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-Al2O3(Al2O3np)/Al7075 composites with a high solid fraction were fabricated by specially made Al2O3np 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.

A novel rheo-rolling process for producing A356 alloy sheet was developed. The influenceof parameters on the microstructure of A356 alloy sheets was investigated. The eutecticfraction and primary α globule size increase with the increment in theroll speed. The globule size and eutectic fraction of A356 alloy sheet increase with theincrement in the casting temperature. The primary α globule sizedecreases and the solid fraction increases with the increment in vibration frequency from30 Hz to 80 Hz. With a roll speed of 0.069 m.s-1, a casting temperature of 660°C, and a vibration frequency of 80 Hz, A356 alloy sheet with a cross-sectionsize of 4 × 160 mm was prepared. The microstructure is mainly composed of fine sphericalor rosette α globules, and the average α globule size is20.5 μm. The ultimate tensile strength of the A356 alloy sheet was 260MPa, and the elongation to failure was 7.9%, which were 2.5 and 3.7 times greater thanthose of the as-cast material.