When selecting a flux for brazing or braze welding, several criteria must be considered to ensure effective and efficient results. These include the compatibility of the flux with the base metal and braze alloy, the ability of the flux to wet and adhere to vertical surfaces, the ease of residue removal, and the specific requirements of the brazing process such as temperature, time, and atmosphere.
Compatibility with Base Metal and Braze Alloy: The melting points of the base metal and the braze alloy are critical. It is essential that the melting range of the braze alloy does not overlap with that of the base metal. Typically, the solidus temperature of the base metal should be at least 55ºC (100ºF) higher than the liquidus temperature of the braze alloy. This ensures that the base metal remains solid while the braze alloy melts and flows into the joint.
Wetting and Adherence: The flux must be capable of wetting the surfaces of the base metal and the braze alloy, and it should remain on these surfaces, especially on vertical or inclined surfaces, to prevent oxidation and promote the flow of the braze alloy. This capability is crucial for ensuring uniform and complete joint formation.
Ease of Residue Removal: After the brazing process, the flux residue should be easily removable from the workpieces. This is important for the integrity and appearance of the final product. If the residue is difficult to remove, it can affect the performance and aesthetics of the brazed components.
Process-Specific Requirements:
- Temperature and Time: The brazing temperature must be sufficiently high to melt the braze alloy but as low as possible to minimize the impact on the base metal. The brazing time should also be minimized to prevent overheating of the base metal and to enhance productivity.
- Heat Input: The heat input to the workpiece must be balanced to ensure that no portion of the joint experiences excessive temperatures, which could lead to distortion or degradation of the base metal.
- Atmosphere: Depending on the materials being brazed, the process might require a specific atmosphere, such as a reducing atmosphere for stainless steel, to prevent oxidation and ensure the integrity of the brazed joints.
Application of Flux: Flux is typically applied as a paste to the joint surfaces. It is preferable to apply the flux before the heating cycle begins to ensure uniform coverage and effectiveness. The quantity of flux applied should be adequate to protect the surfaces but not excessive, which could lead to difficulties in residue removal.
By considering these criteria, the selection of the appropriate flux for brazing or braze welding can significantly enhance the quality and reliability of the brazed joints.
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