Copper-Phosphorous Alloy Advantages

Copper-Phosphorous Alloys Offer Advantages in Brazing Copper

It's self-fluxing characteristic when joining copper to copper is only one of the advantages of this family of alloys.

Copper-phosphorous brazing alloys are used extensively for joining copper, especially refrigeration and air-conditioning copper tubing and electrical conductors. Although the alloys copper-phosphorous and silver-copper-phosphorous were originally patented in this country in 1914, they did not gain extensive use until after WWII. At that time, new refrigerants were developed to replace ammonia. These refrigerants enabled copper to be used for the increased demand for air-conditioned homes, offices and work areas. Credit should be extended to Westinghouse Corp. for its developmental work on the copper-phosphorous system following WWII, thus permitting its rapid growth in the refrigeration and air-conditioning markets.

What is the effect of phosphorous when alloyed with copper?

It lowers the melt temperature of copper. (a temperature depressant)
It increases the fluidity of copper when in the liquid state.
It acts as a deoxidant or a fluxing agent with copper.
It lowers the ductility of copper. (embrittles)

There is a significant effect of phosphorous on the melt temperature of copper. For this discussion, (and the most significant) is the melting point of copper from which the alloy liquidus temperature is obtained depending on the phosphorous content. When heating to this temperature the alloy is completely liquid and when the alloy is cooled below that temperature it is completely solid. 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 in the cost of manufacturing.

There is a misconception that silver improves the ductility of the copper-phosphorous alloys. In reality, silver added to copper acts in a similar manner as phosphorous. The addition of silver to copper lowers the melt temperature (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 at a lower phosphorous content. The lowering of the phosphorous content actually makes the alloy more ductile, not the silver addition.

Likewise, the fluidity of Cu-P and Ag-Cu-P alloys follows in a manner similar to ductility. An increase in phosphorous content increases the fluidity of the liquid metal. A more fluid alloy requires a closer fitting capillary spacing to have the necessary 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 to 0.005 in. for a joint length of less than 1.0 in., and 0.007 to 0.015 in. for a joint length greater than 1.0 in.

A major advantage of the copper-phosphorous alloys is the self-fluxing characteristics when joining copper to copper. They may also be used with the addition of a paste flux on brass, bronze, and specialized applications on silver, tungsten and molybdenum. The self-fluxing characteristic in joining copper to copper is related to the phosphorous element possessing a greater affinity for oxygen than it does for the element copper. Also, the phosphorous will combine with the oxygen in the air and in the flame if an oxyfuel torch is used.

In addition to oxidation of the phosphorous during application, the phosphorous will also diffuse or amalgamate with the surface with the surface of the parts being joined. Any loss in phosphorous if it is by diffusion or oxidation, results in the alteration to 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 BCup alloys, whether it is wire, rod or as a preform, care must be taken 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.

Whether it is selection of the proper BCup alloy or troubleshooting an existing problem, the suggested approach is a review of the desired phosphorous content in the liquid metal and how it is being altered during the application. In torch brazing, a slight change in the oxygen-fuel ratio can affect the joint quality and leak tightness.