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9/27/01 AC 43.13-1B CHG 1 replace the bar-shaped electrodes. The metal to be welded is moved between them, and electric pulses create spots of molten metal that overlap to form the continuous seam. 4-88. BRAZING. Brazing refers to a group of metal-joining processes in which the bond ing material is a nonferrous metal or alloy with a melting point higher than 425 C (800 F), but lower than that of the metals being joined. Brazing includes silver brazing (erroneously called silver soldering or hard soldering), cop per brazing, and aluminum brazing. NOTE: Never weld over a previously brazed joint. a. Brazing requires less heat than welding and can be used to join metals that are dam aged by high heat. However, because the strength of brazed joints is not as great as welded joints, brazing is not used for structural repairs on aircraft. In deciding whether braz ing of a joint is justified, it should be remem bered that a metal, which will be subjected to a sustained high temperature in use, should not be brazed. b. A brazing flux is necessary to obtain a good union between the clean base metal and the filler metal. There are a number of readily available manufactured fluxes conforming to AWS and AMT specifications. c. 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. d. A neutral flame is used in most brazing applications. However, a slightly oxidizing flame should be used when copper-zinc, cop- per-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 re duced flame is preferred to an oxidizing flame. e. 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 at traction. In braze welding, the rod should continue to be added, as the brazing pro gresses, 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 of the rod and torch as possible. f. When the job is finished, the metal should be allowed to cool slowly. After cool ing, remove the flux from the parts by im mersing them for 30 minutes in a lye solution. (1) Copper brazing of steel is normally done in a special furnace having a controlled atmos phere, and at a temperature so high that field repairs are seldom feasible. If copper brazing is attempted without a controlled atmosphere, the copper will probably not completely wet and fill the joint. Therefore, copper brazing in any conditions other than appropriately con trolled conditions is not recommended. (a) The allowable shear strength for copper brazing of steel alloys should be 15 thousand pounds per square inch (kpsi), for all condi tions of heat treatment. (b) The effect of the brazing process on the strength of the parent or base metal of steel alloys should be considered in the structural design. Where copper furnace brazing is em ployed, the calculated allowable strength of Par 4-87 Page 4-59PDF Image | AFS-640
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