Comparison of the semisolid squeeze casting and gravity casting process on the precipitation behavior and mechanical properties of the Al-Si-Cu-Mg alloy

The differences in microstructure, ageing behavior and mechanical properties of the Al-17.8Si-4.3Cu-0.4Mg alloy formed by semisolid squeeze casting and gravity casting processes were investigated. It was found that a nearly equiaxed microstructure of alpha-Al, finer and denser Si phases, and a higher dislocation density exist in the semisolid extruded sample. In the peak ageing state, 0" and Q' phases are mainly observed under the two forming processes, while the precipitate increases and the size decreases after semisolid extrusion. According to the thermodynamics, the binding energy and enthalpy values of Q ' phase are lower than those of the 0' and 0 '' phases. Therefore, among the three metastable phases, the Q ' phase precipitated before the 0 '' and 0' phases. The semisolid extruded alloy exhibits higher strength and ductility in the as-cast state and T6 state. This might be caused due to more uniform microstructure and finer precipitating phase. Double ageing hardness peaks were found in the age-hardening curves of Al-17.8Si-4.3Cu-0.4Mg alloys under both processes, which were related to the influence of the precipitation sequence of the alloy.