Mechanisms of strain rate effect on pure Ti deformation: Application in the bipolar plates forming process

Abstract

In the present research, the mechanical behavior of pure Ti was analyzed at room temperature under low strain rates (LSRs, 0.001 s⁻¹, 0.01 s⁻¹ and 0.1 s⁻¹) and high strain rates (HSRs, 1160 s⁻¹, 1670 s⁻¹ and 2310 s⁻¹). The results demonstrated that as the strain rate increased from 0.001 s⁻¹ to 2310 s⁻¹, the elongation increased significantly by 27 %. The underlying mechanisms governing the strain rate dependence of plasticity were elucidated through microstructural analysis. The frequency of deformation twinning increased notably with strain rate. This effect was especially obvious during the transition from LSR to HSR. Deformation twinning inhibited dislocation motion and enhanced the work hardening capacity. Simultaneously, hard oriented grains (HOG) were found to play a significant role in the plastic deformation under HSR loading, enhancing the deformation coordination between grains and improving material deformation uniformity. This improvement in deformation uniformity mitigated premature localized strain and necking. Further, based on the plasticity-strengthening mechanism induced by HSR loading, an electro-hydraulic forming (EHF) method for the fabrication of pure titanium bipolar plates (Ti-BPPs) at room temperature. In comparison to the conventional LSR loading rubber pad forming (RPF) method, the EHF process significantly improved formability and addressed manufacturing challenges associated with pure Ti-BPPs. The proposed method successfully achieved the formation of complete flow channels with dimensional accuracy within a ± 10 μm tolerance. Additionally, the average thickness reduction in corner regions decreased from 34.2 % to 15.7 %. These findings provide theoretical insights that could guide the manufacturing of pure Ti-BPPs with thin walls and small structural sizes in industrial application.