Effects of Cu Addition on Aging Behavior of Al-0.3%Mg-0.5%Si Alloy

The aging behavior of Al-0.3mass%Mg-0.5mass%Si alloys with or without Cu was investigated at the working temperature of radiators (i.e., 90°C). It was observed that Cu addition was not necessarily effective in strengthening the alloys in the cases where these alloys were aged for a long time at the working temperature of radiators. The Cu-free alloy stored at 90°C for 4300 h possessed approximately the same yield strength as the alloy containing 0.5%Cu or 0.8%Cu. In contrast, when the alloys were stored at room temperature or at 180°C, the yield strength of the alloy containing Cu was larger than that of the alloy not containing Cu. Results of DSC analyses and TEM observations showed that the dominant phases in age hardening were as follows: (i) formation of Mg-Si (-Cu) clusters at room temperature, (ii) formation of clusters and β′′-phase precipitates at 90°C, and (iii) formation of needle-shaped β′′-phase precipitates at 180°C.