Study On The Effect Of Ultrasonic Deep Rolling On The Low-and High-cycle Fatigue Behavior Of Ti-6Al-4V Alloy

In the manufacturing industry of aero engine blades,the surface integrity has always been an important factor limiting the components from a longer serving life and a more aggressive working environment.On the other hand,research on surface treatments have attracted more and more attention in the past several decades.Technologies of surface strengthening treatments have been developed to enhance the engineering reliability and expand the serving life.However,there still needs further study for the aspects of effects of surface treatment on surface integrity.It was well known that the application of surface strengthening treatment would lead to the generation of residual stresses and obvious impacts to the microstructure within the surface region of peened materials.When fatigue loads as well as other mechanical loads were applied to the peened material,the relaxation of residual stress and evolution of microstructure would take place simultaneously.In the present research,a distinct surface strengthen treatment,i.e.,ultra sonic surface deep rolling treatment,was employed to improve the mechanical properties of Ti-6Al-4V alloy.The aim of the present research is to examine the effect of rolling treatment on the integrity of Ti-6A1-4V alloy and investigate the fatigue performance of strengthened material.The mechanism of residual stress relaxation under cyclic loading was studied experimentally.Simple phenomenological models were proposed based on the experimental results.Detailed researches were as follows,(1)Surface deep rolling treatments with different passes were conducted to examine the influence of rolling passes on the surface integrity.Experimental results showed that surface integrity was significantly improved with increasing the rolling passes.For instance,surface roughness were decreased as treatments proceeds.Meanwhile,the micro-hardness and compressive residual stress magnitude were increased at the same time.The formation mechanism of nano grains during surface strengthening treatment was clarified.Plastic deformation and dislocation movement were found to be responsible for the grain refining process.(2)Fatigue experiments with different controlling mode were carried out to examine the low cycle fatigue performance of peened Ti-6A1-4V alloy.The experimental results showed that the surface deep rolling treatment could effectively improve the low cycle fatigue life of peened materials in both stress-and strain-controlled cases.However,this enhancement effect would gradually reduce as loading level rise.The ultra sonic surface deep rolling treatment could retard the initiation and propagation of fatigue cracks.The effect of surface roughness.residual stress and microstructure on fatigue life was also investigated through experimental approach.The application of ultra sonic deep rolling treatment would induce the weakened cyclic softening characteristic of Ti-6A1-4V.Consequently,after ultra sonic deep rolling treatment,the difference between damage magnitude in cases of stress-controlled and strain-controlled fatigue loads would reduce,especially within the high load region.(3)High cycle fatigue experimental results indicated that ultra sonic surface deep rolling treatment could not improve the high cycle fatigue life of Ti-6A1-4V alloy.The fracture observation showed that the treatment could suppress the iniation of fatigue cracks from the surface.Howerever,with the high frequency of cyclic loading,the specimen after peening were much easier to overheat than the as-received material.This might be the reason of ultra sonic surface deep rolling treatment could not improve the high cycle fatigue life.(4)Researches were carried out to study the origination of residual stress due to ultra sonic surface deep rolling treatment,and its relaxation process under fatigue loads.The evolution of micro/nano structure has also been investigated.For both cases of stress-and strain-controlled fatigue experiments,the residual stress was released in some degree within the first load cycle.And the subsequent relaxation process was obviously affected by the fatigue load level.Experimental result of micro/nano structure indicated that the grain size of nanocrystalline increased with increasing the number of cyclic load.The plastic deformation was induced by the application of fatigue loads,leading to the relaxation of residual stress and the growth of nanocrystals.(5)On the basis of previous modeling works,three modified empirical models were developed in the work.These models could be used in the prediction of residual stress relaxation on Ti-6Al-4V under cyclic loading.