Abstract
Quench sensitivity in age hardenable aluminum alloys has been attributed to solute loss on heterogeneous nucleation sites during slow cooling after extrusion. However, recent work on the influence of natural ageing at room temperature on the artificial ageing response suggests that quenched-in vacancies play a significant role in clustering and precipitation behavior of strengthening precipitates. Two different high-strength Al-Mg-Si alloys were cooled at different rates and artificially aged at 175oC after 30 min or 24 h natural ageing at 18oC. While natural ageing was confirmed to have a negative effect on the age-hardening response of fast cooled samples, the ageing response of slow cooled samples was independent of natural ageing time. Therefore the effects of quench sensitivity are less apparent after prolonged natural ageing. It is concluded that quench sensitivity is also affected by changes in ageing kinetics due to loss of quenched-in vacancies during slow cooling.
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