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 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 a lot of research effort over decades 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.
Pure copper is the optimal material for electric current conductors. It combines high electric conductivity and a reasonable price. But many wire and cable applications require a strength which exceeds the strength attainable with pure copper wire, e.g. connector pins. In these cases the use of copper alloys becomes necessary. Strength increase in alloys is possible by two different metallurgical effects, solid solution hardening and precipitation hardening. Brass and bronze are widely used solid solution hardened alloys. Certain high copper alloys with low contents of alloying elements, e.g. Ni, Si, and Cr, are precipitation hardened and offer an interesting combination of high strength, good electrical conductivity and relaxation resistance.
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.
Solid Solution Hardened Copper Alloys
Copper lattice is able to dissolve a certain amount of atoms of other metals, e.g. Sn, Zn and Mg. These atoms take the lattice sites of copper atoms which is called solid solution. The copper lattice in the vicinity of the atoms is distorted by expansion if the atoms which are bigger than copper atom, e.g. Zn and Mg. If the atoms are smaller than copper, e.g. Sn, Ni and Al, the lattice distortion is a contraction.
In both cases the resistance of the material against deformation is increased compared to pure copper, in other words the material becomes harder. This kind of alloys is called “solid solution hardened alloys. Some elements can be dissolved in copper in high percentages. According to the equilibrium phase diagrams, the maximum solubility of Zn is 39.0 % and of Sn is 15.8 %. Standard brasses with 30 or 36% Zn and bronzes with 5, 6 and 8% Sn are often applied in the electronic industry if the strength of pure copper is not sufficient.
To achieve high strength wire, in addition to the solid solution hardening effect a high degree of cold drawing deformation is necessary. Thin wires of phosphor bronze easily achieve strength values of 1000 MPa due to cold drawing. But lattice distortion due to alloyed elements decreases electrical conductivity. The disadvantage of solid solution hardened alloys is the low electrical conductivity, e.g. of about 25% IACS (brass CuZn36) and about 14% IACS (phosphor bronze CuSn6). This decrease of electric conductivity is due to the lattice distortion caused by alloying atoms.
In order to minimize the drop in electrical conductivity and to increase strength, solid solution hardened high copper alloys with low content of alloyed elements are applied. Examples are CuSn0.15, CuSn0.3 and CuMg0.1. A weakness of all solid solution hardened alloys is their insufficient relaxation resistance at slightly increased service temperatures, beginning at about 60°C. To overcome this, the application of precipitation hardened alloys becomes necessary.
Precipitation Hardened High Copper Alloys
The ability to dissolve other types of atoms in general is increased at elevated temperatures. If temperature decreases the limit of solubility is undershot. This fact may be utilized to generate precipitations by an annealing procedure at temperatures below the solubility limit. The atoms form precipitations, a second phase, an intermetallic. The size of these particles usually is lower than 100 nm. As the atoms leave the lattice, the lattice distortion is undone and the electric conductivity of the material increases.
Cold deformation after the solution annealing but previous to the age annealing supports the formation of small-sized and homogeneously distributed precipitations. On the other hand the precipitates increase the base strength of the material and influence the strengthening behavior. They harden the material. For this reason such kind of alloys are called “precipitation hardened alloys”.
A big advantage of precipitation hardened alloys is their relaxation resistance. If the material is exposed to elevated service temperatures the precipitates do not dissolve and the increased base hardness is maintained. Wires of two alloys, CuFe2P and CuNi3SiMg, are already being used for connector pins. As the content of alloyed elements in these alloys is low, these alloys are classified as high copper alloys. They are called “precipitation hardened high copper alloys”.
Relaxation Resistance
Rough service conditions require high performance materials e.g. if automotive industry designs connectors in the vicinity of the engine:
- Impacts require high strength and ductility
- Vibrations require high fatigue strength
- Elevated temperature requires relaxation resistance.
The strength of solid solution hardened alloys is due to the lattice deformation during the cold drawing process. During exposition to elevated temperatures the deformation thermally recovers (relaxes) and the strength is decreasing towards the value of the material’s base strength. As this base strength is low, solid solution hardened alloys suffer from severe relaxation. In principle, precipitation hardened alloys are subjected to the same degradation. However, their base strength is much higher and hence they exhibit good, and some alloys even very good relaxation resistance.
Applications of High Copper Alloys
Few typical uses are given below, each encompassing 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.
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.
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.
Integrated circuit lead frames. These are made from specialty alloys designed for both their connector-related properties and for compatibility with packaging requirements.
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. They are 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.
The use of strip metals for connector springs, terminals, contacts, and switches is well accepted in the interconnection industry. This is particularly true for copper alloys, because of their desirable combination of conductivity, strength, and formability.
The connector designer will approach each new assignment with information on connector’s desired mechanical characteristics, operating environment and life expectancy. His task is to find a specific design which can be reliably and economically manufactured. To accomplish this goal he must find the copper alloy and temper which will allow the connector to be manufactured, assembled and used successfully throughout its intended life, all at minimum cost.
Summary
A new generation of high performance copper alloy wire is attracting attention of the electronic industry. Excellent material properties have been obtained due to the effect of precipitation hardening. The alloys are known in the electronic industry as precipitation hardened copper alloy strip for connectors and electrical contacts. Now these alloys are available as wire which is often further processed to square shaped connector pins.
Wire made of the precipitation hardened high copper alloy such as UNS C70250 attracts more and more attention of connector designers. It is found to be suitable for application as square-shaped connector pin as it offers a lot of interesting properties:
- High strength (up to 900 MPa)
- Reasonable electrical conductivity (45 to 55 % IACS)
- High relaxation resistance
- Excellent bending properties
- Sufficient ductility for further processing
- Easy to galvanize.
