High pressure die castings usually contain gas porosity due mainly to the entrapment of air or gas in the melt during the very high speed injection of the molten metal into the cavity. In this paper, the advantages of using an evacuated die cavity during mould filling were evaluated. ASTM standard die casting tensile specimens of three Al alloys, Al¨C5%Si, Al¨C8%Si and Al¨C18%Si, were cast on a Buhler shot control die casting machine equipped with a Fondarex vacuum assist system. The effect of vacuum assistance on the porosity distribution and mechanical properties of the produced castings were studied in detail. Selected specimens were also subjected to a T6 tempering treatment to evaluate the formation of surface blisters and effects on the mechanical properties.
It was found that the volume of gas porosity and the pore sizes in the castings were significantly reduced by using vacuum assistance during die casting. As a result, the density and the mechanical properties, particularly the tensile strength and ductility, were improved markedly. An optimum injection speed was also identified for producing high performance castings. After heat treatment, vacuum assisted die cast parts showed much less surface blistering when compared to conventional die cast parts, demonstrating that vacuum assisted die castings are promising for heat treatment at elevated temperatures to improve the mechanical properties.