Materials Science Forum Vol. 1186

Abstract: For the reliable numerical simulation and design of compound forging processes involving dissimilar materials, an accurate representation of thermal boundary conditions is essential. In particular, the heat transfer coefficient (HTC) at the interface of the workpiece and the die strongly influences temperature distribution, material flow, and interfacial integrity. Despite its significance, the HTC is frequently modelled as constant in finite element (FE) simulation due to the lack of experimental data for forging-relevant conditions. Therefore, this study presents an experimental–numerical methodology for determining load-dependent HTCs representative for compound forging. A specialised test setup was used to reproduce the thermal–mechanical boundary conditions of hot bulk forming, by inducing contact pressures both below and above the flow stress of the workpiece material. Temperature histories were recorded using embedded thermocouples and analysed through an inverse numerical approach based on a one-dimensional (1D) finite element (FE) model. The influence of contact pressure, heating atmosphere, and lubrication on the HTC was systematically investigated for a S235JR specimen temperature of 600 °C. The results demonstrate a major pressure dependency of the HTC, whilst increasing for higher contact pressures. Oxide formation and lubrication were shown to significantly affect heat transfer behaviour, particularly while heating under atmospheric conditions. The presented approach provides process-specific HTC data that can substantially improve the predictive capability of numerical simulations for compound forging applications.

Abstract: Isothermal forging is a common method for manufacturing titanium alloys, but it involves complex processes and equipment. The oxidation of titanium leads to the formation of an alpha-case, which in turn promotes increased crack formation. To prevent this, inert gas is typically required. However, by encapsulating the titanium billet (Ti-6Al-4V) in a steel casing made of AISI 316L, a quasi-isothermal process can be achieved without the need for inert gas. This method maintains protection against oxidation while simultaneously reducing cooling. The sealing of the capsules is crucial to ensure that the titanium is effectively enclosed and protected from the surrounding gases. In this study, various encapsulation methods are compared, including rotary friction welding, diffusion bonding, and press-fitting a lid with an interference fit. The investigation involves differing contact conditions between the titanium and steel sleeve, as well as steel wall thicknesses of 2 mm and 4 mm. These factors showed no impact on the material flow or microstructure of the formed components. Encapsulation can prevent the formation of an alpha-case. Intermetallics form between the titanium and the steel capsule, depending on the contact conditions. The use of graphite as a separating agent prevents the formation of them.

Abstract: During the hot bulk forming of long parts, inhomogeneous distributions of deformations and temperatures occur. The gradients of these distributions lead to complex, overlaying residual stresses, which can cause critical geometric deviations and mechanical failures. Common finite element (FE)-simulations for designing a process are in principle capable to predict the thermal, mechanical and metallurgical effects, but require extended material models. Thereby, the total strain increment can be described through the partial strain components of the elastic, plastic, thermal transformation related and transformation plasticity strain. To allow the numerical prediction of the distortion of long hot formed parts, an experimental characterisation of the TRIP and backflow effects is presented for the steel 31CrMoV9. Time temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams are determined with JMatPro and verified by means ofmicrostructure analysis and hardness measurements. Based on these diagrams, the transformation plasticity is investigated through dilatometric tests whereby tensile and compressive loads are applied during the phase transformation. The martensite phase transformation showed the highest amounts of TRIP strains, whilst the bainite transformation exhibited lower strains but a high tensile backflow strain. For perlite the beginning of the phase transformation was delayed and its duration extended due to the induced loads.

Abstract: In forging, parallelism deviations between upper and lower dies lead to asymmetric flash formation and affect forming forces, die filling, and part integrity. The flash gap influences local flow resistance and is closely linked to flow behavior and dimensional precision. Conventional diagnostics often assess such deviations under no-load or quasi-static conditions and therefore may not capture the effective closing state at bottom dead centre (BDC) under process load. While modern approaches such as high-resolution optical tracking of ram deflection can provide valuable insight, they require dedicated and sensitive instrumentation and are often limited in scalability. In contrast, workpiece-based signatures inherently reflect process effects such as elastic deflections, guide clearances, frictional conditions, and thermal influences.This study investigates whether workpiece-related geometric features can serve as diagnostic signatures for detecting and quantifying closing-gap inclinations under load. The focus is on the locally resolved flash thickness, which reflects the effective closing gap at BDC. Because this gap results from both geometric alignment and load-dependent deformation, the evaluation targets the final load-bearing state. Comparative forging trials are performed on a press equipped with active parallelism control, where controlled misalignments are introduced. The resulting flash geometry is measured by laser triangulation to determine the resolution limit and to identify the deviation magnitude at which reproducible signatures can be detected under process-relevant conditions. In the investigated setup, flash-thickness asymmetry shows an increasing trend from closing-gap inclinations of ~0.25°, providing a markedly higher diagnostic sensitivity than the maximum forming force. Designed as a non-invasive and retrofit-capable method, the approach supports inline monitoring in high-volume forging. It further enables scalable, data-driven correlation of machine, process, and product data for condition-aware process optimization.

Abstract: The production of tailored hollow shafts usually requires multiple manufacturing processes such as multi-stage forming processes and subsequently several machining operations, resulting into high costs and high manufacturing times. To address these challenges, a novel cold forging process featuring an adjustable forming zone was developed by the authors. This new approach enables the production of tailored hollow shafts with varying cross-sections in their length direction as well as internal undercuts within one stroke of the ram. In order to achieve the desired target geometry of a hollow shaft, a specific tool kinematic is required to precisely adjust the cross-section of the forming zone during the process. Currently, determining geometry-specific tool kinematics requires a time consuming iterative numerical procedure. In this paper, a machine learning approach for the prediction of the tool kinematics for a given target geometry of a tailored hollow shaft with variable wall thickness in its length direction is presented.

Abstract: This study examines the forging process of an aluminum upper control arm for automotive applications. To address the geometric complexity and forming challenges, a multi-step forging route, comprising of roll forging, two-stage bending, pre-forging, and final forging, is developed. Finite element analysis (FEA) using DEFORM-3D software is employed to optimize key forming process parameters in the pre‑forging stage. The response surface methodology (RSM), combined with the Box–Behnken design, is utilized to construct predictive models and identify optimal parameter combinations. A successful forged upper control arm is subsequently produced using these optimized forming parameters. The findings demonstrate that integrating FEA with statistical process optimization strengthens the predictive accuracy of the process design and supports defect‑free forging of AA6082 upper control arms.

Abstract: Hot forging of nickel-based superalloys involves severe thermo-mechanical loading of the forming dies due to the high strength of these materials, even at elevated temperatures. Under industrial production conditions, forging dies are subjected to repeated heating-cooling cycles, which progressively degrade the mechanical properties of the tool steel and increase the risk of die plastic deformation. Reliable assessment of die performance therefore requires material characterisation that accounts for both temperature effects and service-induced degradation. In this work, an H13 tool steel used in an industrial hot forging application (nickel-based superalloy case study), was experimentally and numerically investigated in both its raw and service-degraded conditions. Hardness measurements, microstructural analysis, uniaxial compression tests, and quasi-static tensile tests were carried out from room temperature up to 600 °C. An artificial degrading heat treatment was applied to reproduce the mechanical state of the most degraded die regions, and the resulting data were used to quantify the temperature-dependent reduction in yield strength with service exposure. Finite element simulations of the industrial forging process were then carried out using deformable dies to evaluate temperature evolution and stress levels in critical die regions. The risk of die plastification was assessed by comparing simulated von Mises stresses with the experimentally determined temperature-dependent yield strengths for the raw and degraded conditions. The results show a significant reduction in yield strength due to both increasing temperature and service-induced degradation, leading to a substantially higher risk of die plastic deformation under production conditions. The study underlines the importance of incorporating degraded material properties into tool design and process assessment, and motivates improved cooling systems to enhance tool life and process stability.

Abstract: A full Profile Contour and Flatness Control (PCFC) model prototype has been developed for hot finishing mills. This model prototype accounts for several physical based sub-models calculating the different contributions to the roll gap profile and allows for offline predictions in both preset and recalculation modes. To evaluate the PCFC model developed, an exhaustive comparison analysis between its calculations, the ones coming from the plant model and measures at the finishing mill exit has been carried out. An industrial mill database composed of different rolling campaign types was applied for this purpose and both (i) strip crown and flatness indicators as well as (ii) full strip profiles results have been used for the comparisons. Encouraging results were obtained from this performance assessment since the PFCF model developed leads to similar behavior compared to the existing plant’s model (from an industrial supplier). As a result, the PCFC model developed shows high potential for online implementation in hot strip mills.

Abstract: Application fields and requirements for roll-cladded cooling plates are continuously rising. Especially as part of the thermal management systems in battery electric vehicles (BEV), the share of roll-cladded cooling plates is growing. A deeper understanding of the deformation regime in the roll bite is needed to completely fulfill the high quality, performance and cost requirements of the automotive industry Whereas most cause-effect relationships in the roll-cladding process have been scientifically evaluated, the influence of separating agents on the deformation regime in partial roll-cladding has not yet been investigated. To examine this relationship, an experimental set up is created and trials are conducted on a laboratory size roll-cladding mill. Two different aluminum alloy blanks are joined together under temperature by roll-cladding without the application of strip tensions and with different separating agent patterns. The results show: Firstly, there is a correlation between the materials’ relative flow stress difference and their relative deformation. Secondly, the separating agents’ areal share over the blank width significantly impacts the deformation regime in the roll bite. Thirdly, in areas with separating agent there is a correlation between the surface elongation of the bottom blank and the elongation of the contact interface between the blanks, which governs the later cooling channel tolerances. To use the results in the industrial application, the impact of so far neglected parameters such as strip tensions have to be considered in future research.