Effects of solution treatment on microstructure and mechanical properties of powder thixoforming 6061 aluminum alloy

A novel processing technology for the preparation of particle-reinforced metal matrix composites, powder thixoforming, is proposed and a 6061 matrix alloy was prepared using this method to study the effects of solution treatment on its microstructure and mechanical properties. The results indicate that the microstructural evolution includes two stages during solutionizing at 535 °C. The first stage involves the quick dissolution of the eutectic phase and the resulting rapid coarsening of both the primary particles and secondarily solidified grains within a period of 0-3 h. The second stage is regarded as the normal growth at a slow rate that starts from 3 h. Owing to the large interdendritic eutectic phase, the microstructural evolution of the permanent mold cast alloy is slower than that of the powder thixoforming. The tensile properties of the powder thixoforming 6061 alloy initially increased from 0 h to 3 h owing to the reduction of the harmful eutectic phase, increased solid-solution strengthening, and composition homogenization. It then decreased due to grain coarsening. After being solutionized at 535 °C for 3 h, the tensile properties attained the highest values, with an ultimate tensile strength of 223 MPa, a yield strength of 178 MPa, and elongation of 13.8%. These values represent increases of 23.9%, 79.8%, and 72.5%, respectively, as compared to the as-powder-thixoformed alloy. Simultaneously, the general fracture regime changed from intergranular mode to a mixture of intergranular and transgranular modes and finally to a transgranular mode. The solution time had similar effects on the mechanical properties of the permanent mold cast alloy, most of which were lower than those of the powder thixoforming alloy.