Effects of specimen and grain size on electrically -induced softening behavior in uniaxial micro-tension of AZ31 magnesium alloy: Experiment and modeling

Various electrically-assisted (EA) plastic forming technologies have been developed for difficult-to-form materials such as magnesium alloys in recent years. However, very few studies have been conducted on EA micro-forming, especially determining the size effect on electrically-induced softening behavior. In this study, uniaxial micro-tension tests at various current densities were conducted to investigate the effects of grain size and specimen size on the electrically-induced softening behavior of magnesium alloy AZ31 specimens. It was found that the electrically-induced softening, parameter (i.e., the ratio of the tensile strength in EA to that in non EA test with the higher softening at the smaller value) followed an inverse-S-shaped function of current density. A relatively lower current density would be sufficient for larger sample sizes and smaller grain sizes to achieve a higher softening effect, indicating that grain number may be an important factor influencing electrically-induced softening. These size effects on electrically-induced softening were used to modify a semi-empirical softening function of current density, which could effectively predict the electrically-induced softening behaviors of five metals. The current density threshold in EA tension was defined and formulated based on the semi-empirical softening function, which nonlinearly increased with grain size, but decreased with specimen size and electrical resistivity.