Effect of TiB2 on microstructure of 7075 Al alloy semi-solid slurry at different solid fraction

Abstract

TiB2/7075 Al matrix composites were fabricated by in situ reaction, and the effect of TiB2 particle content on the microstructure of 7075 Al alloy slurry in semi-solid state at different solid fraction were investigated. The results showed that the grain sizes of TiB2/7075 Al matrix composites firstly decreased then increased and finally decreased again with the increase of TiB2 content because the role of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB2 particles increased due to the decrease of the mean diameter of TiB2 particles. As the holding time increasing in the semisolid state, dendritic grains were necked and remelted at first, then rosette grains melted and shrank into the globular grain, finally the globular grains began to grow, but the number of grains decreased and their mean grain size increased. The minimum grain size of 3.0, 4.5 and 9.0%TiB2/7075 Al matrix composites slurry were 81 μm, 51 μm and 69 μm after holding for 35, 20 and 50 mins at 630°C, respectively. The minimum grain size of 3.0 and 4.5%TiB2/7075 Al matrix composites slurry were 100 μm and 75 μm after holding for 120 and 90 mins at 620°C, respectively. However, the maximum size of globular grains was not associated with the TiB2 particle content, the maximum grain sizes of 3.0%TiB2/7075 and 4.5%TiB2/7075 Al matrix composites slurry at 620°C were approximately 128 μm.