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

The purpose of flux in brazing and silver soldering is to clean the base metal by removing any oxide layers or contaminants that may inhibit proper adhesion and the flow of the filler metal. When metals oxidize or accumulate dirt, these surface impurities can create barriers that prevent the molten solder from properly penetrating the joint. The flux acts chemically to clean the surfaces by dissolving these oxides and ensuring a strong bond between the base metals and the filler metal. This cleaning action is crucial for achieving a sound joint with optimal mechanical properties.

While additional heat can generally assist in the brazing process, it's not the primary role of flux. The cooling of the joint too quickly is not its function, and enhancing the color of the solder is irrelevant to the practical function of flux in metal joining. Thus, the correct understanding of flux focuses on its essential role in preparing and cleaning the surfaces for effective joining.