Effect of compression ratio on the microstructure evolution of semisolid AZ91D alloy

The effect of compression ratio on the microstructure evolution of semisolid AZ91D magnesium alloy manufactured by strain induced melt activation (SIMA) process was investigated. The specimens were compressed with different ratio at 230 °C, and then were isothermal hold at the temperature of 560 °C for 15 min. During the compression process, as the increasing of compression ratios, the microstructures of samples changed from dendritic to catenarian, and then to fibrous shape. These changes mean the breakdown of the branches of casting microstructure, and the storage of the deformation energy. At the isothermal holding stage, liquid phase nucleation occurs in the severe deformed region. And the liquid nuclei grow up rapidly to connect together. After that isolated solid particles were left, and the semisolid slurry with the globular microstructure was formed. The solid fraction reduced gradually with the increase of compression ratio, and the solid particle size reached a minimum at the critical compression ratio. After the particle size, solid fraction and sphericity were considered, the optimal compression ratio for this paper was 40%.