Flux lessons: more is better

“This what makes time travel possible!”
flux capacitor

That’s the flux capacitor, of course, and it’s pretty cool. The flux I’m talking about can’t bend the space-time continuum, but it is pretty cool, too. It’s what makes brazing possible.

Brazing joins metals together by flowing a molten filler metal between the red-hot pieces being joined. It’s similar to a plumber soldering pipes together, but it’s done at higher temperatures. Welding, in contrast, joins metals by melting them together. My pieces untitled #8 and untitled #7 have some brazing work; the piece I’m working on now has a lot more.

In all of these, I’m brazing steel together with bronze as the filler metal. The bronze alloy is formulated to flow well, to be “wet,” when it’s molten. Flux helps with that; it’s like putting a little soap on a bead of water. The bronze needs to get into some tiny spaces—brazing actually works through capillary action between the metals—so the wetter it behaves the better. When the bronze isn’t sufficiently wet or the base metals not hot enough, the bronze will just bead up and either roll off or cool and stick where you don’t want it. Not good.

Flux also keeps the brazed joint clean. At such high temperatures, steel will oxidize and cool with a tough blackened layer on top that takes forever to file off. Flux somehow traps the oxides and keeps them off the metal. The trick is to use enough and to work quickly. Any amount of flux has only so much capacity to absorb oxides, and it’s easy to overdo it.

The flux itself is a powder, Anti Borax #2, that I mix with water to form a paste. Applying it evenly to the tubes is a challenge. Some spots have too much and some too little, so I’m always trying new ways to get more even coverage.

You can see a pretty ugly brazing job below, by yours truly, that left me with a lot of clean-up. The white crusty stuff is the spent flux. You can see how far from the joint the metal blackened, showing that I kept the heat on for too long. There’s excess bronze on the joint because it didn’t flow quickly enough to where it needed to be. The clean areas around the bronze are where there was enough flux; the black patches are where there wasn’t.

an ugly brazed joint not enough flux

Now, to be fair to myself, these tubes are less than a millimeter thick and it’s really easy to apply too much heat, especially with the 0-size welding tip on the torch that’s throwing a seven-inch flame. I’d been holding the flame several inches away and trying to heat the general area slowly to get the flux to melt before going in to get the brazing rod to flow. I’ve since started holding the flame closer in to the joint and working much faster to keep the heat from spreading too much, and to keep within the limits of the flux. When that works, the results are much cleaner. The joint between the two long tubes below needed only minimal clean-up.


Getting better . . .

2 Responses to Flux lessons: more is better

  1. Jason Bosh says:

    I have bronze grazed steel since 2010. I prefer bare bronze rods and a jar of Staysilv flux. I am in total control over amount of flux used. I have done stainless and brass and those metals can be tricky and they require ample amounts of flux. With mild steel, move in with torch to heat quickly. Put fluxes bronze in the joint until it flows then remove heat.

    Staysilv is borax with fluoride in it. Don’t breath the fumes but it comes off easier under hot running water with wire brush. Also, if the bronze sizzles you are burning the zinc in the alloy. Remove torch and let cool.
    I will be teaching myself aluminum brazing. A new adventure in the metal universe.

    • John Osterman says:

      Thanks for the tips. That’s great! Good luck with aluminum. I know welding it requires extra good heat control but don’t know much about brazing it.

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