Effect of rolling on friction stir welded joints of aluminum alloy

The upper and back surface of a friction stir welded joint of 5 mm thick 7050 aluminum alloy were rolled by a self-made rolling head. It was compared with the welded joints without rolling. The effect of rolling on the performance of welded joints was studied. The results showed that the roughness of joint surface was reduced from 9.58 μm (maximum value) to 0.85 μm (mean value). The surface layer happened severe plastic deformation, and the grains were remarkably refined to form a fine grain layer of about 200 μm thick. The grain refinement of the subsurface layer was reduced, and a shear band was generated in the weld nugget zone. The hardness of the joint surface was obviously improved, and the average hardness was as high as HV210, which was 91% higher than the hardness HV110 before rolling. The residual stress of the joint surface changed from the original tensile stress to the compressive stress, and the maximum residual compressive stress field depth was about 200 μm. The source was moved from the surface layer to the subsurface layer, and the fatigue life was significantly improved.