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Aircraft Welding and Steel Tube Fabrication Part 1
A Primer for the Novice Aircraft Builder
By Budd Davisson (originally published in EAA Experimenter, August 1987)
This is a series about learning to use an oxy-acetylene welding torch on aircraft. Period! It doesn't deal with repairing tractors, doing automotive body work or anything else. We are going to talk about nothing but airplanes and how to apply that magic blue and white flame to one without making a complete mess out of it.
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?
What is welding? Right now some folks out there are saying, "What a silly question! Everybody knows." Maybe yes; maybe no. Just so we don't go plowing ahead with everybody working on their own definition, let's run back through a few basics and define exactly what it is (and isn't) we're talking about.
It Isn't Soldering
There's very little chance that anybody is going to confuse aircraft welding with soldering, but to be on the safe side, let's make a blanket statement . . . soldering has absolutely no place in aircraft construction! None whatsoever! Basically, soldering is the joining, not the union, of two pieces of metal by melting a material with a much lower melting point than the parent metal and flowing it into the joint between the two pieces. There are several different types of soldering, the most common of which uses lead as the joining material. Silver alloy is also used, where a higher joint strength is required, such as in mounting gun sights.
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
In some ways brazing might be termed a high-strength technique of soldering, since they both join pieces of metal with a filler metal which has a lower melting point than the parent metal. In brazing, the most common filler material used is a copper/bronze alloy that depends on absolutely clean surfaces to adhere, so a flux is normally applied to the rod of the parent material to aid in cleaning the surface and promoting better adhesion.
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
Without getting into a whole lot of high tech terminology and metallurgical mumbo-jumbo, we can cut right to the bottom line and define welding: "Welding is a technique whereby the two pieces of metal to be joined are melted in a narrow area around the joint and allowed to flow together, thereby becoming one piece when cooled. Extra metal in the form of filler rod is normally melted into the molten area to add greater strength in the form of additional metal thickness." In other words, you are actually fusing the two pieces of metal together by allowing one to flow into the other. Sounds simple, doesn't it? It is, and at the same time, it isn't. There is a reasonable amount of both technique and understanding that go into making a strong, good-looking weld, but fortunately, there is nothing magical about it. Once the technique is learned, welding can be as much fun and as creative as dabbling in oils or sculpting a nude out of a block of pine. The real plus in aircraft welding is that when you're all done, you can hop in your creation and take off for Oshkosh . . . try that with a painting or a sculpture!
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.
Unless you go back quite a few years (to World War II and before), you may find it hard to believe that arc welding was ever used on-aircraft. AC/DC arc welding is the farmer's answer to the hammer . . . since you can't nail steel, you weld it. It's used to build ships and bridges and, believe it or not, at one time it was used to build aircraft.
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.
By far the most common type of welding in aircraft, especially in homebuilt and restoration circles, is oxygen/acetylene, or gas welding. The long, blue flame with the tiny blue-white cone in the middle of it is what most people project on their mind's eye when the world "welding" appears. Gas welding quite simply is using a highly concentrated oxygen/acetylene flame to bring the surface of the two pieces of metal to be joined up to melting temperature so that a portion of the surface melts and flows together, and a separate filler rod is fed into the puddle and melted at the same time, thereby causing the familiar weld "bead."
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)
As common as the gas torch is in the amateur workshop, that's how common the TIG welder is in the professional workshop. Within certain parameters, the TIG will do everything a gas welder will do and a few things more, although there is a good news-bad news side to TIG welding, too.
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.