Over the years, a strange question has sometimes been raised by people who are trying to optimize their vacuum brazing procedures. The question usually sounds something like this:

“If vacuum is good for brazing by pumping out the air from the furnace (thus, also removing a lot of the oxygen from the brazing zone that could hurt brazing), and if paste-fluxes are very good at helping to remove any residual oxides that can get onto metal surfaces that are being heated, then, isn’t it reasonable to consider that the combination of brazing in a vacuum along with the use of a paste-flux on the metal surfaces to be brazed would be highly effective for optimal brazing?”

Nothing Could Be Farther from the Truth

Remember, the purpose of brazing in a vacuum is to allow a sufficient amount of the atmosphere in the furnace brazing zone to be “sucked out” of the furnace, so to speak, by the vacuum pumping system so that there is not a sufficient amount of atmosphere (thus, oxygen) left in the brazing zone to cause any oxidation issues. Period!

Additionally, good surface-prep prior to brazing requires that any residual oxidation and/or contamination that was on the components to be brazed should have been removed from all surfaces to be brazed –- before placing the assembly into the vacuum furnace! Thus, parts going into any brazing furnace should be appropriately clean from the start, and the use of a proper furnace atmosphere (gaseous or vacuum) will then keep them clean during the brazing cycle. Therefore, putting flux onto the surfaces for brazing would serve no useful purpose to begin with.

Additionally, consider the chemistry of typical paste brazing-fluxes. These paste fluxes typically contain fluorides, making the flux a corrosive, acidic paste. When heated in a furnace, the high temperatures of brazing will turn much of the flux into a vapor before being pumped out of the vacuum furnace, and the flux thus turns into a very-hot, highly-corrosive gas throughout the vacuum furnace hot-zone, as well as its pumping system components. This acidic gas can rapidly corrode the metallic materials of the furnace hot-zone heating elements, furnace grates, fixturing, pumping systems, etc., thereby quickly ruining the furnace.

Believe it or not, people have used paste flux during a vacuum-furnace braze, and they have learned the hard way that flux should NEVER be used in a vacuum-brazing furnace! Use it once, and your next stop may be your company’s purchasing department to buy a new vacuum furnace hot-zone and pumping system.

Use a flux in a vacuum brazing furnace? Never, Never!

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