Influence of Cu content on ageing behavior of AlSiMgCu cast alloys

The influence of Cu content on ageing behavior of Al–8Si–0.4Mg–xCu alloys has been investigated by hardness measurement, differential scanning calorimety (DSC) and transmission electron microscopy (TEM) analysis. Experiments have been conducted for Al–8Si–0.4Mg–xCu alloys with 1 wt%Cu, 2 wt%Cu, 3 wt%Cu, and 4 wt%Cu produced in permanent moulds. Hardness has been estimated for ageing times varying from 1 h to 100 h. The results indicate that the maximum hardness increases clearly with the increase of Cu content, but the total increase in hardness during ageing (ΔHVmax) decreases with addition of 1 wt%Cu and has a little increase with Cu content from 1 wt% to 4 wt%. Addition of Cu decreases the age hardening rate. The differential scanning calorimetry measurement and transmission electron microscopy analysis results suggest that addition of Cu to Al–8Si–0.4Mg alloy restrained the precipitation of β″ and β′ phases, but Q″ phase precipitated for Al–8Si–0.4Mg–1Cu alloy, and Q″ and θ′ phases precipitated for Al–8Si–0.4Mg–xCu alloys with more than 1 wt%Cu. The precipitation behaviors and effect of precipitates on age hardening behaviors have been discussed.