Residual stress measurements in a thick, dissimilar aluminum alloy friction stir weld

Plates (25.4 mm thick) of aluminum alloys 7050-T7451 and 2024-T351 were joined in a butt joint by friction stir welding (FSW). A 54 mm long test specimen was removed from the parent plate, and cross-sectional maps of residual stresses were measured using neutron diffraction and the contour method. The stresses in the test specimen peaked at only about 32 MPa and had the conventional “M” profile with tensile stress peaks in the heat-affected zone outside the weld. The asymmetric stress distribution is discussed relative to the FSW process and the regions of highest thermal gradients. The general agreement between the two measurement techniques validated the ability of each technique to measure the low-magnitude stresses, less than 0.05% of the elastic modulus. Subtle differences between the two were attributed to spatial variations in the unstressed lattice spacing (d0) and also intergranular strains affecting the neutron results. The FSW stresses prior to relaxation from removal of the test specimen were estimated to have been about 43 MPa, demonstrating the ability of FSW to produce low-stress welds in even fairly thick sections. To avoid the estimated 25% stress relaxation from removing the test specimen, the specimen would have had to be quite long because the St. Venant’s characteristic distance in this case was more related to the transverse dimensions of the specimen than to the plate thickness.