Description of process 5
Advantages and disadvantages of TIG welding 6
Power sources for TIG welding 7
TIG pulsed-arc welding 10
Areas of application 11
Gases for TIG welding 12
Why use purging gases? 13
TIG AC welding of aluminium 14
Basic principle of TIG AC welding 15
Alternating current (AC) waveforms 16
Tungsten electrodes 17
Handling the torch and the welding filler 19
Material preparation in TIG welding 20
Weld preparation 21
Common mistakes in TIG welding 23
Workplace safety 24
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Originally, an arc was drawn between two non-melting tungsten electrodes under hydrogen,
and used for welding. Invented by the physicist Lanmuir in 1924, this process was known as
“Arcatom” welding. Around 1940, researchers in the USA began to weld with one tungsten
electrode, using helium, which was available there cheaply. This is why the Americans still
refer to "Heliarc" welding.
When it comes to its usability on various different materials, there is no other welding process
that is even remotely comparable with it. It guarantees the very highest welding quality that
it is possible to attain. Although TIG welding is suitable for nearly all welding tasks, its use is
restricted by questions of cost-effectiveness. In particular, this is due to its slower welding
speed and lower deposition rate.
DESCRIPTION OF PROCESS
TIG = Tungsten Inert Gas
In TIG welding, the workpiece material is heated and fused by a non-melting electrode. The
electric arc burns between the electrode and the workpiece. The weld pool and the electrode
are protected by a flow of shielding gas through the gas nozzle. The electrode is positioned in
the centre of the gas nozzle. The shielding gases used are argon, helium or mixtures of
Arc ignition generally takes the form of non-contact ignition, assisted by high-voltage
impulses (high-frequency ignition, RPI ignition).
Direction of welding
Shielding gas Tungsten electrode
Most metals are welded with direct current (DC). Only aluminium is generally welded with
alternating current (AC).
ADVANTAGES AND DISADVANTAGES OF TIG WELDING
• Highly versatile process
• Can be used for many different types of material
• Can be used in all welding positions
• Concentrated, stable arc
• High quality weld-deposit
• Smooth, flat weld-seams
• No spatter
• No slagging
• Filler metal not always necessary
• High welding speeds when working with materials less than 3 - 4 mm
thick (mechanised: TIG hot-wire)
The only real disadvantage of TIG welding is its low cost-effectiveness on materials that are
over 4 mm thick.
POWER SOURCES FOR TIG WELDING
Power sources for TIG welding have a steeply drooping power-source characteristic,
meaning that the welding current stays constant when the arc length changes. It should be
possible to control the power source during welding via a remote-control system (e.g. using a
pedal-operated remote-control unit).
voltage in TIG welding:
U = 10 + 0.04 x I
Steeply drooping power-source characteristic
The power source should have a continuous (stepless) current adjustment and a minimum
current of between 5 and 8A. The power source should also deliver a sufficiently high current
to enable various different thicknesses of material to be worked.
Thyristor-controlled power sources (Frowig)
The secondary rectifier consists of a half-controlled or fully controlled rectifier bridge. This
means that thyristors are used instead of the rectifier diodes. These thyristors are triggered by
ignition impulses in accordance with the power required:
Low power -> Few ignition impulses
High power -> Many ignition impulses
• Good controllability and regulability
• High weight
• Large, bulky size
• Because of the bulky output-smoothing reactor that is needed, the efficiency is
only approx. 70%
• Slow control process
• High current consumption (2-phase technology)
Inverter (Magic Wave)
The mains voltage is rectified immediately after the main switch (hence the name inverter)
and then chopped by a transistor module. Depending on the type of machine, this transistor
module (or “primary module”) works at between 25 and 100kHz, meaning that the welding
transformer is not supplied at 50Hz but at up to 100,000Hz. This results in a significant
reduction in the size of the transformer.
A similarly large reduction is achieved in the output ripple of the welding current, meaning that
only a very much smaller output reactor is needed, or indeed no output reactor at all!
After the welding transformer, the voltage is rectified and sent via the output reactor to the
welding power sockets.
Since a pre-set amperage is not always ideal for the entire duration of a welding task,
pulsating welding current is often used. For example, when welding pipes in cramped
conditions, a change in amperage is often necessary. Should temperatures rise too high,
there is a danger that liquid metal will begin to drop from the welding pool. Too low, and the
workpiece material will not melt sufficiently.
A relatively low welding current (background current IG) rises via a steep up-slope to a
considerably higher value (pulse current I1) and drops again after a pre-set period (duty-cycle)
to the basic setting (background current IG), a process which repeats itself over and over
During the welding process, small sections of the weld zone meld and solidify quickly.
Welding a seam using this method is thus considerably easier to control.
This technique is also used when welding thin sheet metal. Each fusion point overlaps the
next, thus forming a neat and regular seam.
When the TIG pulsing technique is used when welding by hand, the welding rod is applied at
each current peak. (only possible in the lowest frequency range, i.e. 0,25-5Hz). Higher pulse
frquencies are generally used in automatic welding applications and serve mainly to stabilize
the welding arc.
The diagram below illustrates TIG pulsing:
IE..........Final (i.e. “end) curr.
between 2 pulses)
AREAS OF USE
TIG welding can be used for all weldable metals. The largest area of use is for welding
stainless steels, aluminium, and nickel alloys.
The process is mainly used for welding materials in the 0.3 - 4 mm thickness range. On
thicker materials, in some cases the root seam is TIG-welded, while for the fill-in bead, other
more efficient processes such as MIG/MAG or submerged-arc welding are used.
Current needed per millimetre sheet thickness:
Aluminium approx. 40A
Copper approx. 75 - 80A
Low-alloy steel approx. 40A
CrNi steel approx. 40A
(depending on position and operator skill)
GASES FOR TIG-WELDING
Only inert gases (chemically inactive). The main gases used are:
Argon Most often used, good arc-carrier, good ignition characteristics, deep finger-
shaped penetration, better cleaning zone than with He, narrow heat-affected
Helium • Thermal conductivity 9x better than Ar
• Wide, deep penetration
• Especially suitable for Al-Cu, as in many cases it makes preheating
unnecessary, and faster welding speeds possible - fewer pores in Al, but
• Higher welding voltage necessary - unsteady arc
• Helium does not ionise the air-gap, so the rule is: “Ignite with argon - weld
Hydrogen • Thermal conductivity 11x greater than Ar
• Generally only 2 - 5%, rest Ar
• Deep penetration, better degasification
• Not suitable for aluminium welding
The gas consumption will depend on the welding position, the gas-nozzle diameter, the
amperage and the type of gas being used, but as a rule will be 4 - 8 l/min
WHY USE PURGING-GASES ?
When corrosion-resistant materials such as stainless steels are welded, the heated areas of
the seam are oxidised by atmospheric oxygen and are no longer corrosion-resistant.
These oxide layers (referred to as “tarnish discoloration”) can be removed by such measures
as brushing, grinding, sand-blasting or pickling, in order to make the workpiece corrosion-
Another possibility here is to stop the tarnish discoloration happening in the first place. The
use of "forming gases" here keeps atmospheric oxygen away from the heated-up areas of the
weld-seam and prevents oxidation taking place.
Depending on the material and the type of gas, this will also have an influence on the root
The main gases used as “forming gases” are nitrogen (N2) - hydrogen (H2) mixtures. Pure
argon is also possible.
TIG AC-WELDING OF ALUMINIUM
A characteristic feature of aluminium is that because of its great affinity to oxygen, an oxide
layer (-0.1µ) forms immediately on all surfaces that are exposed to the air!
The oxide skin has a melting point of 2015°C, whereas aluminium itself melts at approx.
650°C (depending upon which alloy).
So long as there is an oxide skin on the workpiece, an aluminium welding join will be
impossible! The oxide skin would not melt, and molten aluminium would simply dribble off it.
This is why it is absolutely essential for this skin to be destroyed first!
The oxide skin can be removed:
• chemically (very complicated and labour-intensive)
• by giving the electrode plus-polarity
• with alternating current
There are two theories to explain how the oxide layer is destroyed:
• The cathode spot moving across the weld-pool causes the Al oxide to evaporate, while the
electron emissions from the melt lead to oxide particles being impelled away towards the
edge of the weld, where they occasionally form small lines.
• The ions impacting onto the workpiece have sufficient energy to shatter the oxide skin; the
mode of action is compared with that of a sand-blaster. In support of this theory, its
proponents point to the fact that the cleaning effect is even more pronounced with inert
gases that have a higher atomic weight (argon).
BASIC PRINCIPLE OF TIG AC WELDING
ALTERNATING CURRENT (AC) WAVE-FORMS
1. Original sinusoidal form with high frequency (quiet)
One feature of sinusoidal alternating current is that the current rise, current drop and zero
crossing are relatively slow. Because of this, the arc is not very stable.
To maintain the arc, a high frequency is necessary, and this can lead to major disturbances.
The peak current is far higher than the pre-set welding current, leading to increased stress
and strain on the electrode. The advantage is the relatively quiet arc.
2. Square-wave alternating current without high frequency (loud)
The square-wave alternating current is generated by the intersection of various frequencies.
The current rise, current drop and zero crossing times are much shorter here, which results in
a more stable arc. At the same time, the pre-set welding current is also the peak current. The
arc noise is noticeably louder, so that it is necessary for the welder to wear ear protectors.
3. Combination of sinusoidal and square-wave form without high frequency (quiet)
The power source constantly changes the half-wave forms, resulting in a very stable and
extremely quiet arc. A high-frequency device for bridging the zero crossing is no longer
Tungsten is used for the electrode on account of its high melting point (3380°C).
The electrodes are produced using the sintering process. To improve their properties, they
may also be alloyed with oxidic additives.
Pure tungsten WP: Low rectifier effect; smooth, spherical electrode tip; ignition
problems with DC; low current-carrying capacity
With thorium oxide WT: The higher the thorium oxide content, the better the ignition
properties, the service life and the current-carrying capacity
become. There is a danger of the electrode "fraying" if it is under-
loaded. Thorium is slightly radioactive (alpha emitter).
With cerium oxide WC: Similar properties to thorium, but not radioactive
With lanthanum oxide WL: Longer service life than thorium or cerium oxide, but poor
Type of electrode Colour coded:
W = pure tungsten green
WC 10 = 1% cerium pink
WC 20 = 2% cerium grey
WT 10 = 1% thorium yellow
WT 20 = 2% thorium red
WT 30 = 3% thorium mauve
WT 40 = 4% thorium orange
WZ 8 = 0.8% zirconium white
WL 10 = 1% lanthanum black
Selection of electrode diameter:
Electrode diameter Tungsten: Zirconium alloyed: Thorium/cerium
in mm AC, in amps AC, in amps alloyed:
DC, in amps
1,0 10 - 60 15 - 80 20 - 80
1,6 50 - 100 70 - 150 80 - 160
2,4 100 - 160 110 - 180 120 - 220
3,2 130 - 180 150 - 200 200 - 300
4,0 180 - 230 180 - 250 250 - 400
Recommended electrode tip: α = 15 - 20°
HANDLING THE TORCH AND THE WELDING FILLER
TIG welding is suitable for use in all positions, regardless of the material, the thickness of the
workpiece and the type of current.
Note that in order to prevent oxidation, the molten wire-end must remain within the shielding-
gas cover at all times during welding. If it does not, welding defects may result. The arc-
length is around 1 to 3 mm with DC, and 2 to 4 mm with AC. In order to prevent tarnish
discoloration (oxides) and the reduced service life that this entails, the electrode must not
protrude too far out of the shielding-gas nozzle.
Butt weld Fillet weld Position PF
Position PA Position PB
Handling the torch and the welding filler
The filler metal must be melted off by the weld-pool. Do not allow it to drip into the weld-pool,
as this leads to uneven droplet transfers and weld-ripples.
The tungsten electrode must not come into contact with the filler metal or the weld-pool. An
alloyed electrode must be cleaned (burned-out or re-ground).
PREPARING MATERIALS FOR TIG WELDING
⇒ Absolute cleanliness is vitally important!
⇒ When working with CrNi, use only CrNi tools
⇒ Only work aluminium with CrNi tools that have only ever been used for
aluminium and never for steel
• On butt seams, always slightly round off the root-
penetration side, otherwise an "oxide notch" may result
• A larger weld-preparation angle is needed than on steel
• V-welds: max. 80°, generally with no root-face!
• Gap root width > 2 mm, if possible use a backing
support (CrNi, poss. ceramic - but not Cu)
• Weld I-seams (square butt welds) without a gap.
Degrease sheets - in some cases re-baking will be
necessary, as the oxide skin occasionally contains H2,
(with acetylene flame = reducing effect)
When working with thicker materials, pre-heating may be
necessary because of the large heat radiation that takes
place; with He mixtures, pre-heating will not be necessary
in some cases.
As small a weld-preparation angle as possible, as CrNi is a poor thermal conductor. There is
a risk of very great distortion.
Weld at the coolest temperatures possible -> overheating, burn-off of alloy elements, root-
shield protection is necessary because of oxidation (!)
Aluminium Stainless steel
S ≥ 10 S = 6 - 30
D=0 D = 0 - 0.3
K=2-3 K = 1.5 - 2
ß = 12 - 15° ß = 10°
S = 6 - 30
D = 0 - 0.3
ß = 10°
S ≥ 6 - 30
K = 1 - 1.5
α = 60°
ß = 10°
Double flange-weld I-
Aluminium Stainless steel
S≤ 3 S≤ 2
H = 1.5 x S H = 1.5 - 2
D = 0 - 0.5 D = 0 - 0.5
Aluminium Stainless steel
S≤ 4 S= 1-3
D= 0-1 D= 0-2
Aluminium Stainless steel
S= 5 - 12 S= 4-6
D= 0-1 D= 0 - 0.3
K= 1.5 - 2 K= 1.5 - 2
α= 70° α= 70°
COMMON MISTAKES IN TIG WELDING
Defects caused by incorrect manipulation of torch and rod
Radiation from the arc
The arc emits radiation in the visible range, as well as in the ultraviolet and infra-red ranges.
The infra-red radiation can cause burns. Welders and their assistants must protect
themselves by wearing suitable protective apparel.
On uncovered skin, the ultraviolet rays can lead to sunburn-like reddening, and can also
cause "flash-burning" of the eyes. Welders’ eyes must be protected by safety glasses to DIN
4647. These are classified into different protective categories, depending on how much light
they let through. For TIG welding, protective ratings of over 9 are recommended.
The fumes and ozone given off during the welding process should be removed by means of a
welding-fume extraction system.
DVS: Der Schutzgasschweisser (“The Gas-Shielded Arc Welder”)
DVS: Metall Schutzgasschweisser (“Metal Gas-Shielded Arc Welder”)
DVS: Schweissen von Eisen-, Stahl- und Nickelwerkstoffen (“Welding of Ferrous, Steel and Nickel Materials”)