Surface Modification Process And Mechanism Of 316L Stainless Steel By Laser Shock Peening With 532nm

As a novel technology of surface modification,laser shock peening(LSP)technology uses nanosecond pulsed laser with ultra-high energy density to generate the plasma shock waves and produce plastic deformation of the material surface,forming dislocations strengthening and twin strengthening.At present,the high-energy-density laser is mainly transmitted by the hard-optical path of the mirror,which limits the application environment and operating space.As a flexible medium,optical fiber effectively reduces the loss in laser energy transmission and increases the freedom of laser processing,solving the problem of accessibility of laser,which lays a theoretical foundation for the LSP application of long-distance transmission in special environments.In this paper,nanosecond pulsed laser of low-energy was transmitted to the surface of the 316L stainless steel(ss)by an optical fiber and LSP experiments were conducted with different process parameters.The effect of LSP process parameters on the surface characteristics and microstructure of the material was investigated.To understand the microstructure evolution,the molecular dynamics(MD)was used to simulate the LSP process under different pressure conditions.Moreover,the mechanical properties and corrosion resistance of the LSPed samples were evaluated and the performance strengthening mechanism was analysed.After LSP treatment,the surface roughness of the 316L ss increased,and there were deformation twins,dislocation plugging and stacking faults generated in the grains.As the energy improved,the surface roughness increased and high-energy impacts promoted the microstructure evolution.The impact times caused a minor effect on the roughness,and the increase of pulse density led to a remarkable decrease in the surface roughness.The influence of process parameters on roughness is the same as that of on surface energy.The main factor affecting surface energy was roughness.As the impact times and pulse density increased,the parallel deformation twins turned into thick multidirectional twins with high density.The dislocations entangled to form dislocation walls and dislocation cells,promoting the dislocation strengthening and twinning strengthen.The laser energy was the dominant factor affecting the microstructure evolution.The shock wave propagation and microstructure evolution in 316L ss during LSP process were simulated by molecular dynamics.When the shockwave pressure was lower than the Hugoniot elastic limit(HEL)of the material,it was an elastic wave,which had no effect on the microstructure;on the contrary,there were elastic wave and plastic wave and the plastic wave was dominant.During the plastic deformation,the atomic arrangement formed stacking faults.Grain boundaries could lower the HEL of the material.Plastic deformation caused the dislocation nucleation and dislocation slippage,and Shockley dislocations multiplied and entangled to increase the deformation resistance.As the shockwave pressure increased,the single slip was converted into multiple slips,which promoted the plastic deformation and grain sliding dominated by dislocations in the grain boundary area,forming dislocation loops,stair-rod dislocations and Hirth blockages.And the grains were deformed by twinning,which effectively improved the strength of austenitic ss.LSP could improve the hardness,strength and wear resistance of the material.The increase of laser energy promoted the oxidative wear to change into abrasive wear;the increase of impact times caused the hardening depth and surface hardness to increase first and then decrease.After 3 LSPs,the strengthening effect was the most obvious,surface hardness increased by 30 HV with hardening depth reaching 700?m,and the tensile property was improved by 16.1%.Increasing the pulse density markedly expanded the hardened depth,and improved the wear resistance the most.Furthermore,LSP could reduce the corrosion tendency and improve the corrosion resistance of materials.The improvement of the corrosion resistance of the original surface was lower than that of the polished surface.As the pulse density increased,the corrosion resistance of 316L ss increased first and then stabilized,and the number of pitting pits decreased.The proliferation of the dislocation and deformation twin accelerated the formation of passive film,and enhanced corrosion resistance of austenite ss in NaCl solution.