Solid State Phenomena Vol. 389

Abstract: Bipolar plates are key components of fuel cell systems, as they significantly determine efficiency, power density, and service life. In aerospace applications, their importance is further emphasized due to the dual requirement of corrosion resistance and strict weight reduction. Titanium Grade 1 combines low density and excellent corrosion resistance. However, its industrial application is limited by restricted formability. The aim of this paper is a systematic investigation of the forming behavior of Titanium Grade 1 foil material in order to define forming limits and derive manufacturing-oriented design recommendations for bipolar plates in aviation. Procedure. Sixteen distinct geometry features were developed to represent characteristic forming conditions. In addition to cross-section variations, the flow field angle was systematically altered to assess its influence on local stress and strain distribution. Furthermore, the key process parameters forming speed, forming force, and lubricant amount were varied to evaluate their impact on the forming quality. The assessment focused on form filling and material thinning. For this purpose, metallographic cross-sections were prepared, and optical 3D measurements were conducted using a Keyence system to precisely capture local wall thickness variations. Key findings Process parameters: The forming behavior of Titanium Grade 1 is strongly influenced by the applied forming force and lubrication. Form filling becomes sufficient only above 350 MPa (3.000 kN), while the lubricant amount is decisive for achievable forming depths due to the hydrostatic oil cushion effect. In contrast, forming speed shows no significant influence. Anisotropy remains a critical factor, particularly in 0° rolling direction, where premature thinning leads to fracture. Geometry parameters: Small radii are highly critical, while feature depth leads to expectedly higher thinning. Steeper flank angles improve form filling but at the cost of increased thinning. Pitch shows limited influence, although it may become relevant at very small values. Channel design is challenging, as sharp flow field angles consistently result in severe thinning and pose difficulties in tool manufacturing.

Abstract: Electrohydraulic forming involves a complex energy transfer from the initial plasma explosion through the fluid to the sheet metal, where the pressure wave propagation critically determines the amplitude, distribution, and timing of the resulting acceleration forces. These dynamics can be influenced not only by the explosion itself but also by fluid properties and the geometry and volume of the pressure vessel. To enable systematic process optimization, this study introduced a multi-sensor methodology that captured the transient behavior of the forming process. The approach integrated piezoelectric pressure sensor measurement and high-speed backgroundoriented schlieren imaging to analyze the pressure wave propagation, as well as in-situ two-point laser triangulation to monitor the sheet metal displacement. Experiments using different exploding wires varying discharge energies demonstrate the method’s effectiveness. The results reveal different pressure wave velocities from 800 m/s to 1700 m/s and link displacement data to waveinduced impulses. Additional phenomena such as process-related light emissions and cavitation bubble formation were temporally resolved. The study further introduced that, after an initial learning phase with full sensor integration, selected sensors can be omitted in specific scenarios without loss of essential information, highlighting the efficiency and adaptability of the proposed diagnostic approach.

Abstract: The increasing demand for reliable and high-performance heat-transfer components has stimulated the development of robust joining and forming strategies for thin-walled stainless-steel tubes. In this work, the Controlled Tube Expansion of Plasma Arc Welded AISI 316 stainless-steel tubes was investigated through a combined experimental and numerical approach. Welded tubes with an initial diameter of 130 mm were expanded to 180 mm using a three-step mechanical expansion process, and six different expansion sequences were experimentally evaluated. Finite element simulations were performed using a coupled thermo-mechanical model incorporating a damage-based fracture criterion to predict material failure during expansion. Numerical predictions were in good agreement with experimental observations and allowed the identification of a critical cumulative damage threshold governing tube failure. Based on these results, a processability domain was defined, clearly distinguishing safe and unsafe expansion paths. The study demonstrates that tube expandability is strongly dependent on the deformation path and highlights the importance of progressive expansion strategies for maximizing material ductility while preventing fracture.

Abstract: In precise sheet-metal forming operations such as fine blanking, the closed tool design precludes direct observation during production, which makes indirect monitoring of punch wear necessary. Previous work has shown that the sheared surfaces of the scrap web can be used to infer the punch wear through spatial correlation analysis of areal roughness parameters. However, these correlations have so far each explained only a limited portion of the variance and have only been investigated for a single punch geometry, leaving their robustness and generalizability open to question. In this work, multivariate regression approaches and feature importance analysis are used to combine complementary areal roughness parameters and generate robust indicators of punch wear. The plausibility of these indicators is validated by linking the correlated scrap web sheared surface features to physically interpretable wear mechanisms, such as worn surface area or punch breakage. Furthermore, the approach is extended to multiple punch geometries to examine the extent to which the identified correlation patterns can be transferred to different tool designs and process conditions. The results demonstrate the generalizability of spatial correlation-based indicators across different geometries and process conditions.

Abstract: Integrating functional features into lightweight aluminium components remains a key challenge in advanced manufacturing, particularly when forming operations impose severe local deformation. This study focuses on the sheet injection process, a variant of Sheet-Bulk Forming (SBF), where thickening and lateral extrusion occur in previously bent aluminium sheets. The material investigated is AW6082-T6, a medium-strength Al-Mg-Si alloy widely used in transportation and structural applications due to its good strength-to-weight ratio and corrosion resistance. To improve formability and reduce the risk of defects such as fracture and folding, an approach based on Tailored Heat-Treated Blanks (THTB) was employed. Localised Laser Heat Treatment (LHT) was applied to selectively reduce strength and enhance ductility in critical deformation zones. Mechanical characterisation was performed via compression tests on both as-received (T6) and heat-treated (HT) materials. Experiments were conducted on a flexible SBF demonstrator using a two-stage process: bending followed by sheet injection. Numerical simulations were performed to guide LHT pattern design and predict material flow. Among the tested LHT strategies, one demonstrated superior performance, enabling higher injection volumes and reducing process forces while avoiding failure. The experimental results confirm that THTB are an effective method for extending the process window of sheet injection in AW6082-T6, offering a promising solution for the production of complex aluminium components with enhanced functionality.

Abstract: In carbon-free technologies, thin-walled components with microchannels from ultra-thin sheets, such as bipolar plates, cooling plates, and heat exchangers, are widely utilized. Using such components made of high-strength aluminum alloys further reduces the required wall thickness, thereby enhancing their lightweight potential. However, conventional forming methods for ultra-thin sheets, including elastomer-based deep drawing and hydroforming, are limited by process-induced phenomena such as springback, geometrical inaccuracies and reduced formability as well as localized thinning, which can necessitate a higher wall thickness or the use of a lower strength grade alloy. Gas-based hot sheet metal forming of high-strength aluminum alloys is introduced to improve formability and geometrical accuracy. In the present study, an isothermal, gas-based hot sheet metal forming process is developed for forming microchannels from AlMg3 alloy sheets with a thickness of 0.4 mm. A 100 mm × 100 mm blank is heated to 530 °C and formed under nitrogen gas pressure into a heated die featuring various channel geometries. The effects of blank-holder force, maximum gas pressure, wall angle, channel radius, and maximum channel depth on thinning and form filling are investigated. Additionally, the grain size of the final component is analyzed. A full form filling can be reached under a forming pressure of 200 bar. The thinning is dependent on the micro channel geometry and reaches a maximum of 29 % for a channel depth of 1 mm. The grain size increases during the forming process, dependent on the introduced strain into the material. The proposed method enables forming of components without fracture and with high geometrical accuracy.

Abstract: The sheet injection process, a hybrid sheet-bulk forming technique that integrates bending and injection to create complex features such as ribs, offers significant potential for advanced manufacturing applications. However, its implementation with lightweight aluminium alloys is hindered by their limited room-temperature formability. To address this, locally softened, tailored heat-treated blanks produced via laser heat treatment (LHT) can enhance local ductility while maintaining global strength. This study investigates the mechanical property gradients induced by LHT in 3 mm thick AW6082-T6 aluminium alloy sheets using Profilometry-based Indentation Plastometry (PIP). The laser treatment, performed with a 2.5 kW CO₂ laser, produced localized heat treated regions, whose effectiveness was evaluated through microhardness testing. PIP enabled the direct extraction of local stress-strain curves without the need for specimen extraction and was validated against conventional tensile tests. Results showed that PIP accurately captured local variations in mechanical properties, with heat treated zones exhibiting increased ductility compared to the T6 condition.

Abstract: Achieving high geometric accuracy is crucial in stamping and bending processes. Key influencing factors include the position of the blank within the coiled sheet, leveling, strip lubrication, contour cutting, and the sequence of bending. Currently, process design and parameter selection rely largely on the expertise of experienced engineers. To enable a data-driven approach, the exact effects and interactions of process parameters must be analyzed using combinations of settings, machine data, and measurements of all manufactured parts. This study presents variance, interaction, and quality analyses based on long-term production trials for busbars. Process parameters were combined according to a statistical experimental design. The results indicate positive effects from slight pre-bending of sheet strips, active leveling, material removal through contour cutting, and simultaneous bending operations.

Abstract: Hybrid hot sheet forming routes that integrate heat treatment within the forming tool offer a promising pathway to manufacture complex geometries from precipitation‑hardenable 7xxx aluminum alloys, but the resulting local deformation and thermal histories may generate pronounced spatial property variations. In this work, a gas-based hybrid forming process is demonstrated for EN AW‑7020 sheets, combining in‑tool solution heat treatment, isothermal forming at 500 °C with gas calibration and active pushing, followed by water quenching and artificial aging. A thermo‑mechanically coupled finite‑element model is used to identify regions of distinct equivalent plastic strain in a representative demonstrator geometry and to guide local specimen extraction. Tensile tests from low‑ and high‑strain regions reveal clear location-dependent stress‑strain responses after aging, with a reduction in ultimate tensile strength exceeding 20 % in the more heavily deformed zones compared with reference material. Microstructural observations by optical microscopy indicate differences in grain morphology between component regions, consistent with the non‑uniform thermo‑mechanical history. The results highlight the need to account for local strain and process history when designing hybrid‑formed 7xxx components and motivate targeted strategies for controlling property gradients through process parameter tuning and tailored post-forming heat treatment.

Abstract: Geometric deviations remain a major barrier to the widespread industrial adoption of incremental sheet forming (ISF). Compared with conventional toolpath compensation that rely on extensive data generation and trial-and-error procedures, variation of toolpath styles offers a more direct and efficient strategy for mitigating geometric defects. In this study, multiple curvilinear toolpath strategies were investigated for a standard closed-contour ISF part to evaluate their effectiveness in reducing geometric deviations. Six toolpaths were examined, including three established types – convex, concave, and wavy – and three novel toolpaths proposed in this work: adaptive, cusp, and sine. The convex toolpath achieved the largest side-wall springback reduction relative to the linear baseline but introduced a significant bottom pillow effect and reduced formability. While the cusp toolpath effectively suppressed both springback and pillow formation, it resulted in local thickening and degraded surface finish. Overall, the sine toolpath provided the most balanced performance, achieving effective reduction of all major geometric defects. Numerical simulations reveal an inherent tradeoff between side-wall springback reduction and bottom pillow formation, as positive residual bending moments formed in the pillow region contribute to springback mitigation by promoting outward bending of the side walls.