Investigation of the welding parameter dependent microstructure and mechanical properties of friction stir welded pure copper

▶ The commercial purity copper plates were FSW processed within wide welding parameter including the changes of welding speed, rotation speed and the applied load. The process window for FSW of copper was first obtained. ▶ The microstructure evolutions in the stir zone and across the cross-section perpendicular to the welding direction were characterized in detail. The average grain size in the stir zone can be refined to about 3.8 μm by increase of the applied load to 1500 kg. The increase of applied load reveals much larger influence on the microstructure refinement than the reduction of rotation speed. ▶ The increase of the applied load to 1500 kg results in the hardness increase to about 105 HV and the samples finally fractured in the base metal during tensile tests. Some special hardness distribution was investigated and analyzed such as the extremely high hardness value in TMAZ. The process window for friction stir welding of commercially pure copper was obtained, which included a welding speed ranged from 200 to 800 mm/min, a rotation speed ranged from 400 to 1200 rpm and an applied load ranged from 1000 to 1500 kg. In the stir zone, a remarkably refined microstructure with average grain size of 3.8 μm can be obtained by increasing the applied load to 1500 kg. In addition, higher applied load can promote the formation of dislocation cells, while annealing twins and dislocation entanglements are easy to form under lower applied load. The mechanical properties of the joints can be improved further by increasing the applied load, rather than only decreasing the rotation speed at lower applied load. The mechanism of the mechanical property changes in the copper joints were put forward and clarified from the viewpoint of microstructural evolution.