Effect Of The Evolution Behavior Of Debris Particles On The Performance Of TC4 Alloy In Fretting Wear

Ti-6Al-4V(TC4) alloy has been developed into a mature material for manufacturing blades of gas turbine engine in aerospace owing to its excellent mechanical characteristics such as low density,high specific strength,good corrosion resistance,high and low temperature properties and processing and forming performance.However,TC4 alloy is extremely sensitive to the fretting damage resulted from the lower surface hardness,poor wear resistance,weak plastic shear resistance and work-hardening rate,the contact interface of dovetail joints of the turbine blade and disk is prone to fretting wear resulted in the fatigue life of blade reduced,especially the fretting displacement amplitude is far less than the contact radius,leading to the damage with latent,concealed and catastrophic.With the rapid development of aerospace industry,fretting damage have been paid much more attention by researchers and engineers in China and abroad.In order to have a better understanding of fretting tribo-characteristics parameters and expand its extensive applications in aerospace for TC4 alloy,laboratory studies based on the actual working conditions have become the focus of researches.In the present study,the systematic investigation had been conducted on the fretting wear behaviors of TC4 alloy by the high-precision SRV-IV fretting tester,the generation and evolution behaviors of fretting debris particles at various displacement amplitudes and contact temperatures were analyzed by means of the optical microscopy(OM)?scanning electronic microscopy(SEM),backscattered electron microscopy(BSE),laser confocal scanning microscopy(LCSM),energy dispersive spectroscopy(EDS)and 3D non-contact surface profiler.According to systematically investigation the influence of surface distribution of debris particles and in-situ formation of tribo-oxide layers on the fretting operation areas,the tribo-characteristic parameters and worn morphologies,as well as the relationships between the evolution behaviors of fretting debris particles and damage mechanisms of TC4 alloy in different factors were established.The main contents and conclusions are listed as follows:(1)Distribution of fretting debris particles:The distribution of fretting debris particles in the contact interface was firstly in-depth analyzed as the entrance to investigate the fretting behaviors of TC4 alloy at room temperature.The displacement amplitude had been selected as the experimental variable,the debris particles had been used as the experimental media and the distributions law of debris particles in the contact interface induced by fretting wear had been systematically analyzed.The obtained results showed that the‘flake-like'delamination formed by the central adhesive teared was stacked into a‘ridge'in a direction perpendicular to the fretting at a relative low amplitude,and laminar debris accumulated along the direction perpendicular to the fretting under relatively minute displacement becomes the movement barrier.At a medium amplitude,the debris were scattered over the contact surface.A large number of debris particles generated under moderate displacement had been dispersed in the contact region,leading to the contact area expended and coefficient of friction(COF)tend to be stable.When the amplitude was even increased,‘cluster-like'debris were accumulated into a‘ridge'paralleled to the direction of the fretting.At higher displacement,oxidized debris overflowed the contact center and gathered along the direction to the fretting.The COF,wear volume,and wear depth increased rapidly.The various distribution of fretting debris particles resulted the change in the wear mechanisms from the adhesion to abrasive with slight adhesion,and finally the abrasive accompanied with the oxidative wear dominated.(2)In-situ formation of tribo-oxide layers:In-situ formation of tribo-oxide layers of TC4 alloy with various counter-bodies in fretting gross slip regime(GSR)were firstly in-depth investigated.Three commonly kinds of counter-bodies(GCr15?Si₃N₄ and Al₂O₃)were selected to systematically study the fretting operation area,the generation of fretting debris particles and the evolution of tribo-characteristic parameters such as coefficient of friction,wear volumes,wear rates,wear scar profiles and worn morphologies at various temperatures to established the in-situ formation procedure of tribo-oxide layers of TC4 alloy in fretting gross slip regime.The experimental results showed that with three various counter-bodies and temperatures,fretting wear of TC4 alloy were always in typical GSR,meanwhile there was obvious relative sliding in the contact area,generated severe plastic deformation of substrate and a large number of debris particles,which could be the good material foundation for the formation of tribo-oxide layers.At room temperature,COF of TC4 alloy had undergone periodic evolutions,no tribo-oxide layers had been produced on worn surfaces and wear rates were higher.When the samples were heated to 260?,the COF of TC4/GCr15 tribo-pair appeared the earliest transition from a‘concave-convex'dynamic to an approximately‘straight-line'stable state,the earliest severe-mild wear transition occurred and tribo-oxide layer had generated resulted in the wear rate reduced significantly.However,TC4/Si₃N₄ and TC4/Al₂O₃ tribo-pairs had no tribo-oxide layers formed,and the wear rate climbed sharply to the maximum value.Severe-mild wear transitions of TC4 alloy against with various counter-bodies had occurred while the samples were heated to 450?,the tribo-oxide layers differed from the matrix were formed on worn surface and the wear rates were lower reached a minimum value.The excellent wear performance of TC4 alloy in fretting at elevated temperatures could be attributed to the generation of a tribo-oxide layers with uniform distribution,continuous compressed and good adhesion.The wear mechanisms of TC4 alloy were dominated by adhesive and abrasive wear before the severe-mild transition,while controlled by oxidative with slightly abrasive after the transition beginning.(3)Evolution of tribo-characteristic parameters:The influence of worn situation on the properties of TC4 alloy in fretting partial slip regime(PSR)were firstly comparative investigated at RT and 450?.The test results showed whatever the counter-bodies(GCr15?Si₃N₄ and Al₂O₃)and temperatures,fretting wear of TC4 alloy were always in typical partial slip regime,and no transition to other two regimes(mixed and gross-slip regimes)occurred.The variety of tribo-characteristics parameters in PSR were significantly affected by the worn situation:the effect on the degradation of fretted surface was the most critical,the morphologies of worn scars were varied from a clearly distinguishable adhesive and slightly slippery zone to a circular intersection with severe plastic deformation generated newly;the profiles were transformed from a concave-convex peak curve to a regular“U”-shape with a sharp center and rough edges.The influence on damage mechanisms was minor,gradually transitions from adhesive to oxidation and adhesive wear.The effect on coefficient of friction was almost negligible,the curve evolved regularly,and the stability values was basically the same.