Effect of heat treatment on microstructure and tensile properties of die-cast Al-Cu-Si-Mg alloys

Abstract

The effects of T5 and T6 heat treatment on the microstructural development and mechanical properties of newly developed die-cast Al-xCu-2.2%Si-1.1%Mg (x=5, 6.6 and 10.6, in wt%) alloys were investigated. After T5 heat treatment to peak hardness, the main precipitates changes from β’’ in Al5Cu to β’’ and θ’ in Al6.6Cu and then to θ’ and Q’’-type precipitate in Al10.6Cu, due to the various Cu content in the α-Al matrix. The highest yield strength of 395±5 MPa and lowest elongation of 1.9±0.15% have been achieved in Al10.6Cu after peak-aged T5 condition as compared to other Al-Cu-Si-Mg alloy compositions. The solution treatment of Al-Cu-Si-Mg alloys for 30–60 min at 500 °C dissolves a sufficient quantity of Cu (~3 wt%), and spheridises the Al2Cu and Al5Cu2Mg8Si7 phases. A large number of θ’ and Q’’ coexist inside the α-Al matrix after T6 heat treatment, contributing to the exceptionally high mechanical strength. The T6 peak ageing time increased from 10 h in Al5Cu to 14 h in Al10.6Cu. These newly developed aluminium die-cast alloys having excellent mechanical properties that exceed those in existing aluminium die-cast alloys after heat treatment. Especially, the excellent yield strength of 427 MPa and 444 MPa, as well as elongation of 4.4% and 4.1%, were achieved in Al5Cu and Al6.6Cu after T6 heat treatment, respectively.