Brazing Services
Brazing is the joining of metal and ceramic components using a molten filler metal that melts above 850ºF (450ºC). Filler metals include Al-Si, Cu-Ag, Cu-Mn, Cu-P, Cu, Ag, Au, and Ni-Cr-Si and Ni-Cr-Si-B as well as other less used metal fillers that will wet and adhere to metals and in special cases to ceramics. When brazing to ceramics, active elements such as Ti, Hf, Zr and V have been added to filler metals. The addition of these reactive elements permits them to react with the ceramic surfaces during brazing, but only in a vacuum atmosphere.
When brazing the component metal surfaces, they must be heated over the filler melting temperatures, typically above 850ºC, and even higher for the Ni-based filler metals. When heating to these temperatures, a flux or atmosphere that chemically or physically reduces the surface oxides that forms the metals is essential for brazing to work. In order for a molten filler metal to “wet” [flow evenly and adhere as a liquid] the metal surfaces have to be free of oxides before a joint can be made. Once the molten filler metals spread or, via capillary action, is pulled into a joint, the heat source is removed to permit the filler metal to solidify and develop a bond to the metal.
There are two major types of brazing with sub-classifications as shown below:
Flux brazing
- Torch (or other concentrated heat source)
- Resistance
- Furnace
- Dip Brazing (mainly used for aluminum)
Flux brazing normally utilizes more intense heating torches which are normally different that those used in soldering due to the need to melt higher temperature metal filler metals. As such, the higher component surfaces are subjected to more discoloration, distortion, and softening than with soldering. The fluxes are normally much more aggressive than solder fluxes due to the need to remove more oxide and to provide a “local” atmosphere as the flux heats and interacts with the component surfaces and the molten filler metals. Upon cooling, fluxes often slag to the surface and form glass-like deposits that may need to be cleaned.
Controlled atmosphere brazing
- Exo-atmospheres (normally used in belt furnaces)
- Nitrogen / Hydrogen (belt of batch furnaces)
- Vacuum (batch furnace)
Controlled atmosphere brazing provides an environment that is non-oxidizing and typically reducing in order to clean the metal surfaces being brazed. The cleanest and least oxidizing furnace atmospheres are vacuum; as such, vacuum brazing is being used more often for the braze joining of many components. In the case of active brazing and high temperature Ni-based alloy brazing, vacuum brazing is the only process that works.
Note that brazed joints can have strengths up to the component metal strength, especially when braze/joint gaps are small and controlled. Thus brazing is used in much more critically stressed joints as opposed to solder bonding. Joint design and component fixturing are more critical in brazing due to the higher temperatures where thermal expansion and thermal expansion mismatch play a larger role.
Contact Us for assistance in assessing your bonding needs and to meet your brazing requirements. We offer a range of brazing services at our facilities or through those of our partners’. If we cannot offer the service our engineers will refer you to companies that can meet your needs.
