Effects of Sm addition on microstructural evolution of Mg-6Zn-0.4Zr alloy during semi-solid isothermal heat treatment

The application of segmental semi-solid thixoforming of magnesium alloys is confined due to the dimensional distinction existing in solid particles of the alloy billet from edge to center zones. In the present study, the effects of Sm addition on the microstructural evolution of Mg-6Zn-0.4Zr and Mg-6Zn-4Sm-0.4Zr alloys by semi-solid isothermal heat treatment were investigated, to obtain optimum semi-solid microstructures for the subsequently thixoforming. The results indicate that the grains of the Sm-bearing alloy are evidently refined and gradually evolve from dendritic to globular and elliptic particles. In addition, the distinctly dimensional effect of the Mg-6Zn-0.4Zr alloy is eliminated with 4% Sm addition; the particle sizes in all zones from center to the edge of the billet are almost identical. With the increment of isothermal heat treatment temperature, the dendritic microstructures completely disappear, and meanwhile, the irregular and globular particles gradually form. The size, morphology and the distribution of solid particles mainly depend on the formation and permeation of the liquid phase in the process of isothermal heat treatment. As the isothermal temperature increases from 570 °C to 610 °C, the average size and shape factor of solid particles of both the alloys with and without Sm addition gradually decrease while the liquid fraction gradually increases.