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Vacuum brazing flux selection

Borax can be used as a flux and is widely used in vacuum brazing. However, some brazing practitioners do not fully understand the properties of borax flux, and often equate borax with the concept of flux. When it comes to the need to use flux, the first thing that comes to mind is the use of borax, which leads to many brazing problems or defects.
The melting temperature of single borax is high (741℃), the melted borax has high viscosity and poor fluidity.
Therefore, when single borax is used as a flux, it is only suitable for working conditions where the vacuum brazing temperature is very high, such as the vacuum brazing temperature exceeding 900°C.
The liquidus temperature of conventionally used silver-based solder is generally between 650°C and 800°C, and the vacuum brazing temperature generally does not exceed 850°C. Therefore, the usage temperature of borax flux does not match that of silver-based solder.

If you force the two to match, it will result in:
1. When used at low temperature, borax does not melt or has poor fluidity after melting, resulting in the solder not wetting on the material or having poor wetting and spreading properties, resulting in brazing failure.
2. When used at high temperatures, the workpiece is seriously overheated, and the low-temperature advantage of the silver solder is not fully utilized; the zinc in the silver solder is severely volatilized at high temperatures, and the pores in the brazed joint increase, affecting the brazing quality.
Therefore, when silver brazing, you should choose a suitable silver flux (as shown in the table below) instead of borax flux.
In order to reduce costs, many companies will consider replacing silver-based solder with low-priced brass solder, thereby quickly reducing procurement costs. Especially for some refrigeration industry manufacturers involved in the brazing of copper, carbon steel or stainless steel, there is achievable space for using brass brazing material if operational training and control are strengthened. Therefore, some companies will directly replace them after learning about this point. However, the final result was poor brazing and a sharp increase in leakage rates. The defective rate and leakage rate of its vacuum brazed parts is close to 90%.
The reason is that after replacing the silver-based solder with brass solder, OKNA White Flux silver solder is still used.
The active temperature range of OKNA White Flux is 600℃-870℃. When the heating temperature of the flux exceeds 870°C, the flux will quickly react with oxygen and over-burn. The over-burned flux will turn into black slag and adhere to the surface of the base material, unable to play the role of the flux; and the black formed The slag will not be wetted by the brazing material, thereby hindering the combination of the brazing material and the base metal, resulting in a reduction in the connection area of the brazed joint.
Generally, the liquidus temperature of brass solder exceeds 880°C, and its brazing temperature is generally above 900°C. At this time, it has exceeded the effective temperature range of OKNA White Flux, and overburning of the flux is inevitable. As the melting temperature of the selected brass brazing filler metal increases, the overburning of the brazing filler metal becomes more serious and the quality of the brazed joint becomes worse.
Therefore, if brass solder is selected to reduce costs, the matching brazing flux should also be replaced accordingly, such as OKNA Hi-Temp flux, OKNA Hi-Temp Black flux, etc.
If induction heating is used, you can also directly choose OKNA FC CDA680 SMRing brass flux cored welding ring to facilitate flux removal after welding.
Therefore, please pay special attention to the fact that brazing material and flux are used in a matching manner. When selecting the appropriate brazing material, do not neglect the selection of flux.