March 13, 2007
It is important to be able to identify when brazing is suitable for joining copper or copper alloys, how it is applied, and which filler metals to use.
Four processes to consider when joining copper and copper alloys are mechanical couplings, welding, soldering, and brazing. Brazing is suitable for small parts and when high joint strength is required. According to the American Welding Society (AWS), the strength of a brazed joint can meet or exceed that of the metals being joined. It is important to know when to choose brazing and how to perform the process.
From a process standpoint, soldering and brazing are essentially the same. The only differences are the filler metals used and the amount of time and heat required to complete the joint. AWS defines soldering as a joining process that takes place below 840 degrees F, while brazing takes place above 840 degrees F but below the melting point of the base metal. In actual practice for copper systems, most soldering is done at temperatures from about 450 degrees F to 600 degrees F, while most brazing is done at temperatures from 1,100 degrees F to 1,500 degrees F. When brazing copper tube, however, the annealing of the tube and fitting that results from the higher heat can cause the rated pressure of the system to be less than that of a soldered joint.
Copper's melting point is 1,981 degrees F (liquidus) and 1,949 degrees F (solidus). For brazing, it is important to know the melting points of the metals to be joined and the filler metal. The difference between the solidus and liquidus state is the melting range, which may be important when selecting a filler metal. It indicates the width of the working range for the filler metal and the speed with which the filler metal solidifies after brazing. Filler metals with narrow ranges, with or without silver, solidify more quickly and, therefore, require careful application of heat. The liquidus temperature is the minimum at which brazing will take place. See Figure 1 for the melting ranges of some common brazing metals.
According to Lucas-Milhaupt's What Brazing Is All About (www.lucasmilhaupt.com), the choice to braze comes down to five factors:
Brazing fluxes for copper are water-based, dissolve and remove residual oxides from the metal surface, protect the metal from oxidation during heating, and promote wetting of the surfaces to be joined. Brazing fluxes also provide you with an indication of temperature (see Figure 2).
The most commonly used fluxes and brazing filler materials for copper and copper alloys are shown in Figure 3, and a guide to their use is shown in Figure 4. This and other detailed information can be found in The Welding Handbook, 8th Edition, Vol. 8, published by the American Welding Society and available from the Copper Development Association under the title Welding Copper and Copper Alloys, A1050-72/97.
The same basic steps are used for brazing as for soldering, with the only differences being the use of fluxes, filler metals, and the amount of heat used.
In general, both lap and butt joints can be made. Be sure to remove all oxides and surface oils with abrasive cloth, pads, or brushes before joining the metals. Such contaminants interfere with the proper flow of filler metal and may lessen the joint strength or cause failure. Chemicals cleaners may be used if they are thoroughly rinsed off, but be sure you don't touch the clean surface with bare hands or oily gloves.
Apply a thin, even coating of flux with a brush to both surfaces soon after cleaning. Do not apply the flux with your fingers because the chemicals in the flux can be harmful if it comes in contact with your eyes, mouth, or open cuts. Copper-phosphorus and copper-silver-phosphorus metals (BCuP) are considered self-fluxing on copper-base metals.
Support the surfaces securely and ensure an adequate capillary space between them for the flow of the molten brazing filler. Excessive joint clearance can lead to cracking under stress or vibration. A joint clearance of 0.001 to 0.005 in. will develop the maximum joint strength and soundness.
Use only the amount of heat necessary to melt and flow the filler metal. Overheating the joint or directing the flame into the capillary space can burn the flux, destroying its effectiveness and preventing the filler metal from entering the joint properly. Apply the heat around the joint area to draw the filler metal into the capillary space. When dealing with an open flame, high temperatures, and flammable gases, safety precautions as described in ANSI/AWS Z49.1, "Safety in Welding, Cutting and Allied Processes," must be observed.
Allow the completed joint to cool naturally. Shock cooling with water may stress or crack it. When it is cool, clean off any remaining flux residue with a wet rag and test all completed assemblies for joint integrity.
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