Microstructure and mechanical properties of the semisolid slurries and rheoformed component of nano-sized SiC/7075 aluminum matrix composite prepared by ultrasonic-assisted semisolid stirring

摘要

An innovative method so-called ultrasonic-assisted semisolid stirring was proposed to fabricate semi-solid slurries of the nano-sized SiC/7075 aluminum matrix composite. In this method, ultrasonic treatment and semisolid stirring were combined together to disperse the nano-sized SiC and break the primary dendrites of the matrix. The microstructure and mechanical properties of the semisolid slurries and rheoformed cylinder component were investigated. The results show that ultrasonic treatment dispersed the nano-sized SiC particles well due to the effect of transient cavitation and acoustic streaming. With increase of the stirring time the amount of the spheroidal grains of semisolid slurries increases, indicating achieving a better semisolid slurry. When stirring temperature is 620 ℃, the 20 min stirring time led to a desirable semisolid microstructure consisting of a large number of spheroidal grains surrounded by liquid phase. High-quality semisolid slurries were only achieved at the stirring temperatures of 615 ℃ and 620 ℃ upon the 20 min stirring time. Liquid fraction in the regions near to lateral surface is larger than those in the top, bottom and central regions due to main dependence on liquid flow and flow of liquid incorporating solid grains. Yield strength and ultimate tensile strength of rheoformed cylinder components of the nano-sized SiC/7075 aluminum matrix composite increased as compared with those of the 7075 matrix. Yield strength, ultimate tensile strength and elongation of the rheoformed cylinder components of the nano-sized SiC/7075 aluminum matrix composite without T6 are 264 MPa, 357 MPa and elongation of 7.5%, respectively. T6 heat treatment led to a significant improvement of yield strength, ultimate tensile strength and elongation. The mechanical properties of the rheoformed cylinder components with T6 of the nano-sized SiC/7075 aluminum matrix composite involve yield strength of 381 MPa, ultimate tensile strength of 478 MPa and elongation of 8.5%.