Can you vacuum braze brass?

We were asked earlier this week the best filler metal for brazing brass. Our customer wanted to process a brass part through their vacuum furnace.

As many of you will be aware this is a big no and we obviously warned our customer accordingly but why is this such a big cause for concern?

With brass it is the zinc content that causes the big issue because it can easily turn into gas vapour in the furnace when heated which can badly contaminate your furnace. However, if you are brazing with nickel, silver or copper based filler metals it also applies to a few other metals such as cadmium, lead and magnesium.

Vapour Pressure Curves (courtesy AWS Brazing Handbook)

The chart above shows how the boiling points of the elements is affected by the atmospheric pressure. As the vacuum pumps on your furnace create a stronger vacuum likewise the pressure in the furnace drops and the boiling point of the element also drops. You will note zinc drops from c.900C at STP right down to c.320C.

Since you are heating your furnace to probably at least 950C for most nickel or silver based BFMs you can see that zinc would have possibly completely outgassed by this point resulting in gross contamination of your furnace and entire pumping systems.

Some people perhaps might try to work around by using a BFM with a lower melting point, however these lower melting point alloys often have a lower melting point precisely because they contain zinc or cadmium. Likewise you may try to increase the pressure in your furnace with argon or nitrogen. This will be ineffective as the outgassing will be more powerful than any suppressing attempt (also see our previous post about nitrogen and vacuum brazing!)

There are no effective work-around solutions that we know to avoid these problems. Our rule of thumb is simply to never vacuum-braze any zinc containing metals or use any BFMs that contain zinc or cadmium.