Laser Shock Peening Induced Surface Gradient Structural Evolution And Crack Growth Characteristics Of TC4 Titanium Alloy

Laser shock peening(LSP)technology can regulate the residual stress distribution and induce the gradient microstructure of the material surface through high-pressure,ultra fast and non-contact laser precision processing on the material surface,so as to significantly improve the fatigue performance of the material.It has broad application prospects in the field of aviation anti-fatigue manufacturing in the future.However,there is still a lack of in-depth understanding of the surface microstructure characteristics and the formation of gradient structures of metallic materials induced by LSP.There is still a lack of sufficient research on the surface residual stress regulation technology of complex engineering structures.The intrinsic relationship between surface residual stress distribution induced by LSP and fatigue performance is still unclear.In this paper,the microstructure evolution and residual stress regulation method of TC4 titanium alloy induced by LSP are systematically studied by means of theoretical analysis,numerical simulation and experimental research.On this basis,the crack growth behavior and retardation characteristics of TC4 titanium alloy in gradient residual stress field are discussed.Taking the fastening holes of typical aircraft structures as the research object,the service life enhancement mechanism of engineering structural details treated by LSP was explored,and the integrated simulation method of service life prediction treated by LSP was proposed,and the engineering application prospect of laser shock and shot peening composite strengthening process in fatigue resistance performance is explored.The main contents and results are as follows:(1)The effects of different laser process parameters on the crystal characteristics(grain size,orientation distribution and texture characteristics),phase composition and dislocation evolution of TC4 titanium alloy in surface and depth directions were systematically studied.The microscopic mechanism of grain refinement process induced by LSP was analyzed,and the gradient microstructure evolution characteristics induced by LSP was obtained.The effects of LSP on the local(microhardness)and overall(tensile strength)mechanical properties of TC4 titanium alloy were studied.The distribution of microhardness after LSP with different process parameters was tested.The synergistic strengthening mechanism of LSP was determined based on the microstructure characteristics.On this basis,the corresponding relationship between the microstructure and mechanical properties in the depth direction induced by LSP was revealed.(2)A three-dimensional finite/infinite hybrid element laser shock physical model was established to avoid the reflection of shock wave at the boundary of the model.The calculation results of hybrid element and pure finite element models were compared,and a model which can accurately reflect the distribution of surface residual stress and improve the operation efficiency was obtained.Using the finite element model of single point laser shock,the convergence of the model mesh was analyzed,and the optimal three-dimensional mesh size was determined.Based on the development of commercial software subroutine,the numerical simulation of multi-point laser shock was realized.On this basis,the influence rules of laser process parameters(peak pressure,impact time,spot size and overlap rate)on the gradient distribution of residual stress induced by LSP were studied.(3)The crack growth behavior of TC4 titanium alloy treated by LSP was deeply studied.The effects of laser energy and spot shape on the surface residual stress distribution of compact tensile(CT)specimens were analyzed,the evolution of the residual stress field before and after the crack growth test was explored,and the gain effect of LSP on service life was analyzed.The relationship curve of da/d N-?K was used to determine the conditions for the generation of crack retardation effects in compressive residual stress environment.The fracture characteristics and crack trajectories of LSP were studied in depth,the inhibition mechanism of crack growth rate was revealed.The crack growth rate of LSP was predicted by combining the superposition principle and the weight function method,and the effect of residual stress in the crack growth process was studied.(4)The service life of typical connecting hole structure of TC4 titanium alloy strengthened treated by LSP was studied comprehensively.Through the analysis of microscopic fracture morphology and typical fatigue striation spacing,the effect of LSP with different laser energy on the service life of connecting hole samples and the fracture characteristics were studied.The influence mechanism of LSP on the service life of titanium alloy hole structure was revealed.The three-dimensional residual stress distribution characteristics around the hole were analyzed by numerical simulation method.Based on commercial software,an integrated numerical simulation study of "residual stress-cycle loading-service life" was carried out to study the service life problem of LSP.The results show that increasing the surface compressive stress and the depth of the influence layer can significantly improve the service life.(5)The effect of LSP and shot peening(SP)composite process on the surface integrity and service life of TC4 titanium alloy was explored.The influence law and influencing factors of composite strengthening on residual stress and microstructure were analyzed theoretically.The surface morphology,microhardness,residual stress field and microstructure of TC4 titanium alloy under the three processes of LSP,SP and composite strengthening were compared and analyzed.Taking the connecting hole structure of TC4 titanium alloy as the object,the life gain effect and fracture morphology characteristics of three processes of LSP,SP and composite strengthening were studied,and the improvement mechanism of composite peeing on the service life of the connecting hole structure was determined.