Friction Stir Welding of Aluminum 7136-T76511 Extrusions for Aerospace Applications

This research program evaluates the residual properties of 7136-T76511 aluminum extrusions joined through friction stir welding (FSW). AA 7136 is a new aluminum alloy developed by Universal Alloy Corporation for high strength aerospace applications that also demand good corrosion resistance, such as those on the Boeing 787 or the Airbus A380. Mechanical and corrosion testing were performed on the baseline material and on panels friction stir welded at 175, 225, 250, 300, 350 and 400 RPM (all other welding parameters were held constant). Mechanical test results demonstrate that the highest joint efficiency, 74%, is achieved at 350 RPM, but for each weld condition, the elongation of the welded material is significantly reduced, 50 – 75%, from the baseline value. Fracture of the tensile specimens consistently occurred on the retreating side of the weld along the interface between the heat affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ), independent of the rotational speed. Examination of fracture surfaces through SEM revealed microvoid nucleation and coalescence around secondary phase particles in the microstructure, as well as numerous stepped or laminar facets characteristic to both the baseline and welded conditions. Exfoliation corrosion testing revealed a performance gradient across the weld with the weld nugget rating the poorest at EC and the heat affected zone rating the best at EA. Qualitative assessment of corrosion resistance is supported by mass loss calculations between the baseline and welded conditions.