Enhanced high cycle fatigue resistance of Ti-17 titanium alloy after multiple laser peening without coating

High cycle fatigue failure of titanium alloy components on the aircraft is a serious problem that affects the flight safety. The low energy laser peening without coating (LPwC) has been demonstrated for the improved fatigue resistance of metallic materials via the introduction of compressive residual stress and microstructure modifications. Therefore, in the present study, LPwC with different impacts were conducted on the Ti-17 titanium alloy using the Mianna-Q laser with the wavelength of 532 nm and energy of 85 mJ. The surface and depth-wise residual stress and micro-hardness distributions were presented on the samples with and without LPwC treatment. High amplitude compressive residual stress was introduced, and the hardness was significantly improved. The microstructures of the Ti-17 samples after LPwC was characterized using the X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, high cycle fatigue performance of the as-cast, LPwCed sample with 1 impact and LPwCed sample with 3 impacts was evaluated via the tension-compression fatigue test. The fatigue strength of the LPwCed specimen was increased from 390 to 475.4 MPa (1 impact) and 490.3 MPa (3 impacts). Then, the fracture morphology of all the specimens were characterized by the SEM. Finally, the strengthening mechanism was discussed based on the microstructural evolution and residual stress distribution. It was concluded that the enhancement of high cycle fatigue strength was attributed to the combined effects of LPwC-induced compressive residual stress and high-density dislocations.