Filler Metal and Fluxes used in Brazing

Commonly filler metals are used in brazing are listed in below Table 1, along with the principal base metals on which they are typically used.


To be suitable as a brazing metal the following characteristics are needed:

1. the melting temperature must be compatible with the base metal,
2. chemical and physical interactions with base metal must be avoided,
3. surface tension in the liquid phase must be low for good wettability,
4. the fluidity of the molten metal must be high for penetration into the interface, and
5. the metal must be capable of being brazed into a joint of adequate strength for the application.

Table 1: Common filler metals used in the brazing process and the base metals on which they are used.

Filler metal	Typical Composition	Brazing Temperature	Base metals
Aluminum and silicon	90% Al, 10% Si	600° C	Aluminum
Copper	99.9% Cu	1120° C	Nickel Copper
Copper and Phosphorous	95% Cu, 5% P	850° C	Copper
Copper and Zinc	60% Cu, 40% Zn	925° C	Steels, Cast irons, nickel
Gold and silver	80% Au, 20% Ag	950° C	Stainless steel, nickel alloys
Nickel alloys	Ni, Cr, others	1120° C	Stainless steel, nickel alloys
Silver alloys	Ag, Cu, Zn, Cd	730° C	Titanium, Monel, Inconel, tool steel, nickel

Filler metals are applied to the brazing operation in various ways, including wire, rod, sheets and strips, powders, pastes, performed parts made of brazing metal designed to fit a particular joint configuration, and cladding on one of the surfaces to be brazed.

Several techniques for applying filler metal in brazing:
(a) torch and filler rod;
(b) ring of filler metal at the entrance of gap; and
(c) the foil of filler metal between flat part surfaces.,
Sequence: (1) before, and (2) after.

Brazing fluxes have a similar purpose as in welding; they dissolve, combine with, and otherwise inhibit the formation of oxides, other unwanted byproducts during the brazing process. The use of flux does not substitute for the cleaning steps described above.

Characteristics of a good flux include:

1. low melting point temperature,
2. low viscosity so that the filler metal can displace it,
3. facilitates wetting, and
4. protects the brazing joint until the filler metal gets solidified.

The flux should be easy to remove after the brazing operation. Typical ingredients for brazing fluxes include borax, borates, chlorides, and fluorides. Wetting agents are also part of the mix to reduce the surface tension and to improve the wettability of the molten filler metal. Different forms of flux include powders, pastes, and slurries. Alternatives to using a flux are to perform the operation in a vacuum or a reducing atmosphere that inhibits oxide formation.