Study On Laser-arc Weak Coupling Welding Process Of High Nitrogen Steel

This article aims at the problems of nitrogen pore escape and the decrease of solid solution nitrogen content in the weld caused by the change of nitrogen solubility in the welding process of high nitrogen steel,a laser-arc two heat source weakly coupled welding method is proposed.The effect of heat source distance and laser defocusing amount on the stability of high nitrogen steel welding process,weld formation and microstructure characteristics,weld pore defects and nitrogen behavior are studied.Observing each sample under an optical microscope and a scanning electron microscope,it is found that the weld width increases with the increase of the heat source spacing,the remaining height of weld increases first and then decreases,and the forming quality is the best when the heat source distance D_LA=4 mm.With the continuous downward movement of the beam focus,the weld width and the residual height show a trend of continuous reduction.When the defocus F=-3mm or-4mm,the weld surface uniformity is poor and collapse and undercut occur.The weld structure of high-nitrogen steel is austenite+δ-ferrite dual phase structure.Under the light microscope,the ferrite shows black fine dendrites or strips,distributed in the light-colored matrix austenite dendrite gap.As the distance between the laser and the arc increases,the continuity of the growth of the weld dendrite becomes worse,and its number and size decrease.With the continuous downward movement of the focal plane,the number of dendrites in the weld increases obviously and the size becomes larger.Through the analysis of high-speed camera images and welding electrical signals,it is shown that when the heat source distance D_LA=4～6mm and the defocusing amount F=0～-2mm,the droplet transition mode is jet,droplet or both mixed transition mode,the current and voltage waveforms are fine and uniform,the overall fluctuation is relatively small,and the welding process is stable.When the heat source distance D_LA=7mm,8mm and the defocus F=-3mm,-4mm,the droplet transition mode will change to particle transition,and the welding electric signal waveform will appear obvious spikes and turbulence,and the welding stability will become worse,and when the heat source spacing is 7mm or 8mm,the overall shape divergence size of the composite plasma becomes larger.The pores of high-nitrogen steel welds are mainly precipitated nitrogen pores caused by the decrease of solubility when the molten pool is solidified,and process pores caused by the unstable laser keyhole.With the gradually increase of heat source spacing from 4 mm to 8mm,the porosity of weld decreases first and then increases,and when the heat source distance D_LA=5mm,the minimum is 4.47%.Compared with a defocus of 0mm,the number of pores in the weld in the negative defocus mode is significantly reduced.When the defocus F=-3mm and-4mm,the porosity is 0.35%and 0.56%,respectively.The distance between the heat sources and the amount of defocusing affect the dissociation degree of nitrogen molecules in the arc space,the volume and surface area of the composite molten pool,and then the diffusion and escape of nitrogen in the weld.The nitrogen content in the arc zone and the laser zone of the weld decreases with the increase of the heat source distance,the arc zone decreases from 0.45%to 0.39%,and the laser zone changes little,with the content around 0.36%.With the continuous downward movement of the beam focus,the nitrogen content in the arc zone of the weld has a small change,fluctuating around 0.44%,while the change in the laser zone is obviously increasing,up to 0.42%.