Effect of Mg level on the microstructure and mechanical properties of die-cast Al-Si-Cu alloys

The effect of Mg level on the microstructure and mechanical properties was investigated for die-cast Al-Si-Cu alloys under the as cast, the solutionised, and the solution and aged conditions. SEM and TEM analysis were used to identify phase formation and strengthening mechanism. It was found that the Mg additions resulted in an effective strengthening to the alloy under both the as-cast condition and the solution and aged condition. In the die-cast Al-Si-Cu alloys, the lamellar and blocky Al₂Cu(Si) were formed over all the experimental alloys, but the irregular AlCuMgSi intermetallics were emerged when Mg content was higher than 0.32wt%. The majority of the Al₂Cu and AlCuMgSi phases were dissolved into the primary α-Al phase during the solutionising for a short period of time (510°C for 30min). However, the solution treatment did not change the morphology and size of the α-AlFeMnSi intermetallic phase. The addition of Mg offered extra strengthening after the solution and ageing. θ'-Al₂Cu phase was the only phase precipitated during ageing to strengthen the Mg-free Al-Si-Cu alloys. The Mg addition introduced the Q'-AlCuMgSi precipitating phase on top of the precipitation of θ'-Al₂Cu phase during ageing, resulting in the co-operative precipitation strengthening to the alloy. Mg content can be controlled at a level up to 0.73wt% for increasing the strength with acceptable ductility under as-cast and heat treatment conditions.