Research On Strengthening Mechanism Of Cryogenic Laser Peening And Vibration Fatigue Properties Of TC6 Titanium Alloy

Aiming at the problem of vibration fatigue failure of aeroengine blades,the process of cryogenic laser peening(CLP)was proposed based on the coupling effect of ultra-low temperature and ultra-high strain rate.The aerospace titanium alloy TC6 was taken as the research object,the microstructural evolution and the residual stress distribution under cryogenic laser peening process were explored using the combination of theoretical analysis,numerical simulation and experimental research.On the basis,the influence mechanism of cryogenic laser peening on the vibration performance and fatigue life of structural parts was explored.The main work and innovation achievements of the paper are as follows:(1)The microscopic modification mechanism of the coupling effect of cryogenic temperature and laser peening was theoretically studied.The mechanism of cryogenic temperature on inhibiting the recovery and annihilation of dislocation structures was analyzed.The mechanism of cryogenic laser peening on improving dislocation density and stability was revealed.Based on the twin nucleation mechanism of the coupling effect of ultra-low temperature and ultra-high strain rate,the influence mechanism of cryogenic laser peening on suppressing dislocation slip,improving the nucleation driving force of twins and the formation of nano twins was explored.(2)A molecular dynamics model of plastic deformation of single crystal titanium induced by cryogenic laser peening was established.The dislocation generation and proliferation,as well as the formation process of deformation twin and amorphization under cryogenic laser peening was simulated.The microstructural characteristics,such as dislocation density and configuration,grain boundary orientation angle and grain size distribution,as well as deformation twin of TC6 titanium alloy under cryogenic laser peening were investigated using XRD,TEM and EBSD and other experimental methods.The plastic deformation mechanism of dislocation slip,multi-directional twin interlacing,as well as the coordination between dislocation and twin induced by cryogenic laser peening was revealed.(3)A mathematical analytical model of compressive residual stress amplitude and plastic strain depth was constructed.Based on the material elastoplastic deformation at cryogenic temperature and high strain rate,the distribution of peak pressure and residual stress induced by cryogenic laser peening was analyzed.The distribution of residual stress induced by cryogenic laser peening under different laser power densities was analyzed,and the optimal laser power density parameters was obtained.The release process of the compressive residual stress on the surface of the cryogenic laser peening specimen with the cycles of vibration loading was explored.Based on the relationship between compressive residual stress relaxation and microstructural evolution under vibration loading,the strengthening mechanism of deformation twins induced by cryogenic laser peening to enhance the stability of compressive residual stress by suppressing dislocation slip,reducing vibration stress and reducing residual stress relaxation rate was revealed.(4)The adjustment and control method of vibration modal parameters was proposed based on the dislocation structures and deformation twins induced by cryogenic laser peening.Based on the nanoindentation test technology,the effect of cryogenic laser peening on the elastic modulus of the materials was analyzed,and the influence mechanism of elastic modulus on the natural frequency was clarified.The modal analysis method was used to measure the natural frequency and damping ratio of the vibration samples subjected to cryogenic laser peening process.On the basis,the effect of different damping ratios on the amplitude of standard vibration structural parts was experimentally studied.Based on the theory of dislocation damping and twin damping,the influence mechanism of metal damping behavior on vibration attenuation was revealed.(5)Based on deformation twins and compressive residual stress,the coupling strengthening mechanism of cryogenic laser peening process was proposed.The SEM test method was used to analyze the vibration fatigue fracture morphologies and the fracture characteristics of each stage,as well as the vibration fatigue fracture criterion of metal materials was obtained.The dislocation and twin morphologies,as well as the grain size of the vibration fatigue specimens near the fracture surface were analyzed using TEM and EBSD test methods.And the anti-vibration fatigue life extension mechanism by the coupling effect of cryogenic temperature and ultra-high strain rate was proposed.It shows that the high-density dislocation and deformation twins,as well as the high-amplitude compressive residual stress induced by cryogenic laser peening can effectively suppress the initiation and propagation of vibration fatigue cracks,thus significantly improving the vibration fatigue life of metal materials.