Surface modification and its effect on fatigue performance of nickel-based superalloy treated by ultrasonic surface rolling process

In the process of material removal, the metallurgical and mechanical properties of the surface layer of GH4169 superalloy have changed significantly, which has an adverse effect on the fatigue life. Therefore, surface strengthening process is indispensable as the last process of part manufacturing. In the paper, once and four times of ultrasonic surface rolling process (USRP) are carried out on GH4169 superalloy. The results show that USRP treatment improves the degree of grain refinement, the proportion of low angle grain boundaries (LAGBs) and dislocation density of the surface layer. The USRP treatment with four times forms a preferred orientation (< 101 >//TD) on the surface layer, and, the grain refinement mechanism is revealed. Moreover, USRP treatment changes the crack propagation mode from transgranular to intergranular. The fatigue tests indicate that the fatigue life of samples treated with USRP with once and USRP with four times increase by 55.6% and 138.6%, respectively. It can be inferred that the surface topography, residual stress, microstructure and degree of working hardening are the main factors affecting the low cycle fatigue properties of USRP treated samples.