- Original Article
- Open Access
Gas Pool Coupled Activating TIG Welding Method with Coupling Arc Electrode
© The Author(s) 2018
- Received: 19 October 2016
- Accepted: 9 November 2018
- Published: 21 November 2018
Gas pool coupled activating TIG (GPCA-TIG) welding put forward in-house can dramatically enhance weld penetration of TIG welding through introducing active element oxygen to reverse the Marangoni convection flow in the molten pool. In order to further improve the welding productivity, the normal solid tungsten electrode is replaced by a kind of coupling arc electrode. The changes of arc pressure distribution along anode surface and the weld appearance were evaluated. On this basis, the dependences of weld shape characterized with weld depth, width and undercut on the main welding parameters were discussed. The results indicate, the substitution of coupling arc electrode can lead to an obvious decrease of arc pressure. Compared to hollow tungsten electrode and twin tungsten electrodes, the coupling arc electrode is much easier to manufacture and has more compacter structure. Combined with the symmetric distribution of arc pressure in different directions, this electrode has extensive adaptability. In the GPCA-TIG welding with coupling arc electrode, both the substitution of coupling arc electrode and the introduction of outer active gas oxygen can reduce the possibilities of producing humping bead and undercut. Their joint action makes this welding method have the capability of realizing high travel speed and deep penetration welding.
- GPCA-TIG welding
- Coupling arc electrode
- Arc pressure
- Weld shape
- Welding parameter
High welding production is one of the desirable objectives to pursuit for more and more manufacturing enterprises and welding research institutes. Especially for the Tungsten Inert Gas welding, it is well-known for its high welding quality among the common welding methods due to the inert shielding gas and the non-melting tungsten electrode. Because of the low current allowable value to prevent tungsten electrode from melting and the phenomenon of arc divergence resulting from the low heat conductivity of argon shielding gas, however, low welding productivity has become its biggest shortcoming. It is believed that, improving the welding production by combination of high welding speed and deep weld depth can greatly broaden the application field of TIG welding.
Increasing welding speed must eliminate several weld forming defects, such as undercut and humping bead. It is generally accepted that the occurrences of undercut and humping bead have similar mechanism. Some theories had been proposed to reveal this reason. Mills and Keene  suggested that the presences of humping bead and undercut attribute to the Marangoni force created in molten pool, which is based on the observation that humping bead is affected by sulfur content in the weld metal. Paton et al.  believed that, there exists a pressure balance between the welding arc and the molten metal. Arc pressure increases with the welding current to the point of an imbalance in forces and the defects begin to form. A supercritical flow model was proposed by Yamamoto and Shimada . Under conditions of high current and welding speed, weld depression forms because of the high arc pressure, and the front of molten pool turns into a thin liquid layer called gouging region followed by a trailing region. Humping bead will occur when the velocity of the thin layer is beyond the critical value for a hydraulic jump. Soderstrom and Mendez  modified this model. Humping bead produces when the arc pressure extends the gouging region beyond the heat influence of the arc and premature freezing of the thin liquid film occurs. The backfilled trailing region then solidifies. To solve the weld forming problems in high speed TIG welding, depressing the arc pressure is believed a good way. Both hollow tungsten electrode  and twin tungsten electrodes [6, 7] had been used in this field. Moreover, some kinds of external magnetic fields were also applied by some researchers [8, 9] to weaken the backward drag of arc and improve the weld formation in high speed TIG welding. Qin et al.  employed an assistant TIG arc to prevent the premature solidification of thin liquid layer and the backward molten metal flow.
In order to enhance the weld penetration of TIG welding, many innovative welding processes had been proposed. CSIRO  developed a keyhole mode TIG (K-TIG) welding to realize high current TIG welding. Great efforts were also made to combine the TIG arc heat source with other energy sources, such as laser , external magnetic field , ultrasonic vibration  and another arc heat source  to reach the goal of deep depth. Besides these, introducing active elements into the arc-pool coupling system using activating flux or activating gas is believed the most effective way to improve the weld penetration , meanwhile the energy cost is greatly decreased. This kind of welding method is collectively referred to as activated TIG welding. More and more researches [17–19] revealed the increase of weld depth contributes to either the arc constriction or the reverse of Marangoni convection in the molten pool surface. Both of them can make the arc heat more effectively transferred to the bottom of molten pool.
Brushing flux on the weld surface is the common way to introduce active elements. According to the variable properties of base metal and activating fluxes, several brushing methods [20–22] were put forward to welding different metal. Among these, the method called Activating flux TIG (A-TIG) welding pre-coating flux on the whole weld surface is the most deeply investigated and widely applied in industrial fabrication. To overcome the shortcoming of restricting weld efficiency and affecting the weld quality resulting from the flux coating process above-mentioned, Huang et al. [23, 24] creatively transferred the activating flux into the arc-pool coupling system through the argon shielding gas of TIG welding.
Another way to introduce activating elements is utilizing gas containing active elements. For ferroalloys, activating elements, such as oxygen and sulfur, can decrease the surface tension of molten metal and reverse Marangoni convection flow, eventually obtain a much deeper weld depth. Accordingly, Heiple and Burgardt  added a small amount of SO2 into the argon shielding gas and obtained obviously increased weld penetration. Fujii et al. [26, 27] developed an Advanced A-TIG (AA-TIG) welding method to prevent oxidation consumption of the tungsten electrode by using a double shielding gas torch instead of the normal TIG welding torch. An assisted TIG arc with Ar+O2 or Ar+CO2 mixed shielding gas and low current was used by Fan and Huang [28–30] to form a thin oxide layer to introduce oxygen element before the common TIG welding and dramatically increase weld penetration in the end. A similar thin oxide film was produced using laser irradiation with oxygen gas by Mizutani and Katayama .
In this study, a novel variation of activated TIG welding method called Gas Pool Coupled Activating TIG (GPCA-TIG) welding with coupling arc electrode is put forward to actualize a TIG welding process simultaneously with deep penetration and high welding speed. This process is based on a specialized double layers gas shielded TIG welding method named GPCA-TIG welding  developed in-house. The inner inert shielding gas is to prevent tungsten electrode from oxidation. The outer pure/mixing gas contains oxygen and/or nitrogen elements. Oxygen is introduced to dramatically increase weld depth, while nitrogen to improve weld quality. The relative position between the outer and inner convergent nozzles, simplified as a variable h named coupling degree, can be changed to adjust the coupled area and position between the outer activating gas and molten pool surface and control the weld shape and properties. The kind of coupling arc electrode was developed in-house to decrease arc pressure by cutting the tip of normal solid clubbed tungsten into four identical parts. Through analyzing the arc pressure and the influences of welding parameter on weld appearance, the feasibility of this process is discussed in this paper.
Due to the aimed application field of large current and high speed welding, a tungsten electrode with diameter de of 5 mm and gap width Wg of 1.4 mm was selected to performance a series of bead-on-plate welding experiments on 5 mm thickness plate of SUS304 stainless steel. The basic welding specifications are current 400 A, travel speed 700 mm/min, electrode extension 4 mm, arc length 6 mm, flow rate of inner argon 13 L/min, flow rate of outer oxygen 6 L/min, coupling degree 0. Comparison of weld shapes between conventional TIG welding, TIG welding with coupling arc tungsten electrode and GPCA-TIG welding with coupling arc tungsten electrode were made. During this process, the weld shape is characterized with the weld depth D, weld width W and the undercut U. The value of undercut U is referred as the larger surface depression in the weld brim. Based on these, the influences of major welding parameters on weld shape were investigated.
It is also found in Figure 7 that the arc pressures decrease with the increases of the electrode diameter and gap width. These phenomena are similar to those in twin tungsten electrodes TIG welding [34, 35].
4.1 Comparison with Different Welding Process
4.2 Effects of Welding Parameters
The distributions of arc pressure in different directions were evaluated. The results indicate the welding torch developed in-house has good structure symmetry and the GPCA-TIG arc can be used to weld in any direction.
When a normal solid tungsten electrode is substituted by a coupling arc electrode, the arc pressure produced is decreased dramatically. With the increases of the electrode diameter and gap width, the arc pressure decreases.
In the GPCA-TIG welding method with coupling arc electrode, both the substitution of coupling arc electrode and the introduction of outer oxygen can reduce the possibilities of producing humping bead and undercut. This process has the capable of realizing high travel speed and deep penetration welding.
The effects of main welding parameters on weld shape were evaluated. Within the experimental range, travel speed compared with other specifications exhibits much more obvious influence on weld depth and width. Concerning the weld undercut, travel speed, oxygen flowrate and coupling degree present complicated and prominent influences, while arc length and electrode extension have weaken straight-forward effects.
YH was in charge of the whole trial; YH wrote the manuscript; R-LL and Y-ZH assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.
Yong Huang, born in 1972, is currently a professor at State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, China. He received his PhD degree from Lanzhou University of Technology, China, in 2007. His research interests include advanced welding technology, welding physics and welding numerical analysis.
Rui-Lin Liu, born in 1987, is currently a teaching assistant at Chengdu Aeronautic Polytechnic, China. She received her master degree on material processing engineering from Lanzhou University of Technology, China, in 2012.
Yan-Zhao Hao, born in 1983, is currently an assistant engineer at Technology Center of Great Wall Motor Company Ltd, China. He received his master degree on material processing engineering from Lanzhou University of Technology, China, in 2013.
The authors declare that they have no competing interests.
Supported by National Natural Science Foundation of China (Grant No. 51265029).
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