Refining Mechanism of 7075 Al Alloy by In-Situ TiB2 Particles

The nucleation undercooling of TiB2/7075 Al matrix composites, the microstructure observed after solidification at different cooling rate, and the size and distribution of TiB2 particles were investigated. The experimental results have shown that the grain sizes of TiB2/7075 Al matrix composites firstly decreased, then increased, and finally decreased again with the increase of TiB2 content. The nucleation undercooling of TiB2/7075 Al matrix composites first increased, then decreased, and finally increased again with the increase of TiB2 content when the cooling rates was 5 and 10 °C/min respectively, but kept decreasing with the increase of TiB2 content at a cooling rate of 20 °C/min. The melting and solidification process showed no significant change with the decrease of cooling rate in 9.0% TiB2/7075 Al matrix composites. Most small particles can act as heterogeneous nucleus, which induced grain growth and were captured into the grain by the solid/liquid interface. At the same time, most of the larger particles and a minority of the small TiB2 particles are pushed into the grain boundary; locating in the grain boundary can hinder the Al atoms from diffusing during the solidification process and restrain α-Al phase growth. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB2 particles increased