Effects of SiC addition on microstructure of AZ91D alloy initial billet with various size during semisolid isothermal treatment

AZ91D magnesium alloy was remelted; then, 0.3%(mass fraction) SiC particles as grain refiner were introduced into the melt; and finally, the melt was poured into cavities with different diameters to form ingots. Those ingots were cut into some billets with determined diameters, and heated to semisolid temperature. The effect of SiC addition on microstructure of mutative-size AZ91D initial billet during semisolid isothermal heat treatment was studied. The results show that there is an obvious dimensional effect on microstructure for semi-solid AZ91D alloy without SiC. In other words, the microstructures from center to edge for the AZ91D billet are different. The size of solid particles gradually changes from large to small. For example, the size of solid particles of the billet with d70 mm changes from 150 μm to 110 μm, and the particles tend to be gradually spheroid and more liquid. With increasing diameter of the billet, the difference of microstructures from center to edge for the billet further increases. For AZ91D alloy with SiC addition, the average size of solid particles becomes smaller than that without SiC addition after isothermal treatment. The size of solid particles of the billet with d 70 mm is not larger than 80 μm. And the solid particle sizes of the billet with SiC from edge and center areas are almost of identical dimension, between 70 μm and 80 μm and in spheroid shape with homogeneous distribution. Casting defects for AZ91D alloy with SiC addition, such as porosity, decrease significantly after SiC semisolid die-casting.