Brazing flux is used to chemically remove surface oxides, prevent new oxides from forming, and promote the free flow of the brazing filler metal by reducing surface tension and enhancing wetting of the base materials. It forms a protective barrier, absorbs oxides, and ensures a clean surface for the filler metal to bond effectively. Fluxes are critical for most brazing and soldering operations, except in protective atmospheres like vacuum or furnace brazing, or when using self-fluxing copper-phosphorus alloys.
Functions of Brazing FluxOxide Removal:Fluxes chemically react with existing oxides on the metal surfaces, dissolving them to expose a clean, oxide-free surface for the filler metal. Oxidation Prevention:As the metal is heated, the flux creates a protective barrier that shields the base metal and filler metal from atmospheric oxygen, preventing the formation of new oxides that would hinder the brazing process. Enhanced Wetting and Flow:By reducing the surface tension of the molten filler metal, flux helps it to spread, wet, and flow into the joint, ensuring a complete and strong bond. When Flux is RequiredYou need flux for most brazing and soldering applications, including brass, steel, and other metals. It is essential when the brazing filler metal itself does not contain a self-fluxing agent. When Flux is Not RequiredVacuum Brazing:In a vacuum, there is no oxygen to cause oxidation, so flux is unnecessary. Protective Atmosphere Brazing:Brazing in a controlled atmosphere, such as in a furnace, also prevents oxidation. Copper-Phosphorus Alloys:When brazing copper to copper, filler metals containing phosphorus (like BCuP alloys) can act as their own fluxing agent. How to Use Flux1. Application:Apply flux to the joint surfaces before assembly and heating. It can be applied as a paste by brushing or by dipping the parts into the flux. 2. Activity:During heating, the flux melts, becomes active, absorbs any oxides, and protects the base metal. 3. Filler Metal Flow:The molten filler metal then flows over the cleaned and protected surface, flushing away the flux residues. 4. Post-Brazing Cleaning:After cooling, the flux residues must typically be removed mechanically or chemically to prevent corrosion.