Microstructural evolution of thixomolded AZ91D magnesium alloy with process parameters variation

A prototype system of Thixomolder® was used to manufacture the AZ91D magnesium alloy. The effect of various processing parameters on the microstructure was investigated using an image analyzing system. The solid particle size was found to be reduced with increasing barrel temperature while the particle morphology was not essentially affected. The size reduction and the increase of the solid particle sphericity followed the elevated screw rotation speed because of the accelerated solute diffusion. Furthermore, the refinement of the solid particles occurred because of the high cooling rate induced by low injection velocity or low mold temperature. The mechanism of nucleation and growth of the solid particle was put forward to explain the generation and evolution of non-dendritic microstructure including the particle with entrapped liquid, or with rosette shape in semisolid slurry under forced convection. It was believed that there was a critical solid fraction about 10–20%, at which the particle grew intensively, considered as the change point of coalescence and Ostwald ripening.