05 influence of shielding gas composition on arc properties

Influence of Shielding Gas Composition on Arc Properties

in TIG Welding Process

Manabu Tanaka1, a, Toyoyuki Satoh2,b and Anthony B. Murphy3,c 1JWRI, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

2Taiyo Nippon Sanso Corp., 3054-3 Shimokurosawa, Takane-cho, Hokuto-shi,

Yamanashi 408-0015, Japan

3Materials Sci. & Eng., CSIRO, PO Box 218, Lindfield, NSW 2070, Australia

[email protected], [email protected], [email protected]

Abstract. 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 addition 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, Modeling, Thermal plasma

Introduction

The properties of a TIG welding arc, and of the welds performed using the arc, are greatly

influenced by the choice of shielding gas. While argon remains the most widely-used gas, because

of its inertness and relatively low cost, the heat flux to the anode is low in argon arcs. For this

reason, helium and mixtures of argon with helium, hydrogen and nitrogen are also used. Recently,

the use of a CO2-shielded arc was proposed [1].

We have developed a two-dimensional computational model of TIG welding, including the

electrodes. In particular, melting and fluid flows in the work-piece (the anode in TIG welding) are

taken into account, allowing the weld depth to be calculated [2].

We also have developed a computer code and database that allow calculation of the

thermodynamic properties and transport coefficients of all the standard welding gases, and any

mixtures of these gases.

In this paper, we use these capabilities to investigate the influence of the shielding gas

composition on the properties of the welding arc, including arc voltage, temperature and flow fields,

and heat flux density, shear stress and temperature at the work-piece or anode. We compare these

properties for arcs in argon, helium, nitrogen, hydrogen, and mixtures of argon with hydrogen,

helium, nitrogen. We investigate which thermodynamic properties and transport coefficients are

responsible for changes in arc and work-piece parameters for the different gases.

Methods

The composition of the plasma was calculated assuming local thermodynamic equilibrium (LTE)

using the method of minimization of Gibbs free energy. Thermodynamic properties were calculated

using standard methods, and the transport coefficients were calculated using the Chapman–Enskog

method of solution of the Boltzmann equation [3-6].

The two-dimensional arc model is described in [2]. Conservation equations of mass, momentum,

energy and charge are solved by a finite-difference method over a domain that includes the cathode,

arc and work-piece. In all cases, the following parameters were used: an arc current of 150 A, an arc

length of 5 mm, a 3.2 mm diameter thoriated tungsten cathode with a 60° included angle, a water-

cooled copper anode, a gas flow rate of 10 L/min, and atmospheric pressure

Thermodynamic and transport properties

Figure 1 shows the specific heat,

viscosity, thermal conductivity and electrical

conductivity of argon, helium, hydrogen and

nitrogen plasmas. Argon has the lowest

specific heat and thermal conductivity of all

the gases and gas mixtures. All the gases

except helium have similar electrical

conductivities; the electrical conductivity of

helium is lower. Argon and nitrogen have

similar viscosities; that of helium is greater,

and that of hydrogen is lower.

Mixtures of argon and the other gases

have properties intermediate to those of the

pure gases [3-6].

Arc properties

Figure 2 shows the calculated arc and

anode temperatures and flow velocities of

arcs in argon, helium and hydrogen. It can be

seen that the arc temperature and flow

velocity, the arc voltage and the anode

temperature are all larger for helium and

particularly hydrogen arcs. The properties of

an arc in nitrogen (not shown) are

intermediate between those of arcs in argon

and helium, with an arc voltage of 19.8 V, a

maximum arc temperature and velocity of

25 000 K and 690 m/s respectively, and a

maximum anode temperature of 1200 K.

The heat flux density to the anode for arcs in all four gases is shown in Fig. 3. This is again

much lower for argon than the other gases.

The shear stress at the anode surface is shown in Fig. 4. The shear stress for helium is lower, but

all the other gases have a higher shear stress, owing to the higher flow velocities, whose origin is

discussed in Section 5. In the case of hydrogen, its low viscosity also contributes.

As might be expected, the properties of arcs in mixtures of argon and the other gases are almost

always intermediate to those of the pure gases. The only exception is the shear stress at the anode of

argon–helium mixtures; addition of up to about 50 mol% helium to argon increases the shear stress,

while further addition of helium decreases the shear stress to below that of argon.

The dependence of the arc voltage and anode surface temperature on axis on the composition of

arcs in mixtures of argon with hydrogen, helium and nitrogen is shown in Fig. 5. Addition of only a

few mole percent of hydrogen in particular can greatly influence the anode surface temperature.

Demixing, which is neglected in these calculations, will lead to even larger increases in heat flux to

the anode and therefore the anode surface temperature in argon–hydrogen arcs [7].

Discussion

The results given in Section 4 clearly show that the arc properties are strongly dependent on the

properties of the shielding gas, in agreement with published experimental results, e.g., [1,2]. It is

clear, for example, that there is an increase in the arc voltage, the maximum arc temperature, the

102

103

104

105

106

0 5000 10000 15000 20000 25000

10-1

100

101

102

103

104

Ar

He

H2

N2

Temperature (K)

Electrical cond. (S/m)

0.01

0.1

1

10

100

Thermal cond. (W/m/K)

10-5

10-4

10-3

Viscosity (kg/m/s) Specific heat (J/kg/K)

Fig. 1 Specific heat, viscosity, thermal conductivity and

electrical conductivity of argon, helium, hydrogen and

nitrogen at atmospheric pressure.

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

most responsible for the changes in arc behavior.

In an effort to examine this question, we modeled arcs with ‘artificial’ thermodynamic and

transport properties. In each case, we used all but one of the properties of argon, and replaced the

0

2

4

6

8

10

12

-2

-4

-6Axial distance (mm)

Max. 217 m/s

Ar 150 A, 10.8 V

Water cooled Cu

0

2

4

6

8

10

12

-2

-4

-6

3000 K, Interval 2000 K

17000 K

3300 K



600 K

0

2

4

6

8

10

12

-2

-4

-6

Axial distance (mm)

Max. 298 m/s

He 150 A, 19.9 V

Water cooled Cu

0

2

4

6

8

10

12

-2

-4

-6


19000 K

3500 K



1000 K

246810

Radial distance (mm)

0

2

4

6

8

10

12

-2

-4

-6

Axial distance (mm)

Max. 4332 m/s

0 2 4 6 8 10

H2

150 A, 35.5 V

Water cooled Cu

0

2

4

6

8

10

12

-2

-4

-6



27000 K

3500 K



2500 K

300 K

1350 K

Max. 55 cm/s

Fig. 2 Temperature and velocity fields for arcs in argon,

helium and hydrogen

Arc current: 150 A Arc gap: 5 mm Cathode: 2%ThO2-W

Anode: Water cooled Cu

H2 Temp. N2 Temp.

H2 Volt.

N2 Volt.

5

10

15

20

600

800

1000

1200

1400

0 20 40 60 80 100

Arc voltage (V)

Anode surface temperature at axis (K)

H2/He/N

2 content in Ar (%)

He Volt.

He Temp.

Fig. 5 Dependence of arc voltage and on-axis anode

surface temperature on the mole percentage of hydrogen,

helium or nitrogen in the argon plasma gas.

Ar (1230 W)


Anode: Water cooled Cu

H2 (5042 W)

He (2594 W)

N2 (2643 W)

0

1 104

2 104

3 104

4 104

5 104

0 2 4 6 8 10

Heat intensity (W/cm2)

Radius (mm)

Fig. 3 Radial distribution of the heat flux density to the

work-piece in different welding gases. The total heat

flux is also given.

.

0

20

40

60

80

100

120

140

0 2 4 6 8 10

Shear stress at anode surface (Pa)

Radius (mm)

Ar


Anode: Water cooled CuH2

He

N2

Fig. 4 Radial distribution of the shear stress at the anode

surface in different welding gases..

other property (either the specific heat, the

thermal conductivity, or the electrical

conductivity) by the equivalent property of

helium. Like all the other gases considered,

helium has a higher specific heat and higher

thermal conductivity than argon. It also has a

lower electrical conductivity than argon, while

the other gases have similar electrical

conductivities to argon (see 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. 6.

We 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 previously (e.g., [7])

that increasing the thermal conductivity will

increase the current density J near the arc axis,

and hence increase arc constriction.

The mechanism by which increasing the

specific heat constricts the arc has been

discussed by Tanaka and Lowke [2]. 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 current I , 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 AvhIV ρ= , where ρ , h

and v are respectively mass density, enthalpy

and velocity averaged over the arc cross-

sectional area A . For the same values of I and

V , an increase in the value of the enthalpy (or

equivalently the specific heat pc , since

pc dh dT= ) will lead to a decrease in A , and

hence a more constricted arc. This effect was

termed the thermal pinch effect [2].

A reduction in electrical conductivity leads to

greater arc constriction, since it restricts the area

through which current flows, thereby increasing J near the arc axis. This is likely to be the most

important factor in the case of the helium arc, since hρ is similar for argon and helium, because of

the lower density of helium. For all the other gases, hρ is much larger than for argon.

Decreasing the arc cross-sectional area increases the velocity of the arc via the pinch effect

(since the pressure gradient is proportional to J × B , where B is the self-induced magnetic field

strength), and the arc temperature due to the greater ohmic heating. Together these changes increase

the heat flux to the anode and the shear stress at the anode surface.

0

2

4

6

8

10

12

-2

-4

-6

Axial distance (mm)

Max. 888 m/s

Ar (High Cp) 150 A, 21.5 V

Water cooled Cu

0

2

4

6

8

10

12

-2

-4

-6


27000 K

3500 K



1100 K

0

2

4

6

8

10

12

-2

-4

-6

Axial distance (mm)

Max. 173 m/s

Ar (High Tc) 150 A, 14.5 V

Water cooled Cu

0

2

4

6

8

10

12

-2

-4

-6


15000 K

3500 K



900 K

246810


0

2

4

6

8

10

12

-2

-4

-6

Axial distance (mm)

Max. 1219 m/s

0 2 4 6 8 10

Ar (Low Ec) 150 A, 22.0 V

Water cooled Cu

0

2

4

6

8

10

12

-2

-4

-6



27000 K

3500 K



900 K

Fig. 6 Temperature (right) and velocity (left) fields for

arcs in argon, but with (from top to bottom) specific

heat, thermal conductivity and electrical conductivity

changed to that of helium.

In the calculations presented here, a water-cooled copper anode was chosen for simplicity. In

welding, the anode will not, of course, be water-cooled. Since higher anode temperatures and higher

shear stresses at the anode will lead to a greater weld depth, we can conclude that the weld depth

will be increased by adding any of the other gases to argon.

Conclusions

We have shown that the arc temperature and velocity, the heat flux to the anode and

consequently the anode temperature, are increased by adding helium, hydrogen or helium to the

argon shielding gas. It is found that two properties of the gases are responsible for these changes in

arc parameters – the increased specific heat relative to that of argon, via the thermal pinch effect,

and in the case of helium, the lower electrical conductivity relative to that of argon. Addition of

helium up to about the 50 mol% level, and any amount of the other gases, also increases the shear

stress at the anode. All the other shielding gases and gas mixtures are expected to give a greater

weld depth than pure argon.

References

[1] M. Tanaka, S. Tashiro, M. Ushio, T. Mita, A.B. Murphy, J.J. Lowke, Vacuum 80, 1195 (2006).

[2] M. Tanaka, J.J. Lowke, J. Phys. D: Appl. Phys. 40, R1 (2007).

[3] A.B. Murphy, C.J. Arundell, Plasma Chem. Plasma Process. 14, 451 (1994).

[4] A.B. Murphy, Plasma Chem. Plasma Process. 15, 279 (1995).

[5] A.B. Murphy, IEEE Trans. Plasma Sci. 25, 809 (1997).

[6] A.B. Murphy, Plasma Chem. Plasma Process. 20, 279 (2000).

[7] A.B. Murphy, Phys. Rev. E 55, 7473 (1997).

05 influence of shielding gas composition on arc properties

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