We're having trouble brazing a pure copper part to 304L stainless in a vacuum furnace with silver-based brazing filler metal. The stainless comes out a dark color, and the silver looks a bit spongy, gray and doesn't flow on the stainless. What's wrong?
Silver-based brazing filler metals (BFMs) are a good choice for joining pure copper to 304L stainless steel, and the process can actually be done in a vacuum furnace as long as the base-metal chemistries contain no zinc or cadmium. BFMs such as BVAg-6b, BvAg-18 or BAg-21 are examples of silver-based BFMs that can be used to join the copper to stainless in an atmosphere furnace or vacuum furnace without the use of flux.
NOTE: Flux should NEVER be used in a vacuum furnace! I've seen vacuum furnaces destroyed when such attempts were made!
To vacuum braze copper to stainless, two things are essential:
1. The stainless steel should be nickel-plated since the chromium in the stainless oxidizes extensively during heating, and those oxides can’t be "reduced" (removed by chemical reaction or by dissociation) until temperatures of at least 1850-1900°F (1010-1040°C) are reached. Silver brazing is usually performed at temperatures well below that. Electrolytic nickel plating is therefore suggested. Once the areas to be brazed have been electrolytically nickel-plated, you can braze them in the 1400-1700°F (760-925°C) temperature range normally used for furnace brazing with silver-based BFMs.
2. The vacuum furnace will require a partial pressure of clean, dry, inert gas to prevent the copper and silver materials from outgassing during the brazing cycle. Since both copper and silver have relatively high vapor pressures, they can easily begin to volatilize as the pressure in the vacuum furnace is reduced and the brazing temperature is approached. It is common to backfill the vacuum furnace with an inert gas such as very clean, dry argon until a partial pressure of about 100-200 microns (millitorr) is built up and maintained continuously during the brazing cycle.
By taking these two precautions, brazing shops should have no problems joining these two metals together in a vacuum furnace.