Part fit-up variations are critical to the joint quality produced by joining operations like friction stir welding (FSW). Typical fit-up issues may include: gaps, misalignment between the weld path and joint line, setup related variations, voids or holes in the weld path, mismatch between materials being joined, etc. These and other factors potentially result in variations in mechanical properties between the start and finish of the weld as well as lead to the possible formation of defects along the joint line. The purpose of this study was to determine the robustness of the FSW process to selected imperfections in joint fit-up between plates in butt-welding. Specifically, it includes a study of the effect of part fit-up variations on the transverse joint tensile strength of 0.125" thick friction stir butt-welded 7075-T73 aluminum plate. Welds were performed on an MTS® ISTIR™ PDS friction stir weld machine at the National Institute for Aviation Research in Wichita, KS. It was found that the FSW process was capable of creating a sound defect-free joint for a range of typical fit-up discontinuities. The tolerance zone for weld path misalignment to the joint line was related to the formation of fine nugget grain structure at the bottom of the weld, with twothirds of the tolerance zone located on the advancing side of the pin. It was also determined that a certain amount of plate gap or voids in the weld path can be tolerated without a decrease in tensile strength; beyond that point, volumetric defects form in the nugget. Otherwise, there is little variation in tensile properties along the weld joint when care is taken to ensure gap and alignment tolerances are not exceeded.
Nomenclature σ= engineering stress HAZ = Heat-Affected Zone TMAZ = Thermo-mechanically Affected Zone FSW = Friction Stir Welding