High Copper Alloys - High Strength Coppers for Demanding Electrical Applications

Copper Applications in
Electrical & Metallurgy of Copper Areas

By Vin Callcut

• Introduction
• Applications of High Copper Alloys
• Alloy Development
• Forms Available
• Hardening of Copper
• Casting
• Machining
• Joining
• Heat Treatment
• Electrical Conductivity/Resistivity
• Links

Introduction

Pure copper has the highest electrical conductivity of any commercial metal. This property makes it the preferred material for power and telecommunications cables, magnet (winding) wire, printed circuit board conductors and a host of other electrical applications. Copper has sufficient strength, ductility and hardness for these applications at operating temperatures up to 210 F (100 C). For many other applications, however, the demands of electrical technology require copper to have higher mechanical properties and to be capable of use at elevated operating temperatures while still retaining the good conductivity for which it is selected in the first place.

The copper industry has invested much research effort over the years to create materials capable of meeting these demanding needs. The products of this research are found in the large variety of high copper alloys, materials whose properties are equal to or, in some cases, higher than those of many other engineering metals, yet, which have conductivities high enough for electrical applications. In terms of composition, and for wrought products forms (rod, bar, sheet, strip, etc.), these alloys were originally defined as having designated copper contents less than 99.3% but more than 96% and do not fall into any other copper alloy group. "Other alloy group" means they are not generally categorized as, say, bronzes or copper-nickels. Cast high copper alloys (C81400-C83299) have designated copper contents in excess of 94%, to which silver may be added for special properties. Not surprisingly, it is primarily their relatively high copper content that gives this family of copper alloys their high conductivity.

The high-copper alloy family includes, in wrought forms, cadmium coppers (C16200 and C16500), beryllium coppers (C17000-C17500), chromium coppers (C18100-C18400), zirconium copper (C15000), chromium-zirconium copper (C14500) and combinations of these and other elements. Alloy C18000, another member of the group, contains nickel, silicon and chromium. There are fewer cast high-copper alloys, although the beryllium copper family is well represented.

Applications of High Copper Alloys

Here are a few typical uses. Each encompasses a very wide variety of designs and demands to meet product needs. Several of the applications listed are ordinarily satisfied by one of the electrical coppers, UNS C10100-C12000, and high copper alloys are used only when their enhanced properties and needed and when a somewhat lower electrical or thermal conductivity can be tolerated. A searchable database containing a complete listing of coppers and copper alloys is available at CDA's Web site in the Properties section.

Terminals and connectors for electrical, electronic and automotive applications. The bulk of these are made from brass, or, for somewhat more demanding applications, phosphor bronze. High copper alloys such as beryllium copper, copper-nickel and others are reserved for more severe duty, especially with regard to stress relaxation resistance. As always, factors such as formability, strength and conductivity play a role in the materials selection decision. Designers typically work with alloy suppliers when it comes to detailed property requirements. Additional information is available in CDA's Electronic Connector Alloys Design Guide.

Springs for relay contacts and switchgear. Here, too, less-costly alloys are used for commodity-type products, and high copper alloys are used when the need arises. Additional information is also given in the above-mentioned Web site.

Integrated circuit leadframes. These are made from specialty alloys designed for both their connector-related properties and for compatibility with packaging requirements. A discussion is given on the connector Web site.

Busbars. Unless welded, busbar products (rod, bar, plate) are typically made from electrolytic tough pitch copper, C11000, or for maximum conductivity, electronic or oxygen-free high conductivity coppers. Where mechanical requirements demand higher strength, dilute alloys such as silver-bearing copper or high copper alloys can be considered. Welded or brazed busbars require either oxygen-free copper or a deoxidized copper.

Rotor bars. These are normally made from pure copper unless strength requirements dictate higher mechanical properties.

Armatures. Also made from pure copper unless higher strength or annealing resistance needed, in which case a silver-bearing copper can be considered.

Commutators. Silver-bearing copper (C11400) is used for its annealing resistance.

Spot welding electrodes, seam welding wheels. Various grades listed by the Resistance Welding Manufacturers Association (RWMA) include chromium coppers (Class II) and dispersion-strengthened coppers (Class III), among several others.

Heavy electrical switchgear. Chromium copper, zirconium copper, beryllium coppers and other high copper alloys can be specified, depending on strength requirements.

Molds for continuous and static casting. Generally made from cast high-conductivity copper, oxygen-free or deoxidized if welded.

Tuyéres for steelmaking blast furnaces. Oxygen-free high-conductivity copper successfully applied.

The list, above, includes components that can weigh from less than an ounce to more than a ton. Mechanical properties required also vary widely, although any or all may be higher than those of pure copper. Aside from conductivity, properties that a designer looks for in a material can be combinations of:

• Strength, at operating temperatures.
• Hardness, sufficient to give good wear or deformation resistance.
• Springiness, especially for all contacts and relay components.
• Ductility, to enable products to be formed in manufacture.
• Fatigue strength, especially for springs.
• Creep strength, important in rotating electrical generators and motors.
• Stress relaxation resistance, necessary for severe-duty electrical and electronic contacts.
• Wear resistance, needed in switchgear and casting molds.
• Corrosion resistance, for good electrical contact and protection of insulating coatings.
• Good surface condition for plating, soldering or dip tinning.
• Dimensional tolerances (in semi-fabricated products such as rod and bar, sheet, strip and plate, etc.) that meet the needs of advanced technology.

The large number of possible variables, together with other considerations specific to individual applications. explain the sizeable number of alloys now available. Many high copper alloys are, in fact, specifically tailored to suit individual product demands.

Alloy Development

The first high-strength copper alloy gave its name to an era, the Bronze Age, which followed the Copper (Chalcolithic) Age more than 4,000 years ago. Early bronzes consisted of copper and tin, and the "phosphorus bronzes" that we now use for electrical purposes (which contain around 5% tin and very little phosphorus) have been called the direct descendents of these primitive alloys. Phosphorus bronzes cannot be regarded as having high conductivity, since their conductivity is less than 10% that of pure copper. Nor are they classified as high copper alloys, since they contain too little copper. Instead, they are assigned a separate designation under the Unified Numbering System (UNS) for metals and alloys managed by the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE).

Another group of very dilute alloys that falls to the leaner-alloyed side of the conventional UNS definition for high-copper alloys (and the one that can be thought of as the natural progenitor of the high-copper alloys), are the silver-bearing coppers. In some cases, these are coppers to which silver is intentionally added; originally, they are derived from ores from which silver - a common impurity - was not removed during refining. In either case, the silver-bearing coppers retain much of the high conductivity of the pure metal, but more important, they exhibit significantly better high-temperature strength and creep resistance. This attribute makes silver-bearing coppers useful materials for the manufacture of, for example, electric motor commutators, which must retain their shape under high centrifugal forces at the operating temperatures brought about by the current and sparks from the brushes. Annealing ( i.e., softening) resistance is also useful in radiator fin stock, which must retain its properties during brazing or soldering.

Because silver is a precious metal, silver-bearing coppers are expensive, and one fundamental objective in the development of high-copper alloys has, therefore, been to devise alloys with properties that are equivalent to or higher than those of silver-bearing coppers but at equivalent or lower cost. Today, the compositions we think of as high-copper alloys contain additions of cadmium, chromium, magnesium, titanium, zinc and other metals, alone or in combination, in relatively small quantities.

Cadmium gives excellent strength and conductivity, and cadmium coppers have been widely used, for example, in the manufacture of wear-resistant overhead trolley wire and coaxial cable shields. Unfortunately, fumes from molten cadmium are hazardous, and use of the metal is now forbidden in some countries. Used properly, the alloys themselves pose no risk to health.

Copper-chromium alloys were first described in the late 1930s. They have good strength, and when hardened, have a conductivity 80% that of pure copper. This makes them ideal for heavy-duty electrical busbars and switchgear. Chromium copper is also used to form the rotor bars for large electric motors, where a guaranteed long, maintenance-free life is required. Similarly, many electrodes (RWMA Class II) used for spot- and seam- resistance welding, which must resist compression (mushrooming) at operating temperatures and pressures, are commonly made from copper-chromium alloys.

There are currently a series of related alloys containing additions of other elements to the basic chromium copper composition, which includes 0.5%-1% Cr. These elements include silicon, added for better machinability; zirconium and/or titanium, for extra resistance to softening at high temperatures, and magnesium to avoid a ductility trough in creep strength at moderately elevated temperatures. The chromium coppers and their derivatives are heat-treatable, as described below.

Another early heat treatable high copper alloy (developed in the late 1930s) is copper-nickel-phosphorus. Neither its strength nor conductivity are as favorable as those of copper-chromium, but the alloy's high ductility is extremely useful where considerable deformation must be performed during manufacture, as is the case with certain switchgear components. A variant of this alloy is copper-nickel-silicon, which is sufficiently stronger to permit the alloys to be used for the piston crowns of high-speed "Deltic" diesel engines. More recently, copper-nickel-tin alloys have been developed that harden by spinodal decomposition of the precipitate. Some of these alloys rival the beryllium coppers in terms of strength.

The copper-beryllium alloys, or beryllium coppers, have exceptional strength and hardness, in some cases approaching the levels attained in heat-treated steels. These alloys set the benchmark for electrical spring materials for many years, and they are still extensively used. The potentially hazardous nature of the fume that evolves from the molten metal was not fully appreciated at the time the alloys were developed, and resulting worker illnesses (berylliosis) gave the alloys themselves an unwarranted reputation as being unsafe. A better understanding of the phenomenon and the imposition of mandatory safety precautions have significantly reduced any risks involved. As with cadmium coppers, the solidified alloys are safe when used properly. (One would not, to offer an obvious example, use them for cooking utensils.) Additions of elements such as cobalt and nickel provide various improvements in properties. The alloys can be made free-machining by adding lead.

Most copper-beryllium alloys were originally patented and given trade names. Competitors had to find alternative materials, and this need has resulted in ongoing research, both to improve the alloys and to develop new ones to meet successive challenges set by the electrical, electronic and automotive industries. Iron was an obvious addition to consider since it is inexpensive, but the technology to get the addition metallurgically correct took a long time to develop. For example, iron cannot be used with conventional tough pitch copper containing oxygen, but iron can be added if the copper is first deoxidized with phosphorus, in which case, the addition works well. There are now twelve standard ASTM alloys based on this system.

Forms Available

Most high copper alloys are made by conventional production techniques, but because the alloys have special compositions and closely controlled properties, they are best obtained from recognized producers that, as a rule, work closely with customers to meet designers' needs. The alloys are also readily available in all of the common product forms:

Rolled flat strip, commonly used in the manufacture of:

• Terminals and connectors for electronic and automotive applications
• Springs for relay contacts and switchgear and
• Integrated Circuit Leadframes

Extruded and drawn bars, used for:

• Busbars
• Rotor bars
• Armatures
• Commutators and
• Spot welding electrodes

Mechanical wire, for:

• Pin-type electrical connectors and
• Fasteners

Forgings, often used for:

• Seam welding electrodes
• Heavy electrical switchgear
• Molds for continuous and static casting, and
• Tuyéres for steelmaking

Castings, which find various uses in:

• Heavy electrical equipment
• Molds for continuous and static casting
• Dies for plastic injection molding and
• Tuyéres for steelmaking

Hardening of Copper

There are four common ways to harden (strengthen) copper. A fifth, spinodal composition, is currently used commercially only in certain copper-nickel-tin alloys. Combinations of strengthening mechanisms are often used to provide higher mechanical properties in high-copper alloys.

• Strain Hardening

The application of cold work, usually by rolling or drawing, hardens copper and copper alloys. Strength, hardness and springiness increase, while ductility decreases. Conductivity is reduced to a small extent, normally not to the extent that it hinders use of the alloys in electrical products. The effect of cold work can be removed by annealing, in which case full conductivity returns. Strain hardening is the only strengthening mechanism that can be used with pure copper.

• Solid-Solution Hardening

Alloying elements that remain dissolved in solidified copper strengthen the lattice structure. If the addition is within the limit of the element's solid solubility, no secondary phases form, and the appearance under the microscope is similar to that of pure copper. All dissolved additions to copper reduce electrical conductivity, making the balance between strengthening gained and conductivity lost necessarily a compromise. The extent of this effect on conductivity varies widely from element to element. Cadmium additions, for example, affect conductivity least, while others, such as phosphorus, tin and zinc, are more detrimental. In any case, cold working can be used to increase strength beyond the limits of solid solution hardening, and the two strengthening mechanisms are frequently used in combination.

Some elements, especially those that are very sparingly soluble in solid copper, are deliberately added in quantities greater than their solution limit. Examples of this form of addition include lead, sulfur, tellurium and bismuth. The intent here is not alloying, but rather to form a dispersion for reasons other than increased strength. The elements separate from the copper matrix upon solidification, leaving a (usually finely divided) second phase that is visible under the microscope. The second phase in these so-called free-cutting coppers acts as a chip breaker during machining operations, in some cases also providing internal lubrication that cools the face of the cutting tool. While such additions improve machinability, they generally have little effect on mechanical properties other than a small and usually tolerable decrease in ductility. If the addition fully segregates from the matrix, i.e., if essentially none remains in solid solution, its effect on conductivity will be negligible.

• Precipitation Hardening

Some alloying elements exhibit higher solubility in solid copper when hot than when cold. This means they can be dissolved by solution treatment (solution annealing) at high temperatures, around 1740-1800 F (950-1000 C), and then removed from solution by a precipitation (or "aging") treatment at a lower temperature, commonly around 1200 F (650 C). This practice produces a fine precipitate throughout the metal that strengthens the matrix without spoiling the conductivity. In fact, conductivity improves as precipitates drop out of solution. Beryllium, chromium and zirconium are common examples of this type of addition. Combinations of nickel with silicon or phosphorus are also useful. When hardened, precipitation strengthened alloys may have a limited capacity for cold work, thus; severe mechanical deformation is normally performed while the metal is in the solution-annealed condition, prior to final hardening. The precipitation hardening mechanism is well suited to forgings and castings, where no cold deformation is normally performed; however, certain "mill hardened" tempers applied to strip alloys are achieved by combining precipitation hardening with controlled amounts of cold rolling.

• Dispersion Strengthening

Particles of insoluble or even inert materials can also be finely distributed within a copper matrix by metallurgical, mechanical or chemical means, i.e., without having to resort to heat treatment. Being insoluble, the particles have little effect on electrical conductivity. Particles of refractory oxides (alumina being the most common example, as in UNS alloys C15715-C15815) are useful in this regard, especially for the retention of strength and hardness at elevated temperatures. Oxides cannot simply be stirred into the melt when casting (they segregate during solidification), and other methods have been devised to yield the desired dispersion. One is powder metallurgy, in which mixtures of very fine powdered copper and oxides are compacted and sintered to form solid metal. The method more commonly used with copper today is internal oxidation, which forms oxides of a reactive alloy constituent in situ. Finally, dispersions of sparingly soluble elements such as iron, niobium (columbium) or tungsten can be generated during solidification or by powder metallurgical means. These materials are not alloys, but are more accurately classified as metal-matrix composites.

Casting

Copper alloys are well known for the relative ease with which they can be cast to shape using any of several casting methods. But while casting may be relatively straightforward for, say, bronzes or brasses, the high-copper alloys that are the subject of this chapter must be processed using methods suited to their constituents, if specified properties are to be maintained. Only experienced, specialist manufacturers are able to produce them consistently with specified mechanical and electrical properties.

There are currently only three types of cast high copper alloys listed under the UNS numbering system: beryllium coppers (UNS C81400 and C82000-C82800), a chromium copper (C81500) and a nickel-silicon-chromium alloy, C81540. The alloys are used where very high strength and toughness in combination with one or more of copper's physical or mechanical properties are required. Applications include heavy-duty pole line and other electrical hardware, specialty marine and oil-field equipment, and non-sparking tools.

Some of the elements added to high copper alloys readily react with oxygen ( e.g., beryllium, chromium, zirconium, aluminum and silicon) and are, therefore, easily lost as oxides, if melting and casting techniques are not well controlled. If foundry practice is poor, these elements can either disappear in the dross that floats to the surface of the ladle or remain suspended in the liquid metal, transforming it to a semi-viscous sludge that can only be cast with difficulty. Chemical analysis may confirm that a casting is within compositional specification, but the alloying elements may be present as compounds that will not dissolve when subjected to solution treatment. Others elements, such as chromium, easily form carbides and will, therefore, react with the graphite in plumbago crucibles or the charcoal commonly used to reduce the oxidation of liquid copper.

The use of controlled atmospheres for casting is recommended for alloys such as copper-zirconium. Casting of copper-beryllium alloys is not recommended, unless the foundry is fully approved to meet statutory health requirements for the control of beryllium fumes. Table 1 provides typical casting temperatures.

Machining

The machining characteristics of copper-base alloys are classified according to the type of chip generated. Thus, Type I chips are small, fracture readily and are easily cleared from the cutting tool area. Machining requires relatively little energy, and machined surfaces are of high quality, providing other variables are correctly controlled. Several of the high-copper alloys contain lead or other elements that produce this "free-cutting" behavior. Among them are beryllium coppers C17300, C17455 and C17465, which contain small quantities of lead.

Recommendations for machining high copper alloys are given by leading manufacturers. A comprehensive publication on CD-ROM (Copper Rod Alloys for Machined Products-Handbook, Pub. #: A7017, PDF format) covering all rod and bar products is available from CDA. An equally thorough treatment of the subject is available online from CDA (UK). The publications describe tool geometry, feeds, speeds and lubricants for various machining operations.

When in the solution annealed condition, many high copper alloys machine very much like pure copper, producing long, stringy Type III turnings. Harder alloys may exhibit Type II behavior, and chips will be short, tough and coiled. Free-machining grades (Type I behavior) of most alloys are available and should be specified when extensive machining is anticipated.

Chromium, silicon, zirconium, aluminum and beryllium produce refractory oxides that may create a very hard layer on the surface of as-cast or as-heat-treated surfaces. When turning, it is beneficial to make the first cut fairly deep and slow to get through the coating at the first cut with little tool wear. Table 2 provides machinability ratings for coppers and high copper alloys.

Joining

• Soldering

All copper alloys can be soldered with relative ease, most often using tin-based solders. The low temperatures involved especially suit the process to heat treat alloys, such as chromium or beryllium coppers, which might suffer degradation of mechanical properties when exposed to welding or brazing temperatures. Good fluxing is important in any event but especially when working with alloys containing chromium, aluminum, zirconium and beryllium, since the refractory oxides these elements tend to form on the metal surface prevent complete wetting. When excessive reaction between the solder and underlying copper is to be avoided, it is helpful to pre-coat copper with nickel, silver or gold.

• Welding and Brazing

Brazing is practical with all high-copper alloys and shielded arc welding techniques can be used with most materials. The welding of alloys containing beryllium must be done with adequate ventilation and the use of protective masks and clothing, to avoid exposure to toxic fume emissions. With all other materials, appropriate regulations should be observed.

The high-copper alloys are readily weldable, but there are reservations. The main factor to bear in mind is that high temperatures in the heat affected zone can wipe out the effect of heat treatment by over-aging the alloy. This phenomenon affects all high copper alloys except cadmium copper, which is not heat treated. However, cadmium copper is strengthened by a combination of solid-solution strengthening and cold working, and heating anneals out the effects of cold work. Thus, In order to attain maximum properties, heat treatable alloys must be welded in the annealed state before heat treatment. Heat treatment is applied after joining.

For repairs, or if maximum as-heat-treated properties are not required, alloys can be welded with aluminum bronze, AWS 5.7 ECuAl-A2 or silicon bronze, AWS 5.7 ECuSi. The alloys can also be welded using deoxidized copper, AWS 5.7 ECu, but the weld joint will have lower strength than when aluminum bronze or silicon bronze are used.
Because high-copper alloys have somewhat lower thermal conductivities than pure copper, they require lower welding currents (less heat) and less preheat than pure coppers.

• Resistance Welding

It is not easy to resistance weld metals such as pure copper and high copper alloys, because or their high electrical and thermal conductivities. In general, resistance weldability improves with decreasing electrical conductivity, and alloys with conductivities less than about 45% IACS are more readily suited to the process. The high copper alloys can generally be spot and flash-butt welded, given clean surfaces and a good heat input. Seam welding is not recommended.

• Other Welding Processes

Copper and high copper alloys can be welded successfully using plasma-arc and electron-beam techniques, the latter having been shown to be useful in sections as thick as one inch, and possibly more. Both processes are normally autogenous, i.e., applied without filler metals. The metals can also be joined by friction welding and so-called "cold" welding, in which case, clean surfaces are simply brought together with the application of force sufficient to produce severe plastic deformation.

Heat Treatment

When extensively cold-worked, copper and copper alloys may become sufficiently hardened so that full annealing is required before further deformation is possible. If additional cold work is not needed, it may only be necessary to relieve residual stresses, retaining hardness and springiness. In either case, annealing times and temperatures vary with the type of alloy, section thickness, extent of cold work and amount of softening needed. General guidelines are given in Table 3.

High copper alloys resist oxidation, and heat treatments can ordinarily be carried out in air furnaces without a protective atmosphere. Water quenching is usually sufficient. It is sometimes possible to combine hot working with solution treatment, for example by extrusion into a water trough that comes up to the die.

Cold working is frequently carried out in the solution treated condition when the material is softer. Precipitation treatment then gives extremely good properties. For production of spring materials, further cold rolling gives the highest of spring tempers. Table 4 provides some heat treatment temperatures.

Recommendations for particular materials can be obtained from the manufacturers.

Note that for some materials full strength and hardness is obtained by a combination of heat treatment and cold work. If these are subsequently overheated, such properties cannot be regained by heat treatment only.

Electrical Conductivity/Resistivity

In this article, all comparisons of electrical properties have referred to the International Annealed Copper Standard, for which pure copper is said to have 100% IACS conductivity. This is equal to a value of 58 MS/m (Megasiemens per meter). The easiest value to measure accurately is that of the mass resistivity of a meter length of wire. The standard value for this is 0.15328 ohm g/m2. Conductivity is the reciprocal of resistivity.

The 100% IACS value was established in 1913. Since then the purity of commercial copper has been improved and the amount of oxygen present reduced. This means that commercial copper is likely to have a conductivity of about 101.5% in practice. Measurements on solid alloy samples may be made by other methods and be accurate to only ±3%. The effects of composition and heat treatment on thermal conductivity values are proportional to those of electrical conductivity.

The conductivity of high copper alloys ranges from more than 90% IACS to less than 20% IACS, the particular value being determined by the type and degree of alloying, heat treatment, and, to a lesser extent, cold work. As mentioned earlier, mechanical and electrical properties tend to be inversely proportional, and it is up to the designer to select materials that offer the most advantageous combination of these properties to satisfy the product's requirements. Special properties, such as stress-relaxation and corrosion resistance may also have to be considered. Still, it has often (and correctly) been stated that copper alloys offer the best combination of physical, chemical and mechanical properties among all engineering metals.

Compositions and Properties of High Copper Alloys

For current standard compositions, see the ASTM Standard values quoted in the CDA Alloy Designations section for alloys C16200 through C19900.

European Alloys and Usage

Table 5 provides a summary of European high copper alloys and usage.

Links

1. Suppliers of high copper alloys can be found using the Suppliers Database of CDA within USA.
2. Trade literature and supplier Web sites will give up-to-the minute information on available materials.
3. Please contact CDA with your questions about copper, its mining, consumption and general trends in copper usage to the CDA.
4. Microstructures of Copper and Copper Alloys.
5. The database of the CDA (UK) is available with the complete contents of 52 publications on-line. Direct links to relevant publications, with URL bookmarks:
   • High Conductivity Coppers for Electrical Engineering Pub 122
   • Equilibrium Diagrams P94
   • Copper in Electrical Contacts TN23
   • High Conductivity Copper - Technical Data TN27
   • Copper for Busbars Pub 22
   • Beryllium Copper P54
   • Phosphor Bronze PB102 - Technical Data TN40
   • Machining Brass, Copper and Copper Alloys TN44

Also in this Issue:

• High Copper Alloys - High Strength Coppers for Demanding Electrical Applications

Archives: