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 0.0003-10s^-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 Ca_xMg_yZn in the matrix. The map exhibited two dynamic recrystallization (DRX) domains for hot working alloy: (1) 300-400°C/0.0003-0.001s^-1 and (2) 400-500°C/0.0003-0.3s^-1. Microstructural observations and fracture features confirmed the occurrence of DRX in both the domains. Apparent activation energy values of 181 and 225kJ/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.