Experiment And Simulation Research Micro-deformation Of Metal Nickel Treated By Laser Shock Peening

Metal nickel shows good corrosion resistance,processing performance and welding performance,also has high electric vacuum performance and electromagnetic control performance.Nickel is a face-centered cubic metal,with good ductility and strength.Nickel also has a large capacity for absorbing hydrogen,and the finer the grains,the stronger the ability to absorb hydrogen.Laser shock peening technology is a new surface-strengthening improvement technology that uses high-density,short-pulse laser to impact on the surface of material,which would generate a plasma,forming a high-pressure shock wave that propagates into the material,causing the incresement of dislocation density and grain refinement.Thereby the multi-faceted properties of the material are improved.In this paper,laser shock peening of nickel was carried out by experimental methods with different experimental parameters.The changes of surface microstructure and cross-sectional mechanical properties of nickel under laser induced shock wave were studied and analyzed.The plastic deformation behaviors of nickel under ultra-high strain induced by laser were investigated.After the construction of monocrystal and polycrystalline nickel models,molecular dynamics simulation method was used to further reveal the internal micro structure dislocation evolution of nickel.The research's main contents and conclusions are as follows:Firstly,the mechanisms of laser-induced shock wave impacting on the surface of the material and causing the plastic deformation of the material were theoretically analyzed,and the microstructure evolution mechanism of the materials after laser shock peening were studied.Secondly,the effects of laser shock peening nickel under different laser energies and shock times were experimentally analyzed.The three-dimensional morphology of the surface and cross-sectional microhardness distribution were analyzed.It was found that the degree of surface undulation after a single laser shock was comparable.Multiple impacts caused severe plastic deformation,resulting in larger surface roughness.At the same time,increasing the times of shocks can increase the depth of the affected layer of the material,but serveral impacts,the microhardness of the material reached a state of saturation.Thirdly,SEM technology was used to analyze the microscopic morphology of metallic nickel in the direction of the impact cross section.It can be seen that the surface of the metallic nickel underwent obvious plastic deformation after laser shock peening.Grain distortion had occurred.Compared to the grains in the unshocked region,the grains in the shocked region were relatively small,and the grains were refined after laser shock peening.After multiple laser shocks,the degree of plastic deformation of the material was increased which would promote the formation of new grain boundaries and grain refinement.Fourthly,molecular dynamics simulation method was used to investigate the plastic deformation mechanism of monocrystal and polycrystalline nickel under different shock rates.Results of monocrystal nickel after shock wave propagation showed that the movement of dislocations made plastic deformation possible during the early stages of plastic deformation.And with increasing stress,dislocations can be promoted.Partial dislocations reached a saturated state which caused the formation of stacking faults.Finally,it can be found that the grain boundaries play an important role in the plastic deformation of polycrystalline nickel under shock effect.The plastic deformation roughly went through the following process: The first stage is the elastic deformation process,in which the pressure induced by the shock wave is smaller than the Hugoniot elastic limit of the polycrystalline nickel,mainly caused elastic deformation.The second stage was plastic deformation induced by grain boundaries.At this stage,atoms in the grain boundary underwent slip.With the incresement of stress,plastic deformation occurred in the grain boundary region,resulting in grain sliding and grain boundary deformation,etc.Subsequently,with the further increasement of shock pressure,a plastic deformation mechanism dominated by dislocations was caused.Partial dislocations nucleated near the grain boundaries,continuing to grow,and expanded into grains which caused grain refinement.