A-TIG welding - the effects of the shielding gas

TWI Bulletin, July - August 1995

Paul Anderson was a Senior Research Welding Engineer in the Arc Welding Section of the Arc, Laser and Sheet Processes Department at TWI. After obtaining BEng(Hons) in Materials Technology at Coventry Polytechnic, he spent a year with British Steel, at the Welsh Laboratories, Port Talbot. He joined TWI in 1990 and was actively involved with the development of the gas-shielded arc welding processes. His more recent activities include the development of shielding gases for the TIG welding of duplex stainless steels and the assembly of a prototype top face penetration control system.

Richard Wiktorowicz is currently employed as a Principal Cylinder Applications Engineer, within the European Technology Group at Air Products plc. After obtaining a BSc(Tech)Hons in Materials Technology from Sheffield University in 1982, he spent 4 years working for a Refractories company, employed within the R&D Department.

He joined Air Products in 1986 and has been involved in several different research and development programmes, ranging from the fluorination of plastic containers through to his current activity of managing gas-related projects within the welding and cutting section.

It has been reported that the depth of penetration in TIG welding can be significantly increased by using an activated flux. Also, it is widely acknowledged that joint completion rates can be increased by using argon-helium or argon-hydrogen shielding gas mixtures. Paul Anderson and Richard Wiktorowicz outline the benefits of combining the flux with an appropriate shielding gas.

The penetration capability of the arc in TIG welding can be significantly increased by application of a flux coating containing active ingredients on to the joint surface prior to welding. In particular, recent attention has focused on the A-TIG welding process, developed by the E.O. Paton Electric Welding Institute (PWI). It has been claimed that the A-TIG process can achieve, in a single pass, a full penetration weld in C-Mn steel up to 12mm thickness, without using a bevel preparation or filler wire. ^[1] Furthermore, the weldment mechanical properties and soundness are claimed to be unaffected. A-TIG fluxes are available for welding a wide range of materials including carbon steel, low alloy steel and stainless steel.

The A-TIG process has been exploited to improve production efficiencies in a wide range of industries in the former Soviet Union, including power generation, chemical and aerospace. There is a significant level of interest in exploiting the potential benefits of the flux outside the Commonwealth of Independent States.

TWI is currently carrying out a Group Sponsored Project to develop knowledge for the use of this technology with argon shielding gas. ^[2]

Welding details

Welding trials were carried out to assess the weld penetration of the TIG welding process using the A-TIG flux, with a range of argon-helium and argon-hydrogen shielding gas mixtures.

The programme of work involved a series of autogenous TIG welds in stainless steel on bare (uncoated) plate and plate coated with a grade of A-TIG flux developed specifically for stainless steel by PWI, with each shielding gas composition.

The welds were carried out in the flat position using the mechanised TIG welding process. The joint preparation was a close fit, square edge butt. One weld was manufactured for each combination of flux and shielding gas composition. The first half of the weld was made on the bare plate, to provide a reference point for the comparison of flux performance. The second half of the weld was coated with an estimated 0.1 to 0.25 g/m of flux. The thin layer of flux was deposited by painting on a suspension of the flux in acetone. The acetone evaporated leaving the flux coating on the surface.

The welding parameters were modified to ensure that the arc energy (2.5 kJ/mm) was similar for all of the welds without flux. The arc length and welding current (200A) were held constant, with the travel speed adjusted to compensate for changes in the arc voltage due to the shielding gas composition.

Effect of the flux on weld bead penetration

There was a clear change in the weld bead profile between the bare and flux-coated sections of the welds, Fig.l. For welds produced at a fixed arc energy using shielding gas mixtures containing argon and/or helium, a significant increase in the depth of weld penetration was observed, Fig.2, along with a reduction in the weld bead width at the top face. Typically, the welds on the bare plate were of a shallow U profile. The profile of the welds on the coated plate tended towards a 'peanut shell' shape.

Fig. 1 Weld profiles:

1A) Bare plate, Ar shielding gas;

1C) A-TIG flux, Ar shielding gas;

4A) Bare plate, Ar-70%He shielding gas;

4C) A-TIG flux, Ar-70%He shielding gas;

7A) Bare plate, Ar-3%H ₂ shielding gas;

7C) A-TIG flux, Ar-3%H ₂ shielding gas

The mechanism for the change in weld profile and depth of weld penetration is not well understood. Proposed explanations include the effect of the flux constituents on the material flows within the weld pool and/or the constrictionof the arc and concentration of the heat source.

Fig. 2 Effect of shielding gas and flux on weld penetration

Fig. 3 Influence of shielding gas composition on travel speed (arc energy 2.5 kJ/mm, welding current 200A, arc length - 2mm)

Effect of the shielding gas

The travel speed which may be achieved when welding the flux-coated plate at a given arc length and arc energy, increases with the proportion of helium in the shielding gas, Fig.3. This is due to the increase in arc voltage with the 'hotter' arc plasma, which for a given arc length requires an increase in the travel speed to maintain a fixed arc energy. Compared to argon shielding gas, pure heliumallows a 33% increase in travel speed for a given arc energy. Although helium is more expensive, this can result in a reduction in the cost of fabricating a joint.

If the efficiency in generating weld metal with each of the shielding gas mixtures was similar, it would be expected that, given the fixed arc energy, and with a similar depth to width ratio, the depth of penetration would be thesame irrespective of the composition of the shielding gas. Whilst the limited number of data points is not statistically conclusive, it appears that there may be a tendency for a slight increase in penetration with an increase in thehelium content of the shielding gas, although this appears to be a secondary effect, behind the flux.

The welds produced with a combination of flux and a shielding gas containing a proportion of hydrogen exhibited significant levels of porosity, Fig.l. It therefore appears that the use of a combination of flux and a hydrogen-bearing shielding gas should be avoided.

Summary

For the autogenous TIG welding of stainless steel:

• Compared to the penetration on the bare plate, the use of A-TIG flux can change the weld profile and significantly increase the weld penetration.
• The use of a shielding gas mixture containing helium offers up to 33% increase in the joint completion rate compared to argon, which could reduce the fabrication cost of the joint.
• The use of a combination of A-TIG flux and a shielding gas mixture containing a proportion of hydrogen creates porosity.

Further work is required to establish the mechanism by which the flux changes the weld bead profile and affects the depth of penetration. TWI is currently carrying out a Group Sponsored Project to develop practical weldingprocedures for ferrous metals and to establish the effect of the flux on the mechanical properties and soundness of the weld. ^[2]

Acknowledgements

The authors would like to thank the E.O. Paton Electric Welding Institute for providing the flux and Air Products Plc for its permission to publish this work.

References