Author: Mike Pankratz, Training Department, Miller Electric Mfg. Co.
Gas Tungsten Arc Welding (GTAW or TIG) has a long list of advantages. However, it may not be the correct process to use in all applications. The following questions may be used as a helpful guide in determining when to use this process.
Q: How do I determine when to use TIG vs. Stick or MIG?
A: One factor to consider is weld bead appearance. When manufacturing highly visible consumer products, the choice would be TIG, for its better appearance. Another factor is code requirements. Such jobs as nuclear work, piping, and high profile consumer goods often require at least the root pass (the first weld in the pipe joint) to be TIG welded for a good bond. In some cases all the passes on a multi-pass pipe weld may have to be done with TIG, depending upon quality and code requirements.
When appearance or “X-ray” quality are not required, but speed is, MIG welding may be the best choice. This is not to say that MIG is a “poor quality” welding process, it is simply a reference to the generally accepted concept in the welding field that, properly made, a TIG weld will often be a better quality weld than a MIG weld.
Q: Can you give me an example?
A: Consider the following example of a possible welding job: A small assembly within a car engine is made of 1/8″ (3.2 mm) aluminum. You would not choose Stick because it is rarely used due to the fumes and poor weld results on aluminum. While MIG could complete the job faster, the welding engineers decided in this case that the precision of the TIG welding process is needed.
Q: Does metal thickness play a role in determining what process is best for an application?
A: Yes. When considering welding materials in the five-thousandths inch range, you would not be able to use the common welding processes, MIG or Stick, because they would put too much heat into the metal. However, when welding thick metal, such as ½ in. (13 mm), you probably wouldn’t consider TIG at all, but rather MIG or Stick, as they can weld ½ in. material much faster.
Q: When selecting TIG equipment, what should I look for?
A: Decide how much power and sophistication are needed for the job and how much can be afforded, then think beyond the jobs at hand and anticipate what might be welded in the future. Hobbyists or occasional users could probably satisfy their needs with an easy-to-use, affordable unit like Miller’s Econotig package. On the other end of the scale, very versatile, high duty cycle units like Miller’s Syncrowave series are more appropriate when looking for an industrial power source to do automatic welding with external fixturing control and software programming. An industrial power source is also required for applications requiring extremely low amperages, such as welding wafer-thin bellows at three amps.
Q: Do I need an AC/DC power source?
A: Aluminum and magnesium are two common metals that are best welded using the AC output from the power source. Steels and stainless steels are most often welded with DC output. To weld a variety of metals, use a combination AC/DC machine. Also available are machines which are both Constant Current (Stick and TIG), and Constant Voltage (MIG and Flux-cored). A general rule would be: If your application is strictly TIG, get a TlG-only machine. Get a combination CC/CV only if you are sure you want to do MIG or flux-cored welding along with TIG.
Q: What are my options for a portable TIG welder?
If the power source will be moved around the shop, or from site to site (ex. TIG welding a sign out along the highway), then a portable welder is needed. There are two basic ways to accomplish portability: inverters and engine-driven welders. Inverters are now available that weigh about 30 pounds (13.5 kg) and have handles for portability. Some even run on standard 115 volt household current. (Inverters are power converters that allow a machine to be smaller and lighter than conventional machines.) Engine-driven machines are used when a welder has no access to primary power for welding. Engine-driven power sources can be used for in-the-field maintenance, pipe welding, or construction work, and many also have power generating capabilities.
Q: Why is welding power source duty cycle important?
A: The duty cycle of a welding power source is the amount of time you can weld at a given output without having to worry about overheating or burning up the power source. In the United States and some other countries, duty cycle is based upon a 10 minute period of time. For example, if a machine is rated at 300 amps, 32 volts, at 60% duty cycle, welding at 300 amps, 32 volts for six out of ten minutes is the correct cycle. For the remaining four minutes the power source needs to idle and cool. The duty cycle for many machines goes up as the amperage goes down.
A typical duty cycle for a hobbyist-type TIG welder would be 20%. An automatic setup for TIG, on the other hand, can require a 100% duty cycle because of the long weld times possible. Duty cycles of about 40% – 60% are often sufficient for many hand-held TIG applications in construction and industry.
Q: When is a “high frequency,” “scratch start,” or some other method of starting the arc used?
A: Arc starting is important with TIG because the tungsten electrode can contaminate the base metal if they touch, and vice-versa. However, there are occasions in non-critical work where the tungsten is scratched along (touched to) the base metal to initiate the arc, and is then lifted away from the metal to make the weld. The drawback is that this method may not be acceptable for critical, X-ray quality requirements, because tungsten strikes to the base metal may cause discontinuities or defects in the weld.
A “high frequency” arc start for TIG provides a non-touch arc ignition suitable for code requirements. A high frequency arc start provides an electrical path to the arc area, so that the current can “jump” across the arc between the tungsten electrode and the base metal. Because this high frequency voltage likes to wander in certain conditions while welding, arc starting devices must be installed and used under strict guidelines. Always follow owner’s manual instructions on how to use high frequency arc starting, as it may interfere with other electrical equipment in your shop. Other starting methods, such as “touch start” or “lift arc” have been developed to avoid scratching the electrode. However, these methods may not be allowable by some codes.
Information courtesy of Miller Electric