Enhanced mechanical properties of 70/30 brass joint by rapid cooling friction stir welding

The effect of rapid cooling on the microstructure, texture and mechanical properties of friction stir welded 70/30 brass was investigated. Liquid CO2 was used as the cooling medium during the process. Without the rapid cooling, the welded joint showed many micropores and Zn/Cu oxides in the stir zone accompanied by a relatively wide heat-affected zone. The stir zone showed relatively coarse grains with a mean diameter of 9.7 μm and these grains had low dislocation density. The crystallographic texture whose dominant component of {1 0 0} (// shear plane) 〈0 0 1〉(// shear direction) was also found. On the other hand, when the rapid cooling was carried out during the welding process, a sound joint with no obvious heat-affected zone was achieved. The stir zone showed the fine-grained structure whose mean grain size is 1.2 μm with high dislocation density. The texture in the stir zone was completely different from that in the joint without the rapid cooling. These differences between the joints with and without the rapid cooling indicate the significant post-annealing effect, including both recovery and static recrystallization can be prohibited by the rapid cooling. As a result, enhanced yield strength with adequate ductility of the stir zone was obtained.