OxyFuel Aluminum Brazing

April 30th, 2008

a. General. Many aluminum alloys can be brazed. Aluminum brazing alloys are used to provide an all-aluminum structure with excellent corrosion resistance and good strength and appearance. The melting point of the brazing filler metal is relatively close to that of the material being joined. However, the base metal should not be melted; as a result, close temperate control is necessary. The brazing temperature required for aluminum assemblies is determined by the melting points of the base metal and the brazing filler metal.

b. Commercial Filler Metals. Commercial brazing filler metals for aluminum alloys are aluminum base. These filler metals are available as wire or shim stock. A convenient method of replacing filler metal is by using a brazing sheet (an aluminum alloy base metal coated on one or both sides). Heat treatable or core alloys composed mainly of manganese or magnesium are also used. A third method of applying brazing filler metal is to use a paste mixture of flux and filler metal powder. Common aluminum brazing metals contain silicon as the melting point depressant with or without additions of zinc, copper, and magnesium.

c. Brazing Flux. Flux is required in all aluminum brazing operations. Aluminum brazing fluxes consist of various combinations of fluorides and chlorides and are supplied as a dry powder. For torch and furnace brazing, the flux is mixed with water to make paste. This paste is brushed, sprayed, dipped, or flowed onto the entire area of the joint and brazing filler metal. Torch and furnace brazing fluxes are quite active, may severely attack thin aluminum, and must be used with care. In dip brazing, the bath consists of molten flux. Less active fluxes can be used in this application and thin components can be safely brazed.

d. Brazed Joint Design. Brazed joints should be of lap, flange, lock seam, or tee type. Butt or scarf joints are not generally recommended. Tee joints allow for excellent capillary flow and the formation of reinforcing fillets on both sides of the joint. For maximum efficiency lap joints should have an overlap of at least twice the thickness of the thinnest joint member. An overlap greater than 1/4 in. (6.4 mm) may lead to voids or flux inclusions. In this case, the use of straight grooves or knurls in the direction of brazing filler metal flow is beneficial. Closed assemblies should allow easy escape of gases, and in dip brazing easy entry as well as drainage of flux. Good design for long laps requires that brazing filler metal flows in one direction only for maximum joint soundness. The joint design must also permit complete postbraze flux removal.

e. Brazing Fixtures. Whenever possible, parts should be designed to be self-jigging. When using fixtures, differential expansion can occur between the assembly and the fixture to distort the parts. Stainless steel or Inconel springs are often used with fixtures to accommodate differences in expansion. Fixture material can be mild steel or stainless steel. However, for repetitive furnace brazing operations and for dip brazing to avoid flux bath contamination, fixtures of nickel, Inconel, or aluminum coated steel are preferred.

f. Precleaning. Precleaning is essential for the production of strong, leaktight, brazed joints. Vapor or solvent cleaning will usually be adequate for the nonheat treatable alloys. For heat treatable alloys, however, chemical cleaning or manual cleaning with a wire brush or sandpaper is necessary to remove the thicker oxide film.

g. Furnace Brazing. Furnace brazing is performed in gas, oil, or electrically heated furnaces. Temperature regulation within 5°F (2.8°C) is necessary to secure consistent results. Continuous circulation of the furnace atmosphere is desirable, since it reduces brazing time and results in more uniform heating. Products of combustion in the furnace can be detrimental to brazing and ultimate serviceability of brazed assemblies in the heat treatable alloys.

h. Torch Brazing. Torch brazing differs from furnace brazing in that heat is localized. Heat is applied to the part until the flux and brazing filler metal melt and wet the surfaces of the base metal. The process resembles gas welding except that the brazing filler metal is more fluid and flows by capillary action. Torch brazing is often used for the attachment of fittings to previously weld or furnace brazed assemblies, joining of return bends, and similar applications.

i. Dip Brazing. In dip brazing operations, a large amount of molten flux is held in a ceramic pot at the dip brazing temperature. Dip brazing pots are heated internally by direct resistance heating. Low voltage, high current transformers supply alternating current to pure nickel, nickel alloy, or carbon electrodes immersed in the bath. Such pots are generally lined with high alumina content fire brick and a refractory mortar.

WARNING

The acid solutions used to remove aluminum welding and brazing fluxes after welding or brazing are toxic and highly corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling the acids and solutions. Do not inhale fumes. When spilled on the body or clothing, wash immediately with large quantities of cold water. Seek medical attention.

Never pour water into acid when preparing solutions: instead, pour acid into water. Always mix acid and water slowly. These operations should only be performed in well ventilated areas.

j. Postbrazing Cleaning. It is always necessary to clean the brazed assemblies, since brazing fluxes accelerate corrosion if left on the parts. The most satisfactory way of removing the major portion of the flux is to immerse the hot parts in boiling water as soon as possible after the brazing alloy has solidified. The steam formed removes a major amount of residual flux. If distortion from quenching is a problem, the part should be allowed to cool in air before being immersed in boiling water. The remaining flux may be removed by a dip in concentrated nitric acid for 5 to 15 minutes. The acid is removed with a water rinse, preferably in boiling water in order to accelerate drying. An alternate cleaning method is to dip the parts for 5 to 10 minutes in a 10 percent nitric plus 0.25 percent hydrofluoric acid solution at room temperature. This treatment is also followed by a hot water rinse. For brazed assemblies consisting of sections thinner than 0.010 in. (0.254 mm), and parts where maximum resistance to corrosion is important. A common treatment is to immerse in hot water followed by a dip in a solution of 10 percent nitric acid and 10 percent sodium dichromate for 5 to 10 minutes. This is followed by a hot water rinse. When the parts emerge from the hot water rinse they are immediately dried by forced hot air to prevent staining.

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OxyFuel Aluminum Welding

April 30th, 2008

a. General.

(1) General. Aluminum is readily joined by welding, brazing, and soldering. In many instances, aluminum is joined with the conventional equipment and techniques used with other metals. However, specialized equipment or techniques may sometimes be required. The alloy, joint configuration, strength required, appearance, and cost are factors dictating the choice of process. Each process has certain advantages and limitations.

(2) Characteristics of aluminum. Aluminum is light in weight and retains good ductility at subzero temperatures. It also has high resistance to corrosion, good electrical and thermal conductivity, and high reflectivity to both heat and light. Pure aluminum melts at 1220°F (660°C), whereas aluminum alloys have an approximate melting range from 900 to 1220°F (482 to 660°C). There is no color change in aluminum when heated to the welding or brazing range.

(3) Aluminum forms. Pure aluminum can be alloyed with many other metals to produce a wide range of physical and mechanical properties. The means by which the alloying elements strengthen aluminum is used as a basis to classify alloys into two categories: nonheat treatable and heat treatable. Wrought alloys in the form of sheet and plate, tubing, extruded and rolled shapes, and forgings have similar joining characteristics regardless of the form. Aluminum alloys are also produced as castings in the form of sand, permanent mold, or die castings. Substantially the same welding, brazing, or soldering practices are used on both cast and wrought metal. Die castings have not been widely used where welded construction is required. However, they have been adhesively bonded and to a limited extent soldered. Recent developments in vacuum die casting have improved the quality of the castings to the point where they may be satisfactorily welded for some applications.

(4) Surface preparation. Since aluminum has a great affinity for oxygen, a film of oxide is always present on its surface. This film must be removed prior to any attempt to weld, braze, or solder the material. It also must be prevented from forming during the joining procedure. In preparation of aluminum for welding, brazing, or soldering, scrape this film off with a sharp tool, wire brush, sand paper, or similar means. The use of inert gases or a generous application of flux prevents the forming of oxides during the joining process.

b. Gas Welding.

(1) General. The gas welding processes most commonly used on aluminum and aluminum alloys are oxyacetylene and oxyhydrogen. Hydrogen may be burned with oxygen using the same tips as used with acetylene. However, the temperature is lower and larger tip sizes are necessary (table 11-5). Oxyhydrogen welding permits a wider range of gas pressures than acetylene without losing the desired slightly reducing flame. Aluminum from 1/32 to 1 in. (0.8 to 25.4 mm) thick may be gas welded. Heavier material is seldom gas welded, as heat dissipation is so rapid that it is difficult to apply sufficient heat with a torch. When compared with arc welding, the weld metal freezing rate of gas welding is very slow. The heat input in gas welding is not as concentrated as in other welding processes and unless precautions are taken greater distortion may result. Minimum distortion is obtained with edge or corner welds.

(2) Edge preparation. Sheet or plate edges must be properly prepared to obtain gas welds of maximum strength. They are usually prepared the same as similar thicknesses of steel. However, on material up to 1/16 in. (1.6 mm) thick, the edges can be formed to a 90 degree flange. The flanges prevent excessive warping and buckling. They serve as filler metal during welding. Welding without filler rod is normally limited to the pure aluminum alloys since weld cracking can occur in the higher strength alloys. In gas welding thickness over 3/16 in. (4.8 mm), the edges should be beveled to secure complete penetration. The included angle of bevel may be 60 to 120 degrees. Preheating of the parts is recommended for all castings and plate 1/4 in. (6.4 mm) thick or over. This will avoid severe thermal stresses and insure good penetration and satisfactory welding speeds. Common practice is to preheat to a temperature of 700°F (371°C). Thin material should be warmed with the welding torch prior to welding. Even this slight preheat helps to prevent cracks. Heat treated alloys should not be preheated above 800°F (427°C), unless they are to be postweld heat treated. Preheating above 800°F (427°C) will cause a “hot-short” and the metal strength will deteriorate rapidly.

(3) Preheat temperature checking technique. When pyrolytic equipment (temperature gauges) is not available, the following tests can be made to determine the proper preheat temperatures:

(a) Char test. Using a pine stick, rub the end of the stick on the metal being preheated. At the proper temperatures, the stick will char. The darker the char, the higher the temperature.

(b) Carpenter’s chalk. Mark the metal with ordinary blue carpenter’s chalk. The blue line will turn white at the proper preheat temperature.

(c) Hammer test. Tap the metal lightly with a hand hammer. The metal loses its ring at the proper preheat temperature.

(d) Carburizing test. Carburize the surface of the metal, sooting the entire surface. As the heat from the torch is applied, the soot disappears. At the point of soot disappearance, the metal surface is slightly above 300°F (149°C). Care should be used not to coat the fluxed area with soot. Soot can be absorbed into the weld, causing porosity.

(4) Welding flame. A neutral or slightly reducing flame is recommended for welding aluminum. Oxidizing flames will cause the formation of aluminum oxide, resulting in poor fusion and a defective weld.

(5) Welding fluxes.

(a) Aluminum welding flux is designed to remove the aluminum oxide film and exclude oxygen from the vicinity of the puddle.

(b) The fluxes used in gas welding are usually in powder form and are mixed with water to form a thin paste.

(c) The flux should be applied to the seam by brushing, sprinkling, spraying, or other suitable methods. The welding rod should also be coated. The flux wil1 melt below the welding temperature of the metal and form a protective coating on the surface of the puddle. This coating breaks up the oxides, prevents oxidation, and permits slow cooling of the weld.

WARNING

The acid solutions used to remove aluminum welding and brazing fluxes after welding or brazing are toxic and highly corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling the acids and solutions. Do not inhale fumes. When spilled on the body or clothing, wash immediately with large quantities of cold water. Seek medical attention. Never pour water into acid when preparing solutions; instead, pour acid into water. Always mix acid and water slowly. These operations should only be performed in well ventilated areas.

(d) The aluminum welding fluxes contain chlorides and flourides. In the presence of moisture, these will attack the base metal. Therefore, all flux remaining on the joints after welding must be completely removed. If the weld is readily accessible, it can be cleaned with boiling water and a fine brush. Parts having joints located so that cleaning with a brush and hot water is not practical may be cleansed by an acid dip and a cold or hot water rinse. Use 10 percent sulfuric acid cold water solution for 30 minutes or a 5 percent sulfuric acid hot water (150°F (66°C)) solution for 5 to 10 minutes for this purpose.

(6) Welding technique. After the material to be welded has been properly prepared, fluxed, and preheated, the flame is passed in small circles over the starting point until the flux melts. The filler rod should be scraped over toe surface at three or four second intervals, permitting the filler rod to come clear of the flame each time. The scraping action will reveal when welding can be started without overheating the aluminum. The base metal must be melted before the filler rod is applied. Forehand welding is generally considered best for welding on aluminum, since the flame will preheat the area to be welded. In welding thin aluminum, there is little need for torch movement other than progressing forward. On material 3/16 in. (4.8 mm) thick and over, the torch should be given a uniform lateral motion. This will distribute the weld metal over the entire width of the weld. A slight back and forth motion will assist the flux in the removal of oxide. The filler rod should be dipped into the weld puddle periodically, and withdrawn from the puddle with a forward motion. This method of withdrawal closes the puddle, prevents porosity, and assists the flux in removing the oxide film.

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OxyFuel Cutting with MAPP Gas

April 30th, 2008

Cutting with MAPP gas.

(1) Quality cuts with MAPP gas require a proper balance between preheat flame adjustment, oxygen pressure, coupling distance, torch angle, travel speed, plate quality, and tip size. Oxyfuel ratios to control flame condition are given in table 11-4.

(2) MAPP gas is similar to acetylene and other fuel gases in that it can be made to produce carburizing, neutral or oxidizing flames (table 11-4). The neutral flame is the adjust most likely to be used for flame cutting. After lighting the torch, slowly increase the preheat oxygen until the initial yellow flame becomes blue, with some yellow feathers remaining on the end of the preheat cones. This is a slightly carburizing flame. A slight twist of the oxygen valve will cause the feathers to disappear. The preheat cones will be dark blue in color and will be sharply defined. This is a neutral flame adjustment and will remain so, even with a small additional amount of preheat oxygen. Another slight twist of the oxygen valve will cause the flame to suddenly change color from a dark blue to a lighter blue color. An increase in sound also will be noted, and the preheat cones will become longer. This is an oxidizing flame. Oxidizing flames are easier to look at because of their lower radiance.(3) MAPP gas preheat flame cones are at least one and one-half times longer than acetylene preheat cones when produced by the same basic style of tip.(4) The situation is reversed for natural gas burners, or for torches with a two-piece tip. MAPP gas flame cones are much shorter than the preheat flame on a natural gas two-piece tip.

(5) Neutral flame adjustments are used most cutting. Carburizing and oxidizing flames also are used in special applications. For example, carburizing flame adjustments are used in stack cutting, or where a very square top edge is desired. The “slightly carburizing” flare is used to stack cut light material because slag formation is minimized. If a strongly oxidizing flame is used, enough slag may be produced in the kerf to weld the plates together. Slag-welded plates often cannot be separated after the cut is completed.

(6) A “moderately oxidizing” flame is used for fast starts when cutting or piercing. It produces a slightly hotter flame temperature, and higher burning velocity than a neutral flame. An oxidizing flame commonly is used with a “high-low” device. The large “high” oxidizing flame is used to obtain a fast start. As soon as the cut has started, the operator drops to the “low” position and continues the cut with a neutral flame.

(7) “Very oxidizing” flames should not be used for fast starting. An overly oxidizing flame will actually increase starting time. The extra oxygen flow does not contribute to combustion, but only cools the flame and oxidizes the steel surface.

(8) The oxygen pressure at the torch, not at some remotely located regulator, should be used. Put a low volume, soft flame on the tip. Then turn on the cutting oxygen and vary the pressure to find the best looking stinger (visible oxygen cutting stream).

(a) Low pressures give very short stingers, 20 to 30 in. (50.8 to 76.2 cm) long. Low-pressure stingers will break up at the end. As pressure is increased, the stinger will suddenly become coherent and long. This is the correct cutting oxygen pressure for the given tip. The long stinger will remain over a fairly wide pressure range. But as oxygen pressures are increased, the stinger returns to the short, broken form it had under low pressure. (b) If too high an oxygen pressure is used, concavity often will show on the cut surface. Too high an oxygen pressure also can cause notching of the cut surface. The high velocity oxygen stream blows the metal and slag out of the kerf so fast that the cut is continuously being started. If too low a pressure is used, the operation cannot run at an adequate speed. Excessive drag and slag formation results, and a wide kerf often is produced at the bottom of the cut. (9) Cutting oxygen, as well as travel speed, also affects the tendency of slag to stick to the bottom of a cut. This tendency increases as the amount of metallic iron in the slag increases. Two factors cause high iron content in slag: too high a cutting oxygen pressure results in an oxygen velocity through the kerf high enough to blow out molten iron before the metal gets oxidized; and too high a cutting speed results in insufficient time to thoroughly oxidize the molten iron, with the same result as high oxygen pressure. (10) The coupling distance is the distance between the end of the flame cones and the workpiece. Flame lengths vary with different fuels, and different flame adjusts. Therefore, the distance between the end of the preheat cones and the workpiece is the preferred measure (fig. 11-19). When cutting ordinary plate thicknesses up to 2 to 3 in. (5.08 to 7.62 cm) with MAPP gas, keep the end of the preheat cones abut 1/16 to 1/8 in. (0.16 to 0.32 cm) off the surface of the work. When piercing, or for very fast starts, let the preheat cones impinge on the surface. This will give faster preheating. As plate thicknesses increase above 6 in. (15.24 cm), increase the coupling distance to get more heating from the secondary flame cone. The secondary MAPP gas flame will preheat the thick plate far ahead of the cut. When material 12 in. (30.48 cm) thick or more is cut, use a coupling distance of 3/4 to 11/4 in. (1.91 to 3.18 cm) long.

(11) Torch angle (a) Torch, or lead angle, is the acute angle between the axis of the torch and the workpiece surface when the torch is pointed in the direction of the cut (fig. 11-20). When cutting light-gauge steel (up to 1/4 in. (0.64 cm) thick) a 40 to 50 degree torch angle allows much faster cutting speeds than if the torch were mounted perpendicular to the plate. On plate up to 1/2 in. (1.27 cm) thick, travel speed can be increased with a torch lead angle, but the angle is larger, about 60 to 70 degrees. Little benefit is obtained from cutting plate over 1/2 in. (1.27 cm) thick with an acute lead-angle. Plate over this thickness should be cut with the torch perpendicular to the workpiece surface.

(b) An angled torch cuts faster on thinner-gauge material. The intersection of the kerf and the surface presents a knife edge which is easily ignited. Once the plate is burning, the cut is readily carried through to the other side of the work. When cutting heavy plate, the torch should be perpendicular to the workpiece surface and parallel to the starting edge of the work. This avoids problems of non-drop cuts, incomplete cutting on the opposite side of the thicker plate, gouging cuts in the center of the kerf and similar problems.  (12) There is a best cutting speed for each job. On plate up to about 2 in. (5.08 cm) thick, a high quality cut will be obtained when there is a steady “purring” sound from the torch and the spark stream under the plate has a 15 degree lead angle. This is the angle made by the sparks coming out of the bottom of the cut in the same direction as the torch is traveling. If the sparks go straight down, or even backwards, it means travel speed is too high. (13) Cut quality. (a) Variations in cut quality can result from different workpiece surface conditions or plate compositions. For example, rusty or oily plates require more preheat, or slower travel speeds than clean plates. Most variations from the ideal condition of a clean, flat, low-carbon steel plate tend to slow down the cutting action.(b) One method to use for very rusty plate is to set as big a preheat flame as possible on the torch, then run the flame back and forth over the line to be cut. The extra preheat passes do several things. They spall off much of the scale that would otherwise interfere with the cutting action; and the passes put extra preheat into the plate which usually is beneficial in obtaining improved cut quality and speed.(c) When working with high strength low alloy plates such as ASTM A-242 steel, or full alloy plates such as ASTM A-514, cut a little bit slower. Also use a low oxygen pressure because these steels are more sensitive to notching than ordinary carbon steels.

(d) Clad carbon-alloy, carbon-stainless, or low-carbon-high-carbon plates require a lower oxygen pressure, and perhaps a lower travel speed than straight low-carbon steel. Ensure the low carbon-steel side is on the same side as the torch. The alloyed or higher carbon cladding will not burn as readily as the carbon steel. By putting the cladding on the bottom, and the carbon steel on the top, a cutting action similar to powder cutting results. The low-carbon steel on top burns readily and forms slag. As the iron-bearing slag passes through the high-carbon or high-alloy cladding, it dilutes the cladding material. The torch, in essence, still burns a lower carbon steel. If the clad or high-carbon steel is on the top surface, the torch is required to cut a material that is not readily oxidizable, and forms refractory slags that can stop the cutting action.

(14) Tip size and style. (a) Any steel section has a corresponding tip size that gives the most economical operation for a particular fuel. Any fuel will burn in any tip, of course. But the fuel will not burn efficiently, and may even overheat and melt the tip, or cause problems in the cut. For example, MAPP gas will not operate at peak efficiency in most acetylene tips because the preheat orifices are not large enough for MAPP. If MAPP gas is used with a natural-gas tip, there will be a tendency to overheat the tip. The tips also will be susceptible to flash back. A natural-gas tip can be used with MAPP gas, in an emergency, by removing the skirt. Similarly, an acetylene tip can be used if inefficient burning can be tolerated for a short run. (b) The reasons for engineering different tips for different fuel gases are complex. But the object is to engineer the tip to match the burning velocity, port velocity, and other relationships for each type of gas and orifice size, and to obtain the optimum flame shape and heat transfer properties for each type of fuel. Correct cutting tips cost so little that the cost of conversion is minute compared with the cost savings resulting from efficient fuel use, improved cut quality, and increased travel speed.

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