Aluminum brazing continues to grow, and, as I mentioned in my previous blog article, I sometimes receive questions from people asking me: "Can I use available furnace time in my high-temperature aerospace vacuum furnace to perhaps do some aluminum brazing?" I mentioned before that it is very fortunate for them that they asked BEFORE they actually tried to do it, since companies should NEVER use their regular high-temperature vacuum furnaces to braze aluminum base metals!
In my previous blog article, we examined the difference between those two types of vacuum furnaces in terms of the very tight temperature tolerance control needed for aluminum brazing.
In this part, let’s look more closely at the contamination issues presented by outgassing of aluminum brazing products.
Aluminum brazing filler metals (BFMs) often require the presence of magnesium in the furnace chamber to act as a “getter” for excess oxygen present in the furnace. Magnesium may be present in the aluminum base metal or perhaps in the aluminum BFM or both. Sometimes it is added separately as a powder (or chips) placed in a small crucible in the furnace hot zone.
Unfortunately, both aluminum and magnesium can easily outgas and contaminate a vacuum furnace. This could be a major issue for aerospace furnaces used for stainless steels since any contamination from aluminum alloys can totally ruin future furnace runs for stainless steels and superalloys.
I have personally seen companies make the mistake of trying to braze with aluminum-based BFMs in regular aerospace furnaces. They use as their reason for doing so the fact that they were merely trying to use the aluminum-BFM to braze much higher melting-temperature base metals such as when joining titanium components together. So, they were not concerned by the lack of overall temperature control of their standard high-temperature vacuum furnace since the melting point of the aluminum BFM was well below the melting temperature of the titanium alloy being joined.
However, the outgassing contamination from the BFM – and from the magnesium getters they mistakenly added to the furnace to supposedly improve the furnace atmosphere for aluminum brazing – ruined the hot zone, aluminized the insulators and put a lot of magnesium coating on the furnace walls. This coating continually outgassed whenever the furnace was used, rendering the aerospace furnace useless for regular aero-brazing.
If you desire to do any kind of aluminum brazing, whether it’s to join aluminum base metals together or other metals such as titanium, I strongly recommend that you NEVER use a regular aerospace-type vacuum furnace for that purpose. Instead, use only a vacuum furnace built specifically and only for aluminum brazing.