By Joe Escobar
Welding is used in many aircraft applications. One of the most popular methods is gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding. In this article, we will discuss the basic theory of GTAW and cover different equipment setups available.
In gas tungsten arc welding, a non-consumable tungsten electrode is used to establish an arc on the base metal. The heat of the arc melts the base metal and produces a weld pool. In contrast to normal arc welding, in GTAW the weld area is shielded by an inert gas in order to prevent air from contaminating the weld. The gas prevents oxidation of the tungsten electrode, the molten weld puddle, and the heat affected zone adjacent to the weld bead.
In a typical GTAW setup, an AC/DC welding machine is used with a flow of a shielding gas. This shielding gas goes through a regulator and flow meter and on to the torch. The torch has a collet/collet body combination that holds the electrode. A heat-resistant cup or ceramic nozzle surrounds the electrode and controls the gas shield.
Although no metal spatter is produced with GTAW, it still generates intense heat and light. In fact, the clearer atmosphere around the GTAW arcs can cause up to twice the amount of infrared and UV rays compared to normal arc welding. Any exposed skin will be damaged similar to an extreme sunburn. Welders must wear a welding helmet. Welder's protective gloves and clothing should also be used. Fire-resistant cloth and leather clothing and accessories are recommended. Cotton should not be used since it doesn't provide sufficient protection and it deteriorates quickly under the infrared and UV rays produced by the welding process. In addition, dark clothing should be used to reduce reflection of light behind the helmet.
Other safety precautions
The following general precautions need to be observed to protect yourself and co-workers from the dangers associated with GTAW:
- Ensure electrical connection leads are in good condition and tight prior to use. They should be protected to prevent accidental damage from hangar traffic.
- Make sure you have adequate ventilation. Since GTAW uses inert gases during the process, if used in an enclosed area it can displace breathing air and can be harmful without proper ventilation. In addition, ozone is produced during the welding process, and varies with type of electrode used, amperage, and argon flow. In poorly ventilated areas, ozone levels can increase to harmful levels. Whenever possible, draw fumes and contaminated air away when welding.
- Flammable materials should not be carried in clothing pockets.
- Shielding curtains should be placed around all jobs so that workers in the area are not exposed to the welding arcs.
In GTAW, inert gas is used to create a protective shield of gas around the welding arc and hot metals while welding is being done. An inert gas is a gas whose atomic structure does not allow it to react with metals or other gases. Argon, helium, or an argon-helium mixture is used as the inert gas.
Argon is a relatively heavy gas. It has several benefits when used in GTAW. It requires a lower arc voltage than other shielding gases for a given arc length and current used (ideal for thin metal welding). It provides easier arc starting. Its heavier weight as compared to helium provides for good shielding with lower flow rates.
In contrast to argon, helium is the lightest of the inert gases. Because of its light weight, about two to three times more helium is needed as compared to argon to shield the weld area. Helium's advantage over argon is that it can be used with greater arc voltages. Because of this, it is preferred when working with thick metal sections.
Another difference in the two gasses is their cleaning ability. Both helium and argon allow for good cleaning action when using DC. However, when using AC, argon provides for better cleaning action. Argon also provides better arc stability than helium when using AC.
Types of machines
Typically, GTAW machines are either DC or AC/DC. Mike Sammons of Miller Electric shed a little light on the difference in machines.
"Your AC/DC machines will work for just about any weldable material that exists. Typically an aerospace operation will use an AC/DC machine because it allows them to attack whatever material is presented to them on the aircraft. AC machines are typically used for aluminum and magnesium. The majority of the welding is probably DC though. Typically a guy is going to buy a DC machine if he's dealing with non-aluminum type materials. DC machines are typically smaller, more economical, and you can get three-phase or single-phase power. The majority of DC machines are actually three-phase DC, SCR controlled, because you get a better arc performance off of three-phase power than you do single-phase power."
Balanced vs. unbalanced waves
An AC waveshape is broken down into two pieces - electrode-positive and electrode-negative. Electrode-positive removes the oxide that is very common in both aluminum and magnesium. The electrode-negative side of the half cycle provides the work, i.e. melts the material. Sammons talked about wave balance. "In a balanced wave condition, you have equal amounts of both cleaning as well as working. By going to an unbalanced condition, you are actually manipulating the time of each half-cycle leaning more towards the electrode-negative side. That way, you are spending more time welding or working than you are cleaning. For the most part we say you don't want to run in a balanced condition. You want an unbalanced condition to get better arc control, put more heat into the material or get more work done and less time cleaning. Spending more time on the cleaning cycle puts more heat on the tungsten, which means the ball on the tungsten gets larger, the puddle gets larger, and becomes more uncontrollable."
Many different electrode types are available including thoriated, lanthaniated, ceriated, and pure tungsten. When choosing an electrode, follow manufacturers' recommendations and choose the one that is best for you. Some characteristics to consider in an electrode are good ignition and reignition properties, constant arc, long lifetime, and high current-loading capacity.
As a safety note, 2 percent (thoriated) tungsten contains thoria, a radioactive element. When grinding on these electrodes, it is advisable to use some sort of collection device in the form of a vacuum or a liquid bath so that the grinding dust does not become airborne.
Advancements in technology
Sammons states that probably the biggest advancement in the GTAW process is the power supply technology. "The introduction of inverter technology has created a better tool for the welder. From a DC pulsing application, it was a significant improvement. The inverter technology is several hundred times faster than your SCR designs, which allow you to pulse much quicker in a much more square wave shape, which can increase arc stability. From an AC perspective, inverter technology has taken a giant step for mankind so to speak. The advantage is a significant increase in arc stability."
The second improvement Sammons pointed to is an increase in balance control. "In a conventional tirestor SCR power supply, your balance is limited to 68 percent electrode-negative with 32 percent electrode-positive. With inverter technology, it has allowed us to go to 95 percent electrode-negative with 5 percent electrode-positive. So what that does is allows us to spend more time welding and less time cleaning, i.e. more productivity."
This article touched on the basics of GTAW. For more information, or to inquire about training opportunities, contact the manufacturer.
The Lincoln Electric Co.
Miller Electric Manufacturing Co.