Effect of carbonaceous hybrid reinforcement and extrusion temperature on the microstructure and mechanical properties of AA7075 matrix hybrid composite prepared by semi-solid casting

The improvement level of mechanical properties of aluminum matrix composites reinforced to carbonaceous materials is lower than expected. The intrinsic characteristics and production methods are effective parameters on microstructure and efficiency of these composites. In this study, AA7075/CNTs+GNPs hybrid composites were fabricated by flake powder metallurgy process combined to solution method, semi-solid casting and high temperature extrusion. After determination of optimum VGNP/VCNT ratio in hybrid carbonaceous reinforcement (= 0.167), its effect on microstructure and mechanical properties of AA7075 alloy was investigated. The semi-solid casting process led to destroy the network structure of carbonaceous reinforcements and to entrap them mechanically into the aluminum matrix and to segregate alloy elements on their surface. In optimum temperature of extrusion process (= 400 °C), the uniform distribution of carbonaceous reinforcement and the decrease in the average grain size (~39%) were obtained. Hardness, tensile and compressive strengths of the hybrid composite were improved by 17%, 51% and 28% compared to AA7075 matrix alloy. The extraordinary efficiency of tensile strength (~3952%) was obtained in the hybrid composite as a result of effective load transfer due to effect of the pulling out and the bridging of carbonaceous reinforcements.