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1, remove the welding surface of the oxide, reduce the solder melting point and surface tension, as soon as possible to reach the brazing temperature.
2, to protect the weld metal in the liquid from the surrounding atmosphere is not affected by harmful gases.
3, so that liquid solder with a suitable flow rate to fill the brazing seam.
The melting point of the flux should be lower than the melting point of the solder 10-30 ℃, in particular, the flux can also be higher than the melting point of solder. If the melting point of the flux is too low, the flux is prematurely melted so that the flux is lost when the flux is melted due to evaporation and the effect of the base metal.
The choice of flux is usually dependent on the nature of the oxide film. Alkaline oxide films such as Fe, Ni, Cu and the like often use acidic boron-containing boron-enriched (B2O3) fluxes, and the acidic oxide film, for example, to the cast iron containing high SiO2 oxide film commonly contains alkaline Na2CO3 Of the flux to produce fusible Na2SiO3 into the slag.
Some of the fluoride gas is also commonly used as flux, they react evenly, leaving no residue after welding. BF3 is often mixed with N2 for brazing stainless steel at high temperatures.
The brazing flux used at 450 ° C is a softener. Softener is divided into two kinds, one is water-soluble hydrochloric acid and phosphate is usually a single or Sox salt aqueous solution, high activity, corrosive, after welding need cleaning. The other is water-insoluble organic flux, usually rosin or artificial resin-based, adding organic acids, organic amines or their HCL or HBr salt, to improve the membrane capacity and activity.