I was asked to find a flux for use when we are vacuum brazing. Is there any such product on the market today? Do they work?
No, there are no fluxes available in the marketplace created for use when vacuum brazing. Attempts at trying to develop or use such a product have been futile and have always proven to be destructive to the vacuum furnace and its pumping systems.
Over the years, this question has been asked by a number of people who are somewhat new to brazing, and especially new to vacuum brazing. Their reasoning usually falls along this line:
“If vacuum is good for brazing by eliminating a lot of the oxygen from the brazing zone that could hurt brazing and fluxes are very good at helping to remove any residual oxides on metal surfaces being joined, then the combination of the two would seem to be a marriage made in heaven.” Nothing could be farther from the truth.
In reality, 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. And any residual oxidation and/or contamination that was on the components being brazed should have been removed in the first place by proper cleaning of the parts prior to assembling them for brazing. 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, the use of an added flux for brazing would serve no useful purpose to begin with.
Additionally, since a flux is a corrosive, acidic paste, the high temperatures in a brazing-furnace atmosphere, coupled with the greatly reduced pressure in the vacuum chamber, will cause the flux to turn to a high-temperature corrosive gas. This acidic gas can rapidly corrode the metallic materials of the furnace hot-zone and pumping systems, 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!