Significant improvement in tensile shear properties of friction stir lap welded nano TiB₂/2024 Al composite joints via a novel tool pin geometry design

A novel designed tool pin with two shoulders and pins (stepped pin, S-pin) was performed to fabricate the friction stir lap welded nano-TiB₂/2024 Al composite joints for the interface migration optimization and tensile shear strength improvement purposes. Compared with the conventional tapered pin (T-pin) joints, broken and dispersedly distributed hook, significantly restrained cold lap were observed in the S-pin joints. Submicron or hundreds of nanometers grains were detected in the stir zone (SZ), which should be attributed to the existence of nano-TiB₂ hindering the movement of grain boundaries and limiting the growth of recrystallized grains. A wedge-shaped microhardness distributions of the S-pin joints were found, which would be better for the joint bearing capacity. Tensile shear loads of the S-pin joints were much higher than those of the T-pin joints. The optimal average tensile shear strength, 7.3 kN or 484 N/mm, were obtained when the S-pin was performed, dominantly fracturing in the heat affected zone (HAZ) rather than the SZ.