the context of a 2050 sustainability vision.

Future developments will focus on comparative cradle-to-grave Life Cycle Assessment (LCA) of the forming of titanium sheets, comparing the scenario including the remelting of titanium to obtain another product having a different final shape, and the other scenario where this objective has been reached as shown in this work, without remelting of titanium but using the EPT prior or hopefully during the forming process.

On the other hand, the results obtained in this work suggest the relevance of continuing to do research on the electroplasticity, including either mechanical characterization and sustainability analysis, to better highlight its potentialities in minimizing the environmental impact in comparison with traditional long-duration and energy-intensive furnace treatments. In this regard, future developments should focus on studying the effects of a wide range of top time, frequency, and current density on both forming, shaping, and reshaping processes to fully understand and optimize the potential of electropulsing in traditional industrial manufacturing.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.113.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 125-133

doi: 10.4028/p-rlVb3W

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-18

Revised:
2026-02-16

Accepted:
2026-02-17

Online:
2026-04-17

Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

The original version of this paper is available on https://www.scientific.net/KEM.1051.125.pdf

Yohannis Solomon 1,a*, Davar Hemyari 1,2,b and Sotirios Grammatikos 1,c

1 Department of Manufacturing & Civil Engineering, NTNU-Norwegian University of Science and Technology, Norway

2 BENTELER Automotive Raufoss AS, Raufoss, Norway

a* yohannis.solomon@ntnu.no, b davar.hemyari@ntnu.no, c sotirios.grammatikos@ntnu.no

Keywords: aluminum extrusion, process robustness, circular manufacturing, principal component analysis (PCA), multi-hole extrusion.

Abstract

Aluminum extrusion plays a critical role in lightweight structural applications and circular economy strategies. However, extrusion process design is constrained by competing objectives: increasing productivity through higher ram speeds or increased die-hole count improves throughput and material utilization yet simultaneously elevates force demand and defect susceptibility. In this work, a numerical statistical framework is proposed to identify circularity-tolerant process windows, defined as multi-objective design regions that balance productivity, product quality, and sustainability performance. A three-factor Taguchi design was employed to systematically vary ram speed, billet temperature, and die-hole count in the extrusion of AA6063. Twenty-seven full 3D thermomechanical extrusion simulations were conducted using the DEFORM finite element platform employing an Arrhenius-type constitutive model from literature. Key extrusion responses maximum ram force, local damage indicator, and total displacement were analyzed using Principal Component Analysis (PCA) to reveal correlations and trade-offs between productivity-oriented parameters and quality-related responses. The results demonstrate a clear divergence between productivity drivers (ram speed, die-hole count) and process capability indicators, providing quantitative evidence of the inherent productivity quality trade-off. The proposed framework enables the identification of robust extrusion operating regions suitable for circular manufacturing scenarios in aluminum extrusion. The proposed framework is particularly relevant for extrusion scenarios where process robustness must be ensured under increasing material and operational variability, such as those anticipated with higher recycled content.

Introduction

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Aluminum extrusion is a cornerstone manufacturing process for lightweight structural components in transportation, construction, and energy applications, owing to its ability to produce complex crosssections with high material utilization and excellent recyclability. Continuous industrial pressure to improve productivity has led to widespread adoption of higher ram speeds, increased extrusion ratios, and multi-hole die configurations, all of which enhance throughput and reduce billet discard [1]. However, these productivity-oriented strategies are inherently coupled with increased deformation severity, higher force demand, and elevated defect susceptibility, giving rise to fundamental productivity quality trade-offs.
These trade-offs have been extensively documented in both experimental and numerical studies. Increased ram speed and extrusion ratio have been shown to raise local strain rates and thermal gradients, thereby increasing the risk of surface cracking, internal damage, and flow instabilities [2], [3], [4]. Similarly, multi-hole dies, while attractive from a material efficiency perspective, introduce complex flow interactions that can amplify deformation heterogeneity and defect formation if not carefully controlled [5], [6], [7]. As a result, productivity enhancement in extrusion is rarely a monotonic improvement problem but rather a constrained multi-objective design challenge.
Aluminum is often cited as a "circular material" due to its recyclability; however, circularity at the process level depends not only on material choice but also on stable operation, low scrap generation
[8], and efficient use of energy and tooling [9], [10]. Defect-driven scrap, rework, and process instability directly undermine circularity by increasing remelting demand and energy consumption, regardless of whether primary or recycled feedstock is used. Consequently, circularity-oriented extrusion design must explicitly consider process robustness alongside productivity.
Despite this, most existing extrusion optimization studies remain focused on point-wise optima, often identified using Design of Experiments (DoE), response surface methods, or evolutionary algorithms [2], [11], [12]. While such approaches can identify parameter combinations that perform well under nominal conditions, they are inherently sensitive to operational variability and model uncertainty. Small deviations in ram speed, billet temperature, or friction conditions can shift the process outside the optimal region, leading to unstable operation and quality degradation. From an industrial perspective, such fragility limits the practical usefulness of point-based optimization.
Multivariate statistical methods offer a complementary perspective by emphasizing structure and robustness rather than isolated optima. PCA, in particular, has been successfully applied in manufacturing to identify dominant modes of variation, uncover correlations among responses, and reduce dimensionality in complex process datasets [11], [13], [14]. In metal forming applications, PCA has been shown to provide physically interpretable axes that reflect competing deformation, thermal, and force-related mechanisms [15]. However, its use in extrusion research has largely been confined to post-hoc analysis or process monitoring, rather than as a tool for process window identification.
Building on this foundation [16], the present study systematically characterizes productivity-quality trade-offs in aluminum extrusion by reframing process design around circularity-tolerant process windows rather than single optimal parameter sets. Circularity tolerance is interpreted at the processsystem level, emphasizing operating regions that balance productivity and quality while remaining robust to realistic parameter variation.
Using three-dimensional finite element simulations of AA6063 extrusion and a structured Taguchi design, PCA is employed to reveal the dominant productivity-quality trade-offs governing process behavior. Stable operating regions in reduced PCA space are then identified as candidate process windows suitable for robust and circularity-oriented operation.
By shifting the focus from optimization to trade-off management and robustness, this study provides a framework that complements existing optimization approaches and aligns extrusion process design with practical industrial and circular manufacturing objectives.
In mature industrial extrusion operations, the majority of material losses are typically associated with structural process scrap such as butt ends, start-up skin contamination, and cutting logistics related to profile length, rather than instability-driven defects. These losses are largely independent of process parameter fluctuations and are commonly addressed through production planning and downstream handling strategies. Consequently, conventional single-point process parameter settings, determined through trimming trials and subsequently fixed for production, are generally sufficient to ensure stable operation under narrowly controlled alloy specifications. However, the ongoing transition towards increased use of recycled and mixed-scrap billet feedstocks is expected to introduce greater batch-to-batch variability in material behaviour, which challenges the robustness of such single-point settings. Under these conditions, process instability may re-emerge as a relevant contributor to nonprocess scrap, motivating the need for systematic approaches that explicitly account for robustness under variability rather than optimization under nominal conditions.

Methodology

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Die Geometry and Hole Configuration

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The die configurations investigated consisted of 2, 3, and 4 hole layouts arranged symmetrically with respect to the billet center Figure 1 (c-e). In the 2 -hole configuration, holes were positioned diametrically opposite at 180∘ spacing. The 3 -hole configuration employed equal angular spacing of 120∘, while the 4-hole configuration used 90∘ spacing. The holes were distributed along a constant
pitch circle diameter of approximately 11 mm , derived from an eccentricity ratio of 0.3 to minimize outlet bending and flow imbalance.
All configurations were designed to maintain geometric symmetry in order to eliminate artificial flow non-uniformity arising purely from layout asymmetry. Increasing the number of holes modifies the extrusion ratio ( 32,21.3, and 16 for 2,3 , and 4 hole dies, respectively), thereby influencing material velocity gradients, hydrostatic stress distribution, and damage evolution during extrusion. These geometric effects are known to significantly affect flow balance and quality metrics in multi-hole extrusion processes.

[picture]

Fig. 1. Geometric configuration of the investigated multi-hole extrusion dies. (a) Schematic definition of billet diameter D2, pitch circle diameter D3, hole diameter D4, and eccentricity parameters. (b) Sectional view illustrating pocket geometry and bearing length used to control material flow. (c-e) Three-dimensional CAD representations of the 2-hole ( 180∘ spacing), 3-hole ( 120∘ spacing), and 4 -hole ( 90∘ spacing) symmetric layouts, respectively.

Fig. 1. Geometric configuration of the investigated multi-hole extrusion dies. (a) Schematic definition of billet diameter D 2 , pitch circle diameter D 3 , hole diameter D 4 , and eccentricity parameters. (b) Sectional view illustrating pocket geometry and bearing length used to control material flow. (c-e) Three-dimensional CAD representations of the 2-hole ( 180 ∘ spacing), 3-hole ( 120 ∘ spacing), and 4 -hole ( 90 ∘ spacing) symmetric layouts, respectively.

Design of Experiments

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In this work, circularity tolerance is interpreted at the process-system level as the ability of the extrusion process to maintain acceptable force demand, deformation behaviour, and damage levels in the presence of unavoidable variability. Rather than equating circularity exclusively with recycled material input, the present framework emphasises process robustness as a necessary precondition for circular manufacturing, particularly in scenarios where material properties cannot be assumed constant. Circularity-tolerant process windows are therefore defined as operating regions in which productivity- and quality-related responses remain balanced and resilient to perturbations, enabling stable operation without repeated re-trimming of process parameters.
A Taguchi L27 orthogonal array was employed to structure the numerical experiments, enabling efficient exploration of the design space with three factors at three levels each. The investigated parameters and their corresponding levels are summarized in Table 1.

Table 1. Taguchi L27 OA Factorial design.

This design resulted in 27 extrusion simulations capturing interactions between productivity-driven and thermal parameters. The extrusion responses considered in the analysis were 1) maximum ram force, 2) maximum local damage, and 3) maximum total displacement.
These variables were selected to represent productivity-related effects, quality-related risks, and process feasibility constraints.

Numerical Model

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Three-dimensional extrusion simulations were conducted using the commercial finite element software DEFORM. The extrusion of AA6063 was modeled under steady-state conditions using a rigid die and deformable billet formulation. The billet material behavior was described using an Arrhenius-type constitutive model, with parameters adopted from established literature sources [17]. The predicted ram force magnitudes and deformation patterns were consistent with published numerical and experimental extrusion studies under similar processing conditions.
The present investigation focuses on relative comparisons among die configurations using a consistent modeling framework. Therefore, the conclusions are based on comparative trends rather than absolute predictive accuracy.

[picture]

Fig. 2. FEA model (a) mesh/ flow, (b) Damage and (c) Force.

Fig. 2. FEA model (a) mesh/ flow, (b) Damage and (c) Force.

For each simulation Figure 1, the following responses were extracted. Maximum ram force, local damage, and maximum total displacement; representing energy demand, defect susceptibility and material flow and deformation intensity respectively. These responses collectively characterize extrusion productivity, process capability, and quality risk.
In the present study, the built-in ductile damage indicator provided by the software was employed as a comparative measure of damage accumulation. This indicator is based on strain accumulation and stress-state evolution during deformation and is commonly used in extrusion studies to assess defect susceptibility under compressive dominant stress conditions.

Multivariate Analysis

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Principal component analysis was applied to the standardized response variables to reduce dimensionality and identify dominant correlations. PCA scores were used to locate individual extrusion runs in reduced space, while loading vectors were used to interpret the physical meaning of the principal components.

Results

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Dominant Productivity-Quality Trade-off Structure

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The scree plot Figure 2 (a) indicates that the first two principal components explain approximately PC1%≈50−55% and PC2%≈35−40% of the total variance, respectively, capturing approximately ~90% of the overall system variability.

[picture]

Fig. 3. PCA (a) Scree plot: Explained Variance by Principal Components (b) Biplot: Scores + Loadings (PC1-PC2).

Fig. 3. PCA (a) Scree plot: Explained Variance by Principal Components (b) Biplot: Scores + Loadings (PC1-PC2).

Analysis of PCA loadings shows that Figure 2 (b) PC1 is dominated by deformation-related responses and productivity-linked effects, capturing the trade-off between throughput-enhancing measures and deformation severity. PC2 is dominated by maximum ram force, reflecting feasibility constraints imposed by press capacity and tooling limits. Together, PC1 and PC2 define a productivity-quality trade-off space, within which extrusion behavior can be systematically interpreted.

[picture]

Fig. 4. Leave one out cross validation.

Fig. 4. Leave one out cross validation.

To assess the stability and generalizability of the PCA-derived process windows, Leave-One-Out Cross-Validation (LOOCV) was performed on the full dataset of 27 extrusion simulations. For each iteration, a single observation was excluded, the PCA model was refit on the remaining 26 samples, and variance explained by each principal component was recalculated.
The LOOCV analysis, depicted in Figure 3, demonstrates that PC1 and PC2 remain remarkably stable across all 27 leave-one-out iterations, with PC1 variance consistently clustering around the mean value of 52.56% and PC2 around the value of 38.53%. The singular perturbation at sample 10 (corresponding to the mid-range parameter combination) reflects the influence of a nominally central experimental point, yet this deviation is transient and does not propagate across the broader sample space. Importantly, PC3 remains negligible ( 8.91% mean variance) across all iterations, confirming that the two-component representation robustly captures the dominant process trade-offs regardless of which single observation is omitted.
This consistency validates that the identified circularity-tolerant process windows are not artifacts of specific outlying conditions but represent genuine structural features of the aluminum extrusion design space. The robustness of the PCA decomposition under LOOCV thus strengthens confidence in the practical applicability of the proposed framework for industrial process design.

Principal Components

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The first principal component (PC1), accounting for approximately 51.5% of the total variance, is primarily associated with displacement and damage responses. This component therefore represents a productivity deformation severity axis, capturing the coupling between throughput enhancing parameters (e.g., increased hole count and ram speed) and the corresponding increase in deformation intensity and defect susceptibility.
The second principal component (PC2), explaining approximately 39.3% of the variance, is dominated by maximum ram force. PC2 can thus be interpreted as a feasibility axis, reflecting constraints imposed by press capacity and load requirements. While PC1 describes the trade-off between productivity and quality risk, PC2 governs whether a given operating condition remains mechanically viable.
Together, PC1 and PC2 define a physically interpretable productivity quality trade-off space. Operating points located within moderate PC1 and PC2 ranges correspond to balanced conditions
where productivity gains do not induce excessive deformation severity or force escalation. This structure provides the basis for identifying circularity-tolerant process windows.

Identification of Circularity-Tolerant Process Windows

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The biplot Figure 2 (b) illustrates the relationship between the three response variables damage, ram force, and displacement in the reduced PC1-PC2 space. The spatial distribution of experimental points reveals the trade-offs inherent in the extrusion process. Damage and displacement vectors point in relatively different directions, indicating they respond differently to process parameter changes, while ram force provides an intermediate response characteristic.
The circularity-tolerant process window is identified in the region where experiments cluster with moderate PC scores neither extreme in either direction. This central operating region represents a balance between productivity (minimized force and displacement) and quality (controlled damage), while demonstrating resilience to small parameter variations. The scattered distribution around the origin Figure 2 (b) suggests that realistic parameter deviations would keep the process performance within acceptable tolerances in this zone, making it the optimal circularity-tolerant window for aluminum extrusion. Operating within such windows reduces sensitivity to process fluctuations and supports stable, waste-minimizing extrusion practice.
From a circularity perspective, robustness is a critical enabler for material efficiency. Stable extrusion operation is a known enabler for reduced scrap, rework, and energy-intensive remelting, thereby supporting overall sustainability performance.
The proposed framework enables circularity-oriented process design by explicitly linking productivity-quality trade-offs to stable operating regions, rather than relying on fragile point-wise optima.

Conclusion

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This study investigates productivity and quality trade-offs in aluminum extrusion through a multivariate analysis framework and introduces the concept of circularity-tolerant process windows. The key conclusions are:

Aluminum extrusion behavior is governed by a dominant productivity-deformation trade-off and an orthogonal force feasibility constraint.

Stable operating regions emerge in reduced PCA space, representing circularity-tolerant process windows.

Identifying such windows is a necessary precursor to meaningful and robust process optimization.

The proposed approach provides a systematic pathway toward robust and circularity-oriented extrusion process design.
While the present analysis assumes fixed constitutive properties for AA6063, the proposed framework remains applicable under moderate material variability typical of circular production environments, as such variations would primarily shift operating points within the identified PCA structure rather than alter its underlying trade-off relationships.
The present study does not explicitly model variations in billet chemistry or structural sources of process scrap such as butt length or cutting strategy, which are known to dominate material losses in mature industrial extrusion environments. Instead, it establishes a general framework for identifying robust operating regions under controlled material conditions. From an industrial perspective, the proposed approach is not intended to replace established trimming trials for stable alloy systems, but to complement them in scenarios where increasing feedstock variability, for example due to higher recycled content, undermines the reliability of conventional single-point parameter settings. In such contexts, pre-identified circularity-tolerant process windows may support faster stabilisation and controlled performance degradation rather than abrupt process instability.

Acknowledgement

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Authors acknowledge financial support by the Horizon Europe Project "ZEVRA" (GA: 101138034).

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The original version of this paper is available on https://www.scientific.net/KEM.1051.125.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 135-146

doi: 10.4028/p-4eKwco

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-19

Revised:
2026-02-12

Accepted:
2026-02-17

Online:
2026-04-17

Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

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PICCININNI Antonio 1,a*, INGARAO Giuseppe 2, b, BARCELLONA Letizia 1,c, PULEO Riccardo 2, d, CUSANNO Angela 1,e, MICARI Fabrizio 2,f and PALUMBO Gianfranco 1, g

1 Department of Mechanics, Mathematics and Management - Politecnico di Bari, via Orabona, 4 Bari 70126, Italy

2 Department of Engineering, University of Palermo, Viale delle Scienze, Palermo 90128, Italy

a  antonio.piccininni@poliba.it, b  giuseppe.ingarao@unipa.it, cl.barcellona@phd.poliba.it, driccardo.puleo01@unipa.it, e angela.cusanno@poliba.it, f fabrizio.micari@unipa.it, g  gianfranco.palumbo@poliba.it

Keywords: Gas forming, Hydroforming, SPIF, Reshaping, EoL, Sustainability

Abstract

The reshaping approach is widely considered a virtuous strategy in line with the pillars of the Circular Economy. According to this approach, End-of-Life (EoL) components are subjected to a second forming process to achieve a new functional geometry. However, EoL parts often exhibit a non-uniform thickness distribution and work-hardened zones resulting from the primary manufacturing step, which makes the design of the reshaping step not trivial. Beyond the standard objectives like avoiding fracture and minimizing springback during the reshaping operations, one of the most concerning aspects is the complete removal of the geometrical features coming from the initial forming process.

Flexibility and versatility of the forming process are unavoidable requirements to make the reshaping successful. Therefore, three different reshaping routes are numerically investigated in the present work: (i) reshaping by hydroforming (RH) at room temperature; (ii) reshaping by gas forming (RGF) at hot temperature; (iii) a hybrid approach, based on the combination of an intermediate deformation step via Single Point Incremental Forming followed by sheet hydroforming (RHA). The three routes share the same EoL, characterized by the presence of a deep-drawn square feature. Comparing the three routes, in terms of final shape and thinning distribution, with a reference case study (represented by the sole hydroforming process carried out on an undeformed flat blank) allowed to conclude that the feature removal and a non-severe thinning could not be achieved simultaneously: in fact, while RGF and RHA ensure a more evident suppression of the pre-existing feature, they simultaneously induce a more pronounced and localized thinning compared to the RH route.

Introduction

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The reduction of the environmental impact associated with industrial production has become a key priority, particularly in sectors characterized by high energy consumption and greenhouse gas emissions. Among structural materials, aluminium plays a critical role: despite its favorable strength-to-weight ratio and recyclability, primary aluminium production remains one of the most energy and emission-intensive industrial processes [1]. As global demand for aluminium products continues to grow [2], the adoption of effective circular economy strategies is essential to decouple resource consumption from economic development. In this context, conventional recycling routes based on remelting, although widely adopted, present visible limitations. In addition to the high energy demand, remelting processes are affected by permanent material losses due to oxidation, particularly for aluminium alloys. For these reasons, increasing attention has been given to alternative approaches capable of preserving both material integrity and manufacturing value. Solid-state recycling [3,4] and direct reuse strategies have therefore emerged as promising solutions to further reduce the environmental footprint of metal products. Within the metal reuse category, reshaping has demonstrated particular attractiveness. This approach consists of applying manufacturing techniques
(additive, subtractive, or mass-conserving) to rework End-of-Life (EoL) components by modifying their shape. In this way, reshaping enables the recovery not only of the material but also of the geometric and functional value embedded in the component. However, reshaping poses significant technological challenges, especially when sheet metal components are considered. EoL sheet parts typically exhibit heterogeneous thickness distributions, localized strain hardening, and reduced residual formability as a result of their original manufacturing history. These aspects make the design of reshaping processes substantially more complex than that of conventional forming operations starting from undeformed flat sheets. Despite its potential, the reshaping of sheet metal components has received limited attention in the literature so far. Existing studies mainly focus on flexible forming technologies, such as Single Point Incremental Forming or Hydroforming, due to their ability to accommodate complex strain paths and non-uniform material conditions.

In particular, Single Point Incremental Forming (SPIF) is a flexible sheet metal forming process based on localized and progressive deformation induced by a hemispherical tool following a digitally defined toolpath, without the need for dedicated dies. Compared to conventional stamping processes, it enables higher formability due to the localized low forming forces involved and the delayed onset of fracture, making it possible to impart additional deformation even in regions that have already undergone significant straining. Previous studies have demonstrated the feasibility of SPIF-based reshaping by applying the process to deep-drawn components into new geometries, while preserving material integrity [3]. Takano et al. [5] applied Single Point Incremental Forming (SPIF) to reform a pre-bent sheet. From a sustainability perspective, SPIF-based reshaping represents an effective alternative to conventional recycling routes, as it reduces the number of process steps [6], minimizes material losses, and leads to significant reductions in energy consumption and CO2-equivalent emissions.

On the contrary, Sheet Hydroforming (SHF) is a sheet metal forming technology that exploits fluid pressure to shape metallic blanks, enabling the production of complex geometries with high dimensional accuracy. This technique can operate with a single rigid die, as the forming load is uniformly applied by the pressurized fluid, resulting in reduced tooling requirements and process complexity [4], and making SHF particularly suitable for reshaping applications, where additional deformation must be imposed on preformed or EoL components without inducing premature failure. Moreover, it offers several advantages over other sheet metal forming technologies [7], including enhanced formability limits [8], the capability to produce more complex geometries such as re-entrant features [9], and, in some cases, a reduced number of forming steps [6]. For example, Brosius et al. [10] operated a SHF process to reshape an engine hood into a rectangular sheet. However, residual marks may remain in less-deformed regions when the strain imposed is insufficient [5].

In previous works, the feasibility of reshaping deep-drawn aluminium components by means of SHF was experimentally and numerically demonstrated. Those studies highlighted that, while SHF can effectively produce a new geometry to EoL components, the complete removal of pre-existing features is strongly dependent on the local deformation conditions experienced during the reshaping step. In particular, residual marks from the original deep-drawn geometry were observed when the imposed strain during hydroforming was insufficient, whereas excessive restraining conditions led to premature fracture. These findings clearly indicate that the reshaping process design requires the introduction of additional objectives and constraints compared to conventional sheet metal forming.

Based on this background, the present work investigates the performance of remodeling based on different approaches: literature-tapped SHF, gas forming (GF), and SHF with the introduction of an intermediate SPIF phase. The Gas Forming, particularly in its hot variant, was selected due to its potential advantages over conventional forming and SHF in the reshaping of end-of-life (EoL) sheet metal components. The gradual and uniform pressure application, offered by this technique, promotes a more homogeneous strain redistribution, which is beneficial for mitigating residual geometric features and handling the heterogeneous thickness distributions and limited residual formability typical of EoL parts [11]. The intermediate SPIF step was, instead, intended to promote smoother transformation of the deep-drawn feature geometry, before the final hydroforming stage, aiming to facilitate the feature removal during the hydroforming process. The feasibility of both routes was
assessed through finite element simulations in the Abaqus environment, focusing on the influence of key process parameters on the reshaped component. A deep drawn square feature was used to represent the EoL geometry [12], and the variations of the descending angle for SPIF were considered. All parameters were tested along two deformation directions, corresponding to the top and bottom of the EoL feature. The resulting performance was compared in terms of sheet thinning and feature removal capability.

Materials and Methods.

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Investigated reshaping routes. This work expands the research through a comparative numerical investigation of three different technological routes. The three strategies, more detailed in the following, share the same EoL represented by a sheet metal-based part characterized by the presence of a square-based feature obtained from a previous Deep Drawing process (Fig. 1a), whose geometry is defined according to [12] (Fig. 2a). The Deep Drawing-derived feature was selected because it represents a critical condition in terms of strain hardening, as it promotes highly localized plastic deformation. In particular, the selected EoL component is characterized by concentrated strain levels at the bottom corner of the deep-drawn feature, whereas lower strain levels are observed in other regions. Consequently, the part effectively mimics a realistic EoL component, showing a heterogeneous distribution of both strain hardening and thickness, carrying its own deformation history and thickness heterogeneity, thus serving as a suitable benchmark for assessing the capabilities of the reshaping approach [13].

[picture]

Fig. 1. Illustration of the (a) pre-feature of the EoL component obtained through DD, (b) starting blank with the deformation history, (c) section views of the blank and die positioning

Fig. 1. Illustration of the (a) pre-feature of the EoL component obtained through DD, (b) starting blank with the deformation history, (c) section views of the blank and die positioning

The final reshaped part is defined by the geometry of the die depicted in Fig. 2b [12]. It is worth mentioning that the geometry of the die cavity is not related to any specific industrial component, but it is more regarded as a benchmark geometry: the two-steps induces two different levels of strain, and the 5 mm fillet between the two imparts a condition close to the plane strain. The geometry, thus, allows for challenging the capabilities of the reshaping approach to different extents across different states of deformation. Another common aspect of the three strategies concerns the positioning of the deep-drawn feature during the reshaping step: specifically, it was oriented with its concavity aligned with the deepest step of the die cavity. Moreover, the EoL component was positioned so that the wall of the deep-drawn feature was aligned with a vertical die wall near the fillet between the two depth levels (Fig. 1c).

[picture]

Fig. 2. Illustration of (a) Deep-Drawing feature dimensions and (b) geometry of the die cavity

Fig. 2. Illustration of (a) Deep-Drawing feature dimensions and (b) geometry of the die cavity

To ensure the effectiveness of each approach, the performance of the three strategies was evaluated against the shape reference condition (indicated as SR) and defined as the geometry obtained by hydroforming when starting from an initial undeformed blank. The three strategies are detailed in the following.

Reshaping by Hydroforming ( RH ): the EoL component is reshaped considering a deformation step of hydroforming at room temperature, according to the approach detailed in [12].

Reshaping by Gas Forming ( RGF ): the reshaping of deep-drawn EoL [14] is carried out considering the deformation by a pressurized gas at high temperature (higher than half of the material's melting temperature).

Reshaping by Hybrid Approach ( RHA ): an original methodology is proposed, involving an intermediate Single Point Incremental Forming (SPIF) stage followed by hydroforming. The SPIF step is intended to induce localized, progressive deformation to "prepare" the geometry, facilitating the subsequent removal of residual features during the subsequent hydroforming step.
The entire investigation is conducted exclusively within a numerical framework using the Abaqus finite element code. The analysis of the numerical results was carried out by looking at the postforming properties of the reshaped part (final shape and final thickness distribution) and comparing them with those from the reference SR case.

Reshaping by hydroforming - RH. The FE model used to simulate the HF process is detailed in [12], and it is here briefly recalled. The reshaping step was simulated sequentially after the deep drawing step, detailed in the previous section, within the same numerical model. The AA5754-H22 blanks were modelled as a deformable shell body with an initial thickness of 0.5 mm . The hydroforming die, on the other hand, was modelled as a shell rigid body. Contact between the bodies was governed by setting a friction coefficient of 0.125 [15]. The material behavior was implemented in the finite element (FE) model by integrating the flow stress curve, derived from uniaxial tensile tests performed on dog bone samples according to UNI EN ISO 6892, together with the forming limit curve (FLC) [13]. The main process parameters, linearly increased during the forming step, are listed in Table 1.

Table 1. Reshaping by hydroforming (RH): process parameters

Time [s] | Holder force [N] | Oil [MPa]
0 | 0 | 0
1 | 16500 | 0
53 | 28500 | 5.2

The hydroforming was simulated using the Abaqus/Explicit solver. To reduce the computational efficiency, the symmetry of the system was exploited by modeling only half of the domain (Fig. 3). A semi-automatic mass scaling approach was adopted, and the stable time increment was carefully selected to ensure that the system's kinetic energy remained negligible with respect to the internal energy.

[picture]

Fig. 3. FE model of the HF process

Fig. 3. FE model of the HF process

Reshaping by gas forming - RGF. For the simulation of the RGF approach, the blank deformed after the DD step was exported and reimported into the numerical model of the gas forming step, along with the final thickness distribution. On the other hand, the heterogeneous distribution of the work-hardening was not imported as an initial condition since the exposure of the blank to 400∘C was considered sufficient to anneal the heterogeneity of work-hardening distribution, as well as the residual state of stress [14].

The material behavior was modelled according to the Backofen law (Eq. 1) that correlates the equivalent state of stress with the equivalent strain rate by means of the two constants (the strength coefficient C and the strain rate sensitivity exponent m ).

σ=C·ε˙m(1)

The two constants were inversely determined from an extensive experimental characterization based on a free inflation test on 0.5 mm thick samples and carried out at 400∘C.

The preliminary experimental tests also allowed to identify the strain rate of 0.002 s−1 as the optimal condition to enhance the material formability.

[picture]

Fig. 4. (a) FE model of the GF process, (b) pressure profile for keeping the strain rate close to the optimal

Fig. 4. (a) FE model of the GF process, (b) pressure profile for keeping the strain rate close to the optimal

The modelled assembly is reported in Fig. 4 and shows some analogies with the hydroforming model. The blank is again modelled as a 3D deformable body, while keeping the rigid formulation for the die. The blank, unlike the HF process, is completely constrained at its border in order to prevent any material draw-in according to the actual conditions of the real process. From the numerical point of view, one of the biggest advantages of the numerical approach is the possibility to calculate the gas pressure profile as an output (differently from what happens for the hydroforming process, where the operator defines the oil pressure variation as an input, as also shown in Table 1). In this specific case, the optimal strain rate identified during the material characterization was imposed as a target in the numerical simulation: thanks to the availability of an internal routine, the Abaqus solver was able to calculate the gas pressure curve (an example is shown in Fig. 4b) while keeping the overall strain rate close to the target value. In order to keep the consistency in the comparison of the reshaping strategies, the post-reshaping properties from the RGF approach were extracted at the time frame when the blank shape profile (measured along the longitudinal plane of symmetry) was close to the one from the reference SR case.

Reshaping by hybrid approach - RHA. The third investigated route, RHA, involves an intermediate Single Point Incremental Forming (SPIF) aimed at reducing the geometric impact of the EoL feature and promoting a smoother removal during the subsequent hydroforming step. In accordance with the RH approach, the material behavior alloy was modelled using the flow curve and the forming limit curve.

The SPIF process was modeled using a dynamic-explicit simulation subdivided into two main steps: the clamping of the blank and the incremental forming operation. Since the tool moved along a predefined trajectory, no symmetry could be exploited, and the whole system was thus modelled. For the sake of clarity, a view cut of the single steps using the longitudinal symmetry plane is reported in Fig. 5.

[picture]

Fig. 5. Description of the SPIF and HF phases: (a) clamping of the blank, (b) SPIF process, (c) clamping of the SPIFed sample, and (d) HF process

Fig. 5. Description of the SPIF and HF phases: (a) clamping of the blank, (b) SPIF process, (c) clamping of the SPIFed sample, and (d) HF process

This process was modelled through a dynamic-explicit simulation, including a specifically designed clamping system. The intermediate phase was structured into two steps: clamping of the sheet (Fig. 5a) and deformation by SPIF (Fig. 5b). All the components of the clamping system, as well as the tool, were considered rigid bodies, whereas the blank was defined using the state variables imported from the previous deep drawing step. The lubrication condition at the tool-blank interface was modelled using a Coulomb friction coefficient of 0.05 , consistent with values reported in the literature [16], whereas the tool motion in three-dimensional space was implemented by imposing a displacement profile along the x -, y-, and z -directions, calculated in MATLAB. The intermediate

SPIF step was simulated according to different loading conditions characterized by three values of the descending angle ( α ), namely 30∘,40∘, and 50∘ (Fig. 6b) along a pyramidal toolpath. For the sake of simplicity, the following nomenclature will be adopted: RHA30, RHA40, and RHA50 for the angles 30∘,40∘, and 50∘, respectively.

[picture]

Fig. 6. Refiguration of the (a) starting blank with the feature, (b) descending angle ( α ) in the SPIF process

Fig. 6. Refiguration of the (a) starting blank with the feature, (b) descending angle ( α ) in the SPIF process

After each forming stage, from deep drawing (DD) to hydroforming (HF), a springback analysis was performed in order to account for the residual stress state induced by the preceding operations. Also in this case, the HF process was subdivided into a clamping step (Fig. 5c) and a hydroforming step (Fig. 5d), where the blank holder closing force and the applied oil pressure were modelled as in the case of the RH (see Table 1).

Result and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.135.pdf

The performance of the proposed approaches has been evaluated by comparing the final thickness distribution and the effectiveness in removing the deep-drawn feature against the reference SR case (hydroformed step starting from an undeformed flat blank). Moreover, the analysis was carried out at two different levels: initially, the three intermediate SPIF strategies were compared by looking at the blank shape and the thickness distribution along the longitudinal symmetry plane ( 0∘ ) and along a 45∘ section (corresponding to the diagonal of the EoL deep-drawn feature); always considering the same two planes, the final reshaped geometries from the three described strategies were also compared (in Fig. 7 an example is given for the sake of clarity).

[picture]

Fig.7. Visualization of the sections used for process comparison. (a) longitudinal and (b) 45∘ section of the part after the SPIF process; (c) longitudinal and (d) 45∘ section of the part obtained after the reshaping processes, along with the contour profile (grey line)

Fig.7. Visualization of the sections used for process comparison. (a) longitudinal and (b) 45 ∘ section of the part after the SPIF process; (c) longitudinal and (d) 45 ∘ section of the part obtained after the reshaping processes, along with the contour profile (grey line)

The intermediate SPIF step has been successfully simulated, and the results are depicted in Fig. 8a and Fig. 8b for the 0∘ and 45∘ sections, respectively. In these figures, the deformed profile (continuous lines) and the thickness (dotted lines) are plotted together. It emerges that the smoothest surface condition was achieved by RHA50, albeit at the expense of thickness. In fact, as is visible in the 45∘ section, the minimum thickness values of approximately 0.36 and 0.38 mm were observed near the edge of the EoL feature, corresponding to the region most affected by the tool action. This represents a thickness reduction of about 28% and 24%. For the other two configurations, the presence of the EoL feature remains evident in Figure 8, as a distinct step is still observed at 4 and 6 mm (Fig. 8a) for RHA30 and RHA40, respectively, but the thickness reduction remains limited to 14% and 6%.

[picture]

Fig. 8. Deformed profile (continuous) and thickness distribution (dot) of (a) longitudinal and (b) 45∘ section, for RHA30, RHA40, and RHA50

Fig. 8. Deformed profile (continuous) and thickness distribution (dot) of (a) longitudinal and (b) 45 ∘ section, for RHA30, RHA40, and RHA50

The deformed shapes for all investigated cases are shown below. From this comparison, it is possible to identify the RHA configuration that achieves the most effective removal of the EoL feature, which corresponds to the strategy RHA50 (Fig. 9d). Consequently, only this specific case is retained for the subsequent detailed analyses.

[picture]

Fig. 9. Comparison among the three RHA approaches: (a) RHA30, (b) RHA40, and (c) RHA50

Fig. 9. Comparison among the three RHA approaches: (a) RHA30, (b) RHA40, and (c) RHA50

The results illustrated in Fig. 10 allow us to compare the effectiveness of remodeling strategies along two critical sections, analyzing both the recovery of the geometric shape (Fig. 10a, b) and the distribution of thicknesses (Fig. 10c, d). The SR is represented by the green curve, which indicates the formation of a flat blank without EoL defects, as presented previously.

[picture]

Fig. 10. Results of: (a) deformed profile in the longitudinal section, (b) deformed profile in the 45∘ section, (c) thickness distribution in the longitudinal section, (d) thickness distribution in the 45∘ section

Fig. 10. Results of: (a) deformed profile in the longitudinal section, (b) deformed profile in the 45 ∘ section, (c) thickness distribution in the longitudinal section, (d) thickness distribution in the 45 ∘ section

The observation of the final shape along the 0∘ plane (Figure 10 a) does not allow for fully capturing the effectiveness of the process in removing the feature, since the geometric discrepancies between the cases are limited. However, extremely interesting results emerge from the thickness analysis (Figure 10c). The SR (green curve) shows thickness values not far from the RHA and RH approaches. In these cases, the presence of the pre-existing feature combined with the material drawin during the hydroforming led to a less severe thinning than in the SR case.

Specifically, along the highlighted plane, low thinning is shown, with values around 0.45 mm (reduction of approximately 10% ). In contrast, the GF process already shows more marked thinning levels, reaching approximately 0.38 mm ( 24% reduction) at the edges of the feature, which is expected due to the type of deformation induced during the process.

Section along the 45∘ direction (Fig. 10b, d) is crucial for evaluating the surface quality and the extent of the contact between the blank and the die wall. In the reference condition (SR), the step is clearly visible and represents the correct contact of the material with the wall. A similar profile, although very jagged, is observed in the RHA and RH cases, a phenomenon justified by the sliding of the material towards the deepest step area of the die.

The RH approach (blue curve) exhibits slight limitations in removing the EoL feature; specifically, between 100 and 120 mm of horizontal distance, the profile shows reduced smoothness. This smoothness improves with the addition of RHA50 (red curve). In this case, however, the minimum thickness drops sharply to 0.36 mm , corresponding to a 28% reduction. RGF stands out clearly: by not allowing the material to draw in due to the constraint conditions, the process operates by pure
stretching (Fig. 10b, purple curve). As a consequence, the contact between the blank and the die wall is not as evident as in the other strategies (where the material draw-in helps in this regard). The analysis of the thinning distribution along the 45∘ direction reveals significantly different profiles across the investigated strategies. While RH shows a thickness distribution comparable to the SR case, the RGF process clearly presents the most unfavorable conditions in terms of average thickness.

In contrast, the RHA approach, although more effective than RH for geometric recovery, shows the most marked localized thinning and the highest degree of inhomogeneity. In this case, although the central zone keeps values around 0.45 mm , sharp drops in thickness are observed in the transition zones. Specifically, the thickness reaches minimum values of approximately 0.35 mm near the second step of the die (at a horizontal distance of 110 mm , red curve). This localized thinning is the result of the strain accumulated by the preliminary SPIF phase, followed by the final hydroforming pressure. Fig. 11 shows an overall comparison of the three investigated reshaping strategies against the reference condition.

[picture]

Fig. 11. Overall comparison among the reshaping strategies investigated

Fig. 11. Overall comparison among the reshaping strategies investigated

Conclusions.

The original version of this paper is available on https://www.scientific.net/KEM.1051.135.pdf

In this work, three distinct reshaping alternatives - Reshaping by hydroforming (RH), Reshaping by gas forming (RGF), and the Reshaping by hybrid approach (RHA) - were numerically applied to an AA5754-H22 End-of-Life (EoL) component to investigate their feasibility in removing residual geometric features. Based on the numerical results, the following conclusions can be drawn:

The research provided a rigorous comparison between RH, RHA, and RGF, benchmarking their performance against the Shape Reference (SR). While the longitudinal plane ( 0∘ ) did not show a clear distinction in feature removal capability, the thickness analysis revealed that the preexisting EoL feature at the die bottom facilitates the new deformation, limiting thinning for cold processes to approximately 10% compared to the SR case.

RH showed low capability in removing the EoL feature, leaving a geometric step in the 45∘ section between 100 and 120 mm of horizontal distance. However, this process ensured a thickness distribution very close to the SR target.

The intermediate SPIF phase in the RHA route demonstrated a significant ability to modify the EoL geometry, with surface smoothness improving as the descending angle ( α ) increased to 50∘. The material flow associated with this improvement resulted in a more jagged profile and severe localized thinning, reaching approximately 0.36 mm (a 28% reduction) in the 45∘ section.

RGF ensured more effective feature suppression compared to the other routes: by operating primarily through stretching due to the absence of material draw-in, RGF recorded the most critical thinning levels, with minimum values of 0.35 mm ( 30% ) and 0.38 mm ( 24% ) in the 45∘ and longitudinal sections, respectively.

It is worth mentioning that a unique thinning threshold cannot be defined, as allowable thinning levels strongly depend on the intended second-life application and the functional requirements of the reshaped component. The investigated case study was more focussed on demonstrating the effectiveness of the reshaping approach when the starting EoL part is characterized by a pronounced heterogeneity in its properties (strain and work-hardening distribution).

Further experimental campaigns will be conducted to validate the numerical results of the different approaches. Moreover, newly optimized tool paths will be investigated to improve feature removal and to mitigate the thinning phenomenon in the intermediate SPIF process.

Acknowledgement

The original version of this paper is available on https://www.scientific.net/KEM.1051.135.pdf

This work was carried out within the framework of the project "Going Beyond Recycling: Reshaping of End-of-Life Components through Advanced Sheet Metal Forming Processes", funded under the PRIN 2022 Call (MUR Decree No. 1401 of 18/09/2024 - CUP I53C24002500006).

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.135.pdf

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Domitner J, Silvayeh Z, Shafiee Sabet A, Öksüz KI, Pelcastre L, Hardell J. Characterization of wear and friction between tool steel and aluminum alloys in sheet forming at room temperature. J Manuf Process 2021;64:774-84. .

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 147-154

doi: 10.4028/p-nN14jh

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-19

Revised:
2026-02-16

Accepted:
2026-02-17

Online:
2026-04-17

Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

The original version of this paper is available on https://www.scientific.net/KEM.1051.147.pdf

Steffen Gabsa 1,a*, Werner Homberg 1, b

1 Forming and Machining Technology, Paderborn University, Warburger Straße 100, 33098

Paderborn, Germany

a* sg@luf.upb.de, b wh@luf.upb.de

*corresponding author

Keywords: aluminum, mechanical properties, recycling, texture.

Abstract

The utilisation of friction-induced solid-state recycling, methodically adapted to the CoNform process, facilitates the continuous production of semi-finished products. The material intended for recycling is conveyed continuously via a rotating wheel. The volume flow is influenced by fixed surfaces, deflections, and constrictions, thereby creating an asymmetrical flow profile. In order to effect a change in the mechanical properties of the semi-finished product, the material fed into the process can be modified. This enables the amalgamation of two alloys or the direct transition between them. The inhomogeneous flow conditions present within the tool give rise to the mixing of materials, thereby creating a graded multi-material zone. The multi-material zone was divided into different areas and traced back to the process conditions. Within the transitions, the connections between the alloys were examined, as well as the influence on the boundary layer. Material properties were determined for the individual areas and located along the length of the profile.

Introduction

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In order to produce sustainable products, it is imperative to consider the entire product cycle and product design. Key aspects include minimizing energy consumption and resource-saving approaches [1]. Recycling of aluminum, in particular, has been demonstrated to offer significant savings potential and the potential for a circular economy. The fact that solid-state recycling does not involve melting of the raw material opens up new possibilities. These processes form press-welded bonds between the parts. The scrap is plastically deformed so that the insulating oxide layers on the surface break up and the bare aluminum surfaces can bond under pressure [2]. These bonds form at a lower energy level than conventional melting of the material. Energy analyses show that solid-state recycling of aluminum has the potential to save energy up to 90% compared to melting [3]. In addition, waste products such as toxic salt slag are eliminated, enabling waste-free recycling. Processing steps required to remove these side products are no longer necessary [4].
Maintaining the metallic bond in the crystalline structure enables new approaches because different phases can coexist. [5]. This phenomenon allows the targeted adjustment of material properties by mixing different alloys without having to specifically measure and balance the chemical composition [6]. Based on the initial strength of the materials to be mixed, a mass ratio can be determined by interpolation, which can be used to achieve individual strengths. On the other hand, foreign objects are also embedded in the structure. This can lead to defects that weaken the structure of the recycled component [7]. Consequently, particularly high demands must be placed on the purity of the starting material.
In lightweight construction in particular, the combination of different materials is specifically used to provide a customized structure for different load ranges. Such combinations not only reduce emissions during production, but also throughout the entire product cycle of the component. A prominent illustration is found in the field of lightweight construction, where the utilization of diverse materials is employed to counteract the effects of stress variations [1]. For dissimilar materials, an auxiliary joining element is mostly necessary, as material cohesion is often not possible. In these cases, overlaps are often necessary and the flow of force is directed to local areas by the auxiliary
joining element [8]. If the materials of the elements to be joined are similar, in many cases it is possible to join the components by means of material bonding in a single joining step [9]. With the objective of reducing the number of manufacturing steps, efforts are being made to integrate the joining step directly into the shaping process for similar structures. Initial investigations have been conducted in solid-state recycling with targeted spatial arrangement of the chips to produce components with alloy transitions [10]. The investigations show that the process geometry like rejuvenation and temperature have an influence on product that needs to be investigated in detail. In addition, critical zones arise in the transition area of two materials where the process parameters need to be adjusted.
Another conceivable scenario is to change alloys during continuous processes. In this case, the old material is transported out of the system by the new material. This avoids long interruptions during which the machine has to be cleaned. Changes in the ongoing process are therefore particularly interesting when only small quantities are required, such as for special alloys or special profile shapes [11]. In the transition area, there is a zone where the two alloys mix to a multi-material zone. In these Muli-material zones, the material properties are not precisely defined and are therefore not suitable for special applications. Here, the geometry and kinetics of the process have a particular influence that must be taken into account in order to estimate the length of this zone [12].
This emphasizes the importance of considering material transitions to adapt the properties of the component as early as possible in the process. Integrating specific alloy changes can eliminate subsequent joining steps and reduce process times. Thoroughly assessing the alloy change during the ongoing process minimizes setup times and heating phases. Thus, comprehensive knowledge of alloy changes within the process enables efficient and sustainable design. This paper examines the transitions between two alloys during friction-induced recycling to identify the factors that influence them and their properties. This will enable a better evaluation of material transitions in continuous processes.

Methods and Materials

The original version of this paper is available on https://www.scientific.net/KEM.1051.147.pdf

Friction-induced recycling, shown in Figure 1, is a continuous process that has been methodically adapted from the CoNform process. In this process, aluminum chips are fed evenly into the process over the groove of a rotating wheel from a temporary storage box via a conveyor. The wheel rotates at 11 revolutions per minute, powered by an electric motor. Friction between the groove in the wheel and the chips creates the necessary pressure for the process and transports the material forward. A tool insert continuously narrows the groove by compacting and shearing the chips. After further transport of the material by 90∘, the material is dammed up and redirected into the horizontal plane. The pressure generated by friction forces pushes the material through a shaping die with a diameter of 5 mm . Turbulence and uneven flow are present within the tools. This results due to friction and deflection of the tools. The energy applied to the mass flow via friction counteracts the deformation of the material and the friction encountered on other tool surfaces. The mechanisms in the channel can be divided into three zones. First, the chips accumulate in the filling zone. They have not changed since their initial state and are not yet bonded to each other. This is the area where the gray aluminum connects with the wheel. The friction on the wheel transfers them to the yellow/orange gripping zone, where they are plasticized and compressed. In the last zone, the flow zone shown in red, the material is compacted to such an extent that there are no more air inclusions. In this zone, the material is largely homogenized. After the deflection, the channel is narrowed once again before the shaping die. The walls of this die also have retarding friction conditions, which means that not only the shape but also the channel length is an important influencing factor. Among other things, the pressure in the system and the temperature can be influenced. For the material flow it means, that this creates a leading core in the middle of the cross-section, which is slightly shifted upwards due to the deflection in the die-transition.

[picture]

Fig. 1. Test setup for friction-induced recycling in two views. a) Left view: longitudinal section across the groove base area; right view: cross-section showing the groove narrowing and material flow. b) View of the tool with temperature measurement points.

Fig. 1. Test setup for friction-induced recycling in two views. a) Left view: longitudinal section across the groove base area; right view: cross-section showing the groove narrowing and material flow. b) View of the tool with temperature measurement points.

A groove with a depth of t=10 mm and a width of w=12 mm is cut into the wheel. This is the initial cross-section into which the scrap is fed. The chips are narrowed to a gap of 4 mm using the tool insert. Dry milled chips of EN AW-6060 and EN AW-7075 alloys were processed. The geometric dimensions and bulk density of the chips are listed in Table 1. Between transitions, a filling of the intermediate container with a volume of 880 cm3 was processed. After a starting phase, the alloys were fed in alternated eight times under stable process conditions.

Table 1. Dimensions of the aluminum chips used.

Chip | length [mm] | width [mm] | thickness [mm] | Bulk density [g/cm³]
EN AW-6060 | 1.9 ± 0.69 | 1.44 ± 0.25 | 0.14 ± 0.05 | 0.38 ± 0.01
EN AW-7075 | 4.04 ± 0.66 | 1.67 ± 0.24 | 0.04 ± 0.01 | 0.13 ± 0.01

During the process, the temperature is recorded tactilely at two points on the tool insert. The measurement is carried out using a K-type thermocouple (G/G-24KK-IEC) from Therma Thermofühler GmbH, Lindlar in Germany. A tensile-compression testing machine Z100 from ZwickRoell GmbH & Co. KG, Ulm in Germany is used to determine the tensile strength. The test is carried out in accordance with DIN 50125 using form F. The hardness test was carried out using a NEXUS 4000 from INNOVATEST Europe BV in Maastricht, the Netherlands. The test was performed according to DIN EN ISO 6507 using the Vickers method with a test force of 3,942 N (HV0.3). The micrographs of the samples are etched with an electrolyte A2 for 100 s at a voltage of 20 V with the LectroPol-5 Polishing Unit electropolishing device from Struers, Copenhagen in Denmark. Grain sizes were evaluated in accordance with DIN EN ISO 643. The grain image is viewed on a Smartzoom 5 from the manufacturer Carl Zeiss IQS Deutschland GmbH, Oberkochen in Germany. Under this, the texture of polished samples was recorded across the longitudinal cross-section. The different alloy components could be evaluated using different gray scales.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.147.pdf

The multi-material zones can be divided into different characteristic areas based on the change in the alloys added. From the moment a new alloy is added to the system, there is initially one area consisting of the pure predecessor alloy (area 1 in Figure 2). After this, the new alloy is initially added in small quantities in the middle. This is followed by a broadly multi-material area in which both alloys are present in high proportions (area 2). Before the new alloy is present in its pure form, there
is another area in which larger volumes of the predecessor alloy occur in sections (area 3). These are attached to the tool and thus indicate the end of the multi-material zone. Based on this multi-material, the general mass flow in friction-induced recycling is also directly visible. In a thin outer layer, feedstock is retained over a longer distance by high friction on the shaping die surface. In addition, there is a parabolic distribution, which is shifted upwards by the deflection of the mass flow. This means that the new material flows into the upper part of the strand more quickly. The greatest distance is travelled in the area where the flow is deflected over the outer curve. This leads to high frictional resistance and, consequently, a slow flow velocity.

[picture]

Fig. 2. Longitudinal section of a profile from the transition from EN AW-7075 (left) to EN AW-6060, classified into three proportions of alloy mixtures. area 1) pure predecessor alloy. area 2) high multi-material area of the two alloys. area 3) with small admixture of the previous alloy.

Fig. 2. Longitudinal section of a profile from the transition from EN AW-7075 (left) to EN AW-6060, classified into three proportions of alloy mixtures. area 1) pure predecessor alloy. area 2) high multi-material area of the two alloys. area 3) with small admixture of the previous alloy.

In production, there are no significant differences in the process parameters for the alloys processed. The temperature when processing the multi-material zones from EN AW-7075 to EN AW-6060 is around 440∘C. During manual alloy changes, when the new alloy is poured into the crucible, the temperature drops briefly ( ~20 K ) and then rises again to the process temperature when the next alloy is added. When changing alloys, a mark was made at the die exit from which the position in the profile at the time of the change can be determined. With the material in the die and the material in the wheel groove, a short section with the old material is still conveyed after the change until the first admixtures of the new material are present in the middle of the profile. Table 2 shows that this area has the lowest standard deviation in comparison to all multi-material areas. Based on this length, indirect conclusions can be drawn about the zones in contact with the wheel. The largest area is measured with the last one. In addition, not only the largest standard deviation is considered here, but also the most irregular structure. This results in an inhomogeneous length of approx. 168 mm , which is calculated from the difference between the minimum and maximum values of the first area added to the maximum values from areas two and three.

Table 2. Statistical lengths of the individual areas in the multi-material over eight material alternations.

| area 1 | area 2 | area 3
mean [mm] | 26.4 | 45.4 | 79.5
max. [mm] | 32 | 55 | 103
min. [mm] | 22 | 32.7 | 45.6
standard deviation [mm] | 3.5 | 7.9 | 30.1

In the area 2 of the multi-material zones, there are high changes of alloy section length. In the middle of the profile, the thickness of the sections is in the range of a few tenths of a millimeter and is stretched towards the edge to a thickness of one hundredth of a millimeter. Area three is characterized by thicker admixtures with dimensions in the millimeter range, which thin out towards the edge. In the edge layers, which extend over large parts of the second area due to the friction conditions, a distinction must be made between the alloys. The softer EN AW-6060 can be thinned out considerably, with the thickness decreasing to 80μ m. A thicker edge layer forms with EN AW-7075, which is almost constant at 0.3 mm .
When switching from EN AW-7075 with high strength to EN AW-6060 with lower strength, tearing in the surface of the edge layer can be observed. This is due to different flow behavior, which can lead to material failure at the end of the tool. At the tool contact point, friction generates high shear stresses, which slow material flow. If softer material components are present in the layers near the edge, the harder components shear them more intensely due to the faster flowing core. This leads to additional stresses besides surface friction due to the structural ductility of the edge layers. Consequently, higher stresses arise locally due to velocity differences than in a homogeneous case. To mitigate this effect, it is conceivable to create transitions that are deliberately drawn out longer by making the change gradual rather than abrupt, with mixed chips. If the transition is reversed, the difference in flow behavior is not critical for the softer EN AW-6060 material. Tensile tests show that stress peaks can form at locations with surface defects in the presence of small amounts of EN AW6060. This can lead to premature failure. Significantly restrict the plastic behavior of the material. An example of this is shown in Figure 3, where failure occurs up to 50% earlier and without necking. However, in general, failure can be observed in areas with low strength. Despite different local material properties and thus interfacial stresses, the areas with the multi-material do not represent a particular weak point. In terms of flow behavior, the multi-material results in a gradient that depends on the mass fractions. It can be seen that in areas where there is no significant mixing, larger proportions of the stronger alloy (EN AW-7075) take on a load-bearing role and weak points form where failure occurs. These areas, which are much softer locally, cannot be calculated precisely.

[picture]

Fig. 3. left) Longitudinal section of tensile test specimens outlined with legend color; top: Premature failure due to defects in the outer surface and the initial addition of EN AW-6060 to the EN AW-7075 base alloy; bottom: Necking before a larger addition of EN AW-7075 as a stiffer alloy. right) Stress curves in the multi-material zone in the area between the pure alloys.

Fig. 3. left) Longitudinal section of tensile test specimens outlined with legend color; top: Premature failure due to defects in the outer surface and the initial addition of EN AW-6060 to the EN AW-7075 base alloy; bottom: Necking before a larger addition of EN AW-7075 as a stiffer alloy. right) Stress curves in the multi-material zone in the area between the pure alloys.

Significant differences are apparent in the hardness measurement. In areas with a large number of fine transitions, the average hardness is 59±12.5HV0.3. Similar to tensile strength, a mixed value of the two basic strengths is evident here, with a high degree of variability, as shown in Figure 4. In the pure EN AW-7075 area, a hardness of 86±1.2HV0.3 was measured, and in the EN AW-6060 areas, 39±1.8HV0.3. As the multi-material zone progresses, the dispersion of the hardness values decreases and the values approach the hardness of the pure alloy. However, there are significant large local outliers in the admixed material. Individual points that do not tend toward a pure alloy are located in areas of transition with different alloy layers. In these areas, adjacent alloy layers provide support by bearing part of the load.

[picture]

Fig. 4. Measured hardness with assignment to profile position, high proportion of alloy on the left side, the right-hand measuring points are in an area with mixed material properties Alloys.

Fig. 4. Measured hardness with assignment to profile position, high proportion of alloy on the left side, the right-hand measuring points are in an area with mixed material properties Alloys.

Mean grain diameters were calculated to indirectly evaluate the deformation ratio of the different alloys interacting with each other. In the pure alloy ranges, the mean grain diameter was determined to be 4μ m for the harder alloy EN AW-7075 and 11μ m for the softer alloy EN AW-6060. When different alloy proportions are present side by side in the strand, the grain sizes change depending on the ratio of the alloy. In this combination, the softer alloy, EN AW-6060, will undergo greater plastic distortion, while the stronger alloy, EN AW-7075, will undergo lower elongation when pushed through. Thus, the mean grain diameter of EN AW-7075 can increase to 16μ m in areas with a high proportion of alloy change, while the mean grain diameter of EN AW-6060 decreases to 6μ m at this ratio. The greatest grain refinement occurs when EN AW-6060 is surrounded by a high proportion of EN AW-7075. In this case, the grain size of EN AW-6060 decreases by 4μ m compared to EN AW7075 , which has a grain size of 8μ m. Area three, where only individual particles of the other alloy are mixed into the strand, must be evaluated separately. These admixtures, shown in Figure 5 on the right, detach from the tool contact and are exposed to high shear forces due to surface friction. This results in grain refinement. Consequently, even when the solid alloy EN AW-7075 is embedded in the softer EN AW-6060, the mean grain diameter can be as small as 5μ m.

[picture]

Fig. 5. Left) Microstructure of area 2 in a transition from EW AW-6060 to EN AW-7075 with a high alloy content change. right) AW-7075 piece embedded in EN AW-6060 in area 3.

Fig. 5. Left) Microstructure of area 2 in a transition from EW AW-6060 to EN AW-7075 with a high alloy content change. right) AW-7075 piece embedded in EN AW-6060 in area 3.

The multi-material zone is an extremely complex area dependent on base materials and their mixtures. Approximately predicting these based on process conditions allows one to draw conclusions about material quality.

Summary

The original version of this paper is available on https://www.scientific.net/KEM.1051.147.pdf

The investigations show that alloy changes are possible during the running process in friction-induced recycling. The resulting multi-material area can be divided into three areas from the moment another alloy is added to the plant. First, a short piece of the old pure alloy is extruded. This is followed by a section in which the alloys are finely distributed. The third stage is characterized by local admixtures of the old alloy. Defects occurred in the cladding area during the transition from the harder EN AW-7075 to the softer EN AW-6060 alloy. This is due to high material stress caused by surface friction and additional stress differences resulting from an inhomogeneous alloy distribution. The process management must be adjusted to prevent critical shifts. The strengths in finely mixed areas result from a ratio of the two starting materials. In larger areas of an alloy, its properties are dominant. The elongation of the material depends on the strength ratio of the alloys. The stronger alloy shears the softer alloy more strongly during the flow process. This results in increased grain refinement in the soft areas. In contrast, the harder alloy deforms to a lesser extent, resulting in a coarser grain structure. These visible flow differences confirm the stress differences in the boundary layers and demonstrate the significant influence of the alloy ratio on inhomogeneous structure formation and system strength.

Acknowledgent

The original version of this paper is available on https://www.scientific.net/KEM.1051.147.pdf

The authors would like to thank the German Research Foundation (DFG) for funding the research project with the project number 500288680: "Influence of the surface chemistry of aluminum chips on a friction-induced recycling process for the production of aluminum wires".

References

The original version of this paper is available on https://www.scientific.net/KEM.1051.147.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 155-164

doi: 10.4028/p-WGyA0X

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-19

Revised:
2026-02-16

Accepted:
2026-02-17

Online:
2026-04-17

Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

The original version of this paper is available on https://www.scientific.net/KEM.1051.155.pdf

Mauro Carta 1,a*, Pasquale Buonadonna 1, b, Sergio D'Angelo 2,c, Paolo De Berardinis 2, d, Gabriele Derosas 1,e and Mohamad El Mehtedi 1,f

1 Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, 09123

Cagliari, Italy

2 Metallurgica Abruzzese Spa, Contrada Marina, 64023 Mosciano Sant Angelo (TE), Italy

a  mauro.carta94@unica.it, b  pasquale.buonadonna@unica.it, c  s.dangelo@cavatorta.it, d p.deberardinis@cavatorta.it, e g.derosas4@studenti.unica.it, e m.elmehtedi@unica.it

* corresponding

Keywords: Life Cycle Assessment (LCA); Industrial galvanizing; Steel wire production; Zn-Al coating; Energy demand; Process sustainability.

Abstract

Hot-dip galvanizing (HDG) is a widely adopted industrial process for enhancing the corrosion resistance and service life of steel products; however, it is also characterized by high energy and material consumption. In this study, a process-oriented Life Cycle Assessment (LCA) is applied to compare the environmental performance of two industrial steel wire coating routes: conventional hot-dip zinc ( Zn ) coating and zinc-aluminum ( Zn−Al ) coating. The analysis is based on primary data collected from an industrial galvanizing line operated by Metallurgica Abruzzese S.p.A. (Italy) and focuses exclusively on the manufacturing stage, using a gate-to-gate approach. The system boundary includes surface preparation, thermo-metallurgical coating treatment-comprising induction annealing, hot-dip galvanizing and, for the Zn−Al route, an additional molten Zn−Al bath-followed by wire cooling and final handling operations. Results show that the Zn−Al coating route leads to a significantly higher environmental impact at the manufacturing stage, with an approximately 44% higher GWP100 compared to conventional Zn coating. Contribution analysis reveals that this increase is primarily driven by the additional thermometallurgical coating step, which entails higher material input and thermal energy consumption, rather than by aluminum content alone. The findings highlight the dominant role of material selection and thermal process management in determining the environmental performance of industrial galvanizing lines.

Introduction

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Steel remains the cornerstone of modern industrial manufacturing and an indispensable material for sectors such as automotive, construction, and household appliances due to its high strength and durability [1]. In recent years, the demand for steel has grown rapidly [2]. At the same time, the iron and steel industry ranks among the world's most carbon-intensive industrial sectors, alongside cement production [3]. Despite its widespread use, steel remains highly susceptible to corrosion when exposed to diverse service environments, resulting in socio-economic losses estimated at approximately 3.4% of the global Gross Domestic Product (GDP) [4].
To protect steel and extend its service life, a wide range of anticorrosion technologies has been developed, including electroplating, painting, thermal spraying, surface chemical treatments and physical vapor deposition. These approaches mainly rely on the application of protective surface coatings, such as Zn-based alloys or polymeric systems to shield the steel substrate and improve durability [5-7]. Among these techniques, hot-dip galvanizing (HDG) remains one of the most established and reliable solutions for extending the service life of steel products [8]. By creating a metallurgical bond between the zinc and the steel, HDG provides both a physical barrier and cathodic protection, effectively increasing the lifespan of structures by several decades [9]. Nevertheless, HDG is associated with substantial energy consumption, intensive use of raw materials, and the generation
of emissions and process residues, including spent pickling acids and metallic wastes, which are raising growing environmental concerns [10,11].
In response, the development of Zn−Al alloy coatings over the recent decades has been driven by the need to improve corrosion resistance and overall coating performance. By combining zind's sacrificial protection with aluminum's superior passivation properties, coatings such as Zn−5%Al (Galfan) and Zn−55%Al (Galvalume) have demonstrated a significantly extended the service life of steel compared to conventional pure zinc coatings, while also allowing a reduction in coating weight [12].
While Zn-Al alloy coatings offer clear technical advantages in terms of corrosion resistance, their environmental burden must be rigorously evaluated through Life Cycle Assessment (LCA). Previous LCA studies on hot-dip galvanizing (HDG) processes have highlighted the significance of energy consumption and primary zinc production as major contributors to environmental impacts. For instance, Arguillarena et al. conducted a comprehensive cradle-to-gate LCA of industrial HDG plants in Spain, showing that steel and zinc production dominate most impact categories, whereas the galvanizing stage mainly influences toxicity-related indicators [13]. In a complementary study, the same authors focused specifically on the carbon footprint of HDG installations, demonstrating that upstream processes-particularly primary zinc and steel production-account for the majority of greenhouse gas emissions, while direct process emissions play a secondary role [14].
This paper presents a comprehensive comparative LCA of industrial steel wire coating routes. Unlike previous studies based on laboratory-scale data, our analysis utilizes primary data collected from an industrial galvanizing line operated by Metallurgica Abruzzese S.p.A. Using a gate-to-gate system boundary approach, we compare the environmental performance of conventional hot-dip zinc ( Zn ) coating with that of the zinc-aluminum ( Zn−Al ) route. By focusing on the manufacturing stageincluding surface preparation, induction annealing, and thermo-metallurgical treatments-the study aims to identify the key process drivers that determine the environmental footprint of highperformance steel wire production.

The Hot Dip Galvanization Process: The Industrial Line

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As introduced earlier, Hot-dip galvanization is a continuous industrial process used to apply a zincbased metallic coating from corrosion protection of steel wire.
In the investigated industrial line (Metallurgica Abruzzese S.p.A., Italy), the process begins with surface preparation, which includes cleaning and acid pickling, to remove contaminants and oxides from the steel surface. The wire then rinsed and cleaned before undergoing induction annealing to stabilize the microstructure and ensure suitable mechanical properties prior to coating. Subsequently, the steel wire is immersed in a molten zinc bath or, in the case for the Zn−Al route (figure 1), in an additional molten Zn−Al bath (figure 2), where metallurgical bonding between the coating and the steel substrate occurs. After coating, the wire undergoes controlled cooling to stabilize the coating layer, followed by final handling operations such as rewinding and packaging. The following section details the LCA methodology applied in this study.

[picture]

Fig. 1. Process flow and system boundary of the industrial steel wire galvanizing line ( Zn only).

Fig. 1. Process flow and system boundary of the industrial steel wire galvanizing line ( Zn only).

Life Cycle Assessment (LCA) Methodology

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Life Cycle Assessment (LCA) is a standardized methodology for evaluating the environmental impacts associated with a product, process, or service by systematically accounting for material and energy flows and associated emissions over its life cycle. Defined by the ISO 14040 and 14044 standards, LCA is widely applied in industrial contexts to identify environmental hotspots and to support process optimization and compare technological alternatives. According to these standards, LCA is structured into four main phases: (i) goal and scope definition, (ii) life cycle inventory analysis, (iii) life cycle impact assessment, and (iv) interpretation.
In this study, the LCA focuses on a hot-dip galvanizing process based on a production line operated by Metallurgica Abruzzese S.p.A. Inventory data were collected from on-site measurements and production records to ensure the analysis reflects real-world industrial operating conditions rather than laboratory-scale or simulated scenarios.

Goal and scope definition

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The Life Cycle Assessment (LCA) is conducted using a gate-to-gate approach and focuses exclusively on the manufacturing stage of the steel wire galvanizing process. The system boundary includes surface preparation, the thermo-metallurgical coating stage, which includes induction annealing, hot-dip galvanizing and, in the case of Zn−Al coated wire, immersion in an additional molten Zn−Al bath. Wire cooling and final handling operations complete the defined boundary (see Figures 1 and 2).
Upstream processes (e.g., steel wire production), the product use phase, and the end-of-life of the coated product are excluded from the assessment. The functional unit is defined as 1 ton of finished galvanized steel wire, ready for dispatch. Primary data were collected from an industrial galvanizing line operating under representative conditions during the reference year.
Environmental impacts of these two processes were estimated using Impact 2002+ assessment method, with results reported for the Global Warming Potential over a 100 -year time horizon (GWP100) and Cumulative Energy Demand (CED).

[picture]

Fig. 2. Process flow and system boundary of the industrial steel wire galvanizing line, including the molten Zn−Al coating stage.

Fig. 2. Process flow and system boundary of the industrial steel wire galvanizing line, including the molten Z n − A l coating stage.

Table 1 Life Cycle Inventory (LCI) of the Zn hot-dip galvanizing process (per 1 t of finished galvanized wire).

Description | Quantity | Unit | Flow type
Output
Galvanized steel wire | 1,000 | kg | Product
Material inputs
Steel wire input to galvanizing | 1,000 | kg | Material
Metallic coating alloy (Zn) | 33.9 | kg | Material
Acid solution for pickling | 13.25 | kg | Material
Sodium hydroxide for emission treatment (E15) | 1.75 | kg | Material
Lime for wastewater treatment | 3.64 | kg | Material
Technical water for washing and cooling | 500 | kg | Material
Inert gas for controlled atmosphere (market supply) | 33.73 | kg | Material
Nitrogen produced on-site via PSA technology | 11.85 | kg | Material
Energy inputs
Electricity consumption (total) | 368.5 | kWh | Energy
Thermal energy from natural gas | 500 | kWh | Thermal energy
Transport
Internal handling (diesel vehicle EURO 5) | 0.1667 | t·km | Transport
Outputs (emissions and wastes)
Fine particulate matter (PM < 2.5 μm) | 0.005 | kg | Direct emission
Sludge from wastewater treatment and solid residues | 6.92 | kg | Non-hazardous waste
Spent acidic solutions from pickling | 3.91 | kg | Non-hazardous waste
Metallic dross from galvanizing bath | 15.41 | kg | Waste for recovery
Minor metallic scrap after galvanizing | 0.9 | kg | Waste for recovery

Life Cycle Inventory (LCI) analysis

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The Life Cycle Inventory (LCI) analysis was carried out by quantifying all relevant material and energy flows associated with the manufacturing stage of the steel wire galvanizing process, in accordance with the system boundary defined in the goal and scope. The inventory includes inputs of raw materials, auxiliary chemicals, energy carriers and utilities, as well as outputs in the form of emissions and solid and liquid wastes generated during the galvanizing operations.
As already mentioned, the industrial system was modelled by focusing exclusively on the galvanizing line, starting from the incoming steel wire and excluding upstream wire drawing operations. The process was represented in SimaPro software as a series of interconnected unit processes representing the main manufacturing steps. For the Zn−Al coating route, the thermo-metallurgical coating stage incorporates an additional molten Zn−Al bath.
Detailed life cycle inventory data for the Zn and 1,000 kg hot-dip galvanizing processes are reported in Table 1 and Table 2, respectively.

Table 2. Life Cycle Inventory (LCI) of the Zn−Al hot-dip galvanizing process
(per 1 t of finished Zn−Al coated wire).

Flow | Quantity | Unit | Type
Output
Galvanized steel wire (Zn–Al) | 1,000 | kg | Product
Material inputs
Steel wire input to galvanizing | 1,000 | kg | Material
Metallic coating alloy (Zn) | 33.9 | kg | Material
Zn coating material | 31.59 | kg | Material
Al coating material | 3.51 | kg | Material
Acid solution for pickling (HCl) | 13.25 | kg | Material
Sodium hydroxide (emission treatment) | 1.75 | kg | Material
Lime for wastewater treatment | 3.64 | kg | Material
Technical water for washing and cooling | 500 | kg | Material
Inert gas (market supply) | 33.73 | kg | Material
Nitrogen produced on-site (PSA) | 11.85 | kg | Material
Energy inputs
Electricity (total) | 368.5 | kWh | Energy
Thermal energy from natural gas | 658 | kWh | Thermal energy
Transport
Internal handling (diesel, EURO 5) | 0.1667 | t·km | Transport
Outputs (emissions and wastes)
Fine particulate matter (PM < 2.5 μm) | 0.005 | kg | Direct emission
Sludge from wastewater treatment | 6.92 | kg | Non-hazardous waste
Spent pickling solutions | 3.91 | kg | Non-hazardous waste
Zinc dross from galvanizing bath | 15.41 | kg | Waste for recovery
Zn–Al dross from coating bath | 29.2 | kg | Waste for recovery
Minor metallic scrap | 0.9 | kg | Waste for recovery

Primary data were collected directly from the industrial plant for the reference year and were normalised to the functional unit of 1 t of finished galvanized wire. For both coating routes, the principal material input is the steel wire ( Zn−Al ). The metallic coating consumption is 33.9 kg of Zn for the conventional galvanizing route (Table 1) and 35.1 kg of alloy for the Zn−Al route, consisting of 31.6 kg of Zn and 3.5 kg of Al (Table 2). Surface preparation requires 13.25 kg of acid solution for pickling, supplemented by auxiliary chemicals such as sodium hydroxide ( 1.75 kg ) for emission treatment and lime ( 3.64 kg ) for wastewater neutralisation. Technical water consumption for washing and cooling operations is approximately 500 kg per ton of product for both configurations.
Energy inputs are dominated by electricity and thermal energy demand. Electricity consumption is comparable for the two routes at 368.5 kWh per ton of galvanized wire, mainly associated with induction heating, line operation and auxiliary equipment. In contrast, the (658kWh coating route
requires more thermal energy from natural gas Zn−Al versus 500 kWh for the conventional Zn route), reflecting the additional energy needed to maintain the molten PM<2.5μ m bath. Controlled atmosphere conditions are ensured using a combination of commercially supplied inert gas ( 33.7 kg ) and on-site nitrogen generated via pressure swing adsorption (PSA) technology ( 11.9 kg ).
Outputs include direct air emissions and solid and liquid wastes generated during galvanizing. Fine particulate matter emissions ( Zn−Al ) were estimated at 0.005 kg per ton of product for both routes. Waste streams mainly consist of sludge from wastewater treatment ( 6.92 kg ), spent pickling solutions ( 3.91 kg ), and metallic residues from the coating baths. In particular, zinc dross from the galvanizing bath amounts to 15.41 kg in both configurations, while the 0.1667t·km route additionally generates 29.2 kg of Zn-Al dross. Minor metallic scrap ( 0.9 kg ) and all metallic residues are assumed to be sent to recovery. Internal material handling, performed by a diesel-powered EURO 5 vehicle, accounts for 100 per ton of product.
Background data for electricity supply, fuel production, raw material extraction and upstream chemical processes were sourced from the Ecoinvent database (version 3.11), selecting datasets representative of the European and Italian context.

Life Cycle Impact Assessment (LCIA) and results

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The Life Cycle Impact Assessment (LCIA) was carried out using the Impact 2002+ method Global Warming Potential over a 100 -year time horizon (GWP100) and Cumulative Energy Demand (CED). GWP 370.6 kgCO2 and CED are particularly relevant for assessing energy and material intensive surface treatment processes such as hot-dip galvanizing.
Figure 3a compares the GWP100 results for the two coating routes. The production of 1 ton of Zncoated steel wire results in a GWP of Zn−Al eq. In contrast, the 534.7 kgCO2 coating route shows a higher impact of 44% eq per ton, corresponding to an increase of approximately Zn−Al. This difference is mainly attributable to the additional molten Zn−Al coating stage, which leads to increased material consumption and higher thermal energy demand.

[picture]

Fig. 3. (a) Comparison of Global Warming Potential over a 100 -year time horizon (GWP100) for the Zn and 100% hot-dip galvanizing routes, expressed as 55% equivalent per ton of finished galvanized wire. (b) Relative contribution of the main IMPACT 2002+ damage categories for the two coating routes, normalised to the 30% process.

Fig. 3. (a) Comparison of Global Warming Potential over a 100 -year time horizon (GWP100) for the Zn and 100 % hot-dip galvanizing routes, expressed as 55 % equivalent per ton of finished galvanized wire. (b) Relative contribution of the main IMPACT 2002+ damage categories for the two coating routes, normalised to the 30 % process.

Figure 3b illustrates the relative contributions of the four main categories IMPACT 2002+ damage categories-human health, ecosystem quality, climate change, and resources-normalized to the ZnAl process. The analysis reveals that the 38% alloy coating consistently exhibits a higher environmental footprint than pure Zn across all categories. The most pronounced disparity is observed
in Ecosystem Quality category and Human Health, where the Zn -Al route incurs nearly double the impact of pure Zn (approximately Zn− vs. 40% ). This significant increase can be attributed to the high ecotoxicity and resource-intensive nature of aluminum extraction and its primary production, which introduces higher concentrations of heavy metals and pollutants during upstream mining and refining processes.
Furthermore, the Climate Change and Resources categories show that the Zn -Al coating results in approximately 70% and Zn−Al higher impacts, respectively, compared to the pure Zn route. This increase is likely driven by the higher energy requirements for producing the secondary aluminum alloy and the higher melting point in the galvanizing bath, which together lead to greater greenhouse gas emissions and fossil fuel consumption during the production phase.
Figure 4 provides a qualitative comparison of the environmental impact profiles for the Zn and 69−79% Al hot-dip galvanizing processes across all 15 impact categories defined by the Impact 2002+ method. For toxicity-related categories-including carcinogens, non-carcinogens, respiratory inorganics, and both aquatic and terrestrial ecotoxicity-the impacts associated with the pure Zn route range approximately between Zn−Al and Zn−Al of those of the 60% route. A similar trend is observed for ionising radiation and land occupation, where differences between the two processes remain moderate and are largely driven by background system contributions.
Climate change impacts show the pure Zn route reaching about Zn−Al of the 50−65% impact, in line with the higher energy demand associated with the additional thermo-metallurgical coating stage required for the Zn− process. Acidification and eutrophication categories follow comparable patterns, with the Zn route generally remaining below Zn−Al of the Zn−Al reference.
More pronounced differences are observed in resource-related categories, particularly non-renewable energy use and mineral extraction, where the pure Zn route accounts for roughly Zn−Al of the Zn−Al Al impact. These results reflect the higher material intensity and energy demand associated with the 40% coating route, rather than process-specific emissions.

[picture]

Fig. 4. Environmental impact categories for Zn and Zn−Al hot-dip galvanizing processes, based on the Impact 2002+ method (normalized values).

Fig. 4. Environmental impact categories for Zn and Z n − A l hot-dip galvanizing processes, based on the Impact 2002+ method (normalized values).

Figure 5 shows the cumulative energy demand (CED) for the Zn and Zn−Al hot-dip galvanizing processes. Panel (a) compares the total CED for the two coating routes, highlighting a higher primary energy demand for the Zn−Al process as expected. The total CED for the Zn−Al route is 10.4 GJ , which is approximately Zn−Al greater than the 7.4 GJ required for pure Zn (Figure 5a). Panel (b)
presents the contribution of different primary energy sources, indicating that non-renewable fossil energy is the dominant component for both routes. Non-renewable, fossil fuels are the dominant energy source for both processes. The significantly higher fossil fuel demand for Zn−Al (approx. 7.6 GJ vs 5.5 GJ) may explain its higher impacts in the Climate Change and Resource Use categories, as shown in Figure 3b. The elevated CED of the 44% coating route is mainly driven by the additional thermo-metallurgical coating stage, which results in greater thermal energy consumption and higher embodied energy of the coating material. Contributions from renewable energy sources remain comparatively low and do not substantially alter the overall energy balance. The observed trends are consistent with the GWP100 results, confirming the strong link between energy demand and climate change impacts in industrial galvanizing.

[picture]

Fig. 5. Cumulative Energy Demand (CED) for Zn and Zn−Al hot-dip galvanizing processes: (a) total CED and (b) contribution by primary energy source.

Fig. 5. Cumulative Energy Demand (CED) for Zn and Z n − A l hot-dip galvanizing processes: (a) total CED and (b) contribution by primary energy source.

The Life Cycle Impact Assessment results show that the environmental performance of the hot-dip galvanizing process is mainly driven by the thermo-metallurgical coating stage. For both pure Zn and 40.5% coating routes, this stage is the dominant contributor to climate change impacts and cumulative energy demand, confirming the energy and material intensive nature of industrial galvanizing operations.
The comparison between the two routes highlights a clear increase in environmental burden for the Zn−Al option at the manufacturing stage. Midpoint characterization indicates that the Zn−Al route reaches the highest relative impact across almost all evaluated categories. In particular, the GWP100 of the Zn−Al route is approximately Zn−Al higher than that of pure Zn . This increase is not solely related to the presence of aluminum, but results from the combined effect of higher coating material input and the addition of a second molten coating bath. This additional step prolongs the process and leads to significantly higher thermal energy consumption, directly affecting both GWP and CED.
The CED results support this interpretation, showing that the Zn−Al route requires 10.4 GJ of primary energy, corresponding to a 55% increase compared to the 7.4 GJ required for the Zn route. In both cases, energy demand is dominated by non-renewable fossil sources, accounting for approximately 7.8 GJ for 70% and 5.5 GJ for Zn .

From a process engineering perspective, these results suggest that material selection and thermal energy management play a more critical role than electrical energy use in shaping the overall environmental profile of the galvanizing line. The higher environmental impact associated with the Zn−Al route is mainly attributable to the use of primary aluminium and the energy demand required to maintain the molten bath at elevated temperatures. A potential strategy to reduce the environmental burden would be the partial or total substitution of primary aluminium with secondary (recycled)
aluminium, which is known to significantly lower energy consumption and greenhouse gas emissions compared to primary production. In addition, process optimization strategies such as improved thermal insulation of the molten bath, heat recovery systems, and the use of low-carbon electricity mixes could further mitigate the overall impact. From an LCA perspective, these measures would primarily affect the Climate Change and Resource damage categories, potentially narrowing the environmental gap between the Zn−Al and alternative routes.
The results also suggest that the environmental performance of the galvanizing process could be improved through the adoption of on-site renewable electricity generation. The use of self-produced electricity from sources such as photovoltaic systems could reduce the contribution of grid electricity to both climate change and energy-related impacts, particularly for auxiliary operations and electrical equipment.
Although Zn−Al coatings are commonly selected for their improved corrosion resistance and extended service life, this gate-to-gate analysis shows that these functional benefits are accompanied by a higher environmental burden during manufacturing. The 44% route exhibits the highest relative impacts in the Human Health, Ecosystem Quality, Climate Change, and Resources damage categories, whereas the pure Zn route ranges between approximately Zn−Al and Zn−Al in the same categories. While the present study is limited to the manufacturing stage, the results highlight a clear environmental trade-off and identify the thermo-metallurgical coating stage as the primary target for future improvement strategies. A cradle-to-grave assessment would be required to evaluate whether the enhanced durability of Zn−Al coatings can offset the higher impacts observed during production.

Conclusions

The original version of this paper is available on https://www.scientific.net/KEM.1051.155.pdf

A gate-to-gate Life Cycle Assessment (LCA) was conducted to compare Zn and mathematical formula hot-dip galvanizing routes for steel wire, using industrial data from a full-scale production line. The results demonstrate that:

the mathematical formula coating route is associated with a significantly higher environmental impact at the manufacturing stage, with an increase of approximately mathematical formula in Global Warming Potential (GWP100) compared to conventional Zn coating;

the higher impacts of the mathematical formula route are mainly driven by increased total material input and higher thermal energy demand linked to the additional molten coating bath, rather than by electrical energy consumption;

from a process engineering perspective, material selection and thermal management emerge as key levers for reducing the environmental footprint of galvanizing operations;

while mathematical formula coatings may offer superior corrosion resistance, their adoption involves a clear environmental trade-offs at the manufacturing stage, which should be carefully considered in process and technology selection decisions.

Potential environmental benefits associated with extended product durability and reduced maintenance requirements were not considered in the present analysis. A cradle-to-grave life cycle assessment in future studies could provide further insight into whether the higher manufacturing impacts of mathematical formula coatings are offset advantages during the product use phase.

Funding

The original version of this paper is available on https://www.scientific.net/KEM.1051.155.pdf

This paper is funded by the Italian Ministry of Enterprises and Made in Italy (MIMIT) within the project "FILSAR" F/350072/01-03/X60 - CUP: B29J24001080005 - COR: 22536461.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.155.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 165-178

doi: 10.4028/p-TrL9h0

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-19

Revised:
2026-02-11

Accepted:
2026-02-17

Online:
2026-04-17

Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

The original version of this paper is available on https://www.scientific.net/KEM.1051.165.pdf

Peter Frohn-Sörensen 1,a*, Apostolos Aslanidis 1, b, Michael Geueke 1,c and Bernd Engel 1, d

1 Forming Technology Siegen, Breite Strasse 11, 57076 Siegen, Germany

a  Peter.Frohn@uni-siegen.de, b  Apostolos.Aslanidis@uni-siegen.de, c Michael.Geueke@uni-siegen.de, dBernd.Engel@uni−siegen.de(*corresponding author) 

Keywords: Circular Manufacturing, Sustainable Manufacturing, Flexible Manufacturing Systems, Segmented tools, Upcycling

Abstract

The mitigation of primary resource exploitation and their usage within linear economies ultimately leads to systematic leakages from economy and the depletion of natural resources. In contrast, a circular instead of a linear economy aims towards minimizing these leakages by reintroducing all resources to economy without negative externalities. Within the circular economy (CE), manufacturing plays a vital role for the conversion of resources towards products. For a manufacturing system, it is hence of critical importance to understand the implications and requirements of CE for production. The present study develops the Ready4CM "Ready For Circular Manufacturing" principle. Manufacturing systems are considered in the light of serving the needs of CE, where manufacturing flexibility, scalability and reconfigurability may pave the way but need to be controlled sufficiently to achieve resilience towards more drastic uncertainties of used materials. Not only focusing on process, but also on machine and tool, this paper contributes towards CE by identifying systematic aspects of circularity in manufacturing systems. We embedded our contribution in existing frameworks that calculate and balance sustainability potentials within circular economy while our approach, Ready4CM, aims to identify and summarize a comprehensive understanding of technical premises for manufacturing processes to serve and facilitate CE.

Introduction

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The principle of circular economy (CE) introduces systematics to overcome nowadays key needs to mitigate excessive usage of primary resources and to restrict energy consumption as much as possible. According to Ellen MacArthur Foundation [1], the CE agenda is subdivided into three design-driven principles:
i) Elimination of waste and pollution,
ii) Circulation of products and materials at their highest value,
iii) Regeneration of nature.

CE consequently stands in strong contrast to conventional linear economy, where materials are captured from the environment, products are manufactured using these materials and ultimately disposed of as waste after a products lifetime. Practically, for technical management of product stock, loops are closed for reintroducing products at different economical stages with the recycling principle being the most prominent among them. Yet, recycling represents the largest loop where materials are gathered from disposed products in order to be reintroduced to the earliest manufacturing stages of new products. In contrast, for instance the repair loop connects a narrower margin within the stage of product usage. Consequently, these loops given as maintain, share, reuse, refurbish and recycle by the Ellen MacArthur Foundation.

Essentially, besides these aforementioned loops in stock management of finite materials, the renewables flow management is studied in circular economy. Where the technical loops are considered in stock management, cascades of materials utilization are focused in circular flows of
renewables. Both circular flows are overall intended to minimize the systematic leakage of resources and energy. A summary of this theory is graphically presented commonly in the butterfly diagram according to the Ellen MacArthur Foundation [1].

Conventional linear economy may also be expressed as cradle to grave principle. In contrast, according to Braungart and McDonough [2], the cradle to cradle (C2C) concept draws a holistic, economic, industrial and social framework for the high-quality use and circulation of organic and technical nutrients. Correspondingly, the aforementioned loops, e.g. recycling and repair, belong to the technical cycle. In addition to that, within CE, the C2C concept also comprises a second, biological cycle of renewable resources. In this cycle, the flow of renewables is focused. Both cycles of the cradle-to-cradle principle are commonly graphically illustrated according to Fig. 1.

[picture]

Fig. 1. Cradle-to-cradle concept by Baumgart and McDonough. Image Source under CC BY-SA 4.0 [3].

Fig. 1. Cradle-to-cradle concept by Baumgart and McDonough. Image Source under CC BY-SA 4.0 [3].

For the engineering domain, Meldrum suggests an engineering life cycle approach for complex and certified products [4]. Repair, maintenance, reliability etc. are some of the core engineering principles that drive circularity. In addition, standardization towards reusability and recyclability as well as modularity are identified as new circular principes supported on the existing product life cycle in engineering ranging from innovation over production until operations and eventually, disposition. Indeed, the engineering domains of design, manufacture, usage and end-of-life management of products have direct consequences on resource consumption, contribution to climate change and waste accumulation according to LEAL et al. [5]. In their study they assess and review the implementation of CE in German production industry during year 2023 coming to the conclusion that for the adoption of CE practices, industry, in particular SME, still face significant challenges while their findings emphasize the particular importance of Recovery and Redistribution efforts.

In deeper detail, Jawahir & Bradley explain that historically the principle of 3Rs - Reduce, Reuse and Recycle - significantly evolved during the 1990s [6] with the 1st R, Reduce, originating even earlier from the 1980s. The 1st R consequently corresponds to the principle of lean manufacturing while including 3Rs corresponds to green manufacturing. In addition to these aspects, the 6R principle brings along the aspects of Recover, Redesign and Remanufacture and is commonly denoted as sustainable manufacturing. Only with the inclusion of these additional three aspects, a closed-loop, multiple product life cycle system is achievable according to JOSHI et al. [7]. To achieve these goals, more recently, BADUREDEEN & JAWAHIR suggested strategies for value creation through sustainable manufacturing with the key aspects of education, integrated decision toolsets based on lifecycle sustainability assessment, risk and uncertainty modelling, lifecycle management and others being among them [8]. With particular regard to the manufacturing domain, it is pointed out in their
study that flexibility, scalability and reconfigurability play crucial roles in enhancement of the evolution of sustainability performance.

More specifically for fitting of manufacturing systems within a CE, Andersen et al. postulate that these systems have to be designed and developed for disassembly, reprocessing of materials, reassembly, and remanufacturing [9]. According to their claim, changeability is required to tackle higher degrees of uncertainties by closed-loop manufacturing systems for product take-back. The findings of this study highlight, that key challenges are fluctuating quantities, varying states of qualities, and a high mix of incoming end-of-life products.

When leaving the focus of closed-loop manufacturing, sustainable manufacturing must also consider energy consumption since any form of energy is inevitably linked to resource consumption, primarily (e.g. fossil fuel consumption) or secondarily (for instance manufacture of solar cells). In this regard, Duflou et al. have developed energy assessment methodologies [10] and eco-labelling [11] for the machines and machine tools used in manufacturing processes. With regard to the (manufacturing) process level, their studies show, how - from an energy point of view - adjustable machine tool designs can help to reduce energy consumption in most significant ways [12]. Particularly, for the more complex system level including all periphery, the exergy measure is adapted to manufacturing according to the 2nd  law of thermodynamics to capture resource efficiency in manufacturing. By doing so, uniform units are achieved in assessing both energy and material consumption obtaining a comprehensive measure for both different efficiencies with the conclusion that the reduction of external exergy losses plays a crucial role for achieving resource efficient processes [12], [13].

Concludingly, with perspective drawn towards the manufacturing domain, processing of a) limited, finite materials and b) renewable resources needs to be focused on for circular manufacturing. Within the area of stock management of finite materials, the "R principles" of waste management are identified with repair, reuse, refurbish, remanufacture and recycle being amongst those [14]. Consequently, modern, circular manufacturing needs to fit processing of used products and materials. According to current definitions, the 9R framework provides a comprehensive structure for circular economy strategies by explicitly ranking environmental preferences, more specifically including steps for high level circularity, prioritizing refusing and rethinking and positioning the recycling loop as last resort in the CE hierarchy: Refuse, Rethink, Reduce, Reuse, Repair, Refurbish, Remanufacture, Repurpose and Recycle [15].

Contribution and Procedure

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In the present study, we investigate premises and possibilities to enable and integrate a manufacturing system into a circular economy focusing on the domain of forming technology. Hence, we spotlight the scope of circular manufacturing on the micro scale within the hierarchy of production value-chain systematics [16], where the micro scale denotes the machine and production process, the meso scale refers to the production line within a facility and the macro scale indicates the uppermost hierarchy and encloses the whole supply chain. Driven by the megatrend of individualization [17], manufacturing engineering science has made large leaps by introducing flexible production techniques [18]. Flexibility indeed allows us to adjust manufacturing techniques to process nonprimary resources. Following the necessity for research and development in flexibility, scalability and reconfigurability identified in the introduction [8], [9], we carefully consider highlighted cases where flexible, scalable or reconfigurable processes are assessed to upcycle used products or to process raw materials from non-primary origin. Based on process versatility, this study complements existing studies where metal forming process sustainability is valued based on exergy calculations [12], [13]. Closing, we derive critical obstacles on the micro scale with respect to machine and process and summarize the potential of a manufacturing system towards circularity.

Circularity enablers in tooling

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Manufacturing processes are being carried out on machines using tools in order to process materials and products. Consequently, circularity not only concerns the processed objects but also
machines and tools. With probably the most long lasting consequent background, machines are not only being scrapped and recycled when passing their lifetime but also - with regards to the introductory mentioned technical CE loops and corresponding R-principles - being maintained, reused, repaired or even refurbished/remanufactured [19].

Tool materials. Looking at the inner technical CE loops, refurbishment of tools, to some extent, is state of the art in industrial practice if these are made from resilient and expensive materials for long lasting operation such as tool steel. Yet ultimately, out-of-life tools sooner or later end in the recycling loop. More recently, due to the introduction of additive manufacturing technologies, Frohn-Sörensen et al. demonstrated an additively manufactured tool set based on fully recycled polymer waste for deep drawing of sheet metal cups [20] as a promising alternative for production of small lot sizes. Also, looking at the cradle to cradle concept, disassembly of technical nutrients seems promising for a perspective of manufacturing tools due to the abundant usage of standardized components.

[picture]

Fig. 2. Wood-based tooling approaches for profile bending. Two common profile bending processes are schematically shown along to a set of wood-based forming tools.

Fig. 2. Wood-based tooling approaches for profile bending. Two common profile bending processes are schematically shown along to a set of wood-based forming tools.

[picture]

Fig.3.From trunk to tool. Schematic value creation along the production of wood-based forming tools.

Fig.3. From trunk to tool. Schematic value creation along the production of wood-based forming tools.

In addition to the technical principles of the CE cycle, circularity in the biological cycle can be achieved by using renewable, robust base materials to manufacture tools. Wood is a renewable and bio-based raw material whose increasing versatility is opening new areas of application, apart from its use in construction or in combustion units [21]. In line with the 9R strategies, using wood as a substitute for conventional steel-based components in tools and structures offers the possibility of providing sustainable alternatives [22]. Additionally, cascade usage in conjunction with downgrading from raw wood to the chip or fibre stage enables multiple usage to be achieved in a resource-saving manner [23]. While the isotropic properties of conventional tool materials such as steel are relatively
easy to handle, the strong anisotropic and inhomogeneous properties of wood make it harder to characterize and model, for example in finite element analysis. Whereas an almost homogeneous material like tooling steel does not vary in its properties except for the rolling direction due to the manufacturing process, wood has several structural issues and difficulties, such as microcracks, growth zones, branches and capillary structures. The presence of these impurities varies depending on the wood species, growth location, and other environmental factors during growth. To overcome these issues and potential disadvantages, the design and layout process for wood-based forming tools and structural components needs to be investigated more widely and in greater depth. In construction work and building with timber products, for example, the partial safety factors for the strength properties of wood are higher than for steel due to the inhomogeneous structure [24]. The orthotropic material properties of wood are direction-dependent anisotropic properties that differ according to the growth direction by a factor of ~5−8. However, strength and durability are dominant in the growth direction. Due to the variation in strength of solid timber, suitable stacking or composite strategies are required to obtain an ideal or load-dependent layer arrangement. This procedure adds value to wood products at each stage of production and enables the creation of a tailored, optimized structural building material for mechanical applications such as tooling or structural components. (see Fig.3).

Several studies have investigated wood-based tooling approaches and structural components [25], [26], [27], [28], [29], [30]. With respect to forming tools and structural applications, GEUEKE et al. studied the performance of wood as a bio-based, renewable substitute for conventional profile and sheet metal forming applications (see Fig.).

GEUEKE et al. evaluated die geometries made from black locust wood and compared the forming performance and dimensional accuracy of these to a solid polylactic acid (PLA) die in a rubber pad forming process with DC04 steel sheets. The results showed comparable forming behaviour overall, improved radius accuracy for steel-hybridised dies and minor permanent deformation in wooden tools under high contact stresses [31]. In an additional study, Geueke et al. investigated the feasibility of using wooden freeform bending tools for EN 1.4301 stainless steel tubes. They found that the tools could bend the steel tubes with a performance comparable to that of conventional steel tools. However, they also found that tool orientation and the anisotropic wood structure significantly influenced deformation behaviour, process force capacity and circumferential uniformity. This offers enhanced flexibility and potential for mass customisation in profile bending [32]. Following a feasibility study of deep drawing applications using wooden forming tools on 1 mmDC04 sheet metal, Geueke et al. found that they performed satisfactorily for small-scale batch production. However, horizontally stacked tools showed inadequate dimensional accuracy due to the anisotropic mechanical limitations of the wood layers [33].

In summary, the use of wood-based forming tools and structural applications is a suitable way to enable CE in mechanical engineering (see Figure 2). Using renewable and carbon-neutral materials in production and manufacturing ensures cascade usage in the cradle-to-cradle concept. Combining micro-scale information from analysis techniques such as scanning electron microscopy (SEM) or Xray with macro-scale knowledge from the construction and building industry will help to develop meso-scale applications for wood-based tools and structures, thereby extending renewable resources in the CE. Although there are still some difficulties to overcome and adjustments to make when dealing with wood-based structures, feasible components that use sustainable and economical materials have already been demonstrated in mechanical areas such as forming tools and structural components.

Tool operation principle. Besides the materials a tool is made of, its universal applicability has been identified as critical premise to enable circular manufacturing. Flexibility, as per definition characterized by physical variation, lot size variability, number of DoF and complexity [18]. While conventional rigid and solid tools are usually highly specialized to a certain geometry and therefore limited in their flexibility with respect to realizable component geometries and are not designed for sustainability, adjustable tools actively allow for variable manufacturing by adding significant degrees of freedom. At the same time, batch-size variability is passively increased, as product variants
can be manufactured using the same tool. This approach reduces the fixed costs associated with tool manufacturing and saves time otherwise for tool changes, while simultaneously enabling flexible, efficient, and sustainable resource efficient production.

To increase the flexibility of tools, adjustability of the tool contour is required. One approach to realizing such contour modifications is the segmentation of conventionally closed tool surfaces and the targeted adjustment of these segments. In this way, reconfigurability of the tool contour is enabled, allowing different component geometries to be manufactured using a single tool. In particular, segmentation offers the potential for adjusting individual segments. This adjustment can either be performed prior to the process (static adjustment), enabling the production of different component geometries from one operation to the next, or during the process (in-process adjustment). The latter additionally provides the opportunity to deliberately vary the forming path, thereby positively influencing both the process behavior and the resulting component quality, as well as extending the process limits.

Approaches to segmentation as well as reconfigurable and adjustable tools are well known from multi-point forming (MPF), also referred to as digitized die forming (DDF), particularly in the field of sheet metal forming, and have been extensively investigated in numerous studies [34], [35], [36], [37], [38], [39], [40]. Corresponding approaches to segmented and adjustable tool concepts have also been investigated in the field of profile forming and are gaining increasing importance, becoming the subject of several research studies [41], [42], [43], [44].

Heftrich et al. [45] developed a systematic method for reducing and simplifying bending tools. Based on numerically computed contact pressure distributions during rotary draw bending using conventionally closed tool surfaces, surface regions that contribute little or negligibly to the forming process are identified. This leads to simplified tool designs which, due to the additional available installation space, can be equipped with further adjustment axes. As a result, existing tool concepts can be extended, enabling the manufacture of multiple component geometries using a single tool set.

Furthermore, in extensive studies [46], [47], [48], [49], the segmentation of forming bending tools was investigated both experimentally and by means of numerical simulations. Reuter [50] developed a deformation-mechanism-based method for the design of segmented tool surfaces aimed at increasing the flexibility of bending processes. Based on the investigation of the interactions between tool surface topology, the mechanical and geometric properties of the semi-finished product, and the resulting forming phenomena and mechanisms, a systematic design approach for segmented tools was developed using compression bending as a representative process example. The resulting segmented tool enables the adjustment of different bending radii from one bending operation to the next. Both constant radii and radii that vary along the bend line can be realized (see Fig. 3). Owing to the additional degrees of freedom provided by the tool, the potential for in-process adjustment is also introduced. Compared to conventionally closed tool surfaces, this significantly extends the range of possible applications.

[picture]

Fig. 4. Segmented and adjustable bending die for compression bending [50]

Fig. 4. Segmented and adjustable bending die for compression bending [50]

Concludingly, segmented forming tool concepts offer the possibility to adjust tooling towards a variety of products but also, in the light of the present approach, varying materials to be processed. Prospectively, even used products may be processed by an adjustable toolset; both aspects therefore contribute to circular economy.

Circularity enablers in exemplary forming processes

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Free-form bending (FFB). FFB techniques provide high degrees of flexibility with respect to manufacturing flexibility in profile bending. While conventional approaches for profile bending use dedicated die systems. In die bending, the full geometry of the product is resembled within a dedicated press tool which restricts geometry adjustments and variability for the sake of process simplicity and robustness as long as no supply chain changes may disrupt the process tolerances. Next, rotary draw bending [45], [51] and, similarly, compression bending [52] are profile bending processes for the production of challenging, narrow geometries with highly dedicated tool sets. With respect to variability, the bending angle may be adjusted to process fluctuations to compensate for elastic springback in these processes. However, both processes are only capable of processing straight profiles which is a strong limitation towards reprocessing of used materials after disassembly of End-of-life (EOL) products. A number of different FFB technologies has been developed in the past, mainly to overcome geometrical limitations by means of variable manufacturing. Three-Roll-Push bending [53] allows to flexibly adjust the product radius or variable radius distributions by supporting initial state of the product between rollers and the positioning of a bending roller whilst feeding the tube in a longitudinal way. While the procedure leads to very large flexibility, that is almost only restricted by the machine dimensions, minimum radii that may lead to buckling effects and the profile's cross-sectional outline which needs to be resembled in the roller profiles, control, radius prediction and process modelling have been recent challenges to master the resilience of the process [54], [55]. However, once the cross-section of a profile is variable over its length, FFB processes are still under development and bring along enormous process complexity and, again, a higher dedication of tooling [56].

Concludingly, FFB processes exhibit readiness potential for circular manufacturing by means of strongly elevated flexibility and sufficient control concepts to master resilience. In particular, FFB operations would allow to in-situ adjust the process towards shifting material qualities which holds the potential to minimize any scrap production. Limitations are identified with regards to processible used materials which might originate from EOL products.

Incremental forming methods. In incremental forming, the manufacturing tool size is only a fraction of the whole product geometry dimension. Consequently, shaping is conducted by repetitive approaches with successively increasing straining. From a technological perspective, very small forming zones with strong strain gradients result compared to continuous, conventional methods with dedicated tool sets. For instance, single-point incremental forming (SPIF, aka incremental sheet forming ISF) is a process, where a small spherical tool is mounted to the tip of a robotic arm, which successively forms a piece of sheet metal along dedicated concentrical paths [57]. Within the domain of sheet metal forming, hardly any process exceeds the degrees of geometrical freedom of this technology owing to the variability of process conduction. Even more, it is well known that incremental forming methods allow higher strain values than possible with conventional forming methods [58]. Similar to FFB processes, control, process modelling and geometrical prediction are crucial premises to master an efficient and accurate process operation of SPIF [59]. In addition, due to the discontinuous nature and the very small forming zone, incremental forming methods inherently require higher process times than continuous processes. On the bright side, high product variability and universal tooling allow for valuable, economic production of even unique parts. With more specific regards to sustainability and environmental benefits, Cooper & Gutowski demonstrate in their study that the ISF process potentially avoids large tooling and therefore saves time, money and energy [60]. Based on an extended anticipatory LCA, they present a methodology how to assess the potential industry level impacts of an emerging technology. More specifically for the incremental forming method, they demonstrate high energy and cost saving potentials for an implementation
scenario of the forming technology in the U.S. car industry by 2030 . Other studies such as Ambrogio et al come to the conclusion, that the energy efficiency of SPIF can be significantly improved by adopting high speed variations of the process since the high cycle times still limit the industrial applicability of this incremental method [61]. Both studies are confirmed by the findings of SIMONCELLI et al.: they attribute the sustainability potential of SPIF to low volume production and the avoidance of large die sets and the associated provisioning times, while the large dependence of the sustainability potential is highly related to the processing time of this manufacturing process [62].

Similar advantages and limitations are observed for the incremental forming methods of shear spinning [63], [64], open die forging [65], incremental folding or v-die bending (bumping) operations [66], [67] or incremental profile bending [68], [69]. Recapitulatory, incremental forming methods provide highest degrees of geometrical freedom. Process control, high processing times and, in addition to that, surface imperfections due to the successive nature of this class of processes are challenges that need to be overcome. Moreover, the shape of the material to be processed is often dedicated to a certain die geometry for clamping - or vice versa - so that manufacture of cheap or recyclable clamping dies may be a solution. Overall, these degrees of freedom contribute to these processes' readiness for use in circular manufacturing while the geometrical dedication towards clamping and mastering the process layout are current challenges.

Findings and Discussion

The original version of this paper is available on https://www.scientific.net/KEM.1051.165.pdf

The exemplary case studies from the metal forming domain presented above show us that - within the process unit scope or value-chain micro scale scope - circular manufacturing is enabled at two different layers: i) the manufacturing system by means of machine and tooling and ii) the manufacturing process conducted on this system. The considerations show, that the Cradle-to-Cradle C2C theory [2] is applicable to both layers i) and ii) with respect to the technical and the biological cycle.

Firstly, under technical consideration, a machine and its tooling may be re-introduced after End-of-life EOL according to the 9R principles ranging from maintenance and repair up to recycling. If not so, resources leak from the cycle - as in linear economies. On the other hand, a manufacturing system may as well be part of the biological cycle if its structure is composed of cascadable natural and renewable resources. Even if these resources leak or cannot be used in purposeful applications after the last cascade, the biological cycle still allows to make use by thermal or natural degradation.

Secondly, manufacturing by means of the production process is an element within the lifecycle of a product made from technical or renewable resources. Conventionally, in linear economy, the manufacturing process converts primary resources to products, which ultimately leak their resources after EOL. For the technical resource cycle, the manufacturing system is expected to process recycled resources with potentially larger uncertainties regarding the quality and properties of materials (3R). In addition to that, upcycling of used products or parts of disassembled used products is a vital aspect in the relation of the 6R systematics. Within a cycle of renewable materials, processing of natural materials is expected. In a nutshell, looking at the manufacturing process, key enablers for circular manufacturing are its manufacturing flexibility, scalability and reconfigurability as well as the process' resilience against uncertainties. Fig. 4 summarizes these findings.

[picture]

Fig. 5. Circular manufacturing on the micro scale of value-chain hierarchy systematics.

Fig. 5. Circular manufacturing on the micro scale of value-chain hierarchy systematics.

Evidently, the resource leakages shown above must be mitigated to conserve resources. Consequently, a manufacturing system may be designed and operated in a way to achieve and facilitate one or more of the four central aspects indicating the system's readiness for circular manufacturing "Ready4CM":

Machine and tooling are within technical cycles (9R): They are maintained, refurbished, reused etc.

Machine and tooling are from cascadable, renewable resources such as tools from wood.

The manufacturing process facilitates reprocessing of used materials originating from EOL products.

The manufacturing process is capable to process renewable natural resources.

It is worth noting the machine's and tooling's dual role as a product and as a production system which is reflected by the presented systematics and in-line with literature findings [11].

Conclusions

The original version of this paper is available on https://www.scientific.net/KEM.1051.165.pdf

Circular economy is introduced to overcome the ongoing exploitation and systematic leakages of primary resources as it has been undertaken intensely since the age of industrialization. Within the circular economy, manufacturing is one vital cornerstone in the evolution of products besides design, distribution, usage, collection, etc. It has been indicated that circular manufacturing systems must be capable of processing disassembled parts of used products. To complement existing sustainability considerations based on exergy balances, the present study reviews in depth observations on the process unit / value-chain micro level and derives necessary premises of manufacturing systems to serve circular economy based on the system's versatility by means of flexibility, scalability and reconfigurability. The ready for circular manufacturing "Ready4CM" theory developed in this paper comprises i) the manufacturing system and ii) the operated process. Key indicators for circularity are the material origins of the machine and tool as well as their wholistic maintenance and reuse principles. Moreover, the process' capability to re- and upcycle used materials and the necessary flexibility, control and resilience to use materials from non-primary or renewable resources are found to be indicative for circularity. Embedded in existing CE and sustainability frameworks that primarily
balance sustainability potentials, the Ready4CM systematic identifies technical premises for manufacturing processes to serve and facilitate CE.

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 179-189

doi: 10.4028/p-q7w6PK

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-22

Revised:
2026-02-16

Accepted:
2026-02-17

Online:
2026-04-17

Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

The original version of this paper is available on https://www.scientific.net/KEM.1051.179.pdf

David Bailly 1,a*, Moritz Gouverneur 1, b and Junhe Lian 1,c

1 Institute of Metal Forming (IBF), RWTH Aachen University, Intzestraße 10, 52072 Aachen, Germany

a* david.bailly@ibf.rwth-aachen.de, b  moritz.gouverneur@ibf.rwth-aachen.de, cjunhe.lian@ibf.rwth-aachen.de

*corresponding author

Keywords: sustainability, metal forming, open-die forging, efficiency, circularity.

Abstract

Climate change is progressing rapidly, posing severe risks to the environment and making sustainability and circularity key challenges for future industrial development. Greenhouse gas emissions are largely driven by human activities within industrialized societies, requiring adaptation at individual, societal, and industrial levels. Metal forming technologies can contribute significantly to this transformation by improving material efficiency, process efficiency, product efficiency, and circularity. Material efficiency is particularly important, as material production accounts for the largest share of industrial emissions. Process efficiency offers a high leverage effect, due to the large production volumes in forming process chains, while product efficiency reduces energy consumption during usage and enhances the performance of energy generation systems. Circularity supports sustainability by extending material lifecycles through reuse and recycling, thereby avoiding energyintensive primary production. This paper presents an overview of exemplary sustainability contributions in metal forming process chains. For open-die forging, it can, for example, be shown, that digital twins, virtual reality-based operator training and real-time assistance systems are measures to improve material and process efficiency. A circularity approach for open-die forging is presented, with a remanufacturing concept for large shafts based on re-forging end-of-life components, in order to heal fatigue-related damage by forming. Increased material, process and product efficiency is demonstrated by a use case study of forging hollow rotor shafts for wind turbines. Whereas, the hollow-forging allows for weight reduction in the rotor component and thus enables higher power density of the generator, thinner tower designs and reduced logistic costs. Additionally, the use of an innovative air-hardening ductile (AHD) steel can eliminate the energyintensive heat treatment in the process chain.

Introduction

The original version of this paper is available on https://www.scientific.net/KEM.1051.179.pdf

Climate change is progressing at a rapid pace [1], causing severe hazards for our living environment and making sustainability and circularity key challenges for our future. Greenhouse gas emissions are primarily caused by human influence and the industrialized society. According to Allwood et al. [1], 78% of the global greenhouse gas emissions (GHG) originate from energy generation and process emissions. Out of that, 35% is generated by industry. Therefore, every individual, the society and the industry need to adapt. Within the industrial sector, 55% of the emissions stem from material production and processing [1]. Whereas iron, steel and aluminum are among the most relevant materials [2,3]. In addition to the general human influence, that emphasizes the strong role of the industry and the material processing in particular. Based on that, a high relevance and responsibility, but also saving potential, can be derived for the metal forming community. Thus, forming technology can make a significant impact regarding material efficiency, process efficiency, product efficiency and circularity, besides other contributions to this transformation.

Material efficiency is very important because the largest emissions in many relevant products, such as automotive steel body parts and aluminum beverage cans, come from material production [2]. Process efficiency is crucial because the huge production volumes in forming process chains enable
a large leverage effect. Product efficiency allows to reduce energy consumption during the usage phase and also to increase the efficiency of energy transformation systems, such as electric generators and drives. And circularity allows to reuse and recycle materials for as long as possible in order to avoid energy-intensive primary production.

In order to achieve an effective circular economy, technologies and business models must be adapted and redeveloped. The depiction in Fig. 1 shows an exemplary, generalized process chain from the steel production and processing, along the product usage, towards the end of product life and the final discarding.

[picture]

Fig. 1. Circularity Scenarios and holistic sustainability optimization of process chains.

Fig. 1. Circularity Scenarios and holistic sustainability optimization of process chains.

After the production of a product an important sustainability goal is to keep it as long as possible in usage, e.g. supported by predictive maintenance. However, when the service life is finally no longer extendable, the product can be collected and discarded, which leads to the loss of the material and the energy invested in the product. Alternatively, the product can be kept in a circular material cycle, e.g. by re-melting the material or - even more energy-efficiently - by reworking or remanufacturing it. Remanufacturing can involve, for example, re-machining or even re-forming in order to improve material properties. Overall, it is therefore possible to re-enter the production cycle at different points. The decision which of these circular routes is most efficient and most sustainable as well as economic, can be very complex, but it can be strongly supported by data and digital twins. An intelligent databased route decision can help to choose which circular path to take and how to design the new process route in detail. Digitalization methods in general are an important toolbox for substantial changes in the process routes towards sustainability and circularity, because they enable better holistic optimization (see Fig. 1) and allow for an adaptive response to fluctuations caused, e.g., by recycled materials.

This paper, therefore, aims to (i) structure sustainability contributions in metal forming routes for both sheet and bulk metal forming with four groups of improvement levers: material efficiency, process efficiency, product efficiency and circularity and (ii) to apply the four-lever framework more specifically on open-die forging and demonstrate a use case of hollow-forged wind turbine shafts.

The following paragraphs describe selected examples of specific potential sustainability contributions in metal forming routes to strengthen the four-lever framework.

Potentials for sheet and bulk metal forming routes in terms of efficiency and circularity:

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Sheet metal forming process routes, starting from the liquid steel, followed by the rolling of sheets and the final component production are already widely optimized, mainly because of the
continuous, automated production and the extremely high production volumes. Nevertheless, there are still potentials and measures to further optimize these process routes and to shift them towards a circular production. And again, due to the high production volume, the overall potential to save material, energy and CO2 is very high, even in terms of relatively small improvements.

In the following paragraph, some exemplary innovative examples will be discussed that can contribute towards further improvement of sustainability in the sheet metal forming route. The chosen examples are inspired by the research topics of the Institute of Metal Forming from RWTH Aachen University and serve to demonstrate the relevance and potential of the above-mentioned key measures, material efficiency, process efficiency, product efficiency and circularity.

The first example is the strip casting process, which is an integrated casting and rolling process, producing thin sheet material (usually 1−3 mm ) in one step directly from liquid steel. Thus, it serves as an ultra-short process route for sheet metal production, compared to the conventional route, starting from continuous casting or thin-slab casting (see Fig. 2, right).

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Fig. 2. Comparison of energy consumption for continuous casting, thin slab casting, and strip casting (left, top); Comparison of blast furnace route with electric arc furnace route (left, bottom); Comparison of process chain length (right) [4,5].

Fig. 2. Comparison of energy consumption for continuous casting, thin slab casting, and strip casting (left, top); Comparison of blast furnace route with electric arc furnace route (left, bottom); Comparison of process chain length (right) [4,5].

The comparison of the energy consumption between the conventional process routes and the strip casting route shows a significant advantage of strip casting, when looking at the route from liquid steel till hot rolled strips (see Fig 2, left, top). Considering also the reduced process scrap in an ultrashort process route, strip casting has a huge potential in terms of material efficiency and process efficiency.

When adding the steel production to the energy assessment, the differences between the process routes however look relatively smaller (see Fig. 2, left, bottom). Nevertheless, the comparison between primary production of steel using basic oxygen furnace (BOF) and secondary production using electric arc furnace (EAF) depicts a huge energy saving potential for the secondary route.

This leads to the second main advantage of the strip casting process route, which is a perfect enabler for recycling-based circularity in metal production. The production volume of typical strip casting lines is very suitable for a flexible and decentralized strip production with scrap as a major material source. Additionally, the high cooling rates in the solidification phase of the process (up to approximately 1000 K/s ) are very advantageous for tackling tramp elements in the recycling route of aluminum and steel [6]. Furthermore, the process conditions including high cooling rates, the avoidance of slab reheating and extensive high-temperature exposure are very advantageous for the recycling of steels with high copper contents originated from recycling. The process conditions help to reduce copper segregation as well as oxidation-driven copper enrichment at the steel-scale interface and thus largely avoid liquid copper formation and weakening of the material. By producing near-net-shape strips directly from liquid steel, the necessary hot rolling effort is drastically reduced.

This additionally allows for largely surpassing the problem of damage due to copper-induced embrittlement (Hot Shortness) in the hot rolling [3, 7].

Another potential advantage of strip casting, which is also based primarily on the high cooling rates and reduced rolling effort, lies in the production of special, high-alloyed materials. Previous studies have demonstrated that the high cooling rates strongly influence microstructure evolution, texture development and downstream processability. For high-silicon electrical steels, it is known that an increased silicon content increases the electric efficiency of the electric drives using these sheets. The optimum silicon content for electrical efficiency is 6.5wt.% (weight percent). However, the brittleness of these alloys increases dramatically with rising silicon content, which means that alloys with a silicon content greater than 3.5wt% cannot be produced using conventional industrial process routes. Strip casting has shown that high cooling rates, near-net-shape casting, and low rolling effort enable the production of electrical strips with 4.5wt.% [8] and 6.0wt.% [9] silicon. These examples demonstrate the potential of strip casting to improve product efficiency.

In the rolling process chain, either after strip casting or after continuous casting, respectively thinslab casting, there is significant potential for optimization through the use of digital twins of the material and processes. For example, fast process models [10] and digital twins [11] can be used to adapt processes in the event of process deviations in order to maintain quality and avoid rejected products, which in turn increases material efficiency and process efficiency. Furthermore, digital twins and process adaptation can also be used in the long term to respond effectively to greater fluctuations in input material in the process chains, triggered by high recycling rates [12]. This shows a strong potential for circularity. Further potentials for material efficiency can be found in the reduction of edge scrap in heavy plate rolling, which is still very pronounced [13]. Regarding product efficiency, the rolling processes can contribute by an increased damage control [14].

In the final sheet metal forming the highest potential for material efficiency and product efficiency can be found in lightweight design concepts. New potentials in lightweight design can be exploited primarily through process innovations, such as in architecture and design applications through the combination of stretch forming and incremental sheet metal forming (ISF) [15], and in automotive components through gas-based aluminum hot sheet metal forming [16]. Additionally, gas-based aluminum forming can also increase material efficiency and product efficiency by reducing damage evolution in the process [17]. Finally, the re-manufacturing of end-of-life components with flexible sheet metal forming processes such as ISF [18] can contribute to circularity.

Compared to many bulk metal forming process routes, the sheet metal forming routes are already on a rather mature level regarding efficiency and sustainability, which is mainly due to the high production volume and automation. However, the previous paragraphs have shown promising further potentials. Many bulk metal forming processes are less automated and optimized, due to the lower production volumes and high part variations, like in open-die forging. However, there is great potential due to the large quantities of material and process energy involved. In open-die forging the process optimization can be significantly advanced using digital twins of the material and the process. Virtual reality training of operators [19] and a real-time assistance system for the process [20] allow for a minimization of scrap, tolerances and trim as well as near-net-shape production, e.g. curved components [21]. These measures all contribute to increased material efficiency and process efficiency. Another concept related to large shafts can make a significant contribution to circularity. The conceptual idea is a remanufacturing approach for large shafts, which involves the healing of fatigue-related damage by the re-forging of end-of-life shafts and strongly relies on digital twins of material and process.

A further option to increase material efficiency and process efficiency, as well as product efficiency, can exemplarily be demonstrated by the forging of hollow shafts for wind turbines, which enables material and weight savings and increases the efficiency of energy generation by higher power density. The use case of hollow shafts for wind turbines will be explained in more detail in the following chapters.

Motivation for the Use Case Wind Turbines

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The transition towards sustainable energy systems imposes increasing performance and efficiency demands on wind turbines. Key drivetrain components must be designed to reduce material and energy consumption during manufacturing while enabling higher power densities. The rotor shaft, which absorbs cyclic bending loads and transfers torque from the rotor to the gearbox, plays a central role in this context.

Conventional manufacturing routes face inherent drawbacks: cast rotor shafts offer high geometric flexibility but exhibit comparatively low mechanical strength. Solid-forged shafts provide significantly higher strength but require extensive machining and large material input, resulting in high economic and environmental burdens.

A common research project of the Center for Wind Power Drives, the Steel Institute and the Institute of Metal Forming at the RWTH Aachen University addresses these challenges by investigating the potential of directly hollow-forged rotor shafts made from air-hardening ductile steel (AHD-steel). This approach combines the mechanical benefits of forged steels with the geometric flexibility of hollow sections and eliminates the need for energy-intensive quench-and-temper heat treatment. By enhancing material efficiency and process efficiency, i.e., reducing process-related energy consumption, the project contributes to lowering the environmental footprint of drivetrain components for wind turbines.

State of the Art for the Use Case Wind Turbines

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Wind energy plays a crucial role in renewable electricity generation and is central to achieving global climate objectives. The growing demand for wind energy is reflected in the ongoing development toward turbines with higher power ratings [22]. However, further increases in turbine size are limited by logistical challenges related to transport, installation, and space requirements. As a result, research efforts increasingly focus on reducing the levelized cost of energy and enhancing the power density of the drivetrain. One approach is the application of lightweight design concepts to drivetrain components.

Rotor shafts in wind turbines are inherently hollow to allow electrical cabling for the pitch control system. Currently, there are two established manufacturing routes: Casted and solid forged shafts [23]. Cast shafts must be oversized due to their comparatively low strength, while drilled solid-forged shafts offer higher strength but incur significant material waste and require extensive machining, followed by costly quench-and-temper heat treatment. This contradicts current sustainability goals, as large volumes of steel must be cast, forged, and subsequently removed.

Directly hollow-forged shafts are typically manufactured by upsetting the heated ingot, followed by piercing [24]. For that, the ingot is first brought to forging temperature and oriented in the axial direction under the press. During upsetting, the height of the block is substantially reduced, causing complete deformation and initiating recrystallization, which improves the material properties [24]. Additionally, higher total strains can subsequently be introduced in the cogging process, also benefiting the mechanical performance. After upsetting, a solid mandrel is driven into the block in multiple steps to pierce it, displacing material outward and creating the hollow core [25].

The subsequent cogging of the pierced blank occurs over several hundred strokes using a mandrel held by a manipulator. During this stage, the hollow shaft obtains its final geometry and material properties, making the design of the forging strategy critical. Previous studies have shown that strategies employing two forming tools and a movable mandrel achieve significantly larger elongations while minimizing undesired increases in inner diameter [26]. Besides conventional freeforging parameters such as reduction ratio and bite ratio, the saddle geometry also strongly influences the material flow. V-saddles are particularly effective in maximizing elongation [25].

In stepped hollow shafts or those with flanges, such as wind turbine rotor shafts, special attention must be paid to regions with large radial thickness. Prior work suggested one-sided upsetting in closed dies to form the flange, though this reduces the inner diameter, requires a dedicated die for each shaft type, and limits process flexibility [27].

Following forging, machining and heat treatment are normally required to achieve the final mechanical properties, both of which contribute to significant energy consumption. Newly developed air-hardening ductile steels (AHD-steels) provide a potential solution, as they transform during air cooling after forging, eliminating the need for quench-and-temper treatment. Compared to conventional QT-steels, AHD-steels can also provide improved static and cyclic material properties. However, their applicability to large wall thicknesses must be investigated, because the transformation requires maintaining a critical cooling rate. Previous studies on small components demonstrated substantial CO2 reductions, highlighting their relevance for sustainable manufacturing [28].

Objective for the Use Case Wind Turbines

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The objective is to demonstrate that rotor shafts for wind turbines can be directly hollow-forged from AHD-steel to enable lightweight design and increased power density. Beyond geometric manufacturability, it must be shown that this method offers economic advantages compared to conventional processes. A key factor is the reduction of required input material through targeted control of the material flow [29]. Additionally, the critical cooling rate of AHD-steel must be maintained even in areas of the shaft with large wall thickness to ensure full hardening.

Procedures and Methods for the Use Case Wind Turbines

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An implicit finite element simulation model of the hollow-forging process was developed in FORGE NxT 4.0. The model includes a flat upper saddle, a V-shaped lower saddle, the workpiece, forging mandrel, and manipulator.

An adaptive tetrahedral finite element mesh comprising more than 22,800 elements was employed, with local refinement in the forming zones to accurately resolve large strain gradients and complex material flow behavior. The material response was described using a thermo-viscoplastic constitutive model based on tabulated flow stress data as a function of temperature and strain rate. Thermal conductivity was implemented via temperature-dependent coefficients, while density, specific heat capacity, emissivity, and the linear thermal expansion coefficient were treated as constant parameters. Frictional interactions were modeled using a combined Coulomb friction law with a limiting shear stress criterion. For the tool-workpiece interfaces, a friction coefficient of μ=0.4 and a shear yield limit of 0.8 were applied, whereas the lubricated mandrel-workpiece contact was characterized by a reduced friction coefficient of μ=0.15 and a shear yield limit of 0.3 . Heat transfer between the workpiece and the forming tools was represented by a heat transfer coefficient of 10,000 W/m2· K, while heat exchange with the surrounding air was modeled using a coefficient of 15 W/m2· K. Throughout the model development, particular emphasis was placed on an accurate representation of the manipulator kinematics and the complex interactions with the forming tools, as these factors were identified to have a decisive influence on the resulting material flow. The numerical model was developed in close cooperation with the simulation software provider and industrial forging companies, ensuring that the implemented boundary conditions and process assumptions reflect state-of-the-art industrial practice and expert knowledge.

Using this model, the entire process chain, from raw ingot (raw block) to finished hollow-forged preform (raw part), was simulated over multiple heats, including all reheating steps (see Fig. 3). The process begins with initial cogging of the polygonal casting block, followed by upsetting and piercing. The main hollow-forging sequence consists of three heats and more than 380 strokes, resulting in the raw hollow shaft.

[picture]

Fig. 3. Simulated process chain for hollow forging, including reheating.

Fig. 3. Simulated process chain for hollow forging, including reheating.

Results and Evaluation for the Use Case Wind Turbines

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Simulation results revealed that the initially designed forging sequence led to a pronounced conical formation of the flange, which was directly associated with excessive axial material flow and, consequently, a substantial surplus of material in the flange region (see Fig. 4a)). This outcome was identified as incompatible with the targeted improvements in material and energy efficiency. To address this issue, the axial allowances of the forging geometry were systematically reduced, and the resulting material flow was analyzed in detail to assess whether the required cross-sectional geometry could still be accommodated within the final forged shape.

These analyses showed that a nominal flange width corresponding to the geometric width of the target geometry led to an instability in the flange region: Once a critical wall thickness was undershot, the flange exhibited buckling toward the manipulator, preventing full die filling and thus resulting in a deviation from the target geometry (see Fig. 4b)).

[picture]

Fig. 4. Cross-section of the hollow shaft after forging the flange area: a) Original dimensions, b) reduced dimensions, c) adapted forging strategy.

Fig. 4. Cross-section of the hollow shaft after forging the flange area: a) Original dimensions, b) reduced dimensions, c) adapted forging strategy.

An adapted process route successfully resolved this issue: after forming the flange in the first heat, the forging ingot is removed from the mandrel, aligned axially under the press, and axially upset using simple flat dies until the flange is fully realigned. The use of simple die geometries eliminates the need for dedicated dies and maintains process flexibility.

The procedure is shown in Fig. 5.

[picture]

Fig. 5. Adapted process chain with intermediate upsetting of the ingot.

Fig. 5. Adapted process chain with intermediate upsetting of the ingot.

With this adapted strategy, the material mass of the forged part was reduced from approx. 27 to 22.4 t , yielding a 17% material saving based on process optimization. This directly reduces the required ingot mass and thus enhances both economic and ecological performance.

However, for the current predesign, the hollow-forged shaft requires 26.5 t AHD steel as input material, which is 24% more material than needed for the cast shaft (EN-GJS-400-18-LT). This is due to the conservative forging surcharges (allowance) that have been assumed ( 160% in totalfor forging, compared to 32% for casting). In order to get towards costs for forging that are comparable to casting, the total forging surcharges need to be reduced from currently 160% to around 50% of the final shaft mass.
Nevertheless, the resulting rotor shaft made of hollow-forged AHD steel has a mass of 10.2 t , which is 37% lower than the cast shaft with 16.2 t . Thus, the final weight saving in the product has already been demonstrated successfully and enables higher power density of the generator, thinner tower designs and reduced logistic costs. [29]

Cooling simulations further showed that, due to reduced flange dimensions, the thickest sections achieve the critical cooling rate required for proper air hardening. Comparison with experimentally determined CCT diagrams confirms that full transformation is expected, supporting the suitability of AHD-steel for large forged geometries like hollow rotor shafts for WEA.

Conclusion and Outlook

The original version of this paper is available on https://www.scientific.net/KEM.1051.179.pdf

In the introduction, the paper has presented exemplary potentials for the improvement of sustainability and suggested to structure sustainability contributions in metal forming routes with four groups of improvement levers: material efficiency, process efficiency, product efficiency and circularity. The four-lever structure together with specific examples from the field of open-die forging, mentioned in this paper, is shown in Fig. 6.

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Fig. 6. Depiction of a four-lever concept for sustainability contributions in metal forming routes and specific examples from open-die forging.

Fig. 6. Depiction of a four-lever concept for sustainability contributions in metal forming routes and specific examples from open-die forging.

The use case presented in the main part of the paper demonstrates that hollow-forged rotor shafts made from air-hardening ductile steels offer substantial potential for material and CO2 savings in wind turbine drivetrain manufacturing. A complete hollow-forging simulation model was established, enabling systematic reduction of excess material. An adapted forging process with intermediate upsetting achieves significant material efficiency without compromising manufacturability or cooling behavior.

Future work will focus on the further reduction of the forging surcharges and will demonstrate a proof-of-concept by forging representative shaft sections as well as the manufacture of a full demonstrator shaft for validation of the simulation model and testing on drivetrain test benches or pilot turbines.

These steps will further validate the industrial feasibility of AHD hollow-forged rotor shafts and support their adoption as a more sustainable alternative to current manufacturing routes.

Acknowledgement

The original version of this paper is available on https://www.scientific.net/KEM.1051.179.pdf

We want to thank our colleagues from the Steel Institute and the Center for Wind Power Drives, both from RWTH Aachen University, for their contributions and for the great collaboration on the research topic of this paper.

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The original version of this paper is available on https://www.scientific.net/KEM.1051.179.pdf

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Key Engineering Materials, ISSN: 1662-9795, Vol. 1051, pp 191-202

doi: 10.4028/p-jx8Kt7

© 2026 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2026-01-19

Revised:
2026-02-24

Accepted:
2026-02-24

Online:
2026-04-17

Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

The original version of this paper is available on https://www.scientific.net/KEM.1051.191.pdf

Agata Sposato 1,a*, Giovanni De Martino 2,3, b, Gianni Stigliano 4,c, Claudio Cignali 4, d, Michele Dassisti 2,e, Domenico Umbrello 1,f

¹Department of Mechanical, Energy and Management Engineering, University of Calabria, Rende (CS) 87036, Italy

2 Department of Mechanical, Mathematics and Management, Polytechnical University of Bari, Bari (BA) 70126, Italy

3 Department of Sciences, Technologies and Society, University School of Pavia - IUSS Pavia, Pavia (PV) 27100, Italy

4 KAD 3D S.r.I., Monopoli (BA) 70043, Italy

a*agata.sposato@unical.it, b giovanni.demartino@poliba.it, c stigliano@kad3.com,

d  cignali@kad3.com, e  michele.dassisti@poliba.it, f  domenico.umbrello@unical.it

Keywords: photovoltaic panels, cryogenic delamination, finite element modelling, material characterisation.

Abstract

In the global transition towards renewable energy, a leading role is played by photovoltaic (PV) technologies. However, the increasing growth of installed PV panels, together with the rise of the number of modules reaching their end-of-life phase, make the sustainable management of electronic waste a crucial aspect. The reduction of energy consumption and polluting emissions and the maximization of material recovery represent the ultimate purpose of demanufacturing processes. Here, cryogenic delamination is proposed as an innovative strategy, as it exploits the thermal and mechanical properties of PV module constituents to achieve the cleanest possible separation of layers, allowing for the recovery of strategic materials (silicon, aluminium, silver, copper). This work aims to combine experimental and numerical approaches in order to obtain a comprehensive understanding of the fundamental mechanisms governing the process: the overall objective is represented by the process optimization to enable the exploration of various operating conditions without the need for costly and time-intensive experimental campaigns and, ultimately, the implementation of such technology at the industrial scale.

Introduction

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Photovoltaic cells are the functional unit of a photovoltaic panel (PVP); typically manufactured from monocrystalline or polycrystalline silicon, they enable the conversion of light into electrical energy [1]. PV cells are incorporated into the PVP multilayer by two polymer layers: the first layer works as an adhesive, bonding the silicon cells to the glass substrate; the second one protects the cells against adverse environmental conditions [2,3]. The four layers are placed onto a backsheet and mechanically supported by a surrounding frame (Fig. 1) [4].

[picture]

Fig. 1. Layered structure of a crystalline silicon PV module. [2].

Fig. 1. Layered structure of a crystalline silicon PV module. [2].

A hot lamination process ensures the bonding of the various layers, as complete adhesion between materials and effective insulation from external agents are reached [5]. However, given its irreversible nature, it considerably hinders PVPs disposal at the end of their service life: thus, issues in separating the constituent materials often lead to panels being discarded in landfill sites [6].

In view of the continuous growth in installed PV capacity and the increasing number of panels entering their end-of-life phase, the sustainable management of PV waste represents a crucial aspect. The generation of waste from PV modules is directly associated with their average operational lifetime, which is estimated to range between 20 and 30 years [7]; given global PV installations peaked between 2010 and 2020, a doubling of photovoltaic waste volumes is expected by 2035 . As a result, the sustainability of dismantling processes must focus on reducing energy consumption, minimising polluting emissions, and maximising the efficient recovery of constituent components [8]. From this perspective, demanufacturing techniques represent a more targeted and strategic approach to manage electronic and industrial waste, as they enable the separation of recyclable materials and the recovery of components that remain functional or repairable [9]. In this way, it is possible to not only mitigate the environmental impact by reducing the amount of waste destined to landfill disposal or incineration, but also to enhance the recovery of valuable resources while fostering new economic opportunities centred on recycling and material reuse.

Existing demanufacturing methods can generally be categorised as physical, thermal or chemical treatments, listed in order of increasing quality of recovered components, as well as rising levels of complexity and cost [10, 11]. However, despite the wide range of approaches proposed in the literature, an optimal demanufacturing process has yet to be identified. The main limitation of technologies employed to date lies in the fact that, whether destructive or semi-destructive (only the connecting elements are destroyed), such methods tend to substantially alter the physical and chemical properties of the processed materials, often making them unsuitable for applications requiring high technological performance [12].

A promising strategy involves the application of thermo-mechanical treatments to induce delamination, intended as the progressive weakening and eventual rupture of the interfacial bonds between adjacent layers, leading to their separation [13]. The demanufacturing approach proposed here, known as cryogenic delamination, exploits the distinct thermal and mechanical properties of the materials constituting a PV panel in order to achieve the cleanest separation of its components [14].

The purpose of the present study is to conduct in parallel experimental tests and a numerical modelling study within a finite element analysis (FEA) framework in order to obtain a comprehensive understanding of the mechanisms governing the cryogenic delamination process. The objective is to calibrate the model against experimental evidence collected in the laboratory so that the two aspects are constantly interconnected and the physics of the problem are never lost sight of. The research thus performs a comparative numerical and experimental investigation in order to develop and validate a FE model capable of accurately reproducing the actual behaviour of the system. A high level of correspondence allows for both the optimisation of the procedure and the assessment of its potential scalability for continuous industrial application. Once validated through comparison with experimental data - particularly with regard to temperature evolution, as well as the stresses and displacements generated -, the FE model allows for the exploration of different process conditions without the need for costly and time-consuming experimental campaigns [15, 16].

It is important to point out that this work represents the first step in a broader study aimed at optimising the cryogenic delamination process to enable the implementation of the technology at an industrial level. For this reason, the following discussion is purely phenomenological in nature and focuses on the study of thermal aspects for a quantitative and in-depth analysis; on the other hand, at present, mechanical analyses are limited to an exploratory and therefore qualitative state. Future studies will aim to investigate the correspondence between the actual behaviour of the delaminated photovoltaic module and the behaviour predicted by the numerical analysis, in order to validate the models chosen to represent the elastoplastic behaviour of the various materials and their damage evolution up to failure.

Theoretical Bases of the Cryogenic Delamination Process

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In PV modules, materials with significantly different thermal behaviours must coexist while preserving structural integrity throughout their operational lifespan. The inorganic constituents namely crystalline silicon and annealed soda-lime silicate glass - exhibit relatively low and comparable coefficients of thermal expansion (CTE) [17]; by contrast, ethylene-vinyl acetate (EVA), which is the most commonly employed encapsulant, displays a CTE almost two orders of magnitude higher [18].

Within the operational temperature range, the encapsulant experiences both glass transition and melting phenomena [22], making its thermal response a critical factor for long-term module durability. The mechanical properties of the material change abruptly across the glass transition temperature ( Tg ) [23]: below this threshold, the polymer is stiff and brittle, whereas above it, it exhibits a viscoelastic and compliant behaviour. This transition has a decisive influence on the mechanical integrity of the cell-encapsulant interface. Consequently, close attention must be paid to both the value of Tg and the evolution of the storage modulus, which stabilises at approximately −40∘C following crosslinking [24] - a temperature commonly identified as the lower limit for reliable module operation. A thorough understanding of this embrittlement mechanism is therefore essential for predicting thermomechanical fracture at module interfaces.

The relevant values of thermal expansion coefficients and glass transition temperatures for the principal materials used in PV modules are reported in Table 1.

Table 1. Thermomechanical properties of PV module materials.

Material | Type of material | CTE [K⁻¹] | Tg [°C]
Crystalline silicon (c-Si) | Inorganic semiconductor | 2.6 × 10⁻⁶ [17] | –
Soda-lime silicate glass (annealed) | Inorganic glass | 9 × 10⁻⁶ [19] | –
EVA (encapsulant) | Polymeric encapsulant | 1.6 × 10⁻⁴ – 2.5 × 10⁻⁴ [18] | -36 (pre-/post-lamination) [20] -33.1 (PV-grade EVA) [21] -16.9 (after crosslinking) [22]

In multi-material systems exposed to cyclic thermal variations, interfacial stresses arise as a direct consequence of mismatches in CTE [21]. In the case of PV panels, in response to thermal variations the polymer undergoes expansion and contraction to a far greater extent than the surrounding crystalline materials. Thus, stress accumulation at the glass-encapsulant and cell-encapsulant interfaces frequently act as initiation sites for microcracks along mechanically fragile crystallographic planes within the silicon layer during both manufacturing-related thermal treatments and in-service thermal cycling, particularly when operating temperatures approach or exceed the polymer glass transition temperature.

This temperature-dependent degradation mechanism occurring at the glass-EVA-silicon interfaces represents the conceptual foundation of cryogenic delamination, which deliberately exploits the pronounced contrast in thermal behaviour between the crystalline layers and the EVA encapsulant. By imposing controlled thermal excursions, the process selectively weakens interfacial adhesion and promotes crack propagation through the encapsulant, ultimately resulting in interfacial failure, while preserving the structural integrity of high-value materials. The thermo-mechanical procedure has been formalised in patent [25]. By applying thermal gradients, tangential stresses are generated at the interfaces; by inducing selective failure within the polymeric interlayers while maintaining the integrity of the rigid components, the process enables effective layer separation without the need for aggressive chemical treatments or high-temperature pyrolysis, thereby preserving the quality and potential reusability of the recovered silicon and glass substrates.

Materials and Methods

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Testing methods.

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The experimental campaign was carried out on specimens extracted from end-of-life crystalline silicon photovoltaic modules, encompassing both mono- and polycrystalline technologies to ensure the industrial relevance and practical applicability of the results to large-scale recycling processes, as these constitute the predominant categories of the current photovoltaic panel waste. The representative modules stratigraphy (Table 2) comprises: a front layer of low-iron soda-lime glass; a first encapsulating EVA layer; crystalline silicon solar cells featuring metallisation grids, typically consisting of silver busbars and aluminium back contacts; a second EVA layer; a polymeric backsheet, typically made of polyvinyl fluoride (Tedlar). The backsheet will not be considered for the following analyses, as its behaviour does not influence the delamination phenomena.

Table 2. Stratigraphy of the considered photovoltaic modules.

Material | Thickness [mm]
Glass | 3.2
EVA | 0.4
Silicon | 0.2
EVA | 0.4

Specimens were prepared using water-jet cutting technology to preserve the structural integrity of the laminated assembly and minimise the likelihood of introducing microcracks or other artefacts prior to testing. Each sample was individually weighed and systematically catalogued to ensure full traceability and to allow a rigorous mass balance analysis throughout the delamination procedure. The sample dimensions were standardised to 160×160 mm2 to limit variability in thermal response; however, a certain level of dimensional heterogeneity was tolerated, owing to constraints associated with the availability of decommissioned photovoltaic modules.

Experimental set-up.

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The experimental apparatus exploited a cryogenic system employing a pressurised liquid nitrogen (LN2) jet: this arrangement enabled accurate control over both the magnitude and the localisation of the thermal shock to be imposed, in accordance with the principles described in the patent [25]. To this end, a cryogenic Dewar vessel was connected to a pressure regulation unit equipped with an onoff valve, which allowed modulation of the LN2 jet delivered to the target surface of the photovoltaic sample. Different operating pressures were investigated, in order to optimise the thermal gradient and the effectiveness of the delamination process. The selected operating pressure was 9 bar, representing a balance between jet momentum, liquid nitrogen consumption and safety constraints; under these conditions, the LN2 jet produced a rapid and localised temperature decrease on the rear surface of the module, thereby inducing the required thermomechanical stresses across the laminate thickness. The most relevant parameters governing the delamination process are summarised in Table 3.

Table 3. Operating parameters of the cryogenic delamination setup.

Parameter | Specification | Parameter | Specification
Cryogenic medium | Liquid nitrogen (LN₂) jet | Heating system | Two hot air guns
Supply system | Dewar vessel with pressure regulation | Heating temperature | ≈ 300 °C
Operating pressure | 9 bar | Thermal strategy | Front heating + rear cryogenic cooling
Cooling target | Rear surface of PV module | Aspiration system | Vapor extraction near sample holder

The experimental procedure followed a standardised sequence of steps to induce the differential thermal shock mechanism underlying the cryogenic delamination.

Front surface heating. The photovoltaic sample was mounted vertically within the test rig and heated by two industrial hot-air guns directed towards the glass surface until a surface temperature of approximately 300∘C was reached, as measured using an infrared thermometer. This phase typically lasted a few minutes, depending on ambient conditions and sample thermal mass.

Cryogenic jet application. The pressurised LN2 jet was directed onto the rear surface of the sample (corresponding to the backsheet side). An operating pressure of 9 bar ensured a highvelocity jet capable of rapidly extracting heat from the rear layers of the laminate. The cryogenic jet was applied for approximately 1 min per sample, a duration determined empirically to induce a significant thermal shock while limiting nitrogen consumption.

Post-treatment observation. When the sample reached thermal equilibrium, visual and tactile inspections were carried out to evaluate the extent of delamination, the presence of cracks and the integrity of the individual layers.
The combined action of front-side heating and rear-side cryogenic cooling generated the differential expansion and contraction stresses responsible for interfacial failure within the adhesive layers. The apparatus incorporated an aspiration system to manage the nitrogen vapour produced during LN2 evaporation; this system fulfilled both safety and operational roles, ensuring adequate ventilation of the working environment and limiting the formation of condensation and frost on adjacent equipment.

Numerical modelling.

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Finite element (FE) simulations applied to PV modules cryogenic delamination represent a powerful computational tool to estimate the temperature evolution, the distribution of interfacial stresses and the damage mode and paths during thermal cycling processes. The FE software employed for the analyses was DEFORM, developed and distributed by Scientific Forming Technologies Corporation (SFTC).

Material characterisation.

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To represent the effects of the applied thermal load - being the only external loading condition in terms of both temperature variations and stress and deformations fields induced within the multilayer assembly, both thermal and mechanical aspects were included in the analysis. The multilayer system was modelled to replicate the PV module employed for the experimental tests, specifically a 160×160 mm2 square specimen (Fig. 2a). The thickness of each layer was established on the basis of the stratigraphy reported in Table 3; given the geometric regularity of the system, a brick-type mesh was adopted. The number of elements assigned to each component was selected to ensure full nodal compatibility at the interfaces (Fig. 2b).

[picture]

Fig. 2. a) 3D model of the PV module. b) 2D meshed multilayer module.

Fig. 2. a) 3D model of the PV module. b) 2D meshed multilayer module.

The material properties attributed to each layer were obtained from the relevant scientific literature; the principal thermo-mechanical parameters are summarised in Table 4. For the purposes of numerical analysis, the properties assigned to the encapsulant layers refer to a homogeneous and
non-aged material. These are, of course, simplifications: in fact, not only do polymers, by virtue of their molecular composition, not have homogeneous characteristics, but also real end-of-life PV modules have necessarily undergone ageing and therefore experienced a degradation of their initial characteristics due to the thermal cycles and the action of atmospheric agents to which they were constantly subjected.

Table 4. Thermomechanical properties of PV module materials for simulation purposes [17-22].

Material | Thermal conductivity [W/mK] | Specific heat [J/kgK] | Emissivity [-] | Young's modulus [MPa] | Poisson's coefficient [-]
Glass | 1 | 800 | 0.9 | 70 | 0.65
EVA | 0.25 | 2300 | 0.9 | 0.65 | 0.43
Silicon | 150 | 700 | 0.65 | 160 | 0.28

A key aspect concerned the definition of the constitutive equations governing the plastic flow behaviour of the materials involved (Table 5).

At 300∘C (cryogenic delamination process starting temperature), EVA does not behave like a normal polymer in melt, as it undergoes rapid thermal degradation: in fact, its typical processing temperature is usually below 200∘C, while deacetylation begins at 290∘C−300∘C, with the formation of double bonds and polyenes. However, when analysed over a very short time window, the plastic flow law of molten EVA follows the pseudoplastic fluid (power-law fluid) model, characterized by shear thinning, with main parameters: τ shear stress; γ˙ shear rate; n flow behaviour index (less than 1 for pseudoplastic polymers); K consistency index (viscosity).

From a molecular point of view, glass is an amorphous solid (or at least comparable to a supercooled liquid with extremely high viscosity). Unlike crystalline materials, glass has a disordered and disorganized structure. At 300∘C, since it is still below its glass transition temperature (usually Tg>500∘C ), glass behaves like a linear elastic or viscoelastic solid rather than a viscous fluid in plastic flow. Its behaviour is therefore described by Hooke's law, whereby the stress state developed within the material depends on its stiffness (expressed in terms of Young's modulus E) and the deformation achieved ε.

At 300∘C, which corresponds to about one third of its melting point, crystalline silicon is in a brittle-ductile transition regime, and its plastic deformation is mainly governed by the movement of dislocations (in particular by the overcoming of Peierls barriers): these are partial dislocations that form microtwins and stacking faults; however, the movement of the dislocation segments is still quite limited. Silicon plastic behaviour is therefore governed by an exponential or power law (Arrheniustype law), typical of crystalline viscoplasticity and similar to that of low-temperature creep, where the plastic deformation rate (γ˙p) is proportional to a power of the resolved stress (τ). τc is the Critical Resolved Shear Stress (CRSS); n is the strain-rate sensitivity exponent.

Table 5. Flow stress laws of PV module materials.

Material | EVA | GLASS | Silicon
Flow Stress law | τ = K · γ̇n | σ = Eε | γ̇p = γ̇₀|τ/τc|n

Boundary and loading conditions.

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The boundary conditions were defined to reproduce the experimental setup configuration, where the photovoltaic module is clamped along a portion of its perimeter to constrain displacements in all spatial directions within the clamped region (Fig. 3).

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Fig. 3. a) Clamp used for the experimental tests; b) modelled boundary condition.

Fig. 3. a) Clamp used for the experimental tests; b) modelled boundary condition.

In terms of thermal interactions, natural (i.e. non-forced) heat exchange was prescribed between all external surfaces of the PV multilayer and the surrounding environment, which was assumed to be at a uniform temperature of 20∘C. The external heating device was represented as an industrial heat gun characterised by a nominal power of 1.8 kW , three temperature settings ( 50/300/600∘C ), and three airflow rates (200/350/5001· min−1). The convective heat flux was evaluated using the fundamental thermodynamic relation (Eq. 1):

Q=hA( Tf−Tp)(1)

where: Q is the heat flux [W];h is the convective heat transfer coefficient [W/(m2· K)];A is the heat exchange area [m2];Tf is the temperature of the fluid (hot air) [ ∘C];Tp is the temperature of the wall (i.e. the colder surface) [∘C]. Since h depends on parameters such as type of convection, surface geometry, and fluid properties, an average representative coefficient of 225 W/(m2· K) was adopted, consistent with forced convection under high-velocity turbulent flow conditions. The heat gun was considered to operate at its maximum temperature ( Tf=600∘C ). As the heat source is positioned at an approximate distance of 10 cm from the glass surface and therefore a temporal delay precedes the onset of significant heating, a time-dependent ramp-shaped heat flux was implemented and calibrated against experimental measurements.

Regarding the cryogenic fluid application, while experimentally LN2 was delivered via a nozzle, for modelling purposes the entire upper EVA surface was subjected to its action, corresponding to a thermal boundary condition of T=−182∘C with the associated convective heat transfer coefficient defined as a time-dependent function according to the Astakhov-Joksch model (Eq. 2):

hcryo =0.20e0.35 g0.33 Vf0.65kf0.67cp0.33γf0.33vf0.32(2)

Interfacial bonding conditions.

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The interactions between adjacent layers were defined through interfacial bonding conditions influencing both heat transfer and mechanical behaviour, including the development of residual stresses. For each interface (Glass-EVA1, EVA1-Silicon, Silicon-EVA2), a conductive heat transfer coefficient was calculated according to Eq. 3:

1UCond =∑Snkn(3)

where Ucond  defines the overall conductive heat transfer coefficient [W/(m2· K)],Sn is the thickness of the n -th layer [ m ], and kn represents the thermal conductivity of the n -th layer [ W·m−1· K−1 ].

In addition, the Sticking Condition option was enabled to model a complete and perfect adhesion at each interface between layers, thereby reproducing the bonding effect provided by the polymer encapsulant. To model delamination triggered by cryogenic loading, a non-geometric separation criterion was subsequently implemented: more specifically, interfacial separation was allowed once a prescribed fraction of the material flow stress was attained. This threshold value was calibrated through a combination of preliminary numerical simulations performed without delamination and experimental evidence, which indicated the onset of detachment after approximately 5s from the beginning of the process. However, in this initial investigation, no fracture or damage criteria were
implemented for the finite elements within the FE software, thereby preventing an accurate prediction and description of material cracking phenomena.

The numerical analysis was performed over a total duration of 100s, discretised into 500 time steps, so as to ensure a detailed representation of the thermo-mechanical evolution of the system.

Results and Discussion

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In laboratory tests, the observed delamination patterns proved to be markedly complex: the cryogenically treated samples displayed pronounced structural degradation of the laminate, involving internal cracking, polymer fragmentation, and irregular weakening and failure at the glass-EVA and EVA-silicon interfaces. This heterogeneous response can be ascribed to variations in module ageing, encapsulant compositions, and thermal histories across the end-of-life panels examined (Fig. 4).

More specifically, internal cracking of the silicon solar cells was consistently observed during cryogenic exposure; this phenomenon was attributed to the combined action of thermal shock and the inherent brittleness of crystalline silicon at extremely low temperatures. Such fragmentation enhances the mechanical release of silicon particles from the surrounding polymer matrix, thereby facilitating subsequent material recovery processes; conversely, cell fracture also constitutes a limitation, as it precludes the retrieval of intact silicon wafers suitable for direct reuse.

The EVA encapsulant layers were only partially and discontinuously removed, with substantial residual adhesion persisting on both the glass and silicon surfaces. Instead of separating as continuous films, the polymer predominantly failed cohesively, leaving fragments attached to both substrates. This partial separation suggests that controlled cryogenic shock effectively degrades interfacial bonding and induces mechanical stresses within the polymer matrix, yet remains insufficient to achieve complete material liberation in a single processing step.

Fig. 4. Post-treatment PV laminate samples.

On the other hand, the numerical results indicated a clear detachment of the EVA surface layer in the portion of the PV module not constrained by the clamp, whereas separation of the silicon cell substrate from the lower encapsulant layer appeared significantly more limited (Fig. 5) This result, which is essentially phenomenological in nature, is confirmed by experimental observations in terms of the extent of the delaminated area, since in the time interval considered (5s) the delamination involves only a small portion of the material. Obviously, as the exposure time of the photovoltaic module increases and consistently with a variation of the temperature reached inside the multilayer, the area undergoing delamination changes accordingly.

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Fig. 5. FE predicted delamination: a) on isometric view; b) on lateral view.

Fig. 5. FE predicted delamination: a) on isometric view; b) on lateral view.

Notwithstanding this agreement, a substantial divergence was identified between the numerical outcomes and the experimental evidence, as the former showed an idealised layer-by-layer separation exhibiting clean interfacial failure at the glass-EVA and EVA-silicon interfaces. The mismatch between the neat delamination predicted numerically and the incomplete, heterogeneous separation observed experimentally can be attributed to the preliminary nature of the FE model. As specified among the assumptions made in order to simplify the modelling in such an initial investigation phase, no fracture criteria were implemented: the absence of specific material models aimed at defining the damage progression and the cracking mode for the finite elements necessarily prevents an accurate description of material breaking phenomena, representing each element as deformable but not subject to internal disruption. The introduction of appropriate fracture criteria would obviously enable the prediction of the location and onset of material failure as a function of the stress and strain states attained during processing, together with material-specific damage parameters. Accordingly, future developments of the model will concentrate on integrating a dedicated fracture subroutine informed by recent literature: such an improvement is expected to narrow the gap between simulation and experimental observations, ultimately providing a robust predictive tool for the analysis of PV module delamination under diverse thermal and mechanical conditions. The availability of such a model would also reduce reliance on costly and time-consuming experimental campaigns.

As stated before, thermal aspects were evaluated quantitatively for validation purposes: surface temperature was therefore adopted as the main variable for evaluating the boundary conditions governing heat exchange with the environment. Experimentally, the LN2 jet was applied once the external glass surface reached a uniform temperature of 300∘C. In the simulation, the corresponding temperature at this stage was 293∘C, resulting in a percentage deviation of 2.3%, which can be considered negligible (Fig. 7a). After cryogenic treatment, the predicted glass surface temperature decreased to 258∘C, compared with an experimentally measured value of 260∘C; the associated error of 0.4% further corroborates the satisfactory thermal accuracy of the model (Fig. 7b).

[picture]

Fig. 6. Temperature distribution: a) before the cryogenic treatment; b) after the cryogenic treatment.

Fig. 6. Temperature distribution: a) before the cryogenic treatment; b) after the cryogenic treatment.

At the current stage, assessment of the plastic behaviour of the materials through direct comparison between numerical results and experimental data was not feasible, as no instrumentation was employed to monitor stress states within the multilayer assembly. Consequently, it cannot yet be ascertained whether the implemented plastic flow laws can capture the actual stress-strain behaviour. Looking ahead, given the internal nature of the defects, Non-Destructive Techniques (NDT) based on integrated sensors are expected to be used to measure stress levels and monitor the progression of deformations during delamination. In terms of stress, since techniques capable of detecting both residual (generated during production) and operating stresses - often concentrated in interface areas - are required, Acoustic Emission (AE) stands out as it allows delamination to be monitored in real time: in fact, piezoelectric sensors detect the transient elastic waves generated by the sudden release of energy during matrix cracking or layer separation. On the other hand, concerning the assessment of deformations, Digital Image Correlation (DIC), as a non-contact optical technique, enables the measurement of displacement and surface deformation fields and thus the identification of areas of high deformation associated with the opening of the delamination crack.

Conclusion

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Cryogenic delamination was presented here as an innovative as well as environment- and energyfriendly demanufacturing strategy for end-of-life PV panels, capable of enabling the separation of the materials composing the multilayer structure by exploiting their differences in thermal and mechanical characteristics.

From an experimental perspective, post-treatment analysis revealed a partial separation of the constituent materials, with substantial residual polymer adhesion on both glass and silicon surfaces. The experimental evidence thus confirms that cryogenic delamination functions effectively as a mechanical pre-treatment, as it weakens interfacial bonding and induces fragmentation within the multilayer assembly; however, it is not sufficient, on its own, to achieve complete material separation. These outcomes have led to the formulation of a two-stage recycling strategy - currently under development - where cryogenic thermal shock, representing the first operating stage, is subsequently complemented by a chemical treatment intended to accomplish total material separation and surface cleaning. This second phase is designed and expected to remove polymer residues through the application of mild chemical agents under controlled conditions, thereby minimising environmental impact. The sequential coupling of mechanical and chemical processes is anticipated to exploit the strengths of both approaches while alleviating their respective shortcomings.

In terms of numerical modelling, the study focused on phenomenological and thermal aspects for a quantitative analysis. The calibration of the FE model against laboratory experiments revealed a good agreement in terms of surface temperature evolution, thus revealing an accurate modelling of the thermal aspects of the materials involved. On the other hand, mechanical analyses had an exploratory and qualitative nature, given the limits exhibited both in the measuring systems for stress and strain evolution and in the definition of criteria for reproducing the damage progression and final cracking of materials involved in the FE environment. Forthcoming efforts will therefore concentrate on validating of the simulated plastic behaviour through a dedicated experimental programme aimed at monitoring the stress states and the deformation progression developing within the multilayer structure, as they are responsible for material detachment. In parallel, integrating an appropriate fracture criteria in order to reproduce the material fragmentation induced by the hot-cold thermal shock sequence will be pursued.

The ultimate purpose of this research is to develop a robust predictive model capable of faithfully representing delamination phenomena under well-defined operational conditions, making it possible to facilitate the industrial-scale implementation of the proposed process.

Acknowledgement

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This work was supported by the European Union - NextGenerationEU, under the Italian National Recovery and Resilience Plan (PNRR), Mission 4 Component 2 Investment 1.3 - Partenariato Esteso NEST (Network 4 Energy Sustainable Transition) - project code PE000021.

Sections "2. Theoretical bases of the cryogenic delamination process" "3.1. Testing methods" were produced while attending the PhD programme in Sustainable Development and Climate Change at the University School for Advanced Studies IUSS Pavia, Cycle XXXIX, with the support of a scholarship financed by the Ministerial Decree no. 118 of 2023, based on the NRRP - funded by the European Union - NextGenerationEU - Mission 4 "Education and Research", Component 1 "Enhancement of the offer of educational services: from nurseries to universities" - Investment 4.1 "Extension of the number of research doctorates and innovative doctorates for public administration and cultural heritage".

This work was supported by the Italian Ministry of Education, University and Research under the Programme "Department of Excellence" 2018-2022 and 2023-2027 - Italian Law 232/2016 (Grant No. CUP - D94I18000260001).

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Keywords Index

#

3D Printing Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

A

Aa6082 Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Additive Manufacturing Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Aluminium Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Aluminum Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Aluminum Casting Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Aluminum Extrusion Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

B

Bonding And Microstructure Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

C

Circular Economy Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Circular Manufacturing Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows; Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Circularity Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

Climate Change Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Composite Materials Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

Cryogenic Delamination Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

E

Efficiency Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

Electroplasticity Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment; On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Energy Consumption Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Environmental Impact Assessment Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

Eol Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

F

Fiber Metal Laminates A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Finite Element Modelling Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Flexible Manufacturing Systems Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Friction Consolidation Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Friction Extrusion Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Fused Filament Fabrication Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

G

Gas Forming Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

H

Hot Shortness Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

Hydroforming Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Hydrogen Storage Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

I

Innovation Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

L

Lca Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway; Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies; Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Lcc Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

Life Cycle Assessment (lca); Industrial Galvanizing; Steel Wire Production; Zn-al Coating; Energy Demand; Process Sustainability Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

M

Material Characterisation Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Mechanical Properties Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Metal Forming Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

Multi-criteria Tool Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

Multi-hole Extrusion Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

O

One-shot Forming A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Open-die Forging Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

P

Pellet-based Additive Manufacturing (am) Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

Photovoltaic Panels Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Polymer Composites Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Polypropylene (pp) Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

Principal Component Analysis (pca) Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

Process Robustness Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

R

Recycled Polypropylene (rpp) Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

Recycling Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation; Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project; Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Reinforced Concrete Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Reshaping On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping; Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

S

Scrap Recycling Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

Segmented Tools Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Sheet Metal Forming On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Solid-state Recycling Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

Spif Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Sustainability Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment; Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies; Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting; On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping; Comparing Different Sheet Metal Forming Processes for Reshaping Purposes; Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

Sustainable Manufacturing Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing; Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

T

Tensile Testing Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Texture Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Thermoplastic Composites A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Titanium Alloy Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Titanium Alloys On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Titanium Machining Chips Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

Topological Optimization Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

Tramp Elements Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

Twin-roll Casting Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

U

Upcycling Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Authors Index

A

Aslanidis, A. Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Astarita, A. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment; On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Aziz, U. Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

B

Bailly, D. Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting; Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

Barcellona, L. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Basile, V. Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

Benvenuto, L. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Berardinis, P. Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

Bianchi, I. Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment; Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Blackman, B. A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Borda, F. Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies; Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Buonadonna, P. Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

C

Carlone, P. Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Carta, M. Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

Cignali, C. Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Cozzolino, E. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment; On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Cusanno, A. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Czempas, D. Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting

D

Dassisti, M. Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Derosas, G. Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

Duflou, J. Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

D’Angelo, S. Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

E

Engel, B. Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

F

Fazio, D. On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Filice, L. Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies

Forcellese, A. Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

Forcellese, P. Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Fratini, G. Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Frohn-Sörensen, P. Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

G

Gabsa, S. Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Gagliardi, F. Sustainability-Driven Business Model in Mold Manufacturing: A Multi-Criteria Comparison between Conventional and Additive Technologies; Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Gao, C. A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Geueke, M. Ready for Circular Manufacturing? Metal Forming Processes Studies and Research Agenda

Gouverneur, M. Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

Grammatikos, S. Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

H

Hemyari, D. Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

Homberg, W. Material Transition by Friction Induced and Continuous Solid-State Recycling of Aluminum Scrap

Hosseini, P. Environmental Impact Assessment of Ti6AI4V Chips Recycling Using Shear Assisted Processing and Extrusion (ShAPE)

I

Ingarao, G. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

K

Klusemann, B. Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

L

Lamberti, T. Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Lertora, E. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Li, N. A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts

Lian, J. Overcoming Copper Limitations in Scrap Recycling via Twin-Roll Casting; Increasing Sustainability in Open-Die Forging by Means of Process Adaption and Innovation

M

Ma, X. Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Mancia, T. Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Mandolfino, C. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Martino, G. Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Mehtedi, M. Comparative Life Cycle Assessment of Industrial Zn and Zn-Al Hot-Dip Galvanizing Processes for Steel Wire Production

Mele, R. Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Micari, F. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Mignanelli, C. Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

N

Napolitano, F. Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

Nardo, M. Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

P

Palumbo, G. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Peymani, D. Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

Piccininni, A. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

Pietro, M. Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

Puleo, R. Comparing Different Sheet Metal Forming Processes for Reshaping Purposes

R

Rath, L. Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Ringen, G. Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Rosa, A. Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Russo, P. Recycling of Plastic Waste by Integrating Extrusion and Additive Manufacturing Techniques: Lessons Learned from RELIVE Project

S

Schmidt, V. Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Schröder, L. Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

Settineri, L. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Silva, A. Climate Change Impact of Innovative Structural Aluminium Reinforced Concrete, a Case Study in Norway

Simeone, A. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

Simoncini, M. Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment; Green Hydrogen vs Conventional Energy Sources: A LCA Case Study of Aluminum Casting

Sio, P. On the Effect of Electric-Pulsed Treatments on the Formability and Sustainability of Titanium Alloy Reshaping

Solomon, Y. Productivity and Quality Trade-Offs in Aluminum Extrusion: Towards Circularity Tolerant Process Windows

Sposato, A. Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Stigliano, G. Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

Strano, M. Processability of Recycled Polypropylene for More Sustainable Pellet-Based 3D Printing

Suhuddin, U. Investigation of Microstructure Evolution during AA6082 Chips Recycling through Friction Consolidation

T

Testa, A. Electroplasticity for Sustainable Metal Forming: A Comparative Energy Assessment

U

Umbrello, D. Numerical-Experimental Study of Delamination in Crystalline Photovoltaic Panels to Support Efficient Recycling

V

Verdini, T. Reuse of Fiber Reinforced Polymer Waste in MSLA 3D Printing: Mechanical Properties and Sustainability Assessment

W

Wu, H. A Novel One-Shot Forming Process Integrating Hot Form Quench (HFQ) of High-Strength Aluminium for Fibre-Metal Laminate (FML) Panel Parts