Effect of asynchronized and synchronized laser shock peening on microstructure and mechanical properties of the Ti–6Al–4V laser joint

Welding processing is an important method to connect precision component, but the internal stress and crystal anisotropy reduce its performance, which are difficult to solve due to the specific demand on regional post-treatment. In the present research, the asynchronized laser shock peening (ALSP) and synchronized laser shock peening (SLSP) processing technologies were applied to treat the laser joined Ti-6Al-4V thin plates. The microstructure and mechanical properties of the as-welded joint, ALSP and SLSP processed joint were investigated. The results revealed that the as-welded joint presents obvious anisotropic characteristics, and high strain level exists on the prior β grain boundary, which causes stress concentration. The ultimate tensile strength (UTS) and elongation (EL) values of as-welded joint just reach the 92.18% and 72.33% of as-received plate, respectively. ALSP processing could result in the deformation with the depth more than ten microns and introduce compress stress in the top layer, but has little effect on decreasing the anisotropy. The grain boundary becomes obscure and the strain distribution is relatively uniform, accompanying with some dynamic recrystallized laths exist along the grain boundary. Its values of UTS and EL are 5.23% and 5.59% higher than as-welded joint, respectively. With the SLSP processing, the deformation depth exceeds twenty microns, and the crystal isotropy could be greatly promoted. Moreover, higher compressive stress is introduced from top layer to center area of joint. The grain boundary becomes obviously obscure, which is completely replaced by recrystallized lath structures. The residual strain in the top layer is partly eliminated, and the stress concentration phenomenon along grain boundary is dramatically decreased. The UTS and EL values are 6.16% and 20.58% higher than as-welded joint, and reach 97.86% and 87.22% of as-received plate, respectively.