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. A shielding gas (argon helium, nitrogen, etc.) is used to avoid atmospheric contamination of the molten weld pool. A filler metal may be added, if required.
Principle of Operation :
Welding current, water and inert gas supply are turned on. The arc is struck either by touching the electrode with a scrap metal tungsten piece or using a high frequency unit. In the first method arc is initially struck on a scrap metal piece (or a tungsten piece) and then broken by increasing the arc length.
This procedure repeated twice or thrice warms up the tungsten electrode. The arc is then stuck between the electrode and precleaned job* to be welded. This method avoids breaking electrode tip, job contamination and tungsten loss. In the second method, a high frequency current is superimposed on the welding current.
The welding torch (holding the electrode) is brought nearer to the job. 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.
1. Weld puddle is developed due to arc action on the job.
2. Welding torch is moved back.
3. Filler rod is moved ahead and filler metal is added to the weld puddle.
4. Filler rod is withdrawn.
S. Torch is moved to the leading edge of the puddle. TIG welding is carried out in this sequence.
Alter striking the arc, it is allowed to impinge on the job and a molten weld pool is created. The welding is started by moving the torch along the joint as in oxyacetylene welding. At the far end of the job, arc is broken by increasing the arc length. The shielding gas is allowed to impinge on the solidifying weld pool for a few seconds even after the arc is extinguished.
This will avoid atmospheric contamination of the weld metal. The welding torch and filler metal are generally kept inclined at angles of 70-80° and 10-20° respectively with the flat work piece. A leftward welding technique may be used. Filler metal, if required, should be added by dipping the filler rod in the weld pool. When doing so, the tungsten electrode should be taken a little away from weld pool. During welding operation alternatively filler rod and tungsten electrode will withdraw and come closer to the weld pool (in above figure).
This procedure will avoid contamination from the tungsten electrode. Introducing and withdrawing of filler rod into the molten weld pool may disturb the inert gas shielding, entrain air, oxidise filler rod end and thus contaminate the weld pool. In order to avoid these problems, it is preferred to keep the heated end of the filler rod always within the inert gas shield even when withdrawing the same from weld pool during welding. In table given that TIG welding parameters for different materials (Approx. values).
- No flux is used, hence there is no danger of flux entrapment when welding refrigerator and air conditioner components.
- Because of clear visibility of the arc and the job, the operator can exercise a better control on the welding process.
- This process can weld in all positions and produces smooth and sound welds with less spatter.
- TIG welding is very much suitable for high quality welding of thin materials (as thin as 0.125 mm).
- It is a very good process for welding nonferrous metals (aluminium etc.) and stainless steel.
- Under similar applications, MIG welding is a much faster process as compared to TIG welding, since TIG welding requires a separate filler rod.
- Tungsten if it transfers to molten weld pool can contaminate the same. Tungsten inclusion is hard and brittle.
- Filler rod cnd if it by chance comes out of the inert gas shield can cause weld metal contamination.
- Equipment costs are higher than that for flux shielded metal arc welding.
- Welding aluminium, magnesium, copper, nickel and their alloys, carbon, alloy or stainless steels, inconel, high temperature and hard surfacing alloys like zirconium, titanium etc.
- Welding sheet metal and thinner sections.
- Welding of expansion bellows, transistor cases, instrument diaphragms, and can-sealing joints.
- Precision welding in atomic energy, aircraft, chemical and instrument industries.
- Rocket motor chamber fabrications in launch vehicles.
References : A textbook of Welding Technology by O. P. Khanna