Effect Of Shot Peening Combined Plasma Electrolytic Oxidation On Microstructure And Properties Of TC11 Titanium Alloy

TC11 titanium alloy is often used as moving parts material in high temperature environment like pistons because of its low density and high specific strength.However,when used at high temperature above 500℃,the oxygen-rich layer is easy to be formed on the surface of TC11,which leads to the embrittlement of the alloy and the decrease of its fracture toughness,which leads to the decline of the mechanical properties of TC11.Hot-end components like pistons are exposed to long-term high temperature,high pressure,and high load conditions,requiring them to possess exceptional mechanical properties and high-temperature oxidation resistance.A single surface protection method is insufficient for meeting the demands of the challenging working environment.Consequently,a composite protection approach has emerged as the most promising means of enhancing the overall performance of TC11.This study focuses on enhancing the fatigue properties and oxidation resistance of TC11 titanium alloy through the application of a combined approach of shot peening and plasma electrolytic oxidation(PEO-SP).The investigation employed X-ray diffraction(XRD),metallography(OM),scanning electron microscopy(SEM),and transmission electron microscopy(TEM)techniques to examine the impact of shot peening gradient nanostructure and PEO-SP composite treatment on the surface microstructure,high-temperature oxidation properties,and fatigue properties of TC11.The study also analyzed the results before and after exposure to high-temperature oxidation environments.It is found that shot peening(SP)forms a gradient micro-nanostructure with a depth of220μm on the surface of TC11 titanium alloy,which is nanocrystalline layer,twin layer,dislocation layer and matrix layer.The minimum microstructure of low density dislocation deformation tends to be close to the matrix;with the decrease of depth,the type variable increases and the dislocation density increases.Under the action of dislocation entanglement and pinning,dislocation slip is difficult,forming deformation twins.With the decrease of depth and the increase of shape variables,the average grain size is about 21.8nm nanocrystals near the surface,and small nanocrystals with average grain size about 6.7nm are formed on the surface.The grain refinement mechanism of α phase is the interaction of dislocation slip cutting and twinning,while the grain refinement mechanism of β phase is affected by dislocation cutting ofα phase.After oxidation at 650℃,the oxidation behavior of matrix and shot peening(SP)samples followed the linear parabola law.Compared with the matrix samples,the oxidation resistance of SP samples decreased by 5.72%.At the same time,the thickness of oxide particles and oxide film of SP sample are larger than that of matrix sample,and the depth of oxygen diffusion is deeper than that of matrix sample.The main oxide phases of both are rutile(R)Ti O2.The oxide film defect in the matrix sample reduces as the oxidation time increases.However,in the shot peening strengthened sample,the oxidation of the oxide film initially increases and then decreases with time.The matrix and SP treated samples were treated by plasma electrolytic oxidation(PEO).It is found that after PEO treatment,the surface porosity and thickness of the film of the SP sample are higher than those of the matrix sample,and the main phase of both is anatase(A)type Ti O2.The high temperature oxidation properties of the two samples were tested,and it was found that the high temperature oxidation resistance of PEO-SP treated samples was 5.7%higher than that of PEO treated samples after oxidation at 650℃ for 100 h.Compared with the matrix sample oxidized at 650℃ for 100 h,the oxidation resistance of PEO-SP composite treatment at high temperature is increased by 17 times,and the main phases of both are(R)Ti O2 and(A)Ti O2.The fatigue properties and fracture toughness of matrix and SP treated samples were tested.It was found that SP treatment improved the fracture toughness and fatigue properties of TC11 titanium alloy.Under the same alternating stress,the fatigue strength coefficient of SP treated sample was lower than that of matrix sample,and the fatigue strength index of SP treated sample was higher than that of matrix sample.When the stress was 600 MPa,SP treatment increased the fatigue life of TC11 titanium alloy by 43.6%.The fatigue crack propagation mode of matrix specimen is through β phase,while that of shot peening is through α phase and βphase,and the fatigue fracture mode of shot peening treatment of TC11 titanium alloy is ductile fracture.The fatigue properties and fracture toughness of PEO and PEO-SP treated samples were tested,it was found that PEO treatment decreased the fracture toughness,while PEO-SP treatment increased the fracture toughness of TC11 titanium alloy;PEO treatment reduced the fatigue life of TC11 titanium alloy,while the gradient structure and work hardening formed by SP improved the fatigue resistance of PEO samples.Under the stress of 600 MPa,the fatigue life of composite treated samples was 4.8% higher than that of TC11 matrix.The fatigue properties and fracture toughness of matrix,SP,PEO and PEO-SP treated samples were tested.It was found that the fracture toughness and fatigue properties of TC11,SP,PEO and PEO-SP decreased after oxidation at 650℃ for 10 h,and the fatigue sources all appeared multi-source characteristics.