PROCESS AND TECHNOLOGY
OXYGEN FREE HIGH CONDUCTIVITY COPPER (Cu-OFHC) have a minimum copper content of 99.99%. For this kind of product, the norm establishes a maximum level of oxygen of 10 ppm. In our case, we normaly obtain a maximum oxygen level of 5 ppm.

Cu-ETP (Electrolytic Tough Pitch) has a minimum copper content of 99.90%. Oxygen is intentionally alloyed with the copper in production of Cu-ETP and is controlled to around 200-400 ppm. The oxygen acts as a scavenger for dissolved hydrogen and sulphur and will react also with most other impurities, to form insoluble oxides at the grain boundaries. This prevents these from dissolving in the copper matrix and adversely affecting conductivity and annealability of the rod and drawn wire. Conversely, however, the presence of occluded oxides in the copper wire rod, while solving one problem, can lead directly to another, as oxides tend to form hard particles and lead to wire breaks in fine wire drawing.

Production of oxygen-free copper is thus more demanding of the quality of the in-going cathode feedstock used in the process. Conversely, there will be little or no formation of oxides along the grain boundaries which, as mentioned earlier, is often a cause of wire breaks later on.

ADVANTAGES OF OFHC
By comparison to Cu-ETP, the market for Cu-OFHC is relatively small, but it is growing. Cu-OFHC will serve any of the purposes fulfilled by Cu-ETP, but has some additional properties of importance in particular applications.

1. The absence of occluded oxides at the grain boundaries in Cu-OFHC results in a more ductile material. This can be important in such applications as in aerospace, automotive wiring harnesses, robot arms and other similar applications, where the copper conductor core of a wire or cable is subjected to repeated flexing.

2. The absence of occluded oxides at the grain boundaries in Cu-OFHC produces less "noise" and is thus of increasing interest in such applications as high quality sound recording systems.

3. The elimination of the risk of hydrogen embrittlement in copper through the use of Cu-OFHC is an important characteristic in welding applications.

4. Another important market trend which is favouring Cu-OFHC is the explosive growth of the market for electronic devices and the progressive miniaturisation of electronic components. This has resulted in a demand for ever finer wire gauges.

5. Flexible Power Cables.

Fine wires are now being regularly drawn on multi-wire machines with up to 32 strands. More and more attention is now being given to the causes of wire breaks in drawing, as demand for finer and finer wires increases. The use of multiwire drawing machines, processing up to 32 strands simultaneously greatly increases the efficiency of the fine wire drawing process, but only so long as wire breaks can be routinely avoided. Conversely, the whole machine must be stopped and rethreaded when a wire break occurs.

The purity of the metal and the absence of oxide particles in the structure become matters of serious concern. Here too, Cu-OFHC has distinct advantages over Cu-ETP. In production, the cast material exits the casting die and cooler assembly at around 80°C, well below the surface oxidation temperature. Cu-OFHC thus has a very thin layer of surface oxides, significantly less than Cu-ETP.
PROCESS
The process uses a single furnace in which to melt, hold and cast the copper. All the elements of the furnace and the melted copper are immersed in a n nitrogen atmosphere free of oxygen. All parameters of operation are permanently monitored and controlled by a microprocessor to ensure stability and optimize mechanical and electrical properties of the rod.

This is probably the best metallurgic system to melt, hold and cast the copper in the world..

This contrasts with other systems, which almost invariably use ceramic refractory lined, induction-heated furnaces, often positioned in tandem, one for melting and the second for casting, with the copper being poured from one to another.

Characteristics which favour the use of graphite include the purity of this material as elemental graphitised carbon, its machinability, its thermal conductivity, its naturally reducing function, whereby oxygen present in the molten metal will react with the carbon and be eliminated from the melt and its excellent high temperature stability and strength. Graphite is not wetted by copper in the molten state. Synthetic graphite under non-oxidising conditions is the highest temperature-stable elemental solid known.

Crucibles are machined from solid blocks of vibration moulded graphite. The significant mass represented by the heated crucible itself, forms part of the potential energy of the furnace design and contributes significantly to furnace temperature stability.

The flow of molten copper is thus necessarily first-in-first-out and the design incorporates a graphite filter bed at the base of the casting chamber to ensure that the de-oxygenating process is complete, before the metal enters the casting dies at the top of the casting chamber.

A layer of high purity graphite flake is used to protect the surface of the molten metal and an inert gas atmosphere is maintained inside the furnace to protect the graphite hot-working parts from erosion.

Furnace heating is by electric resistance by means of a chain of graphite heating elements positioned adjacent to the wall of the graphite crucible, with the heat being transferred to the copper by radiation and convection. This results in a still metal bath and an ideal condition from which to cast.

The effect of the cathode preheating is to dry off any surface moisture, condensation or surface-trapped electrolyte and thus to ensure that hydrogen from those possible sources is eliminated. It avoids metal splashing at the feed point and contributes to the overall melting burden of the furnace itself.
CATHODES
The quality and cleanliness of the feedstock used have critical importance in production of good quality copper rod. The specification set by Rautomead (furnace manufacturer) calls for the use of LME grade A cathode (Cu-CATH-1). Maximum impurities permitted are 0.0065%.

Composition is influenced by the mining source of the copper ore and by the technology of the refinery where the cathode is produced. The most critical impurities in relation to wire drawing, graded as to their influence on wire breaks are as follows:

Severe effect:
Bi Te Se (cause grain boundary cracks).

Deleterious:
Pb As Sb S (cause grain boundary cracks)

Low effect:
Cr Fe Sn P Si Ag (affect annealability)

Hydrogen also accentuates the detrimental effect of other impurities which may be present at grain boundaries.
Cathode selection and handling, furnace atmosphere, as well as melt surface protection, all are carefully regulated to minimize hydrogen pick-up.