Copper is one of the few metals that finds most widespr […]
Copper is one of the few metals that finds most widespread use in the pure form, rather than as an alloy. There are approximately four dozen different wrought alloys that contain a minimum copper content of 99.3 weight percent (and therefore designated as "coppers"), albeit only a handful are used industrially as electrical conductors. The most widely used of these dilute alloys is known as electrolytic tough pitch (ETP) copper, which consists of extremely high purity metal that has been alloyed with oxygen in the range of 100 to 650 ppm. ETP copper is not recommended for use in hydrogen environments due to its susceptibility to hydrogen embrittlement when exposed to these temperatures. Under these environmental environments, either oxygen-free (OF) or oxygen-free electronic (OFE) grades of copper should be used. Silver bearing copper (OFS) finds limited use in power transformers because of its higher strength and softening resistance at elevated temperature.
Prior to the 1970s nearly all copper rod was made by a batch process, which included pouring and solidification of molten copper into special shaped ingots known as wirebars, reheating the bars in a slightly reducing protective atmosphere, and breaking up the cast dendritic structure by hot rolling in air to a rod form. This was followed by pickling in 10 percent sulfuric acid to remove oxides, and by butt welding of one end to another to form larger coil lengths. Today, a continuous casting and rolling process produces virtually all copper rod. Benefits of continuous casting include less microsegregation of impurities, reduction of copper oxide particles on the surface, fewer steel inclusions resulting from contact with mill rolls, almost total elimination of welds, and lower overall processing costs.
Oxygen is intentionally alloyed with copper to act as a scavenger for dissolved hydrogen and sulfur to form the gases H 2O and SO 2 in the melt. If the oxygen content is kept under control, microscopic bubbles form throughout, and under ideal conditions will offset the approximately 4% shrinkage in volume associated with the liquid-to-solid transformation. If the resulting pores are not too large, they are completely eliminated during hot rolling.
Most continuous casting and rolling units contain non-destructive equipment (eddy-current) that is used on-line to detect surface defects such as cracks and oxides. For certain high quality applications, several mils of metal are oftentimes removed from the rod surface by mechanical shaving.
Most round and square copper products are manufactured by wire drawing using either conventional manmade polycrystalline dies or natural single crystal diamond dies. Copper has excellent formability, and can be easily drawn from rod into very fine wire sizes without the need for intermediate process anneals. In spite of this desirable characteristic, common practice in the magnet wire industry is to limit the area reduction during drawing to about 90%, followed by an anneal.
Beyond that level of reduction, metallurgical structure changes can occur which can degrade the wire's mechanical properties. Copper winding wire is often produced by the so called "in line process" which involves "slow" speed wire drawing followed in line by continuous annealing performed in tandem with enameling. The final wire products are improved appreciably by limiting the area reduction between anneals to about 90%.