Methods to Increase the Fatigue Life of Friction Stir Lap Welds in No-load Transfer Coupons Using a Retractable Pin Tool

The purpose of this study was to obtain a better understanding of crack initiation at friction stir weld exit holes in no-load transfer coupons as a first-order approximation of the potential performance of friction stir welding (FSW) joints employed for aircraft fuselage applications. In order to meet this goal, the effects of weld exit location on fatigue life in discontinuous friction stir lap-welded panels were documented. Sheets of aluminum alloys 7075-T6 and 2024-T3 were chosen for the FSW lap welds. Constant amplitude fatigue tests were carried out in order to determine the effect of various welding scenarios on fatigue crack initiation. A number of weld exit strategies were investigated, as well as another FSW tool design called a retractable pin tool (RPT). This tool was shown to eliminate the exit hole and reduce the stress concentration around the weld exit location. Prior to this work, none of the attempted exit strategies was found to delay or eliminate the initiation of fatigue cracks at the exit holes of discontinuous FSW lap welds. Only by using the RPT, which is able to eliminate the exit hole completely, was it possible to produce fatigue life results in coupons with a tool extraction zone that were comparable to those in baseline coupons containing no extraction zone.