Influence Of Ultrasonic Surface Rolling Process On The Multi-scale Fatigue Crack Propagation In Ti-6Al-4V Alloy

Fatigue failure reached 60%～80%in material failure behavior and fatigue crack initiates in the surface of material in most case.Nano gradient structure in the material surface introduced by surface nanocrystallization can effectively improve the fatigue properties of material.In order to clarify the fatigue crack propagation behavior in nano gradient structure,select ultrasonic surface rolling process(USRP)as surface nanocrystallization method,the following studies are performed:(1)A nano gradient layer which thickness could reach around 300μm has been obtained in the surface of Ti-6Al-4V alloy by designing unique USRP machining path.The nano gradient layer became uniform and stable with the increase of USRP times.The grain refinement was due to the synergistic effect of mechanical twinning and dislocation slide;(2)USRP improved the microhardness effectively and introduced high compressive residual stress in the surface layer of Ti-6Al-4V alloy.USRP also decreased the surface roughness of Ti-6Al-4V alloy.Moreover,the fatigue life of Ti-6Al-4V alloy was significantly improved after USRP treatment;(3)By means of corner crack defects to locate the position of crack initiation and using replica method to observe the crack propagation path.The results showed that the crack propagation path of Ti-6Al-4V alloy changed and deflected visibly after USRP treatment and the crack propagation rate was decreased obviously after USRP treatment;(4)The fatigue life of USRP specimens and as-received specimens were all improved with the stress ratio increased.The fatigue life of USRP specimens were higher than as-received specimens and the crack propagation rate of USRP specimens were lower than as-received specimens in all stress ratios.