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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