|
Tungsten Inert Gas TIG Gas Tungsten Arc Welding GTAW |
|
Tungsten Inert Gas TIG 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. 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 Operations
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 struck 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 super imposed on the welding current.
|
|
The welding 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.
After 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. 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, oxidize 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. Table gives TIG welding parameters for different materials.
| Material |
Current (Amps) |
Diameter of W Electrode (mm) |
Diameter of fileer rod (mm) |
Flow rate of Argon Litres per minute (lpm) |
| 1. Mild, low alloy and Stainless Steel |
250 350 (DCSP) |
3.0 |
3-4 |
7 |
| 2. Gray cast iron |
160 (AC/DCSP) |
3.0 |
5.0 |
8 |
| 3. Aluminium |
200 - 350 (AC) |
4.5 |
3-5 |
9 |
| 4. Copper |
250 - 375 (DCSP) |
3.0 |
3.0 |
7 |
| 5. Magnesium |
100 - 150 (AC) |
2.5 |
4.0 |
10 |
| 6. Silicon bronze |
150 - 200 (DCSP) |
2.5 |
3-4 |
9 |
|
Equipment
(a) welding Torch, Tungsten electrode and filler metal.
(b) Welding power source, high frequency unit, DC suppressor unit and cables.
(c) Inert gas cylinder, pressure regulator and now meter.
(d) Cooling water supply.
(e) Water and gas solenoid valves.
Welding torch which may be air or water cooled, energises the tungsten electrode held in a collet and feeds shielding gas to the weld puddle. When welding above 150 Amps, generally a water-cooled torch is preferred.
The electrode material may be tungsten, or tungsten alloy, i.e., thoriated tungsten or zirconiated tungsten. Alloy tungsten electrodes possess higher current carrying capacity, high resistance to contamination and produce a steadier arc, as compared to pure tungsten electrodes.
|
| |
When welding copper in nitrogen atmosphere, alloy tungsten electrodes are preferred because nitrogen attacks liquid (pure) tungsten; but, for the welding of aluminium under argon shielding pure tungsten electrode is more suitable. The electrode geometry for AC and DC welding.
A filler rod is generally used when welding thicker pieces with Ig'es prepared. Preferably the filler rod should possess the similar chemical composition and be of the diameter same as the thickness of the work piece to be welded. Filler metals up to 4.5 mm diameter in the form of straight lengths or coils are available for TIG welding of different metals and alloys.
Both DC and AC welding machines with good current control can be used for TIG welding. DC is preferred for welding of stainless steel, nickel, copper and copper alloys whereas DCRP or AC is used for welding magnesium, aluminium and their alloys. Reverse polarity of current removes oxide film on aluminium and magnesium.
|
When using AC a high frequency unit is employed to keep the arc ignited and stabilized. A DC suppressor unit is incorporated in the electrical circuit to balance the current wave. A DC suppressor unit reduces the effect of DC component of current which comes into operation due to the fact that arc voltage is more when electrode is positive than when it is negative.
In the absence of a DC suppressor unit, arc may blow and behave erratically and the transformer may become saturated, over heated and derated.
Inert gases ordinarily used in TIG welding are:
(i) Argon
(ii) Helium
(iii) Argon-helium mixtures
(iv) Argon-oxygen fixtures
(v) Argon-hydrogen mixtures
Pressure regulator and flowmeter are used respectively to step down the inert gas pressure from cylinder pressure (approx. 140 kg/cm2) to working pressure (1-1.5 kg/cm2) and to feed the same at a definite flow rate (4-12 lpm) to the welding torch.
For welding mild steel, aluminium and its alloys, copper, nickel and their alloys and stainless steel, generally argon, helium or a mixture of argon and helium can be used. Argon is preferred for welding bronzes and argon or helium can be employed for the welding of titanium and magnesium.
Gas and water solenoid valves if incorporated in the system, control the respective flows, i.e., they start the gas and water flow before the arc ignites and stop the same after the welding is over, tungsten electrode has cooled and weld metal has solidified.
Base Metals Welded
(i) Carbon and alloy steels,
(ii) Stainless steels,
(iii) Heat resisting alloys,
(iv) Refractory metals,
(v) Aluminium alloys,
(vi) Copper alloys,
(vii) Magnesium alloys,
(viii) Nickel alloys, etc.TIG welding is well adapted to welding thicknesses up to 6 mm.
Joint Design
Butt, Lap, Corner Edge and T joint are all used in TIG welding.A square groove butt joint is used for smaller thicknesses. A single V groove butt joint is required for base metal thicknesses between 4.8 and 9.6 mm. The included angle of V groove is 60° and root face 3.2 to 6.4 mm.
A double V groove butt joint is generally used on base metals thicker than 12.5 mm. A corner joint is used for fabricating boxlike structures. An edge joint does not usually require the addition of filler metal.
|
|
