Research On Process Optimization Of High Nitrogen Steel By Laser Welding

The research object was the thickness of 8mm high nitrogen austenitic stainless steel,the joints were analyzed by the optical microscopy?OM?,scanning electron microscopy?SEM?,X-ray flaw detector?XFD?,X-ray stress tester?XST?and so on.The experiment was carried out the bead on plate by the single factor optimization test,the effect of process parameters such as laser power,welding speed,defocus amount and shielding gas on the macroscopic morphology,microstructure of the joint were analyzed,the microhardness and porosity of weld joints,welding stress were tested,to obtain a set of optimal combination of parameter.Research results show that with the increasing of laser power,the quality of weld face was declined,the waist of section was obvious,the weld penetration,the average hardness and the porosity of weld were increased,The mass fraction of Cr and Mn,and the number of ferrite were increased above the weld waist.With the increasing of welding speed,the quality of macroscopic morphology was reduced,the waist of weld section was not apparent,the weld penetration and width were reduced,the average hardness was increased and the porosity of weld increase first and then start to decrease,the mass fraction of Cr and Mn were decreased above the weld waist,and the ferrite grains were refined.When defocusing amount was negative,the weld penetration was larger,the waist of section was significantly,the average hardness of weld was higher,the porosity was lower.When the defocusing amount was-3mm,the quality of weld face was fine.the more precipitates generated in weld when defocusing amount was-1mm,and the defocusing distance was less than+5mm,the weld grain coarse.When the shielding gas composition were40%He and 40%Ar,the quality of weld surface was fine,the waist of section was not remarkable,the weld penetration was the biggest,meanwhile,The porosity reach the lowest.The porosity was higher when the shielding gas was N₂.With the increasing of the volume fraction of He,the average hardness of the weld increases.When the shielding gas was free of N₂,the number of ferrite was more and evenly distributed above the weld waist.Simulating the temperature field and stress field of laser welding of high nitrogen steel by SYSWELD welding simulation software.Obtaining the weld shape,the temperature cycle curve of the fusion zone and the results of the residual stress of the joint.The comparison and analysis of the actual measured data reach in good agreement.The analysis of the residual stress of the weld showed that the residual stress was positively correlated with the laser power and negatively correlated with the welding speed.The greater the absolute value of the defocus,the greater the residual stress in the weld.The optimization of process parameters such as laser power,welding speed,defocus amount,and shielding gas,as well as finite element simulation analysis,yields the optimum process parameters:laser power 3.0kW,welding speed 0.9m/min,defocusing distance-3mm,protection gas of 60%He+40%Ar.