Effect of pre-ageing on natural secondary ageing and paint bake hardening in Al-Mg-Si alloys

The effect of pre-ageing (PA) time and temperature on subsequent natural secondary ageing (NSA) and paint bake (PB) hardening of an Al-Mg-Si alloy was systematically investigated, especially when both parameters change. A wide range of PA conditions was covered, including temperatures from 80 degrees C to 160 degrees C and times from several minutes to several days depending on the PA temperature. Hardness and electrical resistivity measurements, positron annihilation lifetime spectroscopy, and differential scanning calorimetry were used to characterise the alloy after various ageing stages. It is found that both the delay of NSA clustering and PB hardening are closely related to the PA conditions. PB hardness can be enhanced primarily by increasing PA temperature, whereas NSA stability (resistance to clustering) is improved by prolonging PA time or by lowering PA temperature. In particular, when PA hardness is constant, one can use PA temperature to trade PB hardness for NSA stability and vice versa. One can also tailor PA conditions to minimise hardness after a given NSA time while maximising PB response, which is a way to improve industrial practices.