Aircraft Building:Welding Methods

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Aircraft Building WELDING IS ESSENTIALLY A method of joining metals without the use of fasteners. The actual process Joints and techniques for success involves melting the edges of two pieces of metal that RON ALEXANDER are joined and then allowing the molten material to flow together so the two pieces review the different types of welds become one. How metal is joined and weld joints so you will better understand the mechanics together is our topic this month. There are three methods of weld- involved. Prior to initiating the ing that are typically used for air- welding process, you must place the craft construction and repair: oxy- pieces of metal together properly. acetylene (gas welding), gas metal This is called “fit up.” Following fit arc welding (MIG), and gas tungsten up the members are held in place arc welding (TIG). As an aircraft using a tack weld. A tack weld is a builder or restorer, you’ll choose the small weld that holds the parts type that best suits your needs for while they are being welded during building or repairing. You will want the final assembly operation. There to look at the advantages and disad- are five basic types of weld joints vantages of each, the cost, the com- that you will encounter in aircraft plexity, the original fabrication building (see Figure 1). These five technique used in construcFigure 1. Types of weld joints.demo. tion, etc. One word of caution: You will encounter varied opinions as to which method is best for use on aircraft. Try to sift through all of the information you receive Butt joint before making your decision. Our discussion will concentrate mainly on gas welding Corner joint and TIG welding. MIG welding is often used in production work, but it’s not well suited for final assembly use by the amateur welder workT-joint Lap joint ing on aircraft metals. MIG, however, is an excellent process for tacking components together before final TIG welding.

types of joints can be connected using two common types of welds: a groove weld or a fillet weld (see Figure 2). Fusion welding consists of melting one piece of metal into the other without using any type of filler material and is best applied to very thin metals. An edge weld on an aluminum fuel tank is an example of a fusion weld on an aircraft part. A fillet weld is one whose size is approximately a triangular crosssection joining two surfaces that are at roughly right angles to each other. Typical places for this type of weld in an airplane are flanges, gussets, tabs, etc. A groove weld is a weld in a channel on or between work piece surfaces, or on or between base metal edges or surfaces. The butt joint consists of welding two pieces of metal edge to edge and uses a groove weld. Generally, the pieces are separated by about half the thickness of the metal. If you are welding metal more than 1/8-inch thick, it’s advisable to bevel the edges so you achieve an increased penetration of the weld into the base material. A lap joint is simply one piece of metal overlapping the other. As a rule of thumb, the minimum amount of overlap is five times the thinnest base member, but in no case less than 1 inch. Lap joints in sheet and plate are done with fillet welds. Another example of a lap joint is the

Welding Methods

Types of Welds and Joints Before we begin our comparison of welding methods, let’s

Edge joint

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Aircraft Building union of two pieces of tubing of different sizes using an internal sleeve as reinforcement. When splicing tubes in this manner, you’ll drill holes in the outer tube and weld it to the inner tube through this hole. In addition, you will weld along the scarf of the outer tube. This method of joining different size tubing is known as a plug weld or a rosette weld. A properly welded joint will be much stronger than the base metals that have been combined. Thorough and complete fusion is the most important characteristic of a sound weld. Penetration in welding is defined as the depth of fusion by the weld into the base material. Penetration is affected by the thickness of the metal being joined, the size of the filler metal, and how the filler is added. As an example, in a butt weld the penetration should be 100 percent of the thickness of the base metal. In a fillet weld, the penetration requirements are 25 to 50 percent of the base metal. Now that you have the basic idea of the types of welds, which method should you use to create the weld? Oxyacetylene The oxyacetylene process is considered a chemical welding method because a flame creates the energy. The equipment for gas welding is relatively simple, consisting of an oxygen tank, acetylene tank, pressure regulators, a torch, flash back arrestors, and connecting hoses. Just a word about torches and hoses. For the types of welding in aircraft construction and repair, a torch with a small body makes it easier to manipulate and to control the welding process. A small aircraft torch is shown in Figure 4. Light duty hoses also make the torch easier to hold and less tiring. The oxygen cylinder used in gas welding is a high100 OCTOBER 2003

pressure container capable of withstanding a pressure in excess of 2,000 psi. The acetylene cylinder typically has a pressure of approximately 375 psi. Attached to the top of each cylinder is a pressure regulator that reduces the high internal tank pressure to a working pressure. Care should be exercised to not allow the acetylene gas to reach working pressures above 15 psi because it will become unstable. Hoses connect the regulators to the torch itself. The welding torch includes a mixing section where the oxygen and acetylene combine to provide the necessary mixture for the flame. The ratio of these two gases can be altered by the adjustment of two separate control valves located on the torch. After the gases are mixed, they flow through a detachable tip. These tips are made in a variety of sizes to allow welding of different metal thicknesses. Oxyacetylene welding has been used extensively in aircraft welding for many years and is a popular and economical method of joining metals—especially thin materials. An entire setup for gas welding can be

purchased for less than $400. The tanks are normally leased from a local welding supplier. Gas welding outfits are portable and no electrical power is required. While the basics of gas welding are easy for the amateur to learn and become proficient at, oxyacetylene welding does have limitations. First, the adjustment of the flame to produce a neutral atmosphere takes practice and skill. If too much oxygen is used, the flame removes carbon, which makes the weld weaker. Too much acetylene adds carbon, and then the weld becomes carburized and brittle.

Gas Metal Arc Welding While gas metal arc welding is the formal designation for this process, it’s commonly referred to as “MIG” welding. A solid wire electrode fed continuously through a welding gun characterizes gas metal arc welding. An arc is created between this wire and the work piece to heat and melt the base and filler materials. Once molten, the wire becomes deposited in the weld joint. MIG welding uses carbon dioxide, or a carbon dioxide argon gas mixture, providing a “shield” that protects the weld from Figure 2—Types of butt joints using groove welds being contaminated by the surrounding air. The electrodes used for MIG are solid wires supplied on spools or reels of various sizes. This wire must be kept clean. If Single-square-groove weld allowed to remain out in the open, the wire may become contaminated with rust, oil, or moisture and foul the weld. The welding machine Single-bevel-groove weld itself is relatively inexpensive. You can purchase a 110-volt outfit for less than $400. And learning to use a MIG welder is relatively easy. A MIG welder Single-V-groove weld is a great tool for home

PHOTO COURTESY OF HARRIS CALORIFIC, A LINCOLN ELECTRIC COMPANY

Figure 3—Small body aircraft oxy-fuel torch

and workshop use. However, it’s more difficult to master this type of welding for aircraft use. My suggestion is to use the MIG process for tacking parts together if you so desire, and leave the finish MIG welding to experts who use the technique in production every day. Gas Tungsten Arc Welding Gas tungsten arc welding (GTAW), commonly referred to as “TIG” welding, is the new term for what has often been called “heli-arc” welding. TIG means tungsten inert gas. TIG welding uses an inert gas to protect the weld zone from the sur-

rounding atmosphere. It provides a strong weld with a neat appearance. Because TIG welding outfits are becoming more affordable, TIG is being widely used within the aircraft building community. In TIG welding, the necessary heat is provided by an intense electric arc that passes from a non-consumable tungsten electrode to the piece being welded. Welding rod is used as a filler material in TIG welding just as it is with gas welding. The welding rod is fed into the arc and melted with the base metal using the arc between the tungsten electrode and base as the source of heat.

See Figure 5. Why is an inert gas needed in the process? In any type of welding, the best possible weld results when the weld has the same physical properties as the base metal itself. This can only be obtained when the molten weld puddle is protected from the surrounding air or atmosphere. If ambient air is allowed to gather around the melting puddle during the welding operation, atmospheric oxygen and nitrogen will combine with the molten weld metal and result in a weak, porous weld. In simpler terms, the area you are welding must be protected from the sur-

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Aircraft Building rounding air to produce a good weld. In TIG welding, the area being welded is shielded from the air by an inert gas (usually argon) fed through the welding torch. Shielding the area also provides a conducting path for the plasma column of the arc as it melts the base material. Argon is usually the gas of choice, but helium may also be used, which explains the earlier term heli-arc. Because TIG welds are protected from the surrounding air, the resulting welds are stronger and have better performance characteristics than welds made with other processes—including gas welding. There is a minimum of sparking or spattering with TIG, and it’s a controllable and precise welding method. More and more aircraft builders are finding TIG welding to be effective on 4130 steel, stainless, and even aluminum. Although more pricey than other units, a TIG welding outfit can be purchased for less than $1,500. TIG welding has one major advantage in aircraft construction. It allows an amateur to become proficient in welding aluminum. Aluminum can be welded with gas, but it’s much easier to do with TIG, and the resulting welds are more precise. One significant advance in welding technology is the auto-darkening helmet. No longer do you put the helmet down and pray that your welding skill is instrument rated. The helmet allows you to see the weld joint through the helmet lens before you strike the arc. To learn more about welding, attend the weekend workshop on gas or TIG welding through an EAA SportAir workshop (check out www.sportair.org) or visit a welding workshop at EAA AirVenture Oshkosh each summer. Either of these education opportunities will provide EAA members with a solid overview of welding. 102

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