A Primer on Selecting Cast Copper Alloys
Traditionally, cast copper alloys were classified by a variety of systems including the ASTM letter-number designation based on nominal composition, by trade names, and by descriptive terms such as “ounce metal,” “Navy M” and so forth. However, with technological developments, new alloys were produced and existing alloys modified, making the old designation systems inadequate and misleading.
A new system was developed based on a precise description of the composition range for each alloy, which is now the accepted alloy designation system used in North America for cast copper and copper alloy products. Originally developed as a three digit system by the copper and brass industry, the designations have now been expanded to five digits that follow a prefix letter “C,” and have been made part of the Unified Numbering System (UNS) for Metals and Alloys. The UNS is managed jointly by the American Society for Testing and Materials, and the Society of Automotive Engineers.
The UNS designations are expanded versions of the former designations. For example, copper alloy no. 836 in the original three digit system became C83600 in the UNS system. The previous designations will continue to be used over a transition period of several years. Because the old numbers are incorporated in the new UNS numbers, there is little chance for confusion. This system is also being used in Australia and Brazil.
The designation system is administered by the Copper Development Assn, Inc. New designations are assigned as new coppers and copper alloys come into commercial use, and designations are discontinued when an alloy ceases to be used commercially. Anyone may submit requests for changes in the list.
New designations are assigned if a copper or copper alloy meets three criteria:
- 1. The complete chemical composition is disclosed.
- 2. The copper or copper alloy is in commercial use or is proposed for commercial use.
- 3. The composition does not fall within the limits of an already designated composition in the list. (Exceptions may be made for standard alloys for countries outside North America.)
With this system, cast alloys are numbered from C80000 through C99999. The compositions are grouped into the following families of coppers and copper alloys:
Coppers-Metals which have a designated minimum copper content of 99.3% or higher.
High-Copper Alloys-The cast high-copper alloys have designated copper contents in excess of 94%, to which silver may be added for special properties.
Brasses-These alloys contain zinc as the principal alloying element, with or without other designated alloying elements present such as iron, aluminum, nickel and silicon. The cast alloys comprise four major families of brasses:
- copper-tin-zinc alloys (red, semi-red, and yellow brasses)
- manganese bronze alloys (high-strength yellow brasses)
- leaded manganese bronze alloys (leaded, high strength yellow brasses)
- and copper-zinc-silicon alloys (silicon brasses and bronzes).
As noted, the designations are based on precise compositional limits; however, the ingot analysis may differ slightly from the composition shown (which is for the final casting). This is to allow for the loss of elements by oxidation or volatilization, or the possible pickup of impurities when the ingot is remelted to make the casting.
Bronzes-Broadly speaking, bronzes are copper alloys in which the major alloying element is not zinc or nickel. Originally, bronze described alloys with tin as the only or principal alloying element. Today, the term is generally not used by itself but with a modifying adjective. The cast alloys include four main families of bronzes:
- copper-tin alloys (tin bronzes)
- copper-tin-lead alloys (leaded and high-leaded tin bronzes)
- copper-tin-nickel alloys (nickel tin bronzes)
- and copper-aluminum alloys (aluminum bronzes). The family of alloys known as manganese bronzes, in which zinc is the major alloying element, is included in the brasses category.
Copper-Nickels-These are alloys use nickel as the principal alloying element, with or without other designated alloying elements present.
Copper-Nickel-Zinc Alloys-Known commonly as “nickel silvers,” these are alloys that contain zinc and nickel as the principal and secondary alloying elements, with or without the presence of other designated elements.
Leaded Coppers-These comprise a series of cast alloys of copper with 20% or more lead, usually with a small amount of silver present but without tin or zinc.
Special Alloys-Alloys whose chemical compositions do not fall into any of the above categories are categorized as special alloys.
The use of these standard designations is optional. They do not preclude the manufacture or supply of products that do not conform to them.
The designated compositional limits do not preclude the possible presence of other unspecified elements. By agreement between supplier and consumer, an analysis may be required, and limits may be established for elements not cited. Percentage contents of elements shown as “remainder” are normally calculated by difference.
Mechanical properties are not considered in this designation system, which is based solely on chemical composition. The mechanical properties are a function of the manufacturing method as well as of composition, as shown in the examples in Table 1.
This example emphasizes the fact that the designation system is not a specification to which products are manufactured. Thus, before manufacturing starts, reference must be made to the current relevant specifications.
This section covers in tabular form, various properties of commonly used cast copper-base alloys. Typical properties, as opposed to the minimum values are dealt with here.
Table 2 covers such mechanical and physical properties as tensile and yield strengths, modulus of elasticity, Brinell hardness, compression strength, impact strength, electrical and thermal conductivity and specific gravity.
|Table 1. Mechanical Properties of Select Copper-Base Alloys|
|Casting Method||Sand or Centrifugal¹||Continuous²||Sand or Centrifugal¹||Continuous²||Sand or Centrifugal¹||Continuous²|
|0.5 Yield Strength||14||19||16||19||12||20|
|Elongation % in 2 in.||20||15||24||18||15||6|
1. ASTM B584-79, ASTM B271-73
2. ASTM B505-81