Effectiveness of Hot Isostatic Pressing on Healing Oxide Film Defects in Cast Aluminium Alloys

Abstract

Hot isostatic pressing (hipping) of cast aluminium alloys is recognized as a means of providing improved internal soundness and mechanical properties of the castings. However, little is known about the limitations and potential benefit of the hipping process to produce reliable aluminium alloy castings containing oxide film defects due to turbulently filling of moulds. In this study, oxide film defects and fatigue lives of turbulent filled unfiltered AI-7Si-Mg alloy castings have been characterised in as-cast and hipped. conditions to quantify the effectiveness of hot isostatic pressing on healing oxide film defects in the castings. The average fatigue life and fatigue life at 0.1 percent failure rate of AI-7Si-Mg alloy castings improve substantially following hipping treatment that also reduces significantly the scatter of the fatigue life data of the castings. This indicates that solid state hipping is a robust process to de-activate oxide film defects in AI-7Si-Mg alloy castings in which transformation of oxide film structure does promote the diffusion bonding across oxide film interfaces. This, however, contrasts with the behaviour of oxide inclusions in Al-10Mg alloy castings in which magnesium oxide film, whose structure is stable and does not transform during hipping, is possibly resistant to form effective bonding across the oxide film interfaces.