Effect of TiB₂ Particles on Microstructure of 7075 Al Alloy in Semi-Solid State Near Liquidus

Abstract

TiB₂/7075 Al matrix composites were formed by in situ reaction, and effect of the TiB₂ particles content on microstructure of 7075 Al alloy in semi-solid state near liquidus were investigated. The results have shown that the average grain size decreased with increased the amount of TiB₂, and the microstructure became uniform rosette grains. The morphology of grains became big dendrite grains when the particles content were over 4.5 mass%. Globular grains of 7075 Al alloy whose size reached 94 µm can be got for 7.5 min at 630°C. The minimum grain size of 3 mass%TiB₂/7075, 4.5 mass%TiB₂/7075 and 9 mass%TiB₂/7075 Al matrix composites at 630°C were 88 µm, 51 µm and 69 µm for 30 min, 20 min and 50 min, respectively. The TiB₂ particles located in the grain boundary can hinder the Al atoms from diffusing within the semi-solid state of the 7075 Al alloy and restrain α-Al phase growth, but it can be conducive to improve the stability of semi-solid slurry in near-liquidus thixoforming.