Influence of shielding gas composition on arc

properties in TIG welding

M. Tanaka*1, S. Tashiro1, T. Satoh2, A. B. Murphy3 and J. J. Lowke3
The influence of shielding gas composition on arc properties including temperature, voltage, heat
flux and shear stress at the anode and also the weld depth as indicated by the maximum
temperature of a water cooled anode is investigated. It is found that the additions of helium,
hydrogen and nitrogen to argon all increase the arc and anode temperature. For helium, this is
due to the lower electrical conductivity; in the other cases, it is due to the higher specific heat.
Keywords: Arc welding, Transport coefficients, Gas mixtures, Modelling, Thermal plasma

Introduction composition. This was calculated assuming local ther-

modynamic equilibrium (LTE) using the method of
The properties of a tungsten inert gas (TIG) welding arc, minimisation of Gibbs free energy. The required
and of the welds performed using the arc, are greatly thermodynamic data were calculated from spectroscopic
influenced by the choice of shielding gas. While argon data, for example, those tabulated by Moore,14 or taken
remains the most widely used gas owing to its inertness from the JANAF tables.15 The following species were
and relatively low cost, the heat flux to the anode is low considered: Ar, Arz, Ar2z and Ar3z for argon; N2,
in argon arcs. For this reason, helium and mixtures of N2z, N, N–, Nz, N2z and N3z for nitrogen; H2, H2z,
argon with helium, hydrogen and nitrogen are also used. H, H– and Hz for hydrogen; and He, Hez and He2z
Recently, the use of a CO2 shielded arc was proposed.1,2
for helium, as well as the electron in all cases. The
While arc welding has been modelled for many years,
composition of the plasma was determined using the
in most early work only the arc3,4 or only the weld
method of minimisation of the Gibbs free energy.16
pool5,6 was treated. Unified models of the arc and the
Thermodynamic properties such as enthalpy and
electrodes have been developed relatively recently.7–11 A
specific heat were calculated using standard methods.16
two-dimensional numerical model of TIG welding,
The transport coefficients were calculated using the
including the electrodes, has been developed. In
Chapman–Enskog method of solution of the Boltzmann
particular, melting and fluid flows in the workpiece
(the anode in TIG welding) are taken into account, equation.17,18 The viscosity was calculated to the first
allowing the weld depth to be calculated.12 The authors level of approximation (where the level of approxima-
also have developed a computer code and database that tion is the number of terms used in the finite Sonine
allow the calculation of the thermodynamic properties polynomial expansion that is the basis of the Chapman–
and transport coefficients of all the standard welding Enskog method). The thermal conductivity was written
gases, and any mixtures of these gases.13 as the sum of three components, i.e. the translational,
In the present paper, the authors use these capabilities internal and reaction thermal conductivities respectively.
to investigate the influence of the shielding gas The translational thermal conductivity, owing to the
composition on the properties of the arc in TIG welding, transport of translational energy, is divided into
including arc voltage, temperature and flow fields, and contributions from heavy species (ions, atoms and
heat flux density, shear stress and temperature at the molecules), and electrons. The former is calculated to
workpiece. They compare these properties for arcs in the second level of approximation, and the latter to the
argon, helium, nitrogen, hydrogen, and mixtures of third order, using the method of separation of heavy
argon with hydrogen, helium, and nitrogen, and species and electrons developed by Devoto.19 The
investigate which thermodynamic properties and trans- internal thermal conductivity, owing to the transport
port coefficients are responsible for changes in arc and of internal energy, is calculated to a second level of
workpiece parameters for different gases. approximation, and the reaction thermal conductivity,
owing to the release of reaction enthalpy when species
Methods recombine after diffusing from a higher to a lower
The calculation of the thermodynamic and transport temperature region, is calculated using the expressions
properties of a plasma requires knowledge of the of Butler and Brokaw.20,21 The electrical conductivity
was calculated to a third order of approximation,
1
JWRI Osaka University, Osaka, Japan
neglecting the contribution of the ions, as described by
2
Taiyo Nippon Sanso Corp., Yamanashi, Japan Devoto.19 The calculation of the transport coefficients
3
CSIRO Industrial Physics, Sydney, Australia required a determination of collision integrals for
*Corresponding author, email tanaka@jwri.osaka-u.ac.jp interactions between all the species listed above.22–25

ß 2008 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute
Received 11 August 2007; accepted 5 December 2007
DOI 10.1179/174329308X283929 Science and Technology of Welding and Joining 2008 VOL 13 NO 3 225
 Tanaka Influence of shielding gas composition on arc properties in TIG welding

1 Specific heat, thermal conductivity, electrical conductivity and viscosity of argon, helium, hydrogen and nitrogen at
atmospheric pressure

Net radiative emission coefficients were determined water cooled copper anode, a gas flowrate of
using the method of Cram,26 assuming a 1 mm 10 L min21, and atmospheric pressure. The model has
absorption length. been benchmarked against measurements of arc tem-
The two-dimensional arc model is described in perature distribution and weld depth and shape, for the
detail.12 Conservation equations of mass, momentum, case of argon shielding gas and a stainless steel work-
energy and charge were solved by the finite volume piece,12 and against measurements of weld depth in the
method of Patankar27 over a domain that includes the case of helium and carbon dioxide shielding gases.1,12
cathode, arc and anode (or workpiece). The thermo-
physical (i.e. thermodynamic, transport and radiative) Thermodynamic and transport
properties appear in the conservation equations as
follows: density in the mass conservation equation; properties
density and viscosity in the momentum conservation Figure 1 shows the specific heat, viscosity, thermal
equation; density, specific heat, enthalpy, thermal conductivity and electrical conductivity of argon,
conductivity, electrical conductivity and radiative emis- helium, hydrogen and nitrogen plasmas. Argon has the
sion coefficient in the energy conservation equation; and lowest specific heat and thermal conductivity of all the
electrical conductivity in the charge conservation equa- gases and gas mixtures. The peaks in the specific heat of
tion. It is therefore expected that at least some of these hydrogen at 3500 K and nitrogen at 7000 K correspond
properties will have a significant influence on the arc to the energy required to dissociate the respective
parameters. molecules. The peaks for argon, hydrogen and nitrogen
A key point is treatment of the anode sheath, where at y15 000 K correspond to the energy required to
the LTE diffusion approximation of Lowke and ionise the respective atoms; the peak owing to ionisation
Tanaka, which has been shown to provide results of helium does not occur until temperatures .20 000 K.
equivalent to more complicated models in argon arcs, There are corresponding peaks in the thermal conduc-
is used.28 The thermionic cathode sheath is treated by tivity; these are due to the large peaks in the reaction
the method of Lowke et al.10 The electrodes are thermal conductivity associated with the dissociation
modelled using the same conservation equations as for and ionisation reactions.
the arc, but with different material properties. In all All the gases except helium have similar electrical
cases, the following parameters were used: an arc current conductivities; the electrical conductivity of helium is
of 150 A, an arc length of 5 mm, a 3?2 mm diameter much lower since the electrical conductivity is closely
thoriated tungsten cathode with a 60u conical tip angle, a related to the electron density, and since helium ionises

Science and Technology of Welding and Joining 2008 VOL 13 NO 3 226

 Tanaka Influence of shielding gas composition on arc properties in TIG welding

2 Temperature and velocity fields for arcs in argon, helium, hydrogen and nitrogen at arc current of 150 A and arc
length of 5 mm

at a higher temperature than the other gases, as noted hydrogen and nitrogen. It can be seen that the arc
above. temperature and flow velocity, the arc voltage and the
Argon and nitrogen have similar viscosities: that of anode temperature are all larger for helium, nitrogen
helium is greater at high temperatures and that of and particularly hydrogen arcs. The hydrogen arc is very
hydrogen is lower at all temperatures. Viscosity is intense, with an arc voltage of 35?5 V, a maximum arc
proportional to the square root of the atomic mass, temperature and velocity of 27 000 K and 4332 m s21
and inversely proportional to the collision integral. The respectively, and a weld pool which appears owing to the
lower viscosity of hydrogen is due to its lower mass. The maximum anode temperature of 2500 K being larger
weak collision cross-sections for interactions between than the melting point of copper. Corresponding figures
helium atoms indicate that the collision integrals are for the argon arc are 10?8 V, 17 000 K, 217 m s21 and
lower than those for other gases; this counteracts the 600 K respectively. Values for the nitrogen and helium
influence of the low mass of helium atoms. arcs are between those of the argon and hydrogen arcs.
Mixtures of argon and the other gases have properties The heat flux density to the anode for arcs in all four
intermediate to those of the pure gases; full details are gases is shown in Fig. 3. The heat flux density is
given in Refs. 22–25. calculated as the sum of the fluxes owing to thermal
conduction and the electron flux; the latter is calculated
as jeww, where je is the electron current density and ww
Arc properties is the work function of the anode material. Other
Figure 2 shows the calculated arc and anode tempera- contributions, such as radiative flux, are negligible.
tures and flow velocities of arcs in argon, helium, Figure 3 shows that the heat flux density is much lower

Science and Technology of Welding and Joining 2008 VOL 13 NO 3 227

 Tanaka Influence of shielding gas composition on arc properties in TIG welding

3 Radial distributions of heat flux density to workpiece in

different shielding gases: total heat flux is also given 5 Dependence of arc voltage and on axis anode surface
temperature on mole percentage of hydrogen, helium
or nitrogen in argon shielding gas

for argon than for the other gases. The hydrogen arc always intermediate to those of the pure gases. The
gives a very intense heat flux density which can melt the only exception is the shear stress at the anode of argon–
copper anode, as shown in Fig. 2. helium mixtures; an addition of up to y50 mol.-%
The shear stress at the anode surface is shown in helium to argon increases the shear stress, while further
Fig. 4. Shear stress is a measure of the plasma flow addition of helium decreases the shear stress to below
parallel to the anode surface. The flow velocity in most that of argon.
of the arc is almost perpendicular to the anode surface; it The dependence of the arc voltage and anode surface
is only within y1 mm of the anode that the flow temperature on axis on the composition of arcs in
direction changes significantly. A high shear stress mixtures of argon with hydrogen, helium and nitrogen
corresponds to a large flow velocity near the arc axis is shown in Fig. 5. The addition of only a few mole
before the flow direction changes. The shear stress for per cent of hydrogen in particular can greatly influence
helium is lower than that for the other shielding gases, as the anode surface temperature. Demixing, which is
a consequence of the higher viscosity of helium. neglected in these calculations, will cause a concentra-
Viscosity is a measure of the transfer of momentum tion of hydrogen near that arc axis, leading to even
transverse to the flow, so a high viscosity promotes the larger increases in heat flux to the anode and therefore
broadening of the flow profile, and hence a lower flow the anode surface temperature in argon–hydrogen
velocity on axis approaching the anode. All the other arcs.13,29–31
gases have a higher shear stress, owing to the higher flow
velocities, whose origin is discussed in the section on Discussion
‘Discussion’. The low viscosity of hydrogen also
contributes to the high shear stress in the hydrogen arc. The results given in section on ‘Arc properties’ clearly
As might be expected, the properties of arcs in show that the arc properties are strongly dependent on
mixtures of argon and the other gases are almost the properties of the shielding gas, in agreement with
published experimental results, e.g. Refs. 1 and 12. It is
clear, for example, that there is an increase in the arc
voltage, the maximum arc temperature, the maximum
arc flow velocity, the heat flux density at the anode and
the anode surface temperature, for all gases and gas
mixtures considered, relative to pure argon. Since all the
thermodynamic and transport properties depend
strongly on the gas or gas mixture used, it is not
straightforward to determine which properties are the
most responsible for the changes in arc behaviour.
In an effort to examine this question, the authors
modelled arcs with ‘artificial’ thermodynamic and
transport properties. In each case, they used all but
one of the properties of argon, and replaced the other
properties (the specific heat, the thermal conductivity, or
the electrical conductivity) by the equivalent property of
helium. The specific heat of helium was used to 9000 K,
and that of carbon dioxide was used above .9000 K
(Fig. 6). Like all the other gases considered, helium has
4 Radial distributions of shear stress at anode surface in a higher specific heat and a higher thermal conductivity
different shielding gases than argon. It also has a lower electrical conductivity

Science and Technology of Welding and Joining 2008 VOL 13 NO 3 228

 Tanaka Influence of shielding gas composition on arc properties in TIG welding

6 Composite specific heat of helium (for T,9000 K) and

carbon dioxide (for T.9000 K) used for ‘high Cp’ ima-
ginary gas calculation shown in Fig. 7: specific heat of
argon is shown for comparison

than argon, while the other gases have similar electrical

conductivities to argon (Fig. 1). Hence this numerical
experiment allows the influence of changes in properties
of all the gases to be investigated. The results are shown
in Fig. 7.
It was found that the higher specific heat, and to a
lesser extent, the lower electrical conductivity, of helium,
clearly led to increased constriction of the arc, and a
consequent increase in the maximum arc temperature
and flow velocity. The higher thermal conductivity has
the opposite effect. This is contrary to expectations,
since it has been argued previously29 that increasing the
thermal conductivity will increase the current density J
near the arc axis, and hence increase the arc constriction.
It is postulated, on the basis of the results by the
authors, that increasing the thermal conductivity
increases the cross-sectional area through which heat
can flow, thereby leading to a broader temperature
profile.
The mechanism by which increasing the specific heat
constricts the arc has been discussed by Tanaka and
Lowke.12 They argued that at any axial position, the
total enthalpy of the plasma flowing towards the work-
piece can be approximated by the product of the arc
currentI, and the potential difference V between the
cathode and the plasma at the axial position. This
neglects the relatively small radiation and cathode
conduction losses. _Tanaka and Lowke then used
__ _ _ _
IV ~rhvA, where r, h and v are mass density, enthalpy
and velocity averaged over the arc cross-sectional area A
respectively. For the same values of I and V, an increase
in the value of the enthalpy (or equivalently the specific
heat Cp, since Cp5dh/dT) will lead to a decrease in A,
and hence a more constricted arc. This effect was termed
the thermal pinch effect.12 In comparing the high specific
heat arc shown in Fig. 6 with the argon arc shown in
Fig. 2, it is noted that the voltage, and therefore IV, has
approximately doubled. However, the specific heat, as
shown in Fig. 6, is approximately 10 times greater than 7 Temperature and velocity fields for arcs in argon, but
that of argon over most of the temperature range, which with specific heat (high Cp), thermal conductivity (high
dominates the voltage increase, so a large decrease in A Tc) and electrical conductivity (low Ec) changed to that
is expected. of helium

Science and Technology of Welding and Joining 2008 VOL 13 NO 3 229

 Tanaka Influence of shielding gas composition on arc properties in TIG welding

8 Experimental weld profiles for low sulphur (40 ppm) and high sulphur (220 ppm) stainless steel, after exposure to
150 A arcs in argon and helium for 20 s

A reduction in electrical conductivity leads to greater weld depth will be increased by adding any of the other
arc constriction, since it restricts the area through which gases to argon.
current flows, thereby increasing J near the arc axis. This Experimental weld profiles for 150 A argon and
is likely to be the most important factor in the case of the helium arcs are in agreement with this prediction.12
helium arc, since rh is similar for argon and helium, Figure 8 shows weld profiles for low sulphur stainless
because of the lower density of helium. For all the other steel, and high sulphur stainless steel after 20 s exposure
gases, rh is much larger than for argon. to argon and helium arcs. For low sulphur steel, the
Decreasing the arc cross-sectional area increases the Marangoni effect drives circulation upwards on the axis
velocity of the arc via the magnetic pinch effect (since of the weld pool, while for high sulphur steel, the
the pressure gradient is proportional to JB, where B is circulation is downwards. The weld pool is therefore
the self-induced magnetic field strength), and the arc deeper for high sulphur steel. In both cases, the depth of
temperature owing to the greater ohmic heating. the weld is significantly greater for the helium arc.
Together these changes increase the heat flux to the
anode and the shear stress at the anode surface. Note
that the velocity also plays a role in arc constriction,
__ _ Conclusions
since according to the relation IV ~rhvA introduced
above, the arc cross-sectional area A will decrease when It has been shown that the arc temperature and velocity,
_
the velocity v increases for constant arc power. the heat flux density to the anode and consequently the
In the calculations presented here, a water cooled anode temperature, are increased by adding helium,
copper anode was chosen for simplicity. In welding, hydrogen or helium to the argon shielding gas. It is
the anode will not, of course, be water cooled, and the found that two properties of the gases are mainly
molten region (the weld pool) will be present for all the responsible for these changes in arc parameters: the
shielding gases considered. The depth of the weld pool increased specific heat relative to that of argon, via the
will be influenced by the heat flux density, the shear thermal pinch effect, and in the case of helium, the lower
stress on the anode, and the Lorentz force owing to the electrical conductivity relative to that of argon. Addition
current flowing in the weld pool (as well as factors that of helium up to y50 mol.-% level, and any amount of
are independent of the shielding gas, such as weld pool the other gases, increases the shear stress at the anode.
circulation drive by the Marangoni effect). Increasing All the other shielding gases and gas mixtures are
the heat flux density will increase the weld depth, as will expected to give a greater weld depth than pure argon,
an increase in the Lorentz force in the weld pool as a since the higher heat flux density is likely to dominate.
result of a more constricted arc. A higher shear stress This has been confirmed experimentally in the case of a
will tend to decrease the weld depth by promoting flow helium arc. It is noted that, as well as the generally
parallel the surface in the weld pool; however, flow desirable effect of increasing the weld depth, addition of
driven by the Marangoni effect tends to dominate this.12 molecular gases to argon may have deleterious effects on
The heat flux density is increased by adding helium, the weld pool. These could include the promotion of
hydrogen or nitrogen to argon; the influence is reflected porosity and introduction of contaminants, and these
in the present calculations by the higher anode factors of course have to be considered in determining
temperatures. The authors therefore expect that the the optimum shielding gas composition.

Science and Technology of Welding and Joining 2008 VOL 13 NO 3 230

 Tanaka Influence of shielding gas composition on arc properties in TIG welding

15. M. W. Chase, Jr, C. A. Davies, J. R. Downey, Jr, D. J. Frurip, R.

References A. McDonald and A. N. Syverud: J. Phys. Chem. Ref. Data, 1985,
1. M. Tanaka, S. Tashiro, M. Ushio, T. Mita, A. B. Murphy and J. J. 15, (Suppl. 1), 1–186.
Lowke: Vacuum, 2006, 80, 1195–1198. 16. M. I. Boulos, P. Fauchais and E. Pfender: ‘Thermal plasmas:
2. S. Tashiro, M. Tanaka, M. Ushio, A. B. Murphy and J. J. Lowke: fundamentals and applications’; 1994, New York, Plenum.
Vacuum, 2006, 80, 1190–1194. 17. J. O. Hirschfelder, C. F. Curtiss and R. B. Bird: ‘Molecular theory
3. K. C. Hsu, K. Etemadi and E. Pfender: J. Appl. Phys., 1983, 54, of gases and liquids’, 2nd edn; 1964, New York, Wiley.
1293–1301. 18. S. Chapman and T. G. Cowling: ‘The mathematical theory of non-
4. M. Goodarzi, R. Choo and J. M. Toguri: J. Phys. D, 1997, 30D, uniform gases’, 3rd edn; 1970, Cambridge, Cambridge University
2744–2756. Press.
5. T. Zacharia, S. A. David, J. M. Vitek and T. DebRoy: Metall. 19. R. S. Devoto: Phys. Fluids, 1967, 10, 2105–2112.
Trans. B, 1990, 21B, 600–603. 20. J. N. Butler and R. S. Brokaw: J. Chem. Phys., 1957, 26, 1636–
6. R. T. C. Choo and J. Szekely: Weld. J., 1992, 71, 77s–93s. 1643.
7. P. Zhu, J. J. Lowke and R. Morrow: J. Phys. D, 1992, 25D, 1221– 21. R. S. Brokaw: J. Chem. Phys., 1960, 32, 1005–1006.
1230. 22. A. B. Murphy and C. J. Arundell: Plasma Chem. Plasma Process.,
8. L. Sansonnens, J. Haidar and J. J. Lowke: J. Phys. D, 2000, 33D, 1994, 14, 451–490.
148–157. 23. A. B. Murphy: Plasma Chem. Plasma Process., 1995, 15,
9. P. Zhu, J. J. Lowke, R. Morrow and J. Haidar: J. Phys. D, 1995, 279–307.
28D, 1369–1376. 24. A. B. Murphy: IEEE Trans. Plasma Sci., 1997, 25, 809–814.
10. J. J. Lowke, R. Morrow and J. Haidar: J. Phys. D, 1997, 30D, 1– 25. A. B. Murphy: Plasma Chem. Plasma Process., 2000, 20, 279–297.
10. 26. L. E. Cram: J. Phys. D, 1985, 18D, 401–411.
11. M. Tanaka, H. Terasaki, M. Ushio and J. J. Lowke: Metall. Mater. 27. S. V. Patankar: ‘Numerical heat transfer and fluid flow’; 1980, New
Trans. A, 2002, 33A, 2043–2052. York, McGraw-Hill.
12. M. Tanaka and J. J. Lowke: J. Phys. D, 2007, 40D, R1–R23. 28. J. J. Lowke and M. Tanaka: J. Phys. D, 2006, 39D, 3634–3643.
13. A. B. Murphy: J. Phys. D, 2001, 34D, R151–R173. 29. A. B. Murphy: Phys. Rev. E, 1997, 55E, 7473–7494.
14. C. E. Moore: ‘Atomic energy levels’, Circular no. 467, Vol. 1; 1949, 30. A. B. Murphy: J. Phys. D, 1998, 31D, 3383–3390.
Washington, DC, US National Bureau of Standards. 31. A. B. Murphy and K. Hiraoka: J. Phys. D, 2000, 33D, 2183–2188.

Science and Technology of Welding and Joining 2008 VOL 13 NO 3 231