Effect of heat treatment and Fe content on the microstructure and mechanical properties of die-cast Al-Si-Cu alloys

Abstract

The effect of solution and ageing heat treatment on the microstructure and mechanical properties of the die-cast Al-9 wt.%Si-3.5 wt.%Cu alloys containing 0.1-1.0 wt.% Fe was investigated. The results showed that the dendritic primary α-Al phase was varied from 20 to 100 μm in size and the globular α-Al grains were smaller than 10 μm in size. The Fe-rich intermetallics exhibited coarse compact or star-like shapes with the sizes from 10 to 20 μm and the fine compact particles at an average size of 0.75 μm. The solution treatment of the alloys could be achieved in a short period of time, typically 30 min at 510 °C, which dissolved the Cu-rich intermetallics into the primary α-Al phase and spheroidised the eutectic Si phase. During the subsequent ageing treatment, numerous fine precipitates of θ' and Q' phases were formed to provide effective strengthening to the α-Al phase, significantly improving the mechanical properties. Therefore, Fe content in the die-cast Al-Si-Cu alloys needs to be controlled at a low level in order to obtain the improved ductility and strength under solution and aged condition.