brazing

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brazing,

method of joining metal parts using nonferrous filler metals with high melting points such as copper, silver, and aluminum alloysalloy
[O. Fr.,=combine], substance with metallic properties that consists of a metal fused with one or more metals or nonmetals. Alloys may be a homogeneous solid solution, a heterogeneous mixture of tiny crystals, a true chemical compound, or a mixture of these.
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. Brazing differs from soldering (see soldersolder
, metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i.e.
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) by using a higher temperature; and unlike weldingwelding,
process for joining separate pieces of metal in a continuous metallic bond. Cold-pressure welding is accomplished by the application of high pressure at room temperature; forge welding (forging) is done by means of hammering, with the addition of heat.
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, the parts are not melted. Brazing is best for dissimilar or thinner metal parts and for parts difficult to weld or solder.

Brazing

A method of joining metals, and other materials, by applying heat and a brazing filler metal. The filler metals used have melting temperatures above 840°F (450°C), but below the melting temperature of the metals or materials being joined. They flow by capillary action into the gap between the base metals or materials and join them by creating a metallurgical bond between them, at the molecular level. The process is similar to soldering, but differs in that the filler metal is of greater strength and has a higher melting temperature.

When properly designed, a brazed joint will yield a very high degree of serviceability under concentrated stress, vibration, and temperature loads. It can be said that in a properly designed brazement, any failure will occur in the base metal, not in the joint. There are many design variables to be considered. First among them is the mechanical configuration of the parts to be joined, and the joint area itself. All brazements can be categorized as having one of two basic joint designs: the lap joint or the butt joint. Others are adaptations of these two.

Design considerations should include the informed selection of the base and filler metals. In addition to the basic mechanical requirements, the base metals used in the brazement must retain the integrity of their physical properties throughout the heat of the brazing cycle. No universal filler metal that will satisfy all design requirements is possible, but there are many types available, ranging from pure metals such as copper, gold, or silver to complex alloys of aluminum, gold, nickel, magnesium, cobalt, silver, and palladium.

There are 11 basic brazing processes. In torch brazing, heat is applied by flame, from some type of torch, directly to the base metal. A mineral flux is normally used. The brazing filler metal may be preplaced in the joint, or face-fed into the joint. In induction brazing, brazing temperatures are developed in the parts to be brazed by placing them in or near a source of high-frequency ac electricity. Flux and preplaced filler metals are normally employed. Resistance brazing employs electrodes, which are arranged so that the joint forms a part of an electric circuit. Heat is developed by the resistance of the parts to the flow of the electric current. In dip brazing, the brazing filler metal is preplaced in or at the joint, and the assembly is immersed in a bath of molten salt or flux until the brazing temperature is achieved. In a variation of this process, the assembly is prefluxed and dipped into a bath of molten brazing filler metal. Infrared brazing is a process in which high-intensity quartz lamps are directed on the metals to be joined.

Furnace brazing is a widely used technique, especially useful where the parts to be brazed are machined or formed to their final dimensions, or constitute a complex assembly that has already been lightly joined or fixtured. The atmosphere within a brazing furnace is usually controlled, which permits a great deal of flexibility. An important advantage is that potential distortion of metal, created by heating and cooling, can be predicted and controlled and thereby minimized or eliminated. Also the capacity for automation is facilitated in the furnace brazing process.

Diffusion brazing, unlike furnace brazing, is defined not by the method of heating but rather by the degree of mutual fillermetal solution and diffusion with the base metal resulting from the temperature used and the time interval at heat. In diffusion brazing, temperature, time, in some cases pressure, and selection of base and filler materials are so controlled that the filler metal is partially or totally diffused into the base metal. The joint properties then closely approach those of the base metal.

Other, less used processes include arc brazing, block brazing, flow brazing, and twin carbon arc brazing.

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CREATIVE FLOW: Liverpool Community College plumbing students visiting BZT School, in Rohr-Kloster, Germany, as part of a fact-finding mission, included Danni-Lee Walsh, 18, from Speke, who is seen doing specialist silver soldering, a technique used in the installation of solar thermal systems; and, inset, below, Jake Hogg, 17, from Maghull, who is pictured on an interactive workboard which allows students to design green technologies within new buildings