Ultrasonic fatigue performance of the shot-peened TC4 titanium Alloy: Improved or deteriorated?

The mechanism behind the fatigue life improvement or deterioration of the shot peened TC4 titanium alloy rod used as ultrasonic scalpel is a valuable scientific issue. In this paper, the ultrasonic fatigue property of the shot peened TC4 titanium alloy rod specimen was investigated based on microstructure characterization , mechanical tests and ultrasonic fatigue tests . The experimental results show that obvious grain refinement can be observed in the topmost surface layer of the shot peened TC4 titanium alloy rod specimens. Moreover, a large number of crater defects left on the surface of specimens leading to the increase of surface roughness. A maximum residual compressive stress of approximately 450 MPa was introduced into the surface layer of the specimens. The microhardness distribution is very discrete at the same depth, and thus the contribution of microhardness enhancement to suppressing fatigue crack initiation is limited. Surprisingly, the fatigue lives of the shot peened TC4 titanium alloy rod specimens are slightly deteriorated as compared with that of the un-shot peened specimens . The fatigue risk factor model was used to evaluate the difficulty of fatigue crack initiation. By the calculation, the fatigue risks of the shot peened TC4 titanium alloy rod specimens are generally higher than that of the un-shot peened specimens under the same stress amplitude , which can be mainly attributed to the effect of the competition between the beneficial effects, i.e., the introduction of residual compressive stress, and the harmful effects, i.e., stress concentration caused by crater defects, on the initiation of fatigue cracks. • Limited and uneven microhardness improvement in shot-peened TC4 specimen. • Contrary to previous studies, the fatigue life of shot-peened TC4 specimen reduced. • Crater defects deteriorate the fatigue property and increase fatigue risk. • Controlling crater defects is the key to enhancing ultrasonic fatigue life.