Evolutions of microstructure of AZ91D magnesium alloy in lost foam casting at different pouring temperatures under vibration and vibration+semi-solid heat treatment conditions have been observed. The results show that the microstructure of the AZ91D magnesium alloy in lost foam casting can be obviously refined by vibration at different pouring temperatures

and the finest grain size can be observed by vibration force of 1.5 kN at the pouring temperature. The microstructures of both the vibrated and common samples treated by semi-solid isothermal condition can be spheroidized

and the microstructures would be coalesced to growth

in which the optimized semi-solid heat treated parameters are at holding time for 10 min at 570 ℃.