TIG Welding
Tungsten inert gas (TIG) welding became an overnight success in the
1940s for joining magnesium and aluminium. Using an inert gas shield
instead of a slag to protect the weldpool, the process was a highly
attractive replacement for gas and manual metal are welding. TIG has
played a major role in the acceptance of aluminium for high quality
welding and structural applications.
Process characteristics
In the TIG process the arc is formed between a pointed tungsten
electrode and the workpiece in an inert atmosphere of argon or helium.
The small intense arc provided by the pointed electrode is ideal for
high quality and precision welding. Because the electrode is not
consumed during welding, the welder does not have to balance the heat
input from the arc as the metal is deposited from the melting electrode.
When filler metal is required, it must be added separately to the
weldpool.
Power source
TIG must be operated with a drooping, constant current power source -
either DC or AC. A constant current power source is essential to avoid
excessively high currents being drawn when the electrode is
short-circuited on to the workpiece surface. This could happen either
deliberately during arc starting or inadvertently during welding. If, as
in MIG welding, a flat characteristic power source is used, any contact
with the workpiece surface would damage the electrode tip or fuse the
electrode to the workpiece surface. In DC, because arc heat is
distributed approximately one-third at the cathode (negative) and
two-thirds at the anode (positive), the electrode is always negative
polarity to prevent overheating and melting. However, the alternative
power source connection of DC electrode positive polarity has the
advantage in that when the cathode is on the workpiece, the surface is
cleaned of oxide contamination. For this reason, AC is used when welding
materials with a tenacious surface oxide film, such as aluminium.
Arc starting
The welding arc can be started by scratching the surface, forming a
short-circuit. It is only when the short-circuit is broken that the main
welding current will flow. However, there is a risk that the electrode
may stick to the surface and cause a tungsten inclusion in the weld.
This risk can be minimised using the 'lift arc' technique where the
short-circuit is formed at a very low current level. The most common way
of starting the TIG arc is to use HF (High Frequency). HF consists of
high voltage sparks of several thousand volts which last for a few
microseconds. The HF sparks will cause the electrode - workpiece gap to
break down or ionise. Once an electron/ion cloud is formed, current can
flow from the power source.
Note: As HF generates abnormally high electromagnetic emission
(EM), welders should be aware that its use can cause interference
especially in electronic equipment. As EM emission can be airborne, like
radio waves, or transmitted along power cables, care must be taken to
avoid interference with control systems and instruments in the vicinity
of welding.
HF is also important in stabilising the AC arc; in AC, electrode
polarity is reversed at a frequency of about 50 times per second,
causing the arc to be extinguished at each polarity change. To ensure
that the arc is reignited at each reversal of polarity, HF sparks are
generated across the electrode/workpiece gap to coincide with the
beginning of each half-cycle.
Electrodes
Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria
to improve arc ignition. Alternative additives are lanthanum oxide and
cerium oxide which are claimed to give superior performance (arc
starting and lower electrode consumption). It is important to select the
correct electrode diameter and tip angle for the level of welding
current. As a rule, the lower the current the smaller the electrode
diameter and tip angle. In AC welding, as the electrode will be
operating at a much higher temperature, tungsten with a zirconia
addition is used to reduce electrode erosion. It should be noted that
because of the large amount of heat generated at the electrode, it is
difficult to maintain a pointed tip and the end of the electrode assumes
a spherical or 'ball' profile.
Shielding gas
Shielding gas is selected according to the material being welded. The
following guidelines may help:
- Argon - the most commonly-used shielding gas
which can be used for welding a wide range of materials including
steels, stainless steel, aluminium and titanium.
- Argon + 2 to 5% H2 - the addition of hydrogen to
argon will make the gas slightly reducing, assisting the production
of cleaner-looking welds without surface oxidation. As the arc is
hotter and more constricted, it permits higher welding speeds.
Disadvantages include risk of hydrogen cracking in carbon steels and
weld metal porosity in aluminium alloys.
- Helium and helium/argon mixtures - adding helium
to argon will raise the temperature of the arc. This promotes higher
welding speeds and deeper weld penetration. Disadvantages of using
helium or a helium/argon mixture is the high cost of gas and
difficulty in starting the arc.
Applications
TIG is applied in all industrial sectors but is especially suitable for
high quality welding. In manual welding, the relatively small arc is
ideal for thin sheet material or controlled penetration (in the root run
of pipe welds). Because deposition rate can be quite low (using a
separate filler rod) MMA or MIG may be preferable for thicker material
and for fill passes in thick-wall pipe welds.
TIG is also widely applied in mechanised systems either autogenously
or with filler wire. However, several 'off the shelf' systems are
available for orbital welding of pipes, used in the manufacture of
chemical plant or boilers. The systems require no manipulative skill,
but the operator must be well trained. Because the welder has less
control over arc and weldpool behaviour, careful attention must be paid
to edge preparation (machined rather than hand-prepared), joint fit-up
and control of welding parameters.
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