New technique of self-refilling friction stir welding to repair keyhole

A new technique of self-refilling friction stir welding (SRFSW) relying on non-consumable joining tool has been developed to repair the keyhole left at the end of 316L stainless steel friction stir welding/friction stir processing (FSW/FSP) seam. The conventional FSW process was transformed by adopting a series of non-consumable tools with gradual change in geometry to create a solid state refilled joint step by step. Using the combined plastic deformation and flow of the material around the keyhole, the SRFSW process is able to repair the keyhole with both metallurgical and mechanical bonding characteristics, and the FSW/FSP seam can be achieved without keyhole or other obvious macro defects. Microstructural observation results showed that the grains in the refilled zone were significantly refined by the tool. Tensile test results showed the refilled joint fractured at the base metal side, and the relative tensile strength and elongation are 112 and 82% of the base metal respectively. Moreover, no sigma phase but few Cr carbides were found in the refilled zone, which would not result in obvious corrosion resistance degradation of 316L stainless steel.