The processing phenomena of semisolid Mg–9% Al–1% Zn alloy at ultra high contents of the unmelted phase

A prototype system was used to process the semisolid Mg–9% Al–1% Zn alloy with a content of 75–85% of the unmelted phase. The influence of operating parameters on mold filling characteristics, integrity and microstructure of the component with a weight of 0.5 kg, varying geometry and wall thickness, was examined. The mold filling time was found to be the primary factor controlling the process. Although the turbulent flow created porosity at intermediate stages of mold filling, highly dense structures were obtained unless the injection velocity was high enough to preserve the semisolid state until the final densification. A laminar flow of the slurry, observed for lower injection velocities, resulted in a low porosity at all stages of the mold filling. The final microstructure was dominated by near globular primary α-Mg particles with a size of 40 μm which, for thicker walls, evolved into equiaxed grains. For high injection velocities and large distances from the gate, an inhomogeneity in distribution of α-Mg was detected. The eutectics of the secondary α-Mg and Mg17Al12 with a volume fraction of 15–25%, existed as thin films at particle boundaries and islands at triple junctions. Some suggestions for the processing of magnesium alloys at ultra high solid contents are defined.