Hot tearing behavior of AZ91D magnesium alloy

Hot tearing is a serious destructive solidification defect of magnesium alloys and other casting metals. Quantitative and controllable measurements on the thermal and the mechanical behavior of an alloy during its solidification process are crucial for the understanding of hot tearing formation. We developed a new experimental method and setup to characterize hot tearing behavior via controlled cooling and active loading to force hot hearing formation on cooling at selected fractions of solid. The experimental setup was fully instrumented so that stress, strain, strain rate, and temperature can be measured in-situ while hot tearing was developing. An AZ91D magnesium alloy, which is prone to hot tearing, was used in this study. Results indicate that when hot hearing occurred, the local temperature, critical stress, and cumulative strain were directly affected by strain rate. Depending on the applied strain rate, hot tearing of the AZ91D magnesium alloy could occur in two solidification stages: one in the dendrite solidification stage (f_S ∼ 0.81–0.82) and the other in the eutectic solidification stage (f_S ∼ 0.99). AZ91D alloy exhibited distinct mechanical behaviors in these two ranges of fraction solid.