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.

In-situ TiB₂ particle reinforced 7075 Al alloy was produced by molten salt method (LSM). The effects of TiB₂ content on the microstructure, the melting behavior and the viscosity of 7075 Al alloy were investigated. The experimental results have shown that the microstructure became uniform rosette grains and the grain size decreased with increasing TiB₂ particles. The morphology of grains became bigger with dendritic structure when the TiB₂ particles content is more than 4.5%_. The liquidus and solidus temperatures of the TiB₂/7075 composites were firstly increased and then decreased with the increase of the content of TiB₂ particles. The semisolid processing temperature window of 7075 Al alloy became narrow after adding TiB₂ particles, and the liquid fraction of TiB₂/7075 composites with increasing temperature was more sensitive than 7075 Al alloy. The apparent viscosity of the 4.5%TiB₂/7075 composite was the lowest among 3.0, 4.5 and 9.0%TiB₂/7075 aluminum matrix composites respectively. The apparent viscosity of the 9.0%TiB₂/7075 was less than that of 7075 alloy but was greater than that of 4.5%TiB₂/7075 composite under the same conditions. The viscosities of the composites decreased sharply with increasing shear rate, and the change with the shear rate were more sensitive than that of the 7075 alloy.

In this study, TiB₂/7075 aluminum matrix composites were prepared via in situ synthesis. It was found that TiB₂ particles are mainly quadrate. Large TiB₂ particles (1–2 μm) agglomerate at grain boundaries, but most of the particles are on the submicron scale. Adding 4.5 wt.% TiB₂ particles effectively optimizes α-Al grains in the 7075 aluminum alloy. By combining in situ reinforcing particles with the self-stirring effect of a serpentuator, rheocasting of the 7075 aluminum alloy was achieved in a simple and economical way. The average grain size of the specimen after rheocasting and heat treatment was smaller than 33 μm, and the shape factors were greater than 0.85. The wear resistance of the 4.5 wt.% TiB₂/7075 aluminum matrix composite that was prepared via rheocasting and gravity casting was tested with loads of 30, 60, 90, and 120 N at a friction speed of 0.15 m/s for a duration of 30 min. Because of the optimized microstructure and increased hardness, the wear resistance of the 4.5 wt.% TiB₂/7075 aluminum matrix composite was significantly better than that of the 7075 aluminum alloy, and the wear resistance of the rheocast TiB₂/7075 aluminum matrix composite was better than that of the gravity cast one.

TiB 2 /7075 Al matrix composites were fabricated by in situ reaction, and the effect of TiB 2 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 TiB 2 /7075 Al matrix composites firstly decreased then increased and finally decreased again with the increase of TiB 2 content because the role of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB 2 particles increased due to the decrease of the mean diameter of TiB 2 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%TiB 2 /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%TiB 2 /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 TiB 2 particle content, the maximum grain sizes of 3.0%TiB 2 /7075 and 4.5%TiB 2 /7075 Al matrix composites slurry at 620 C were approximately 128 μm.

Sn37Pb solder bumps with Ø0.6 mm ball were used to conduct thermal shock test, microstructure and property of solder bumps were investigated. The experimental results have shown that the shear strength of solder bumps was 62.89 MPa after reflow, then dropped to between 47.31 MPa and 50.86 MPa with increasing of thermal shock cycles, but finally reached to 61.96 MPa at 2000 cycles again. Interfacial IMCs were typical scallop-type and loose in the solder bumps, but were serrated and become smoother and compact after thermal shock with 1500 cycles. The IMCs thickness of solder bumps was about 1.80∼3.05 µm, but the composition of IMCs was Cu₆Sn₅ whether thermal shock or not. 1/25 of solder bumps at 200∼1000 cycles, 2/25 of solder bumps at 1500 cycles and 3/25 of solder bumps at 2000 cycles were failure respectively. Obvious tearing crack and fiber tissue was observed in the solder bumps of as-received, and then became large after thermal shock, but dimples of solder bumps deepened after thermal shock at 2000 cycles.