Brazing

Brazing is the process of joining two pieces of metal by using a nonferrous filler metal that has a lower melting point than the joined metals. Unlike soldering, brazing is done at temperatures above 425° C (800°F); and unlike welding, no melting occurs in the parts to be joined, although a limited amount of alloying may occur between the filler metal and the parts. The filler metal must wet the surfaces of the parts being joined and must be held and distributed in the joint by capillary action to fill the free space. Brazing is most advantageous for joining metals that are difficult to weld, for dissimilar metals, and for very thin sections of metals. The process is rapid, and the resulting joint is neat and easy to finish.

Brazed joints can have considerable strength, depending on the metals to be joined, the filler alloy, the amount of alloying that occurs in the joint, the cleanliness of the materials, the use of a flux, and the method of heating and cooling the assembly. The brazing flux protects the surfaces to be joined against oxidation and reduces the surface tension of the melted filler metal.

The four filler materials commonly used are copper, copper alloys, silver alloys, and aluminum alloys. Copper brazing is used with ferrous metals and other high-melting-point alloys, including high-speed steel and tungsten carbide. The product exhibits high strength. Copper brazing is usually done in a furnace at 1095 to 1150° C (2000-2100° F) in a protective atmosphere. Various copper alloys with melting points from 870 to 1290° C (1600-2250° F) are used to braze steel, cast iron, and copper. Alloys such as Tobin bronze, some silicon bronzes, and manganese bronzes, are applied with a torch and offer good strength at high temperatures.

Silver alloys, which are more expensive, have melting points ranging from 630 to 845° C (1165-1550° F). They are used extensively in the electrical and jewelry industries. Alloys range from silver-brass to silver-copper-zinc-cadmium. Aluminum brazing alloys are used exclusively for the brazing of aluminum and have a melting range of 570 to 640° C (1060-1185° F). For parts made of certain aluminum alloys no satisfactory brazing technique has yet been devised. Many special brazing alloys have been developed for high temperature applications such as gas turbines. One such alloy commonly used is nickel-chromium-iron-boron.

The proper design of joints for brazing is important and quite detailed. The three basic joints are butt, scarf, and lap joints, each of which can be used with flat, round, or tubular members. Lap joints are preferred for high strength or pressure tightness. To speed up production in brazing large numbers of identical parts, the filler metal is prepared as rings, rods, or other special shapes to fit the joint being brazed. This ensures that the proper amount of brazing alloy is present where it is needed.

New methods of brazing nonmetallic materials such as ceramics, carbides, and diamonds to metals in a one-step operation are carried out in a reducing atmosphere, in an inert gas, or in a vacuum. Either of two techniques may be used to produce a brazing alloy that will wet both metal and nonmetal. In the first method, hydrides of titanium, zirconium, tantalum, or columbium are used to coat and wet the surfaces to be brazed, and then a suitable solder such as copper-silver, silver, or aluminum is placed in contact with the hydride surfaces and melted. In the second method, special brazing alloys are employed, including aluminum-zirconium, silver-zirconium, and aluminum-silver-zirconium. '

Heating for brazing is accomplished in several ways: torch heating is the most common manual method and employs oxyacetylene or oxyhydrogen torches; furnace heating is used when a controlled atmosphere is needed; induction heating is useful for repetitive work and brazing in open air; other techniques include dipping in a metal bath, electric-arc heating, and resistance heating.

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