Effect of tool rotation rate on microstructure and mechanical properties of friction stir welded copper

Defect free copper welds were achieved by friction stir welding (FSW) carried out at a constant welding speed of 100 mm min(-1). The influence of tool rotation rate on microstructure, mechanical properties and fracture location was investigated. As the tool rotation rate increased, the grains of nugget zone grew significantly, the thermomechanically affected zone became indistinct and the grain size increased, but the effect of tool rotation rate on the grain size of heat affected zone was limited. Both ultimate tensile strength (UTS) and elongation increased first and then decreased with increasing rotation rate and the UTS achieved a highest value of 282 MPa at the rotation rate of 400 rev min(-1) together with the welding speed of 100 mm min(-1), which was on the level of the base metal. The fracture occurred at the cavity defect on the advancing side of the joint when the FSW was performed at a low tool rotation rate, while it occurred on the retreating side when the tool rotation rate was relatively high.