Copper

>> Download publication: High Conductivity Copper for Electrical Applications

Today, over two thirds of the copper produced is used in electrical and electronic applications and this leads to a convenient classification of the types of copper into:

• Electrical (high conductivity)
• Non-electrical (engineering).

High Conductivity Coppers (Electrical)

Wrought high conductivity copper has excellent ductility and, as a result, is available in forms such as wire, tube, extrusions, bar and sheet.

The main grade of copper used for electrical applications such as building wire, motor windings, cables and busbars is electrolytic tough pitch copper CW004A (was C101) which is at least 99.90% pure and has an electrical conductivity of at least 101% IACS minimum. Tough pitch copper contains a small % of oxygen (0.02 to 0.04%) so if the high conductivity copper is to be welded or brazed or used in a reducing atmosphere, then the more expensive oxygen free high conductivity copper CW008A (was C103) may be used.

Wrought high conductivity coppers (CW004A and CW008A) can only be strengthened and hardened by cold working such as occurs on cold drawing or bending (typically tensile strength 250 N/mm² with 12% elongation). They cannot be strengthened by heat treatment so, if a stronger grade of high conductivity copper is required, then small amounts (less than 1%) of alloying elements such as silver, cadmium, magnesium or tin are used.  These additions give solid solution hardening and contribute to work hardening when the alloys are cold drawn into wires or tubes or rolled into sheet. However, there is a small loss of conductivity. These alloys, with 90 to 100% IACS values are typically used for overhead conducting and catenary wires on railway and tram systems to transmit electric current to the electric motors of trains and trams.

High conductivity copper may also be produced as die and sand castings (CC040A), typically for electrical switchgear and electrode holders.

The electrical conductivity of castings may be slightly lower than in wrought copper, however a minimum value of 93% IACS is guaranteed but values up to 100% are reached.

Higher Strength Alloys
Small additions of silver, cadmium, magnesium or tin are used to give a small increase in the strength of copper conductors but, for significant increases in strength, the heat treatable copper chromium (0.5-1.2% Cr) and copper chrome zirconium (0.1% Zr) alloys have been developed in cast and wrought form. These alloys combine high strength (tensile strength 450 N/mm² with 10% elongation, up to 400^oC) with high electrical conductivity (75 to 78% IACS) and high thermal conductivity. The properties of these alloys are developed by a heat treatment process which involves heating to 950-1000^oC (solution treatment), followed by water quenching then reheating to 425-500^oC (precipitation hardening). In addition to this heat treatment wrought alloys are strengthened by cold working. Applications for these alloys include resistance welding electrodes, switchgear, heat sinks, current carrying arms and steel casting moulds where copper would be ideal from the electrical and thermal conductivity standpoint but is simply not strong enough.

Free Machining Copper
An addition of approximately 0.5% tellurium or sulphur raises the machinability rating from 20% to 90%, based on a scale where free machining brass is rated at 100%. The particles of copper telluride or copper sulphide act as chip breakers leading to excellent machinability without substantially affecting the electrical conductivity which is rated at 93% IACS. Free machining copper is used where a large amount of repetitive machining at high rates is required. One example is in the production of gas, laser and plasma cutting nozzles which involves the drilling of small holes in rods followed by cold forming to the finished shape. Other applications include screws, fasteners, contacts, connectors, clamps and bolts used in the electrical and semi-conductor industries.

Engineering Copper (Non-Electrical)

The usual grade of copper used for engineering applications is CW024A (was C106). Many of the applications of copper depend upon properties other than its high electrical conductivity.

The properties which make copper the standard material for engineering, including architecture and plumbing, are:

• Thermal conductivity – the thermal conductivity of copper, 394 W/mK, is about twice that of aluminium and thirty times that of stainless steel. This means that copper is used for components where rapid heat transfer is essential. Examples include saucepan bottoms, heat exchangers, car and vehicle radiators and heat sinks in computers, disk drives and TV sets.
• Corrosion resistance – copper is non-reactive and does not rust or become brittle in sunlight
• Ease of joining – by brazing or soldering. The latest technology called CuproBraze® is used to fabricate strong and reliable brazed copper/brass heat exchangers for cooling in vehicles which include cars, trucks, locomotives, tractors and JCBs.
• High ductility – tubes are easily bent even when hard
• Toughness – does not become brittle at sub zero temperatures
• Heat resistance – withstands fire well, melting point is 1083^oC
• Hygienic – copper’s ease of shaping, corrosion resistance and hygienic properties make it ideal for brewing vessels.
• Range of colours and malleability – widely used by designers and architects for exterior and interior applications.
• Recyclability – copper is infinitely recyclable without loss of properties. The price of scrap copper is high.