Experimental investigation on surface integrity and fatigue performance of Ti60 alloy under ultrasonic impact treatment

The surface integrity and fatigue properties of Ti60 alloy under different ultrasonic impact processes were investigated. Ultrasonic impact treatment (UIT) with various parameters enhanced the surface integrity of the turned specimen. The R a values of the UIT specimens were all less than 0.380 mu m, with the surface stress concentration factor ( K st ) values ranging from 1.075 to 1.120. The compressive residual stress values of the UIT specimens were enhanced, leading to the localization of the maximum residual stress ( sigma rsm ) in the subsurface. Increases in surface residual stress ( sigma s ) values exceeded 21.9 % and sigma rsm values were improved by more than 74.4 % compared to the turned specimen. The microhardness values showed improvement after UIT, with surface microhardness ( HV s ) values ranging from 408.5 HV 0.025 to 461.6 HV 0.025 . The orientation of the grains was deflected in UIT specimens, and the depths of the plastic deformation layer ( h p ) values experienced increases of more than 17 times. The fatigue life ( N f ) value of the Comb. U2 with the best surface integrity was approximately 9328.4 % higher than that of turned and about 4362.1 % higher than that of Comb. U1 with the worst surface integrity among the UIT specimens. Observing the fatigue fractures, the crack initiation locations of the UIT specimens were relocated to the subsurface at a distance of more than 150 mu m from the surface. A higher fatigue life of the specimen was achieved when there was better surface integrity and when the fatigue crack source was located further away from the surface. The crack source in the UIT specimen exhibited a large and irregular shape, and there were numerous small, evenly distributed dimples and a limited number of cleavage planes in fatigue instantaneous fracture region. Additionally, it was found that the subsurface fatigue life prediction model demonstrates superior accuracy for shorter fatigue life.