Studies Of Mechanical Properties And Microstructural Refinement Of Austenitic Stainless Steel And Its Weldment Subjected To Laser Shock Peening

Laser shock processing(LSP)is a promising surface treatment technique with high energy,high pressure(GPa~TPa)and ultra-high strain-rate(107~108S-1).LSP can obtain high residual stress layer and high density dislocation on the top surface of metallic material,which can improve the fatigue property,abrasive resistance and stress corrosion resistance.The improvement of surface properties of the metal material subjected to LSP mainly depends on the microstructure evolution of the surface metal.In this paper,the mechanical properties and microstructure refinement of stainless steel and its welding parts subjected to laser shock peening are studied.The following four aspects are studied in this paper: association of residual stress distribution and microstructure at a laser spot of stainless steel subjected to different laser shock peening impacts,the effects of massive LSP treatment with different pulse energies on surface roughness and microstructural evolution in the surface layer of stainless steel,finite element analysis on stress and strain of laser shock peened stainless steel weldment with different pulse energies,microstructural evolution in the welding zone of laser shock peened stainless steel weldment,some significant conclusions and original achievements of this work were listed as follows:Firstly,association of residual stress distribution and microstructure at a laser spot of stainless steel subjected to different laser shock peening impacts was investigated systematically.A method for obtaining uniform surface residual stress from single point to multi point laser shock peening is studied.After triple LSP impacts,residual stress value of spot center and the affected depth reach to a peak,and compressive residual stress keeps a constant and the affected depth also reaches to a saturated value.The underlying influence mechanism of typical substructure induced by laser shock wave on residual stress distribution at a laser spot was revealed.Refined grain and ultrahigh strain rate are two important factors to generate high-level compressive residual stresses in the top surface of AISI304 stainless steel.Most of the spot of the treated surface are impacted by laser pulse at least three times when the laser impact overlapping rate is 50%.Large numbers of submicron triangular blocks are generated on the top surface of AISI304 stainless steel and microstructures become uniform.The residual stress,nano-hardness,elastic modulus are distributed uniformly on the extended surfaces.Secondly,the effects of massive LSP treatment with different pulse energies on surface roughness and microstructural evolution in the surface layer of stainless steel were further investigated systematically.Massive LSP treatment causes an increase in surface roughness of the as-received sample.Meanwhile,massive LSP treatment with 6 J pulse energy results in a relative smooth surface with a surface roughness of 0.947 ?m compared with that of 1.226 ?m from 3 J pulse energy.This phenomenon can be attributed to the fact that the 6 J pulse energy presents a relatively large effective deformation area compared with the 3 J pulse energy.Compared with 9% peak appearing at 60° misorientation angle for the as-machined sample,46% for LSPed sample with 3 J pulse energy and 40% for LSPed sample with 6 J pulse energy.Furthermore,a number of deformation twinnings of ~60°<111> were found in the surface layer of AISI304 subjected to LSP.These findings indicate that massive LSP treatment refines the coarse grains in the surface layer of AISI304 by twinning division.Accompanying the existence of MTs,?-martensitic transformation also occurs during the plastic deformation induced by laser shock wave.On the basis of the impact time at micro-region and the inherited crystal structure of austenite stainless steel,we present an understanding on the maximum of twining planes in FCC alloys with a low SFE.Furthermore,a new grain refinement mechanism based on MT–MT intersection with four directions is developed under the mechanical effect of laser shock wave.Thirdly,finite element analysis on stress and strain of laser shock peened stainless steel weldment with different pulse energies was investigated systematically.Results show that LSP can generate a compressive residual stress layer to eliminate the tensile residual stress induced by welding process.With increasing of pulse energy,the depth and value of compressive residual stress increase,but the increment rate turns smaller and smaller.The peak value of strain on the top surface of LSPed weldment increases with increasing of the laser energy.After 60 ns,the strain decreases as laser shock wave decays to be disappear.There are similar trends for three kinds of dynamic strain-time curves of the LSPed weldments with different pulse energies.Finally,microstructural evolution in the welding zone of laser shock peened stainless steel weldment was studied based on the weldment laser shock peening simulation.Based on the microstructure results,a grain refinement mechanism during massive LSP treatment in the near–surface layer of the welding zone is proposed.It involves the development of dislocation movement in original grains,the formation of dislocation tangles and dislocation walls due to the pile up of dislocation lines,the development and formation of subgrain boundaries in both vertical and horizontal directions,and the evolution from subgrain boundaries to new grain boundaries.During massive LSP treatment,the coarse grain in the near–surface layer in HAZ of the welded specimen is divided mainly by dislocation movement,while the coarse grain in base metal of the welded specimen is refined mainly by mechanical twins in multiple directions in nanometer scale.