Brazing Filler Metal Selection
Proper selection of a brazing filler metal for the intended application depends upon a number of factors, including component design, base metal alloy(s), and service environment. Brazing filler metals are typically classified according to chemical composition. HASTELLOY® and HAYNES® alloys may be successfully brazed using a variety of nickel-, cobalt-, silver-, copper-, and gold-based filler metals; some of the possible brazing filler metals are listed in Table 4. The exact alloying content of the brazing filler metal determines the temperature range between the liquidus and solidus, i.e. the melting temperature range. The magnitude of the melting temperature range indicates the potential filling capability, and a brazing filler metal with a larger melting range is generally more capable of filling a larger joint clearance. If the brazing filler metal melts at a specific temperature, it is referred to as a eutectic filler metal. As a result, eutectic filler metals have less filling capability and require tight joint clearances. Examples of eutectic filler metals are the AWS A5.8 BAg-8, BAu-4, and BCu-1 classifications.
Filler metals are commonly applied as a powder mixed with a liquid binder. The brazing filler metal powder can also be mixed with a water-based gel suspension agent to produce a paste. Filler metals are also available as foil and tape. Every effort should be made to confine the brazing filler metal to the joint area as any spatter upon non-joint surfaces could severely degrade the environmental resistance at that location, particularly if it is exposed to service temperatures above the melting point of the brazing filler metal. Since most brazing filler metals do not possess the same level of corrosion resistance as Ni-base corrosion-resistant alloys, it is preferable that brazing is used for joining only when the brazed joint will be isolated from the corrosive environment.
Nickel-based brazing filler metals can be utilized for high-temperature service applications up to 2000°F (1093°C). They generally have additions of boron, silicon, and manganese to depress the melting range and accommodate brazing at various temperatures. The boron-containing brazing filler metals are used for aerospace and other applications subject to high temperature and stress conditions. However, they are susceptible to the formation of brittle borides. These brazing filler metals may also contain chromium to provide for more oxidation-resistant joints.
Cobalt-based brazing filler metals are typically useful for achieving compatibility with Co-base alloys, and obtaining good high-temperature strength and oxidation resistance.
Silver-based brazing filler metals have been successfully used for brazing Ni-base corrosion-resistant alloys intended for service applications below approximately 400ºF (204ºC). They are known for excellent flow characteristics and ease of usage. Filler metals containing low-temperature constituents, such as zinc and tin, are difficult for furnace brazing since they will evaporate prior to reaching the brazing temperature. Most furnace brazing with silver-based filler metals should be conducted in an argon atmosphere. It should be cautioned that most Ni-base alloys are subject to stress-corrosion cracking when exposed to molten silver-rich compositions, so it is imperative that the base metal be stress-free during brazing when utilizing silver-based filler metals. This liquid metal embrittlement form of cracking occurs catastrophically at the brazing temperature.
Copper-based brazing filler metals tend to alloy rapidly with Ni-base alloys, raising the liquidus and reducing fluidity. Therefore, they should be placed as close to the joint as possible, and the assembly should be heated rapidly to the brazing temperature. Copper-based brazing filler metals are only suggested for joining components to be used at service temperatures below 950°F (510°C). Copper-based brazing filler metals that contain significant amounts of phosphorus should be used with caution since they tend to form nickel phosphides at the bond line that promote brittle fracture. Copper-based filler metals should not be used for brazing Co-base alloys.
Gold-based brazing filler metals are mostly used when joining thin base metals due to their low interaction with the base metal. They are also useful when good joint ductility and/or resistance to oxidation and corrosion are primary concerns.
For more detailed information on different brazing filler metal classifications, please refer to: AWS A5.8M/A5.8, Specification for Filler Metals for Brazing and Braze Welding, American Welding Society. There are also numerous proprietary brazing filler metals and alloy compositions that are commercially available. It is suggested that brazing filler metal manufacturers be consulted when selecting a filler metal for a specific base metal alloy or application.