Cast copper alloys are known for their versatility. They are used in a wide range of applications, such as plumbing fixtures, ship propellers, power plant water impellers and bushing and bearing sleeves, because they are easily cast, have a long history of successful use, are readily available from a multitude of sources, can achieve a range of physical and mechanical properties and are easily machined, brazed, soldered, polished or plated.
Following is a list of physical and mechanical properties common to cast copper alloys. Although not every property is applicable to every alloy, this range, which occurs in unique combinations, isn’t found in any other alloy group:
- good corrosion resistance, which contributes to its durability and long-term cost-effectiveness;
- favorable mechanical properties ranging from pure copper, which is soft and ductile, to manganese-bronze, which rivals the mechanical properties of quenched and tempered steel. In addition, almost all copper alloys retain their mechanical properties, including impact toughness, at low temperatures;
- high thermal and electrical conductivity, which is greater than any metal except silver. Although the conductivity of copper drops when alloyed, copper alloys with low conductivity still conduct both heat and electricity better than other corrosion-resistant materials;
- bio-fouling resistance, as copper inhibits marine organism growth. Although this property (unique to copper) decreases upon alloying, it is retained at a useful level in many alloys, such as copper-nickel;
- low friction and wear rates, such as with the high-leaded tin-bronzes, which are cast into sleeve bearings and exhibit lower wear rates than steel;
- good castability, as all copper alloys can be sand cast and many can be centrifugal, continuous, permanent mold and diecast;
- good machinability, as leaded copper alloys are free-cutting at high machining speeds, and many unleaded alloys, such as nickel-aluminum bronze, are readily machinable at recommended feeds and speeds with proper tooling;
- ease of post-casting processing, as good surface finish and high tolerance control is readily achieved. In addition, many cast copper alloys are polished to a high luster, and plating, soldering, brazing and welding also are routinely performed;
- large alloy choice, since several alloys may be suitable candidates for any given application depending upon design loads and corrosivity of the environment;
- comparable costs to other metals due to their high yield, low machining costs and little requirement for surface coatings, such as paint.
Using Copper Alloys
Cast copper alloys are identified by the Unified Numbering System (UNS) in which each alloy is assigned a number ranging from C80000 to C99999. From a metallurgical viewpoint, many cast copper alloys are single-phase solid solutions in which the alloying elements such as zinc, tin and nickel are substituted for copper in the copper matrix. Examples of cast single-phase solution alloys are red brass, which contains up to 6% zinc and 2% tin, copper-nickel, which contains up to 10% nickel, and tin-bronze, which contains up to 8% tin and 4% zinc.
As the alloy content increases, a second phase may form. In the case of brass, when the zinc content is increased, a hard second phase (called beta) forms with the copper-rich matrix. This phase is found in yellow brass, which contains up to 41% zinc. In addition, this phase impairs room temperature ductility but increases elevated temperature ductility.
Role of Lead
Lead is commonly added to many cast copper alloys. Because of the low solubility of lead in copper, true alloying does not occur to any measurable degree. During the solidification of castings, some constituents in a given alloy form crystals at higher temperatures relative to others, resulting in tree-like structures called dendrites. The small spaces between the dendrites can interconnect to form micropores. This microporosity is a consequence of the solidification process. The role of lead is to seal these intradendritic pores. This results in a pressure-tight casting, which is important for fluid handling applications.
Lead also allows the machining of castings to be performed at higher speeds without the aid of coolants because it acts as a lubricant for cutting tool edges and promotes the formation of small, discontinuous chips that can be cleared easily. This results in improved machined surface finishes. Lead also plays a role in providing lubricity during service, as in cast copper bearings and bushings. Lead does not have an adverse effect on strength unless present in high concentrations, but it does reduce ductility. Although lead-containing copper alloys can be soldered and brazed, they cannot be welded.
Following is a list of the various cast copper alloy families.
Coppers (C80100 to 81200)—These alloys are pure copper (99.7% minimum) with traces of silver (for annealing resistance) or phosphorus (a de-oxidizer for welding). These alloys are used in high thermal and electrical conductivity applications, such as electrical connectors.
High Coppers (C81400 to C82800)—High copper alloys (more than 95.1% copper) are unique in that they combine high strength with high thermal and electrical conductivity.
Chromium-Copper (C81400 to 81540)—Containing up to 1.5% chromium, the strength of these alloys is twice that of pure copper, but its electrical conductivity is 80% of pure copper. Applications include welding clamps and high-strength electrical connectors.
Copper-Beryllium (C82000 to C82800)—These alloys contain 0.35-2.85% beryllium as the major alloying element and are age- or precipitation-hardened. They achieve high strength due to the precipitation of a fine second phase during heat treatment. Copper beryllium alloys either achieve high conductivity at moderate strength or moderate conductivity at high strength.
The brasses (C83300 to C85800 and C89320 to C89940) are the most common casting alloys and are made of copper and zinc.
Red Brass (C83300 to 83810)—The red brasses are alloys of zinc (1-12%) and tin (0.2-6.5%) and may contain lead (0.5-7%). In red brass, lead is present to promote pressure tightness in service and to facilitate free machining during manufacturing. The red color is due to low zinc content. The highest volume red brass alloy (C83600) has been used commercially for hundreds of years and accounts for more tonnage than any other alloy.
Semi-Red Brass (C84200 to C84800)—Semi-red brass has higher zinc content than the red brasses, which reduces corrosion resistance, lowers raw material costs and lightens the color (but has little effect on strength). Because of their outstanding aqueous corrosion resistance, red brass and semi-red brass often are used in plumbing fittings, such as unions, valves and water meters.
Yellow Brass (C85200 to C85800)—The yellow brasses are lower in cost than the red brasses because their zinc content is higher (20-41%). In addition, they have good castability, with some alloys being permanent mold cast or diecast. Yellow brass has a pleasant yellow color that can be polished to a high luster.
Copper-Bismuth and SeBiLOYS (C89320 to C89940)—The copper-bismuth and selenium-bismuth (SeBiLOY) alloys are low-lead brass alloys that are used in food process and potable water applications. The three SeBiLOY alloys were developed to minimize lead leaching into potable water and to replicate the high machinability and pressure tightness of leaded brass. This is realized by substituting selenium and bismuth for lead. SeBiLoy I and II are red brasses and SeBiLOY III is a yellow brass.
Bronze is an imprecise term. It originally referred to alloys in which tin was the major alloying element. Under the UNS system, the term bronze (C86100 to 87800, C90200 to C95900) applies to a broad class of alloys in which the principal alloying element is neither zinc nor nickel. Nevertheless, bronze is the common name for a number of alloys that contain little, if any, tin.
Manganese-Bronze (C86100 to C86800)—Manganese-bronze, which contains zinc (22-42%) as the major alloying element, is among the strongest cast copper alloys and is used for gears, bolts and valve stems. Where economically feasible, aluminum-bronze replaces manganese-bronze because it offers high strength in combination with better corrosion resistance.
Silicon-Bronze and Silicon-Brass (C87300 to C87800)—Silicon-bronze and silicon-brass are alloys of zinc and silicon that have low melting points and high fluidity, which favor permanent mold and diecasting. Because of its low lead content, silicon-bronze often is a replacement for leaded plumbing brasses, but its limited machinability inhibits use in high-volume potable water systems. It is currently being used as a substitute for semi-red brass in immersed pumps.
Tin-Bronze (C90200 to C91700)—Tin-bronze is an alloy of copper and tin with good aqueous corrosion-resistance. Additional attributes include high strength, good wear resistance and a low friction coefficient compared to steel. This accounts for its use in bearings, piston rings and gear parts.
Leaded Tin-Bronze (C92200 to C92900)—These alloys are a tin-bronze containing 0.3-6% lead. Leaded tin-bronze offers the additional advantage of free cutting.
High-Leaded Tin Bronze (C93100 to C94500)—This is a tin-bronze containing 2-34% lead. High-leaded tin-bronze is used in sleeve bearings and bushings because the additional lead provides improved lubricity.
Nickel-Tin-Bronze (C94700 to C94900)—This is a tin-bronze containing 4-6% nickel. Nickel-tin-bronze is a versatile alloy that has the good wear resistance and corrosion resistance found in tin-bronzes with improved strength. Nickel-tin-bronze is used in many applications including bearings, gears, wear guides, and pump and valve components, and in motion and translation devices, such as shift forks and circuit breaker parts.
Aluminum-Bronze (C95200 to C95900)—Aluminum-bronze has a complex metallurgical structure that imparts both strength and oxidization resistance due to the formation of alumina-rich protective films. These alloys are wear-resistant and exhibit good casting and welding characteristics. Their corrosion resistance is superior in seawater, chloride and dilute acids. Applications are varied and include propellers and valves, pickling hooks, pickling baskets and wear rings. The aluminum bronze alloys that contain nickel are desirable for fluid-moving applications, such as pump impellers, because of superior erosion, corrosion and cavitation resistance.
Copper-Nickel (C96200 to 96950)—These alloys are simple solid solutions of nickel in copper without lead. The copper-nickel alloys have excellent corrosion resistance in seawater, high strength and ease of manufacturing. Their various applications include pumps, valves, ship tail shaft sleeves and other marine applications.
Nickel-Silver (C97300 to C97800)—The presence of nickel accounts for these alloys’ silver luster. These alloys, which do not contain silver, offer good corrosion resistance, ease of castability and good machinability. Despite their high degree of alloying, these alloys are simple solid solutions. Major uses include hardware for food processing, seals, architectural trim and musical instrument valves.
Leaded-Coppers (C98200 to C98840)—These are essentially pure copper or high-copper alloys containing lead. The leaded-coppers offer the moderate corrosion resistance and high conductivity of the copper alloys, in addition to the lubricity and low friction characteristics of high-leaded bronzes.
Special Alloys (C99300 to C99750)—These are alloys with unique characteristics, such as Incrament 800 (C99300), which has high oxidation resistance due to aluminum, good thermal fatigue resistance and high hot hardness. This alloy was developed for glass processing including glassmaking molds and plate glass rolls.
Design for Manufacturing
The choice of alloy and casting method (sand, permanent mold, die or investment casting) determines the mechanical and physical properties, section size, wall thickness and surface finish that can be achieved. Each alloy and casting process combination results in a different set of properties.
If metalcasting facility and design engineers can work together on the “raw” or ideal component, all options will be considered early in the design process, resulting in a design and component that take advantage of the versatility that copper alloys offer.
--Harold T. Michels, Copper Development Assn. Inc.