Gas-tungsten arc welding (GTAW) uses a permanent, non-consumable tungsten electrode to create an arc to a workpiece. This electrode is shielded by an inert gas, such as argon or helium (or a mixture of the two), to prevent electrode degradation; hence the older, common names tungsten-inert gas (TIG) and heli-arc welding. As shown in Figure below, current from the power supply is passed to the tungsten electrode of a torch through a contact tube. This tube is usually (but may not be) water-cooled to prevent overheating. The gas- tungsten arc welding process can be performed with or without filler (autogenously). When no filler is employed, joints must be thin and have a close fitting square-butt configuration.
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Gas Tungsten Arc Welding (GTAW)- Tungsten Inert Gas (TIG) welding current or operating modes
The GTAW process, as well as several other arc welding processes (e.g., SMAW, GMAW, and FCAW), can be operated in several different current modes, including direct current (DC), with the electrode negative (EN) or positive (EP), or alternating current (AC). These different
current or power modes result in distinctly different arc and weld characteristics. When the workpiece or weldment is connected to the positive (+) terminal of a direct current power supply, the operating mode is referred to as direct current straight polarity (DCSP) or direct current electrode negative DC - or DCEN). When the workpiece is connected to the negative terminal of a direct current power supply, the operating mode is referred to as direct current reverse polarity (DCRP) or direct current electrode positive (DC + or DCEP). In DCSP, electrons are emitted from the tungsten electrode and accelerated to very high speeds and kinetic energies while traveling through the arc. These high-energy electrons collide with the workpiece, give up their kinetic energy, and generate considerable heat in the workpiece. Consequently, DCSP results in deep penetrating, narrow welds, but with higher workpiece heat input. About two-thirds of the net heat available from the arc (after losses from various sources) enters the workpiece. High heat input to the workpiece may or may not be desirable, depending on factors such as required weld penetration, required weld width, workpiece mass, susceptibility to heat-induced defects or degradation, and concern for distortion or residual stress. In DCRP, on the other hand, the heating effect of the electrons is on the tungsten electrode rather than on the workpiece. Consequently, larger water-cooled electrode holders are required, shallow welds are produced, and workpiece heat input can be kept low. This operating mode is good for welding thin sections or heat-sensitive metals and alloys. This mode also results in a scrubbing action on the workpiece by the large positive ions that strike its surface, removing oxide and cleaning the surface. This mode is thus preferred for welding metals and alloys that oxidize easily, such as aluminum or magnesium.
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The DCSP mode is much more common with nonconsumable electrode arc processes than the DCRP mode. There is, however, a third mode, employing alternating current or AC. The AC mode tends to result in some of the characteristics of both of the DC modes, during the corresponding half cycles, but with some bias toward the straight polarity half-cycle due to the greater inertia (i.e., lower mobility) and, thus, greater resistance of large positive ions. During this half-cycle, the current tends to be higher due to the extra emission of electrons from the smaller, hotter electrode versus larger, cooler workpiece. In the AC mode, reasonably good penetration is obtained, along with some
oxide cleaning action. Figure below
summarizes the
characteristics of the various current or operating modes of the GTAW process described above. (Incidentally, many of these effects are far less pronounced with other electric arc welding processes employing consumable electrodes. Most particularly, there is little difference in penetration between DCSP and DCRP. This is so since the concentration of heat at the electrode with RP aids in melting the consumable electrode, as is
desired, but this heat is returned to the weld when the molten metal droplets transfer to the pool. On the other hand, the cleaning action of the RP mode at the workpiece still takes place.)
TIG Welding at Various Currents Diagram
In modern welding power supplies designed specifically for GTA welding, there is the added capability for square-wave AC and for wave balancing. In square-wave AC, solid-state electronic devices reshape the sinusoidal wave provided as input to the power supply from line voltage to give it a square shape; positive for half a cycle and negative for half a cycle. This shape turns out to be advantageous during the transition from one half-cycle to the other, where the voltage and resulting current pass through zero. For normal sinusoidal waveforms, as this transition is taking place, the voltage just before and just after the reversal approaches zero relatively slowly compared to the rate of change for a square wave. The effect of the much more rapid (essentially instantaneous) reversal with a square wave is to avoid possible momentary loss and subsequent difficulty of reestablishing the arc.
In wave balancing, there is the capability of shifting the relative magnitude of the straight and reverse half-cycles, thereby shifting the characteristics of the altered waveform. This is done by applying a DC bias voltage to the AC, whether of sinusoidal or square waveform. The advantage is the ability to fine-tune the waveform for the particular material being welded, obtaining just the degree of straight (penetrating) or reverse (cleaning) half-wave behavior desired. Regardless of mode or waveform,
power supplies for GTAW are generally of a constant current (CC) type.
Square and normal sinusoidal wave forms and wave balancing are illustrated schematically in Figure below.
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| a. square VS normal sinusoidal wave AC forms b. wave balancing in the AC operating mode |
Electron Emission Improvements of tungsten electrodes in TIG
The electron emission of tungsten electrodes can be occasionally enhanced by adding 1-2% thorium oxide or cerium oxide (or other rare-earth oxides) to the tungsten. This addition improves the current-carrying capacity of the electrode and consequently there is less chance for contamination of the weld by expulsion of tungsten due to localized electrode overheating and melting, and allows for greater arc stability and easier initiation, As mentioned earlier, both argon and helium are used for shielding with the GTAW process. Argon offers better shielding since it is heavier and tends to
stay on the work. Arc initiation is also easier, since the binding energy (i.e.work potential) for electrons in the completely filled outermost electron shell (some of which must be stripped from this shell to provide a conducting a plasma) is lower than for helium.
The advantage of helium in TIG is a hotter arc, which is the result of the higher work potential compared to argon. By using mixtures of these two inert gases, mixed characteristics can be obtained. In summary, the GTAW process is good for welding thin sections due to its inherently low heat input (especially in the DCRP mode), offers better control of weld filler dilution by the substrate than many other processes (again due to low heat input), and is a very clean process (as a result of the excellent protection afforded by inert argon or helium or argon-helium mixtures). Its greatest limitation is its slow deposition rate (only about 1-2 Ibs. or 0.5 1 kg. per hour), although this can be overcome by employing a “hot wire” variation in which the filler wire is resistance heated by being included in the circuit at a lower potential than the electrode. Deposition rate can also be increased to compete with GMAW, SMAW, and FCAW by using much larger, water-cooled electrodes with much higher currents (e.g., upward of a thousand amperes versus around a hundred amperes), or by using a fairly recent variation of the process that employs supplemental flux (fluxed gas- tungsten arc welding). In both of these variations, the process must be mechanized, however, to deal with the greater volumes of molten weld metal.
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