Effect of holding temperature on microstructure of an AS91 alloy during semisolid isothermal heat treatment

A semisolid microstructure of an AS91 alloy is successfully achieved by isothermal heat treatment process, and the effects of holding temperature on semisolid microstructure of the AS91 alloy, especially on the semisolid microstructure evolution and the modification of Chinese script shaped Mg₂Si phase are investigated. The research results indicate that it is possible to produce the AS91 alloy with non-dendritic microstructure by the semisolid isothermal heat treatment. After treated at 560-575 °C for 20 min, a non-dendritic microstructure with a 37.7-51.7 vol.% liquid content was obtained in the AS91 alloy with increasing treatment temperature, the morphologies of α-Mg grains changed from the initial dendritic shape to spherical types and their average sizes increased from 54.3 μm to 81.5 μm. In addition, the semisolid isothermal heat treatment can modify the Chinese script shaped Mg₂Si phases in the experimental alloy. After treated at 560-575 °C for 20 min, the morphology of Mg₂Si phases in the AS91 alloy changed from the initial Chinese script shape to granule and/or polygon shapes.