Nuts a Bolts
Aircraft Building
What's TIG Welding?
WHEN IT COMES TO BUILDING or restoring airplanes, especially those with steeltube structures, welding is an essential skill. It's also the source of many quesAnother tions, and one newcomers often ask is Can I learn how to do it, or should I hire it out? Certainly, you can learn to weld, and in the March "Aircraft Building" we discussed how to set up a shop for oxygen-fuel welding (OFW). But in learning about welding you've probably heard about TIG welding, which has given you a new series of questions to ask, like What's TIG, and what equipment do you need to do it? TIG is an acronym for tungsten inert gas welding, and the American Welding Society has assigned it a new acronym— GTAW, for gas tungsten arc welding. When Union Carbide Corporation created the process in the 1940s to weld difficult materials it was called "heli-arc" welding, because it uses an inert gas (like helium) to protect the weld zone from the surrounding atmosphere. It creates a strong, neat weld, and now that the equipment is becoming more affordable, its use in homebuilding is growing because builders are finding that GTAW is an effective way to weld 4130 steel and aluminum. To create the heat necessary to weld, an intense electric arc jumps from a nonconsumable tungsten
electrode to the metal being welded. Because the electrode doesn't melt to create a filler material, you feed welding rod into the welding zone, just as you name for Gas-Tungsten Arc Welding do with OFW. Why does GTAW use an RON ALEXANDER inert gas? In any type of welding, the best possible weld results when it has the same physical properties as the base metal, and the best way to achieve this is to protect the molten weld puddle from the surrounding atmosphere. When in contact with the air, the atmospheric oxygen and nitrogen combine with the molten weld metal. The result is a porous, weaker weld compared to a weld protected from the atmosphere. GTAW shields the weld by creating a "bubble" of inert gas (usually argon) around it. The gas flows through the handle, and the gas shield also provides a conducting path for the arc current. Because resulting welds are protected from the surrounding air, they are stronger and more corrosion resistant than welds made with other processes, including OFW. Because GTAW is very controllable and precise, with little sparking or spattering, it creates a weld with a neat appearance. '
It creates a strong, neat weld, and now that the equipment is becoming more affordable, its use in homebuilding is growing.
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APRIL 2000
GTAW Fundamentals
Like gas welding, GTAW uses a torch, but instead of a hot flame, an electric current passes through its tip, arcing between the tungsten elec-
Don't Guess! Fly Safe with Angle-of-Atta6IP" trode and the piece being welded. GTAW is also more complicated. In the torch's metal body a clamp, called the collet, holds the tungsten tip in place with a tightening cap. There are different sizes of tungsten tips, and which one you use depends on the thickness and type of metal you're welding. The tip does not melt (the reason it's made from tungsten), and you can adjust its length inside the collet. A ceramic cup surrounds the tungsten tip and directs the inert gas to the material being welded; the cups come in different sizes, too. In addition to holding the electrode and directing the inert gas to the arc and weld zone, the handle insulates the user from the electricity flowing through it. Besides protecting the weld from the atmosphere, the inert gas is often used to cool the handle's internal components, which get hot from all the electricity flowing through them. Water often cools larger torches—those rated for more than 150 amps—but few homebuilders will encounter these machines. A cable and a hose connect the torch to the welding cart, which holds the electric power unit and bottle of inert gas. Like an OFW cart, make sure the gas cylinder is securely chained to the cart. Another cable connects the power unit to a good source of electricity. Some of the smaller GTAW units require 120-volt current, but most of the machines homebuilders will want to use require 240 volts, and these units use a standard 240-volt, 50-amp plug.
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Power Settings
With the equipment securely in place, plugged in, and ready to go, your next step is to set up the unit for the proper current. If you're welding aluminum or magnesium, you'll select AC (for alternating current). If you're welding steel, you'll select DC (direct current) negative. The negative is important. Direct current travels in one direction (straight polarity), from positive to
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negative, and the DC negative setting concentrates most of the heat in the part being welded. With the type of current selected, you next dial in how much current, measured in amps, you'll need. Ideally, your welding unit will have a way to remotely control the arc current. This lets you change the
amount of heat (current) passing through the tip. You set the maximum current before welding, and this current passes through the tip when you move the thumb or foot switch to its maximum travel. Generally, the formula for determining the maximum power needed is 1 amp per 0.001 inch in metal
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thickness. For example, if you are welding 0.040, your maximum setting would be about 40 amps. To start welding, you turn on the flow of argon to 15-20 cubic feet/hour, turn on the power unit, hold the torch close to the weld zone, and push on the foot pedal. It sounds simple, and it is. Equipment & Cost
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What materials you want to weld influences the GTAW equipment you'll need and its cost. If you are not going to weld aluminum or magnesium, you can get a DC-only power source, and some are available for less than $500. If you later find that you need to weld aluminum or magnesium, you can get a power source conversion kit for around $200. Less expensive units require you to "scratch start" the current flow, which means you must scratch the welding tip over the metal you'll be welding to start the arc. More sophisticated units have built-in highfrequency circuitry that starts the current flow without tapping the tip on the metal. A good all-around unit would have AC and DC settings with a top end of at least 150 amps. These units are capable of welding most metals, including steel, aluminum, magnesium, titanium, etc. They come with a stick electrode holder so you can do "stick welding," which uses a consumable electrode, in addition to TIG welding. One example of this kind of unit is the Lincoln Square Wave TIG 175, which costs $1,400 to $1,600. The Square Wave unit comes with a foot-operated current control, and a thumb-controlled switch is an option. When you move your toe downward on the pedal, the current flow increases, stopping at the preset maximum amperage. When you release pressure on the foot control, the flow decreases or completely shuts off.
Your GTAW torch should have a 1/16inch and a 3/32-inch collet and electrode for most aviation applications. Shown is Lincoln Electric's K859-1 torch.
For most aviation applications, your GTAW torch should have a 1/16-inch and a 3/32-inch collet and electrode. For welding aluminum and magnesium, use a pure tungsten electrode; for most other metals, use a thoriated tungsten electrode. To complete your unit you'll need a bottle of argon gas, which you can lease from your local welding supply company; a regulator; and a hose to connect the cylinder to the torch. You'll need a welding hood with a lens in a shade rated from 8 to 12, and a bench grinder to dress or clean up the tungsten tips after they have been used.
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GTAW Safety
Eye protection is the paramount GTAW safety concern because the ultraviolet light emanating from the arc can burn the eyes and skin. Eye damage can be permanent, and the skin burn is comparable to a severe sunburn. Long sleeves and long pants, as well as leather gloves, an apron, and boots, will protect your skin. Don't wear synthetic fabrics because they melt and stick to your skin when a stray spark lands on them, and that goes for sneakers, too. Wear boots so errant sparks won't land in the gap between lowcut shoes and your socks. A comfortable, lightweight fullface welding hood with a lens of the appropriate shade protects your face
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Aircraft Building can Welding Society publishes a chart containing this information. Typically, when using fewer than 50 amps, a shade 8 lens is the minimum. If you're using 150 amps, you'll need a shade 10 to 12 lens. A good rule of thumb is to start welding with a shade that is too dark to see the weld zone, then go to a lighter shade that will provide sufficient view of the weld without going below the minimum (shade 8).
Set the metal type and thickness to weld and the simple slide rule-type TIG welding guide will show you the appropriate electrode diameter, filler rod, amperage, and gas flow.
Regular welding goggles used for gas welding are not satisfactory for TIG. Don't forget to protect the eyesight of others who may be around when you're welding. Be sure onlookers are wearing adequate eye
The most reliable, rugged, metal-working equipment euer built. protection. To protect those who might wander into the shop, always work behind a welding curtain. This protects your pets as well; they can suffer eye damage just like humans. Be sure to keep safety caps on all tanks. Argon tanks are pressurized to about 2,200 psi. A broken valve on an argon tank will create a rocket. The tanks should be secured before removing the safety caps. Be sure the tanks are secured when welding.
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You can use GTAW on most aircraft metals, including stainless steel, aluminum, 4130 steel, magnesium, and titanium, and very few equipment changes are required to weld different types of metal. You can also weld very thin metal. The welding arc is very hot but is easily controlled. This means the heat is localized and causes less metal distortion compared to gas welding. The welding joints are very high quality with excellent penetration and high strength. Anyone with gas welding experience can easily adapt to TIG welding, and you can learn it even if you have no previous welding experience. Like all new skills, it requires a basic knowledge and practice. •
