Enhanced mechanical properties of 70/30 brass joint by multi-pass friction stir welding with rapid cooling

70/30 (Cu/Zn) brass plates with a 2 mm thickness were jointed by repeated rapid cooling friction stir welding. The joints from each FSW cycle showed the typical construction of a microstructure which includes the stir zone and the thermo-mechanically affected zone, but the morphology and boundary characteristics in these zones changed with the different cycles. In the stir zone, the grain size decreased and the number fraction of the high angle boundaries increased with the increasing number of FSW cycles. The texture analysis suggested that the post-annealing effect, which frequently occurred after the FSW process, was remarkably restricted by the liquid CO₂ cooling, which accelerated the refinement of the microstructure. As a result, a joint with an ultrafine grained structure (0·8 μm) and an excellent strength ductility matching (548 MPa and 34% respectively) can be achieved by multi-pass rapid cooling FSW process.