Microstructural evolution of Mg9AlZnY alloy with vibration in lost foam casting during semi-solid isothermal heat treatment

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 alpha-Mg grains fabricated by semi-solid isothermal heat treatment(SSIT) at 530 degrees C and 570 degrees C holding different time were studied. The results show that the main constituent phases of Mg9AlZnY alloy are alpha-Mg, beta-Mg17Al12 and Al2Y, and the Y can greatly refine alpha-Mg grains. The distribution of alpha-Mg grains equivalent diameters between 20 and 100 mu m is up to 87%, and the average roundness of alpha-Mg grains reaches 1.37 in the specimen obtained at 570 degrees C 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 alpha-Mg grains change ceaselessly. When the liquid fraction reaches equilibrium, the alpha-Mg grains are gradually spheroidized under the interfacial tension, and then the alpha-Mg grains begin to combine and grow. Evolution of alpha-Mg dendritic grains on SSIT process is obviously different from that of equiaxed grains.