Pressures Produced by Gas Tungsten Arcs
M. L. LIN and T. W. EAGAR
The pressure of gas tungsten welding arcs has been measured for currents from 300 to 600 amperes
using argon and helium gases. Although the measurements are generally consistent with previous
results at lower currents, the present work shows that the pressure exerted by helium is a strong
function of arc length. Several different scaling laws for the maximum pressure as a function of arc
current and electrode tip angle are discussed.
-
I. INTRODUCTION
ARC
pressure in Gas Tungsten Arc Welding (GTAW) is
caused by the momentum transfer of the impinging plasma
jet on the weld pool and has been thought by many to be a
major factor in producing puddle surface depression and
weld defects; hence, a number of investigators have studied
its origin and have measured its value.I4 The present work
has extended these measurements and suggests some scaling
laws to predict the arc pressure at even higher currents than
are practical for an experimental study.
When current flows through a conductor, it generates a
circumferential magnetic field. The interaction between the
current and this self-induced magnetic field produces a body
force, called the Lorentz force, which is toward the central
axis of the azimuthal magnetic field loop as shown in
Figure l(a). In GTAW, the conductor is an ionized shielding
gas. The constricting Lorentz force is balanced by the radial
pressure gradient in the arc acting in the opposite direction,
which can be expressed as
If the arc has a divergent current distribution as shown in
Figure l(b), the current density J near the tungsten electrode (cathode) will be higher than the current density near
the base metal (anode). Thus, the static gas pressure at the
cathode end is higher than the gas pressure at the anode end.
The difference of static gas pressure between anode and
cathode will produce a plasma jet toward the anode in the
GTAW process. The static gas pressure in both the radial
and axial directions of an arc is illustrated in Figure 2. The
stagnation pressure generated when the plasma jet is arrested
on the anode surface is called the arc pressure.
In overhead welding, arc pressure has a beneficial effect
because it can support the liquid metal above the arc torch
without dropping or sagging. At very low currents, the arc
pressure is too small and the arc may become very unstable.
High frequency current pulsing is used to increase the arc
pressure, thus stabilizing the arc.5 However, arc pressure
may also be detrimental to the quality of a weld, especially
at high currents. Weld defects such as humped beads, finger
penetration, and undercutting have been explained as the
results of high intensity arc p r e s ~ u r e . ~It, ~is' ~found that
blunt electrode tips, hollow tungsten electrodes, and increasing the amount of helium in the Ar-He shielding gas
mixture may reduce the magnitude of arc pressure in
\
.
/
M.L. LIN, Postdoctoral Associate, and T. W. EAGAR, Associate Professor, are with Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139.
Manuscript submitted August 15, 1985.
METALLURGICAL TRANSACTIONS B
Fig. 1- ( a ) Azimuthal magnetic field B generated by current density 3.
The arrows toward the center of the loop represent the direction of Lorentz
force J X 5. (b) Divergent arc.
GTAW.6'9These facts have been found to reduce the occurrence of weld defects in many cases. When a fluid jet impinges on the surface of a liquid, it may induce flow motion
in the l i q ~ i d .Thus,
' ~ ~arc
~ ~pressure
~ ~ ~ may also influence the
penetration profile and shape of the liquid-solid boundary
due to the induced liquid motion in the weld pool.
Since previous ~ t u d i e s l had
~,~~
measured arc pressure
only to 400 amperes and there was some discrepancy in
the measured values, a new study was made to extend the
range of measured values and to resolve differences among
investigators.
11. EXPERIMENTAL PROCEDURES
Figure 3 shows the apparatus used to measure the arc
pressure while Figure 4 shows the detail of the water-cooled
copper plate. Considerable effort was needed to optimize the
design of plate thickness and cooling water flow such that
the highest currents could be used. It was found that a thick
VOLUME 17B. SEPTEMBER 1986- 601
copper plate
mm
water inlet
-
water o u t l e t
+
Fig. 2-The electromagnetically induced static gas pressure in both radial
direction and axial direction. The pressure difference between cathode and
anode in GTAW generates the plasma jet.
TO pressure transducer
Fig. 4-Detail
(6 mm) copper plate permitted better radial heat dissipation
than a very thin plate. Thicker plates produce too shallow an
axial temperature gradient and hence result in surface melting. It is believed that the plate thickness used in the present
study is near the optimum for typical welding arcs. A 4 rnm
diameter 2 pet thoriated tungsten electrode was used. The
arc length was maintained at 8 mm at high currents in order
to prevent melting of the water-cooled copper plate. Three
different electrode tip angles: 30 deg, 60 deg, and 90 deg
were chosen. All tests were made with DC, electrode nega-
of water-cooled copper plate.
tive. Argon arc pressures were measured up to 600 amperes;
however, due to the high heat intensity, the maximum current was limited to 400 A with helium. The travel speed was
constant at 40 mm/min which is high enough to prevent
melting of the water-cooled copper plate and low enough
to give a good dynamic response for the arc pressure
transducer.
Each measurement of arc pressure was repeated five
times. Since a slight misalignment between the center of
9
Shielding
Gas
Water
11
1
W a t e r - c o o l e d Copper P l a t e
3
1
Pressure Transducer
11
Fig. 3 -Set-up of experimental apparatus.
602-VOLUME
17B, SEPTEMBER 1986
METALLURGICAL TRANSACTIONS B
^-s
\
tungsten cathode and that of the central hole in the watercooled copper plate can greatly reduce the magnitude of arc
pressure, the measured arc pressure data were not averaged.
Instead, the maximum value of the measured arc pressure
was taken in this experiment. However, all of the measured
data are within 10 pet of the listed arc pressure.
Silicone oil was used as the medium between the arc and
pressure transducer because of its stable thermal properties
and high dielectric constant. A 600 ampere D.C. analog
transistor regulator was used to maintain constant current
within 1 pet. The pressure transducer uses a variable capacitance sensor, which has a stainless steel diaphragm and an
insulated electrode as the variable capacitance plates.
ARC LENGTH : 8 mm
T I P ANGLE : 60'
SHIELDING GAS
: At-
111. RESULTS AND DISCUSSION
Figures 5,6, and 7 show the behavior of arc pressure with
radial distance from the arc axis with 30 deg, 60 deg, and
90 deg tip angles for currentsranging from 300 A to 600 A
in argon. It is possible to simplify this information by considering the maximum pressure or the total force exerted by
the arc. It can be seen in Figure 8 that the maximum arc
pressure increases linearly with current. The data from
Reference 13 are also given in Figure 8 for comparison.
Though the measured arc pressure data in this study for the
90 deg tip angle are lower than those from Reference 13, the
measured-datafor both 30 deg and 60 deg tip angles in this
study are higher.
As noted in the exnerimental section. there are a number
of problems encountered in averaging the measured arc
1
:
2
:
3 :
4 :
ARC
LENGTH
8
2 .a
4.0
6.0
4
mm
:
:
:
:
300A
400A
500A
60BA
ARC LENGTH I 8 rnm
T I P ANGLE : 90'
SHIELDING GAS : A r
Ar
8.0
8.0
Fig. 6-Arc pressure distribution of different currents at 60 deg tip angle
of electrode.
I
2
3
:
6.0
4.0
RADIAL DISTANCE Crnm:)
0.0
a. 0
2.0
300A
400A
500A
600A
TIP ANGLE : 30'
SHIELDING GAS :
4.0
0.0
2.0
4.0
6.0
8.0
RADIAL DISTANCE Crnrn:)
R A D I A L DISTANCE Crnrn?
Fig. 5-Arc pressure distribution of different currents at 30 deg tip angle
of electrode.
METALLURGICAL TRANSACTIONS B
Fig. 7-Arc pressure distribution of different currents at 90 deg tip angle
of electrode.
VOLUME 17B, SEPTEMBER 1986-603
strong dependence on the current. In addition, Allum's'
numerical solution has shown that the axial plasma jet velocity increases faster than
His experimental velocity
data show that the relationship is almost linear. Hence, it
would appear that Eq. [3] gives the best scaling behavior.
Combining Eqs. [2] and [3] gives:
0.0
0.0
1.0
2.0
I
1
I
I
3.0
4.0
5.0
6.0
CURRENT < A )
7.0
xia2
Fig. 8 -Maximum arc pressure vs current at different electrode tip angles.
Solid symbols represent the data from Ref. 13, and hollow symbols represent data in this study.
pressure. Yamauchi and Taka did not describe how they
chose their values of maximum arc pressure. We have chosen the highest measured values as explained previously.
The dependence of arc pressure on the current can be
explained as follows. Since the arc pressure is the stagnation
pressure of the plasma jet arrested at the anode surface, this
can be expressed as
Since there is no metal transfer in GTAW, there is no arc
pressure from the inertia of a stream of metallic drops. Two
different relationships between arc current and the velocity
of the plasma jet along the axis of arc have been expressed:
Using a current density, dynamic viscosity, and density
of argon of 7 X
A/m2,I7 2.2 X
kg/ms,18 and
0.05 kg/m3,19respectively, at 300 A, 90 deg tip angle, and
6 mm arc length, the arc pressure calculated from Eq. [5] is
0.22 kN/m2, while an arc pressure of 4.2 kN/m2 is calculated using Eq. [4] combined with Eq. [ Z ] . Since the experimental data give a value of 0.9 kN/m2 as shown in
Figure 8, the calculated arc pressure from Eq. [3] is underestimated by a factor of 4 while Eq. [4] overestimates the
pressure by a similar factor. The fundamental dependence of
arc pressure on current as predicted from Eq. 131 is more
consistent with the experimental data than the dependence
predicted from Eq. [4], hence, Eq. [3] is believed to be
more realistic than Eq. [4].
From Eq. [ 5 ] ,it is seen that arc pressure is proportional
to both the current and the current density; thus, arc pressure
depends not only on the current but also on the current
distribution. As an approximation, the current density J is
assumed to be uniform within the radius R of the arc. Thus,
the current density can be expressed by
Combining Eqs. 151 and [6] gives
Parc
=
^0I2
4^r2R2
Thus, the arc pressure depends not only on the square of the
current but also on the square of the arc radius.
To test this hypothesis, we may use the data of Tsail' who
measured the radial distribution of current in the arc. He
approximated the current density by a Gaussian relationship:
If we define the radius of the arc (R) as the distance at
which J(R) = 1 pet Jmm,then R is equal to 3ue.Tsai found
that when current increases from 100 A to 200 A using an
argon arc with a 75 deg tip
- angle and a 5.5 mm arc length,
the distribution parameter of current (q)
increases from
2.18 mm to 2.5 mm, which is approximately a 15 pet increase. From Eqs. 171 and [8], we know that arc pressure is
proportional to 12/u2.Thus, if we assume that the arc distribution parameter (u,,) increases 15 pet when current becomes twice as large, it can be estimated that when arc
current increases from 300 A to 600 A, the arc pressure
should increase by 22/(1.15)2, or about a factor of three.
Comparing this value with Figure 8, it can be seen that
the arc pressure increases 283 pet when current increases
from 300 A to 600 A with a 90 deg tip angle. However,
the increase of arc pressure is only 216 pet when the current increases from 300 A to 600 A with 30 deg and 60 deg
tip angles.
-
and
Equation [3] is given by Maeckerl' based on Bernoulli's
theorem. Equation [4]is given by Squirei6assuming a laminar jet due to a point source of momentum. Combining
Eq. [2] with Eq. [4],it can be seen that the increase of arc
pressure is proportional to the 4th order of current while
Eq. [3] combined with Eq. [2] shows that arc pressure increases with the square of arc current. Comparing these
relationships with Figure 8, Eq. [4]may be very unrealistic
because the measured arc pressure data do not show such a
604-VOLUME
17B, SEPTEMBER 1986
/"^
METALLURGICAL TRANSACTIONS B
-
A possible reason for the rapidly increasing rate of arc
pressure with current for a dull electrode is that the current
distribution parameter (we) increases slowly with increasing
current. Thus, the radius of the arc (R) does not increase by
a large amount when current increases, and Eq. [7] would
indicate a large increase of arc pressure. However, for a
sharp electrode, the current distribution parameter ue increases more rapidly with increasing current. Thus, the arc
pressure in Eq. [7] does not show as large an increase when
current increases because of the larger increase of arc radius
with the sharper electrode. As a result, a sharp electrode
gives a slower rise in arc pressure than a dull electrode when
current increases.
It is seen in Figure 8 that the extended lines for 30 deg
and 60 deg tip angles pass through the origin while the line
for 90 deg tip angle does not. The reason for this behavior
is not clear; however, it may be that wider electrode tip
angles produce jets which are not well focused along the
axis of the arc especially at low currents.
The relationship between integrated arc force and current
at different electrode tip angles is illustrated in Figure 9. It
can be seen that integrated arc force increases almost parabolically with current, especially for a 90 deg tip angle,
rather than linearly as does arc pressure. The integrated arc
force F is given by
PI
From this eauation, the dependence of arc force on the
current is derived by conve;i2~ as follows:
0.0
A
:
!3
:
0
:
1.0
3 0 T I P ANGLE
60' T I P ANGLE
913" T I P ANGLE
2.0
3.0
4.0
CURRENT CA.)
-
5.0
6.0
7.0
x I a*
Fig. 9-Integrated arc force vs current at different electrode tip angles.
The total arc force is zero at zero current; hence curves are drawn through
the origin.
METALLURGICAL TRANSACTIONS B
- CURRENT
: 300A
T I P ANGLE : 90'
ARC LENGTH : 8 mm
-----
0.0
2.0
CURRENT : 200A
T I P ANGLE : 30'
ARC LENGTH : 3 mm
4.0
6.0
8.0
R A D I A L DISTANCE Cmm)
Fig. 10-Comparison of arc pressure distribution of argon and helium.
Solid lines represent data in this study, and dotted lines represent data
from Ref. 13.
The experimental results in this study show that arc force
is a function of the square of current only, which is consistent with the fact that arc force is proportional to the square
of current as seen in Eq. [lo].
Figure 10 shows the distribution of arc pressure of both
argon and helium arcs at 300 A. The arc pressure distribution of helium is smaller and wider than that of argon. Since
the arc pressure is a function of velocity and density of
the gas, and the density of He is lower than that of Ar,
the difference of the plasma jet velocity may also play an
important role. Because the plasma jet velocity is inversely
proportional to the vis~osity,'~
it is necessary to discuss the
difference of viscosity between argon and helium.
At low temperatures, the viscosity of He is lower than that
of Ar because the Van der Waals force between He molecules is weaker than between Ar molecules. However, when
the temperature is over 12,000 K, the viscosity of He
becomes higher than that of Ar because of the ionization
effect of the gas at high temperatures." From Glickstein's
calculated d a t a , the maximum temperature of He and Ar
at 100 A is 17,000 K and 15,500 K, respectively. Based
on these values, the viscosities of the He and Ar are
2.2 x l o 4 kg/m-sec and 1.15 x l o 4 kg/m-sec, respectively." Thus, excluding the effect of shielding gas density,
the plasma jet velocity of He is only one-half of that of Ar
because He has a viscosity about two times larger than that
of Ar. Since the difference of viscosity of He and Ar at the
same welding parameters is always less than a factor of 2,"
VOLUME 17B, SEPTEMBER 1986-605
A
:
the velocity of He and Ar should not differ by more than a
factor of 2. Although it can be seen from Eq. [2] that both
the density and the velocity of the plasma are important in
determining the arc pressure, it should be noted that the
density may be more important than viscosity because the
density of Ar is about 10 times larger than that of He.
The spread of the plasma jet is proportional to r)'/p;I6
thus, the lower density and higher viscosity of He compared
to that of Ar at high temperature gives a broader distribution
of He arc pressure than that of Ar as shown in Figure 10.
Figure 11 shows the maximum arc pressure vs arc length
at 300 A for both argon and helium shielding gases. The
maximum arc pressure of argon is almost independent of the
arc length while the maximum pressure of helium decreases
with increasing arc length. Since the spreading rate of the
He plasma jet is larger than that of Ar asshown in Figure 10,
the radial momentum dissipation in helium is expected to be
greater. Therefore, the influence of arc length on the arc
pressure is stronger in a He plasma than in Ar. The arc
pressure of helium vs radial distance for different arc lengths
is shown in Figure 12.
He
TIP ANGLE : 90"
CURRENT : 300A
IV. CONCLUSIONS
2.0
0.0
4.8
8.0
6.B
10.0
ARC LENGTH C m m l
Fig. 11-Maximum arc pressure vs arc length of argon and helium. Solid
symbols represent data from Ref. 13, and hollow symbols represent data in
this study.
ARC LENGTH
ARC LENGTH
: ARC LENGTH
: ARC LENGTH
CURRENT : 300A
TIP ANGLE : 90'
SHIELDING GAS :
1
2
3
4
:
:
8 mm
6 mm
4 mm
2 mm
The magnitude of the maximum arc pressure in GTAW
increases linearly with an increase of current because the arc
pressure is a function of both the magnitude and the distribution of the current. However, the integrated arc force
increases parabolically with increasing current. The arc
pressure of helium is smaller than that of argon because
of the lower density and higher viscosity of He at high temperature as compared with Ar. The arc pressure distribution
of He is wider than Ar due to the greater radial momentum
dissipation in He.
LIST OF SYMBOLS
B
F
I
J
He
Jam
p
pa
r
R
R,
R2
v
z
-
r)
/AO
I
0.0
I
I
2.0
4.0
I
6.0
p
a,
I
magnetic flux density (weber/m2)
integrated arc force (N)
current (A)
current density (A/m2)
maximum current density at the center of arc (A/m2)
static gas pressure in the arc ( ~ / r n ~ )
arc pressure (stagnation pressure of the plasma jet)
(N/m2)
radial distance (m)
radius of arc (m)
radius of arc at the cathode region (m)
radius of arc at the anode region (m)
velocity of the electromagnetically-induced plasma jet
(mlsec)
axial distance from tip of cathode (m)
viscosity of the shielding gas (kg/m-sec)
permeability in free space (henry/m)
density of the shielding gas (kg/m3)
current distribution parameter (m)
8.0
R A D I A L DISTANCE C m m >
Fig. 12-Arc
arc lengths.
pressure distribution of 300 A helium arc at different
606-VOLUME
17B. SEPTEMBER 1986
ACKNOWLEDGMENT
The authors are grateful for support of this work by
the Office of Naval Research under Contract N00014C-230-0384.
METALLURGICAL TRANSACTIONS B
REFERENCES
1. C. J. Allum: Journal of Physics D:Applied Physics, 1981, vol. 14,
pp. 1041-59.
2. C. W. Chang, T. W. Eagar, and J. Szekely: Arc Physics and Weld Pool
Behavior, The Welding Institute, London, 1979, pp. 381-88.
3. G. Seeger and W. Tiller: Arc Physics and Weld Pool Behavior, The
Welding Institute, London, 1979, pp. 2 15-26.
4. J. B. Wilkinson and D. R. Milner: British Welding Journal, 1960,
vol. 7, pp. 115-28.
5. W. Shimada and T. Gotoh: I.I.W. Document 212-388-77, 1977.
6. N. Yamauchi, T. Taka, and M. OK: Proceedings of International
Conference on Welding Research in the l980's, Osaka University,
October, 1980, pp. 25-30.
7. W. F. Savage, E. F. Nippes, and K. Agusa: Welding Journal, 1979,
vol. 58, no. 7, pp. 212s-24s.
8. K. Ishizaki: Weld Pool Chemistry and Metallurgy, The Welding Institute, London, 1980, pp. 65-76,
9. V. N. Selyanenkov, V. V. Stepanov, and R. Z . Saifiev: Welding
Production, 1980, vol. 27, no. 5. pp. 6-8.
10. A. V. Bradshaw and D. Wakelin: in Heat and Mass Transfer in Pro-
METALLURGICAL TRANSACTIONS B
cess Metallurgy, The Institution of Mining and Metallurgy, London,
1967.
11. J . Szekely and S. Asai: Metall. Trans., 1974, vol. 5 , pp. 463-67.
12. M. Salcudean and R. I. L. Guthrie: Metall. Trans. B , 1979, vol. 10B.
pp. 423-28.
13. N. Yamauchi and T. Taka: I.I.W. Document 212-452-79, 1979.
14. S. Harada: private communication, Osaka Transformer Co., Osaka,
Japan, 1980.
15. H. Maecker: 2. Phys., 1955, vol. 141, pp. 198-216.
16. H. B. Squire: Quarterly Journal of Mechanics and Applied
Mathematics, 1951, vol. 4, pp. 321-29.
17. N. S. Tsai and T. W. Eaaar: Metall. Trans. B . 1985, vol. 16B,
pp. 841-46.
18. Mondain-Monval: I.I.W. Document 212-264-73. 1973.
19. A . B . Cambel: Plasma Physics and ~a~ne'to-fluidmechanics,
McGraw-Hill, New York, NY, 1963.
20. J. Converti: "Plasma Jets in Arc Welding," Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1981.
21. S. S. Glickstein: Arc Physics and Weld Pool Behavior, The Welding
Institute, London, 1979, pp. 1-16.
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VOLUME 17B, SEPTEMBER 1986-607