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How to apply the flux

Flux Form - Premixed
It is recommended that flux should be applied as a paste to the parts to be joined.

Paste or Powder? 
Powders can be made into pastes by stirring in water until the mixture has the consistency of thick cream. A few drops of liquid detergent added to the mixture will often improve the wetting of flux onto clean parent metals. 

The flux should be applied to both joint surfaces before assembly. Application of flux after assembly places great demands on the fluidity of the molten flux and its ability to penetrate capillary joints.

Brushing
Brushing is an effective method of applying a thin film of paste to the joint itself and to surrounding component surfaces.

Dipping 
Brazing flux may be applied by dipping one or more components of an assembly into a container of flux. This is most effective with a paste of a thin consistency.

Automatic Application
Automatic application of flux is possible and can be used for mass production. Consult a Johnson Matthey representative for more details.

Hot Rodding
This is a technique in which a warm brazing rod is dipped into flux powder and the flux adhering to the rod is transferred to the joint area.  This is an effective fluxing method but difficult to achieve good penetration of capillary joints. A refinement on this technique is the use of flux coated brazing rods.


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Why use a flux?

A molten brazing filler metal will only wet and flow over a parent metal if both are substantially free of surface oxide. Simply removing surface oxide before brazing is not effective, since a new oxide layer is rapidly formed on heating.  Thus, to achieve an oxide free surface, it is necessary either to:-

a) remove oxide as it is formed by the use of a suitable brazing flux.
b) prevent oxidation during brazing by heating in a protective atmosphere or vacuum.
c) use a self-fluxing brazing alloy - only applicable when brazing copper to copper.

Brazing fluxes are only designed to remove oxide films. Where other contaminants such as oil, paint and lacquer are present these should be removed before brazing, using either mechanical or chemical methods.


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Factors which affect how a flux works
The ability of any flux to perform satisfactorily during a brazing operation will depend on the brazing  temperature and time, the parent metals, and the volume of flux applied.
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The effect of temperature on how a flux works

To be effective the flux must be both molten and active before the brazing alloy melts, and it must remain active until the brazing alloy flows through the joint and solidifies on cooling. The working ranges of Johnson Matthey brazing fluxes are indicated in the products section of this site. It is good practice to select a flux which is active at least 50˚C below the solidus of the brazing alloy and which is still active at a temperature at least 50˚C above the liquidus of the brazing alloy. This will ensure that the flux is effective during the brazing operation.


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The effect of time on how a flux works

The flux has to remove the oxides on the component and must continue to remove fresh oxide until the completion of the brazing cycle. There is a limit to the amount of oxide that the flux can dissolve. The longer the heating cycle the more likely it is that the flux will become exhausted and the residues and components will exhibit a blackened appearance. There is no fixed time for which a flux will be effective since this is dependent on the operating temperature and the type of parent metal. With long heating cycles flux exhaustion may occur and the use of flux with a higher working range would be recommended.

For short rapid heating cycles it may be possible to use a flux above its recommended maximum working temperature.


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The effect of parent metals on how a flux works

Johnson Matthey fluxes are suitable for use on copper, brass, mild steel and most other common engineering materials.

Special purpose fluxes exist for aluminium bronze, stainless steel, tungsten, molybdenum and tungsten carbide. Silver brazing fluxes are not effective on aluminium, magnesium, titanium or their alloys.


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The effect of flux volume on how a flux works
The volume of flux required will vary depending on the nature of the application. Usually it is sufficient to coat the joint faces and the surrounding component surfaces with a layer of paste using a brush.  Using an excess of flux is in no way detrimental to the quality of the brazed joint, and can assist flux removal. Application of flux to surfaces away from the joint helps to prevent oxidation of the components.  The use of too little flux can lead to flux exhaustion resulting in unsound brazed joints. It is always best to use too much rather than too little flux!
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The stages of heating flux to brazing temperatures

 

 

 

 The flux becomes white and solidifies as the water is driven off

The flux becomes clear and watery as it approaches brazing temperature 

The flux protects the component from oxidation as the brazing alloy flows

 

 

 

 


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