Effects of strain rate, temperature and sheet thickness on yield locus of AZ31 magnesium alloy sheet

Magnesium alloys usually exhibit low ductility at the room temperature due to its hexagonal close-packed structure, but it will be improved at elevated temperature. Therefore, warm press-forming of magnesium alloy sheets is quite attractive. In order to determine the optimum condition of press-forming for magnesium alloy sheets, in the present work, the effects of strain rate, temperature and sheet thickness on the yield locus were experimentally investigated. The yield loci of magnesium alloy (AZ31) sheets with different sheet thickness (0.5 and 0.8 mm) were obtained by performing biaxial tensile tests, using cruciform specimens, at temperatures of 100, 150, 200, 250 and 300 °C at strain rates of 10−2, 10−3 and 10−4 s−1. Based on the experimental results, the effects of strain rate, temperature and sheet thickness on the yield locus were discussed. The size of yield locus drastically decreases with increasing temperature and decreases with decreasing strain rate. In contrast with the temperature and strain-rate dependence of the yield locus, sheet thickness has no influence on the yield locus. The shape of the yield locus of magnesium sheet is far from the predictions calculated by the yield functions of von Mises, Hill and Cazacu. Instead of these, the yield functions of Logan-Hosford or Barlat is a better choice for the accurate description of biaxial tension stress–strain responses at high temperature.