The heat treatment of Al–Si–Cu–Mg casting alloys

The mechanical properties of aluminium–silicon casting alloys containing Cu and Mg are known to be improved by heat treatment. Over 60 papers are reviewed here in order to clarify the sequences of microstructural changes which occur during heat treatment, and their influence on the mechanical properties. It is shown that the changes occurring during solution treatment are relatively well understood, and that the equilibrium phase diagram can be used to predict the stability of phases at the solution treatment temperature. The influence of quench rate and natural ageing on subsequent artificial ageing needs to be studied further, but some conclusions can be drawn. These include: (1) An increase in quench rate above 4 °C/s gives a small increase in yield strength after ageing, while the concomitant influence on elongation is more complicated and depends on the alloy. (2) Natural ageing is shown to have a large influence on subsequent artificial ageing response of Al–Si–Mg alloys, while there is a significant lack of knowledge for Cu-containing alloys. Artificial ageing of Al–Si–Mg alloys in the temperature range 170–210 °C gives peak yield strengths of the same level, while Cu-containing alloys show a decrease in peak yield strength with increasing ageing temperature. The precipitation sequences in Al–Si–Mg and Al–Si–Cu alloys are relatively well known. In Al–Si–Cu–Mg alloys several precipitation sequences are possible, which need further investigation.