Effects of Si and Ni Content and Preparation Process on Microstructure and Thermal Properties of Al-Si-Ni Alloy

Three kinds of alloys, Al-11Si-5Ni, Al-22Si-10Ni and Al-33Si-15Ni, were prepared by ordinary solidification, water-cooled copper mold sub-rapid solidification and water-cooled copper mold sub-rapid solidification combined with heat treatment (520 ℃×6 h). The microstructures and thermal properties of the alloys were studied. The results show that only Al-Si-Al₃Ni ternary eutectic reaction occurred in the solidification process of Al-11Si-5Ni eutectic alloy, and the microstructure was ternary eutectic structure composed of α-Al, eutectic silicon and eutectic Al3N phases. The Al-22Si-10Ni and Al-33Si-15Ni alloys were hypereutectic alloys, the microstructure consisted of primary silicor phase, primary Al₃Ni phase and ternary eutectic structure, and the solidification process was divided into three stages: the primary silicon phase precipitation, the primary silicon and Al₃Ni phase co-precipitation, and Al-Si-Al₃Ni ternary eutectic reaction. With the simultaneous increase of Ni and Si content, primary silicon and Al₃Ni phases were coarsened. Compared with the ordinary solidification process, the water-cooled copper mold sub-rapid solidification process could refine the alloy structure, and the heat treatment made the eutectic silicon phase and the eutectic Al₃Ni phase spheroidize. With the simultaneous increase of the content of Ni and Si, the alloy's thermal conductivity and the thermal expansion coefficient decreased. The water-cooled copper mold sub-rapid solidification combined with heat treatment could effectively improve the thermal conductivity of the alloy. Al-22Si-10Ni alloy prepared by the water-cooled copper mold subrapid solidification combined with heat treatment had the best comprehensive performance with the room temperature thermal conductivity of 129.9 W · m⁻¹ · K⁻¹, the thermal expansion coefficient at 100 ℃ of 13.8×10⁻⁶ K⁻¹, and the average thermal expansion coefficient in the temperature range of 25–100 ℃ of 12.9×10⁻⁶ K⁻¹.