Mechanical properties of hypoeutectic, eutectic, and hypereutectic Al-Si automotive alloys under ageing treatment

The role of different levels of silicon on mechanical properties has been investigated through tensile and impact tests along with fractography of Al-Si automotive alloys. Cast alloys are given precipitation strengthening through T6 heat treatment. It has been experimentally determined that ageing these alloys leads to the enhancement of tensile properties, which is mainly caused by the precipitation of the intermetallics such as Al2Cu, Mg2Si, and Al2CuMg within the Al-matrix. The ductility of these alloys primarily exhibit a diminishing trend as the ageing temperature increased. This diminishing trend can be attributed to the formation of fine precipitates like GP zones and metastable phases. However, after reaching a certain temperature, the ductility started to increase again. This is caused by the coarsening of the precipitates. At higher strain rates, the tensile strength is found to be identical for higher Si-added alloys due to their brittleness. The microstructures of the alloys reveal that as Si content increases, eutectic phases become more apparent. Fractography of the base alloy exhibits a small dimple structure, and higher Si-added alloys initiate the crack propagation. Beyond eutectic composition, the mechanical properties of the alloys became distorted due to block-like primary Si into the Al-matrix.