Influence of heat treatment on microstructure and thermo-physical properties of Al-50wt.%Si alloy fabricated by selective laser melting

Herein, we investigate the influence of heat treatment on microstructure and thermo-physical properties of Al-50wt.%Si alloy fabricated by selective laser melting (SLM). Microstructure characterizations reveal that SLM and heat-treated Al-50wt.%Si alloys are all characterized by the primary Si and eutectic structure consisting of dominant Al matrix and tiny eutectic Si. However, the size of primary Si monotonously increases with increasing the heat treatment holding time at 550 °C. In addition, the SLM Al-50wt.%Si alloy display a highest coefficient of thermal expansion (CTE) among all examined alloys. As the heat treatment holding time increases, the Si precipitates from the Al(Si)-supersaturated solid solution, leading to the formation of a skeleton-like Si phase, which effectively alleviates the thermal expansion. Furthermore, the SLM Al-50wt.%Si exhibits a lowest coefficient of thermal conductivity (CTC), since the significantly increased size of primary Si with increasing the heat treatment holding time leads to a substantial reduction in the number of grain boundaries, which severely restricts the scattering of heat transfer phonons. Moreover, theoretical models are employed to calculate the CTE and CTC. Results indicate that the Schapery lower model provides a better prediction of CTE. However, two selected models are all not suitable to accurately predict CTC. Those findings can provide new insights to designing Al matrix composites with low CTE and high CTC through microstructural optimization.