Hot tearing of ternary Mg−Al−Ca alloy castings

Abstract

Ternary Mg−Al−Ca alloys are the base of a few new creep-resistant, lightweight Mg alloys for automobiles. Hot tearing in Mg−xAl−yCa alloys was studied, including Mg−4Al−0.5Ca, Mg−4Al−1.5Ca. Mg−4Al−2.5Ca, Mg−4Al−3.5Ca, Mg−5Al−2.5Ca, and Mg−6Al−2.5Ca, by constrained rod casting (CRC) in a steel mold—with a movable pouring cup to keep solidification therein from interfering with the rising tension in the rods. The hot tearing susceptibility, based on measured crack widths and crack locations, decreased significantly with increasing Ca content (y) but did not change much with the Al content (x). An instrumented CRC with a steel mold was developed to detect the onset of hot tearing by monitoring the tension in the rod during casting and the temperature near the cracking site. It was further improved by reducing the rod diameter to detect hot tearing earlier, at a higher temperature, and with a clear peak in the load curve. To further understand the hot tearing susceptibility of these alloys, the secondary phases, eutectic content, solidification path, and freezing range were examined. Alloy Mg−4Al−0.5Ca had the widest freezing range and the lowest eutectic content and was most susceptible to hot tearing, while alloys Mg−4Al−3.5Ca and Mg−6Al−2.5Ca were the opposite. Mg−4Al−0.5Ca had the widest freezing range (183 °C) because its solidification path led to the formation of Mg₁₇Al₁₂ from the liquid at a very low temperature (440°C). The application of the results to die casting was discussed.