How Do You Use Flux When Brazing?

To use flux when brazing, you typically apply it directly to the joint surfaces before heating, covering both surfaces thoroughly, often by brushing the flux on as a paste.

Brazing flux plays a crucial role by cleaning the metal surfaces during heating and preventing oxidation, allowing the filler material to flow smoothly into the joint. Understanding how to apply it correctly ensures a strong, reliable braze.

Forms of Brazing Flux

Brazing flux is available in several convenient forms, catering to different applications and preferences:

• Powder: Often mixed with water or alcohol to create a paste before application.
• Paste: A ready-to-use form, commonly applied directly from the container.
• Liquid: Applied by dipping, brushing, or spraying, sometimes used for specialized applications.

According to best practices, if you are using powder flux, it is recommended to mix the powder into a paste before application for optimal results. This ensures a consistent application and better adherence to the joint surfaces.

Applying Brazing Flux: Step-by-Step

Proper application is key to effective brazing. Here’s how it's typically done:

1. Clean Surfaces: Before applying flux, ensure the surfaces to be joined are thoroughly cleaned, free from dirt, grease, and oxidation.
2. Prepare Flux: If using powder flux, mix it with the appropriate liquid (usually water or a manufacturer-recommended solvent) to achieve a smooth, consistent paste. Paste and liquid fluxes are ready to use.
3. Apply to Joint Surfaces: Both joint surfaces need to be covered with the flux. This is critical because the flux must protect all areas that will reach brazing temperature.
4. Application Method: The most convenient application method is by brushing. Using a brush allows you to evenly distribute the flux over the entire area of both surfaces that will form the joint. Other methods like dipping or squirting can also be used depending on the flux form and joint configuration.
5. Assemble Joint: Fit the parts together after applying the flux. The flux will help hold the parts in place slightly and ensure coverage remains uniform.
6. Braise: Heat the joint area. As the temperature rises, the flux melts, becoming active and cleaning the surfaces, preparing them for the flow of the brazing filler metal.

Key Considerations for Flux Application

• Coverage: Ensure complete coverage of the joint area. Any spot missed may oxidize, preventing the filler metal from bonding.
• Thickness: Apply a layer that is sufficient to protect the joint throughout the heating process. Too thin, and it might burn off prematurely; too thick, and it can become trapped in the joint.
• Joint Design: The design of the joint influences how easily flux can be applied and how effectively it works. Capillary action helps draw both flux and filler metal into tight joints.

Here's a quick summary of flux forms and application:

Applying flux correctly is a fundamental step in achieving a high-quality brazed joint, ensuring proper wetting and flow of the filler material.