Elucidation of interface joining mechanism during friction stir welding through Cu/Cu-10Zn interfacial observations

Abstract A butt friction stir welding (FSW) process was performed on Cu and Cu-10Zn plates. Microstructural evolution along the Cu/Cu-10Zn interface retained in the FSW joint was systematically investigated in order to clarify the interface joining mechanism during the FSW. The initial joining interface is completely unbonded and has lots of large oxides distributed on it. The initial large oxides are then fragmented into small particles and the oxide layers initially formed on the interface are ruptured, which produces fresh surfaces on both the Cu and Cu-10Zn sides. The fresh surfaces are then joined at an atomic level under a compressive force perpendicular to the interface, while the small oxide particles are still dispersed on the interface with voids formed around the particles. A strain-induced grain boundary migration driven by the stored energy difference associated with the dislocation density difference will occur, which assists to extinguish the voids retained beside the oxide particles. Finally, a sound interface joining without oxide particles and large voids remaining on the interface is obtained during the FSW.