Microstructure and tensile property of an Al–Si–Cu–Mg alloy produced by thixocasting process

Thixocasting is focused as a suitable casting process for near net shaping, weight reduction of a product and obtaining good mechanical properties. In the present study, the microstructures and tensile properties of the Al–6 mass%Si–3 mass%Cu–0.3 mass%Mg alloy produced by the thixocasting process were investigated and compared with those of the permanent mold castings. Microstructure of the thixocast parts basically consists of spherical primary α–Al phase particles, fine eutectic Si phases and acicular compounds containing iron. The average size of α–Al phase particles was about 60 μm in diameter. After solution treatment, the average size of Si particle for the thixocast alloy was about 2.6 μm and was smaller than that of the permanent mold castings. At aging temperature between 433 and 473 K, the hardness of thixocast alloy was higher than that of the permanent mold casting and the maximum hardness of thixocast and permanent mold cast specimens aged at 433 K were HB 118 and HB 108. The ultimate tensile strength, 0.2% proof stress and elongation of T6–treated thixocast alloy were 394 MPa, 331 MPa and 7.5% respectively. These value are far superior to those of the permanent mold casting. Such the high tensile properties of thixocast alloy might be caused by this extremely high solidification rate in thixocasting process made eutectic Si phase and Al–Fe system intermetallic compound to be fine, increase in Cu and Mg concentration in α–Al matrix compared with the permanent mold castings.