Effect of room temperature storage time on precipitation in Al-Mg-Si(-Cu) alloys with different Mg/Si ratios

The effect of natural ageing time before artificial ageing has been investigated in four Al-Mg-Si(-Cu) alloys, with 0.4% Mg + 0.8 % Si and 0.8% Mg + 0.4 % Si, both with and without 0.14 at.% Cu. The precipitate microstructure was quantified by means of transmission electron microscopy. Varying the storage time before ageing for 170 min at 200 degrees C, we observe an initial hardness increase after minutes, a decrease after several hours and another increase after weeks. The hardness decrease was most pronounced in the Mg-rich Cu-free alloy, caused by a reduced precipitate volume fraction. Adding Cu produces finer microstructures, higher hardness and reduces the negative effect of natural ageing regardless of the Mg/Si ratio of the alloy. With 1 week storage, an increase in the fraction of the Cu-containing precipitates L and Q' was observed in the Cu-containing Si-rich and Mg-rich alloys respectively.