Influence of the microstructure on the forming behaviour of bipolar plates

The austenitic stainless steels 1.4301 and 1.4404 are often used to produce bipolar plates. Microstructure and chemical composition of these materials influence forming behavior and formability. With regard to microstructure, the grain size must be considered when processing ultra-thin sheets (thickness 50 μm). The forming behavior is significantly influenced by the predominant number of grains over the sheet thickness. Within the study, different alloys of the materials 1.4301 and 1.4404 with different grain sizes and a sheet thickness of 50 μm were characterized and compared to a 1.4404 with a thickness of 100 μm. The mechanical properties were determined by tensile tests and the microstructure was analyzed by light microscopy. A bipolar half-plate geometry was then produced by hydroforming. Varying the fluid pressure enables for different load conditions in the material. The target pressure for forming was set to stress all tested materials to failure, providing insights into localized necking. The formed bipolar half plates were then evaluated comparatively using transmitted light inspection and 3D optical scanning. In addition, micrographs were taken in areas of high strain. It was shown that, in addition to the steel alloy chemical composition, the microstructure is a major factor influencing the strength, the resulting forming behavior and the material failure.