Effects of Shot Sleeve Pre-solidification on the Microstructure and Tensile Properties of High Pressure Die Cast AE44

In cold-chamber high pressure die casting (HPDC), some pre-solidification occurs in the shot sleeve prior to being injected into the die cavity. Here, we study the effects of pre-solidification on the development of microstructure and defects in the HPDC of magnesium alloy AE44 (Mg–4Al–4RE–0.2Mn, wt pct, where RE are mixed La and Ce) and on subsequent tensile properties. Samples with a high fraction of pre-solidification contained both externally solidified crystals (ESCs) and cold flakes, which also induced large pockets of positive macrosegregation. Variations in the morphology and length scale of α -Mg grains and eutectic Al _11 RE _3 are discussed in terms of the different cooling conditions for each microstructural feature using a grain morphology map for α -Mg. A high fraction of pre-solidification resulted in a small decrease in yield stress due to the larger microstructural length scale, and a substantial decrease in ultimate tensile strength and ductility due to the presence of a non-bonded interface between cold flakes and the surrounding material. The results provide insights into microstructure formation in the HPDC of AE44 and highlight the importance of controlling the extent of pre-solidification that is injected into the die cavity.