The influence of natural ageing on the artificial ageing response of Al-Si-Cu-Mg casting alloys

The T6 heat treatment is commonly used to increase the strength of gravity cast Al-Si components containing Cu and/or Mg. The artificial ageing response is known to be affected by the thermal history, such as solution treatment, quench rate, natural ageing and heating rate to the artificial ageing temperature. The influence of natural ageing on the artificial ageing response was investigated for three alloys; Al-8Si-0.4Mg, Al-7Si-3Cu and Al-8Si-3Cu-0.4Mg. Natural ageing had a strong influence on the ageing response of the Al-Si-Mg alloy in the underaged condition and the strength increase was strongly reduced. Despite this, the time to peak yield strength as well as its magnitude were not strongly affected by natural ageing. No clear influence of natural ageing was observed for the Al-Si-Cu alloy. For the Al-Si-Cu-Mg alloy the ageing response seems to depend on the natural ageing time. Natural ageing of 3 weeks shifted the peak yield strength to shorter ageing times and its magnitude was decreased a little compared to direct ageing after quench. Natural ageing of 1 day gave the least beneficial properties after artificial ageing.