Effect of laser shock peening on the microstructural characterization in weld nugget zone of friction stir welded 7050 aluminum alloys

Laser shock peening (LSP) has been used to strengthen the surface of friction stir welding (FSW) joint for aluminum alloys. For FSW joints, the weld nugget zone (WNZ) shows the obvious ultra-fine grain structure, and the microstructure and properties in the WNZ have attracted more and more attention. In this paper, the effect of laser shock peening on microstructural properties in the surface of weld nugget zone of FSW joints of 7050-T7451 alloys was performed. The strengthening phase θ (CuAl2) was distributed in grain or grain boundary in the shock region of WNZ. In WNZ, the hardness at a distance from the upper surface of 1.0–3.0 mm had obvious changes, which shows a stable state. The average hardness is about 140 HV. The residual stress in WNZ generated a relatively high compressive stress after LSP, and the biggest compressive residual stress 56.5 MPa is located in the 0.20 mm depth. The WNZ generated the tensile residual stress region before LSP, and the biggest tensile residual stress is 27.8 MPa.