Tungsten Electrode Erosion And Its Monitoring In High Speed TIG Welding Process Of Thin-walled Stainless Steel Pipes

Tungsten inert gas(TIG)welding is the main welding production process of thin-walled stainless steel pipes because of its advantages of stable welding process,high welding quality and low cost and so on.But in real continuous production for a long time,the tungsten electrode will be seriously eroded,which can deteriorate the welding quality of stainless steel pipes.In order to ensure the welding quality stability of stainless steel pipes,the tungsten electrode will be replaced by workers through observing the electrode erosion status according to their experiences,which is a certain lack of accuracy and timeliness.For realizing the standardization of tungsten electrode replacement in the welding production of thin-walled stainless steel pipes,the phenomenon of the tungsten electrode erosion in the welding process was studied,and the evolution of cathode tip shape and its influence on the arc shape and weld appearance were also elaborated quantitatively.Moreover,the evaluation and early warning of the electrode service life were realized based on rules of cathode erosion and visual sensing in the welding production of stainless steel pipes.Based on the circulating water-cooled copper plate,a fixed-point experiment system of tungsten electrode ignition was designed to study the process of cathode erosion at different welding parameters.The results showed that the cathode erosion can be divided into 3 types of slow erosion,shedding and expansion.While the cathode erosion would cause the tip shape to be the elliptical shape and the truncated cone shape.Different erosion modes and their corresponding tip shapes further affected the arc shape and arc stability.Slow erosion was the common mode of cathode erosion at different welding parameters,and it can cause the arc root width Ra and arc diameter Rb to be slowly increased.The shedding mode would occur in the TIG welding process with the increase of welding current or the small cone angle and diameter of the electrode,which can lead to a rapid increase of 6.3%in Ra.When the initial tip shape of the electrode was the truncated cone shape or the tip exhibited the truncated cone shape after shedding,the phenomenon of expansion would occur.And the expansion can cause the Ra and Rb to decrease by 5.7%and 2.3%,respectively.During the whole stable working process of the tungsten electrode,it was found that the cathode erosion can lead to an increase in Ra and Rb by 13.3%and 3.5%,respectively,but it had a greater influence on the Ra and less influence on the Rb.In order to study the effect of tungsten electrode erosion on arc shape and weld appearance in TIG welding production of thin-walled stainless steel pipes,a vision system to monitor the cathode erosion was built in high speed TIG welding production line of thin-walled stainless steel pipes.The results showed that in the large range of welding current,the electrode would fall off many times,and the erosion amount was large.The axial shortening length can be 0.62 mm,and there was a certain difference in the rules of the tungsten electrode erosion with that in the laboratory.At the same time,the arc shape was expanded and dragged back significantly.Compared with the initial stage of welding,the arc root width Ra and arc diameter Rb of the TIG arc were increased by 63.6%and 12.5%,respectively,when the weld was unqualified.And the ratio of Rb/Ra was decreased by 31.2%.The cathode erosion would strongly reduce the arc pressure with the increase of the degree of cathode erosion.Therefore,the back weld width was reduced or even the defect of incomplete penetration was appeared,while the width of the heat-affected zone was increased.Compared with the initial stage of welding,the widths of the top and back heat-affected zones when the weld was not fully penetrated were increased by 43.5%and 161.0%,respectively.To realize the standardization of electrode replacement and avoid the quality problems caused by the late replacement of tungsten electrodes,a visual monitoring system for electrode erosion was developed to monitor the tungsten electrode status in the TIG welding production process of stainless steel pipes.When the pipes produced were unqualified due to cathode erosion,the erosion amount was defined as the critical value.A critical erosion amount for electrode replacement was derived by determining the safety factor of cathode erosion to replace the tungsten electrode in advance.The construction of the visual monitoring system for tungsten electrode erosion can provide certain technical support for the standardization of tungsten electrode replacement in TIG welding production of stainless steel pipes.At the same.time,it has important engineering application value for reducing the cost and improving the production efficiency of thin-wall stainless steel pipes.