Brazing flux is a chemical substance applied during the brazing process to prevent metal oxidation, dissolve existing oxides, and promote the even flow of the filler metal for a strong, clean joint. Common flux components include various borates, and the flux protects the heated base metals from air exposure, creating a necessary barrier against atmospheric oxygen. The type of flux used depends on the base metals, the filler metal, and the brazing temperature, with formulations available in paste, powder, or flux-coated rods.
What Brazing Flux DoesPrevents Oxidation:It forms a protective coating on the metal surface, shielding it from oxygen in the air, which forms oxides at high temperatures. Dissolves Oxides:It chemically dissolves and absorbs oxides that have already formed on the metal or were present before brazing. Promotes Filler Metal Flow:By ensuring a clean, oxide-free surface, the flux enables the molten filler metal to wet the base metal surfaces and flow smoothly into the joint. Indicates Temperature:For some fluxes, a visual change, such as turning clear, signals that the joint has reached the optimal temperature for brazing. Types and ApplicationForms: Flux is available as a powder, paste, or liquid. Application: It can be applied to the joint area before heating or the end of the filler rod can be dipped into the flux. Flux-coated filler rods incorporate the flux directly into the rod's coating. Components: Fluxes are chemical mixtures, with ingredients like sodium, potassium, and lithium borates, chosen based on the specific metals and application. Key ConsiderationsBase Metal Compatibility:Different fluxes are designed for specific metals and alloys, such as steel, copper, brass, stainless steel, or aluminum bronze. Filler Metal:The type of flux is also matched to the filler metal being used. Temperature:Fluxes are formulated for specific temperature ranges, with some having boron added to extend their active life for high-temperature or prolonged heating cycles. Residue Removal:Some flux residues are soluble in hot water, while others may require specific cleaning processes after brazing.