Investigation Of Fretting Fatigue Damage And Shot-peening Palliative Mechanisms Of TC4 Alloy Dovetail Joints

Blades and discs in an aero-engine are often connected with dovetail joint structures.When the engine is running,the contact surfaces of dovetail joints are subjected to normal pressures combined with tiny relative slips,which are caused by the centrifugal force and strong aerodynamic loads.Such working condition is known as the fretting fatigue environment,which can easily lead to the failure of blades,subsequently resulting in catastrophic consequences.TC4 titanium alloy(Ti-6A1-4V)is a common material for manufacturing the engine fan blades and discs,which occupies an important position in the research of fretting fatigue problems.Therefore,investigations on the damage and palliative mechanisms of TC4 dovtail joints are significant to the national economy and national defense security.The previous studies are mainly focused on the observation of fretting wear morpholygy,the calculation of fretting contact stresses,the evaluation of fretting fatigue life and the development of new fretting palliative technologies.However,the evolution of important fretting characteristics such as fretting slip mode,fretting damage mechanism and fretting crack initiation are still less investigated.In addition,the new fretting palliative technologies are often realized through the combination of traditional techniques.Shot peening is one of the most widely used fretting palliatives in industry,which is often applied as an important composite in many combining treatments for fretting fatigue protection.Nevertheless,a systematic and comprehensive research on the strengthening mechanism of shot peening has not been presented.The effect of surface asperities and surface hardening layer induced by shot peening on the fretting fatigue of materials,especially for TC4 alloy,is still unclear.In this paper,the dovetail joint structure of TC4 titanium alloy was taken as the research object,shot peening and shot peening combined with CuNiln coating were applied as the surface treatment methods.The fretting fatigue test was carried out on the specimens with three surface states:as-received,shot-peened and combining treated.The scanning electron microscopy(SEM),transmission electron microscopy(TEM)and laser confocal microscope(LSCM)were used to characterize the evolution of fretting slip mode,fretting damage mechanism and fretting crack initiation in fretting fatigue process.Furthermore,the fretting fatigue behavior of dovetail joint specimens before and after surface treatment were comparatively analyzed and the effect of different surface integrity factors induced by shot peening were systematically discussed.The fretting damage and palliative mechanisms of TC4 dovetail joints were concluded based on the experimental results.The main conclusions are listed as follows:The surface roughness,surface strain hardening and compressive residual stress are induced by shot peening treatment.A transition zone from the high plastic strain layer to fine crystal layer is formed in the subsurface of a shot peened specimen.The microstructure of the transition zone is characterized by the pile-up,tangles and rearrangement of dislocations.The dislocation movement is extremely active,leading to the formation and evolution of small angle grain boundaries.The distribution of residual stress is closely related to the microstructure.The depth of maximum residual stress is accompanied with the formation of abundant dislocation walls,which could be attributed to the great differences in lattice distortion and system energy between the dislocation rearrangement zone and its adjacent layers.In the early fretting stage,the wear damage of as-received(AS)and shot-peened(SP)specimens are respectively dominated by adhesion and asperity peeling.The wear volume of AS specimen is lower than that of the SP specimen.While the friction coefficient of AS specimen is higher than that of the SP specimen.The friction behavior of AS specimen is related to the morphology of surface asperity.The asperities formed under 0.3 mmA shot peening intensity have relatively large top fillet radius,which is conducive to reducing the mechanical friction coefficient,fM,and improving the fretting wear performance.In the long-term fretting progress,the wear damage of the AS and SP specimens are both dominated by the surface delamination.The wear amount of specimens depend on the delamination thickness.The delamination thickness of AS specimen reaches 8 ?m,while that of the SP specimen are reduced below 6 ?m.The wear volume of AS specimen is higher than the SP specimen.The 0.3 mmA specimen has the minimum delamination thickness(2.5 ?m)and the best wear resistance due to the good combination of surface strength and toughness.In addition,large wear pits are formed in the AS specimen.The wear pits play the role as the surface defects,which are easy to cause the local stress concentration and lead to crack nucleation.While the size of wear pits in SP specimen is significantly reduced and no cracks are observed.The fretting regime of the dovetail joint specimen gradually develops from initial adhesion to partial slip,subsequently to mixed slip,and finally to gorss slip.The fretting damage mechanism progressively evolves from adhesion wear to delamination wear.Multiple kinds of fretting cracks are initiated in the contact region,such as the subsurface cracks,the cracks nucleated in the bottom of the wear pits,and the the cracks initiate in steep angles(50�,80�)to the fretting surface.The crack length increases with the rising of cyclic numbers(14 ?m to 35?m)and the maximum crack number existed under the mixed slip regime,proving that mixed slip regime has adverse effect on the fretting fatigue performance of TC4 dovetail joints.After shot peening treatment,the fretting regime of contact region jumps over the partial slip and mixed slip modes,directly entering the gross slip mode.The damage mechanism rapidly transforms from asperity peeling to surface delamination.The bottom cracks in wear pits disappear.The subsurface cracks are remained.The steep crack initiation angles are replaced by shallow angle(20�).The crack initiation angle is analyzed through the combination of numerical simulation and theoretical calculation based on the modification of Giannakopoulos theory.The results show that the existence of the surface compressive residual stress is the core reason for the change of crack initiation angle.After shot peening,the crack growth is inhibited and the length of crack was maintained at about 20 ?m.The number of crack is also decreased.In summary,the surface asperity,surface strain hardening and surface residual stress could reduce the crack number,decrease the crack type,change the crack initiation angle and inhibit the crack propagation,respectively.The fretting fatigue strength of TC4 dovetail joint is promoted by 28.9%after shot peening treatment.After the combining treatment,the fretting damage of TC4 dovtail joints are dominated by the wear and consumption of CuNiIn coating in the early stage of the fretting fatigue process.The coating is consumed by the initiation and propagation of subsurface cracks and the formation and growth of wear pits.The good plasticity of CuNiIn coating enables it to have a good deformation accommodation ability,which is beneficial to alleviating the stress concentration at the contact edge,modifying the local stress condition and reducing the crack initiation risk.After the coating is worn out,the substrate will be fretted under the strengthening factors induced by shot peening until to the fracture failure.The fretting fatigue strength of TC4 dovetail joint is promoted by 17.3%after combining treatment.The strengthening effect of combining treatment is slightly lower than that of the shot peening treatment.Such phenomenon is probably resulted from two reasons.Firstly,the poor adhesion between the coating and the substrate causes the premature failure of the CuNiln coating.Secondly,the thermal effect of plasma spraying process lead to the relaxation of residual stress introduced by shot peening,weakening the inhibiting effect on crack propagation.In order to achieve a better combining strengthening effect,it is necessary to select the poper composite technologies and find the optimal technological parameters.The key strengthening factors of each technology should be remained so as to realize the complementary advantages of different technologies.Further exploration is still required to improve the combining treatment for fretting fatigue palliative of TC4 dovtail joints.