Note: This article is adapted from the version which originally appeared in the October, 1994 issue of Welding Journal magazine
Probably the most widely used brazing filler metal is an alloy of copper and phosphorus, often with silver added as a third element. The role of silver in these alloys is often misunderstood.
Both phosphorus and silver have significant effects on the properties of these filler metals. Of the two, however, phosphorus is, by far, the more potent. On average, 1% of phosphorus will lower the liquidus temperature (the point at which the alloy is entirely molten) by 100 to 120°F. On the other hand, 1% of silver will lower this temperature by only 10 to 15°F.
The widespread assumption that the addition of silver to the phosphorus-copper filler metals improves ductility is false. Reducing phosphorus content improves ductility. The fact that adding silver in sufficient quantity to lower the melting range permits the reduction of phosphorus has perpetuated this myth.
The addition of silver in significant amounts also tends to widen the melting range when phosphorus is reduced. This characteristic enables an operator to fill larger gaps more readily than with the more fluid alloys of just copper and phosphorus.
Silver’s high market price has prompted many users to seek braze filler metals with a lower silver content than the traditional 15% silver alloy. An alloy of 6% silver and about 6% phosphorus, our Dynaflow®, has captured a large part of this market. It exhibits similar characteristics as the 15% alloy with almost the same melting range, but at considerably lower cost.
Over the years the industry subsequently introduced phosphorus-copper alloys containing as low as 1% silver, but the benefits of these lower silver alloys are, at best, questionable. Most of these low-silver alloys show very little, if any, benefit over the 0% silver alloys. The assumption that they are more ductile is unfounded unless the phosphorus content is reduced.
Users would have a much better idea of how a particular composition works if the identity number were to indicate phosphorus content instead of silver content. The American Welding Society (AWS) has adopted a method in which the numbers designate neither silver nor phosphorus, but refer to an alloy with a specified chemical change. The AWS also urges the industry to adopt the term "brazing filler metal" and avoid the term "silver solder."
While both phosphorus and silver tend to reduce ductility in copper, this is mostly the concern of the producer because these alloys are difficult to manufacture. Users of these filler metals are, or should be, more concerned with the characteristics of the joint. Strong, ductile joints can be made with relatively brittle filler metals if other factors are properly controlled.
Brazing requires that the filler metal flow into a capillary between the two closely aligned base metals to be joined. In the case of phosphorus alloys - these metals are usually copper or brass, and joint integrity is dependent on a number of factors in addition to the choice of filler metal. One of the most important is the design of the joint and the thickness of the capillary. An overlap rather than a butt joint is essential for best results. The filler metal must be compatible with, wet and adhere to the base metal.
The temperature at which the filler metal flows and the skill of the operator in adding filler metal at the proper temperature are important. Proper cleaning of the base metal before brazing and avoiding excess oxidation during heating are contributing factors in joint integrity.
All of the factors that contribute to joint integrity must be properly controlled to assure reliability of a brazed joint.
Intelligent selection of the proper filler metal will yield big dividends in both time and money.
Dick Ballentine, now deceased, worked for The Harris Products Group for 20 years, most recently as Director of Research and Development. Prior to joining Harris, Ballentine spent 40 years with Westinghouse Electric. He was plant manager of their brazing products manufacturing plant where he researched composition and melting of phosphorus copper braze alloys. Ballentine held eight patents related to brazing and soldering process and applications. He was an active member of the AWS C3 Brazing Committee and contributed to the first, and subsequent, editions of the "Brazing Handbook". In 1998 Ballentine was given the honorary title of AWS "Fellow" for contributions to the industry.