What is the primary purpose of flux in brazing and silver soldering?

The primary purpose of flux in brazing and silver soldering is to clean the base metal by removing the oxide film. When two metals are joined together, they often have an oxide layer that forms on their surfaces when exposed to air, which can inhibit the wetting action of the molten filler material. Flux aids in the cleaning process by chemically reacting with the oxides, allowing the molten filler metal to properly flow and adhere to the base metals.

Additionally, flux helps to prevent the formation of new oxides during the heating process, which ensures a strong bond is achieved. The result is a cleaner and more effective joint, facilitating better adhesion of the filler material and thus improving the overall quality of the brazed or soldered connection.