Effect of Al Content on the Solidification Behaviors of Mg–Al Alloys

The variations in force, displacement and temperature over time during solidification and subsequent cooling processes were measured by a force-tested module and a displacement-tested module, respectively. Then, the effects of Al content (0, 5, 10, 15, 19 and 22 wt%) on the solidification behaviors of as-cast Mg–Al binary alloys were obtained. The results showed that the expansion behavior in the mushy zone gave rise to a large change in displacement but a relatively slight change in compressive force. With increasing Al content, the changes of the maximum expansion velocity and maximum compressive force followed the “Λ” shapes, with the largest values of 13.44 × 10−3 mm s−1 and 74.4 N in the as-cast Mg–10 wt% Al alloy, respectively. But the critical Al content for the solidification shrinkage behavior of the as-cast Mg–Al alloy was 15 wt%. That is, with increasing Al content, the mean solidification shrinkage velocity in the mushy zone after expansion decreased significantly on the condition of Al addition being < 15 wt%, while decreased slightly if the Al addition was > 15 wt%. Furthermore, the effect of Al content on the hot cracking tendency of Mg–Al alloys also followed the “Λ” shape, with the highest accumulating stress coefficient of 1.90 × 10−4 MPa·(s °C)−1 in the Mg–15 wt% Al alloy.