Study On High Temperature Oxidation Behavior And Shot Peening Strengthening Structure And Properties Of TC11 Titanium Alloy

Titanium and titanium alloys have the advantages of low density,high strength and good corrosion resistance.They are widely used in automotive industry,shipbuilding,aerospace and other fields.In recent years,titanium alloys as a hot end moving part have a great development prospect in the manufacture of engine piston.Because the piston works under the environment of high temperature,high pressure and high-speed operation,higher requirements are put forward for the high temperature resistance and fatigue resistance of titanium alloy.When TC11 titanium alloy is exposed to the air above 500?for a long time,the surface will produce an oxide layer,and the matrix will form an oxygen rich layer due to the dissolution of oxygen,which seriously affects the mechanical properties of the material,especially the fatigue resistance.The surface modification method can effectively improve the surface microstructure and mechanical properties of the alloy.Shot peening surface modification technology can refine the surface grains and introduce residual compressive stress,effectively improve the fatigue resistance of moving parts and delay the initiation and propagation of fatigue cracks.In this paper,TC11 titanium alloy was taken as the research object to explore the high-temperature oxidation behavior of the alloy at 100?700?;The influence mechanism of shot peening on the microstructure and properties of TC11 titanium alloy was explored.Metallographic microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to investigate the microstructure of high temperature oxide film of TC11 titanium alloy and the effect of shot peening on the microstructure of alloy matrix.The phase composition and element distribution of high temperature oxide film of the alloy were studied by X-ray diffraction(XRD),X-ray photoelectron diffraction(XPS)and energy dispersive spectroscopy(EDS).Residual stress analyzer,micro Vickers hardness tester and fatigue testing machine were used to characterize the surface residual stress,micro hardness and fatigue properties of TC11 titanium alloy before and after shot peening.It is found that the oxidation tendency of TC11 titanium alloy is relatively small when the oxidation temperature is between 100?400?;When the oxidation temperature exceeds 500?,the oxidation tendency of the alloy is very large.When the oxidation temperature is 500?and600?,the oxidation behavior of the alloy conforms to the law of linear parabola;When the oxidation temperature is 700?,the oxidation behavior of the alloy conforms to the parabolic law.The original structure of TC11 titanium alloy is composed of equiaxed and long straight sheet close packed hexagonal structure?Phase and acicular and strip body centered cubic structure?phase composition.At the initial stage of oxidation,the oxidation takes place at the boundary of?/?phase and the edge of?phase;After a continuous and complete oxide film is formed on the alloy surface,the composition and structure of the high-temperature oxide film change with the continuous oxidation of the alloy.At 500?,the alloying element Al in TC11 titanium alloy shows selective oxidation,and the main composition of the oxide film is?-Al₂O₃;When the temperature is above 600?,Al in the oxide film is enriched in the shallow layer of the oxide film,and the outer layer is?-The mixed layer of Al₂O₃ and Ti O₂,and the inner layer is rutile Ti O₂ layer;At900?,the loose Ti O₂ in the mixed layer grows to the top layer of the oxide film,while there are many linear cracks in the inner Ti O₂ layer?-Al₂O₃.When the oxidation time is long enough,the structure of high-temperature oxide film element diffusion layer matrix appears on the alloy section.The diffusion ability of?phase is stronger than?phase,so that in the diffusion layer?,the phase is concentrated on the side close to the matrix.After shot peening,the samples exhibited four gradient transition zones along the impact direction of the projectile,including the nanoscale structure zone,high density parallel dislocation zone,divergent dislocation zone and low density dislocation zone.The Micron,nanoscale structure(about 0?20?m)showed higher density dislocation cell,dislocation plug and sub-boundaries than the matrix before shot peening.In the high-density high-density parallel dislocation region(20?80?m),there were high density parallel dislocation lines,parallel dislocation slippage and dislocation proliferation structure.The divergent dislocations(80?140?m)showed divergent dislocation lines with stress centers and propagated along different phases in the grain.The dislocations with low density(140?200?m)showed lower dislocations with no obvious orientation and dispersion distribution.With the increase of shot peening pressure,the microstructure refinement degree of the larger slice and equiaxed?phase and the smaller strip and lamellar?phase gradually becomes higher,the deformation degree of?phase was higher than that of?phase.With the increase of shot peening diameter,the deformation degree of both?phase and?phase decreased.However,the effect of shot peening diameter was not as great as that of shot peening pressure.With the increase of shot peening pressure,the residual compressive stress on the alloy surface increased.The residual compressive stress decreases decreased with the increase of shot peening diameter.Under the same loading stress,the fatigue cycle of TC11 titanium alloy was increased by 77.9%.The effect of shot peening on the oxidation behavior of TC11 titanium alloy was mainly concentrated in the early oxidation stage,which accelerated the diffusion of Ti in the alloy to the sample surface and formed Ti-rich oxide layer,which had little effect on the oxidation behavior of TC11 titanium alloy in the middle and later oxidation stage.