| The tungsten inert gas(TIG)welding is widely used in the modern manufacturing industry,especially in the production of thin-walled stainless steel pipes,because of its advantages of stable welding process,high quality and low cost and so on.The continuous demands of modern manufacturing industries for high efficiency and low cost require TIG welding to improve production efficiency,therefore,based on the thermo-mechanical control of molten pool by assistant TIG arc,a high-speed tandem TIG arc welding process was developed to realize the high efficient production of thin-walled stainless steel pipes in 2015.However,the thermo-mechanical control effect of assistant TIG arc and its suppression mechanism of weld appearance defects are lack of quantitative analysis.Therefore,it is of great theoretical and engineering significance to quantitatively reveal the thermo-mechanical effect of assistant TIG arc on the molten pool by exploring the heat and mass transfer behavior in the molten pool in high speed tandem TIG welding processAccording to the physical characteristics that the deformation of surface of the molten pool,and which results in the variation of the thermo-mechanical distribution of the TIG arc in high-speed TIG welding,and the differences caused by the different parameters of the two electrodes were considered,the arc heat flux and arc forces models of the main and assistant TIG arcs were founded,respectively,to describe the reasonable distribution of the arc heat and arc force of TIG arcs during high speed welding process,and the heat and mass transfer behaviors of the liquid metal in the molten pool were simulated and anlyzed.At the same time,the reliabilty and accurracy of the founded models were verified by weld shape and temperature in HAZ obtained from experiments.To explore the suppression mechanism of weld appearance defects during tandem TIG welding,the variation of heat and mass transfer behavior of molten pool liquid metal with different welding speed in single TIG welding was analyzed.At the welding speed of 1.5 m/min,the highest backward flow velocity in molten pool is approximately 0.41 m/s,the propotion of backward flow flux from side channel is no more than 55%,and the undercut defect begins to appear.As the welding speed greater than 2.5 m/min,the highest backward flow velocity in molten pool is more than 0.5 m/s,the propotion of backward flow flux from side channel is approximately 60%and the length of depression zone is longer than 3.6 mm,which results in the formation of humping weld defects.It suggested that under the condition that the heat input is basically kept to be constant,with the increase of welding speed,the length of the molten pool first increases and then decreases,the peak temperature decreases,both the maximum backward velocity of lateral channel and the length of the depression zone are increased.The maximum backward velocity of lateral channel and the length of the depression zone in the molten pool play important roles in the formation of undercut defect humping weld defects,respectively.In order to quantitatively reveal the defect suppression mechanism of high-speed tandem TIG arc welding,the heat and mass transfer behaviors of liquid metal in molten pool during single TIG welding and tandem TIG welding at welding speed of 3.0 m/min were calculated and compared,respectively.Compared with single TIG welding,during tandem TIG welding process,the thermal cycle is bimodal,the highest temperature is decreased by 12.4%,the existence time of liquid metal is prolonged by 70%,the length and volume of molten pool increase significantly,and the width of molten pool decreased by 21.1%.In addition,the maximum velocity of liquid metal in the molten pool is reduced by 34.6%,and the maximum backward velocity is reduced by 49.4%.The liquid metal in the molten pool has no obvious inward flow trend,and the forward backflow is significantly increased.The backward flow flux from the lateral channel is significantly reduced,the morphology of the depression zone is stable under the maintenance of the swelling region,while the length of the depression region is significantly reduced.During the experiment,it is found that the welding process parameters such as match of arc energy beween main TIG arc and assistant TIG arc,welding speed,the inclined angle of the two electrode and the space between two TIG arcs have a great influence on the heat and mass transfer behavior of liquid metal in molten pool,and then on the weld formation.Therefore,the heat and mass transfer behaviors of liquid metal in molten pool at different welding process parameters are analyzed.In high speed tandem TIG welding process,the welding parameters with high energy of main TIG arc and low energy of assistant TIG arc should be selected to obtain a stable welding process.With the increase of welding speed,the thermo-mechanical distribution of the two TIG arcs changes significantly,which leads to the weakening of the thermo-mechanical effect of the assistant TIG arc.However,as the welding speed is no higher 4.0 m/min,a stable high-speed welding process can be achieved The inclined angles of the two electrodes at 840 and 75° respectively are the best angle to ensure the stability of the welding process and make the assistant TIG arc fully play its role of thermo-mechanical effect.In addition,the minimum distance between two TIG arcs without interaction is beneficial to stable welding process.The numerical simulation results show that,in high-speed tandem TIG welding process,an appropriate electrode space,higher main TIG arc power,and the inclination angles of electrode are 84° and 75° respectively,can realize the stable and energy-saving high-speed welding at 4.0 m/min. |
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