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
the weld pool or within the flame envelope.
Proper preparation for welding is an important factor in every welding operation. The edges of the parts must be prepared in accordance with the joint design chosen. The method chosen (bevel, groove, etc.) should allow for complete penetration of the base metal by the flame. The edges must be clean.
Arrangements must be made for preheating, if this is required.
Wher preparing an aircraft part for welding, remove all dirt, grease or oil, and any protective coating such as cadmium plating, enamel, paint, or varnish. Such coatings not only hamper welding, but also mingle with the weld and prevent good fusion.
Cadmium plating can be chemically removed by dipping the edges to be welded in a mixture of 1 lb.
of ammonium nitrate and 1 gal, of water.
Enamel, paint, or varnish may be removed from steel parts by a number of methods, such as a steel wire brush or emery cloth, by gritblasting, by using paint or varnish remover, or by treating the pieces with a hot, 10% caustic soda solution followed by a thorough washing with hot water to remove the solvent and residue. Gritblasting is the most effective method for removing rust or scale from steel parts. Grease or oil may be removed with a suitable grease solvent.
Enamel, paint, varnish, or heavy films of oxide on aluminum alloys can be removed using a hot 10% solution of either caustic soda or tri-sodium phosphate. After treatment, the parts should be immersed in a 10% nitric acid solution, followed with a hot water rinse to remove all traces of the chemicals. Paint and varnish can also be removed using paint and varnish remover.
The tip of the filler rod should be dipped below the surface of the weld puddle with a motion exactly opposite the motion of the torch. If the filler rod is held above the surface of the puddle, it will melt and fall into the puddle a drop at a time, ruining the weld.
Filler metal should be added until the surface of the joint is built up slightly above the edges of the parts being joined. The puddle of molten metal should be gradually advanced along the seam until the end of the material is reached.
As the end of the seam is approached, the torch should be raised slightly, chilling the molten steel to prevent it from spilling over the edge or melting through the work.
Chrome Molybdenum The welding technique for chrome molybdenum is practically the same as that for carbon steels, except that the surrounding area must be preheated to a temperature between 300° and 400° F. before beginning to weld. If this is not done, the sudden application of heat causes cracks to form in the heated area.
A soft neutral flame should be used for welding;
an oxidizing flame may cause the weld to crack when it cools; and a carburizing flame will make the metal brittle. The volume of the flame must be sufficient to melt the base metal, but not so hot as to weaken the grain structure of the surrounding area and set up strains in the metal. The filler rod should be the same as the base metal. If the weld requires high strength, a special chrome molybdenum rod is used and the piece is heat treated after welding.
Chrome molybdenum thicker than 0.093 in. is usually electric-arc welded, since for this thickness of metal, electric arc provides a narrow heat zone, fewer strains are developed, and a better weld is obtained, particularly when the part cannot be heat treated after welding.
Stainless Steel The procedure for welding stainless steel is basically the same as that for carbon steels. There are, however, some special precautions that must be taken to obtain the best results.
Only stainless steel used for nonstructural members of aircraft can be welded satisfactorily; the stainless steel used for structural components is cold worked or cold rolled and, if heated, loses some of its strength. Nonstructural stainless steel is obtained in sheet and tubing form and is often used for exhaust collectors, stacks or manifolds. Oxygen combines very readily with this metal in the molten state, and extreme care must be taken to prevent this from occurring.
A slightly carburizing flame is recommended for welding stainless steel. The flame should be adjusted so that a feather of excess acetylene, about 16 in. long, forms around the inner cone. Too much acetylene, however, will add carbon to the metal and cause it to lose its resistance to corrosion. The torch tip size should be one or two sizes smaller than that prescribed for a similar gage of low carbon steel. The smaller tip lessens the chances of overheating and subsequent loss of the corrosion-resistant qualities of the metal.
To prevent the formation of chromium oxide, a 259
flux should be spread on the underside of the joint and on the filler rod. Since oxidation is to be avoided as much as possible, sufficient flux should be used, Another method used to keep oxygen from reaching the metal is to surround the weld with a blanket of hydrogen gas. This method is discussed later. The filler rod used should be of the same composition as the base metal.
Since the coefficient of expansion of stainless steel is high, thin sheets which are to be butt-welded should be tacked at intervals of 1-¼ to 1-½ inches, as shown in figure 6-17. This is one means of lessening warping and distortion during the welding process.
Tack center first
- 1¼ to 1½ FIGURE 6-17. Tacking method for stainless 20/12 When welding, hold the filler rod within the en- steel welding.
velope of the torch flame so that the rod is melted in place or melted at the same time as the base metal. Add the filler rod by allowing it to flow into the molten pool. If the weld pool is stirred, air will enter the weld and increase oxidation. Avoid rewelding any portion or welding on the reverse side of the weld. Such practices result in warping and overheating of the metal.
WELDING NONFERROUS METALS USING OXY-
ACETYLENE Nonferrous metals are those that contain no iron.
Examples of nonferrous metals are lead, copper, silver, magnesium, and most important in aircraft construction, aluminum. Some of these metals are lighter than the ferrous metals, but in most cases they are not as strong. Aluminum manufacturers have compensated for the lack of strength of pure aluminum by alloying it with other metals or by cold working it. For still greater strength, some aluminum alloys are also heat treated.
Aluminum Welding The weldable aluminum alloys used in aircraft construction are 1100, 3003, 4043, and 5052. Alloy numbers 6053, 6061, and 6151 can also be welded, but since these alloys are in the heat-treated condition, welding should not be done unless the parts can be re-heat treated.
The equipment and technique used for aluminum welding differ only slightly from those of methods discussed earlier. As in all welding, the first step is to clean the surface to be welded-steel wool or & wire brush may be used, or a solvent in the case of paint or grease. The welder should be careful not to scratch the surface of the metal beyond the area to be welded; these scratches provide entry points for corrosion. The piece should then be preheated to lessen the strains caused by the large coefficient of expansion of aluminum.
Never preheat aluminum alloys to a temperature higher than 800° F. because the heat may melt some of the alloys and burn the metal. For thin sheet aluminum, merely passing the flame back and forth across the sheet three or four times should be sufficient.
Either of two types of filler rod can be used in welding aluminum alloys. Choosing the proper filler rod is important.
Aluminum and its alloys combine with air and form oxides very rapidly; oxides form doubly fast if the metal is hot. For this reason it is important to use a flux that will minimize or prevent oxides from forming.
Using the proper flux in welding aluminum is extremely important. Aluminum welding flux is designed to remove the aluminum oxide by chemically combining with it. Aluminum fluxes dissolve below the surface of the puddle and float the oxides to the top of the weld where they can be skimmed off. The flux can be painted directly on the top and bottom of the joint if no filler rod is required; if filler rod is used, it can be coated, and if the pieces to be welded are thick, both the metal and the rod should be coated with flux.
After welding is finished, it is important that all traces of flux be removed by using a brush and hot water. If aluminum flux is left on the weld, it will corrode the metal. A diluted solution of 10% sulfuric acid may be used if hot water is not available.
The acid solution should be washed off with cold water.
Thickness of the aluminum alloy material determines the method of edge preparation. On material up to 0.062 in., the edges are usually formed to a 90° flange about the same height as the thickness of the material (gure 6-18A). The flanges should be 260
straight and square. No filler rod is necessary when the edges are flanged in this manner.
Unbeveled butt welds are usually made on aluminum alloy from 0.062 to 0.188 in. thick. It may also be necessary to notch the edges with a saw or cold chisel in a manner similar to that shown in figure 6-18B. Edge notching is recommended in aluminum welding because it aids in getting full penetration and also prevents local distortion. All butt welds in material over 0.125 in. thick are generally notched in some manner.
(A)
10) FICURE 6-18. Edge preparation for welding aluminum.
In welding aluminum over 0.188 in. thick, the edges are usually beveled and notched as shown in figure 6-18C. The included angle of bevel may be from 90° to 120°.
A neutral flame should generally be used to weld aluminum alloys. In some cases a slightly carburizing flame can be used. However, the excess of acety.
lene should not be too great, as it will be absorbed into the molten metal, resulting in a weakened joint.
The torch must be adjusted to give the mildest flame that can be obtained without popping. The use of a strong, harsh flame makes it difficult to control the melting metal, and holes are often burned through the metal.
When starting to weld, the two joint edges should begin to melt before the filler rod is added. The work must be watched carefully for signs of melting. The melting point of aluminum is low and heat is conducted rapidly through the material. There is very little physical or color change to indicate that the metal is reaching the melting point. When the melting point is reached, the metal suddenly collapses and runs, leaving a hole in the aluminum.
A filler rod can be used to test the metal's condition. Aluminum begins to feel soft and plastic just before it reaches the melting point. Any tendency of the metal to collapse can be rectified by rapidly lifting the flame clear of the metal. With practice it is possible to develop enough skill to melt the metal surface without forming a hole.
The flame should be neutral and slanted at an approximate 45° angle to the metal. The inner cone should be about ⅛ in. from the metal. A constant and uniform movement of the torch is necessary to prevent burning a hole through the metal.
The correct integration of torch and rod action is important when welding aluminum. After heating the metal and when melting has begun, the filler rod is dipped into the pool and allowed to melt. The filler rod is lifted and the torch movement continues as the weld progresses. The rod is never lifted out of the outer envelope of flame, but is held there until almost melted and then added to the pool.
Magnesium Welding Many aircraft parts are constructed of magnesium because of its light weight, strength, and excellent machinability. This metal is only two-thirds.as heavy as aluminum and, like aluminum, is very soft in its pure state. For this reason, it is generally alloyed with zinc, manganese, tin, aluminum, or combinations of these metals. Repair of magnesium by welding is limited by two factors:
(1) If the magnesium is used as a structural member, it is usually heat treated and, like heat-treated aluminum, the welded section can never have the strength of the original meta). (As a rule, failures do not occur in the welded area, but in the areas adjacent to the weld, because the heat applied to 261
the metal weakens the grain structure in those areas.)
(2) It is necessary to use flux in making all magnesium welds, and to remove all the flux from the metal after welding or severe corrosion will take place.
The type of joint is limited to those that provide no possibility of trapping the flux-therefore, only butt welds can be made. Magnesium cannot be welded to other metals, and magnesium alloy castings are not considered suitable for stressed welds.
If varying thicknesses of magnesium are to be welded, the thicker part must be preheated. The filler rod should be of the same composition as the base metal and one prepared by the manufacturer to fuse with his alloy. The filler rod comes with a protective plating that must be cleaned off before using.
The method of preparing the butt joint depends on the thickness of the metal. Sheet magnesium alloy up to 0.040 in. thick should be flanged by about ⅔2 in. to the angle as indicated in figure 6-19.
Butt joints on metal from 0.040 to 0.125 in. thick are neither flanged nor beveled, but a /18-in. space should be allowed between the edges of the joint.
For butt joints in metal thicker than 0.125 in.,. each edge should be beveled down 45° to make a 90° included angle for the "V." A 16-in. space should be allowed between the edges of the joint for metal
0.125 to 0.250 in. thick and a ⅛-in. space for metal
0.250 in. and up (figure 6-19).
material as the base metal. The filler rod and both sides of the seam should be covered with flux. Use a neutral or slightly carburizing flame, and hold it at & flat angle to the work to avoid burning through.
Two rod techniques are recommended for the welding of magnesium. One method requires that the filler rod be kept in the puddle at all times; the other method is the same as that used in welding aluminum.
It is preferable to make the weld on one uninterrupted pass, but if oxidation occurs, the weld should be stopped and scraped out before continuing. The joint edges should be tack-welded at the ends at intervals of ½ to 3 in., depending upon the shape and thickness of the metal.
Welding should be accomplished as quickly and with as little heat as possible. Buckling and warping can be straightened while the metal is still hot by hammering with a soft-faced mallet. The metal should be allowed to cool slowly. When the weld is cool enough to handle, the accessible portions should be scrubbed lightly, using & bristle brush and hot water, to remove excess flux. The part should then be soaked in hot water (160° to 200° F.) to float off the flux adhering to any portions not reached by the scrub brush. When soaking is completed, the part should be immersed in a 1% solution of citric acid for approximately 10 min.
After the citric acid bath the part should be drained thoroughly and then rinsed clean in fresh water. The part must be dried quickly and completely to prevent oxidation.
• 3/32" min, # 1/16"
TITANIUM Titanium welding does not have the wide appli- Gages up to .040 .040 to .125 cation which steel has, therefore this handbook will not treat titanium as extensively.
• 90 90° Titanium Welding Titanium can be fusion welded with weld efficien-
• 1/16" 1/8" cies of 100% by arc welding techniques which, in .125 to .250 .250 and up many respects, are quite similar to those used for other metals.
FIcURE 6-19. Preparation of edges for Certain characteristics must be understood in welding magnesium sheet.
order to successfully weld titanium.
Remove oil or grease with a suitable solvent, and SNES then use a wire brush or abrasive cloth to clean and brighten the metal for a distance of ¾ in. back from the weld area. Select a filler rod of the same
1. Titanium and its alloys are subject to severe embrittlement by relatively small amounts of certain impurities. Oxygen and nitrogen even in quantities as low as 0.5% will embrittle a weld so much that it is useless.
Titanium, as it is heated toward its melting point will absorb oxygen and nitrogen from the atmosphere. In order to successfully weld titanium, the weld zone must be shielded with an inert gas such as argon or helium.
2. Cleanliness is necessary as titanium is very reactive with most materials. The metal and the welding area must be clean and free of dust, grease, and other contaminants. Contact with ceramic blocks and other foreign materials must be avoided during welding. Coated arc-welding electrodes, and other fluxing compounds will cause contamination and embrittlement.
3. Titanium, when alloyed excessively with other structural metals, looses ductility and impact strength due to the formation of brittle intermetallic compounds and excessive solid solution hardening,
4. Any fusion welding cycle results in a weld zone containing "as-cast material". Additionally, the high heat will have reduced the ductilities of certain highly heat-treatable titanium alloys to an unacceptable condition.
Equipment Either non-consumable electrode or consumable electrode arc welding equipment may be used for fusion welding of titanium. Whatever type is used the weld must be shielded with an inert gas such as argon or helium.
Titanium may be spot welded with any machine which has accurate control over the four basic spot welding parameters: welding current amperage, duration of welding current (cycles at 60 cycles per second), force applied to the welding electrodes (pounds per square inch), and electrode geometry.
The complexity of titanium welding processes and its limited application outside of specialized titanium fabrication shops does not justify detailed treatment in this handbook.
The foregoing discussion on titanium welding was extracted from Titanium Welding Techniques, published by Titanium Metals Corporation of America.
CUTTING METAL USING OXYACETYLENE Cutting metals by the oxyacetylene process is fundamentally the rapid burning or oxidizing of the metal in a localized area. The metal is heated to a bright red (1,400° to 1,600° F.), which is the kindling or ignition temperature, and a jet of high pressure oxygen is directed against it. This oxygen blast combines with the hot metal and forms an intensely hot oxide. The molten oxide is blown down the sides of the cut, heating the metal in its path to a kindling temperature. The metal thus heated also burns to an oxide which is blown away on the underside of the piece. The action is precise ly that which the torch accomplishes when the mixing head is replaced with a cutting attachment or when a special cutting torch is used.
Figure 6-20 shows an example of a cutting torch.
It has the conventional oxygen and acetylene needle valves, which control the flow of the two gases Many cutting torches have two oxygen needle valves so that a finer adjustment of the neutral flame can be obtained.
Cutting oxygen lever
- Oxygen needle valve Preheat orifices Acetylene needle valve Cutting oxygen oritice FICURE 6-20. Cutting torch.