Effect of pre-ageing and natural ageing on the paint bake response of alloy AA6181A

A pre-ageing treatment of 20 s at 200 °C immediately after quenching effectively retards clustering during subsequent natural ageing in alloy 6181A. This results in a better paint-baking response in alloy 6181A than in 6022 with a 0.3 wt% Cu addition. The formation of solute aggregates with more than 22 detected Mg+Si+Cu atoms during the pre-ageing treatment was found to contribute to an enhanced age hardening response during a subsequent ageing treatment of 0.5 h at 170 °C. Based on a generalised regression model, it was found that for a given fraction of Mg+Si+Cu in solute aggregates, large solute aggregates with more than 75 detected Mg+Si+Cu atoms contribute about 2 times more to the yield strength than small solute aggregates with less than 23 atoms, while intermediately sized solute aggregates appear to have only a negligible strengthening effect. Also, based on another generalised regression analysis, it was confirmed that the electrical conductivity is influenced by both the solute concentration in solution and by differently sized aggregates. For a given volume fraction, small aggregates with less than 23 Mg+Si+Cu atoms were found to have a significantly more negative effect on the conductivity than larger aggregates.