Refilling termination hole in AA 2198–T851 by refill friction stir spot welding

Abstract

Friction stir welding features a joining technology with only few drawbacks. The exit hole inevitably left by the tool is a major disadvantage. In order to eliminate volume defects such as termination holes or voids in friction stir welds, refill friction stir spot welding was used for keyhole repair. The keyhole closure process was employed to 3 mm thick sheets of aluminum-lithium alloy AA 2198-T851. Exit holes with a diameter of 7.5 mm in semi–stationary shoulder bobbin tool friction stir welds were refilled successfully. To characterize the refilling process characteristic process parameters such as position, force and torque of the tool parts have been monitored. The welds have been analyzed with regards to microstructural features, the local and global mechanical performance as well as the thermal cycle. The properties of the repair welds have been compared to friction stir welded butt joints and keyhole closure welds in bare sheet. A systematic investigation comparing and combining the two friction based processes has shown that refill friction stir spot welding is able to be applied as keyhole closure technique in sheet material and in semi–stationary shoulder bobbin tool friction stir welds. Defect free welds with known microstructural features were achieved. The mechanical performance of keyhole closure welds using refill friction stir spot welding in semi–stationary shoulder bobbin tool friction stir welds is comparable to the performance of keyhole closure welds in bare sheets. High-strength keyhole closure welds with efficiencies of 78 % in terms of ultimate tensile strength were achieved.