Microstructural evolution during reheating and tensile mechanical properties of thixoforged AZ91D-RE magnesium alloy prepared by squeeze casting-solid extrusion

摘要

Thixoforging requires feedstock having a non-dendritic, equiaxed microstructure, which in some cases can be obtained by the recrystallization and partial (RAP) melting route. Based on the RAP route, squeeze casting-solid extrusion followed by partial melting is proposed to obtain the semi-solid feedstock for thixoforging. After squeeze casting-solid extrusion, unrecrystallized grains were elongated along the extrusion direction. As the temperature rises into the semi-solid regime, there is a sudden increase in the appearance of spheroidal grains. The occurrence of recrystallized grains is closely associated with the location of the first liquid to form above the solidus. Grain coarsening in the semi-solid state can be described by classical LSW equation in Ostwald ripening. The coarsening rate does decrease with increasing solid fraction,f_s, for f_s less than approximately 0.70, and then increases again with further increasing f_s. With prolonged holding time, the grains coarsen and mechanical properties of thixoforged components decrease. Moreover, the AZ91D-RE thixoforged from starting materials produced by squeeze casting-solid extrusion followed by reheating at 570℃ for 5min have extremely fine and uniform microstructure. The tensile mechanical properties of squeeze casting-solid extrusion formed AZ91D-RE are better than those of AZ91D-RE thixoforged from starting materials produced by squeeze casting-solid extrusion followed by partial melting.