Classification, type and characteristics of flux

The brazing flux (flux) is the flux used in the brazing process, and is used in combination with the brazing filler metal to ensure the smooth brazing process and obtain a dense joint. The function of flux is to remove the oxide on the surface of molten solder and base metal, protect the surface of solder and base metal from further oxidation, improve the wettability of solder to base metal, promote the interface activation, so that it can realize the brazing process smoothly. The reasonable selection of flux and filler metal plays a key role in the quality of brazed joints.

1. Basic requirements for flux

A. The melting point and minimum active temperature of the flux are lower than that of the solder, and there is sufficient fluidity in the active temperature range. Before the solder is melted, the flux should melt and begin to work, removing the brazing gap and the oxide film on the surface of the solder, creating conditions for spreading wetting of the liquid solder.
b, should have good thermal stability, so that the flux in the heating process to keep its composition and function stable and unchanged. Generally speaking, the flux should have a heat stable temperature range of not less than 100 ° C.
c, it can dissolve or destroy the oxide film on the surface of the brazed metal and brazing metal. The vaporization (evaporation) temperature of each component in the flux is higher than the brazing temperature to avoid the loss of flux volatilization.
d, in the brazing temperature range, the flux should be small viscosity, good fluidity, can be well wetted brazing metal, reduce the interfacial tension of liquid brazing filler metal.
E. The density of the product after melting flux and removing the oxide film should be small, which is conducive to floating, and the film layer is uniformly covered on the surface of the brazed metal, effectively isolating the air, promoting the wetting and spreading of the brazing filler metal, and not remaining in the brazing joint to form slag inclusion.
f, molten flux residue should not have a strong corrosion effect on brazed metals and joints, and the toxicity of flux volatiles is small.

2. Classification of flux

The composition of the flux depends mainly on the physical and chemical properties of the oxides to be removed. The composition of the flux can be a single component (such as borax, zinc chloride, etc.), or it can be a multi-component system. The multicomponent system is usually composed of matrix component, membrane removal component and active component.
The classification of flux is compatible with the classification of solder, which is usually divided into soft flux, hard flux, aluminum flux, etc., which is suitable for different occasions.

(1) Soft flux
Soft flux refers to the flux used for brazing below 450 ° C, which is composed of film forming substances, activating substances, auxiliaries, diluents and solvents, etc., and can be divided into inorganic soft flux and organic soft flux two categories.
Inorganic soft flux has high chemical activity, strong ability to remove oxides, good thermal stability, and can promote the wetting of liquid brazing metal to ensure the quality of brazing. This kind of flux is suitable for a wide temperature range of brazing, but its residue has a strong corrosion effect, so it is also called corrosive soft solder, which must be removed after brazing. Inorganic soft flux can be used in stainless steel, heat-resistant steel, nickel-based alloys, etc. Organic soft flux is divided into water-soluble and natural resin (rosin), which is almost non-corrosive to the base material, so it is called non-corrosive soft flux.

(2) Hard flux
Hard flux refers to the flux used for brazing above 450 ° C. The main components of hard flux commonly used for ferrous metals are borax, boric acid and their mixtures. In order to obtain a suitable melting point and enhance the ability to remove oxides, fluoride and fluoroborate of various alkali metals or alkaline earth metals can be added.
Borax (Na2B4O7·10H2O) is a monoclinic white transparent crystal, easily soluble in water, heated to more than 200℃ crystal water can all evaporate. Borax should be used after dehydration. Boric anhydride in borax and metal oxide action to form a fusible borate, and further decomposition to form sodium metaborate and borate to form a mixture with a lower melting point, and to achieve the purpose of removing oxides, so it can be used as a flux.
FB102 flux is the most widely used general flux. FB103 flux has the lowest brazing temperature and is especially suitable for silver copper zinc cadmium brazing metal; FB104 can remain active for a long time at a slow heating rate.

(3) aluminum flux
The oxide film on the surface of aluminum is compact and stable. Special aluminum flux must be used for brazing aluminum and aluminum alloys. The flux for aluminum alloy is divided into soft flux for aluminum and hard flux for aluminum according to its use temperature. Soft flux for aluminum is divided into organic flux and reactive flux, and the basic composition of hard flux for aluminum is alkali metal and alkali earth metal chloride.

(4) Gas flux
Gas flux is a special type of flux, according to the brazing method can be divided into furnace brazing gas flux and flame brazing gas flux. The biggest advantage of this type of flux is that there is no flux residue after brazing, and the brazing head does not need to be cleaned. However, most of these fluxes and their reactants have certain toxicity, and corresponding safety measures should be taken when using them.

3. Model and grade of flux

(1) Flux model
The flux type for brazing is indicated by the letter "FB" and the four codes "1,2,3,4" and the flux serial number according to the main components of the flux; The tail of the model is denoted by capital letters S (powder, granular), P (paste) and L (liquid) respectively.

(2) Grade of flux
The letter "QJ" before the flux grade means brazing flux; The first digit of the brand indicates the use of the flux, of which 1 is for brazing silver solder, 2 is for brazing aluminum and aluminum alloy; The second and third digits indicate different grades of the same type of flux.
There are many varieties of flux, which should be reasonably selected according to the brazing temperature and brazing process requirements.