Influence of natural aging on the precipitation hardening of an AlMgSi alloy

The influence of natural aging on precipitation hardening of an Al–Mg–Si alloy was investigated by mechanical testing and quantitative transmission electron microscopy. It was found that natural aging increases yield stress and reduces ductility, which is attributed to the formation of Mg/Si clusters. The evolution of tensile properties saturates after around 7 days natural aging. Artificial aging for 30 min at 180 °C after solution treatment resulted in an important increase in yield strength and loss of ductility, due to the rapid precipitation of the β′′ intermediate phase. Previous natural aging progressively reduces the strengthening effect of 30 min/180 °C artificial aging, completely suppressing it after 7 days pre-aging. Microhardness evolution during 180 °C artificial aging was followed in samples that were aged immediately after quenching, and with 7 days natural aging before artificial aging. In addition, microstructural characterization using TEM was carried out at specific artificial aging times. Previous natural aging was found to reduce the initial hardening rate, producing a delay in the formation of β′′ precipitates. The peak-aging condition was reached almost at the same aging time. However, longer β′′ precipitates with a lower volume fraction were observed when artificial aging was preceded by natural aging.