Machinery's Handbook, 31st Edition
1568 SOLDERING soldering. In this method flux dissolves the aluminum oxide and keeps it from re-forming. The flux should be fluid at soldering temperatures so the solder can displace it in the joint. In the friction method the oxide film is mechanically abraded with a soldering iron, wire brush, or multi-toothed tool while being covered with molten solder. The molten solder keeps the oxygen in the atmosphere from reacting with the newly exposed aluminum surface; thus, wetting of the surface can take place. The alloys that are used in soldering aluminum generally contain from 50 to 75 percent tin, with the remainder being zinc. The following aluminum alloys are listed in order of ease of soldering: commercial and high-purity aluminum; wrought alloys containing not more than 1 percent manganese or magnesium; and finally, the heat-treatable alloys, which are the most difficult. Cast and forged aluminum parts are not generally soldered. Soldering Magnesium: Magnesium is not ordinarily soldered to itself or other metals. Soldering is generally used for filling small surface defects, voids, or dents in castings or sheets where the soldered area is not to be subjected to a load. Two solders can be used: one with a composition of 60 percent cadmium, 30 percent zinc, and 10 percent tin has a melting point of 315 ° F (157 ° C); the other has a melting point of 500 ° F (260 ° C) and has a nominal composition of 90 percent cadmium and 10 percent zinc. The surfaces to be soldered are cleaned to a bright metallic luster by abrasive methods before soldering. The parts are preheated with a torch to the approximate melting tempera ture of the solder being used. The solder is applied and the surface under the molten solder is rubbed vigorously with a sharp pointed tool or wire brush. This action results in the wetting of the magnesium surface. To completely wet the surface, the solder is kept molten and the rubbing action continued. The use of flux is not recommended. Soldering Stainless Steel: Stainless steel is somewhat more difficult to solder than other common metals because of a tightly adhering oxide film on the surface of the metal and because of its low thermal conductivity. The surface of the stainless steel must be thoroughly cleaned. This can be done by abrasion or by clean white pickling with acid. Mixtures that may be used as fluxes for soldering stainless steel include: muriatic (hydrochloric) acid saturated with zinc, or this mixture combined with 25 percent additional muriatic acid, or with 10 percent additional acetic acid, or with 10 to 20 percent additional water solution of orthophosphoric acid. Tin-lead solder can be used successfully. Because of the low thermal conductivity of stainless steel, a large soldering iron is needed to bring the surfaces to the proper temperature, which is reached when the solder flows freely into the area of the joint. Removal of the corrosive flux is important in order to prevent joint failure. Soap and water or a commercial detergent may be used to remove the flux residue. Ultrasonic Fluxless Soldering.— This more recently introduced method of soldering makes use of ultrasonic vibrations, which facilitates the penetration of surface films by the molten solder thus eliminating the need for flux. The equipment offered by one manufacturer consists of an ultrasonic generator and ultrasonic soldering head, which includes a trans ducer coupling, soldering tip, tip heater, and heating platen. Metals that can be soldered by this method include aluminum, copper, brass, silver, magnesium, germanium, and silicon. Brazing Brazing is a metal joining process that uses a nonferrous filler metal with a melting point below that of the base metals but above 840 ° F (450 ° C). The filler metal wets the base metal when molten in a manner similar to that of a solder and its base metal. There is a slight diffu- sion of the filler metal into the hot, solid base metal or a surface alloying of the base and filler metal. The molten metal flows between the close-fitting metals because of capillary forces. Filler Metals for Brazing Applications.— Brazing filler metals have melting points that are lower than those of the base metals being joined and have the ability when molten to flow readily into closely fitted surfaces by capillary action. The commonly used brazing
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