A-TIG welding- the effects of the shielding gas

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					                     A-TIG welding –
                    the effects of the
                      shielding gas
      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.




T
         he penetration capability of the            TWI is currently carrying out a Group          an estimated 0.1 to 0.25 g/m of flux. The
         arc in TIG welding can be                Sponsored Project to develop knowledge            thin layer of flux was deposited by painting
         significantly increased by               for the use of this technology with argon         on a suspension of the flux in acetone. The
application of a flux coating containing          shielding gas.2                                   acetone evaporated leaving the flux coating
active ingredients on to the joint surface                                                          on the surface.
prior to welding. In particular, recent           Welding details                                      The welding parameters were m6dified
attention has focused on the A-TIG                Welding trials were carried out to assess         to ensure that the arc energy (2.5 kJ/mm)
welding process, developed by the E.O.            the weld penetration of the TIG welding           was similar for all of the welds without flux.
Paton Electric Welding Institute (PWI). It        process using the A-TIG flux, with a range        The arc length and welding current (20OA)
has been claimed that the A-TIG process           of argon-helium and argon-hydrogen                were held constant, with the travel speed
can achieve, in a single pass, a full             shielding gas mixtures.                           adjusted to compensate for changes in the
penetration weld in C-Mn steel up to                 The programme of work involved a series        arc voltage due to the shielding gas
12mm thickness, without using a bevel             of autogenous TIG welds in stainless steel        composition.
preparation or filler wire.1 Furthermore,         on bare (uncoated) plate and plate coated
the weldment mechanical properties and            with a grade of A-TIG flux developed              Effect of the flux on weld bead
soundness are claimed to be unaffected.           specifically for stainless steel by PWI, with     penetration
A-TIG fluxes are available for welding a          each shielding gas composition.                   There was a clear change in the weld
wide range of materials including carbon             The welds were carried out in the flat         bead profile between the bare and flux-
steel, low alloy steel and stainless steel.       position using the mechanised TIG welding         coated sections of the welds, Fig 1. For
   The A-TIG process has been exploited           process. The joint preparation was a close        welds produced at a fixed arc energy
to improve production efficiencies in a wide      fit, square edge butt. One weld was               using shielding gas mixtures containing
range of industries in the former Soviet          manufactured for each combination of flux         argon and/or helium, a significant
Union, including power generation,                and shielding gas composition. The first          increase in the depth of weld penetration
chemical and aerospace. There is a                half of the weld was made on the bare             was observed, Fig 2, along with a
significant level of interest in exploiting the   plate, to provide a reference point for the       reduction in the weld bead width at the
potential benefits of the flux outside the        comparison of flux performance. The               top face. Typically, the welds on the bare
Commonwealth of Independent States.               second half of the weld was coated with           plate were of a shallow U profile. The




FIGURE 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 shilding gas; 7A) Bare plate, A4-3%H2 shielding gas; 7C) A-TIG flux, Ar-3%H2 shielding gas.
                                                                                                            70
                  10
                                                                                                            65




                                                                                     Travel speed, mm/min
                   8
Penetration, mm




                   6                                                                                        60

                   4                                                                                        55

                   2   Without flux
                       A-TIG                                                                                50

                   0   10   20    30   40   50     60   70      80    90   100
                                                                                                            450   10   20   30   40   50 60       70    80    90   100
                                         % He in Ar
                                                                                                                                   % He in Ar
FIGURE 2. Effect of sheilding gas and flux on weld penetration.                    FIGURE 3. Influence of shielding gas composition on travel speed
                                                                                   (arc energy 2.5 kJ/mm. welding current 200A, arc length -2mm).

profile of the welds on the coated plate                     SUMMARY
tended towards a ‘peanut shell’ shape.                       For the autogenous TIG welding of
  The mechanism for the change in weld                       stainless steel:
profile and depth of weld penetration is                        • Compared to the penetration on the
not well understood.           Proposed                           bare plate, the use of A-TIG flux can                                         Paul Anderson
explanations include the effect of the flux                       change the weld profile and                                                   Paul Anderson is a
constituents on the material flows within                         significantly increase the weld                                               Senior Research
the weld pool and/or the constriction of                          penetration.                                                                  Welding Engineer in
the arc and concentration of the heat                           • The use of a shielding gas mixture                        the Arc Welding Section of the Arc,
source.                                                           containing helium offers up to 33%                        Laser and Sheet Processes Department
                                                                  increase in the joint completion rate                     at TWI. After obtaining BEng(Hons) in
Effect of the shielding gas                                       compared to argon, which could                            Materials Technology at Coventry
The travel speed which may be achieved                            reduce the fabrication cost of the j                      Polytechnic, he spent a year with British
when welding the flux-coated plate at a                           joint.                                                    Steel, at the Welsh Laboratories, Port
given arc length and arc energy, increases                      • The use of a combination of A-TIG                         Talbot. He joined TWI in 1990 and has
with the proportion of helium in the                              flux and a shielding gas mixture                          been actively involved with the
shielding gas, Fig. 3. This is due to the                         containing a proportion of hydrogen                       development of the gas-shielded arc
increase in arc voltage with the ‘hotter’                         creates porosity.                                         welding processes. His recent activities
arc plasma, which for a given arc length                     Further work is required to establish the                      include the development of shielding
requires an increase in the travel speed                     mechanism by which the flux changes                            gases for the TIG welding of duplex
to maintain a fixed arc energy. Compared                     the weld bead profile and affects the                          stainless steels and the assembly of a
to argon shielding gas, pure helium allows                   depth of penetration. TWI is currently                         prototype top face Penetration control
a 33% increase in travel speed for a given                   carrying out a Group Sponsored Project                         system.
arc energy. Although helium is more                          to develop practical welding procedures
expensive, this can result in a reduction                    for ferrous metals and to establish the
in the cost of fabricating a joint.                          effect of the flux on the mechanical
   If the efficiency in generating weld                      properties and soundness of the weld.2
metal with each of the shielding gas
mixtures was similar, it would be                            Acknowledgements
expected that, given the fixed arc energy,                   The authors would like to thank the E.O.                                           Richard
and with a similar depth to width ratio,                     Paton Electric Welding Institute for                                               Wiktorowicz
the depth of penetration would be the                        providing the flux and Air Products Plc                                          Richard Wiktorowicz
same irrespective of the composition of                      for its permission to publish this work.                                         is currently employed
the shielding gas. Whilst the limited                                                                                       as a Principal Cylinder Applications
number of data points is not statistically                   References                                                     Engineer, within the European
conclusive, it appears that there may be                     1 Yuschenko K A et al: ‘A-TIG welding                          Technology Group at Air Products plc.
a tendency for a slight increase in                          of carbon-manganese and stainless steel’.                      After obtaining a BSc(Tech)Hons in
penetration with an increase in the helium                   Proc. Conf. Welding Technology Paton                           Materials Technology from Sheffield
content of the shielding gas, although this                  Institute, Abington, October 1993.                             University in 1982, he spent 4 years
appears to be a secondary effect, behind                     2 TWI GSP No. 5663: ‘An evaluation of                          working for a Refractories company,
the flux.                                                    the A-TIG welding process’. January                            employed within the R &D Department.
   The welds produced with a combination                     1994.                                                            He joined Air Products in 1986 and
of flux and a shielding gas containing a                                                                                    has been involved in several different
proportion of hydrogen exhibited significant                                                                                research and development programmes,
levels of porosity, Fig.1. It therefore appears                                                                             ranging from the florination of plastic
that the use of a combination of flux and a                                                                                 containers through to his current activity
hydrogen-bearing shielding gas should be                                                                                    of managing gas-related projects within the
avoided.                                                                                                                    welding and cutting section.