Fluxis a chemical mixture added to the surface of the base material before brazing. The flux is very important during the brazing process because it can prevent the formation of oxides on the surface of the materials in the brazing area during heating. The presence of oxides will prevent the liquid filler metal from wetting the base metal and forming a connection with it.

The brazing area is covered with a layerFlux, which can block air and protect the high-temperature workpiece. At the same time, the flux can remove oxides that have not been completely removed from the workpiece surface and those formed due to oxidation of the high-temperature workpiece during heating.

During brazing heating, the flux will continue to be consumed. The flux can be seen as a substance similar to a 'sponge'. Given a sponge, the amount of water it can absorb is fixed. The flow rate is the same. When the reaction reaches saturation during heating, it will lose its activity.

The amount of flux used should be appropriate. Too little or too much is detrimental to the quality of the brazed joint. The amount of flux should be sufficient to last until the end of the entire brazing heating. For large, thick workpieces, with long heating times, a larger amount of flux is needed; for small and lightweight workpieces, with fast heating speeds, a smaller amount of flux should be used.

Negative effects caused by insufficient flux

1. The liquid filler metal cannot fill the joint and has not connected with the base material.

2. Insufficient penetration or connection area at the joint.

3. The brazing angle is not smooth and continuous.

4. The weld surface is severely blackened, making it difficult to clean after welding.

Negative effects caused by excessive flux

1. The flux can easily mix in the joint forming holes.

2. For certain sealing systems, excessive flux may flow into pipes along the bottom of gaps and cannot be cleaned after welding. Internal flux may lead to corrosion, blockage, and other issues.

3. After welding, there is a lot of residual flux on the joint surface, resulting in more wastewater for cleaning, which is not conducive to environmental protection.

Do not use too little flux, as it is essential for preventing oxidation.

The choice of flux has a significant impact on the quality of brazing and the time required for brazing; therefore, care should be taken when selecting flux. The selection of flux should follow these principles, methods, or criteria:

1. The flux should have the ability to dissolve and remove oxides from both metal substrates and filler metal surfaces;

2. Familiarity with the properties of metal substrates, composition and state of surface oxides is necessary. The selectedFluxmust match and be compatible with base materials, filler metals, brazing equipment, fixtures, and brazing protective atmospheres while being active throughout the entire brazing temperature range;

3. Familiarity with the brazing methods used and their effects on flux composition is important. For example, for induction brazing, a highly active flux should be selected; for furnace brazing, a low-activity flux should be chosen;

4. Fluxes for dip brazing must not contain water;

5. Fluxes used for resistance brazing and induction brazing should have conductivity;

6. The effective temperature range of the flux must fall within that of the filler metal's brazing temperature range. Generally speaking, the melting point of the flux should be lower than that of the filler metal;

7. Residues from the flux should be easy to remove after welding;

8. The corrosive effects of flux on base materials, filler metals, fixtures, and brazing equipment should be minimized;

9. The flux should have good thermal stability;

10. The flux should have properties to prevent moisture absorption and deliquescence.