Evaluation of the gas porosity and mechanical properties of vacuum assisted pore-free die-cast Al-Si-Cu alloy

High pressure die casting (DC) is an effective manufacturing method suitable for high accuracy, mass production. However, its high injection speeds produce an unstable molten metal flow that induces gas porosity; a critical casting defect that limits the application of heat treatment and welding. In this study, we performed casting using conventional DC(CDC), vacuum assisted DC(VDC), pore-free DC(PFDC), as well as a novel vacuum assisted PFDC (VPFDC). We evaluated the gas porosity of the as-cast and heat-treated specimens using computed tomography, microstructure, and mechanical property analysis. The VDC-, PFDC-, and VPFDC-prepared specimens recorded lower inner gas quantities than that the CDC-prepared specimen; the lowest (4.914 cc/100 g) was recorded for the VPFDC-prepared specimen (product section). X-ray diffraction and optical microscopy analysis showed that the specimens all contained alpha-Al, eutectic Si, theta-Al2Cu, and Al2O3. However, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy analysis indicated a higher oxygen content in the PFDC-prepared specimen than the CDC-prepared specimen. Moreover, the tensile properties of the VDC-, PFDC-, and VPFDC-prepared specimens were better than those of the CDC-prepared specimens, and after heat treatment, the tensile properties of the CDC-prepared specimen deteriorated while those of the VDC-, PFDC-, and VPFDC-prepared specimens improved.