Material properties of long term naturally aged die cast Al-Si-Cu alloys

Die cast aluminium alloys, including automotive parts, are generally understood to suffer some degradation of their mechanical properties over time due to corrosion, specifically when exposed to water and sea breeze (salt air). However, in this study, it is shown that in the absence of a corrosive environment, the mechanical properties of die cast aluminium alloys actually increase over a 10 year period due to natural aging. The increment in the mechanical properties is mainly attributed to precipitation hardening brought about by theta '' phase particles. It also appears that any significant change in microstructure affects the mechanical properties, e. g. the change of the needle-like eutectic Si to a spherical form and the joining of discrete alpha-Al phases to make larger grains. The driving force for the microstructural changes upon natural aging is related to the residual stress in the die cast sample accumulated by high casting pressure and rapid cooling rate during injection process.