Solid State Phenomena Vol. 389

Abstract: Bipolar plates are key components in fuel cells, and their performance strongly depends on the geometry of the microchannels used to distribute reactant gases. Producing channels with sufficient depth in thin metal sheets remains challenging, particularly when cost-effective manufacturing routes are required. This work investigates a multi-stage roller embossing process for forming bipolar plate channels using additively manufactured polymer tools. By dividing the total deformation into multiple forming stages, the process reduces tool deflection that typically limits channel depth in single-pass embossing. Experiments conducted on 0.1 mm stainless steel foil show that the multi-stage approach increases the achievable average channel depth from approximately 0.25 mm in a single pass to approximately 0.34 mm, resulting in a maximum aspect ratio (channel depth to width) of 0.314. These results indicate that combining multi-stage forming with 3D-printed tooling provides a practical route for flexible and low-cost fabrication of metallic bipolar plates, especially for low volume production.

Abstract: This study introduces a novel flowformability test aimed at replicating the complex loading conditions of industrial flowforming processes—alternating stress triaxiality, large plastic strains, and high strain rates. A novel Conical Flowformability Test (CFT) configuration was selected for experimental validation due to its ability to achieve a high theoretical thickness reduction while respecting machine constraints. Experiments conducted on AA6061 in O-temper and W+3h states demonstrated substantial thickness reductions. Comparison between the numerical simulations using the software FORGE® and the experimental results is satisfactory despite certain unquantifiable experimental defects such as fish scales and material build-up. The current study paves way to establish a robust framework for assessing material flowformability and damage evolution under realistic process conditions.

Abstract: Hot stamping of manganese–boron steels is widely used in automotive manufacturing to produce ultra-high-strength components with tensile strengths exceeding 1500 MPa . Conventional industrial heating relies on gas-fired roller hearth furnaces, which require 5 – 10 min to reach austenitization and exhibit low energy efficiency . Resistance heating offers a compact and energy-efficient alternative, enabling heating rates above 100 K/s and full austenitization within seconds. However, rapid heating of uncoated steels leads to severe oxidation, and established coating systems such as AlSi are not designed for diffusion-controlled bonding within such short times . This study demonstrates that resistance heating in an XHV-adequate atmosphere – consisting of nitrogen and monosilane – suppresses oxidation while simultaneously enabling adhesion of a pre-laminated aluminum foil to the steel substrate. For coating preparation, 22MnB5 sheets were roughened by corundum blasting, cleaned, and laminated with an aluminum foil using a flat-die pressing tool. The pre-coated blanks were heated in a self-developed resistance-heating chamber, in which the oxygen concentration was reduced to an XHV-adequate level. Several heating profiles were investigated to determine suitable process windows for coating formation. The results show that resistance heating achieves austenitization within a few seconds, reducing heating times by more than an order of magnitude compared to furnace heating. The XHV-adequate atmosphere reliably prevents scale formation, enabling completely oxidation-free surfaces during rapid heating. Under these conditions, the laminated aluminum foil bonds uniformly to the substrate, forming a continuous coating layer. Metallographic cross-sections and SEM analyses confirm the formation of Al–Fe intermetallic phases at the interface, demonstrating robust metallurgical bonding suitable for subsequent hot stamping operations. Overall, the combination of resistance rapid heating and an XHV-adequate atmosphere provides a highly energy-efficient process route for hot stamping while offering an opportunity to integrate aluminum-based protective coatings directly into the heating step. This approach addresses the limitations of current furnace-based heating and coating technologies and opens a promising pathway toward more flexible, sustainable, and functionally integrated hot-stamping process chains.

Abstract: Hydrogen-based energy systems are considered a key pillar of the energy transition, yet the cost-efficient, mass production of metallic bipolar plates (BPPs) for proton exchange membrane fuel cells (PEMFCs) remains challenging, as conventional processes are limited by comparatively long cycle times and forming-related instabilities. This paper investigates the rubber drawing process as a cost-efficient manufacturing method for metallic bipolar plates, proposed as an alternative to the commonly applied hydroforming process, analysing the influence of pressing force, rubber hardness and thickness, tool modifications for varying pressure distribution, and the suitability of additively manufactured tool dies made from Maraging Steel 1 (X3NiCoMoTi 18-9-5) or ceramic-filled UV resin. The results show that precise and stable tool guidance, as well as a well-adapted tool setup, are required to achieve reproducible component quality; targeted adjustments of process and rubber parameters improved channel dimensional accuracy, but revealed limited forming capability in certain areas. Furthermore, concavely and convexly modified rubber dies reduced component warping in specific directions, and steel dies exhibited higher precision and less distortion compared to ceramic-filled UV resin dies. These findings highlight the potential of the rubber drawing process for cost-effective production of bipolar plates, while identifying key parameters for further optimization toward industrial-scale manufacturing.

Abstract: Tailor welded blanks (TWB) are commonly used in the automotive industry to achieve heterogeneous components, particularly for creating high strength, lightweight parts. Laser welding is one method for joining TWB. Laser welding was used to create TWB composed of stainless steel 304L, with varied thicknesses, in a “patchwork quilt” pattern forming quadrants within the sample. The mechanical properties and quality of the weld were evaluated via tensile testing and microscopy. Truncated pyramids were then formed with weld seams along the faces, and springback and mechanical properties after forming were analyzed. Optical microscopy revealed that the weld seams remained intact after forming. The weld seam location in the center of the pyramid walls did not have a significant impact on the geometrical accuracy of the formed parts. The results of this study show promise for the use of SPIF with quilted TWB to achieve optimal formed part properties for the intended part application.

Abstract: Flow forming of metastable austenites is an innovative, incremental metal forming process with special capabilities due to the TRIP effect. However, the TRIP effect during flow forming is significantly affected by disturbances and especially batch fluctuations leading to process uncertainty. This aspect is further analyzed and quantified in this paper to give insights on how to minimize the impact of uncertainty. For this purpose, semifinished parts and resulting flow forming workpieces are systemically characterized concerning their properties and the property uncertainty supported by mathematical methods like correlation analysis and error propagation. A result is that the most influencing impact factor on the strain induced α’-martensite volume fraction as a material property are batch fluctuations, specifically the variations of the chemical composition. Those especially appear from batch to batch, but also within a batch accompanied by e.g. temperature effects. To counter this challenge, different methods from control theory like closed-loop property control and adaptive control can be applied to flow forming. Thus, uncertainty will be reduced to increase process robustness and to enable industrial exploitation of the TRIP effect in flow forming of metastable austenitic steels.

Abstract: Thick-walled longitudinally arc-welded tubes are indispensable in modern infrastructure owing to their exceptional load-bearing capacity and structural integrity. Nevertheless, their fabrication remains highly challenging, as the conventional forming forces demand the use of large-scale industrial presses. To address this limitation, this research introduces a novel process architecture that integrates agile tube roll forming process for tube manufacturing, thereby enabling the production of such tubes using significantly smaller and more flexible manufacturing systems. To this end, three tube support configurations—namely, support-less, dynamic roller support, and static support—were systematically investigated in this study on 7, 9, 11, and 15 mm thick 304 stainless steel. While the supportless condition represents the most economical option, the incorporation of dynamic or static support significantly improves geometric accuracy, yielding near-net cross-sections combined with reductions in tube ovality of approximately 75 and 79 %, respectively, compared to support-less configuration. Considering the straightness of the weld line as a quality indice, the dynamic support provides the highest quality. Using the static/dynamic support strategies, the deformation forces arise between 2 and 3 times compared to support-less strategy.

Abstract: In the present study, the technical feasibility of manufacturing a height-variable profile from an advanced high-strength steel (AHSS) sheet using the roll forming process was investigated through finite element analysis (FEA). The study focused on the production of a cross member used in a B-segment passenger vehicle. For this purpose, the kinematics of the forming rolls in both the height and longitudinal directions were derived and integrated into the finite element model. The sheet metal was modeled using both shell and solid (3D) elements. These two different modeling strategies were evaluated in terms of the formed profile geometry. The results demonstrated that the manufacturing of the selected cross member is feasible with the derived roll kinematics. Additionally, it was observed that the use of shell elements led to higher deviations from the desired geometry compared to solid elements. The analyses are planned to be validated experimentally in the next phase of the study.

Abstract: The six independent axes available for free-form bending enable the production of complex three-dimensional bent tube and profile geometries. In industrial environments, only tangential bending strategies are currently used, which means that the bending head is always positioned parallel to the cross-section of the tube in the current bending section. Therefore, the individually controllable axes make it possible to apply other, non-tangential bending strategies. In so-called overbending, the bending head is rotated more in comparison to tangential bending. However, in order to ensure that the bending radius does not change compared to tangential bending, the translational deflection of the bending head must be reduced at the same time. In contrast, the bending head is rotated less during underbending and the translational deflection is increased. Overbending and underbending offer the possibility of improving the mechanical properties while maintaining the same bending geometry. These strategies allow the components to be optimized for individual load cases. As part of this work, a structural component was produced multiple times using free-form bending. Both conventional tangential bending strategies as well as innovative overbending and underbending strategies were applied. The mechanical stiffness of the bent components was then examined on a test bench. The influence of the bending strategy on the cross-sectional change in the bent area was investigated by using a tactile coordinate measuring machine. Furthermore, residual stress measurements were performed on the bent tubes, which allowed the different mechanical behavior of the tangentially bent, overbent and underbent tubes to be explained.

Abstract: This study investigates a hybrid manufacturing route combining heat-assisted Single Point Incremental Sheet Forming (SPIF) with Tungsten Inert Gas welding (TIG)-based material deposition for the local reinforcement of Mg–Zn–Zr (ZK61) alloy thin sheets. Flat and curved substrates extracted from SPIF-formed geometries were used to examine the influence of substrate thickness, forming temperature, and geometry on TIG deposition morphology and thermal distortion. The results indicate that heat input and substrate thickness strongly affect deposition morphology and dimensional stability, while SPIF sheet forming temperature influences the repeatability of the deposition process. In addition, deposition behavior exhibited limited sensitivity to substrate curvature for single depositions, whereas successive depositions resulted in increased thermal distortion due to cumulative residual stresses. Overall, this work identifies key process sensitivities and constraints associated with TIG deposition on SPIF-formed magnesium alloy sheets, providing a basis for the development of hybrid forming-deposition process chains for localized reinforcement applications.