Residual stress and precipitation of Mg-5Zn-3.5Sn-1Mn-0.5Ea-0.5Cu alloy with different quenching rates

The effect of the quenching rate after solution treatment on the residual stress and precipitation behavior of a high strength Mg-5Zn-3.5Sn-1Mn-0.5Ca-0.5Cu plate is studied. The simulation results show decreasing temperature gradient in the plate with decreasing quenching rate, which leads to weakened inhomogeneous plastic deformation and decreased residual stress. No dynamic precipitation on the grain boundary happens after either cold water cooling or air cooling, however, air cooling leads to dynamic precipitation of Mg-Zn phase on Mn particles around which a low-density precipitate zone develops after aging treatment. Moreover, the fine and densely distributed Mg-Zn precipitates observed after aging treatment of the cold water cooled alloy are replaced by coarse precipitates with low density for the air cooled alloy. Both the low-density precipitate zone near Mn particles and the coarsening of precipitates are the source of the decrease in hardness and tensile properties of the air cooled alloy. The residual stress drops faster than the hardness with decreasing quenching rate, which makes it possible to lower the residual stress without sacrificing too much age-hardening ability of the alloy.