The microstructural development of Mg-9 pct Al-1 pct Zn alloy during injection molding

Ex-situ microstructural analysis was used in order to assess the high-temperature transformations of the Mg-9 pct Al-1 pct Zn alloy within the injection-molding system. In addition to as-cast ingot, chipped feedstock, and final products, the alloy samples at different stages of the process were investigated. It was revealed that the cold deformation during ingot chipping and recrystallization during initial stages of residency inside the barrel led to nucleation and growth of the equiaxed grain structure. Subsequent melting of the Al-rich phase followed by liquid-alloy wetting of α-Mg grain boundaries initiated the formation of the slurry with globular solid particles. The structural evolution of the semisolid alloy under the simultaneous influence of temperature gradient and injection-screw shear is considered, and the mechanisms involved are discussed. It is concluded that during processing of partially remelted alloy the temperature profile along the injection-molding barrel in combination with the feedstock structure are key factors that control both the transformation of the thixotropic slurry and the final morphology of the primary solid phase.