Investigation of the influences of heat treatment on the microstructures and thermal properties of Al-20Si alloy fabricated by powder extrusion

Abstract

Al-20Si alloy with high thermal conductivity and adequately low coefficient of thermal expansion is successfully produced by hot extrusion of pre-alloyed powder and used as a model material in investigating the influences of heat treatment on the thermal properties of the material. In the as-extruded samples, fine Si particles are distributed uniformly in the Al matrix. When the alloy is annealed at 250 °C or 400 °C for 2 h, neither the Al nor Si grains grow significantly, but the internal strain developed in extrusion is relieved. Therefore, thermal conductivity is improved to 148.0 W/(m·K) and the coefficient of thermal expansion is reduced to 17.3 × 10−6/K. With further increasing annealing temperature to 560 °C, the coarsening of Al and Si grains occurs. On one hand, it significantly reduces the grain boundaries and phase boundaries in the material, consequently further increasing the thermal conductivity to 174.8 W/(m·K). On the other hand, the coarsened structures lead to thermal stress during cooling, which offsets the constraining effect of Si on the thermal expansion of Al and results in an increase in the coefficient of thermal expansion.