Gas Tungsten Arc Welding (GTAW) also known as Tungsten Inert Gas (TIG) is one of many welding processes that the name itself self-explains the distinguishable trait of GTAW from any other welding process. This process is manual, so it requires a great deal of skill from the welder, even some will consider this process is harder compared to SMAW.
This welding process is also non-consumable electric arc welding, which means that it employs electric arc as the source of heat while the arc itself is generated from a contact between base metal and non-consumable electrode, an electrode that doesn’t melt to fill the weld gap like those of SMAW but only to generate electric arc to melt the base metal and filler metal. Because of that, it’s possible for this process to weld without applying filler metal. This nonfiller method is called autogenous welding. For the electrode to be able to generate heat without being melted, it must have a higher melting point than the temperature of the electric arc itself. Luckily, we have one element that just refuses to melt at 4000 degree Celsius and just happen to conduct electricity as great as its ability to withstand heat. The element is a metal called wolfram or commonly known as its trade name tungsten.
Gas Tungsten Arc Welding is slowly developed through the 1900s with its peak of development around the Second World War. The development is driven by the demand for welding non-ferrous metal especially aluminum which is known to be very reactive with the surrounding air when melted and causing porosity which reduces the quality of the joint. Applying protective flux like those of SMAW is still not enough to diminish the existence of porosity. Until later on in 1930s heavy inert gas like Argon or just simply inert gas like helium is employed to block the surrounding air from contaminating the weld pool. The said gas also works as the medium for the electric arc to form a plasma. The gas is super-heated thus ionized and in such condition, plasma is allowed to form.
Although this process can produce a very high quality of welding, welding TIG is never so simple. It takes skill from the welder, even higher than the requirement for SMAW process. GTAW welding requires a good coordination skill from the welder because generally the welder weld with two hands, one for guiding the torch and the other for feeding the filler metal. Moreover, TIG is usually used for welding non-ferrous metal that has significantly lower weldability compared to ferrous metal, therefore the welder is required to have the understanding of the material that they’re going to weld. It is why welder with the competence to operate this process is so much valued and will be more likely to have a handsome salary rather than any other welding process.
Gas Tungsten Arc Welding:
The setup of GTAW welding machine is kind of the mix between SMAW and GMAW, while it’s set with gas tank and gas flow mechanism like those of GMAW, the end of the torch is a tungsten rod which similarly resembles SMAW. Here, I’ll try to explain every part of the set and its function.
GTAW Welding Machine
TIG welding machine comes in many different model and variation. Those variations are mainly about the machine capability coverage to operate in different polarities such as AC, DC, or both and the ability to provide square wave electricity or high-frequency voltage to make welding easier. Generally, the machine is a constant current machine, because the torch is handheld by the welder so constant current machine will be able to provide steady arc and heat input when the welder is moving the torch and the voltage is varying depending on the distance of the tip of the electrode to the parent metal.
Variation in polarity can greatly affect the quality of the weld because different material reacts differently if exposed to different polarity. For example, when welding steel, DCEN is more commonly used because it promotes deeper penetration. Because the electron is flowing straight from the electrode to base metal, making the heat more intense in the base metal and allowing for deeper penetration. DCEP will just not work because the heat will be concentrated at the tip of the electrode rather than at the base metal itself.
Therefore there will be no deep penetration like in DCEN. Moreover, the heated tip will also possible to melt and contaminating the weld pool, causing discontinuity known as tungsten inclusion. DCEP has its own unique properties, which is its ability to clean the existing oxide film that commonly found in metal such as aluminum. Making welding aluminum a lot easier. The alternative to mixing deep penetration in DCEN and oxide cleaning properties in DCEP is by using the Alternating Current (AC) that basically will flip between the positive and negative polarity for hundreds of time in a matter of a second, therefore it’s able to retain DCEN penetrating ability and DCEP oxide cleaning ability.
The Alternating Current is suitable for welding aluminum and the polarity that is commonly used to aluminum. The more advanced welding machine even able to engineer the way alternating current behave. Modifying the duration on which each of the polarity will spend to suit the welder. The square wave and high-frequency feature are to smooth out the arc so it will be stable trough promoting ignition when the polarity alternates.
TIG welding torch is very similar to a mix between GMAW and SMAW. It has on-off switch to signal the electricity whether to flow to the electrode or not. There are two cables inside the body of the torch, one for conducting electricity from the power supply, the other is the gas hose for the shielding gas to flow from the gas tank. Some torch model even has an extra hose for water (or air) that serves as a heatsink for the torch. This cooling system will extend the life of the torch and makes the welder weld more comfortably. Inside the torch, you will be able to find collet that serves as the primary holder of the electrode.
This collet will hold the electrode firmly to avoid unnecessary wobbling movement as long as the diameter matched the diameter of the electrode. The electrode itself sometimes extend for a few inches while only the edge is used for welding, the other edge and the extra length is covered in tungsten housing. Gas Lens and Gas Diffuser is placed inside the torch to control the flow of the gas. The cup also comes in different shape, it depends on the welding position and the joint design to promote shielding of the gas and comfortability for the welder. The cup is made from ceramic so it will be able to withstand the intense heat of the welding, however, the new model of the cup can also be made of high purity glass to allow the welder to have an unobstructed vision toward the weld pool.
Tungsten Electrode and Filler Metal
As mentioned before, TIG welding uses metal with high melting temperature as an electrode. The electrode is made of Wolfram (Tungsten) and sometimes are alloyed with another element such as Thorium, Zirconium, Lanthanum, and Cerium. It comes in different size, ranging from 0.5 to 6.4 millimeter in diameter and ranging from 75 to 610 millimeter in length. AWS has already designated a set of codenames (AWS A5.12) for us to easily distinguish one with each other. Here is the summary of each electrode:
- EWP (Electrode Wolfram Pure) it’s made of pure tungsten with green color code. Basically, the cheapest electrode that does not help ease the welder from their job. However, it’s cheapness still make them relevant for use, mainly for AC welding of aluminum or magnesium.
- EWTh (Electrode Wolfram Thorium Oxide) a tungsten electrode with Thorium content 1% (EWTh-1, Yellow) and 2% (EWTh-2, Red). It has excellent arc stability and great ignition making it popular. However, Thorium is a radioactive substance so it has health risk and disposal issue.
- EWCe (Electrode Wolfram Cerium Oxide) a tungsten electrode alloyed with 2% Cerium (EWCe-2, orange). Cerium increases the heat resistance of the electrode, preventing burn-off. It also improves arc stability and ignition although not as excellent as Thorium it doesn’t have radioactive properties, so there won’t be an issue about health risk and disposal.
- EWLa (Electrode Wolfram Lanthanum Oxide) a tungsten electrode alloyed with 1% (EWLa-1, Black), 1.5% (EWLa-1.5, Gold), or 2% (EWLa-2, Blue) of Lanthanum oxide. Pretty much similar in term of heat resistance, arc stability, and ignition with the Cerium alloyed electrode and it’s also not radioactive.
- EWZr (Electrode Wolfram Zirconium Oxide) a tungsten electrode alloyed with 0.3% Zirconium (EWZr-1, Brown). It increases the electrode melting temperature, increasing the current capacity and prolonging its life. It also improves the arc stability and ignition.
Together with the electrode is the filler metal. Filler metal is added especially when welding thicker metal. The rod-shaped filler metal is commonly made from the same material with the welded material or any other that have similar properties. It is added manually to the weld pool by the welder in a manual process. Except for the automatic ones they are stored in coils and being fed to the weld pool in a manner similar to SAW or GMAW.
Due to the reactivity of the materials being welded by this process, the shielding gas being employed is mainly the inert gas. The side effect of lacking gas protection can cause several defects like porosity if the contaminant manages to make contact with the weld pool. The shielding gas also helps to stabilize the arc by conducting heat from the tip of the electrode to the base metal. The two most common shielding gas for GTAW Welding is argon and helium. There are many deciding factors in choosing which shielding gas is to be employed. Welding position and joint design can decide how each type of gas shielding will be effective or not. Due to its greater mass compared to its surrounding air, argon will be more effective for shielding flat positioned welding with a rather exposed joint design. While helium that’s clearly much lighter than air will be great for non-flat or even overhead welding with complicated and not really exposed joint.
The desired weld quality and penetration can also be manipulated with the selection of shielding gas. Argon does not really promote deeper penetration due to its lower ionization energy compared to helium. However, Argon can help the welder to achieve soundness of surface, making the welder looks professional. While Helium does not really help with final appearance, it has good penetration and cleaning action.
TIG welding is a highly advanced welding process with some drawbacks. Although it can produce a high-quality weld joint. It would require a great deal of skill coming from the welder. Otherwise, the result will be much worse from SMAW. TIG welding is very great to be used for welding stainless steel and other non-ferrous material commonly magnesium and aluminum. While it’s also great for welding steel, it’s not really favored there due to its significantly slow welding speed. This process has focused heat input, so problems like distortion and uncontrolled HAZ is minor compared to SMAW or even SAW. TIG welding also uses inert shielding gas to prevent contaminant to cause a visual defect or metallurgical defect.
That’s why TIG or GTAW is an expensive method for joining two metal. Starting from complex equipment on the machine until the torch. Its unusual pair of non-consumable electrode and filler metal. The expensive shielding gas. And the last but not least, the welder that refuse to be paid poorly. Will make you think twice to weld metal with this process. However, after you find out the great quality that this process can achieve, there’s no arguing that this process worth all the cost that is being spent. Moreover, on its development, surely the process will get cheaper and cheaper while sustaining its ability to produce great quality of weld joint.