nuts & bolts
aircraft building
Beginning to Weld Melting metal to build your airplanes Ron Alexander
F
inally, after all of the preparation discussed in the last several issues of EAA Sport Aviation, we’re ready to start welding using a TIG (tungsten inert gas) machine. If you’ve decided to weld with gas, read Budd Davisson’s series of articles at Homebuilder’s Headquarters in the members-only section of the EAA website (www.eaa.org). TIG welding is not new, but it has come full circle in aviation. In 1939, engineer Russell Meredith was asked to develop a way to weld aircraft airframes made of magnesium tubing. At the time, most aluminum and magnesium aircraft materials were gas welded with an oxy-hydrogen flame and a fluoride flux. The flux left a residue that was removed by vigorously scrubbing the parts under hot water. If not removed, the flux residue continued to react (and weaken the weld) on magnesium tubing, and there was no way to remove the residue trapped inside a tube sealed by a closure weld. Needless to say, this did nothing to generate the confidence of fighter pilots whose seats came loose from their aircraft in combat maneuvers. Meredith’s TIG process presented a superior alter-
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native, and was patented in 1942.
TIG Basics Before you turn on the TIG machine, let’s answer some general questions. To review, TIG welding generates heat for melting metal via an intense electric arc that passes through a non-consumable tungsten electrode to the piece being welded. Filler material (welding rod) is fed into the weld zone and melted with the base metal using the tungsten electrode as the source of heat. The weld zone is shielded from the ambient air by an inert gas fed through the welding torch. This shield also provides a conducting path for the plasma column of the arc as it melts the base material. Argon is usual-
ly the inert gas of choice, but helium may also be used. Because TIG welds are shielded from the surrounding air (and the impurities it contains), the resulting welds are stronger and more corrosion resistant. TIG welds are also usually neater because the process produces little sparking or spatter-
ing and is controllable and precise. In the past, many homebuilders didn’t consider using TIG because the equipment was expensive. Thanks to solid-state electronics and computer controls, TIG machines are now more affordable, reliable, and easy to use. A good machine may be purchased for around $1,700.
Because TIG welds are protected from the surrounding air, the resulting welds are stronger and more corrosion resistant than welds made with other processes including gas welding. TIG welding may be used on most aircraft metals including stainless steel, aluminum, 4130 steel, magnesium, and titanium. Little within the equipment itself needs to be changed to weld different types of metals. TIG units are also capable of welding extremely thin metals. At EAA SportAir Workshops I’ve watched Lincoln Electric instructors weld two pieces of aluminum foil together! A TIG arc is super hot but easily controlled, meaning the heat input is localized, resulting in less metal distortion as compared to gas welding. Anyone with gas welding experience can easily adapt to TIG welding. If you have no welding experience at all, you can learn how at an EAA SportAir TIG Welding Workshop, which is presented several times a year in Sport Aviation
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aircraft building the Atlanta, Georgia, area. These weekend workshops teach homebuilders the basic skills so they can go back to their workshop and practice until proficient with the process. Just like gas welding, the welding end of a TIG outfit is called a torch, but rather than a flame, an electric arc between the TIG torch’s tungsten electrode and the piece being welded provides the necessary heat. At the “hot” end of the TIG torch’s insulated handle a clamp assembly known as the collet body, collet, and back cap holds the tungsten electrode. The torch’s “cold” end has connections for electrical power and the shielding gas. The tungsten will not melt because the collet acts as a heat sink, and the shielding gas, guided by a ceramic cup, cools the tungsten as it flows around it. New torches and torch consumables are designed to create a “gas lens” that provides a more uniform, non-turbulent flow of gas that minimizes the mixing of atmospheric air in the gas and plasma column. There are various sizes of tungsten electrodes that are used depending upon the
thickness and type of material both AC and DC is recommendyou are welding. Cups also come ed. These units are capable of welding most metals, and they in different sizes. At the other end of the electri- come with a stick electrode holdcal cable and gas hose is the elec- er allowing you to stick weld with trical power unit itself and a con- the machine in addition to TIG. A unit like the nection to the gas Lincoln Precision bottle. A cart that Regular welding TIG 185 has a micro holds the power start system that unit and gas cylingoggles used for allows the arc to be der is a handy way started with low to safely use and gas welding are amperage somemove the entire not satisfactory where around 5-6 unit. The cylinder amps. This feature must be chained in for TIG. will help you preplace just as you vent melting secure your gas welding bottles. A good source of through the edge on a saddle electrical power will also be notch or a T, Y, or K cluster. Your TIG machine should have required. Some of the smaller TIG units require only 120 volts, but at least a 90 percent duty cycle in most machines, such as the unit the mid range around 90 amps. pictured, will require 240 volts. A Duty cycle is the amount of time 240-volt, 50-amp standard plug is a power source can produce the maximum rated current and voltused. age in a 10-minute cycle. So, if a Required Equipment unit has a 50 percent duty cycle Most homebuilders or aircraft and is a 150-amp machine, it can restorers working on an “average” produce 150 amps for five out of project will need the following every 10-minute period. TIG items: machines in the price range we’re A power source. A good all- talking about may also have a around unit that would have a “pulse” unit built into them, protop end of at least 150 amps with viding a pulse of amperage that makes control of the puddle and TIG welding requires an argon gas cylinder arc more uniform and also gives that must be chained securely in place. the desirable “stacked dime” appearance of most weld faces. Current control. Most TIG welders have a foot control to vary the electrical current (heat). When you press the pedal the current flow increases to the preset maximum amperage. When you release the pedal, the current flow will decrease or completely shut off. The amperage is displayed on an LED display that shows the set amperage and actual amperage. For example, if you set the machine on 50 amps, the foot pedal controls the current from 0 to 50 amps. With this feature, small movements in the pedal do not result in large changes in amperage. 84
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With the right touch and a practiced hand, the TIG torch is controllable and precise.
TIG torch. For aircraft applications, your torch should accept a 1/16 collet and a 3/32 collet—along with appropriately sized electrodes. Electrodes are supplied as pure tungsten for aluminum and magnesium or thoriated tungsten for most other metals. Argon gas cylinder. This may be leased or purchased from your local welding supply company. Regulator for the argon bottle. Hose connection between the argon bottle and the torch. Welding hood with lens of a shading of 8 to 12. With TIG welding you may find that an auto-darkening helmet gives you increased flexibility in controlling the arc when you start to weld. If you wear bifocals, you’ll want to purchase a “cheater lens,” which is a bifocal that fits inside your welding helmet. Bench grinder. This will be used to grind the tungsten electrodes after they have been used. This grinder should be used exclusively for grinding tungsten. Other materials on the wheel will contaminate the tungsten electrode causing an erratic arc.
shade. Typically, with amperage less than 50, a shade 8 would be the minimum. If the amperage is 150, you’ll need a shade 10 to 12. The American Welding Society publishes a chart containing this information. A rule of thumb: Start welding with a shade that is too dark to see the weld zone, and then go to a lighter shade that will provide sufficient view of the weld without EAA SportAir going below the minimum. In Workshops the case of TIG welding, the Learn TIG Welding minimum will be 8. basics in Griffin One item often overlooked (Atlanta area), Georgia. involves preventing other Upcoming Workshops: people from receiving eye February 20-22, 2004 damage. If you’re doing any March 19-21, 2004 type of arc welding or TIG May 14-16, 2004 welding, you should consider June 25-27, 2004 working behind a welding For more information, visit curtain. If a child or neighwww.sportair.com. bor—or even a pet—happens into your workshop while you’re welding, serious eye damage could result. Be sure any onlookers wear eye protection. Next month in our continuing series, we’ll discuss how to set up the machine for welding and the basics of using a TIG torch.
TIG Welding Safety The safety issues with TIG welding are similar to other types of welding. Naturally, eye protection is the most important factor. TIG welding emits strong ultraviolet radiation that will burn unprotected eyes and skin. This type of burn is like severe sunburn. Regular welding goggles used for gas welding are not satisfactory for TIG. Wear a light and comfortable welding hood with the appropriate shade lens that protects the entire face. What welding amperage you use determines the lens
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