The definition of welding flux is very broad, including molten salt, organic matter, active gas, metal vapor, etc., that is, except the base metal and filler metal, it generally refers to the third kind of all substances used to reduce the interface tension between the base metal and filler metal.
Function of flux:
1. Remove the oxide on the welding surface, reduce the melting point and surface tension of the solder, and reach the brazing temperature as soon as possible.
2. Protect the weld metal from harmful gases in the surrounding atmosphere when it is in liquid state.
3. Make the liquid solder have a proper flow speed to fill the solder joint.
Function of flux in submerged arc welding:
Mechanical protection: the welding flux melts into the slag on the surface under the action of the arc to protect the weld metal from the gas in the surrounding atmosphere invading the molten pool when it is in liquid state, so as to avoid porosity inclusion in the weld.
Transfer the necessary metal elements to the molten pool.
The weld surface shall be smooth and straight, and the melting point of the well formed flux shall be 10-30 ℃ lower than that of the filler metal. Under special circumstances, the melting point of the flux can also be higher than that of the filler metal. If the melting point of the flux is too low, it will melt prematurely, which will make the flux lose its activity when the flux melts due to evaporation, interaction with the base metal and other reasons. The choice of flux usually depends on the nature of the oxide film. Alkaline oxide films, such as oxides of Fe, Ni, Cu, etc., often use acidic brazing fluxes containing boric anhydride (B2O3). For example, for iron oxide films containing high SiO2, alkaline Na2CO3 brazing fluxes are commonly used to make molten Na2SiO3 enter the slag. Some fluoride gases are also commonly used as brazing fluxes, which react evenly and leave no residue after welding. BF3 is often mixed with N2 to braze stainless steel at high temperature. The soldering flux used for brazing below 450 ℃ is soft soldering flux. There are two kinds of soft soldering fluxes. One is water soluble, which is usually composed of a single salt of hydrochloride and phosphate or a water solution of soge salt. It has high activity and strong corrosivity, and needs to be cleaned after welding. The other is an organic brazing flux insoluble in water, which is usually based on rosin or artificial resin, and is added with organic acid, organic amine or its HCl or HBr salt to improve the ability and activity of removing film.