Study On Fretting Wear Behavior Of Titanium Alloy In Aero-engine

With the high-speed operation of the aero-engine,fretting wear inevitably occurs between the tenon groove connection of the blade,the lap joint of the damping platform as well as the regulator of the nozzle.Compared with ordinary sliding wear,the biggest feature of fretting wear is small displacement,so it is very concealed.Fretting wear mainly produces two aspects of damage.First,it causes directly surface wear of materials,which may lead to damage of parts.Second,it may induce the initiation of cracks.Once cracks initiate,they may become the crack source of overall fatigue,and expand without detection,bringing disastrous consequences.Titanium alloy is the key material in aero-engine.Therefore,it is of great significance to study the fretting wear of titanium alloy for safe and reliable operation of aero-engine.In this thesis,the fretting wear behavior of titanium alloy was systematically studied,focusing on the identification,prediction and protection of fretting wear.Firstly,the fretting wear behavior between Ti-6Al-4V and Ti-6Al-4V was studied.Experiments under different loads and applied displacements show that with the increase of applied displacement or the decrease of load,fretting wear will gradually transit from partial slip regime to gross slip regime.The morphology of the wear scar is different due to the fretting running conditions,which is shown as follows:in gross slip regime,the central region of the wear scar is higher than that in the other region;in the mixed fretting regime,there is a large accumulation of materials at both ends of the reciprocating motion;in partial slip regime,almost original fretting sample surfaces were observed in the central region,and wear occurred in the edge region.By defining wear scar diameter ratio(α,ratio of actual wear scar diameter to Hertz contact diameter),fretting regime of titanium alloy can be determined.Subsequently,the role of debris in fretting wear was investigated.In terms of the distribution and size of the debris,the debris in the middle of the wear scar is massive,while the debris discharged outside the wear scar is loose granular.The size is concentrated between 0.2 μm and 1.5 μm,and the debris with the size of about 0.5 μm is the most.The wear debris can also significantly affect the friction coefficient and system deformation of fretting wear.After debris was removed and the test restarted,the debris between the contact surfaces was less,and the friction coefficient and system deformation were smaller than those before the removal of the debris.Among them,the decrease was the largest at 20 N load,which was 63%and 41%,respectively.When the surface texture with perpendicular grooves was prepared on the sample surface to capture the debris,there were more debris between the contact surfaces,and the friction coefficient and the system deformation were larger than those without texture.The increase was the largest at 20 N load,which was 21%and 47%,respectively.Then,the system deformation of friction pair in fretting wear was studied.When fretting wear is in different running conditions,the relationship between system deformation and load and applied displacement is also different:in gross slip regime,system deformation is independent of applied displacement,and increases with the increase of load;in partial slip regime,the system deformation is independent of the load,and increases with the increase of the applied displacement.The system deformation ratio(γ,the ratio of system deformation to applied displacement)was defined and it could be concluded that when γ<0.7,the fretting wear runs in gross slip regime;when 0.7≤γ<0.9,the fretting wear runs in mixed regime;when 0.9≤γ<1,the fretting wear runs in partial slip regime.Then,the effects of groove spacing and orientation on fretting wear were investigated,and the groove filling mechanism in fretting wear was also concerned.Compared with the case without surface texture,the surface texture with perpendicular grooves increases the system deformation and makes fretting wear more inclined to partial slip regime.The surface texture with parallel grooves and 45° grooves decreases the system deformation and makes fretting wear more inclined to gross slip regime.The reason for the above results is that the perpendicular grooves hinder the discharge of debris from the contact area,while the parallel and 45° grooves promote the discharge of debris from the contact area.Therefore,the fretting regimes can be controlled to a certain extent by preparing surface textures with different groove orientations.In addition,for the samples with perpendicular grooves,when the groove spacing is 80μm and fretting wear is in the partial slip regime,cracks and spalling appear on the convex platform;for the samples with parallel grooves,the upper material gradually squeezes the lower material into the middle and bottom of the grooves,and then fills the grooves.Finally,the fretting wear behavior of Cu-Ni-In coating and TiNi alloy rubbed with Ti-6Al-4V alloy was compared.It was found that TiNi alloy showed better fretting wear resistance than Cu-Ni-In coating.The friction coefficient of Cu-Ni-In coating can be effectively reduced in gross slip regime,and the friction reduction effect can be achieved.TiNi alloy has smaller friction coefficient,even less than Cu-Ni-In coating,which can play an effective role in reducing friction.Compared with Cu-Ni-In coating,TiNi alloy not only causes less wear to Ti-6Al-4V alloy,but also has stronger wear resistance.In addition,TiNi alloy has a significantly larger system deformation under higher load,that is,more displacement can be absorbed by its own deformation ability,thereby reducing the relative sliding between the two contact surfaces.In some cases where fretting wear is expected to be in partial slip regime,TiNi alloy has potential application value.