Gas Welding Aluminum
(1) 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) Aluminum 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 Aluminum.
(1) 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) Gas welding aluminum 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) Gas welding aluminum 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) Aluminum 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) Aluminum hammer test. Tap the metal lightly with a hand hammer. The metal loses its ring at the proper preheat temperature.
(d) Aluminum 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) Gas welding aluminum 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) Gas welding aluminum 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 aluminum 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 will 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.
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 fluorides. 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) Gas welding aluminum 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.