What is the purpose of using flux in silver brazing?

Using flux in silver brazing primarily serves the purpose of preventing oxidation of metal surfaces. During the heating process, metal surfaces can naturally oxidize, and oxidation creates a barrier that can inhibit the flow and bonding of the brazing filler material to the base metals. This can weaken the joint and reduce the effectiveness of the brazing process.

By applying flux, the oxidation is mitigated, allowing for a cleaner and more effective bonding environment. The flux works by forming a protective layer on the metal surfaces, ensuring that they remain clean and facilitating better wetting of the filler material. This results in stronger joints and overall improved performance of the brazed connections.

The other options do not accurately reflect the main function of flux in this context. For instance, flux does not increase the melting point of the materials being joined, nor does it cool down the joined metals quickly; rather, it is intended to function within a specific temperature range to promote effective brazing. While flux can play a supportive role in the overall joint strength by facilitating better bonding, its primary and most critical purpose is to prevent oxidation.