Baocheng Yang

The torsional behavior of an extruded AZ31 magnesium alloy was investigated by combined axial-torsion mechanical testing with different stress ratios. The SEM-EBSD was used to analyze the microstructure and texture evolution of deformed samples. The results indicate that the axial tension results in a concave-down shape of shear stress–strain curves, while a concave-up shape after yielding is presented during combined compression-torsion loading due to twinning mechanism. Compared to pure shear, the yield strength decreases by 7 MPa and shear strain increases by 1% under σ:τ = −1:1 with same shear stress. Due to the Swift effect, a strain partitioning is displayed for axial strain during combined tension-torsion loading with low stress ratio. The twin volume fraction is 90% under σ:τ = −1:1, and the local dislocation density with a KAM value of 1.1 is maximum under σ:τ = 2:1. The primary twin type is {10-12} twins with axial compression. The main deformation mode changes from basal slip to prismatic slip with increase in axial tension stress. Both basal slip and twinning are activated and the interaction between dislocation slip and twinning contributes to the complex strain hardening behavior during combined compression-torsion loading.

Bipolar plates are one of the most important components of proton exchange membrane fuel cells. With the miniaturization of bipolar plate flow channel sizes and the increasing demand for precision, springback has become a key focus of research in the bipolar plate forming process. In this paper, the hydroforming process for 316L stainless steel bipolar plates was studied, and an FEM model was built to examine the stress and strain at various locations on the longitudinal section of the plate. Modeling accuracy was validated by the comparison of experimental profile and thickness distribution. The effects of forming pressure and grain size on springback behavior are discussed. The results show that with increasing forming pressure, the springback value decreases initially, followed by an increase, but then again decreases. When the forming pressure is 80 MPa–100 MPa, the deformation of the lower element of the upper rounded corner is not uniform with more elastic regions, and the springback is positively correlated with forming pressure. The springback distribution pattern on the cross-section of the bipolar plate changes from a normal distribution to a distribution of “M” shape with increased pressure. The larger the grain size, the lower the yield strength elastic proportion, resulting in a decrease in springback of the sheet. The maximum amount of springback of the bipolar plate is 3.1 μm when the grain size is 60.7 μm. The research results provide a reference for improving the forming quality of metal bipolar plates with different flow channel shapes.