What Is TIG Welding?
TIG welding, which stands for Tungsten Inert Gas welding, is a welding method that employs the use of tungsten gas to execute smooth, precise cuts.
The History of TIG Welding
TIG Welding has a long history and has been a valuable process since its inception. In the early days, it was referred to as Heliarc, a patented process by the Linde Company. The reason for the “Heliarc” nomenclature was that the shielding gas was Helium. This created a very hot arc that could be used for almost any material. Because of the rapidly dissipating heat, aluminum alloys were more easily welded since the heat input was higher than earlier methods.
Helium is still often utilized when welding such alloys as Titanium. Helium-Argon or Helium-Hydrogen mixes provide even more advantages for heat concentration. The high heat lent itself more to a water-cooled design, which could be made smaller than the original bulky air-cooled torches, although more equipment was required.
The use of tungsten has been the key to creating a non-consumable electrode source. Tungsten is a natural metal that is now widely used in welding due to its extreme hardness and temperature resistance.
Since the melting point is above 4,000° F it may be used to join high-temperature alloys without “spitting” and depositing tungsten into the weld. This was an issue that was often seen when Carbon Arc Welding, with non-carbon (non-ferrous) alloys such as nickel-based stainless alloys. It has a tendency to deposit carbon into the weld puddle creating undesirable weldments. Carbon Arc Welding is still used with very low amperage for electric arc brazing of galvanized ductwork.
TIG Welder Uses
The process of using tungsten in welding is now referred to as the tungsten inert gas (TIG) welding and has evolved into the most effective method for high-quality welds in the piping and pressure vessel industry. It is also used heavily in the aircraft industry because of its ability to be finitely controlled. Many welders rely on TIG welding for “root passes”, which is the process of connecting two individual faces to make one structure.
For carbon and stainless applications, the shield gas is almost exclusively Argon. With this gas, the torch remains relatively cool and the operator is able to wear lightweight gloves and protective clothing. For welding, these materials Direct Current Electrode Negative current (DCEN) once referred to as straight polarity is used. The electrode that is commonly used for these alloys contains thorium (EWTH-1 or 2). The “E” represents “Electrode”. The “W” represents “Wolfram”. The “TH” represents “Thorium”. The “1 or “2” represents the percentage of the thorium. The tungsten is sharpened to a point and the arc is highly visible and precise. Much lower amperage is required and the relatively low heat input produces much more suitable Charpy impact values. The welder is able to pinpoint the placement of the weld and observe the puddle more clearly.
For Aluminum and Copper alloys, “pure” tungsten “EWP” is used. It is not sharpened as the EWTH-1 or EWTH-2 is prepared. For this type of welding, alternating current (AC) is used. AC current is said to have a superior cleaning effect to the direct current. To minimize contamination of the tungsten, a ball should be formed on the end of the tungsten by striking an arc on a clean copper piece. A larger cup will need to be utilized for the larger tungsten. All oxide must be removed from the metal that is to be welded, which may be accomplished by grinding, sanding, or by using a chemical such as Zepsolve. A successful weld will not be made without removal of ALL the oxide, or else the arc will wander and will not penetrate the oxide.
TIG Welding Requirements
Cleanliness is imperative with the TIG welding process, even on the carbon and stainless applications. For carbon steel, the mill scale must be removed to eliminate contamination in the weld. The welder will notice dark spots in the puddle and upon solidification, there will be an inclusion that will show up on a radiograph.
For the stainless materials, there will be chromium oxide inclusions if the material is not cleaned. Austenitic stainless steel alloys must be cleaned with chemicals, a brush, or a grinding disc that is produced especially for stainless or nickel alloys.
An advantage of welding with the TIG process is that the 300 series should never be allowed to exceed 350°F and the TIG welding machine is used at lower amperages, thus the heat input is held to a minimum. The 400 (martensitic) or (ferritic) series stainless should be treated similarly to carbon steel. These materials have little or no nickel content. The martensitic series are highly hardenable while the ferritic series is not. In either case, the TIG welding process allows the metallurgical content to remain virtually undisturbed due to a smaller heat-affected zone (HAZ).
For welding copper alloys with the TIG welding process, a large amount of preheating is required. A minimum of 1,100° F is usually required. Ideally, a water-cooled torch is utilized. The welder must use much heavier gloves and protective clothing.
The best scenario is to have one person using the preheat torch while the other one welds. The heat must be maintained at all times during the welding. One advantage of copper alloys is that it is not hardened by heating and cooling. To harden copper it must be “worked”. Bending, compressing, or stretching is suitable as a hardening process this material. Observing the color of the copper is not indicative of the amount of heat input. Instead, a thermometer is required to measure the amount of heat input. A hand-held heat indicator is the most suitable method. The maximum heat should not exceed 1,600°F.
Recommended TIG Welding Equipment
TIG welding requires a power source, preferably with a built-in high-frequency feature. Ideally, an AC-DC power source is used. The high frequency is used to enhance the start of the arc for carbon and stainless applications.
For aluminum and copper alloys, the high frequency is continuous, which aids in cleaning. The continuous high-frequency results in a noticeably cleaner weld on both sides of the weld. It is possible to “scratch start” the arc but there is more of a possibility for contamination of the electrode since the tungsten must contact or nearly contact the workpiece.
With high frequency, the arc may be started without being in contact with the workpiece. The arc is transferred to the electrode through a copper collet which is surrounded by a copper collet body. Since copper has practically no resistance to the current, there is virtually no loss in current between the power cable and the tungsten.
The shielding gas is transmitted through a hose that is located in the center of the cable that is housed inside the torch assembly. To further enhance the gas coverage there are holes in the collet body. This also aids in the cooling of the torch.
A ceramic cup is used to direct the gas into the puddle. The ceramic cup does not allow spatter to stick to it and block the gas flow. The size of the cup is related to the fractional number of the size. A number 4 cup is 4 sixteenths of an inch or 1/4” inside diameter. A number 8 cup is 8 sixteenths of an inch or 1/2” inside diameter.
There are also high-impact cups available that resist cracking. These are slightly more expensive but actually save down-time by minimizing the time it takes to replace the broken or chipped cup. A chipped cup creates turbulence in the weld puddle which may cause a lack of fusion. There is no limit to the length of the cable, but 15 feet is a suitable length to avoid current or gas flow loss.
Overall, the TIG welding process has more advantages than any other except for the rate of deposit. The quality factor makes up for any loss of speed.