Research On Formation Mechanism And Mechanical Properties Of Laser Shock Peening Induced Gradient Microstructure

The microstructure in the surface layer of the metal material determines its performance and service life.The method of surface severe plastic deformation can realize the multi-stage construction of the microstructures in metal materials without introducing other elements,so that the microstructures of the material are gradiently distributed in the depth direction,and excellent strength-plasticity matching can be achieved.Laser shock peening(LSP)introduces compressive residual stress in the surface layer and modifies the surface microstructure of target material,thereby improving fatigue life and wear resistance of the material.Extensive investigations have been carried out on the strengthening effect and mechanism of LSP for various materials by domestic and foreign scholars,and a variety of external field-assisted LSP methods have also been developed.However,further understanding of laser shock induced plastic deformation and microstructure evolution of metal materials,as well as the formation of gradient structures and its plastic deformation mechanism are still not clear.In this work,metal materials with different lattice structures are used as the research objects.Surface integrity and microstructure characteristics in the gradient layer after LSP are systematically investigated.Multi-scale simulation models are established to investigate the dislocation evolution and plastic deformation under LSP induced ultra-high strain rate.Based on experiments and simulations,the synergistic strengthening behavior of LSP induced gradient structures is analyzed.The main research contents and results are as follows:Surface integrity of LSP treated specimens: Scanning microscopy,microhardness tester,nanoindentation and X-ray diffraction were used to investigate the surface macro and microscopic morphology,residual stress and hardness distribution in different materials induced by LSP.The results show that LSP causes the surface roughness of polished sample to increase,but the surface roughness does not change much with the increase of laser energy and layer of LSP.Increasing laser energy or the layer of LSP can increase the amplitude of compressive residual stress and microhardness in the surface layer of specimen,and deepen compressive residual stress layer and plastic influence layer.Based on the nanoindentation test,a calculation method of LSP induced dislocation density is proposed.The quantitative analysis of the X-ray diffraction show that LSP can induce nanocrystallization on the surface of pure nickel and pure iron specimens.Microstructure characteristics of LSP induced gradient layer and grain refinement mechanism: The microstructures of LSP induced gradient layer in materials with different lattices were analyzed by transmission electron microscope(TEM).Grain refinement mechanisms in LSP treated metals materials with high stacking fault energy were proposed based on observed microscopy.The grain refinement mechanisms of face-centered cubic(FCC)pure nickel and body-centered cubic(BCC)pure iron during LSP have the similar process,including: rapid dislocation multiplication and dense dislocations gradually transforming to dislocation tangles and dislocation walls;the original coarse grains being divided by dislocation tangles and dislocation walls to form dislocation cells;the increase of plastic deformation causing the dislocation cells to be compressed and dislocations continuously gathering toward the dislocation cell walls;the dislocation cell walls and dislocation walls gradually transforming into subgrain boundaries and grain boundaries,forming lamellar grains;further plastic strain causing the dislocation walls to transform into grain boundaries perpendicular to the lamellar grain boundaries,and the ultra-fine lamellar grians gradually being refined into equiaxed ultra-fine grians;as the plastic deformation continuing to accumulate,the dense dislocations continuously transforming into dislocation walls and subgrain boundaries,at the same time,dislocations annihilating at the subgrain boundaries,resulting low-angle grain boundaries transforming into high-angle grain boundaries,and finally forming large-angle equiaxed nanograins.Moreover,it is found that for Ti-6Al-4V alloy,mechanical twinning in ? phase may adjust the plastic deformation only in the initial stage of deformation during LSP while the dominant mechanism of plastic deformation is still dislocation evolution.Multi-scale simulation model of LSP process: Dislocation dynamic and finite element models of LSP were established.Dislocation evolution characteristic and the influence of LSP parameters on the strengthening effect were investigated.The results show that under the condition of laser shock induced ultra-high strain rate,the dislocation density of pure nickel increases with the increase of temperature,while the dislocation density of pure iron decreases with the increase of temperature.Plastic deformation dominated by dislocations of single crystal is sensitive to loading direction,strain rate,dislocation damping coefficient and initial dislocation density.The ultra-high strain rate induced by LSP can effectively promote dislocation multiplication and suppress dislocation annihilation,and thus significantly increase the dislocation density under the same strain.Finite element simulation can predict the residual stress and equivalent plastic strain distribution in the subsurface layer,but it is limited to accurately predict the residual stress and equivalent plastic strain on the topmost surface.The increase of laser spot size or overlapping rate can increase the depth of compressive residual stress layer,while reducing the laser spot size or increasing the overlapping rate results in uniformly distributed compressive residual stress and equivalent plastic strain.The greater the strain hardening modulus of the material,the greater the depth of the laser shock induced compressive residual stress layer;the greater the elastic modulus,the lower the equivalent plastic strain and the depth of plastic deformation layer.The comparison between simulation and experimental results shows that the finite element method can effectively predict the residual stress distribution induced by low-energy LSP,but for high-energy LSP,the finite element method has certain limitations.LSP can induce surface nanocrystallization in metal materials under low strain and small strain gradient.Plastic deformation mechanism of LSP induced gradient structure: Quasi-static tensile tests were carried out on pure nickel and pure iron specimens before and after LSP using MTS universal testing machine.The effects of LSP and gradient layer volume fraction on tensile properties were investigated.The results show that increasing the layer of LSP can enhance the yield strength of the specimen but reduce the elongation.After LSP treatment,the yield strength of pure nickel inceases with the decrease of specimen thickness while elongation and strain hardening rate significantly decrease,and specimen thickness has little influence on the ultimate strength of pure nickel.The yield strength and ultimate strength of the pure iron specimen increase with the decrease of the specimen thickness,so does the elongation.Excellent strength and plasticity matching can be achieved by controlling LSP parameters.The change of thickness does not cause the change of necking mode of pure nickel specimen,but the neck of pure iron specimen is significantly affected by the thickness.The gradient structure induced by LSP has a significant synergistic strengthening effect,and the synergistic strengthening effect originates from the transformation of uniaxial stress to biaxial stress,the generated stress and strain gradient between gradient layer and coarse-grained layer during tensile process.