Fig. 1 — Photo of a typical plastic brazing flux jar containing paste flux for brazing
A number of years ago I wrote an article for this website about the use of a brazing flux in a vacuum furnace, in answer to an inquiry I had received about the advantages or disadvantages of using a paste flux to enhance the brazeability of components in a vacuum brazing furnace. It is time to once again bring up that subject for today’s brazing community, in order to be sure that everyone understands that you should NEVER put a brazing flux in a vacuum furnace!
The brazing fluxes that people usually refer to when asking about the use of “a flux” in a vacuum furnace are the same fluxes that they use when hand-brazing with a torch or induction coil out in the open air. Such fluxes, as shown in Fig. 1, are thick paste products, with the consistency of thick mayonnaise, and the paste is either white or black in color.
Brazing pastes that are used for torch or induction brazing usually contain fluorides (an acid) and borates so that the flux paste will be effective at reacting with, and absorbing, oxygen, thereby preventing that oxygen from reacting with the base metals being brazed, and thus preventing the formation of oxides on those base metals. This is necessary since molten brazing filler metal (BFM) will not bond to oxidized surfaces.
In a similar manner, a vacuum furnace used for brazing has an effective pumping system that removes the air from the vacuum chamber, thereby keeping oxygen away from the base metals that are to be brazed.
Thus, some people ask: “If fluxes keep oxygen away from base metals, and a vacuum-furnace pumps air out of the furnace so that oxygen will not be present to react with the base metals, wouldn’t it make sense to combine those two processes in order to take advantage of the oxygen-absorption/oxide-prevention capability of both?” The answer is a resounding “NO” !!
Remember, good surface-prep prior to brazing requires that any residual oxidation and/or contamination on the component surfaces has to be removed before placing the assembly into the vacuum furnace! Thus, parts going into any brazing furnace should have clean braze-surfaces right from the start, and the use of a proper furnace atmosphere (gaseous or vacuum) will then keep them clean throughout the entire brazing cycle. Therefore, nothing would be gained by putting flux onto those brazing-surfaces prior to putting them into the furnace since the furnace atmosphere (vacuum or gaseous) should already be more than adequate to keep those clean surfaces clean and free of oxidation throughout the furnace cycle. If you’re concerned that maybe the furnace atmosphere might NOT be good enough to keep the part free from oxidation, then FIX THE FURNACE! Do not EVER try to compensate for what you think might be a marginal atmosphere by adding flux to the surface of the parts being brazed, since it will be a LOSE/LOSE situation! You won’t gain any additional surface protection against oxidation, since the flux will vaporize in the high-temp vacuum atmosphere and will not be able to protect the metal surface, so you lose on that front. And you’ll lose BIG-TIME when you destroy your vacuum furnace’s hot-zone due to the high-temp reaction of the flux with the metallic elements in your furnace!
Why would flux destroy the inside of your vacuum furnace? Well, consider the chemistry of typical paste brazing-fluxes. As mentioned earlier, 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 entire vacuum-furnace hot-zone, as well as in the furnace’s 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. This is illustrated in Fig. 2, which shows the inside of a small vacuum furnace intentionally used as part of an R&D effort to test and evaluate the effects of a special flux that allegedly was developed by a flux-manufacturer for use in a vacuum brazing furnace.