Effect of modification and subsequent heat treatment on thermal and mechanical properties of near-eutectic Al-Si-Fe-Mg alloys

To investigate the effect of the size, shape and distribution of eutectic Si on the thermal conductivity, electrical conductivity and mechanical properties of near-eutectic Al-Si-Fe-Mg alloys, the morphology of eutectic Si was improved by modification and subsequent T6 heat treatment. The average size and aspect ratio of the eutectic Si decreased from 11.22 μm and 7.92 to 2.09 μm and 2.78 after modification by 0.04% Sr. After subsequent T6 heat treatment, the average size and aspect ratio of eutectic Si further decreased to 0.96 μm and 1.57. And the morphology of eutectic Si transformed from coarse lamellar to highly branched and refined fibrous, which were attributed to fusion and spheroidization of eutectic Si during the T6 heat treatment. The thermal and electrical conductivity of the alloy increased from 155.99 W/(m·K) and 35.64 pct IACS to 172.58 W/(m·K) and 37.05 pct IACS after modification and heat treatment, and the ultimate tensile strength and elongation increased from 162.05 Mpa and 0.78% to 256.16 Mpa and 4.09%, respectively. The shape and size of eutectic Si play an essential role in improving the properties of the alloy, not only by increasing the gaps between eutectic Si, promoting more electron transfer channels and reducing the scattering effect on the electron flow through the α-Al matrix, but also by reducing the cutting effect of the eutectic Si on the matrix and enhancing the ultimate tensile strength and elongation of the alloy.