TIG - Direct-Current Welding with High-Frequency Pulses, an Interesting
Process Variant
2
2.1
Im
Tungsten-inert-gas welding (TIG) is one of the most
important joining technologies in welding-related fabrication. High-quality weld joints without spattering and
slags qualify this welding technology for the major part
of metals. As the filler-metal supply is separated from
the arc, the molten pool can be controlled in the best
way possible - an advantage which ensures the quality
of the execution of the weld but entails a relatively low
deposition rate and welding speed. If the welding
speed is increased, the arc looses some of its stability
and the penetration decreases. Above a critical value,
depending on the welding parameters selected, no
continuous weld can be achieved any more [1].
On the basis of these experiences, several TIG
torches are combined (multi-cathode torch), or TIG
dual-flow torches as well as plasma torches are used
in order to increase the welding speed in automatic
fabrication processes. The high requirements in torch
construction and torch operation turn out to be a disadvantage in this respect.
With laser welding devices, a considerable increase in
the welding speed can be achieved. However, the high
costs and the extremely low efficiency often hinder the
employment of these devices.
TIG direct-current welding with high-frequency pulses
also leads to a considerable increase in the welding
speed. In ranges of material thicknesses up to about 2
mm, this welding method thus represents an interesting low-cost alternative to multi-cathode and laser
welding.
IP
Introduction
tG
tP
IG
1
Welding current I [A]
D. Dzelnitzki, Muendersbach
Time t [s]
1 pulse cycle = 1/f
IG
Im
IP
= basic current
= medium amperage
= pulse current
pulse duty ratio
Fig. 1.
f = pulse frequency
tG = basic time
tP = pulse time
T =
tP x 100%
(tG+tP)
TIG - Pulsed-arc welding, terms
A pulse frequency of 0.5 - 6 Hz should be selected,
because higher values practically do not result in a
temperature difference between pulse and basic level
and thus produce a temperature course similar to nonpulsed TIG welding. In this respect, 40 - 60 % is considered to be a sensible pulse duty ratio [2].
Under these circumstances, we have to face the
question: What is the use of TIG direct-current welding
with high-frequency pulses?
The arc provides the answer. By heterodyning with a
frequency of a few Hertz to 35 kHz, its shape can be
influenced. The arc column is contracted and assumes a cylindrical shape [3]. In consequence, the arc
pressure rises, fig. 2.
Characteristics of high-frequency TIG pulsedarc welding
Process principle
© 2000 EWM HIGHTEC WELDING GmbH
Arc pressure TN
TIG welding with pulsating current has been known for
many years. It is mostly used for fully-mechanised and
automatic welding processes. The welding current
weaves periodically between a high (pulse current Ip)
and a low (basic current IG) value. In the basic-current
phase, the low temperature causes a decrease in the
volume of the molten pool. Thus, the heat input is
reduced and optimum control of the molten bath is
ensured. Of course, the pulse parameters (pulse current Ip , basic current IG, pulse frequency f and the
pulse duty ratio T), fig. 1, must be adjusted precisely to
the respective application in order to guarantee the
desired difference of the temperature between highcurrent and low-current phase. According to [2], the
ratio between pulse and basic current (Ip /IG ) should
be 1.25 - 4.
Fig. 2.
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Pulse frequency f
Arc pressure as a function of the pulse frequency [3],
tungsten electrode: 2,4 mm, 2% thoriated, sharpening
angle of the electrode: 60°,
medium amperage: 50 A, current amplitude: 150 A,
basic current: 5 A, arc length: 2 mm
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penetration TN
Arc length lLB
10 kHz
0 Hz
Pulse current IP
Fig. 3.
Change of the penetration in TIG welding with pulse
heterodyning of the welding current [3]
1 TN = f (IP), 2 TN = f (LB) 10 kHz, 3 PN = f (LB) 0 Hz
Fig. 4b. Arc formation of non-pulsed TIG- welding [4],
=
292
A,
vS
=
2,0
m/min,
IS
shielding gas: 95% Ar + 5% H2
While, up to a frequency of about 5 kHz, this effect is
very strong, further frequency increases do only cause
minimal arc-pressure increases. Besides the pulse
frequency, a growing pulse-current amplitude additionally reinforces the arc pressure on the molten bath
at the same effective value of the welding current [3].
This way, both welding parameters, pulse frequency
and pulse-current amplitude, make the arc column
extremely stiff. This stiffness permits to increase the
welding speed. Even at high speeds a continuous
seam with good penetration is formed, fig. 3.
The figures 4a and 4b offer a comparison between the
arc formation of non-pulsed TIG welding and highfrequency pulsed welding. With non-pulsed TIG welding, the arc silhouette shows deformations in the opposite welding direction, fig. 4b. The 6 kHz-pulsed arc
has a symmetric and more contracted shape, fig. 4a
[4]. In consequence, the arc is more stable.
2.2
For welding applications, two types of power sources
are available, which are designed for a maximum
welding current of 500 A and 1000 A respectively,
fig. 5.
Fig. 5.
Fig. 4a. Arc formation of high-frequency TIG- pulsed-arc
welding [4],
f = 6 kHz, IP =375 A, Im = 292 A, vS = 2,0 m/min
shielding gas: 95% Ar + 5% H2
© 2000 EWM HIGHTEC WELDING GmbH
Power sources and equipment
High-frequency- TIG- pulsed-welding power source
TIG 1000 DC
inverter
They have an inverter power module, which is characterised by high efficiency and insensitivity towards
mains-voltage fluctuations. The pulse parameters are
set via a remote control, fig. 6. It enables the system
to pulse the welding current up to 8 kHz.
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Fig. 6.
Fig. 7a. Weld formation of high-frequency TIG pulsed-arc welding
of an I-seam [4],
parent metal: 1.4301, t = 2 mm, f = 6 kHz,
IP = 375 A, Im = 292 A, vS = 2,4 m/min, shielding gas:
95%Ar + 5%H2, no filler metal
Remote control for high-frequency TIG pulsed welding,
operating elements
A power source only meets the requirements for optimal current shapes (high, rectangular current pulses),
if it has very good dynamic features. However, this
makes anti-noise measures necessary in order to face
the noise pollution.
Special attention must be paid to the torch. The electrode tip should display low surface roughness to
achieve long life at high current load. Equipping the
water-cooled torch with a gas lense is advisable.
3
Practical experiences and prospects
Areas of application of high-frequency TIG pulsed-arc
welding are continuously-fabricating welding automations of half-finished products such as tubes, foils and
strips, as well as the fabrication of parts, waste-gas
tubes, bellows or shapes, for instance.
Weldable metals are e.g. low- and high-alloy steels,
nickel-based alloys, copper and titanium alloys and
aluminium-based alloys These different materials require a careful selection of the shielding gas, since the
thermal flux of the arc is determined by the type and
the composition of the shielding gas [3].
Apart from argon/hydrogen mixtures (95% Ar / 5% H2),
argon/helium mixtures (50% Ar / 50% He) have turned
out to be suitable.
Welding experiments have been carried out mainly
with butt welds, fig. 7a and 7b [4]. The material thickness was 2 mm. There, considerable differences concerning the weld formation could be observed. While a
concentrated penetration was formed with 6kHzpulsed arc welding, fig. 7a, the typical TIG penetration
shape developed in the macrosection, fig. 7b, of nonpulsed welding.
© 2000 EWM HIGHTEC WELDING GmbH
Fig. 7b. Weld formation of non-pulsed TIG welding of an
I-seam [4],
parent metal: 1.4301, t = 2 mm,
IS = 292 A, vS = 1,6 m/min,
shielding gas: 95%Ar + 5%H2, no filler metal
What makes the difference between both methods is
the achieved welding speed. The pulse heterodyning
resulted in a 50% increase under identical circumstances. In spite of an identical medium amperage of
292 A, the welding speed during non-pulsed operation
had to be reduced from 2.4 m/min to 1.6 m/min in
order to achieve complete fusion.
Another high-frequency TIG pulsed-arc welding cycle
at a tube of the same material thickness even brought
about a welding speed of 2.7 m/min,fig. 8.
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Bibliography:
Fig. 8.
[1]
Cui, H. u.a.:
Laserinduziertes Fokussieren des WIG- Lichtbogens.
DVS- Berichte, Band 146, S. 139-143
(Cui, H. and others: Laser-induced focussing of the
TIG arc. DVS reports, volume 146, pp. 139-143)
[2]
Killing, U.:
Geeignete Parameter für das WIG- Impulslichtbogenschweißen.
Jahrbuch Schweißtechnik `94,
Deutscher Verlag für Schweißtechnik
DVS- Verlag GmbH,
Düsseldorf, 1993, S. 108 - 114
(Killing, U.: Suitable parameters for TIG pulsed-arc
welding. `94 Yearbook of welding technology,
German
publishing house for welding technology
DVS GmbH, Düsseldorf, 1993, pp. 108-114)
[3]
Schellhase, M.:
Der Schweißlichtbogen - ein technologisches
Werkzeug.
Fachbuchreihe
Schweißtechnik,
Band
84,
Deutscher Verlag für Schweißtechnik (DVS)
GmbH,
Düsseldorf, 1985, S.86, 97-99
(Schnellhase, M.: The welding arc - a technological
tool. Welding technology book series, volume 84,
DVS GmbH,
Düsseldorf, 1985, pp. 86, 97-99)
[4]
Müller, S.:
Untersuchungen zum Hochfrequenz-WIG- Impulslichtbogenschweißen.
Bericht der Schweißtechnischen Lehr- und Versuchsanstalt Fellbach (1997).
(Müller, S.: Examinations into high-frequency TIG
pulsed-arc welding. Report of the Schweißtechnische Lehr- und Versuchsanstalt Fellbach, 1997)
Weld formation of high-frequency pulsed-arc welding
at a tube;
parent metal: 1.4301, t = 2 mm, vS = 2,7 m/min,
shielding gas: 95% Ar + 5% H2, no filler metal
A prerequisite to turn such a technology successfully
into practice is the integration of power-source technology, on the one hand, and process technology, on
the other hand.
The increase of the penetration at the same welding
speed or a higher welding speed at a given throat
opens up areas of application to TIG direct-current
welding with high-frequency pulses that have been
restricted to multi-cathode or laser systems so far. The
big advantages of this welding process are, above all,
its considerably lower costs and the energy savings
compared with the previously mentioned systems. .
© 2000 EWM HIGHTEC WELDING GmbH
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