Effect of Surface Laser Shock on Fatigue Crack Propagation Rate of Bimodal Structure Ti55531 Titanium Alloy

The fatigue crack propagation rate (da/dN) of bimodal structure Ti55531 titanium alloy before and after laser shock peening(LSP)was investigated. The fracture, microstructure and residual stress of fatigue crack propagation samples were analyzed. The results show that after LSP, the fatigue crack propagation rate decreases. When Delta K < 22.84 MPa center dot root m, the LSPed sample (BM-LSP) has a lower fatigue crack propagation rate than the unLSPed sample (BM). When Delta K=22.84 MPa root m, the crack propagation rates of the two samples are similar, which are 3.92x10(-4) mm/cycle. After LSP, the thickness dispersion and length dispersion of the secondary alpha layer are decreased by 38.9% and 22.9%, and the polar densities of alpha and beta phases are decreased by 37% and 16%, respectively. The passivity of tip of the lamellar alpha and microstructure homogenization alleviate the stress concentration, resulting in a decrease in da/dN. In addition, the LSP process introduces a residual compressive stress layer with a depth of about 900 mu m on the surface of the material. Residual compressive stress is also an important factor to offset tensile stress at crack tip, to enhance crack closure and to slow down crack propagation.