Experimenter - 08/87
By Budd Davisson
Right from the beginning you should understand that aircraft welding is a unique skill in that it takes basic welding techniques and applies them in slightly different ways. The concept of welding an airplane is the same as welding on anything else, but the actual application is different, and it's the slight differences that separate the aircraft welder from the industrial/farm worker.
One last note before you jump head first into this series: Don't expect that by reading this you will automatically become a welder. Welding is not an intellectual exercise; it is a finely balanced combination of intellectual conceptualizing, visually observing and analyzing exactly what is going on, and using that input to guide your hands in orchestrating the eternal union of two pieces of steel. It boils down to three simple words: Welding is practice, practice, practice.
Welding: What is it?
It Isn't Soldering
Soldering is fine for sticking wires together in radios or working on radiators, but it has absolutely no application in primary structures.
It Isn't Brazing
Normal brazing is seldom, if ever, used in primary aircraft structures, although a number of antique aircraft (especially old homebuilts) did make limited use of it. Today it might be used in attaching secondary structures, such as upholstery mounting tabs, to the basic steel structure.
Very recently several new alloys have been developed which use the brazing technique (the parent metal is not melted), but the joint strength is much higher than that of normal brazing. Although none of these products are approved or recommended for primary structural use, they do appear to offer a viable alternative to normal bronze/brass brazing in the secondary structural application. There is even the possibility that, if a lightweight, low-speed ultralight-type airplane were designed specifically to use these new high-strength brazing materials, they could be used in primary structure. However, let the boys with the brains do some testing before anybody starts sticking airplanes together with anything new and untested.
This is Welding
As a general rule when you are talking about aircraft welding, it's assumed that you are talking about oxygen-acetylene (oxy-acetylene) welding. But that's not always the case, because there are at least three distinctly different types of welding concepts that have been, and are being, applied to aircraft. These are gas (or oxy-acetylene), arc, and TIG (Tungsten Inert Gas) welding, commonly referred to as Heli-arc, which is actually a trade name. Let's hit each one of those separately, then home in on gas welding, since that is the most common technique available to the aircraft builder and restorer.
Arc welding is a method in which high-voltage, high-amperage current is fed into a consumable electrode, or welding rod, so that when it is brought close to the workpiece, which is grounded, a fierce arc develops. In one single instant the surface of the workpiece and the tip of the electrode melt, allowing the melted electrode to be fed into the melted joint. It's a very effective method for joining metal . . . if you're building bridges and ships. However, arc welding is extremely difficult to control when working with thin metal, such as that found in aircraft tubing. Although the Stinson Aircraft Company used it very effectively in its wartime L-5 observation airplanes, very few companies have used it successfully since then. The arc that so effectively penetrates into heavy metal can cause .035 wall steel tubing to disappear before your very eyes. It also concentrates the weld heat into an extremely small, narrow band adjacent to the weld, which sets up tremendous stresses in the joint and the adjacent area when the weld cools, which means you have to go back and heat the entire area up to a cherry red to unlock all the stresses. On a ship or a tractor it doesn't make all that much difference, but in an airplane it's critical.
The long and the short of it is, you can forget about using arc welding on airplanes, except in very rare instances concerning extremely heavy plate weldments or when you are building a flying tractor.
There are some significant advantages to gas welding, not the least of which is it is a relatively simple technique to learn and use. Also, when building or repairing aircraft, or anything else for that matter, you will soon find that the acetylene torch is an absolutely indispensable tool around the shop for heating and bending metal. No matter what other type of welding you're using, any well-equipped shop requires a torch just for the oddball jobs that the other types of welding won't do. Gas welding also has the advantage of heating a larger area, and therefore not concentrating the heat in one small area, which in effect partially normalizes (heating up to a temperature and allowing to cool normally), which releases the majority of the locked-in stresses caused by the expansion and contraction of the metal being welded.
The bad news is that a gas torch has limitations in the amount of heat that a given tip can put out, so you find you're constantly changing tips and adjusting flame up and down for the job at hand, i.e. welding two small tubes requires a small tip and a small flame, and a half dozen large tubes coming together requires a much larger tip and a larger flame, which then must be adjusted as you go around it. Once in a while you'll even run into a joint that requires two gas torches to supply enough heat to maintain good welding temperature. Fortunately, however, 99.9 percent of the welding requirements in light aircraft can be met, one way or another, through the use of a single acetylene torch.
Because the technique is simple, the cost of the equipment is low, and the application is wide, the majority of this series will be devoted to the use of the gas torch, as opposed to any other type of welding equipment.
TIG (Tungsten Inert Gas)
Basically, TIG welding is the outgrowth of the technological marriage of the best points of both gas and arc welding. A small, almost wire-like tungsten electrode is contained in a ceramic "gun," which may vary in configuration, but most have the appearance of a woodburning set. A current is fed through the electrode, which then arcs into the workpiece similar to old-fashioned arc welding, except that the tungsten electrode is not consumed.
Also connected to the "gun," or tip, is a mixture of argon and helium gas, which blows out over the electrode and the area being heated, creating a totally inert shield which protects the weld surface and the parent material from any form of oxidizing. The joint area is brought up to temperature, and as it melts, the filler rod is fed into it, similar to gas welding.
The current is hooked up to a foot pedal, or even a finger control, which allows the intensity and heat of the flame to be increased or decreased to match the changing demands of the weld in progress. The advantages are obvious: You can get an extremely good, deep-penetrating weld on anything from the thinnest to the thickest material, and the weld is positively free of any outside contaminants caused by contact with the atmosphere. This is not the case with either gas or normal arc welding.
Again, the bad news: Even though they are coming down in price, a reliable TIG welder will always cost from four to eight times as much as an acetylene torch. Also, TIG concentrates the heat so tightly and the locked-in stresses are so high that it's not unusual for a TIG weld in 4130 aircraft steel to crack almost as soon as it's done. That's why almost all TIG welds must be heated immediately with an acetylene torch, brought up to a red temperature, and allowed to cool slowly; ergo you will still need the acetylene torch to do a proper job of welding with TIG.
The most common application of TIG welding for the amateur builder or restorer is on a "where needed" basis: When you run into an area you can't handle with an acetylene torch, take it down to your local specialty welding shop and let them put a TIG machine on it. But, it's important you pick someone who knows the characteristics of 4130 chromoly steel, so they'll use 308 or 309 stainless rods; it is less prone to cracking, is a little more flexible and can normalize the weld almost immediately.
Next month we'll continue this series with an in-depth look at the various types of equipment which will make the welding job much easier.
Members Home | EAA Flight Planner | e-HOT LINE | Aviation Advisors | Magazine Search | Aircraft Facts
EAA Aviation Center
logos, pictures, and videos are the property of the Experimental Aircraft Association, Inc.