Fatigue Behavior And Strengthening Mechanism Of TC4 Titanium Alloy By Wet Shot Peening

TC4 titanium alloy is widely used in aerospace,national defense industry and other highend fields,as well as in medical,construction,automotive and mechanical fields.The fatigue property of titanium alloy is a key performance index for the design and safe service of loadbearing structural parts.Shot peening is an effective technical means to improve fatigue properties of titanium alloy structural parts.It changes the surface integrity by impact of projectiles.Surface integrity parameters of shot peening include residual stress,deformation structure,work hardening and surface roughening.These parameters have different effects and mechanisms on fatigue performance.Based on the background of wet shot peening treatment of TC4 titanium alloy,this paper tried to quantitatively evaluates the influence of various parameters on fatigue strength,and studied the internal relationship between fatigue crack source location,surface integrity and external loading.In addition,the reasons for the change of crack source location were discussed.,and focused on the analysis of residual stress stability and its influence on fatigue performance during the fatigue process.The main results are as follows:The fatigue crack source location of shot peening titanium alloy was closely related to surface integrity parameters and external loading.The fatigue crack source occurs in the subsurface layer of the material under low load.Under 550 MPa,the crack source was 200 ?m away from the surface.With the increased of loads,the crack source gradually shifted to the surface.The location of crack source was predicted by the relationship between surface integrity parameters and fatigue strength.The results showed that the crack source of shot peening TC4 alloy would transfer from the subsurface layer to the surface weak area with the increasing of loads.As for the roughness of surface integrity parameters,shot peening treatment of TC4 alloys roughened by the impact of projectiles,forming micro-scale concave and convex contours and inducing stress concentration effect.Under the condition of wet shot peening strength of 0.4 mmN and coverage of 200%,the stress concentration factor Kt of surface roughening notch effect was 1.42,and the depth of stress layer reached 50 ?m.When the applied volume load was 550 MPa,the rough surface stress value was 781 MPa,which accelerated cracks initiation and propagation,and had the negative impact on fatigue performance.With regard to work hardening in surface integrity parameters,the hardened layer of TC4 alloy surface was formed due to elastic-plastic deformation during shot peening.Under the condition of wet shot peening strength of 0.4 mmN and coverage of 200%,the 100 ?m hardened layer was formed on the surface of TC4 alloy,and the average hardness increased from 340 HV to 445 HV.According to the quantitative calculation of the relationship between hardness and yield strength,the surface yield strength increased to 1.83 times of the matrix.The stability of residual stresses in surface integrity parameters was closely related to fatigue performance.When shot peening strength was 0.4 mmN and coverage was 200%,the maximum residual stress was-911.3 MPa.According to the principle of fracture superposition,residual compressive stress could inhibit crack growth and improve surface fatigue strength.However,the residual stress relaxed during fatigue.When the applied volume load was 750 MPa,the maximum residual stress decreased to-569.8 MPa,and the strengthening effect decreased.The residual compressive stress field introduced by shot peening had relaxation effect during the fatigue process.When the external load was 550 MPa,the compressive residual stress relaxation mainly occurred in the near surface layer of 0-30 ?m.The sub-surface region at 30-80 ?m relaxed first and then stabilized,while the matrix region remained stable.When the load was 750 MPa,the residual stress relaxation occured at 0-80 ?m,and the relaxation rate was accelerated and sustained.Re-shot peening could restore the residual stress fields of structural parts and prolonged the fatigue life.And the choice of processing time was very important for fatigue life.When the load was 650 MPa,the fatigue life was increased by 65.7% after re-shot-peening at 25% life stages.