All high conductivity coppers contain some oxygen as a result of the copper refining process but certain applications require copper with the highest purity and lowest oxygen levels, this is called Oxygen-free High Conductivity Copper – OFHC.
It is not possible to remove every trace of impurity; no copper is 100% pure, but OFHC has only a trace of oxygen. In practice the oxygen content is typically 0.001 to 0.003% with a total maximum impurity level of 0.0065%.
The addition of silver to OFHC copper raises its recrystallisation (softening) temperature considerably with very little effect on the electrical conductivity. Silver also improves the mechanical properties especially the creep resistance. The resistance to recrystallisation and creep increases with increasing silver content. The material is therefore preferred for electrical machinery when resistance to softening is required as in commutators or when the material is expected to sustain stresses for long times at elevated working temperatures, as in large alternators and motors.
Silver is present naturally in many copper ores so copper-silver alloys will have been in use for thousands of years exploiting their attractive colour, high ductility and ability to be hardened by cold working. The significance of silver as an alloying element to enhance the mechanical and electrical properties of copper was not exploited until the late 1930s.
The ranges of silver contents available are:
- 0.03 – 0.05% (CW017A)
- 0.06 – 0.08% (CW018A)
- 0.08 – 0.12% (CW019A)
Addition of silver greater than 0.12% does not appreciably enhance resistance to softening.
Copper-silvers cannot be hardened by heat treatment. They do not become harder with time and they do not have a ‘shelf life’. Copper-silvers may be stress relieved between 250 and 350oC though this does not soften the copper. If this is required then a full anneal at 350 to 650oC is needed. However, it is rarely called for in the annealed condition since its outstanding property is the retention of work hardness at temperatures up to 250oC and short times at 350oC.
In the annealed (soft) to the hard condition, these alloys have a combination of the following properties:
- Tensile strength: 200-230 N/mm2
- Proof strength: 120-200 N//mm2
- % Elongation: 35-12
- Hardness (HV): 35-115
- Electrical conductivity: 100-101.5% IACS. (This is the highest of the copper silvers.)
- Thermal conductivity: 385 W/moC
| Process || Rating |
| Cold formability |
| Excellent |
| Hot formability |
| Good |
| Soldering || Excellent |
| Brazing || Good |
| Oxyacetylene welding |
| Fair |
| Gas shield arc welding |
| Good |
| Resistance welding |
| Fair |
This copper does not suffer from steam (hydrogen) embrittlement when heated in a hydrogen (reducing) atmosphere.
While copper-silvers, like other pure coppers, cannot be regarded as a free-machining material, it is not difficult to machine, especially in the work-hardened condition in which it is usually supplied. The machinability rating is 20% (free-machining brass is 100%).
Resistance to Corrosion
Like all coppers, Silver-bearing oxygen free coppers have good resistance to atmospheric corrosion.
Resistance to Softening
Softening is time and temperature dependent and it is difficult to estimate precisely the time at which it starts and finishes. It is usual, therefore, to consider the time to ‘half-softening’, i.e. the time taken for the hardness to fall by 50% of the original increase in hardness caused by cold reduction. In the case of silver-bearing oxygen-free coppers, this softening occurs at temperatures above 150oC. It has been established experimentally that such copper would operate successfully at a temperature of 105oC for periods of 20-25 years, and that it could withstand short-circuit conditions as high as 250oC for a few seconds without any adverse effect.
Creep is slow deformation of a material with time at temperature and must be allowed for in the design of any component operating above room temperature. The resistance of silver-bearing oxygen-free coppers to creep at moderately elevated temperatures is considerably better than non-silver-bearing coppers, especially when the material is in the cold-worked condition. Tests have shown that at normal working stresses non-silver bearing coppers will begin to creep at temperatures as low as 130oC, whilst even at 230oC no creep was observed in silver-bearing coppers.
Silver-bearing oxygen-free coppers do not suffer from steam (hydrogen) embrittlement on brazing. This, together with the beneficial effects of silver at high temperatures, and a high purity, lead to applications as detailed below.
- Commutator strips
- Electrical motor parts
- Semi-conductor components and etching plates
- Electrical commutator segments
- Conductivity wire
- Terminal connectors
- Radio parts
- Printed circuit foil
- Industrial chemical process equipment
- Printing rolls
- Heat exchanger tube
The silver content in this alloy is higher, leading to use for more highly stressed components such as rotor windings.
- Electronic applications
- Electrical motor parts
- Semi-conductor components
- Trolley wires
- Commutator segments
- Matrix material for super conductors
- Rotor winding strips
- Cable strips
- Industrial strips
- Transformer strips
These alloys are available in bar, plate, sheet, strip, tube and wire.
Below are the specifications for Europe and the US. Note that for USA, some compositions are not identical. For equivalent standards from other countries visit the Copper Key website.
- UK: C103 (British Standard BS designation). UK Standards are superseded by European Standards.
- Europe: CW017A [CuAg0.04(OF)], CW018A [CuAg0.07(OF)], CW019A [CuAg0.10(OF)] (European Standard EN designation).
- USA: C10400, C10500 (American Society for Testing and Materials ASTM designation).
Further information on silver-bearing oxygen-free coppers, and other conductivity materials, is available at the Copper Alloys Knowledge Base:
Application Example 1: Trolley Wires
Silver-bearing oxygen-free copper is widely used for the overhead conductor trolley wires for tram systems in UK and Europe. The presence of silver (0.10%) gives high temperature resistance to the copper.
Application Example 2: Commutators
Silver-bearing oxygen-free copper is used for commutators because of its combination of high electrical conductivity and strength at temperature in a moving stressed part.