COPPER and its alloys constitute one of the major groups of
commercial metals. They are widely used because of their excellent electrical
and thermal conductivity, outstanding resistance to corrosion, and ease
of fabrication, together with good strength and fatigue resistance.
They are generally nonmagnetic.
They can be readily brazed, and many coppers and copper alloys can be
welded by various gas, arc and resistance methods. For decorative parts,
standard alloys having specific colors are readily available. Copper
alloys can be polished and buffed to almost any desired texture and
luster. They can be plated, coated with organic substances or
chemically colored to further extend the variety of available finishes.
Pure copper is used extensively for cables and wires, electrical
contacts, and a wide variety of other parts that are required to
pass electrical current. Coppers and certain brasses, bronzes and
cupronickels are used extensively for automobile radiators, heat
exchangers, home heating systems, panels for absorbing solar
energy and various other applications requiring rapid conduction
of heat across or along a metal section. Because of their
outstanding ability to resist corrosion, coppers, brasses,
some bronzes, and cupronickels are used for pipes, valves
and fittings in systems carrying potable water, process
water or other aqueous fluids.
In all classes of copper alloys, certain alloy compositions for
wrought products have counterparts among the cast alloys, which
enables the designer to make an initial alloy selection before
deciding on the manufacturing process.
Most wrought alloys are available in various cold worked conditions,
which have room temperature strengths and fatigue resistances that
depend on the amount of cold work more than on alloy content.
Typical applications of cold worked conditions (cold worked tempers)
include springs, fasteners, hardware, small gears, and cams. Certain
types of parts - most notably plumbing fittings and valves - are
produced by hot forging simply because no other fabrication process
can produce the required shapes and properties as economically.
Copper alloys containing 1 to 6% Pb are free machining grades,
and are used widely for machined parts especially those produced in
Copper and its alloys are relatively good conductors of electricity
and heat. In fact, copper is used for these purposes more often than
any other metal. Alloying invariably decreases electrical conductivity
and, to a lesser extent, thermal conductivity. For this reason, coppers
and high copper alloys are preferred over copper alloys containing more
than a few percent total alloy content when high electrical or thermal
conductivity is required for the application. The amount of reduction
due to alloying does not depend on conductivity or any other bulk
property of the alloying element, but only on the effect that the
particular foreign atoms have on the copper lattice.
Commercially pure copper is represented by UNS numbers C10100 to C13000.
The various coppers within this group have different degrees of purity,
and therefore different metal characteristics. Fire refined tough pitch
copper C12500 is made by deoxidizing anode copper until the oxygen content
has been lowered to the optimum value of 0.02 to 0.04%.
Electrolytic tough pitch copper C11000 is made from cathode copper - that
is, copper that has been refined electrolytically. C11000 is the most
common of all the electrical coppers. It has high electrical conductivity,
in excess of 100% IACS. It has the same oxygen content as C 12500, but
differs in sulfur content and in over-all purity. C11000 has less than
50 ppm total metallic impurities (including sulfur).
Oxygen-free coppers C10100 and C10200 are made by induction melting
prime-quality cathode copper under nonoxidizing conditions produced
by a granulated graphite bath covering and a protective reducing
atmosphere that is low in hydrogen.
If resistance to softening at slightly elevated temperature is
required, C11100 is often specified. This copper contains a
small amount of cadmium, which raises the temperature at which
recovery and recrystallization occurs.
High purity copper is a very soft metal. It is softest in its
undeformed, single-crystal form, requiring a shear stress of only
3.9 MPa . Annealed tough pitch copper is almost as soft as high
purity copper, but many of the copper alloys are much harder and
stiffer, even in annealed tempers.
Cold working increases both tensile strength and yield strength, but the
effect is more pronounced on the latter. For most coppers and copper
alloys, the tensile strength of the hardest cold-worked temper is
approximately twice the tensile strength of the annealed temper.
For the same alloys, the yield strength of the hardest cold worked
temper may be as much as five to six times that of the annealed temper.
Hot working. Not all shaping is confined to cold deformation. Hot
working is commonly used for alloys that remain ductile above the
recrystallization temperature. Hot working permits more extensive
changes in shape than cold working, so that a single operation can
replace a sequence of forming and annealing operations.
Annealing. Work-hardened metal can be returned to a soft
state by heating, or annealing. During annealing, deformed and
highly stressed crystals are transformed into unstressed crystals
by recovery, recrystallization and grain growth. In severely
deformed metal, recrystallization occurs at lower temperatures
than in lightly deformed metal. Also, the grains are smaller and
more uniform in size when severely deformed metal is recrystallized.
Grain size can be controlled by proper selection of cold working and
Anneal-resistant coppers. Addition of small amounts of elements
such as silver and cadmium to deoxidized copper increase resistance to
softening at times and temperatures encountered in soldering operations
such as those used to join components of automobile and truck radiators.
The thermal and electrical conductivity of copper are relatively
unaffected by small amounts of either silver or cadmium. Room
temperature mechanical properties also are unchanged. C111000,
C14300 and C16200 (cadmium-bearing coppers) work harden at higher
rates than either C11400 or C11000.
The most common way to catalog copper and its alloys is to divide
them into six families: coppers, dilute copper alloys, brasses,
bronzes, copper nickels and nickel silvers. The first family,
the coppers, is essentially commercially pure copper, which
ordinarily is soft and ductile and contains less than about 0.7% total
impurities. The dilute copper alloys contain small amounts of various
alloying elements that modify one or more of the basic properties of copper.
Solid Solution Alloys. The most compatible alloying elements
with copper are those that form solid-solution fields. These include
all elements forming useful alloy families (Zn, Sn, Al, Si…).
Hardening in these systems is great enough to make useful objects
without encountering brittleness associated with second phases or compounds.
Cartridge brass is typical of this group, consisting of 30% Zn in
copper and exhibiting no beta phase except an occasional small amount
due to segregation, which normally disappears after the first anneal.
Provided that there are no elements such as Fe, cold working
and grain growth relation ships are easily reproduced in practice.
Age-hardenable Alloys. Age hardening produces very high
strengths, but is limited to those few copper alloys in which the
solubility of the alloying element decreases sharply with decreasing
temperature. The beryllium coppers can be considered typical of the
age-hardenable copper alloys. Other age-hardenable alloys include
C15000 (zirconium copper); C18200, C18400 and C18500 (chromium coppers);
C19000 and C19100 (copper nickel phosphorus alloys); and
C64700 (copper nickel silicon alloy).
By combining cold working with heat treatment, higher strengths can
be obtained than can be achieved by either cold working or age
hardening alone. Beryllium copper illustrates well the effects of
heat treatment and cold working: in the soft, solution treated
condition, the tensile strength is about 500 MPa, solution treated
and aged, about 1000 MPa, and solution treated, cold worked and aged,
about 1400 MPa.
Some age-hardening alloys have different desirable characteristics,
such as high strength combined with better electrical conductivity
than the beryllium coppers.
Insoluble Alloying Elements. Lead, tellurium and selenium are
added to copper and its alloys to improve machinability. They, along
with bismuth, make hot rolling and hot forming nearly impossible and
severely limit the useful range of cold working.
An exception here are the high-zinc brasses, which become fully beta
phase at high temperature. The beta phase can dissolve lead, thus
avoiding a liquid grain-boundary phase at hot forging or extrusion
temperatures. Most free-cutting brass rod is made by beta extrusion.
C37700, one of the leading high-zinc brasses, is so readily hot forged
that it is the standard alloy against which the forgeability of all
copper alloys is judged.
Deoxidixers Li, Na, Be, Mg, B, Al, C, Si and P can be used to
deoxidize copper. Ca, Mn and Zn can sometimes be
considered deoxidizers, although they normally fulfill different roles.
The first requirement of a deoxidizer is that it have an affinity
for oxygen in molten copper. Probably the second most important
requirement is that it be relatively inexpensive compared to copper
and any other additions. Thus, although zinc normally functions as
a solid-solution strengthener, it is sometimes added in small amounts
to function as a deoxidizer, because it has high affinity for oxygen
and is relatively low in cost. In tin bronze, phosphorus has
traditionally been the deoxidizer, hence the name "phosphor bronzes"
for these alloys. Silicon instead of phosphorus is the deoxidizer for
chromium coppers because phosphorus severely reduces electrical
conductivity. Most deoxidizers contribute to hardness and other
qualities, which often makes classification as a deoxidizer indistinct.
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