Microstructure and Thermal Stability of A357 Alloy With and Without the Addition of Zr

The principal purpose of this research was to evaluate the effects of Zr on the microstructure and thermal stability of an A357 alloy that has been subjected to an aging treatment (T6) and thermal exposure (250 °C). The results show that the addition of Zr had a significant influence on the refinement of the grain size, which enhanced the hardness and tensile strength of the A357 alloy under the T6 condition. During thermal exposure at 250 °C, the rodlike metastable β’-Mg₂Si precipitates transformed into coarse equilibrium phase β-Mg₂Si precipitates, resulting in a significant drop in the hardness and tensile strength of the T6 heat-treated A357 alloy. However, after thermal exposure, coherent, finely dispersed Al₃Zr precipitates were found to be formed in the T6 heat-treated A357 alloy. The addition of 0.1% Zr played a critical role in improving the high-temperature strength. Consequently, the A357 alloy with the addition of Zr demonstrated better mechanical properties at room temperature and high temperature than the alloy without Zr, in terms of both microstructure and thermal stability.