Do you know the selection and matching of brazing flux and filler metal in vacuum brazing furnace

The effective wetting between the molten solder and the base metal in the brazing process mainly depends on the role of the flux. The strength and corrosion resistance of the brazed joint mainly depend on the role and relationship between the filler metal and the base metal.

Although the direct wettability of filler metal and base metal is greatly related to the nature of filler metal, the role of filler metal in brazing is much less than that of flux. The concept of soldering flux should be broad, including molten salt, organic matter, active gas, metal vapor, etc., that is, except for base metal and solder, it generally refers to the third kind of all substances used to reduce the interface tension between base metal and solder. The molten lead is difficult to be wetted on the aluminum base metal either in the phase diagram or in practice, but it can be completely wetted by the action of appropriate flux, but the strength and corrosion resistance of the joint are very poor.

Selection of flux

The function of brazing flux can be divided into three parts: matrix, film remover and interfacial active agent [3:1]. Some fluxes can be clearly divided into these three functional parts, such as aluminum solder. The functional parts of most fluxes cannot be clearly divided, but these three functions do exist.

The matrix is the main component of the flux, which controls the melting temperature of the flux. It is covered on the surface of the solder joint after melting to isolate the air. It is also the solvent of other functional components in the flux. In order to match the melting point of solder, the melting temperature of solder should be 10~30t lower than that of solder;. Under special application conditions, the melting temperature of solder is slightly higher than that of solder. If the melting temperature of the flux is too low than that of the solder, the premature melting will make the flux lose its activity when the solder melts due to evaporation, interaction with the base metal, especially the reduction of the mass transfer rate between the flux and the base metal, and between the flux and the solder. It can be seen that it is very important to adjust the composition of the matrix to match the melting temperature of the filler metal.

The role of the film remover is to remove, break or loosen the facial mask of the base metal through the physical and chemical process, so that the melted solder can wet the fresh surface of the base metal. The role of the interfacial active agent is to further reduce the interfacial tension between the molten solder and the base metal, so that the molten solder can spread on the surface of the base metal.