In this article we will discuss about:- 1. Meaning of Gas Tungsten Arc Welding (GTAW) 2. Advantages of GTAW Process 3. Polarities 4. GTAW Tungsten Electrodes 5. Shielding Gases.
Meaning of Gas Tungsten Arc Welding (GTAW):
Air during welding, (viz. oxygen, hydrogen, nitrogen and water vapour) tends to reduce the quality of weld. Inert gases are used to keep contaminants away from contacting the metal, and also to remove the contaminants from the metal surfaces as the contaminants like air, dust and metal oxides also reduce the weld quality considerably. Inert arc gas welding is faster, produces cleaner weld and can weld metals considered to be difficult or impossible to weld.
Earlier GTAW was called as:
TIG Welding (Tungsten Inert Gas Welding):
It uses a non-consumable electrode and a separate filler metal with an inert shielding gas.
A GTAW process welding set utilises suitable power sources, a cylinder of organ gas, a welding torch having connections of cable for current, tubing for shielding gas supply, and tubing for water for cooling the torch. The shape of torch is characteristic, having a cap at the back end to protect the rather long tungsten electrode against accidental breakage.
In an arc welding process, three elements (current, voltage and speed of travel) have to be controlled to obtain satisfactory welding operation. In automatic welding, all these variables are present and controlled by the welding equipment.
The important statements about arc welding principles which require due consideration in study of welding machines are reviewed here:
(i) A welding arc has two electrodes, one being the welding rod, and the other being the work-piece to be welded. If direct current is used for arc welding, there are two possibilities (or polarities, as they are called). One is direct current straight polarity (DCSP) in which the work-piece is connected to the positive terminal of the welding machine. The other is DCRP (reverse polarity) in which the work-piece is made negative.
(ii) Arc-voltage is the voltage drop across the arc between the two electrodes. It increases with arc length. If the electrode is shorted to the work-piece, the voltage drop across the arc is zero.
(iii) Penetration is deepest for DCSP (because the electron stream flows to the work, thus concentrating the heat at the work), less for A.C. and least for DCRP. In the case of DCRP good cleaning action is obtained. Due to less penetration in case of DCRP process, it is best suited for thin sections of aluminium, magnesium and other hard-to-weld materials using tungsten electrodes.
(iv) In an arc welding process using direct current, about two-thirds of useful arc heat appears at the anode, and about one-third at the cathode. In A.C. welding the welding heat will be distributed equally between the welding rod and the work-piece.
(v) Deposition rate of filler metal increases with current.
Gas Tungsten Arc Welding (GTAW) process uses a non-consumable tungsten electrode which is mounted in a special electrode holder. This holder is also designed to furnish a flow of inert gas around the electrode and around the arc. Welding operation is done by striking an arc between the work-piece and tungsten electrode in an atmosphere of inert gas such as helium, argon, carbon dioxide.
Gas Tungsten Arc Welding (GTAW) or TIG (tungsten inert gas) welding used a non-consumable tungsten electrode to heat and melt the work-piece. Filler metal can be fed. Molten puddle is shielded from the atmosphere with an inert gas supply feeding from the torch cup.
Advantages of GTAW Process:
(a) It welds more different types of metals and alloys (carbon steel, stainless steel, nickel steels, aluminium, magnesium, brass, copper, bronze, titanium.
(b) Unlike metals can be welded to each other like mild steel stainless steel, brass to copper.
(c) Heat-Affected zone (weak area for failure of sound weld) is very low.
(d) Filler metal need not pass through the superheated electric arc.
(e) Requires no clean up because of absence of slag or spatter.
The GTAW outfit is shown in Fig. 9.41. Generally the water hose, inert gas hose, and welding leads are all in one jacket and form one head. Common practice is to flow the outlet water along the electrode lead. This water cooling permits using a smaller diameter lead, which provides a lighter weight torch and greater flexibility.
The arc welding machine may be either a motor-generator unit, or a rectifier unit. Most of the welding machines use a high frequency superimposed current in the circuits to aid in starting the arc. The use of GTAW is confined mainly to the welding of relatively thin materials upto about 7 mm. It is especially suited for welding aluminium and magnesium-based alloys, stainless steel, titanium, etc.
Polarities Used by GTAW:
GTAW uses all the three polarities:
(Direct current straight polarity) i.e. tungsten electrode – ve and work + ve, is used for applications requiring deep penetration (because 2/3rd heat is at the work) and a narrow bead as in mild steel, stainless steel, copper, and titanium.
(Direct current reverse polarity) i.e. tungesten electrode + ve and work – ve, is limited to preparing the tungsten. Rounded to balled tip is used which gives it more heat carrying capacity). It is used for welding aluminium and heavily oxidised aluminium castings.
(Alternating current high frequency) is used for welding aluminium and magnesium which have oxide coatings that contaminate the molten Al and Mg while welding and ACHF helps to remove these oxides. It may be noted that 50 Hz a.c. is not used alone but a high frequency (15,000—22,000 Hz signal) is super-imposed. Thus when 50 Hz a.c. goes through its zero point, the HF goes through many cycles and prevents the arc from stopping. HF pulses also help in oxide cleaning action.
GTAW Tungsten Electrodes:
Five types of electrodes are given below and the colour by which the case identified is also indicated in bracket:
(i) Pure tungsten (W) (green)
(ii) 1 % Thoriated W (yellow)
(iii) 2% Thoriated W (red)
(iv) Striped W (blue)
(v) Zirconium W (brown).
Pure tungsten electrode is used only on ACHF and designed for aluminium and Mg welding only:
1% thoriated tungsten electrode is used for copper and copper alloys.
2% thoriated tungsten electrode can be used for almost any metal.
Striped tungsten electrode combines pure tungsten and a stripe of 2% thoriated tungsten. Thoriat helps to keep a stabilized arc and increase melting temperature.
Zirconium tungsten reduces the contamination effects of dipping the tungsten into the molten puddle while welding Al and Mg.
The shape of the tungsten electrodes should be as shown in Fig. 9.42 in order to obtain the best results. For starting the unit it should be ensured that sufficient water (as that the temperature rise is about 5°C) and inert gas are flowing (gas flow may be controlled by relays and solenoid valves automatically also).
The extension of electrode out of the cup should, be approximately equal to the inside diameter of the cup of slightly more in case of fillet welds, otherwise the shielding effect will be reduced.
Preventing Tungsten Contamination:
For preventing tungsten contamination, it should be shielded from the atmosphere and should not touch the weld puddle.
To achieve this:
i. Inert gas flow must be sufficient.
ii. Laminar flow of shielding gas must be achieved.
iii. Post-Purge should be long enough to allow tungsten to cool to a non-reactive state.
For striking the arc the torch is held horizontally over the metal starting block or work (or best on the used tungsten electrode) and very quickly tilted and swung to the upright position with the electrode reaching a point about 3 mm above the metal;the arc will jump this gap.
With A.C. the unit needs superimposed high frequency. The tungsten electrode torch should then be warmed by practising on a scrap piece of metal, before starting the weld in order to get good starting results on the job. The correct positions of the tungsten electrode and the filler wire in manual gas tungsten arc welding are shown in Fig. 9.44.
After the arc has been struck and the welding process started, used the smallest circular motion possible, and make a small puddle in the spot where the weld is to begin. The electrode holder is held at an angle of 60 to 80 degrees with the work in order to protect the weld puddle with the inert gas.
The arc can be stopped by lifting the electric arc holder quickly or by reducing the current flow by the foot control. The gas should be continued on till the tungsten is cool, otherwise the tungsten will become corroded and will also be consumed too rapidly.
Shielding Gases for GTAW:
Various gases and gas mixtures used are:
This being heavier than air produces a superior shield. Its low resistance to electricity (ionisation at 17 V) promotes extremely smooth, quiet arc.
It is extremely light gas and produces good shield for molten metal. Due to higher ionisation voltages (24.2 V) it produces a higher volume of heat in the arc stream and thus a wide, deeper penetration puddle.
It combines the characteristics of excellent shielding of argon and the wider are stream and wider/deeper puddle of helium.
The addition of H2 increase the wetting action (fluidity) and makes low thermal conductivity metals to be welded faster and easier.
It is used for carbon steels only.
Usually argon is used as the shielding gas. Sometimes helium is used for welding thicker sections since light are voltages are possible with this gas Helium is, of course, a costly gas.
GTAW can be used to produced welds in the flat, horizontal, vertical and overhead positions. Progress in normally downward when welding in a vertical position. In several uses of GTAW a higher degree of operator skill is required.
The important points requiring due attention for obtaining best results in the GTAW are:
Tungsten electrodes should be kept clean and straight and their ends in the proper condition. Correct size electrodes should be used. If it is too small the end of the tungsten will form into a molten ball larger than the electrode, and this ball may fall into the weld. If it is too large, the arc will wander from one side of the electrode to the other.
A discoloured tungsten electrode usually means that it has been exposed to the air while still very hot. Gas connections must be tight or else leaks may result. Electrode should not extend beyond the cup by less than a bare minimum amount even through it may interfere to some extent with the vision of the weld puddle. Steel welding rods should not be copper coated, as the copper coating will cause spatter and may contaminate the tungsten electrode.