Effect of Ultrasonic Impact on Fatigue Crack Growth Rate of Titanium Alloy Welding Joints

In this study, the effect of ultrasonic impact treatment (UIT) on the fatigue crack growth (FCG) rate of titanium alloy laser-welded joints was investigated to determine the FCG. Accordingly, the surface roughness, surface topography, microstructure, microhardness, and residual stress of specimens treated with two distinct UIT amplitudes (18 and 25 mu m) were investigated as a result. A scanning electron microscope was used to examine the specimens' fatigue fracture. The surface roughness of the UIT-treated specimens was 4.3 times that of the untreated specimens, but the surface scratches were eliminated. Surface grains were refined to form an approximately 100-mu m-thick plastic deformation layer, and compressive residual stress (- 506.4 MPa) was introduced to the surface layer of the specimens. Compared with the untreated specimens, the fatigue life and hardness of UIT-treated specimens increased by 60.6 and 11.5%, respectively, and the hardness influence depth was similar to 0.07 mm. The fracture morphology revealed narrow and thick fatigue cracks, indicating that the FCG rate considerably reduced. Finally, the crack growth process was thoroughly investigated, and the FCG rate and trend were predicted.