The microstructure and mechanical properties of friction stir welded Cu–30Zn brass alloys

The optimized processing map for the friction stir welding of commercial 2 mm-thick single phase Cu–30Zn brass alloy plates was obtained, which included a welding speed that ranged from 200 to 800 mm/min, a rotation speed that ranged from 750 to 1200 rpm and a constant applied load of 1000 kg. A remarkably refined microstructure was formed in the stir zone. The average grain size ranged from 6.4 to 9.6 μm depending on the different welding conditions, which is much smaller than the 28.2 μm of the base metal. Compared with the base metal, the dislocation cells, annealing twins with a smaller twin lamellar width, are easy to form in the stir zone. All the sound welds fractured on the base metal during the tensile tests, indicative of improved mechanical properties in the stir zone. Miniature tensile tests of the stir zone revealed that the highest tensile strength of the stir zone can reach about 380 MPa, much higher than the 340 MPa of the base metal. The mechanism for the mechanical property improvement in the brass alloy joints are discribed and clarified from the viewpoint of microstructural evolution.