Microstructure characteristics in non-modified and Sr modified Al–Si–Cu–Mg 319 type alloys

Abstract

This study was carried out on 319 alloys containing low and high levels of Mg, in the non-modified and Sr modified conditions (150 ppm Sr addition). Single step, two step and triple step heat treatments were applied to identify the optimum solution heat treatment to minimise incipient melting of the copper phases Al2Cu and Al5Mg8Cu2Si6 in these alloys in relation to the alloy properties. In Mg free alloys, no incipient melting of Al2 Cu was observed even in samples heat treated at 520°C. Addition of Sr leads to modification of Si particles but also to an increase in area per cent porosity and pore length, especially when the solution temperature reaches 520°C. Addition of Mg results in a decrease in the Si particle aspect ratio but an increase in particle size. Magnesium was also found to increase the possibility of incipient melting resulting from the formation of the insoluble Al5Mg8Cu2Si6 phase. To some degree, Sr decreases the effect that Mg has in increasing the area per cent porosity and pore length, while Mg impairs the effects that Sr has on modifying Si particles, even though the lowest Al–Si eutectic temperature is obtained for the 319 alloy containing both Mg and Sr.