Solid State Phenomena Vol. 391

Abstract: Incremental forming represents a versatile and cost-effective alternative to conventional sheet forming processes. In recent years, its application has been extended to polymers and composite materials. Among these, natural fiber-reinforced thermoplastics offer several advantages, as natural fibers are widely available, contribute to the semi-biodegradability of the composites, and serve as effective reinforcements for polymer matrices. This experimental study examines the mechanical properties of flax woven fabric-reinforced polypropylene composites, fabricated via compression molding, and their suitability for producing spherical caps through cold incremental forming. A range of features was investigated to assess the effectiveness of incremental forming on these biobased composites and to compare the mechanical performance of undeformed and deformed laminates.

Abstract: The following study examines the experimental determination of the electrochemical machinability of cemented carbides with a neutral electrolyte solution. In previous research, a removal mechanism of tungsten carbide-cobalt was demonstrated applying pulsed electrochemical machining. This work focuses on the material removal of tungsten carbide-cobalt with the further developed modulated pulsed electrochemical machining process according to DIN SPEC 91399. Compared to the preceding investigation, the modulated pulsed technique demonstrates a similar removal mechanism with higher current efficiencies.

Abstract: EU dependence on critical raw materials poses a serious risk of fragility and vulnerability, mainly in strategic sectors such as energy and defense. Electronic boards contain high concentrations of metals, particularly copper and lead, comprising critical raw materials. Urban mining offers the possibility to recover these metals, but continuous innovation is necessary to make extraction processes more efficient, productive and cheap. Contamination is an issue which reduces the extraction performance and increases pollution and costs. Thermo-mechanical disassembly can be used to separate electronic components from boards, by removing the soldering alloy. A thermo-mechanical disassembly process, currently under patenting (namely “impact desoldering”), allows the separation of electronic components and soldering alloys from the boards by impact. It has been applied to 4 kg of waste boards to separate the printed circuit board from the electronic components, and the soldering alloy has been melted to make an ingot. A further thermo-mechanical process has been also applied to extract copper sheets from the disassembled printed circuit boards, by rolling.

Abstract: The increasing adoption of polymer additive manufacturing (AM) in functional applications has intensified the need for reliable hole-making strategies, particularly when printed components require mechanical fastening or post-processing. However, the layered architecture of fused-filament fabrication (FFF) parts typically leads to dimensional inaccuracy and thermally induced defects during drilling. This study examines the quality of holes made directly with FFF technology and those obtained through traditional drilling on PLA components produced with two different layer heights (0.20 and 0.28 mm). Additively manufactured holes were analysed via least-squares circle fitting and roundness evaluation, revealing high repeatability but substantial undersizing (>1 mm deviation from the 6.35 mm nominal diameter). CNC drilling tests were then performed under a full-factorial combination of spindle speed and feed rate, assessing thrust force, torque, infrared-based cutting temperature, and burr formation for both top and bottom sample side. Results show that CNC drilling can restore near-nominal dimensional accuracy, but only within a restricted process parameters window. Low feed rates promote severe thermal-viscoplastic deformation, while specific combinations of spindle speed and entry surface strongly influence burr formation.

Abstract: This paper aims to extend the evaluation of the process of electric discharge machining by analysing the discharges. Therefore, a method for detecting and classifying discharges was developed. To detect different discharge types, experiments were conducted with varying of technology parameters, such as peak current or duty factor. During the experiments, the voltages and the currents were measured via an oscilloscope. For the classification, an unsupervised machine learning method was applied, to cluster and classify the detected discharges and compare them with the measured material removal rate and the measured tool wear rate.

Abstract: Adhesive bonding is increasingly used in lightweight structural applications, though its effectiveness for carbon fiber reinforced polymers is often limited by low surface energy. This study investigates the concurrent effect of infrared laser surface texturing and resin pre-coating on the mechanical performance of carbon fiber-reinforced polymer single-lap joints. Using a CO₂ laser system, micro-dimples were generated on the substrate to promote mechanical interlocking, while various concentrations of an epoxy resin (5 wt%, 10 wt%, and 20 wt%) were applied as precoating to enhance surface wetting and interfacial continuity. To validate the experimental findings, a numerical investigation was performed using a Cohesive Zone Model implemented within a finite element framework. This model utilized a bilinear traction-separation law and a quadratic delamination criterion to predict the progressive debonding and failure of the interface. Experimental results indicate that the 10 wt% resin pre-coating concentration provides the highest lap shear strength.

Abstract: Adhesive bonding is widely employed for joining complex-shaped components due to its ability to distribute stresses uniformly, preserve material integrity, and eliminate mechanical fasteners. However, the permanent nature of conventional adhesives and the difficulty of clean disassembly hinder the reuse of bonded joints, limiting recyclability and sustainability. This study investigates laser cleaning as an enabling technology for re-bonding single-lap joints. An ultrashort pulsed laser was applied to remove residual epoxy adhesive from AA6061 aluminum alloy substrates after debonding. Surface characterization revealed that laser ablation produces micrometric roughness, and laser processing can be further tailored to generate laser-induced periodic surface structures (LIPSS), influencing morphology, chemistry, and wettability. Mechanical testing demonstrated that laser cleaning also improves the tensile lap-shear strength of re-bonded untreated joints. These findings confirm that laser cleaning effectively restores substrate surfaces and, when combined with controlled texturing, can enhance bonding performance. The proposed approach supports repair and recycling strategies, contributing to extended component lifecycles and circular economy objectives.

Abstract: Carbon fiber–reinforced polymer (CFRP) composites are extensively used in aerospace applications; however, their end-of-life management remains a critical challenge. This study investigates an unconventional recycling route based on the direct hot compression molding of CFRP waste powders, aiming to valorize industrial composite scraps without the addition of virgin polymers or binding agents. The material investigated corresponds to the finest fraction (~300 μm) obtained from a sieving process applied to industrial CFRP scrap powders derived from trimming residues and partially cured aeronautical prepregs. The use of this fine powder fraction promotes effective particle aggregation and consolidation during molding, preventing powder loss during demolding and enabling the fabrication of relatively thick panels despite the absence of additional bonding agents. Compression molding was carried out at 250 °C and 1.5 bar for 20 min. Two material configurations were analyzed: uncoated compression-molded panels and panels coated with a thin polyester layer. The recycled materials were characterized through morphological, thermomechanical, and mechanical analyses. The results indicate that the polyester-coated panels exhibit improved mechanical performance compared to the uncoated configuration. In comparison with previous studies focused on coarser powder fractions (≤1 mm), the present work highlights the potential of the finest powder fraction for effective consolidation, demonstrating the strong influence of particle size on the processability and properties of compression-molded recycled CFRP. These findings confirm the viability of direct compression molding as a sustainable and scalable recycling strategy for tailoring CFRP waste reuse as a function of powder size.

Abstract: Fe-Nd-B Rare earth magnets are widely used in high performance applications but suffer from poor corrosion resistance due to their multiphase microstructure and the preferential dissolution of rare earth rich intergranular phases. In this study, the corrosion behaviour of Fe-Nd-B magnets was investigated and the effectiveness of electroless nickel coatings as a protective solution was evaluated. Microstructural and compositional analyses were combined with potentiodynamic polarisation tests, comparing bare and coated magnets with low alloy steel. Coating adhesion was assessed by pull off testing according to ASTM D4541. The results show that Fe-Nd-B magnets are significantly more susceptible to corrosion than steel, while nickel exhibits a passive behaviour in the investigated environment. Electroless nickel coatings provide effective protection and display adhesion comparable to that measured on steel substrates, demonstrating their suitability for corrosion protection of Fe-Nd-B magnets.