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Investigation of the A-TIG Mechanism and the Productivity Benefits in TIG Welding (May 2009)

   

V Kumara, Bill Lucasa, D Howsea, G Meltona, S Raghunathan  and Louriel Vilarinhob

aTWI Limited, Cambridge, United Kingdom
bFederal University of Uberlandia, Brazil

Paper presented at 15th International Conference on the Joining of Materials (JOM 15) and 6th International Conference on Education in Welding (ICEW 6) Helsingor, Denmark, 3-6 May 2009.

Abstract

Tungsten Inert Gas (TIG) welding continues to be one of the major welding processes used in the industry for high quality joints. Numerous developments have taken place in TIG welding technology over the past in power supply, shielding gas, tungsten electrode, and welding torches. Even though the developments such as addition of active elements to the electrode to increase the emissivity, the use of shielding gas mixtures, and the optimisation of the electrode-tip configuration enhanced the arc characteristics and improved the weld bead penetration profile, the productivity improvements were marginal. An increase in productivity can be achieved by increasing the penetration depth, as it helps reducing the number of welding passes. Activated TIG welding process known as A-TIG can be beneficial in this respect.

A-TIG welding process involves a method of increasing the penetration capability of the arc in TIG welding. This is achieved through the application of a thin coating of activating flux material onto the work piece surface prior to welding. The effect of flux is to constrict the arc which increases the current density at the anode root and the arc force on the weld pool. The actual mechanism by which the application of flux constricting the arc is not fully understood. Despite the productivity benefits of A-TIG welding, industry to date has been slow in exploiting this process. This is partially because the use of the flux is seen as an additional cost and its application an additional operation. Furthermore, the commercial fluxes tend to produce an inferior surface finish compared to conventional TIG welding and produce a surface slag residue which is required to be removed. TWI has developed a new activating flux mitigating many of these disadvantages.

In this work, some of the proposed mechanism for A-TIG welding has been discussed and suggestions are made based on experimental results. This work also explores the A-TIG welding process employing TWI flux for improving the weld penetration depth and productivity for stainless steel tube materials. It was found that the consistency in quality, reduced need for edge preparation, reduced distortion and the improved productivity could make the A-TIG welding process more attractive than the conventional TIG process in tube welding.

1. Introduction

The A-TIG welding process involves a method of increasing the penetration capability of the arc in TIG welding.[1] This is achieved through the application of a thin coating of activating flux material onto the joint surface prior to welding. The effect of flux is believed to constrict the arc which increases the current density at the anode and the arc force action on the weld pool. The constricted appearance of the A-TIG arc is compared with the characteristic diffuse appearance of the conventional TIG arc in Figure 1.

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Fig. 1. Comparison of conventional TIG and A-TIG welding arc

The use of activating fluxes for TIG welding was first reported by the EO Paton Institute of Electric Welding in the former Soviet Union in the 1950s. More recently activating fluxes have become commercially available from several sources. These fluxes claim to be suitable for the welding of a range of materials, including C-Mn steel, Cr-Mo steels, stainless steels and nickel-based alloys. The fluxes are generally available in the form of either an aerosol or as a paste (powdered flux mixed with a suitable solvent) which is applied onto the surface with a brush. The activating fluxes can be applied in both manual and mechanised welding, although it is more difficult to control in the former mode of operation.

The specific advantages claimed for the A-TIG process compared with conventional TIG include:

  • Increased productivity due to greater depth of penetration, i.e., up to 8 mm in stainless steel compared to 3mm for conventional TIG welding. Increased productivity is derived through a reduction in welding time and/or a reduction in the number of welding passes.
  • Reduced distortion, ie, the use of a square edge closed butt joint preparation reduces weld shrinkage compared with a conventional multipass V butt joint.
  • Problems of inconsistent weld penetration associated with cast-to-cast material variations can be eliminated. For example, deep penetration welds can be made in low sulphur stainless steel (~0.002%), which would otherwise show a shallow, wide weld bead in conventional TIG welding.

Despite the productivity benefits of A-TIG welding, industry to date has been slow to exploit the process. This is because the use of the flux is seen as an additional cost and its application an additional operation. Furthermore, the commercial fluxes tend to produce an inferior surface finish compared to conventional TIG welding and produce a surface slag residue, which is required to be removed. In order to mitigate these disadvantages, TWI has developed a new activating flux with the following characteristics which have been demonstrated in several experiments:

  • It comprises a relatively simple, readily available flux ingredient.
  • The flux ingredient is non-toxic. It contains no halides or fluorides.
  • Flux performance, including depth of weld penetration in stainless steel, is similar to alternative commercial fluxes.
  • It produces a satisfactory weld deposit surface appearance with minimal slag residue.

In this work, some of the proposed mechanism for A-TIG welding has been discussed and suggestions are made based on experimental results. This work also explores the A-TIG welding process employing TWI flux for improving the penetration depth and productivity in welding stainless steel tube materials. It was found that the consistency in quality, reduced need for edge preparation, reduced distortion and the improved productivity could make A-TIG welding process more attractive than the conventional TIG process in tube welding.

2. Proposed mechanisms of A-TIG welding

The first published reference[2] which relates to activated fluxes for TIG welding dates back to 1965 and refers to the improvement in the penetration of titanium alloys using an oxygen free activated flux. These fluxes were made up of fluorides and chlorides of alkali and alkali earth metals. The effect on the physical characteristics of the process, such as an increase in the welding voltage associated with the flux was noticed. The first paper which refers[3] to a similar effect for steels was published in 1968 and used fluxes which give off vapours of fluorides, chlorides, oxygen compounds and other elements. It was claimed that the ability of the flux to wet the surface of the molten pool had an effect on the suitability and that the composition could be altered to modify the surface tension and produce better wetting.[4] However, it was not entirely clear as to how this wetting of the surface contributes to the effectiveness of the flux.

The wetting of the surface of the weld and altered surface tension of the flux was thought to explain variable penetration in welds made without arcs such as laser and electron beam welds as well as TIG welds. The change in fluid flow is related the Thermal Coefficient of Surface Tension (TCST) of the molten pool. If the TCST is negative, the cooler peripheral regions of the pool will have a higher surface tension than the centre of the weld pool and the flow will be outwards creating a wide shallow weld pool. In materials with a positive gradient, this flow is reversed to the centre of the weld pool and in the centre the molten material flows down.[5] This creates a narrower deeper weld pool for exactly the same welding conditions.

It is known that for stainless steels, sulphur can change the TCST and hence alter the penetration depth of the resulting weld. Other elements such as calcium or aluminium, also affect penetration, although these elements probably tie up the elements affecting TCST, such as sulphur, rather than having a primary affect themselves. It is believed that the activating flux had a similar effect on the weld pool.

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Fig. 2. Distribution of halogen particles in various stages at different zones of the arc as suggested by Simonik. [6]

There are other mechanisms proposed for altered weld bead penetration. One such theory is based on the absorption of electron by the flux constituent forming negative irons and is proposed by Simonik.[6] His observation was based on experiments on titanium with fluxes made up of calcium, and aluminium fluorides. He suggested that the halogen entering the arc will be discharged either as neutral atom, as molecules, and as positive or negative ions. Along the axis of arc discharge, where high energy electrons predominate, the most probable state is a neutral atomic state with an extremely small quantity of positively charged ions (the ionisation potential, U, of fluorine, chlorine and bromine is 16.9, 13.0 and 11.48 eV respectively). Because of the high value of U, there is hardly any ionisation of the halogen atoms, and therefore in the axial zone of the column they do not have an influence on the arc temperature, since all the processes are determined by elements of lower ionisation potential.

The theory also states that the probability of the formation of negative ions by the addition of an electron to an atom is very low in view of the small effective cross section of the atom, and it need not be considered in practice. Negative ions form as a result of the capture of an electron by the molecule of a halogen, whose effective cross-section is larger than the cross-section of an atom. Consequently negative ions can appear only at or below the temperature of molecule formation of the halogen, which in the present case should coincide with the peripheral regions of the arc column. Reduction in the number of electrons at the periphery (the main charge carriers) lowers the conductivity of the periphery of the arc column, and this leads to a constriction of the discharge. A diagram of the state of the particles in different zones of the arc is reproduced in Figure 3. This model does not agree with current theory in which the arc comprised of a central ionised (plasma) column rather than neutral atoms.

Based on this theory for arc constriction, Ostrovskii carried out further work and proposed a theory for the increased penetration based on Lorentz force.[7] This claims that the axial component of the electromagnetic force forms as a result in the cross section of the conductor. The electromagnetic force will depend upon the welding current and on the extent of widening of the current lines. Because the radius of the anode in welding with the activated fluxes is smaller than conventional arc welding, it should be expected that the axial component of the electromagnetic force, forming a directional flow of liquid metal from the surface to the bottom of the weld pool increasing penetration.

Lucas and Howse[1] have applied Simonik's principle of electron absorption to account for the observed constriction of the arc and the increased weld pool penetration. The heat required to form the weld pool is principally derived from the transfer of the kinetic energy of the electrons as they are absorbed into the surface. The amount of heat produced at the surface is determined by the energy acquired on accelerating across the anode drop and their heat of condensation. Constriction of the arc will increase the temperature at the anode because of the increase in current density and the higher arc voltage. It is considered that the vaporised flux will constrict the arc by capturing electrons in the outer regions of the arc in a similar manner to that proposed by Simonik. Electron absorption is affected by the attachment of electrons to vaporised molecules and dissociated atoms to form negatively charged particles. Electron attachment can only take place in the cooler peripheral regions where the electrons have low energy in a weak electric field. Towards the centre of the arc where there is a strong electric field, high temperatures and very high energy electrons, ionisation will dominate. Thus, restricting current flow to the central region of the arc will increase the current density in the plasma and at the anode resulting in a narrower arc and a deeper weld pool.

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Fig. 3. Mechanism proposed by Lucas and Howse for the A-TIG process [1]

The proposed mechanism is supported by the observed relative effectiveness of the flux constituents by Simonik. For example, arc constriction will be promoted by flux constituents whose molecules or atoms have a large electron attachment cross section. Thus, halogen compounds which have a large electron attachment cross section when disassociated will have a strong affinity for electrons. Other compounds, such as metal oxides, which have a lower electron attachment diameter but a higher dissociation temperature, are equally effective in constricting the arc as they can provide a greater number of vaporised molecules and atoms in the outer regions of the arc.

The most plausible theory is that the arc or plasma is constricted by the action of the fluxes and that the associated increase in current density results in increased arc forces which alter the molten pool flow to give increased penetration.

3. Objectives

Objectives of this investigation were to have a greater understanding on the A-TIG welding mechanism and the benefits of employing A-TIG welding process in stainless steel welding. This involved measuring the temperature gradient across the weld in conventional TIG welding process and A-TIG welding process, and the identification composition of the arc medium in both the processes.

4. Experimental programme

Mechanised TIG and A-TIG welding experiments were carried out in the down hand position on 5mm thick 304L grade stainless steel plates. Experiments were also carried out by adding certain additives to the flux. In the A-TIG welding trials, TWI developed flux was applied manually on to the surface using a paint brush. The thickness of the flux coating was measured using PosiTest DFT coating thickness gauge, and ensured uniform flux coating thickness of about 40µm on the surface. The general welding conditions employed are given below:

Material: Stainless steel grade 304L
Thickness: 5 mm
I = 150 A
Arc length = 2 mm
Travel speed: 10 cm/min
Pure Argon @ 12l/min
Purge gas: argon
Electrode W+2%Th diam. 2.4 mm

In order to measure the temperature across the weld, four K-type thermocouples were attached on to the root side of the weld at a spacing of approximately 3mm with the one placed exactly on the weld centreline. The locations of the thermocouples are given in Figure 4. Autogeneous bead on plate welding trial were carried out to determine the temperature distribution across the weld. The temperature was recorded at 50Hz using a Dataq Instrument Inc, Model DI-718BX data logger. The composition of the shielding gas in the arc region was analysed using an Ocean Optics Model HR4000CG-UV-NIC high resolution optical spectrometer with the input focussed on to the centre of the arc.

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Fig. 4. Location of the thermocouples in conventional and A-TIG welding experiments

5. Results

The time-temperature graphs corresponding to conventional TIG and A-TIG welding are shown in Fig.5. The maximum temperature measured in A-TIG welding along the weld centreline was about 950°C against a maximum temperature of 600°C observed in conventional TIG welding. At a distance of about 9mm from the weld centreline, the maximum temperature measured in A-TIG welding was only about 450°C whilst in conventional TIG it was about 600°C. All the thermocouples measured almost same temperature in conventional TIG process whilst significant variations in temperature were measured in A-TIG welding.

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Fig. 5. Time-temperature graph corresponding to conventional TIG and A-TIG process. Note the erroneous reading of thermocouple -7 in conventional TIG welding

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Fig. 6. Spectral patterns corresponding to conventional TIG, A-TIG, and A-TIG with additives

The results of the mass spectroscopic analysis are given in Figure 6. The addition of flux and additives showed additional peaks in the specrum. Analysis showed an increase in intensity of alkali metals and a corresponding decrease in intensity of argon (Figure 7).

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Fig. 7. Changes in intensities of alkali metals due to the addition of flux and flux+ additives

6. Discussion

The measurement of temperature across the weld in conventional TIG welding and A-TIG welding showed significant variations in the peak temperature as well as the temperature distribution across the weld. In conventional TIG welding there was no significant variation in measured temperature across the weld up to a distance of about 9mm, whilst in the A-TIG process a gradually increasing temperature distribution from the outer region to the weld centreline was recorded. Compared to conventional TIG process, thermocouple placed at a distance of 9mm from the weld centreline line recorded much lower temperature (450°C) compared to that recorded at a corresponding location (600°C) in conventional TIG welding process. This was due to the narrow weld obtained in A-TIG welding process. The width of the weld bead in A-TIG welding was only about 6mm compared to a weld width of 9mm obtained in conventional TIG welding process. A deeper penetration in the A-TIG welding might be responsible for the sharp increase in temperature along the weld centreline. The steeper temperature gradient across the weld should have produced a negative TCST producing an outward flow of the molten metal and a shallow weld pool. However a reverse effect has been noticed as revealed by the weld width. So an effect similar to the one caused by the presence of sulphur may be present in A-TIG process. Probably the increase in electromagnetic force due to the arc constriction is so high that it could reverse the flow overcoming the effect of a negative TCST.

Spectroscopic analysis revealed a decrease in intensity of argon lines. At the same time it showed an increase in intensity of metallic irons in the presence of flux. The presence of metallic ion is further increased by the addition of potassium salt to the flux. The ionisation potential of potassium is much lower (4.34eV) than that of Argon (15.76eV). The lower ionisation potential for potassium might have assisted its preferential ionisation facilitating the conduction of the current. At the outer envelop of the arc where the temperature was sufficiently low, the ions condense to form neutral atoms increasing the resistance to the flow of current resulting in arc constriction. So the mechanism with TWI developed flux can be explained to a limited extent in the following way.

The arc results in the vaporisation of the flux applied on the target. The presence of alkali metal with substantially lower ionisation potential results in preferential ionisation of these elements. These ions conduct the current across the arc. At the outer envelop of the arc, where the temperatures are sufficiently low, the cat ions condense to form atoms increasing the resistance to the flow of current. This increase the current density along the axis increasing the electromagnetic force. The increase in electromagnetic force is sufficiently high to reverse the flow of metal overcoming the effect of surface tension temperature gradient resulting in inward flow of the metal and a narrow weld pool.

7. Case studies

The following case studies are aimed at illustrating the benefits of employing A-TIG welding for stainless steel tube welding.

Case Study-1: Experiments on stainless steel tube material of different wall thickness

Orbital welding of stainless steel tube is often difficult due to several problems including the capacity limitation with the welding head, heat build-up in the tube material, weld sagging, and the deflection of the arc from the seam at locations corresponding to the long seam joint resulting in lack of penetration. A-TIG welding process can offer solution to many of these problems. The objective of this investigation was to evaluate the performance of TWI developed A-TIG flux in orbital welding of stainless steel tubes of different wall thickness and diameter.

Orbital welding was carried out using a Swagelok M100 or Arc Machines Inc 227 welding systems depending on the diameter of the tube, together with the appropriate weld head. The flux was applied as water based paste using a painting brush. The thickness of the flux coating was about 40µm. Welding trials were carried out on tube materials of the following dimensions:

  • Seamless 304L stainless steel tube 48mm OD, 4mm WT.
  • 304L stainless steel laser seam welded tube 29mm OD, 1.6mm WT. These tube materials had previously shown a susceptibility to arc deflections during conventional TIG welding. Welding trials were carried out to determine whether the use of an activating flux eliminated this problem.
  • 304L stainless steel tube, 6mm OD 1.6mm WT. A series of welds were carried out to compare the production duty cycles possible with conventional TIG and A-TIG welding. The welds were performed as full penetration melt runs to identify the effect of heat build up in the tube material. In comparison to conventional TIG welding, all the A-TIG weld procedures showed significant reduction in arc energy together with much narrower weld bead profile (Figures8-10). This could help increase production duty cycles for applications where heat build up was a limiting factor. This was demonstrated for the thin wall stainless steel tube. Compared to conventional TIG welding, the operating duty cycle of the orbital TIG weld head could be increased by 50% as a result of the lower heat input required by the A-TIG procedure.

The flux was also effective in eliminating arc deflection when welding laser seam welded tube thus avoiding the possibility of lack of penetration defects which may occur in conventional TIG welding (Figure 11).

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Fig. 8. Transverse weld section of A-TIG and conventional TIG welds in 48mm OD, 4mmWT 304L stainless tube

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Fig. 9. Transverse weld sections of Conventional TIG and A-TIG welds in 29mm OD 1.6mm WT laser seam weld 304L tube

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Fig. 10. Transverse weld sections of A-TIG and conventional TIG welds in 6mm OD, 1.0 WT 304 L stainless tubes

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Fig. 11. Conventional TIG and A-TIG welds in 29mm OD 1.6mm WT laser seam welded 304L tube showing a deflected weld bead in the conventional TIG weld

Main conclusions were: The use of activating flux enabled autogenous orbital TIG welding equipment to be used for butt welding of 4mm thick 304L stainless steel tube; Activating flux eliminated the problem of arc deflection which could occur with orbital TIG welding of laser seam welded tube; Compared to conventional orbital TIG welding, the arc energy was reduced by 25 - 30% for A-TIG, resulting in a narrow weld profile and reduced thermal build up, as a result the operating duty cycles for the weld head could be increased by up to 50% for welding small diameter thin wall stainless steel tube.

Case Study II: Welding of thick pipes in stainless steel material

The objective of this investigation was to produce a weld penetration in excess of 5mm on AISI 316L grade stainless steel tubes of size 70mm OD X 22mm WT using square butt joint configuration keeping the welding current within 150A as limited by the capacity of the welding torch.

Experiments were carried out in the 2G position. Initial bead-on-tube welding trials were carried out by moving the tube relative to the torch at constant speed by holding and rotating it with a chuck. The torch was mounted on an Arc Voltage Controller (AVC) which accommodated any variations in torch height coming from the eccentricity of the tube rotation and hence kept the arc voltage constant. The welding power source was OTC make AccuTIG 300P AC/DC TIG welding machine, which had a maximum current capacity of 300A.

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Fig. 12. Photomacrographs of typical transverse sections of the A-TIG weld at 150A: bead-on- plate and butt welds

The flux was applied manually onto the joint by a painting brush and the thickness of the flux coating was measured with a PosiTest DFT coating thickness gauge. The measured thickness of the flux coating was in the range 37-45µm.

Initial bead-on-tube experiments with the application of flux revealed significant constriction of the arc by the flux coating. The weld sagging was only about 0.35mm. Photomacrographs of typical transverse macrosections of selected welds are given in Figure 12. There was no significant difference in penetration depths between the weld produced on tubes and the weld produced on a butt joint with the application of the flux.

The major conclusions were: Single pass weld providing a depth of penetration in excess of 5mm could be produced on 22mm thick stainless steel tubes by the A-TIG process in 2G position welding at a welding current less than 150A; Flux coating thickness in the range 37-45µm was adequate to provide sufficient arc constriction and the required minimum penetration depth; Close control of flux coating procedure and hence the coating thickness is necessary in achieving uniform weld geometry along the circumference.

Case study III: Full penetration weld in low sulphur containing stainless steel

TIG weld penetration depth in stainless steel depends on its sulphur content. Deep penetration TIG welds are extremely difficult in low sulphur containing stainless steel. Objective of this investigation was to compare the weld penetration depths produced by conventional TIG welding process and A-TIG welding process in a low sulphur containing stainless steel plate.

Welding experiments were carried out on 316L grade stainless steel with sulphur content less than 0.002% in the 2G position. A Miller Maxstar 300 DC TIG/ STICK inverter power source was used with 300A water cooled machine torch. The welding machine had settings for controlling the pre gas shielding time, the base current, the current slope-up time, the peak current, the current slope-down time and the post gas shielding time. TWI developed flux was used. The components for welding were initially de-greased with acetone. The flux was applied directly onto the weld seam manually using a two stroke brush action giving a flux covering to a width of about 10mm on either side of the seam. The thickness of the flux coating was measured with a PosiTest DFT coating thickness gauge and the thickness was in the range 35-50µm.

Photomacrograph of the transverse section of the welds obtained in conventional TIG and A-TIG are shown in Figure 13. The weld produced with A-TIG process was narrower and with little or no sagging compared to the one produced with conventional TIG welding which was shallow with significant sagging.

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 Fig. 13. Photomicrographs of transverse macro sections of weld on 316L grade steel produced at 180A welding current and 85mm/min welding speed with A-TIG and conventional TIG welding

The major conclusions of this study were: TWI developed A-TIG flux was found to be effective in constricting the arc and increasing penetration on low sulphur containing 316L grade stainless steel; Flux coating thickness in the range 37-45µm was adequate to provide sufficient arc constriction and the required minimum penetration depth; A productivity improvement of about 3 times could be achieved by A-TIG welding process employing TWI flux.

8. Conclusions

  • A-TIG welding produces a steeper temperature gradient across the weld than conventional welding.
  • Spectroscopic analysis shows a decrease in intensity of argon lines and an increase in intensity of alkali metals in the arc medium.
  • Arc constriction effect of TWI flux is related to the evaporation of the flux and its preferential ionisation.
  • Preferential ionisation of the alkali metals and its high dissociation temperature are believed to be responsible for the arc constriction.
  • Strong electromagnetic force from the constructed arc is believed to reverse the flow pattern overcoming the effect of TCST in A-TIG welding.
  • Case studies show significantly higher productivity with A-TIG welding compared to conventional TIG welding.

References

  1. Lucas W, Howse DS: 'Activating flux - increasing the performance and productivity of the TIG and plasma processes' Welding and Metal Fabrication, 1996.
  2. Gurevich SM et al: 'Improving the penetration of titanium alloys when they are welded by argon tungsten arc process' Automatic Welding, 1965, 9, pp l-4.
  3. Makara AM et al: 'High-tensile martensitic steels welded by argon tungsten arc process using flux' Automatic Welding, 7, 1968, 78-79.
  4. Voropai NM and Lebedeva: 'Physical properties of welding fluxes based on TiO, formed in melting activated wires' Automatic Welding, 3, 1989, 19-23.
  5. Heiple CR and Roper JR: 'Mechanism for minor element effect on GTA fusion zone geometry' Welding Journal, 1982, 61 (4).
  6. Simonik AG: 'The effect of contraction of the arc discharge upon the introduction of electro-negative elements' Welding Production, 1976, 3, pp 49-51.
  7. Ostrovskii OE et al: 'The effect of activating fluxes on the penetration capability of the welding arc and the energy concentration in the anode spot' Welding Production, 3, 1977, 3-4.

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