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(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.