A. Clifton, Lockheed Martin Space Systems Company, New Orleans, LA
A friction stir weld (FSW) repair method has been developed and successfully implemented on Al 2195 plate material for the Space Shuttle External Fuel Tank (ET). The method includes restarting the friction stir weld in the termination hole of the original weld followed by two reweld passes. Room temperature and cryogenic temperature mechanical properties exceeded minimum FSW design strength and compared well with the development data. Simulated service test results also compared closely to historical data and were in family with legacy FSW data for normal FSW, confirming no change to the critical flaw size for the repaired welds or inspection requirements. Testing of FSW/VPPA fusion intersection weld specimens exhibited acceptable strength and exceeded the minimum design strength. Porosity, when present at the intersection was on the toe of the fusion weld, the “worst case” being 0.700 inch long. These “worst case” porosity conditions were tested “as is” and demonstrated that porosity did not negatively affect the strength of the intersections. Larger, 15-inch “wide panels” were tested to demonstrate strength and evaluate residual stress using photo stress analysis. All results exceeded design minimums, and photo stress analysis showed no stress gradients due to the presence of a restart and multiple rewelds in the wide panels.
Summary: A friction stir weld repair method that yields mechanical properties comparable to initial welds has been developed and implemented on Al 2195 plate material. The method includes restarting the weld in the termination hole followed by two reweld passes. Room temperature and cryogenic temperature mechanical properties exceeded initial weld design strength. A-basis repair allowables were slightly lower than initial weld A-basis allowables. Simulated service test data confirmed that no change to the critical flaw size or inspection requirements were necessary. In addition, repair friction stir weld to VPPA fusion weld intersections showed mechanical properties similar to initial friction stir weld to VPPA fusion weld intersections. Worst-case porosity (up to 0.700 inch) at the fusion weld toe did not affect the intersection strength.
