Copper-Phosphorous Alloys for Copper Brazing
The alloys copper-phosphorous and silver-copper-phosphorous were patented in the U.S. in 1914 and gained popularity when new refrigerants were developed to replace ammonia. These refrigerants enabled the use of copper to facilitate the increased demand for air-conditioned homes, offices and work areas.

What happens when phosphorous is alloyed with copper?

1. The self-fluxing (deoxidant) characteristic of phosphorous provides an advantage in brazing copper to copper.
2. Phosphorous lowers the melt temperature of copper, acting as a temperature depressant.
3. Phosphorous increases the fluidity of copper when in the liquid state.
4. Phosphorous lowers the ductility of (or embrittles) copper.

Self Fluxing
Copper-phosphorous alloys are self-fluxing when joining copper to copper. These alloys may also be used with a paste flux on brass, bronze and specialized applications on silver, tungsten and molybdenum. The self-fluxing characteristic occurs because the phosphorous element possesses a greater affinity for oxygen than for the copper. The phosphorous also combines with oxygen in the air and the flame if an oxyfuel torch is used.

In addition to oxidizing, the phosphorous diffuses, or amalgamates, with the surface of the parts being joined. Any loss in phosphorous, by diffusion or oxidation, alters the properties of the remaining liquid as it flows into the joint. In most cases, the change reflects a more viscous liquid with a higher melt temperature. In the application of the copper-phosphorous alloys, whether wire, rod or preform, take care to minimize any overoxidation of the phosphorous element. A slight change in the phosphorous content has a significant effect on the liquid-metal-flow characteristics.

Melting-Point Depressant
While responsible for the self-fluxing properties of the alloys, phosphorus is also an excellent and inexpensive melting-point depressant when added to copper. At a concentration of 8.25%, phosphorus forms a eutectic phase with copper, with a melting point of approximately 710°C. All the copper-phosphorus alloys used as brazing filler metals contain a certain percentage of the eutectic phase; therefore, all the alloys used for brazing have the same solidus temperature (start melting at the same temperature).

Increasing Fluidity
The percentage of phosphorus in any particular alloy has a marked effect on its liquidus temperature (the temperature at which it becomes fully molten) and hence its flow characteristics. The closer an alloy's phosphorus content is to the eutectic composition, the better its flow properties. In addition, small changes in phosphorus content can also have a marked effect on an alloy's flow properties.

For Cu-P and Ag-Cu-P alloys, an increase in phosphorous content increases the fluidity of the liquid metal. A more fluid alloy requires a closer-fitting space to allow capillary attraction to retain the liquid. With the addition of silver, the phosphorous content can be decreased, resulting in a less-fluid molten liquid. Therefore, a higher-silver alloy with less phosphorous will fill or bridge a larger gap and produce a larger fillet. According to Chapter 2 of the AWS Brazing Handbook, the suggested joint clearance for the BCuP family is 0.001 - 0.005 in. (0.0254 - 0.127mm) for a joint length of less than 1.0 in. (25.4mm), and 0.007 - 0.015 in. (0.178 - 0.38mm) for a joint length greater than 1.0 in. (25.4mm).

Reducing Ductility
The major negative effect of phosphorous on copper is the reduction in ductility. In the range of 8.0 - 8.5% phosphorous content, the material becomes extremely difficult to form in both hot and, especially, cold forming. Any gain in properties at the higher phosphorous content is offset by the cost of manufacturing.

Note regarding silver: There is a misconception that silver improves the ductility of copper-phosphorous alloys. In reality, when silver is added to copper, the effect is similar to phosphorous. The silver lowers the melt temperature (acting as a temperature depressant) and decreases ductility. Fortunately, the rate and amount which silver lowers copper ductility is significantly less than that of phosphorous. Therefore, taking advantage of the temperature-depressant property of silver, an Ag-Cu-P alloy can be selected at approximately the same melt temperature as a Cu-P alloy, but with a lower phosphorous content. Decreasing the phosphorous content-not adding the silver content-actually makes the alloy more ductile.

CONCLUSION:
Copper-phosphorous brazing alloys are used to join copper to copper in HVAC/R applications and industries using electrical conductors. Whether you are selecting the proper copper-phosphorous alloy or troubleshooting an existing problem, Lucas-Milhaupt technicians suggest a review of the desired phosphorous content in the liquid metal and how it is being altered during application. In torch brazing, a slight change in the oxygen-fuel ratio can affect joint quality and leak tightness.

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