Abstract: 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 heataffected 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.