Comparison of mechanical properties of die cast aluminium alloys: Cold v. Hot chamber die casting and high v. Low speed filling die casting

In this paper, the mechanical properties of die cast aluminium alloys made by various die casting technologies were examined. To create high quality aluminium alloy die castings, two die casting processing technologies were employed. These were (a) ultra slow speed filling cold chamber die casting and (b) high speed hot chamber die casting. Significant improvements of the fatigue and mechanical properties were obtained for both die casting systems compared to the normal high speed cold chamber die casting technique. By comparing ultra slow die casting with hot chamber die casting, it was found that the fatigue and mechanical strengths from hot chamber die casting were higher than those for ultra slow filling die casting. The differences in material strength were attributed directly to the material properties, e.g. microstructural morphology and internal defects. Spherical fine dendritic cells in the hot chamber die casting sample gave rise to high fatigue crack growth resistance; the low crack growth resistance for cold chamber die cast aluminium is mostly due to the growth of aluminium rich α phase and the presence of eutectic silicon fibres. The fatigue strength was also related to the number of internal defects, e.g. the lower the defect rate on the fracture surface, the higher the fatigue resistance and mechanical strength. The characteristics of the principal internal defect were different depending on the die casting technology: this showed fine porosity for hot chamber die casting but solidification shrinkage and the scattered chill structure for slow and high speed cold chamber die castings. The reasons for the change of material strength were therefore influenced by the die casting process.