Effect of ageing and thermal exposure on microstructure and mechanical properties of a HPDC Al–Si–Cu–Mg alloy

Mechanical behavior and microstructure evolution of a newly developed high strength die casting Al–13Si-0.25Cu alloy (in wt%) were investigated under as-cast, peak-aged and thermal-exposed conditions. In the as-cast condition, the alloy has a yield strength (YS) of 213 MPa and an elongation (El) of 4.6%. The alloy YS is further improved to 265 MPa after T5 ageing treatment. The enhancement of YS after ageing is attributed to the precipitation of nanoscale β″, Q′ and θ′ phase from the α-Al matrix. After thermal exposure at 350 °C for 100 h, however, these precipitates mostly dissolve into the α-Al matrix and the ultrafine eutectic Si networks undergo breaking and coarsening, reducing YS to 117 MPa while enhancing the El to 10.3%.