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