Gas Tungsten Arc Welding
| By: Michael Francoeur, President of Joining Technologies
The development of the GTAW process was accelerated early in 1940. Initially the process was called ‘‘Heliarc’‘, because Helium was used for the shielding gas. Later when argon was available the process was renamed tungsten inert gas or ‘‘TIG’‘. Now, it is generally and preferably called gas tungsten arc welding (GTAW), as gases other than argon and helium, which are inert, can be mixed with them. Hydrogen, for example, may be included for its special benefits.
The equipment basically consists of the power supply, the welding torch and connecting cables. The torch utilizes a non-consumable tungsten electrode usually alloyed with oxides such as thorium or orcerium to improve the electrode performance. Electrodes are attainable in various diameters ranging from .010’‘ up to .250’‘. To accommodate their installation in the torch an adjustable collet is used. The torch also includes the shielding gas nozzle through which the shielding gas flows to cool the electrode and to shield the weld ‘‘puddle’‘ or pool from oxygen. Nozzles may be selected from a range of sizes, materials and configurations. Material may be ceramic or metal. Fused quartz, which is translucent, is sometimes used to improve the weld pool visibility. Shape or configuration of the nozzles is based on aerodynamic principles and compatibly with special application requirements involving joint accessibility.
The torch assembly installed on the pendant cable is totally mobile for manual welding but is usually ‘‘fixed’‘ for machine welding. In the latter case it is most often stationary but if need be can be mechanized for providing for programmed movement. The torch assembly may be either air or water cooled. Water cooling is needed for the higher amperage equipment. The cables include the electrical conductors, shielding gas hoses and water lines. Tap water is commonly used for cooling but may be recirculated for conservation purposes and refrigerated. Shielding gas can be supplied from individual, portable cylinders, multi-cylinder manifolds or ‘‘tank farms’‘ depending on the required volumes. Switches in the torch handle, gas flow meters and regulators control the gas flow through the torch. For manual welding filler wire is introduced manually. However for machine welding, filler wire is supplied from reels through motor driven wire feed and guide mechanisms.
Power supplies are the constant or drooping output types and may use either DC or AC current with transformers and rectifiers. Direct current is most often used with the torch electrode being either negative or positive. In welding refractory alloys, aluminum, etc., alternating current is preferred for its oxide removal advantage.
For direct current (DC) welding the powers supply can incorporate a pulse forming network. Pulse repetition rates are adjustable as are the pulse profiles. Other pulse features are also controlled. Power supplies may include electronically controlled features such as ‘‘up slopes’‘ and ‘‘down slopes’‘, etc. The latter are necessary to eliminate craters and crater cracking at the beginning and ends of welds or to accommodate thickness changes, etc. For manual welding much of this power supply sophistication is not used, as the skilled welder modifies his technique as he observes variations in the weld pool during the progress of the weld.
Melting followed by fusion of the weld joint results from the flow of electrons, (the arc current) and the very high temperature of the arc plasma through which the current passes and which provides the conductance path. The arc comprises both the current and plasma, along with some generated metallic vapors, etc. Plasma temperature at the electrode can be tens of thousands of degrees Fahrenheit depending on amperage. Within the arc region it can be ten to twenty thousand degrees Fahrenheit. Considerable heat is lost by radiation and in this regard arc efficiency drops.
Depending on circuit polarity, the arc may emanate from the tungsten electrode or from the workpiece. Most often direct current is used but alternating current is advantageous for welding materials which form refractory oxides, for example aluminum. Due to the complex physics of the arc, involving electron collisions, pressures, temperature gradients, etc. the arc forms what is sometimes referred to as an ‘‘umbrella’‘ shape. Because of this, arc length must be very short if narrow welds are to be achieved. Welder skill is therefore challenged to avoid tungsten inclusions in the weld pool and electrode contamination. Since the arc and plasma is not focusable the concentration of energy in the arc cannot equal with that developed in the electron and laser beam processes and therefore, GTAW welds due to the lower rate of energy input are usually wider and have wider heat affected zones. As a result GTAW welds are more prone to distortion, wider heat affected zones and stresses. We must emphasize this is not intended to imply that GTAW welds are lower quality, they are in fact of excellent quality. GTAW welding can be called the workhorse of industrial production welding. The low cost of the equipment and its versatility along with excellent weld quality are the basis of its popularity. In addition, and due to the slower cooling rate GTAW welds may contain less porosity, lack of fusion or the formation of brittle martensite than can result from the concentrated energy beams of either the laser or electron beam processes which are both characterized by very rapid cooling and weld solidification. Rapid cooling is associated with martensite formation - a brittle phase which has the potential for cracking.As in other welding processes there are subtleties. Arc instability, arc blow and wander all seriously affecting the operation and weld quality can result from magnetic influences, improper electrode preparation, poor maintenance, improper electrode size, incorrect arc voltage, base metal surface oxidation, inadequate shielding gas flow rates, coverage and purity. Welds in restricted channel like areas can also reflect the results of arc instability. Shielding gas contaminated with moisture or oxygen, resulting from hose leaks or moisture on interior hose surfaces have their destructive effects on weld quality. Efficiency of shielding gas coverage is important to avoid weld pool and electrode contamination. Physically and chemically dirty joint surfaces inhibit the ‘‘wetting’‘ action of the molten weld pool causing lack of fusion, along with porosity and inclusions.
Gas tungsten arc welding is a thermal process depending on conducted heat through the weld joint materials to achieve penetration. In this respect its conductance mode is similar to laser and electron beam. They differ substantially, however, in the thermal impingement area dimension and their rates of energy input. The impingement areas of the laser and electron beams may be .010’‘ or .020’‘ in diameter whereas the gas tungsten arc impinges on an area many times that and, in addition and unlike laser and electron beam is supplemented by the high temperature radiation and thermal convection of the plasma which result in additional heating of the areas adjacent to the weld. GTAW penetration is limited to materials less than .250’‘ thick. Beyond this the weld joint must be prepared by beveling the joint edges forming a ‘‘V’‘ or ‘‘U’‘ shaped joint geometry when the workpiece details are butted.
Beveling the weld joint by material removal eliminates the need to conduct the arc heat from the joint surface through to the root. In effect, a crucible is configured and filled by filler wire addition in the welding operation. Therefore, deeper penetration is dependent not so much on thermal conductance but by the deposition of filler wire to fill the groove. Welder skill is required. The resultant unbalanced fusion weld geometry, wide at the top and narrow at the bottom can produce significant distortion. Restraining fixtures are most often used to reduce this problem. If restraining fixtures or clamps are used contractual stress generation develops. Depending on the materials and level of stress immediate or delayed cracking can result.At this point we will add to our earlier discussion of GTAW powers supplies. Predominantly direct current is used with the torch electrode the negative terminal or cathode. This arrangement has the added advantage of reducing the temperature generated at the electrode tip thus extending its life. Most importantly since the tungsten electrode is a concentrated source the arc is more constrained, than if the workpiece were negative and the electrode positive. Thus the energy is concentrated and consequently deeper penetration and narrower welds can be achieved.
It may be necessary to reverse the polarity making the electrode positive and the workpiece negative. The electrons will now flow from the workpiece, a broad source to the positive electrode. The result is wider welds and significantly less penetration. Electrode life is now shortened by overheating. When welding materials with refractory oxides, alternating current is recommended because the current reversal tends to break up the oxides. In addition, the tungsten electrode is cooled during the brief interval of low current flow occurring at the midpoint of the sine wave reversals. Refractory oxides melt at temperatures higher than the base metal from which they are formed. For example aluminum melts slightly above 1200 degrees F but its oxide exceeds 3000 degrees F. If its oxide is not removed the weld is subject to oxide inclusions and possible poor fusion by the reduced or inhibited ‘‘wetting’‘ action of the molten weld pool with the oxide contaminated weld joint surfaces. Recognizing this, it is necessary these materials be both physically and chemically cleaned and their cleanliness protected before welding.