Filling friction stir welding for repairing keyhole based on principle of solid state joining

In this paper, a technique of filling friction stir welding (FFSW) for repairing the keyhole was introduced. A semi-consumable tool composed of a tool steel shoulder and a bit was designed. The keyhole was repaired by plastic deformation and flow of the bit during welding. The microstructure of the FFSW zone was classified into four different zones: filled zone, quasi-nugget zone, thermo-mechanically affected zone, and heat-affected zone. The results showed that the ultimate tensile strength and elongation of the FFSW joint were both up to 90% of the original FSW joint. The FFSW process can repair the keyhole and other defects with quasi-equivalence mechanical performances, and FSW seams without keyhole could be achieved.