Research On Surface Shot Peening Modification And Tribological Properties Of Ti-45Al-2V-1Cr Alloy

With the rapid development of aviation and aerospace components lightweight field,the higher performance of engine blades are needed.Compared with nickel-based high temperature alloys,Ti Al alloys have high specific strength,low density,good oxidation resistance,and their density is only about half of that of nickel-based high temperature alloys.So,Ti Al alloys show a huge application perspective in the aerospace field.The service environment of the aero-engine is very harsh,mainly in the following three aspects:（1）during the pressurization process,the blade surface is subjected to large aerodynamic loads which might be up to several tons on a single blade,with rapidly increasing temperature in the blade surface;（2）during the operation of the aircraft,the engine easily inhales water,sand and dust from the air,and furthermore the blades may suffer from salt spray corrosion and other damage due to the large salt content in the air of the marine and coastal areas which lead to the premature fatigue failure of blades;（3）because there is a fit gap between the blade tenon and the wheel groove,the high-speed rotation of the blade is prone to the fretting wear,which induces fatigue crack and fretting fatigue damage in the blade.The shot peening treatment has been used as a traditional surface strengthening technology for the surface hardening and modification of various alloys.In addition,grain refinement can improve the mechanical properties of Ti Al alloys.The previous works show that shot peening treatment leads to residual compressive stress and grain refinement of the surface of Ti Al alloys,which can enhance the fatigue performance and service life of such alloys.In this paper,the effects of shot peening treatment on the hardness,residual compressive stress,plastic strain and so on of the surface and longitudinal section of Ti-45Al-2V-1Cr alloy were investigated,and the optimal shot peening parameters were determined.Then,the heat treatment was used to control the microstructure of the surface.Finally,the effects of shot peening and heat treatment on the friction and wear properties were studied.The details of the study are shown as follows:The shot peening model was built using the finite element software ABAQUS/Explicit.The effects of blasting parameters on the crater profile,residual stress and equivalent plastic strain of the alloy were analyzed,which could provide reference for subsequent physical shot peening experiments.The simulation results showed that the higher shot strength,the larger crater profile.And,that also led to the larger the maximum plastic strain and maximum residual compressive stress which existed in the deeper site from the surface.The more shot peening meant the larger crater profile and the maximum plastic strain which existed in the deeper site from the surface.As the projectile diameter increased,the crater profile became larger and the maximum plastic strain value and the maximum residual compressive stress value became deeper from the surface.In addition,a fretting wear model was established to investigate the influence of fretting parameters（normal load and displacement amplitude）on the equivalent force,frictional dissipation curve and equivalent plastic strain on the surface of such alloy.The Ti-45Al-2V-1Cr alloy was shot peened,and the residual compressive stress and hardness were characterized using a microhardness tester and an X-ray residual stress tester.It was found that the residual compressive stress on the surface was significantly higher.The microhardness of the longitudinal section increased and then decreased with increasing depth from the surface until it was the same as the hardness of the substrate.The surface residual compressive stress and hardness decreased with increasing shot peening strength.Subsequently,different heat treatment experiments were conducted on the shot-peened Ti-45Al-2V-1Cr alloy.It was found that theα₂phase of the alloy increased significantly and the microhardness increased significantly when the heat treatment temperature was 1200°C.In addition,quenching and annealing heat treatments with a holding temperature of 900°C and different holding times（15 minutes,2 hours,8 hours）were conducted.For the both heat treatments,the residual stresses on the surface of the specimens decreased with increasing holding time.For the same holding time,the residual compressive stress of the shot-peened and quenched specimen was lower than that of the shot-peened and annealed specimen.Then,original specimens,shot peened+900°C/quenched（15 minutes）specimens and shot peened+900°C/annealed（8 hours）specimens were studied.These specimens were subjected to frictional wear and fretting wear tests using the MFT-50 frictional wear tester and the MFT-5000 fretting frictional wear tester,respectively.The results of the study showed that in the frictional wear experiments,the shot peened and quenched specimens had shallow craters at a frequency of 1 Hz.The crater depths of the original specimens and the shot peened+annealed specimens at frequencies of 1 and 4 Hz were similar to that of the shot peened+quenched specimen at frequency of 4 Hz.In the fretting friction wear experiments of shot peened+quenched and heat treatment specimens,all specimens had partial slip,mixed slip and complete slip regimes.With the increase of displacement amplitude and the decrease of normal load,the slip regime changed from partial slip to mixed slip and complete slip zone.When the three specimens were located in the partial slip regime,the wear mechanism was dominated by fatigue wear.The wear mechanism was more complicated when the three specimens were located in the mixed slip and complete slip regimes where the oxidation and adhesion phenomena were serious.