Gas Tungsten-arc Welding (GTAW)
Gas Tungsten-arc Welding (GTAW) formerly known as TIG (Tungsten
Inert Gas) welding, the filler metal is supplied from a filler wire as
shown in the figure below. The tungsten electrode is not used during this
welding operation, a constant and stable arc gap is maintained at a constant
current level. The filler metals are similar to the metals to be welded and
flux is not used. The shielding gas used in this welding process is usually
argon or helium (or a mixture of these both gases). Welding with gas
tungsten-arc welding may be done without using filler metals. for example,
in the welding of close-fit joints.
Schematic
diagram of Gas Tungsten-arc welding process
Depending on the type of metals to be welded, the power supply
is either DC at 200A or AC at 500A (see below image). In general, AC is
preferred for welding metals aluminium and magnesium, because the cleaning
action of AC removes oxides and improves weld quality. Thorium or zirconium can
be used in the tungsten electrodes to improve their electron emission
characteristics. The power supply ranges from 8 to 20 kW. Contamination of the
tungsten electrode by the molten metal can be a major problem, particularly in
critical applications, because it can cause discontinuities in the weld.
Contact of the electrode with the molten-metal pool should be avoided.
Equipments used for gas
tungsten-arc welding operations
The gas tungsten-arc welding process is used for a wide variety
of applications and metals, particularly aluminium, magnesium, titanium and the
refractory metals. It is highly suitable for thin metals. The cost of the inert
gas makes this process more expensive than Shielded Metal-arc
Welding but provides welds of very high quality and surface finish.
The equipment
used for gas tungsten-arc welding process is portable.
Applications of Gas
Tungsten-arc Welding:
§ Originally
developed for welding Aluminium and Magnesium.
§ The other metals
are Stainless steel, High carbon steel, Copper, Monel(Ni + Cu+ Fe +
Mg)), Inconel (Cu + Cr + Fe), Brass, Bronze, Silver, Molybdenum etc.
§ This process is
used for joining various combinations of dissimilar metals like brazing and
braze welding.
§ Pipe work required
for high pressure steam lines, chemical and petroleum industries.
§ Welding of air
craft frame, jet engine casing, rocket motor casing.
§ Accuracy welding
of parts in atomic energy.
§ Expansions
bellows, transistors cases, instrument diagrams etc.
Advantages of Gas
Tungsten-arc Welding:
§ Gas Tungsten-arc
welds are stronger and more ductile.
§ No danger
corrosion due to no flux is used.
§ No post weld
cleaning because of no slag.
§ Wide variety of
joints can be made because no flux is used.
§ There is very
little or no smoke, fumes or sparks at all. This helps in making a neat and
sounder weld.
§ As the shielding
gas is transparent, operator can clearly observe the weld.
§ Fusion welds can
be made in merely all commercial metals.
Limitations
of Gas Tungsten-arc Welding:
§ Because of usage
of inert gas, coolant and coolant pump etc the cost of Tungsten
Inert Gas welding is very high.
§ Maximum thickness
of plate which can be joined by this welding process directly is up to 5mm.
§ For welding of
above 5 mm thickness plate additional filler rod must be used.
§ Even though
tungsten electrode is not melting but at high temperature the atoms of tungsten
may get diffused from the tip of electrodes and entering into the weld pool
which will increase the brittleness of weld bead.