Choosing the Right Brazing Alloy
Copper, nickel and silver and the most frequently-used base metals for brazing alloys; aluminum and gold are also used for specific purposes. The table above compares important characteristics and advantages of the most commonly used base metals.
For each base metal, many different alloy compositions are available. The additional metals combined with the base determine the alloy's compatibility for use in joining specific metals and individual process requirements. For example, vacuum brazing requires an alloy free of volatile elements such as cadmium. In addition to the composition of the alloy, other important characteristics to consider are melting range, required joint clearance and ease of flow.
The melting range for a brazing alloy is defined by the minimum temperature at which the alloy will start to melt (“solidus”) and the temperature at which the alloy is 100% liquid (liquidus). For most purposes, the actual brazing temperature is 50°F to 200°F (30°C to 110°C) above the liquidus. The melting range is based on the alloy’s chemical composition, but it is important to note that individual batch characteristics may very slightly. Some alloys (eutectics) have a very narrow melting range while other alloys have a comparatively wide range. Alloys with a narrow melting range are used for filling very narrow gaps while wider range alloys generally work better for filling larger gaps. Wide range alloys have a tendency to separate into their basic components if heated too slowly (liquidation). So it is almost always better heat rapidly through the melting range to reach brazing temperature.
High temperature brazing alloys such as gold, nickel and copper can be used for brazing many joints at once, but care must be taken with the joint design and joint clearance. Heating time should be minimized to the time needed to bring all components to the heating temperature and for the molten alloy to flow quickly. High temperature brazing is often used for joining cobalt or nickel-based superalloys.
Most brazing alloys are normally available in forms such as wire, foil, tape, powder and paste. For links to more information about individual brazing alloys and their characteristics, use GH IA's helpful Brazing Alloy Suppliers Guide.
|Alloy||Brazing Temp.||Joint Clearance||Advantage|
|Copper||2000°F||Interference to 0.002"||Joint strength, low cost|
|Silver||1300° F||0.002 to 0.005"||Low temperature|
|Silver w/Lithium||1300°F||0.002 to 0.005"||Low temperature, self-fluxing|
|Nickel||1900°F||0.002 to 0.008"||Joint strength, corrosion resistance|
|Aluminum||1080°F||0.002 to 0.01"||Only braze for aluminum|
|Gold||1800°F||0.002 to 0.008"||Resistance to oxidation and corrosion|
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