Question:
We are investigating brazing a large cylindrical component to a series of tubes and supporting them in a graphite fixture. Both are manufactured from Inconel 625 alloy. We will use a BNi-3 braze alloy. Can you provide us with any tips on how to proceed?

Answer:
Here are some additional ideas to help.

5. Cycle

A. For assemblies such as the one involved in this application – consisting of both heavy and light sections – preheating is required to reduce thermal gradients and subsequent distortion as well as improve wetting (flow) on the heavier part.
(i) Preheat temperature should be 100°F lower than the melting point (liquidus) of the brazing filler metal.

6. Graphite Fixtures

A. Graphite fixtures should not be allowed to come in contact with the parts due to concern over carbon transfer.

B. The graphite fixtures should be made of high-purity, high-density graphite.

c. Graphite has a lower coefficient of thermal expansion than the base metal.
(i) For example, at 2000°F graphite will expand 0.005 in/in (0.13 mm/m)
(ii) Grooves cut into the graphite to accommodate the metal part must be larger to compensate for the differential in expansion.

D. Store fixtures in a manner so as to minimize moisture pickup.

7. Vacuum Furnace

A. The pumping system should be oversized with respect to the (equivalent) furnace size.
(i) A large pumping system will improve the quality of the brazing.

B. Braze quality also depends on:
(i) Leak rate (5-10 microns/hour maximum). A tight furnace is mandatory.
(ii) Virtual leakage and outgassing.

C. Capability of pumping down into the 10-4 to 10-5 torr range (recommended).
(i) A vacuum level of 10-5 torr (minimum) might work but would be dependent on the nickel-coating thickness.

D. To obtain adequate strength properties from the filler metal it is essential to hold the assembly at brazing temperature for a minimum of 10-15 minutes and to achieve maximum strength, 45-60 minutes.
(i) Soak time must be adequate for full diffusion but not so long as to allow leaching of Ti, Ta, or Al through the nickel-plating causing joint attack.

E. Partial pressure (argon) is required.
(i) Nitrogen partial pressure must be avoided due to the possibility of forming boron nitrides (which has the potential to act as a stop-off).
(ii) Chromium will outgas to some degree at 10-3 to 10-4 torr at 1900-1950°F.
(iii) Aluminum will be vaporized from the surface of the Inconel 625 at 1950°F.

F. A two-bar gas quench is adequate after brazing.
(i) A quench delay, typically for 30 seconds to 1-2 minutes to allow the braze to solidify prior to quenching (so the parts don't move during quench).