Dissimilarity of physical and mechanical properties of Al-(0.2–17.9 wt%) Si automotive alloys in under ageing, peak ageing, and over ageing conditions

Abstract

The current study investigates the impact of under, peak, and over-aging on the mechanical and physical characteristics of Al-based Si-doped automotive alloys. The Al–Si–Cu–Mg alloys with 0.2, 3.5, 6.1, 12.7, and 17.9 wt% Si are cast in a conventional manner, followed by homogenizing, solutionizing, quenching, and ageing. According to the experimental data, peak-aged alloys' increased hardness is mostly caused by the production of hard and brittle metastable Al2Cu, Mg2Si, and iron-rich precipitates inside the Al matrix. A higher level of Si-rich intermetallics makes the alloy of superior strength as it hinders the dislocation movement. Nevertheless, the tensile strength decreases beyond the eutectic composition as the formation of primary Si throughout the alloy distorts the mechanical properties. Additionally, peak-aged conditions show a decreased percentage of elongation and impact energy because of the substantial production of various fine precipitates along the grains. At the over-aged state, the strength drops drastically for precipitation coarsening as coarse particles reduce the effective pinning effect letting the dislocations move. Similarly, the alloys do not offer enough strength in under-aged conditions as the elements remain in a solid solution but are not precipitated. Thermal conductivity decreases with the Si content for a higher level of precipitates. The over-aged condition shows the highest values due to the alloys' stress relieving and grain coarsening effect. An analysis of the alloys' microstructure shows that the solution treatment enhances the distribution of silicon grains. The microstructure of the grain in the over-aged stage reveals that it has totally recrystallized, for which, compared to the eutectic composition, the primary Si is more common in the alloys.