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Published: Friday, March 30, 2018 | Updated: Wednesday, October 03, 2018 How to Flux Brazing Joints

Flux is a chemical compound applied to the joint surfaces before brazing. Flux is essential in the brazing process because it prevents oxides from forming and inhibiting the metallurgical bond in brazing. Oxides are the result of chemical combination between the hot metal and oxygen in the air and form when a metal surface is heated. These oxides must be prevented from forming or they'll inhibit the brazing filler metal from wetting and bonding to the surfaces.

A coating of flux on the joint area, however, will shield the surfaces from the air, preventing oxide formation. And the flux will also dissolve and absorb any oxides that form during heating or that were not completely removed in the cleaning process.

When and How Do You Apply Flux to the Joint?

Typically you apply the flux just before brazing, if possible. That way the flux has least chance to dry out and flake off, or get knocked off the parts in handling.

As for the how, apply the flux any way you can as long as you cover the surfaces completely. Since flux is conventionally made in a paste consistency, it's usually most convenient to brush it on. But as production quantities increase, it may be more efficient to apply the flux by dipping or dispensing a pre-measured deposit of high viscosity dispensable flux from an applicator gun.

Why dispensable flux? Many companies find the repeatable deposit size improves joint consistency, and because typically less flux is used, the amount of residue entering the waste stream is also reduced.

Choosing the Right Flux

Choose flux formulated for the specific metals, temperatures and conditions of your brazing application. There are fluxes formulated for practically every need.  For example, fluxes for brazing at very high temperatures (in the 2000°F/1093°C area), fluxes for metals with refractory oxides, fluxes for long heating cycles, and fluxes for dispensing by automated machines. Fortunately, your inventory problem is considerably simplified by the availability of general-purpose fluxes, such as Handy & Harman's Handy Flux, which is suitable for most typical brazing jobs. (See page 40 for a chart of Handy & Harman/Lucas-Milhaupt fluxes.) Our technical representative can answer any questions you may have and assist you in your choice.

How Much Flux Do You Use?

Enough to last throughout the entire heating cycle. Keep in mind that the larger and heavier the pieces brazed, the longer the heating cycle will take so use more flux. Lighter pieces, of course, heat up faster and so require less flux.

As a general rule, don't skimp on the flux. It's your insurance against oxidation. Think of the flux as a sort of blotter. It absorbs oxides like a sponge absorbs water. An insufficient amount of flux will quickly become saturated and lose its effectiveness. A flux that absorbs less oxides not only insures a better joint than a totally saturated flux, but it is a lot easier to wash off after the brazed joint is completed.

Flux can also act as a temperature indicator, minimizing the chance of overheating the parts. Handy & Harman's Handy Flux, for example, becomes completely clear and active at 1100°F/593°C. At this temperature, it looks like water and reveals the bright metal surface underneath - telling you that the base metal is just about hot enough to melt the brazing filler metal.

Temperature

Appearance of flux

 
 

212°F (100°C)

Water boils off.

 

600°F (315° C)

Flux becomes white and slightly puffy, and starts to "work."

 

800°F (425°C)

Flux lies against surface and has a milky appearance.

 

1100°F (593°C)

Flux is completely clear and active, looks like water. Bright metal surface is visible underneath, At this point, test the temperature by touching brazing filler metal to base metal. If brazing filler metal melts, assembly is at proper temperature for brazing.

 

 

Exceptions To The Fluxing Rules

We've said that fluxing is an essential step in the brazing operation. This is generally true, yet there are a few exceptions to the rule. You can join copper to copper without flux, by using a brazing filler metal specially formulated for the job, such as Handy & Harman's Sil-Fos or Fos-Flo 7. The phosphorus in these alloys acts as a fluxing agent on copper and you can often omit fluxing if you're going to braze the assembly in a controlled atmosphere.

A controlled atmosphere is a gaseous mixture contained in an enclosed space, usually a brazing furnace. The atmosphere (such as hydrogen, nitrogen or dissociated ammonia) completely envelops the assemblies and, by excluding oxygen, prevents oxidation. Even in controlled atmosphere brazing, however you may find that a small amount of flux improves the wetting action of the brazing filler metal.

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