Effect of natural ageing and pre-straining on the hardening behaviour and microstructural response during artificial ageing of an Al–Mg–Si–Cu alloy

Abstract

Al–0.42Mg–0.5Si–1.0Cu alloy was prepared by hot extrusion, followed by quenching and ageing treatment. The keeping and pre-straining immediately after quenching were performed before ageing, to investigate natural ageing and pre-straining effects on the age-hardening and microstructural response of the alloy, hardness measurement, TEM, and HRTEM characterisation were used. The results indicated that, room-temperature keeping led to a loss of peak ageing hardness for subsequent artificially aged alloy. The peak ageing hardness decreased and the peak ageing time increased with increasing keeping time. The formation of clusters/GP zones induced by natural ageing decreased the densities of β″ and Q′ phases, which contributed to the decrease of peak-ageing strengthening. Pre-straining before ageing significantly increased the peak ageing hardness and shortened the peak ageing time of the alloy. Furthermore, it significantly mitigated the effects of natural ageing on the hardening behaviour of the alloy during subsequent artificial ageing by restraining the formation of clusters/GP zones and accelerating the nucleation of β″ and Q′ phases in the early stage of subsequent artificial ageing. In the over-ageing stage, dislocations induced by pre-straining facilitated the coarsening of precipitates and consequently led to an apparent decrease of hardness.