X-ray Computed Tomography Studies on Porosity Distribution in Vacuum Induction Cast Al-7Si Alloys

AbstractPorosity in aluminum alloys is a great concern to the casting and automotive industry. In this publication, porosity formation in air-melted and vacuum induction melted (VIM) aluminum alloys was studied and compared to understand its effect on microstructure and mechanical properties of Al-7Si alloys. Al-7Si alloys were cast at 700°C and 900°C in a muffle furnace and VIM furnace. Microstructural results show that the alloys cast in muffle furnace refined the eutectic silicon compared with the cast samples prepared in VIM furnace. X-ray computed tomography (XCT) was used for three-dimensional (3D) visualization and quantification of porosity in these alloys. The volume fraction of pores was observed to be higher in alloy air-melted at 900°C compared with 700°C. XCT results from VIM alloy samples showed no significant porosity when cast at either 700°C or 900°C. The morphology of large pores in alloys air-melted at 700°C represents the formation of shrinkage porosity due to the incomplete flow of molten metal during solidification. Tensile test results show that the elongation property of VIM alloy was increased by more than 20% compared with air-melted alloy. The tensile strength and elongation were observed to be higher for alloy samples cast at 700°C compared with 900°C for both air-melted and VIM alloys. The findings from microstructure, XCT, and tensile tests show that vacuum induction melting improves the mechanical properties of the alloy compared with air-melted alloy.