GENERAL Metals can be joined by mechanical means (bolting or riveting, or by welding, brazing, soldering or adhesive bonding). All of these methods are used in aircraft construction. This chapter will discuss the methods used to join metals by welding, brazing, Welding Welding is the process of joining metal by fusing the materials while they are in a plastic or molten state. There are three general types of welding: (1) Gas, (2) electric arc, and (3) electric resistance welding. Lach of these types of welding has several variations which are used in aircraft construction.
Welding is used extensively in the repair and manufacture of aircraft. Such parts as engine mounts and landing gear are often fabricated in this manner, and many fuselages, control surfaces, fittings, tanks, etc., are also of welded construction.
Structures that have been welded in manufacture may generally be repaired economically by using the same welding process. Careful workmanship, both in preparation and actual welding, is of utmost importance.
Welding is one of the most practical of the many metal-joining processes available. The welded joint offers rigidity, simplicity, low weight, and high strength. Consequently, welding has been universally adopted in the manufacture and repair of all types of aircraft. Many structural parts as well as nonstructural parts are joined by some form of welding, and the repair of many of these parts is an indispensable part of aircraft maintenance.
It is equally important to know when not to weld, as it is to know when. Many of the alloy steels or high-carbon steel parts that have been hardened or strengthened by heat treatment cannot be restored to 100% of their former hardness and strength after they have been welded.
Gas Welaing Gas welding is accomplished by heating the ends or edges of metal parts to a molten state with a high- CHAPTER 6
AIRCRAFT WELDING temperature flame. This flame is produced with a torch burning a special gas such as acetylene or hydrogen with pure oxygen. The metals, when in a molten state, flow together to form a union without the application of mechanical pressure or blows.
Aircraft parts fabricated from chrome-molybdenum or mild carbon steel are often gas welded.
There are two types of gas welding in common use:
(1) Oxyacetylene and (2) oxyhydrogen. Nearly all gas welding in aircraft construction is done with an oxyacetylene flame, although some manufacturers prefer an oxyhydrogen flame for welding aluminum alloys.
Electric Arc Welding Electric arc welding is used extensively in both the manufacture and repair of aircraft, and can be satistactorily used in the joining of all weldable metals. The process is based on using the heat generated by an electric arc. Variations of the process are: (1) Metallic arc welding, (2) carbon arc welding, (3) atomic hydrogen welding, (4) inert-gas (helium) welding, and (5) multi-arc welding. Metallic arc and inert-gas welding are the two electric arc welding processes most widely used in aircraft construction.
Electric Resistance Welding Electric resistance welding is a welding process in which a low-voltage, high-amperage current is applied to the metals to be welded through a heavy, low-resistance copper conductor. The materials to be welded offer a high resistance to the flow of current, and the heat generated by this resistance fuses (welds) the parts together at their point of contact.
Three commonly used types of electric resistance welding are butt, spot, and seam welding. Butt welding is used in aircraft work to weld terminals to control rods. Spot welding is frequently used in airframe construction. It is the only welding method used for joining structural corrosion-resistant steel.
Seam welding is similar to spot welding, except that power-driven rollers are used as electrodes. A con- 247
tinuous airtight weld can be obtained using seam welding.
OXYACETYLENE WELDING EQUIPMENT Oxyacetylene welding equipment may be either stationary or portable. A portable equipment rig consists of the following:
(1) Two cylinders, one containing oxygen and one acetylene.
(2) Acetylene and oxygen pressure regulators, complete with pressure gages and connec tions.
(3) A welding torch, with a mixing head, extra tips and connections.
(4) Two lengths of colored hose, with adapter connections for the torch and regulators.
(5) A special wrench.
(6) A pair of welding goggles.
(7) A fint lighter.
(8) A fire extinguisher.
Figure 6-1 shows some of the equipment in a typical portable acetylene welding rig.
Oxygen pressure regulator Acetylene pressure Oxygen cylinder regulator Acetylene cylinder Torch - FicuRs 6-1. Typical portable acetylene welding rig.
Stationary oxyacetylene welding equipment is similar to portable equipment, except that acetylene and oxygen are piped to one or several welding stations from a central supply. The central supply mually consists of several cylinders connected to a common manifold. A master regulator controls the pressure in each manifold to ensure a constant pressure to the welding torch.
Acetylene Gas Acetylene gas is a flammable, colorless gas which has a distinctive, disagreeable odor, readily detectable even when the gas is heavily diluted with air.
Unlike oxygen, acetylene does not exist free in the atmosphere; it must be manufactured. The process is neither difficult nor expensive. Calcium carbide is made to react chemically with water to produce acetylene.
Acetylene is either used directly in a manifold system or stored in cylinders. If ignited, the result is a yellow, smoky flame with a low temperature.
When the gas is mixed with oxygen in the proper proportions and ignited, the result is a blue-white flame with temperatures which range from approximately 5,700° to 6,300° F.
Under low pressure at normal temperatures, acetylene is a stable compound. But when compressed in a container to pressures greater than 15 p.s.i., it becomes dangerously unstable. For this reason, manufacturers fill the acetylene storage cylinders with a porous substance (generally a mixture of asbestos and charcoal) and saturate this substance with acetone. Since acetone is capable of absorbing approximately 25 times its own volume of acetylene gas, a cylinder containing the correct amount of acetone can be pressurized to 250 p.s.i.
Acetyione Cylinders The acetylene cylinder is usually a seamless steel shell with welded ends, approximately 12 in. in diameter and 36 in. long. It is usually painted a distinctive color, and the name of the gas is stenciled or painted on the sides of the cylinder. A fully charged acetylene cylinder of this size contains approximately 225 cu. ft. of gas at pressures up to 250 p.s.i. In the event of fire or any excessive temperature rise, special safety fuse plugs installed in the cylinder will melt, allowing the excess gas to escape or burn, thus minimizing the chances of an explosion. The holes in the safety plugs are made small to prevent the flames from burning back into the cylinder. Acetylene cylinders should not be completely emptied, or a loss of filler material may result.
Oxygen Cylinders The oxygen cylinders used in welding operations are made of seamless steel of different sizes. A typi- 248
cal small cylinder holds 200 cu. ft. of oxygen at 1,800 p.s.i, pressure. A large size holds 250 cu. ft.
of oxygen at 2,265 p.s.i. pressure. Oxygen cylinders are usually painted green for identification The cylinder has a high-pressure valve located at the top of the cylinder. This valve is protected by a metal safety cap which should always be in place when the cylinder is not in use.
Oxygen should never come in contact with oil or grease. In the presence of pure oxygen, these substances become highly combustible. Oxygen hose and valve fittings should never be oiled or greased, or handled with oily or greasy hands. Even grease spots on clothing may flare up or explode if struck by a stream of oxygen. Beeswax is a commonly used lubricant for oxygen equipment and fittings.
Pressure Regulators Acetylene and oxygen regulators reduce pressures and control the flow of gases from the cylinders to the torch. Acetylene and oxygen regulators are of the same general type, although those designed for acetylene are not made to withstand such high pressures as those designed for use with oxygen. To prevent interchange of oxygen and acetylene hoses, the regulators are built with different threads on the outlet fitting. The oxygen regulator has a righthand thread, and the acetylene regulator has a lefthand thread.
On most portable welding units, each regulator is equipped with two pressure gages, a high-pressure gage which indicates the cylinder pressure and a low-pressure gage which indicates the pressure in the hose leading to the torch (working pressure).
In a stationary installation, where the gases are piped to individual welding stations, only one gage for oxygen and one for acetylene are required for each welding station, since it is necessary to indicate only the working pressure of the gases flowing through the hose to the welding torch.
A typical regulator, complete with pressure gages and connections, is shown in figure 6-2. The adjusting screw shown on the front of the regulator is for adjusting the working pressure. When this adjusting screw is turned to the left (counterclockwise) until it turns easily, the valve mechanism inside the regulator is closed. No gas can then flow to the torch. As the handle is turned to the right (clockwise), the screw presses against the regulating mechanism, the valve opens, and gas passes to the torch at the pressure shown on the working pressure gage. Changes in the working pressure can Low or torch High or cylinder pressure gage pressure gage outer scale indicates pounds ( pounds per per sq. in.pressure sq. in.) inner scale indicates cu-ft cylinder contents.
Double stage pressure regulator Cylinder connection Hose connection Adjusting screw handle FIcURE 6-2. Typical oxygen pressure regulator.
be made by adjusting the handle until the desired pressure is registered.
Before opening the high-pressure valve on a cylinder, the adjusting screw on the regulator should be fully released by turning it counterclockwise.
This closes the valve inside the regulator, protecting the mechanism against possible damage.
Welding Torch The welding torch is the unit used to mix the oxygen and acetylene together in correct proportions. The torch also provides a means of directing and controlling the size and quality of the flame produced. The torches are designed with two needle valves, one for adjusting the flow of acetylene and the for other adjusting the flow of oxygen.
Welding torches are manufactured in different sizes and styles, thereby providing a suitable type for different applications. They are also available with several different sizes of interchangeable tips in order that a suitable amount of heat can be obtained for welding the various kinds and thicknesses of metals.
Welding torches can be divided into two classes:
(1) The injector type and (2) the balanced-pressure type. The injector-type torch (figure 6-3A) is designed to operate with very low acetylene pressure as compared to the oxygen pressure.
A narrow passageway or nozzle within the torch, called the injector, through which the oxygen 249
Oxygen Acetylene A Injector-type welding torch.
Oxygen shutoff valve X Oxygen Acetylene shutoff valve Acetylene Mixed oxygen and acetylene B Balanced-pressure welding torch.
FICURE 6-3. Welding torches.
passes, causes the speed of oxygen flow to increase to a high velocity with a corresponding drop in pressure. This pressure drop across the injector creates a pressure differential which acts to draw the required amount of acetylene into the mixing chamber in the torch head.
In the balanced-pressure torch, the oxygen and acetylene are both fed to the torch at the same pressure (figure 6-3B). The openings to the mixing chamber for each gas are equal in size, and the delivery of each gas is independently controlled.
This type of torch is generally better suited for aircraft welding than the injector type because of the ease of adjustment.
Welding Torch Tips The torch tip delivers and controls the final flow of gases. It is important that the correct tip be selected and used with the proper gas pressures if a job is to be welded satisfactorily. The nature of the weld, the material, the experience of the welder, and the position in which the weld is to be made, all determine the correct size of the tip opening, The size of tip opening, in turn, determines the amount of heat (not the temperature) applied to the work.
If a tip which is too small is used, the heat provided will be insufficient to produce penetration to the proper depth. If the tip is too large, the heat will be too great, and holes will be burned in the metal.
The torch tip sizes are designated by numbers, and each manufacturer has his own arrangement for classifying them. As an example, a number two tip is made with an orifice of approximately 0.010 in. diameter. The diameter of the tip orifice is related to the volume of heat it will deliver.
Torch tips are made of copper or copper alloy and are made so that they seat well when tightened handtight. Torch tips should not be rubbed across fire brick or used as tongs to position work.
With use, the torch tip will become clogged with carbon deposits and, if it is brought in contact with the molten pool, particles of slag may lodge in the opening. A split or distorted flame is an indication of a clogged tip. Tips should be cleaned with the proper size tip cleaners or with a piece of copper or soft brass wire. Fine steel wool may be used to remove oxides from the outside of the tip. These oxides hinder the heat dissipation and cause the tip to overheat.
A flint lighter is provided for igniting the torch.
The lighter consists of a file-shaped piece of steel, usually recessed in a cuplike device, and a piece of flint that can be drawn across the steel, producing the sparks required to light the torch. Matches should never be used to ignite a torch since their length requires bringing the hand in close to the tip to ignite the gas. Accumulated gas may envelop the hand and, when ignited, cause a severe burn.
Goggles Welding goggles, fitted with colored lenses, are worn to protect the eyes from heat, light rays, sparks, and molten metal. A shade or density of color that is best suited for the particular situation should be selected. The darkest shade of lens which will show a clear definition of the work without eyestrain is the most desirable. Goggles should fit closely around the eyes and should be worn at all times during welding and cutting operations.
Welding (Filler) Rods The use of the proper type filler rod is very important in oxyacetylene welding operations. This material not only adds reinforcement to the weld Brea, but also adds desired properties to the finished weld. By selecting the proper rod, either tensile strength or ductility can be secured in a weld, or both can be secured to a reasonably high degree.
Similarly, rods can be selected which will help retain the desired amount of corrosion resistance. In some cases, a suitable rod with a lower melting point will eliminate possible cracks caused by expansion and contraction.
Welding rods may be classified as ferrous or nonferrous. The ferrous rods include carbon and alloy steel rods as well as cast-iron rods. Nonferrous rods include brazing and bronze rods, aluminum and aluminum alloy rods, magnesium and magnesium alloy rods, copper rods, and silver rods.
Welding rods are manufactured in standard 36- in. lengths and in diameters from 16 in. to ⅛ in.
The diameter of the rod to be used is governed by the thickness of the metals being joined. If the rod is too small, it will not conduct heat away from the puddle rapidly enough, and a burned weld will result. A rod that is too large will chill the puddle. As in selecting the proper size welding torch tip, experience enables the welder to select the proper diameter welding rod.
Setting Up Acetylene Welding Equipment Setting up acetylene welding equipment and preparing for welding should be done systematically and in a definite order to avoid costly mistakes. The following procedures and instructions are typical of those used to assure safety of equipment and personnel:
(1) Secure the cylinders so they cannot be upset, and remove the protective caps from the cylinders.
(2) Open each cylinder shutoff valve for an instant to blow out any foreign matter that may be lodged in the outlet. Close the valves and wipe off the connections with a clean cloth.
(3) Connect the acetylene pressure regulator to the acetylene cylinder and the oxygen regulator to the oxygen cylinder. Use a regulator wrench and tighten the connecting nuts enough to prevent leakage.
(4) Connect the red (or maroon) hose to the acetylene pressure regulator and the green (or black) hose to the oxygen regulator.
Tighten the connecting nuts enough to prevent leakage. Do not force these connections, since these threads are made of brass and are easily damaged.
(5) Release both pressure regulator adjusting screws by turning the adjusting screw handle on each regulator counterclockwise until it turns freely. This is to avoid damage to the regulators and pressure gages when the cylinder valves are opened.
(6) Open the cylinder valves slowly and read each of the cylinder pressure gages to check the contents in each cylinder. The oxygen cylinder shutoff valve should be opened fully and the acetylene cylinder 251
shutoff valve is opened approximately one and one-half turns.
(7) Blow out each hose by turning the pressure adjusting screw handle inward (clockwise) and then turning it out again.
The acetylene hose should be blown out only in a well-ventilated space which is free from sparks, flame, or other sources of ignition.
(8) Connect both hoses to the torch and check the connections for leaks by turning the pressure regulator screws in, with the torch needle valves closed. When 20 p.s.i.
shows on the oxygen working pressure gage and 5 p.s.i. on the acetylene gage, close the valves by turning the pressure regulator screws out. A drop in pressure on the working gage indicates a leak between the regulator and torch tip. A general tightening of all connections should remedy the situation. If it becomes necessary to locate a leak, use the soap suds method. Do this by painting all fittings and connections with a thick solution of the soapy water. Never hunt for an acetylene leak with a flame, since a serious explosion can occur in the hose or in the cylinder.
(9) Adjust the working pressure on both the oxygen and acetylene regulators by turning the pressure-adjusting screw on the regulator clockwise until the desired setings are obtained.
Oxyacetylene Flame Adjustment To light the torch, open the torch acetylene valve a quarter to a half turn. Hold the torch to direct the flame away from the body and ignite the acetylene gas, using the flint lighter. The pure acetylene flame is long and bushy and has a yellowish color. Continue opening the acetylene valve until the flame leaves the tip approximately one-sixteenth of an inch. Open the torch oxygen valve. When the oxy.
gen valve is opened, the acetylene flame is shortened, and the mixed gases burn in contact with the tip face. The flame changes to a bluish-white color and forms a bright inner cone surrounded by an outer flame envelope.
Oxyacetylene Welding Process The oxyacetylene process of welding is a method in which acetylene and oxygen gases are used to produce the welding flame. The temperature of this flame is approximately 6,300° F., which is sufficiently high to melt any of the commercial metals to effect a weld. When the oxyacetylene flame is applied to the ends or edges of metal parts, they are quickly raised to a melting state and flow together to form one solid piece when solidified. Usually some additional metal is added to the weld, in the form of a wire or rod, to build up the weld seam to a greater thickness than the base metal.
There are three types of flames commonly used for welding. These are neutral, reducing or carburizing, and oxidizing. The characteristics of the different kinds of flames are shown in figure 6-4.
Neutral flame B Reducing flame Oxidizing flame FIGURE 6-4. Characteristics of oxyacetylene flames.
The neutral flame (figure 6-4A) is produced by burning acetylene with oxygen in such proportions as to oxidize all particles of carbon and hydrogen in the acetylene. This flame is distinguished by the well-rounded, smooth, clearly defined white central cone at the end of the tip. The envelope or outer flame is blue with a purple tinge at the point and edges. A neutral flame is generally used for welding and gives a thoroughly fused weld, free from burned metal or hard spots.
To obtain a neutral flame, gradually open the oxygen valve. This shortens the acetylene flame and causes a "feather" to appear in the flame envelope.
Gradually increase the amount of oxygen until the "feather" disappears inside a clearly defined inner luminous cone.
The reducing or carburizing flame is shown in 252
figure 6-4B. Since the oxygen furnished through the torch is not sufficient to complete the combustion of the acetylene, carbon escapes unburned.
This flame can be recognized by the greenish-white brushlike second cone at the tip of the first cone.
The outer flame is slightly luminous and has about the same appearance as an acetylene flame burning freely in air alone. This type of flame introduces carbon into the steel.
To obtain a reducing flame, first adjust the flame to neutral; then open the acetylene valve slightly to produce a white streamer or "feather" of acetylene at the end of the inner cone.
An oxidizing flame (figure 6-4C) contains an excess of oxygen, which is the result of too much oxygen passing through the torch. The oxygen not consumed in the flame escapes to combine with the metal. This flame can be recognized by the short, pointed, bluish-white central cone. The envelope or outer flame is also shorter and of a lighter blue color than the neutral flame. It is accompanied by a harsh sound similar to high-pressure air escaping through a small nozzle. This flame oxidizes or burns most metals and results in a porous weld. It is used only when welding brass or bronze.
To obtain the oxidizing flame, first adjust the flame to neutral; then increase the flow of oxygen until the inner cone is shortened by about one-tenth of its length. The oxidizing flame has a pointed inner cone.
With each size of tip, a neutral, oxidizing or carburizing flame can be obtained. It is also possible to obtain a "harsh" or "soft" flame by increasing or decreasing the pressure of both gases.
For most regulator settings the gases are expelled from the torch tip at a relatively high velocity, and the flame is called "harsh." For some work it is desirable to have a "soft" or low-velocity flame without a reduction in thermal output. This may be achieved by using a larger tip and closing the gas needle valves until the neutral flame is quiet and steady. It is especially desirable to use a soft flame when welding aluminum, to avoid blowing holes in the metal when the puddle is formed.
Improper adjustment or handling of the torch may cause the flame to backfire or, in very rare cases, to flashback. A backfire is a momentary backward flow of the gases at the torch tip, which causes the flame to go out. A backhre may be causes by touching the tip against the work, by overheating the tip, by operating the torch at other than recommended pressures, by a loose tip or head, or by dirt or slag in the end of the tip. A backfre is rarely dangerous, but the molten metal may be splattered when the flame pops.
A flashback is the burning of the gases within the torch and is dangerous. It is usually caused by loose connections, improper pressures, or overheating of the torch. A shrill hissing or squealing noise accompanies a flashback; and unless the gases are turned off immediately, the flame may burn back through the hose and regulators and cause great damage. The cause of a flashback should always be determined and the trouble remedied before relighting the torch.
FICURE 6-5. Holding the acetylene torch to weld light-gage metals.
Extinguishing The Torch The torch can be shut off simply by closing both needle valves, but it is better practice to turn the acetylene off first and allow the gas remaining in the torch tip to burn out. The oxygen needle valve can then be turned off. If the torch is not to be used again for a long period, the pressure should be turned off at the cylinder. The hose lines should then be relieved of pressure by opening the torch needle valves and the working pressure regulator, one at a time, allowing the gas to escape. Again, it is a good practice to relieve the acetylene pressure and then the oxygen pressure. The hose should then be coiled or hung carefully to prevent damage or kinking.
rundamental Oxyacetylene Welding Techniques The proper method of holding the acetylene welding torch depends on the thickness of the metal being welded. When welding light-gage metal, the torch is usually held as illustrated in figure 6-5, with the hose draped over the wrist.
Figure 6-6 shows the method of holding the torch during the welding of heavy materials.
the weld is progressing, as illustrated in figure 6-7.
The filler rod is added to the puddle as the edges of the joint melt before the flame. The forehand method is used in welding most of the lighter tubings and sheet metals.
FICURE 6-7. Forehand welding.
FICURE 6-6. Holding the acetylene torch to weld heavy materials.
The torch should be held so that the tip is in line with the joint to be welded, and inclined between 30° and 60° from the perpendicular. The best angle depends on the type of weld to be made, the amount of preheating necessary, and the thickness and type of metal. The thicker the metal, the more nearly vertical the torch must be for proper heat penetration. The white cone of the flame should be held about ⅛ in. from the surface of the base metal.
If the torch is held in the correct position, a small puddle of molten metal will form. The puddle should be composed of equal parts of the pieces being welded. After the puddle appears, movement of the tip in a semicircular or circular motion should be started. This movement ensures an even distribution of heat on both pieces of metal. The speed and motion of the torch movement are learned only by practice and experience.
Forehand welding is the technique of pointing the torch flame forward in the direction in which Backhand welding is the technique of pointing the torch flame toward the finished weld and moving away in the direction of the unwelded area, melting the edges of the joint as it is moved (figure 6-8). The welding rod is added to the puddle between the flame and the finished weld.
Backhand welding is seldom used on sheet metal because the increased heat generated in this method is likely to cause overheating and burning. It is preferred for metals having a thick cross section.
The large puddle of molten metal required for such welds is more easily controlled in backhand weld ing, and it is possible to examine the progress of the weld and determine if penetration is complete.
FICURE 6-8. Backhand welding.
WELDING POSITIONS There are four general positions in which welds are made. These positions are shown in figure 6-9 and are designated as flat, overhead, horizontal, and vertical.
The overhead position is used when the material is to be welded on the underside with the seam running horizontally or in a plane that requires the flame to point upward from below the work. In welding overhead, a large pool of molten metal should be avoided, as the metal will drip or run out of the joint. The rod is used to control the size of POSITION OF WELDING the molten puddle. The volume of flame used should
FLAT HORIZONTAL VERTICAL OVERHEAD not exceed that required to obtain good fusion of the base metal with the filler rod. The amount of heat needed to make the weld is best controlled by selecting the right tip size for the thickness of metal to be welded.
When the parts to be joined are inclined at an angle of more than 45°, with the seam running vertically, it is designated as a vertical weld. In a vertical weld, the pressure exerted by the torch flame must be relied upon to a great extent to support the puddle. It is highly important to keep WELDS the puddle from becoming too hot, to prevent the hot metal from running out of the puddle onto the finished weld. Vertical welds are begun at the bottom, and the puddle is carried upward using the forehand technique. The tip should be inclined from 45° to 60°, the exact angle depending upon the desired balance between correct penetration and FICURE 6-9. Four basic welding positions. control of the puddle. The rod is added from the top and in front of the flame.
Welding is done in the flat position whenever possible, since the puddle is much easier to control in this position. Quite often, though, it is necessary to weld in the overhead, vertical, or horizontal position in aircraft repair.
The flat position is used when the material can be laid flat, or inclined at an angle of less than 45° and welded on the topside. The welding torch is pointed downward toward the work. This weld may be made by either the forehand or backhand technique, depending upon the thickness of the metal being welded.
The horizontal position is used when the line of the weld runs horizontally across a piece of work, and the torch is directed at the material in a horizontal or nearly horizontal position. The weld is made from right to left across the plate (for the right-handed welder). The flame is inclined upward at an angle of from 45° to 60°. The weld can be made using the forehand or backhand technique.
Adding the filler rod to the top of the puddle will help prevent the molten metal from sagging to the lower edge of the bead.
WELDED JOINTS The five fundamental types of welded joints (figure 6-10) are the butt joint, tee joint, lap joint, corner joint, and edge joint.
Butt Joints A butt joint is made by placing two pieces of material edge to edge, so that there is no overlapping, and then welded. Some of the various types of butt joints are shown in figure 6-11. The flanged butt joint can be used in welding thin sheets, 16 in.
or less. The edges are prepared for welding by turning up a flange equal to the thickness of the metal. This type of joint is usually made without the use of filler rod.
A plain butt joint is used for metals from 716 in.
to % in. in thickness. A filler rod is used when making this joint to obtain a strong weld.
If the metal is thicker than ⅛ in., it is necessary to bevel the edges so that the heat from the torch can penetrate completely through the metal. These 255
ULI Butt Lap Plain Single bevel Double bevel joint Edge joint joint FICURE 6-12. Types of tee joints.
Edge Joints Corner An edge joint may be used when two pieces of joint sheet metal must be fastened together and load Tee stresses are not important. Edge joints are usually joint made by bending the edges of one or both parts upward, placing the two bent ends parallel to each FIGURE 6-10. Basic welding joints. other or placing one bent end parallel to the upright unbent end, and welding along the outside of the seam formed by the two joined edges. Figure 6-13 shows two types of edge joints. The type shown in fig. 6-13A requires no filler rod, since the Flanged Plain edges can be melted down to fill theseam. The type shown in fig 6-13B, being thicker material, must be beveled for heat penetration; filler rod is added for Single bevel Double bevel reinforcement.
FICURE 6-11. Types of butt joints.
bevels may be either single- or double-bevel type or single- or double-V type. A filler rod is used to add strength and reinforcement to the weld.
Cracks Repair of cracks by welding may be considered just another type of butt joint. A stop drill hole is made at either end of the crack, then the two edges are brought together. The use of a filler rod is necessary.
Corner Joints A corner joint is made when two pieces of metal are brought together so that their edges form a corner of a box or enclosure as shown in figure
6-14. The corner joint shown in fig. 6-14A requires little or no filler rod, since the edges fuse to make the weld. It is used where load stress is unimportant. The joint shown in fig 6-14B is used on heavier metals, and filler rod is added for roundness and strength. If much stress is to be placed on Tee Joints A tee joint is formed when the edge or end of one piece is welded to the surface of another, as shown in figure 6-12. These joints are quite common in aircraft work, particularly in tubular aircraft metal thicknesses, but heavier thicknesses structures. The plain tee joint is suitable for most (A) (B) require the vertical member to be either single or Thin stock Thick stock double beveled to permit the heat to penetrate deeply enough. The dark areas in figure 6-12 show the depth of heat penetration and fusion required. FIGURE 6-13. Edge joints.
(A) (B) Closed type Open type (C) Braced FICURE 6-14. Corner joints.
the corner, the inside is reinforced as shown in fig.
6-14C.
Lap Joints The lap joint is seldom used in aircraft structures when welding with oxyacetylene, but is commonly used when spot welding. The single lap joint (figure 6-15) has very little resistance to bending, and will not withstand the shearing stress to which the weld may be subjected under tension or compression loads. The double lap joint (figure 6-15) offers more strength, but requires twice the amount of welding required on the simpler, more efficient butt weld.
Single ap Double lap FICURE 6-15. Single and double lap joints.
EXPANSION AND CONTRACTION OF METALS Heat causes metals to expand; cooling causes them to contract. Uneven heating, therefore, will cause uneven expansion, or uneven cooling will cause uneven contraction. Under such conditions, stresses are set up within the metal. These forces must be relieved, and unless precautions are taken, warping or buckling of the metal will take place.
Likewise, on cooling, if nothing is done to take up the stress set up by the contraction forces, further warping may result; or if the metal is too heavy to permit this change in shape, the stresses remain within the metal itself.
The coefficient of linear expansion of a metal is the amount in inches that a 1 in. piece of metal will expand when its temperature is raised 1° F. The amount that a piece of metal will expand when heat is applied is found by multiplying the coefficient of linear expansion by the temperature rise, and that product by the length of the metal in inches. For example, if a 10 ft. aluminum rod is to be raised to a temperature of 1,200° F. from a room temperature of 60° F., the rod will expand 1.75 in.-
0.00001280 (aluminum's coefficient of linear expansion) X 120 (length in inches) X 1140 (temperature rise).
Expansion and contraction have a tendency to buckle and warp thin sheet metal ⅜ in. or thinner.
This is the result of having a large surface area that spreads heat rapidly and dissipates it soon after the source of heat is removed. The most effective method of alleviating this situation is to remove the heat from the metal near the weld, and thus prevent it from spreading across the whole surface area.
This can be done by placing heavy pieces of metal, known as "chill bars," on either side of the weld;
they absorb the heat and prevent it from spreading.
Copper is most ofen used for chill bars because of its ability to absorb heat readily. Welding jigs sometimes use this same principle to remove heat from the base metal.
Expansion can also be controlled by tack welding at intervals along the joint.
The effect of welding a long seam (over 10 or 12 in.) is to draw the seam together as the weld progresses. If the edges of the seam are placed in contact with each other throughout their length before welding starts, the far ends of the seam will actually overlap before the weld is completed. This tendency can be overcome by setting the pieces to be welded with the seam spaced correctly at one end and increasing the space at the opposite end as shown in figure 6-16. The amount of space depends on the type of material, the thickness of the material, the welding process being used, and the shape and size of the pieces to be welded.
(A) (B) (C) FicURE 6-16. Allowance for a straight butt weld when welding steel sheets.
The weld is started at the correctly spaced end and proceeds toward the end that has the increased gap. As the seam is welded, the space will close and should provide the correct gap at the point of welding. Sheet metal under 16 in. can be handled by flanging the edges, tack welding at intervals, and then by welding between the tacks.
There is less tendency for plate stock over ⅛ in.
to warp and buckle when welded because the greater thickness limits the heat to a narrow area and dissipates it before it travels far on the plate.
Preheating the metal before welding is another method of controlling expansion and contraction.
Preheating is especially important when welding tubular structures and castings. Great stress can be set up in tubular welds by contraction. When two members of a tee joint are welded, one tube tends to draw up because of the uneven contraction. If the metal is preheated before the welding operation begins, contraction still takes place in the weld, but the accompanying contraction in the rest of the structure is at almost the same rate, and internal stress is lessened.
CORRECT FORMING OF A WELD The form of the weld metal has considerable bearing upon the strength and fatigue resistance of a joint. The strength of an improperly made weld is usually less than the strength for which the joint was designed. Low-strength welds are generally the result of insufficient penetration; undercutting of the base metal at the toe of the weld; poor fusion of the weld metal with the base metal; trapped oxides, slag, or gas pockets in the weld; overlap of the weld metal on the base metal; too much or too little reinforcement; and overheating of the weld.
Characteristics of a Good Weld A completed weld should have the following characteristics:
(1) The seam should be smooth, the bead ripples evenly spaced, and of a uniform thickness.
(2) The weld should be built up, thus providing extra thickness at the joint.
(3) The weld should taper off smoothly into the base metal.
(4) No oxide should be formed on the base metal close to the weld.
(5) The weld should show no signs of blowholes, porosity, or projecting globules.
(6) The base metal should show no signs of burns, pits, cracks, or distortion.
Although a clean, smooth weld is desirable, this characteristic does not necessarily mean that the weld is a good one; it may be dangerously weak inside. However, when a weld is rough, uneven, and pitted, it is almost always unsatisfactory inside.
Welds should never be filed to give them a better appearance, since filing deprives the weld of part of its strength. Welds should never be filled with solder, brazing material, or filler of any sort. Additional information about weld characteristics is con:
tained in Chapter 10, of the Airframe and Powerplant Mechanics General Handbook, AC 65-9A.
When it is necessary to re-weld a joint, all old weld material must be removed before the operation is begun. It must be remembered, though, that reheating the area may cause the base metal to lose some of its strength and become brittle.
OXYACETYLENE WELDING OF FERROUS METALS Steal Low-carbon steel, low-alloy steel, cast steel, and wrought iron are easily welded with the oxyacetylene flame. Plain, low-carbon steel is the ferrous material that will be gas welded most frequently. As the carbon content of steel increases, it may be repaired by welding only under certain conditions.
Factors involved are carbon content and hardenability. For corrosion- and heat-resistant nickel chromium steels, the allowed weldability depends upon their stability, carbon content, or re-heat treatment.
In order to make a good weld, the carbon content of the steel must not be altered, nor can other chemical constitutents be added to or subtracted from the base metal without seriously altering the properties of the metal. Molten steel has a great affnity for carbon, and oxygen and nitrogen combine with the molten puddle to form oxides and nitrates, both of which lower the strength of steel.
When welding with an oxyacetylene flame, the inclusion of impurities can be minimized by observ.
ing the following precautions:
(1) Maintain an exact neutral flame for most steels, and a slight excess of acetylene when welding alloys with a high nickel or chromium content, such as stainless steel.
(2) Maintain a soft flame and control the puddie.
(3) Maintain a flame sufficient to penetrate the metal and manipulate it so that the molten metal is protected from the air by the outer envelope of flame.
(4) Keep the hot end of the welding rod in 258