Al-Si alloys have been widely used in automotive, aerospace, electronics and communication industries due to their excellent castability, low thermal expansion, and good wear and corrosion resistance. However, the presence of coarse eutectic Si often results in relatively low thermal conductivity. With the rapid development of the electronics and communication industries, the requirements for thermal conductivity and mechanical properties of materials are increasing. In this study, the effects of heat treatment and minor Mg addition on the microstructure, mechanical properties, and thermal conductivity of Al-7Si alloys are systematically investigated.

The results indicate that heat treatment at 300 ℃ after solution treatment promotes the spheroidization of eutectic Si and reduces the solid solubility of solute atoms in the aluminum matrix, thereby enhancing the thermal conductivity and reducing the hardness of the Al-7Si alloy. The three-step heat treatment process (solution treatment +300 ℃ treatment +180 ℃ treatment) not only facilitates the spheroidization of eutectic Si, but also induces the precipitation of nanoscale (Mg, Si) strengthening phases, further reducing the solid solubility of solute elements in the Al-7Si alloy with 0.4%Mg addition. After the three-step heat treatment, the Al-7Si-0.4Mg alloy reaches to 189 W/(m·K) in thermal conductivity and 73.5 HV in microhardness, respectively, which are increased by 11.2% and 62.6% respectively, compared with the as-cast Al-7Si alloy.

According to the Wiedemann-Franz law and the Matthiessen-Fleming rule, the primary factors influencing the thermal conductivity of alloys are solute atoms in solid solution and secondary phases. In this study, a three-step heat treatment process is used to transform the plate-like eutectic silicon in the Al-7Si-0.4Mg alloy into fine spherical particles. Additionally, micrometer-sized silicon particles and nanoscale (Mg, Si) precipitates are induced within the alloy matrix. This microstructural modification simultaneously enhances the thermal conductivity and mechanical properties of the alloy. Our work is expected to inspire the design of Al-Si alloy with high strength and high conductivity.