Tungsten Inert Gas (TIG) or Gas Tungsten Arc welding (GTAW):
It
is an arc welding process wherein coalescence is produced by heating
the job with an electric arc struck between a tungsten electrode and the
job. The electrodes used in this process are made of either Tungsten or
Thoriated Tungsten. Thoriated Tungsten electrodes run cooler than plain
tungsten electrodes and maintain square arcing ends but it is more
expensive.
Shielding gas:
A
Shielding gas (Argon, Helium, Nitrogen, Carbon dioxide or a mixture of
gases etc.) is used to avoid atmospheric contamination of the molten
weld pool. The inert gas blanket shields the entire weld area from the
atmospheric attack eliminating the need for any flux. General inert gas
used is Argon though Helium or a mixture of the two may also be used. A
filler metal may be added if required.
Commercial
grades (Shielding gas) used for welding are 99.9% pure. Argon is 0.38%
heavier than air and about 10 times heavier than Helium. Both gases
ionize when present in an electric arc. This means that the gas atoms
lose some of their electrons that have a negative charge. These
unbalanced gas atoms, properly called positive ions, now have a positive
charge and are attracted to the negative pole in the arc.
When
the arc is positive and the work is negative, these positive ions
impinge upon the work and remove surface oxides or scale in the weld
area. Argon is most commonly used of the shielding gases. So Argon is
suitable for welding thinner material.
Whereas
Helium produces a higher arc voltage than Argon. This makes it more
suitable for welding heavy sections of metal that have high heat
conductivity or for automatic welding operations where higher welding
speeds required.
TIG Welding Polarity:
The
current source may be either DC or AC depending upon application. DC
straight polarity arc is used in metals other than aluminium and
magnesium including copper alloys, cast iron, steel and stainless steel.
It gives good heat concentration and produces welds that are deep and
narrow. Welding rates are high and there is less distortion of base
metal. DC reverse polarity is not generally used because it produces
shallow and wide welds. AC arc is used for welding aluminium, magnesium,
cast iron and a number of other metals. Penetration with AC arc is
midway between the penetration produced by DC direct polarity and DC
reverse polarity.
The
process is most suitable for low gauge sheet metal welding and cannot
compete with conventional shielded metal electrode welding for welding
of heavier gauges due to the cost of inert gas.
TIG Welding Process:
A
high frequency, high voltage (100 kHz to 2 MHz, 2000 volts) low
amperage current supply is often used in TIG welding to initiate the
arc. This is to avoid the contamination of the electrode caused when the
arc is initiated by short circuiting with the work piece. When
electrode tip reaches within a distance of 3 to 2 mm from the job, a
spark jumps across the air gap between the electrode and the job. The
air path gets ionized and arc is established.
The
superimposed high frequency current causes a spark to jump the gap
between the electrode and the work piece. If a high frequency source is
not available the arc can be initiated by scratch starting using copper
striker plate to limit electrode contamination. Tungsten inert gas
welding can be done in any position. Welding speeds of the order of 400
to 1000 mm per minute have been attained by manual process. The process
can also be adapted to mechanized and automatic operation.
Advantages:
1. No flux is used, hence there is no danger of flux entrapment when welding refrigerator and air conditioner components.
2. Because of clear visibility of the arc and the job, the operator can exercise a better control on the welding process
3. This process can weld in all positions and produces smooth and sound welds with less spatter.
4. TIG welding is very much suitable for high quality welding of thin materials (0.125mm)
5. It is a very good process for welding Nonferrous metals (Aluminum etc.) and Ferrous metals.
Disadvantages:
1. Tungsten if it transfers to molten weld pool can contaminate. It is Hard and Brittle.
2. Filler rod end can cause weld metal contamination
3. Equipment costs are higher than that for flux shielded metal arc welding.
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