Numerical Simulation Of Arc Characteristics And Molten Pool Behaviors In Narrow Gap TIG Welding Of 316LN Stainless Steel

316LN Stainless Steel is a kind of structural material used widely in nuclear field.Using narrow gap TIG to manufacture thick-walled products of it can achieve high efficiency while avoid problems such as large welding deformation and poor corrosion resistance of weld beads.However,welding parameters of narrow gap TIG are complex.It also has strong sensibility to poor fusion of side walls as well as lack of inter layer fusion while repairing of them is rather difficult.To make sure welding quality,reasonable configuration of parameters is needed.Numerical simulations were conducted in this study to investigate the arc characteristics and molten pool behaviors during narrow gap TIG welding of 316 LN stainless steel,aiming to know fundamental physical phenomenon in this procedure,and provide theoretical foundation for designing of welding parameters.At first,TIG arc characteristics during butt welding and narrow gap welding were simulated separately to investigate the influence of narrow gap groove itself.In the narrow gap groove,flow direction of protecting gas changes,resulting in a compression to arc plasma.So,the high temperature region of TIG arc get larger than its counterpart in butt welding.The convection between plasma and anode wall gets more adequately,leading to an increase of arc efficiency.The Axial velocity of arc plasma increases,lead to 50% promotion of arc pressure on anode surface.The shape of groove bottom brings about changes of current line direction,so current density of anode surface deviates from gauss distribution.Combining the characteristics of narrow gap TIG process,then the influences of spatial position,shape of anode surface,and welding parameters on arc characteristics were investigated to get better understanding of arc characteristics in narrow gap.It is found that when arc moves towards side wall,high temperature region of arc moves along the same direction and gets larger,while low temperature region moves towards the other side.The efficiency of arc decreases.Near anode surface,arc plasma moves parallel to groove,leading to decrease of pressure,and lateral concentration of electrical channel.When curvature of anode surface decreases,the arc root shrinks and efficiency decreases,while peak value of arc pressure remains the same level.As for the current density on the anode surface,the peak value of it increases,but mutation happens at the edge of electrical channel is crippled.When welding current gets higher,high temperature region of arc gets larger,and pressure of arc plasma is elevated.The peak value of current density also increases with the distribution range of it enlarging.When arc length increases,high temperature region and peak temperature of arc are not affected.But the temperature gradient and axial velocity gradient along the axis decrease.The arc pressure on the anode surface also reduces.Finally,based on an understanding of arc characteristics in narrow gap,numerical simulations of molten pool behavior were conducted.Dynamic surface heat source and arc pressure model were used,and correction of electromagnetic forces using coordinates mapping was conducted to improve the modeling accuracy.The bridge transfer during TIG welding with wire filling was simulated tentatively.The molten pool behaviors during backing TIG welding with or without wire filling were investigated,influence of welding procedure on fluid flow was studied.It is found that under high welding current,the main driving force of molten pool is electromagnetic force.Two inward vortexes form in the cross section,while a single clockwise vortex forms in the longitudinal section.During the formation of molten pool,dynamic evolution of vortex is the main factor.During the backing TIG welding with wire feeding,filler metal transfer leads to acceleration of fluid flow,suppression of inward vortex in cross section,and intensify of vortex in longitudinal section.After filler metal entering the molten pool,the clockwise vortex brings it to the bottom,and then move towards the back side.When the power of heat source increases,fluid flow rate increases,the range of vortex enlarge while moving towards arc center in the cross section.As for the longitudinal section,the clockwise vortex moves backwards.The region in which liquid metal under arc center moves downwards gets larger.With the welding speed increasing,fluid flow in molten pool slows down,vortex shrinks and moves towards surface.In the longitudinal section,flow direction of liquid metal changes from left frontward to right backward.