Effect of calcium addition on the hot working behavior of as-cast AZ31 magnesium alloy

The hot deformation behavior of as-cast Mg-3Al-1Zn-1Ca (AZX311) alloy has been characterized within the temperature range of 300-500 °C and strain rate range of 0.0003-10 s−1, and compared with the behavior of as-cast Mg-3Al-1Zn (AZ31) base alloy through processing map and kinetic analysis. Calcium addition contributed to the formation of thermally stable second phases (Mg,Al)2Ca at the grain boundaries, and of CaxMgyZn in the matrix. The map exhibited two dynamic recrystallization (DRX) domains for hot working alloy: (1) 300-400 °C/0.0003-0.001 s−1 and (2) 400-500 °C/0.0003-0.3 s−1. Microstructural observations and fracture features confirmed the occurrence of DRX in both the domains. Apparent activation energy values of 181 and 225 kJ/mole were estimated in these domains, respectively. The values were higher than those of Mg self-diffusion, suggesting that considerable back stress was generated due to the intermetallic particles within the matrix. Compared with the processing map for base alloy AZ31, Domain 1 was less wide and Domain 2 moved to slower strain rates, which were attributed to the strengthening effect of the matrix particles and the reduced grain boundary migration rates, respectively. A third DRX domain exhibited by AZ31 at higher strain rates was absent in the map of the Ca-containing alloy. As this domain was controlled by grain boundary self-diffusion, the (Mg,Al)2Ca particles at the grain boundaries were effective in diffusion rate reduction.