Introduction Copper is the preferred and predominant choice in the electrical industry because of its high conductivity, both electrical and thermal.
In order to obtain the required properties, unalloyed high purity copper is almost always used. This article discusses the rationale for this choice, and pays particular attention to the underlying metallurgical principles. It is intended to serve as a technical discussion of pertinent developments spanning the past several decades in the copper wire industry. Conductor Requirements Considerable progress has been made in recent years to explain the electronic nature of the noble metals, i.e., copper, silver, and gold.
These elements exhibit high conductivity because their conduction electrons show relatively little resistance to movement under an electric field. Copper in particular is an excellent conductor because outermost electrons have a large mean free path (about 100 atomic spacings) between collisions. The electrical resistivity is inversely related to this mean free path. Several electrically conductive metals are lighter than copper, but since they would require larger cross-sections to carry the same current, are unacceptable if limited space is a major requirement (e.g., in small electric motors). Consequently, aluminum is used mainly when excessive weight could become a problem.
Copper possesses the best characteristics for commercial applications, inasmuch as silver must be dismissed because of its prohibitively high cost. Applications 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. Production of Rod and Wire 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 H2O and SO2 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 wires mechanical properties. Magnet 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%. Role of Impurities Chemistry is one of the most important variables needed for the establishment of high electrical conductivity. The most harmful of these elements can significantly decrease electrical conductivity, increase the mechanical strength of the annealed wire, retard recrystallization, and will sometimes induce hot shortness during the hot rolling process in the production of rod. Numerous investigations have shown that very small additions of solute elements may increase the electrical resistivity (decrease conductivity) of copper in a linear manner as illustrated in Figure 1. Many impurities increase the half-hard recrystallization temperature in a non-linear relationship.
However, the deleterious effects on conductivity are minimized when the impurities are tied up in precipitates or oxides rather than in solution. Resistivity, ohm-meters Percent impurity by weight Figure 1 – Influence of solute elements upon the elecrical resistivity of copper at ambient temperature. Figure 2 shows the effects of various single element additions to a high purity ETP copper containing only 200 ppm of oxygen. In general, the first few parts per million of impurities have their greatest impact upon annealability compared with subsequent equal additions.
However, it should be noted that the purity of commercial copper has improved dramatically since the electrical standards for copper, established in 1913, were represented by a conductivity of 100% IACS. Today, most commercial copper cathodes have conductivities approaching well over 101% IACS. Half-Hard Recrystalization Temperature, C Solute Content, ppm, Weight Figure 2 – Influence of single element additions to ETP copper upon the half-hard recrystallization temperature. Influence of Oxygen Content Oxygen is used as an alloying element to improve the soundness of “as-cast” copper bars through the control of gas-metal reactions. Equally important, oxygen acts as a scavenger in reacting with most of the impurities, which have their most potent effects on properties and annealing response when they are dissolved in the copper matrix. In contrast, harmful effects may be nullified when impurities are tied up as insoluble oxides.
The maximum conductivity of ETP copper occurs at approximately 200 ppm of oxygen as shown in Figure 3. Consequently, oxygen content for ETP copper is generally in the range of 175 to 450 ppm. Lower oxygen values are usually avoided because of a propensity to hot cracking resulting from uncombined impurities. In contrast, oxygen values in excess of this optimum concentration range are not too common because of an adverse effect upon formability. Actual oxygen content is a compromise between attaining better (less sluggish) annealing behavior and avoiding possible drawability problems. Electrical Conductivity, % IACS Oxygen Content, Weight % Figure 3 – Effect of oxygen content on the electrical conductivity of annealed copper.
Importance of Thermal-Mechanical Process Variables In addition to oxides formed from metallic impurities, equilibrium copper oxides can be made to either dissolve or precipitate from a copper matrix by altering the thermal history. These types of solid state reactions may also influence the final grain size because copper oxide inclusions help to promote a small uniform grain size during recrystallization. However, secondary recrystallization (abnormal grain growth) is associated with a duplex grain structure caused by the dissolution of oxides during a high temperature anneal. The propensity for grain coarsening and duplex grains is attributed to solution temperatures in excess of 500 C, and to oxygen concentrations less than 600 ppm. Some of these grain size results are exhibited in Figure 4.
Coarse grains formed prior to wire drawing are not eliminated after the subsequent lower temperature anneals. The rate of cooling from high temperature can also influence the high temperature mechanical properties, particularly when the levels of impurities are high. Rapid quenching results in high, non-equilibrium levels of impurities in solid solution. On the other hand, slow cooling allows for the interaction between impurities and oxygen, which leads to subsequent precipitation from solid solution. Annealed Grain Size, Microns Solution Anneal Temperature, C Figure 4 – Effect of pre-annealing temperature upon subsequent grain size of annealed ETP copper.
The amount of cold work by either wire drawing or rolling between intermediate process anneals is limited for commercial magnet wire. It is desirable to limit the amount of cold work prior to the final anneal in order to have good conformability (the ability of the wire to hold its shape during forming or winding with minimal springback). A high elastic modulus and low yield strength are desired properties because they are both indicative of minimum springback. Annealing Behavior Annealability of copper is a complex characteristic that is governed by large local inhomogeneities which can change with deformation and thermal history, metal purity, and oxygen content.
Impurities play a much smaller role in affecting annealing behavior when they have precipitated, as opposed to being in solid solution. A correlation exists between annealing temperature and the atomic size difference between solvent (copper in this case) and solute (the impurity). Valence of a solute element is also an important parameter affecting annealability. However, due to the complexities associated with the thermodynamic interactions of multiple species, annealability cannot be simply related to such plausible parameters as atomic volume or valence of the so