a. General weldingtheory
(1) Brazing is similar to the soldering processes in that a filler rod with a melting point lower than that of the base metal, but stove 800°F (427°C) is used. A groove, fillet, plug, or slot weld is made and the filler metal is distributed by capillary attraction. In brazing, a nonferrous filler rod, strip, or wire is used for repairing or joining cast iron, malleable iron, wrought iron, steel, copper, nickel, and high melting point brasses and bronzes. Some of these brasses and bronzes, however, melt at a temperature so near to that of the filler rod that fusion welding rather than brazing is required.
(2) Besides a welding torch with a proper tip size, a filler metal of the required composition and a proper flux are important to the success of any brazing operation.
(3) The choice of the filler metal depends on the types of metals to be joined. Copper-silicon (silicon-bronze) rods are used for brazing copper and copper alloys. Copper-tin (phosphor bronze) rods are used for brazing similar copper alloys and for brazing steel and cast iron. Other compositions are used for brazing specific metals.
(4) Fluxes are used to prevent oxidation of the filler metal and the base metal surface, and topromote the free flowing of the filler metal. They should be chemically active and fluid at the brazing temperature. After the joint members have been fitted and thoroughly cleaned, an even coating of flux should be brushed over the adjacent surfaces of the joint, taking care that no spots are left uncovered. The proper flux is a good temperate indicator for torch brazing because the joint should be heated until the flux remains fluid when the torch flame is momentarily removed.
b. Characteristics. For satisfactory use in brazing applications, brazing filler metals must possess the following properties:
(1) The ability to form brazed joints possessing suitable mechanical and physical properties for the intended service application.
(2) A melting point or melting range compatible with the base metals being joined and sufficient fluidity at brazing temperature to flow and distribute into properly prepared joints by capillary action.
(3) A composition of sufficient homogeneity and stability to minimize separation of constituents (liquation) under the brazing conditions encountered.
(4) The ability to wet the surfaces of the base metals being joined and form a strong, sound bond.
(5) Depending on the requirements, ability to produce or avoid base metal-filler metal interactions.
c. Filler Metal Selection. The following factors should be considered when selecting a brazing filler metal:
(1) Compatibility with base metal and joint design.
(2) Service requirements for the brazed assembly. Compositions should be selected to suit operating requirements, such as service temperature (high or cryogenic), thermal cycling, life expectancy, stress loading, corrosive conditions, radiation stability, and vacuum operation.
(3) Brazing temperature required. Low brazing temperatures are usually preferred to economize on heat energy; minimize heat effects on base metal (annealing, grain growth, warpage, etc.); minimize base metal-filler metal interaction; and increase the life of fixtures and other teals. High brazing temperatures are preferred in order to take advantage of a higher melting, but more economical, brazing filler metal; to combine annealing, stress relief, or heat treatment of the base metal with brazing; to permit subsequent processing at elevated temperatures; to promote base metal-filler metal interactions to increase the joint remelt temperature; or to promote removal of certain refractory oxides by vacuum or an atmosphere
d. Aluminum-Silicon Filler Metals. This group is used for joining aluminum and aluminum alloys. They are suited for furnace and dip brazing, while some types are also suited for torch brazing using lap joints rather than butt joints. Flux should be used in all cases and removed after brazing, except when vacuum brazing. Use brazing sheet or tubing that consists of a core of aluminum alloy and a coating of lower melting filler metal to supply aluminum filler metal. The coatings are aluminum-silicon alloys and may be applied to one or both sides of sheet. Brazing sheet or tubing is frequently used as one member of an assembly with the mating piece made of an unclad brazeable alloy. The coating on the brazing sheet or tubing melts at brazing temperature and flows by capillary attraction and gravity to fill the joints. weldingtheory
e. Magnesium Filler Metals. Because of its higher melting range, one magnesium filler metal (BMg-1) is used for joining AZ10A, KIA, and MIA magnesium alloys, while the other alloy (BMg-2a), with a lower melting range, is used for the AZ31B and ZE10A compositions. Both filler metals are suited for torch, dip, or furnace brazing processes. Heating must be closely controlled with both filler metals to prevent melting of the base metal.
f. Copper and Copper-Zinc Filler Metals. These brazing filler metals are used for joining various ferrous metals and nonferrous metals. They are commonly used for lap and butt joints with various brazing processes. However, the corrosion resistance of the copper-zinc alloy filler metals is generally inadequate for joining copper, silicon bronze, copper-nickel alloys, or stainless steel.
g. Copper-Phosphorus Filler Metals. These filler metals are primarily used for joining copper and copper alloys and have some limited use for joining silver, tungsten, and molybdenum. They should not be used on ferrous or nickel base alloys, or on copper-nickel alloys with more than 10 percent nickel. These filler metals are suited for all brazing processes and have self fluxing properties when used on copper.
However, flux is recommended with all other metals, including copper alloys.
h. Silver Filler Metals.
(1) These filler metals are used for joining most ferrous and nonferrous metals, except aluminum and magnesium, with all methods of heating. They may be prep laced in the joint or fed into the joint area after heating. Fluxes are generally required, but fluxless brazing with filler metals free of cadmium and zinc can be done on most metals in an inert or reducing atmosphere (such as dry hydrogen, dry argon, vacuum, and combusted fuel gas).