Fabrication of Al7075–Al_2O_3np-based metal matrix composites with a high solid fraction for thixoforming

For thixoforming, it is necessary to have a good microstructure of fine, uniform, and globular grains in a semisolid range. In this study, the nano-Al_2O_3(Al_2O_3np)/Al7075 composites with a high solid fraction were fabricated by specially made Al_2O_3np containing Mg powders and semisolid ultrasonic vibration (SSUV) process. The influence of SSUV technology on primary α-Al grain and second phase in the composites was investigated. Microstructural studies revealed that a good semisolid slurry with an average grain size of 73 μm, a shape factor of 0.84, and a solid fraction of 0.715 could be obtained. Also, it could be shown that SSUV affected largely the size and type of the second phase as well as growth and nucleation of the primary α-Al grain. TEM analysis revealed that there are well-defined crystallographic orientation relationships between the second phases and α-Al, suggesting enhanced heterogeneous nucleation in Al7075 alloys. Moreover, mechanisms involved in the development of microstructure were discussed.

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