Refill Friction Stir Spot Welding as a Keyhole Closure Technique in BT-FSW Welds of Aluminum-Lithium Alloy 2198-T851

Aluminum lithium alloys are difficult to weld by conventional fusion welding methods. The solid state welding process stationary shoulder bobbin tool friction stir welding (SS_UBT-FSW) was recently used to weld aluminum lithium alloys and showed superior mechanical performance. The BT-FSW process variant is particularly suitable for closed structures such as pipes or hollow structures and therefore very interesting for aerospace applications. A major drawback of most FSW processes is the remaining exit hole at the end of the weld resulting from the extraction of the welding tool.

Recently refill friction stir spot welding (RFSSW) was applied to seal through holes in metals to solve problems such as damage to holes, broken tools or exit holes left at the end of FSW processes. This method shows advantages including defect free welds, superior surface appearance on both sides of the weld and a high reproducibility. Additionally, this method does not need any surface preparation prior to or after the welding process and seals the keyhole in a single welding step.

In this work, a solution for closure of termination holes and flaw repairs in SS_UBT-FSW is presented using RFSSW. To evaluate the performance of RFSSW in welded structures, holes were drilled in AA 2195 SS_UBT-FSW butt welds and subsequently closed by RFSSW.

A systematic investigation comparing and combining both processes has shown that refill friction stir spot welding is able to be applied as a keyhole closure technique in sheet material as well as in semi–stationary shoulder bobbin tool friction stir welds. The results show defect free welds with known microstructural features and a mechanical performance comparable to the initial weld seam without termination hole.

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