Effect of Silicon Addition on the Microstructure and Precipitation of Cast Al-5.3Cu-0.8Mg-0.6Ag Alloys

Al-Cu-Mg-Ag based alloys with different silicon content had been investigated using optical microscope, scanning electron microscope, transmission electron microscopy and hardness testing. It is shown that the addition of excess silicon decreases the hardness and thermal stability of the based alloy, and elongates the peak-ageing time at 185 ℃. Moreover, the excess Si addition completely suppresses the precipitation of Ω phase that dominates of the microstructure of the based alloy. The θ' prime phase and minor σ phase are the predominant hardening phases in the Al-5.3Cu-0.8Mg-0.6Ag-6.0Si (wt. pct) alloy after peak ageing at 185 ℃.