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.
BRAZING METHODS Brazing refers to a group of metal-joining processes in which the bonding material is a nonferrous metal or alloy with a melting point higher than 800° F., but is lower than that of the metals being joined. Brazing includes silver soldering, also called hard soldering, copper brazing, and aluminum brazing.
Brazing requires less heat than welding and can be used to join metals that are damaged by high heat. However, because the strength of brazed joints is not so great as welded joints, brazing is not used for structural repairs on aircraft. In deciding whether brazing of a joint is justified, it should be remembered that a metal which will be subjected to a sustained high temperature in use should not be brazed.
As the definition of brazing implies, the base metal parts are not melted. The brazing metal adheres to the base metal by molecular attraction and intergranular penetration; it does not fuse and amalgamate with them.
In brazing, the edges of the pieces to be joined are usually beveled as in welding steel. The surrounding surfaces must be cleaned of dirt and rust.
Parts to be brazed must be securely fastened together to prevent any relative movement. The strongest brazed joint is one in which the molten filler metal is drawn in by capillary action, thus a close fit must be obtained.
A brazing flux is necessary to obtain a good union between the base metal and the filler metal. A good flux for brazing steel is a mixture containing two parts borax and one part boric acid. Application of the flux may be made in the powder form or dissolved in hot water to a highly saturated solution. A neutral torch flame should be used, moved with a slight semicircular motion.
The base metal should be preheated slowly with a mild flame. When it reaches a dull red heat (in the case of steel), the rod should be heated to a dark or purple color and dipped into the flux. Since enough flux adheres to the rod, it is not necessary to spread it over the surface of the metal.
A neutral flame is used for most brazing applications. However, a slightly oxidizing fame should be used when copper/zinc, copper/zinc/silicon, or copper/zinc/nickel/silicon filler alloys are used.
When brazing aluminum and its alloys a neutral flame is preferred, but if difficulties are encountered, a slightly reducing flame is preferred to an oxidizing flame.
The filler rod can now be brought near the tip of the torch, causing the molten bronze to flow over a small area of the seam. The base metal must be at the flowing temperature of the filler metal before it will flow into the joint. The brazing metal melts when applied to the steel and runs into the joint by capillary attraction. The rod should continue to be added as the brazing progresses, with a rhythmic dipping action so that the bead will be built to a uniform width and height. The job should be completed rapidly and with as few passes as possible of the rod and torch.
When the job is finished, the weld should be allowed to cool slowly. After cooling, remove the flux from the parts by immersing them for 30 minutes in a lye solution.
Silver Solder The principal use of silver solder in aircraft work is in the fabrication of high-pressure oxygen lines and other parts which must withstand vibration and high temperatures. Silver solder is used extensively to join copper and its alloys, nickel and silver, as well as various combinations of these metals, and thin steel parts, Silver soldering produces joints of higher strength than those produced by other brazing processes.
It is necessary to use flux in all silver soldering operations because of the necessity for having the base metal chemically clean without the slightest film of oxide to prevent the silver solder from coming into intimate contact with the base metal.
The joint must be physically clean, which means it must be free of all dirt, grease, oil, and/or paint, and also chemically clean. After removing the dirt, grease, and/or paint, any oxide should be renoved by grinding or filing the piece until bright metal can be seen. During the soldering operation, the flux continues the process of keeping oxide away from the metal, and aids the flow of the solder.
In figure 6-21, three types of joints for silver soldering are shown. Flanged, lap, and edge joints, in which the metal may be formed to furnish a seam wider than the base metal thickness, furnish the type of joint which will bear up under all kinds of loads. If a lap joint is used, the amount of lap should be determined according to the strength needed in the joint. For strength equal to that of the base metal in the heated zone, the amount of lap 264
should be four to six times the metal thickness for sheet metal and small-diameter tubing.
Lap joint Flanged butt joint Edge joint Solder Solder FIGURE 6-21. Silver soldering joints. FIGURE 6-22. Soldering copper and soldering iron.
The oxyacetylene flame for silver soldering should be neutral, but may have a slight exceas of acetylene. It must be soft, not harsh. During both prebeating and application of the solder, the tip of the inner cone of the flame should be held about ½ in. from the work, The flame should be kept moving so that the metal will not become overheated.
When both parts of the base metal are at the right temperature (indicated by the flow of flux), solder can be applied to the surface of the under or inner part at the edge of the seam. It is necessary to simultaneously direct the flame over the seam and keep moving it so that the base metal remains at an even temperature.
SOFT SOLDERING Soft soldering is used chiefly for copper, brass, and coated iron in combination with mechanical seams; that is, seams that are rivited, bolted, or folded. It is also used where a leakproof joint is desired, and sometimes for fitting joints to promote rigidity and prevent corrosion. Soft soldering is generally performed only in very minor repair jobs.
This process is also used to join electrical connections. It forms a strong union with low electrical resistance.
Soft solder yields gradually under a steadily applied load and should not be used unless the transmitted loads are very low. It should never be used as a means of joining structural members.
A soldering copper (called a soldering iron if it is electrically heated) is the tool used in soldering.
Its purpose is to act as a source of heat for the soldering operation. The bit, or working face, is made from copper, since this metal will readily take on heat and transmit it to the work. Figure 6-22 shows a correctly shaped bit.
To tin the copper, it is first heated to a bright red, then the point is cleaned by filing until it is smooth and bright. No dirt or pits should remain on its surface. After the copper has been mechanically cleaned, it should be re-heated sufficiently to melt solder, and chemically cleaned by rubbing it lightly on a block of sal ammoniac. (If sal ammoniac is not available, powdered resin may be used.)
Then solder is applied to the point and wiped with a The last two operations may be combined by melting a few drops of solder on a block of sal ammoniac (cleaning compound) and then rubbing the soldering copper over the block until the tip is well coated with solder. A properly tinned copper has a thin unbroken film of solder over the entire surface of its point.
Soft solders are chiefly alloys of tin and lead.
The percentages of tin and lead vary considerably in various solders, with a corresponding change in their melting points, ranging from 293° to 592° F.
"half-and-balf" (50-50) solder is a general purpose solder and is most frequently used. It contains equal proportions of tin and lead and melts at ap.
proximately 360° F.
The application of the melted solder requires somewhat more care than is apparent. The parts should be locked together or held mechanically or manually while tacking. To tack the seam, the hot copper is touched to a bar of solder, then the drops of solder adhering to the copper are used to tack the seam at a number of points. The film of solder between the surfaces of a joint must be kept thin to make the strongest joint.
A hot, well-tinned soldering copper should be held so that its point lies flat on the metal at the seam, while the back of the copper extends over the seam proper at a 45° angle, and a bar of solder is touched to the point. As the solder melts, the copper is drawn slowly along the seam. As much solder as necessary is added without raising the soldering copper from the job. The melted solder should run between the surfaces of the two sheets and cover the full width of the seam. Work should progress along the seam only as fast as the solder will flow into the joint.
ELECTRIC ARC WELDING Electric arc welding is a fusion process based on the principle of generating heat with an electric arc jumping an airgap to complete an electrical circuit.
This process develops considerably more heat than an oxyacetylene flame. In some applications, it reaches a temperature of approximately 10,000° F.
Variations of the process are metallic arc welding.
inert-gas (helium) welding, and multi-arc welding.
The metallic arc and helium processes have the widest application in aircraft maintenance.
The welding circuit (figure 6-23) consists of a welding machine, two leads, an electrode holder, an electrode, and the work to be welded. The electrode, which is held in electrode holder, is connected to one lead, and the work to be welded is connected to the other lead. When the electrode is touched to the metal to be welded, the electrical circuit is completed and the current flows. When the electrode is withdrawn from the metal, an airgap is formed between the metal and the electrode. If this gap is of the proper length, the electric current will bridge this gap to form a sustained electric spark, called the electric arc.
Welding cable connector Welding cable and the work to be welded. Under the intense heat developed by the arc, a small part of the base metal or work to be welded is brought to the melting point instantaneously. At the same time, the end of the metal electrode is also melted, and tiny globules or drops of molten metal pass through the arc to the base metal. The force of the arc carries the molten metal globules directly into the puddle formed on the base metal, and thus filler metal is added to the part being welded. By moving the metal electrode along the joint and down to the work, a controlled amount of filler metal can be deposited on the base metal to form a weld bead.
The instant the arc is formed, the temperature of the work at the point of welding and the welding electrode increases to approximately 6,500° F. This tremendous heat is concentrated at the point of welding and in the end of the electrode, and simultaneously melts the end of the electrode and a small part of the work to form a small pool of molten metal, commonly called the crater.
The heat developed is concentrated and causes leas buckling and warping of work than gas welding. This localization of the heat is advantageous when welding cracks in heat-treated parts and when welding in close places.
Generator Electrode holder Ground cable Ground cable connector Metal being welded FIGURE 6-23. Typical arc-welding circuit.
Metallic Arc Welding Metallic arc welding is used mainly for welding low-carbon and low-alloy steels. However, many nonferrous materials, such as aluminum and nickel alloys, can be welded using this method.
To form an arc between the electrode and the work, the electrode is applied to the work and immediately withdrawn. This initiates an arc of intense heat. To maintain the arc between the electrode and the work, the metal electrode must be fed at a uniform rate or maintained at a constant distance from the work as it melts.
Metallic arc welding is a nonpressure fusion welding process which develops welding heat through an arc produced between a metal electrode Gas Shielded Arc-Welding A good weld has the same qualities as the base metal. Such a weld has been made without the molten puddle being contaminated by atmospheric oxygen and/or nitrogen. In gas-shielded arc welding, a gas is used as a covering shield around the arc to prevent the atmosphere from contaminating the weld.
The original purpose of gas shielded arc welding was to weld corrosion resistant and other difficultto-weld materials. Today the various gas shielded arc processes are being applied to all types of metal. See fig. 6-24 for typical applications.
The ease of operation, increased welding speed and the superiority of the weld, will lead to shielded arc-welding and oxy-acetelyne welding being replaced with gas shielded arc welding.
Advantages of Gas-Shielded Arc Welding The shielding gas excludes the atmosphere from the molten puddle. The resulting weld is stronger, more ductile and more corrosion resistant. Also welding of nonferrous metal does not require the use of flux. This eliminates flux removal, and the possibilities of gas pockets or slag inclusions.
Shielding gas -Welding torch Shielding gas Molten weld Moiten weld - Nozzie metal metal - •Consumable - Tungsten Solidified weld metal (Machine fed) electrode Solidified weld • Arc electrode metal - Filler wire (Hand fed) Gas metal arc welding (MIG). Gas tungsten - arc welding (TIG).
FIGURE 6-24A. Gas-shielded arc welding process.
Figures 6-24, 6-25, 6-26, and 6-27 courtesy Hobart Bros.
Another advantage of the gas shielded arc is that a neater and sounder weld can be made because there is very little smoke fumes or sparks to control. The weld may be observed at all times.
Weld splatter is held to a minimum, therefore there is little or no metal finishing required. A gasshielded weld does not distort the base metal near the weld. The completed weld is clean and free of the complications often encountered in other forms of metallic-arc or gas welding.
FicuRE 6-24(C). One of the many types of automatic welders.
Tungsten Inert Gas Welding (TIG) In Tungsten Inert Gas (TIG) welding a virtually non-consumable tungsten electrode is used (figure 6-25) to provide the arc for welding. During the welding cycle a shield of inert gas expels the air from the welding area and prevents oxidation of the electrode, weld puddle and surrounding heataffected zone. In TIG welding the electrode is used only to create the arc. If additional metal FIGURE 6-24(B). A semi-automatic welder. is needed, a filler rod is used in the same manner as in oxy-acetylene welding.
Regulator with flowmeter Klectrole holder Tungsten betrole Filler metal held manually, if used Electrical conductor Insulating sheath Gas supply Gas passages Shickling gas Work Power source FIGURE 6-25. Typical TIG welding equipment.
Shielding gas Regulator with flosmeter Wire reel Wire drive may be located in welding gon handle or at wire reel Wire drive Pover saurce Controls for governing wire drive current gas fee:
and cooling water, if wed J Contactor FreURE 6-26. Typical MIG welding equipment.
The type gas used in TIG welding depends upon the metal being welded. Argon, helium or a mixture of the two gases is used. Argon is used more extensively than helium because it is cheaper.
Argon is preferred for several reasons besides the cost. It is heavier therefore provides better cover.
It provides better cleaning action wher welding aluminum and magnesium. The welding arc is quieter and smoother. Vertical and overhead welding arcs are easier to control. Welding arcs 268
are easier to start and for a given welding the weld produced is narrower with a smaller heat-affected zone.
Helium is used primarily in TIG machine welding or when welding heavy material having high heat conductivity. The arc voltage is higher when using helium, therefore a lower current flow is possible to get the same arc power.
Metal Inert Gas Arc Welding (MIG) With the substitution of a continuous feed consumable wire electrode for the non-consumable tungsten electrode used in TIG, the welding process becomes Metal Inert Gas Arc Welding (figure 6-26). The wire electrode is fed continuously through the center of the torch at pre-set controlled speeds, shielding gas is fed through the torch, completely covering the weld puddle with a shield of gas. This tends to complete automation of the welding process. Power, gas flow, wire feed and travel over the work piece are preset when using a welding machine. In semi-automatic welding, the operator controls the travel over the work.
Argon is the commonly used gas. Some metals use small amounts of helium or oxygen. Low carbon steel uses carbon dioxide or argon plus 2% Oz.
Plasma is often considered the fourth state of matter. The other three are gas, liquid, and solid.
Plasma results when a gas is heated to a high temperature, and changes into positive ions, neutral atoms and negative electrons. When matter passes from one state to another latent heat is generated In a plasma torch the electrode is located within the nozzle. The nozzle has a relatively small orifice which constricts the arc. The high-pressure gas flows through the arc where it is heated to the plasma temperature range. Since the gas cannot expand due to the constriction of the nozzle, it is forced through the opening, and emerges in the form of a supersonic jet. This neat melts any known metal and its velocity blasts the molten metal through the kerf (figure 6-27).
TUNCSTEN ELECTRODE
- OUTER (COOL) SHEATH Plasma Arc Welding
ARC CORE Plasma welding is a process which utilizes a - INNER (HOT) SHEATH central core of extreme temperature, surrounded by a sheath of coal gas. The required heat for fusion is generated by an electric arc which has been highly intensified by the injection of a gas into the arc stream.
The super-heated columnar arc is concentrated into a narrow stream and when directed on metal makes possible butt welds up to one-half inch in FicURE 6-27. Plasma welding uses a centrol core of exthickness or more in a single pass without filler treme temperature surrounded by a sheath of rods or edge preparation. cool gas.
In many respects plasma welding may be considered as an extension of the conventional TIG welding. In plasma arc welding the arc column WELDING PROCEDURES AND TECHNIQUES is constricted and it is this constriction that pro- The first step in preparing to arc weld is to make duces the much higher heat transfer rate. certain that the necessary equipment is available and that the welding machine is properly connected The arc plasma actually becomes a jet of high and in good working order. Particular attention current density. The arc gas upon striking the metal cuts through the piece producing a small should be given to the ground connection, since a hole which is carried along the weld seam. During poor connection will result in a fluctuating arc, this cutting action, the melted metal in front of the which is difficult to control.
arc flows around the arc column, then is drawn The electrode should be clamped to its holder at together immediately behind the hole by surface right angles to the jaws. Shielded electrodes have tension forces and reforms in a weld bead. one end of the electrode free of coating to provide 269
good electrical contact. The electrode holder should be handled with care to prevent accidental contact with the bench or work, since such contact may weld it fast.
Before starting to weld, the following typical list of items should be checked:
(1) Is the machine in good working order?
(2) Have all connections been properly made?
Will the ground connection make good contact?
(3) Has the proper type and size electrode been selected for the job?
(4) Is the electrode properly secured in the holder?
(5) Has sufficient protective clothing been provided, and is it in good condition?
(6) Is the work metal clean?
(7) Does the polarity of the machine coincide with that of the electrode?
(8) Is the machine adjusted to provide the necessary current for striking the are?
The welding are is established by touching the plate with the electrode and immediately withdrawing it a short distance. At the instant the electrode touches the plate, a rush of current flows through the point of contact. As the electrode is withdrawn, an electric are is formed, melting a spot on the base metal and the end of the electrode.
The main dificulty confronting a beginner in striking the are is freezing; that is, sticking or welding the electrode to the work. If the electrode is not withdrawn promptly upon contact with the plate, the high amperage will flow through the electrode and practically short circuit the welding machine. The heavy current melts the electrode which sticks to the plate before it can be withdrawn.
There are two essentially similar methods of striking the arc. The first is a touch method, illustrated in figure 6-28, and the second is a scratch method, shown in figure 6-29.
When using the touch method, the electrode should be held in a vertical position, and lowered until it is an inch or so above the point where the arc is to be struck. Then the electrode is touched Touch to work gently and swiftly Withdraw to long arc -1/8" to 3/16" FIGURE 6-28. Touch method of starting the arc.
Long arc immediately after striking 20°-25° Sweeping motion of electrode FIGURE 6-29. Scratch method of starting the arc.
very gently and swiftly to the work, using a downward motion of the wrist, followed immediately by withdrawing the electrode to form a long arc (approximately ⅛ to 16 in. long).
To strike the arc by the scratch method, the electrode is moved downward until it is just above the plate and at an angle of 20° to 25°, as illustrated in figure 6-29. The arc should be struck gently, with a swiftly sweeping motion, scratching the electrode on the work with a wrist motion. The electrode is then immediately withdrawn to form a long arc. The purpose of holding an excessively long arc immediately after striking is to prevent the large drops of metal, passing across the arc at this 270
time, from shorting out the arc and thus causing freezing.
To form a uniform bead, the electrode must be moved along the plate at a constant speed in addition to the downward feed of the electrode. The rate of advance, if too slow, will form a wide bead resulting in overlapping, with no fusion at the edges. If the rate of advance is too fast, the bead will be too narrow and have little or no fusion at the plate. When proper advance is made, no overlapping occurs, and good fusion is assured.
In advancing the electrode, it should be held at an angle of about 20° to 25° in the direction of travel, as illustrated in figure 6-30.
3 Crater FIGURE 6-31. Re-starting the arc.
Multiple-Pass Welding Groove and fillet welds in heavy metals often require the deposit of a number of beads to complete a weld. It is important that the beads be deposited in a predetermined sequence to produce the soundest welds with the best proportions. The number of beads is, of course, determined by the thickness of the metal being welded.
The sequence of the bead deposits is determined by the kind of joint and the position of the metal.
All slag must be removed from each bead before another bead is deposited. Typical multiple-pass groove welding of butt joints is shown in figure
6-32.
20° Techniques of Position Welding to 25* Each time the position of a welding joint or the type of joint is changed, it may be necessary to change any one or a combination of the following:
(1) Current value, (2) electrode, (3) polarity, (4)
- 1/16" to 1/8" arc length, or (5) welding technique. Current values are determined by the electrode arc length size as well as the welding position. Electrode size is FICURE 6-30. Angle of electrode. governed by the thickness of the metal and the joint preparation, and the electrode type by the welding position. Manufacturers specify the polarity to be a crater will be formed at the point where the are If the arc is broken during the welding of a bead, by a combination of the electrode size, welding used with each electrode. Arc length is controlled ends. The are may be broken by feeding the elec- position, and welding current.
trode too slowly or too fast, or when the electrode Since it is impractical to cite every possible variashould be replaced. The are should not be re-started tion occasioned by different welding conditions, in the crater of the interrupted bead, but just shead only the information necessary for the commonly of the crater on the work metal. Then, the electrode used positions and welds is discussed here.
should be returned to the back edge of the crater.
From this point, the weld may be continued by welding right through the crater and down the line of weld, as originally planned. Figure 6-31 illustrates the procedure for re-starting the arc.
Every particle of slag must be removed from the vicinity of the crater before re-starting the arc. This prevents the slag from becoming trapped in the wela.
Flat Position Welding There are four types of welds commonly used in flat position welding. They are the bead, groove, fillet, and lap joint welds. Each type is discussed separately in the following paragraphs.
Bead Welds Welding a square butt joint by means of stringer beads involves the same techniques as depositing 271