Abstract:The nearly equiaxed grains of Mg9AlZnY alloy were obtained by vibrating solidification in lost foam casting(LFC) and the microstructure of Mg9AlZnY alloy was analyzed. On this basis, the morphology and size of a-Mg grains fabricated by semi-solid isothermal heat treatment(SSIT) at 530 ℃ and 570 ℃ holding different time were studied. The results show that the main constituent phases of Mg9AlZnY alloy are a-Mg, β-Mg₁₇Al₁₂ and Al₂Y, and the Y can greatly refine a-Mg grains. The distribution of a-Mg grains equivalent diameters between 20 and 100 μm is up to 87%, and the average roundness of a-Mg grains reaches 1.37 in the specimen obtained at 570 ℃ and holding time 60 min. According to the analysis of solidification kinetics and thermodynamic, binary eutectic with low melting point melts firstly on SSIT process. As the liquid fraction increases with the solute diffusibility, both of the shape and size of a-Mg grains change ceaselessly. When the liquid fraction reaches equilibrium, the a-Mg grains are gradually spheroidized under the interfacial tension, and then the a-Mg grains begin to combine and grow. Evolution of a-Mg dendritic grains on SSIT process is obviously different from that of equiaxed grains.

Key words: lost foam casting (LFC); semi-solid isothermal heat treatment (SSIT); microstructural evolution; magnesium alloy