Yes, you should use flux when brazing aluminum.
The use of flux is crucial in aluminum brazing due to the material's high reactivity with oxygen, which leads to the formation of a stable aluminum oxide layer on the surface.
This oxide layer prevents the filler metal from wetting the base metal, which is essential for successful brazing.
Explanation of the Need for Flux in Aluminum Brazing:
1. Aluminum's Oxidation Characteristics
Aluminum naturally forms a thin oxide layer (Al2O3) when exposed to air.
This layer is highly resistant and prevents the filler metal from adhering to the base metal.
The use of flux is necessary to dissolve or remove this oxide layer, facilitating the wetting and flow of the filler metal across the surface.
2. Role of Flux in Dissolving Oxide Layer
The flux used in aluminum brazing, such as potassium fluoroaluminate (KAlF4), melts at a temperature slightly below that of the filler metal.
This allows it to dissolve the oxide layer before the filler metal is applied.
The flux also helps in maintaining a clean surface by preventing the re-formation of the oxide layer during the heating process.
3. Types of Flux and Atmospheres
There are different methods for aluminum brazing, including using corrosive or non-corrosive fluxes in inert atmospheres or vacuum furnaces.
Non-corrosive fluxes like potassium fluoroaluminate are commonly used in the Nocolok process, which operates in a controlled atmosphere.
This method ensures that the flux remains active only during the brazing process, minimizing any potential corrosive effects on the aluminum parts.
4. Advantages of Using Flux
The use of flux not only aids in the removal of the oxide layer but also enhances the capillary action of the filler metal, ensuring a strong and durable joint.
The flux also contributes to a more uniform thermal distribution, which is critical for maintaining the integrity of the brazed parts.
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