Unless a brazing operation is included in an inert or reducing atmosphere environment (i.e., a vacuum furnace), a flux such as borax is required to prevent the formation of oxides when the metal is heated. Flux is also used to clean any contaminants left on the soldered surface. The flux can be used in any form, including solder paste, liquid, powder or pre-formed solder paste, which combines the flux with the filler metal powder. Flux can also be applied using a solder bar or flux core with a flux coating. In either case, when applied to the heated joint, the flux flows into the joint and is displaced by the molten filler metal entering the joint. Excessive flux should be removed when the cycle is complete because the flux remaining in the joint can cause corrosion, interfere with joint inspection, and prevent further surface finishing operations. When copper is attached to copper, the phosphorus-containing braze alloy can be self-fluxing. Flux is typically selected based on their performance on a particular base metal. In order to be effective, the flux must be chemically compatible with the base metal and filler metal used. If used on iron or nickel, the self-fluxing phosphorus filler alloy produces brittle phosphides. As a general rule, longer brazing cycles should use less active flux than short brazing operations.
A variety of alloys are used as filler metals for brazing depending on the intended use or application. Generally, a brazing alloy is composed of three or more metals to form an alloy having desired properties. The filler metal for a particular application is selected based on the ability to wet the base metal, withstand the required conditions of use, and melt at a lower temperature than the base metal or at a very specific temperature.
Brazing alloys are commonly used as rods, strips, powders, slurries, creams, wires and preforms (eg stamped gaskets).  Depending on the application, the filling material can be placed in advance at the desired location or during the heating cycle. For manual brazing, wire and rod forms are often used because they are the easiest to apply when heated. In the case of furnace brazing, the alloy is usually placed in advance because the process is usually highly automated.  Some of the more common types of filler metals used are
Amorphous brazing foil using nickel, iron, copper, silicon, boron, phosphorus, etc.