BRAZING

process for joining two pieces of metal that involves the application of heat and the addition of a filler metal. This filler metal, which has a lower melting point than the metals to be joined, is either pre-placed or fed into the joint as the parts are heated. In brazing parts with small clearances, the filler is able to flow into the joint by capillary action. The temperature of the molten filler used for brazing exceeds 800 F (430 C). In a related process called soldering, the filler metal remains below that temperature. Brazed joints are usually stronger than soldered joints. Brazing can be carried out on most metals, and the range of available brazing alloys is increasing as new alloys and new service requirements are introduced. Heating by torch in air is satisfactory, provided the joint is adequately fluxed. Other forms of heating include inductive heating, electrical resistance, molten salts, and baths of molten metal. The wide use of these processes has led to the development of special furnaces and automatic equipment, with special attention being given to accurate control of the temperature and regulation of the atmosphere. Jigs and fixtures are necessary for dip brazing. Preparation of the surfaces by mechanical or chemical cleaning is important for brazing. The extensive use of silver-based brazing alloys melting at temperatures below 1,200 F (650 C) required development of fluxes that are fluid and active at 1,100 F (593 C). Combinations of borates, fluoborates, fluorides, chlorides, borax, and boric acid are commonly used as fluxes. Brazed joints are highly reliable and are used extensively on rockets, jet engines, and aircraft parts. See also welding.