Effect of rotation speed on the microstructure and mechanical properties of dissimilar friction stir-welded copper/brass metals

Dissimilar copper and brass metals were friction stir welded at a constant welding speed of 80 mm/min. Groove defects were found on the weld surface with a rotation speed of 1000 rpm, and defect-free welds were successfully obtained with rotation speeds ranging from 1200 to 1600 rpm. The base material of copper consisted of an equiaxed α phase, while that of brass was characterized by a coarse α phase containing certain twins and a β′ phase. Onion rings were developed due to the plastic flow of metal induced by mechanical mixing in the stir zone during friction stir welding. The grain size in the heat-affected zone was refined compared with that in the base material at the brass side of the welded joint, while the situation was completely the opposite for the copper side. With increasing rotation speed, grain size in all zones increased. The weld exhibited higher hardness than the base material. There is little change in tensile properties of all the defect-free welded joints with increasing rotation speed, and all the defect-free welded joints were fractured in the lowest hardness region of the welded joint during tensile tests.