2012 Volume 53 Issue 6 Pages 1094-1099
We have developed a new type of semi-solid injection process that can obtain a high material yield of about 90% for magnesium alloys. The morphology of the solid particles in the semi-solid slurry has significant effects on the rheological behavior. In this study, semi-solid injection moldings of AZ91D magnesium alloy were conducted with injection speeds of 220, 300, and 400 mm/s and for fraction solids of fs = 0.3, 0.4, and 0.5 in order to investigate the effects of shear stress on the microstructure. With a high injection speed, the α-phase solid particles became spherical and the average particle size decreased. The average roundness of the α-phase solid particles was correlated to the shear stress calculated on the assumption that the slurry characteristics correspond to that of the steady state at the nozzle. The reduction of the average particle size suggests that the solid particles were broken up and spheroidized by the shear stress. Under all experimental conditions, the α-phase particles in the plane perpendicular to the flow direction were concentrated at the center of specimen rather than on the surface. On the other hand, distribution of the α-phase particles in the plane parallel to the flow direction depended on the injection speed. A high fraction solid zone was generated at the end of the specimen for a high injection speed.