December 14, 2017

BRAZING FLUX

BRAZING FLUX: HOW TO CHOOSE AND HOW TO USE IT

We have seen in one of the previous posts that, in order to obtain a sound brazed joint, it is necessary to ensure that both the base and the filler metal are clean and free from any superficial film of contaminating material.
These films, in fact, will inhibit the wetting and the flow of the filler metal inside of the joint.
Dust, grease, oil, residues of fabrication process, etc. may be easily removed by simple mechanical means such as brushing or by washing with solvents and/or detergents.
However, the majority of metals are covered, even at ambient temperature, by a film of superficial oxide, coming from the reaction of the metal with atmospheric oxygen, which poses a further obstacle to the flow of the filler metal.
Besides of that, the thickness of this oxide layer grows quickly and considerably when metals are heated up in air, becoming a complete barrier to the successful outcome of the brazing operation.
It is therefore necessary that, throughout all of the process, already present oxides are dissolved and the pieces are protected from further oxidation.
In some instances this may be obtained excluding oxygen from the brazing environment, by surrounding the pieces with inert (argon, nitrogen) or reducing (hydrogen mixtures) atmospheres, or by performing operations under vacuum conditions.
In the majority of the cases, however, these processes are not practical nor convenient, and the heating of the pieces takes place in the open air, without any protective atmosphere.
An effective, economical and easy to use protective medium is in these cases needed. This medium is the flux.
Fluxes are chemical compounds, usually based on complex alkaline salts mixtures, and produced in form of liquids, powders or pastes.
They are deposited and distributed on the surface of the pieces to be brazed, prior of their assembly and heating.
During the heating stage the fluxes become active, by chemically removing the surface oxides, preventing further oxidation and protecting the base metal from contact with the atmospheric oxygen.
Once the brazing temperature has been attained, the molten filler metal will flow and flush away the flux from the joint.
After the cooling stage, in the majority of the cases, the flux residues will have to removedeither mechanically or chemically, as otherwise they could promote corrosion phenomena.
The most important technical property of fluxes is their working temperature range.
This property is indicated by two temperatures: the lower one is the temperature at which the flux starts to be active, the upper one is the temperature at which the flux is exhaustedand will not perform any more its deoxidant and protective function.
It is a good rule to choose fluxes in such a way so that the lower temperature is at least
50 °C lower
than the solidus temperature of the brazing alloy to be used, and that the upper temperature is at least 50 °C higher than the liquidus temperature of the alloy.
As an example, a good general purpose flux for silver based alloys, such as Stella Flux AG4, has a working temperature range of 550 – 850 °C, which make it suitable to be used with a wide range of alloys, approximately from Ag25 to Ag60.
Many different types of fluxes are available, each with different chemical composition, working temperature range and properties, that may be used with different alloys, different family of products and for different applications.
A comprehensive explanation of the different features of each flux will take too long and is out of the purpose of these short notes.
The necessary details can be found in the relevant data sheets of each product.
For more specific advice you can also ask for support to our technical department.
It is however possible to define the following broad families:


– general purposes fluxes for silver alloys (Stella AG series: AG1, AG4, AG7)
– general purpose fluxes for silver alloys with high working temperature (Stella AG series: AG3, AG5)
– high temperature, long life fluxes for silver alloys, with boron addition and typical brown colour (Stella AG8)
– high temperature fluxes for low silver alloys or brass alloys (Stella BR Series: BR1)
– fluxes for aluminium (Stella AL Series)
– fluxes for soft solders (Stella SN Series: SN1, SN2, etc.)

There is also an additional class of liquid fluxes (Stella LI Series), which are used in a particular process in which the flux is directly spread through the torch’s flame.
This flux is generally used with brass filler metals, for the brazing of iron or steel elements, or with copper-phosphorus or silver-copper-phosphorus alloys for the brazing of copper pipes and fittings.
We will address this specific flux in a subsequent post.
One last comment has to be dedicated to the brazing of copper with the copper-phosphorus and silver-copper-phosphorus filler metals.
As it is well known, in this case, no additional flux is needed for the brazing operation.
The reason for this apparent contradiction is simple: it is the phosphorus contained in the alloy that act as the flux, and make these products self fluxing on copper.

STELLA – Technical Department