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

Abstract

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.