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The Metallurgy of Copper Wire

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.

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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

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