The Electrical Conductivity of Wrought Copper and Copper Alloys

In determining the uses of copper and copper alloys, the properties of major significance are electrical conductivity, thermal conductivity, corrosion resistance, machinability, fatigue characteristics, malleability, formability and strength. In addition, copper has a pleasing color, is nonmagnetic, and is easily finished by plating or lacquering. Copper can also be welded, brazed and soldered satisfactorily.

When it is desirable to improve certain of these basic properties, especially strength, and when such an improvement can be effected with the sacrifice of no other properties except those of limited significance in the intended application, alloying often solves the problem, and such widely used commercial materials as the brasses, leaded brasses, bronzes, copper-nickel alloys, nickel slivers, and special bronzes have been developed in consequence. Nominal compositions of the principal alloys are listed in Table 1.

The greatest single field of use for copper results from the high electrical conductivity of the metal. The reasons for the use of copper for electrical conductors and in the manufacture of all types of electrical equipment are so commonly understood that a detailed discussion is unnecessary. However, even in the electrical industry, high conductivity alone does not give copper great economic value; it is rather the combination of this property with high resistance to corrosion and ease of formability. Even with very high electrical conductivity, a material that is unable to be drawn or fabricated with ease or is subject to rapid corrosion when exposed to normal atmospheric conditions would be impractical in the electrical industry.

Electrolytic tough pitch copper is the preferred material for current-carrying members. Conductivity is 101 % IACS (Table 2) in the soft temper with 220 MPa tensile strength, and 97% in spring rolled temper at 345 to 380 MPa tensile strength.

Temperatures above 200°C will soften tough pitch copper to a tensile strength of 300 to 240 MPa. The three silver-bearing coppers resist softening up to about 340°C, and are less susceptible to creep rupture in highly stressed parts such as turbo generator windings and high-speed commutators. Softening characteristics are important for applications such as commutators that are baked or "seasoned" at elevated temperature to set mica between the copper bars. Copper must not be softened by this treatment.

If electrolytic tough pitch copper is exposed to temperatures above 370°C and reducing gases, especially illuminating gas and hydrogen, embrittlement will almost certainly take place. Oxygen-free copper or phosphor-deoxidized copper is then specified, at higher cost.

The tensile properties of all the coppers are similar at room temperature, although slight differences may influence selection of a specific conductor. Deoxidized copper with no residual deoxidant (oxygen-free copper) has excellent ductility and is used for most severe deep drawing and cold working.

A combination of 480 MPa tensile strength with conductivity of 80% and higher, suited to spot welding tips and seam welding wheels, can be obtained with heat treated chromium copper. Where tensile strength up to about 1350 MPa and fatigue strength of 240 MPa are required and where the penalty of 17% conductivity and high cost are tolerable, heat treated beryllium copper can be used, if the combined effect of ambient temperature and electrical resistance of the part holds temperatures below 370°C.

Conducting springs, contacts and similar highly stressed members that also may have to be formed may use either chromium copper or beryllium copper. Parts are shaped soft and then strengthened by heat treatment. Parts that must be highly machined and highly conductive are made from the free-machining coppers. Widely used is tellurium copper, which has 90% minimum conductivity and a machinability rating of 80 to 90 (free-cutting brass = 100). Leaded copper (1% Pb) or sulfurized copper is also used because of the 80% machinability rating, with most other properties similar to copper. If tensile strengths of 440 to 525 MPa are required at 80% machinability, heat-treated and hard drawn forms of tellurium-nickel copper may be chosen, provided electrical conductivity of 50% is permissible.

Telecommunication parts that carry low currents but require good fatigue properties because of the hundreds of thousands of contacts that are made and broken, may be fabricated from cartridge brass to give a suitable compromise between strength and e lectrical conductivity. If corrosion or severe fatigue are factors to be considered, the more expensive but stronger nickel silvers, phosphor bronzes or beryllium coppers will serve.

All values are for the annealed condition. Cold worked alloys may be as much as 5 points lower. Compositions are given in the Table 1.

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