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Copper
has been the most common alloying element almost since the beginning of
the aluminum industry, and a variety of alloys in which copper is the
major addition were developed. Most of these alloys fall within one of
the following groups:
·
Cast
alloys with 5% Cu,
often with small amounts of silicon and magnesium.
·
Cast
alloys with 7-8% Cu,
which often contain large amounts of iron and silicon and appreciable
amounts of manganese, chromium, zinc, tin, etc.
·
Cast
alloys with 10-14% Cu.
These alloys may contain small amounts of magnesium (0.10-0.30% Mg),
iron up to 1.5%, up to 5% Si and smaller amounts of nickel, manganese,
chromium.
·
Wrought
alloys with 5-6% Cu and
often small amounts of manganese, silicon, cadmium, bismuth, tin,
lithium, vanadium and zirconium. Alloys of this type containing lead,
bismuth, and cadmium have superior machinability.
·
Durals,
whose basic composition is 4-4.5% Cu,
0.5-1.5% Mg,
0.5-1.0% Mn,
sometimes with silicon additions.
·
Copper
alloys containing nickel, which can be subdivided in two groups: the Y alloy
type, whose basic composition is 4% Cu, 2% Ni, 1.5% Mg; and the Hyduminiums,
which usually have lower copper contents and in which iron replaces
30me of the nickel.
In most
of the alloys in this group aluminum is the primary constituent and in
the cast alloys the basic structure consists of cored dendrites of
aluminum solid solution, with a variety of constituents at the grain
boundaries or interdendritic spaces, forming a brittle, more or less
continuous network of eutectics.
Wrought
products consist of a matrix of aluminum solid solution with the other
constituents dispersed within it. Constituents formed in the alloys can
be divided in two groups: in the soluble ones are the constituents
containing only one or more of copper, lithium, magnesium, silicon,
zinc; in the insoluble ones are the constituents containing at least
one of the more or less insoluble iron, manganese, nickel, etc.
The type
of soluble constituents formed depends not only on the amount of
soluble elements available but also on their ratio. Available copper
depends on the iron, manganese and nickel contents; the copper combined
with them is not available.
Copper
forms (CuFe)Al6 and Cu2FeAl7,
with iron, (CuFeMn)Al6 and Cu2Mn3Al20 with
manganese, Cu4NiAl,
and several not too well known compounds with nickel and iron. The
amount of silicon available to some extent controls the copper
compounds formed. Silicon above 1% favors the FeSiAl5,
over the iron-copper compounds and (CuFeMn)3Si2Al15,
over the (CuFeMn)Al6 and Cu2Mn3Al20 compounds.
Similarly,
but to a lesser extent, available silicon is affected by iron and
manganese contents. With the Cu:Mg ratio below 2 and the Mg:Si ratio
well above 1.7 the CuMg4Al6compound
is formed, especially if appreciable zinc is present. When Cu:Mg
> 2 and Mg:Si > 1.7, CuMgAl2 is
formed. If the Mg:Si ratio is approximately 1.7, Mg2Si and CuAl2 are
in equilibrium. With the Mg:Si ratio 1 or less, Cu2Mg8Si6Al5,
is formed, usually together withCuAl2.
When the copper exceeds 5%, commercial heat treatment cannot dissolve
it and the network of eutectics does not break up. Thus, in the 10-15%
Cu alloys there is little difference in structure between the as-cast
and heat treated alloys.
Magnesium is
usually combined with silicon and copper. Only if appreciable amounts
of lead, bismuth or tin are present, Mg2Sn, Mg2Pb, Mg2Bi3 can
be formed.
The
effect of alloying elements on density and thermal expansion is
additive; thus, densities range from 2 700 to 2 850 kg/m3,
with the lower values for the high-magnesium, high-silicon and
low-copper alloys, the higher for the high-copper, high-nickel,
high-manganese and high-iron contents.
Expansion
coefficients are of the order of 21-24 x 10-6 1/K
for the 300-4000 K range and 23-26 x 10-6 1/K
for the 300-700 K range, with the higher values for the high-magnesium,
low-copper and low-silicon alloys, the lower ones for the higher
silicon and higher copper contents. At subzero temperatures the
coefficient decreases practically in the same way as that of pure
aluminum. However, release of casting stresses or precipitation and
solution of copper and magnesium produce changes in length of up to
0.2%, which may affect the dimensional accuracy of parts exposed to
high temperature. Subzero treatment of castings to reduce warpage has
been recommended.
Specific
heat of the commercial alloys is practically the same as for the binary
aluminum-copper. Thermal conductivity is little affected by alloying
elements other than copper: for the commercial alloys with 4-12% Cu,
< 4% other elements, it is approximately 70% of that of pure
aluminum at room temperature, some 75-80% at 600 K and 30-35% at 200 K.
Electric
conductivity is very sensitive to copper in solution, and to a much
lesser extent to magnesium and zinc, but is little affected by alloying
elements out of solution. In an alloy with 5% Cu in
solution the conductivity is approximately half that of pure aluminum
(30-33% IACS), but in the annealed state an alloy with 12% Cu and
up to 5% other elements has a conductivity of 37-42% IACS, only 25-30%
lower than that of pure aluminum.
The
mechanical properties of the alloys vary over an extremely wide range,
from those of the sand cast 8% Cu alloys,
which are among the lowest in aluminum alloys, to those of durals or
wrought 5% Cu alloys,
which may reach values of up to 650 MPa.
Higher
purity, special compositions, fabricating techniques or heat treatments
may produce higher properties. Porosity, poor feeding of castings,
excessive amounts of impurities, segregation and poor quality control
in fabrication may reduce the properties well below the determined
limits. Surface defects reduce the properties of castings more than
internal ones. Prestrain or elastic strain during testing have no
effect on properties. Ultrasonic vibration may reduce or increase them;
and irradiation at cryogenic temperatures may slightly increase
strength. Dynamic loading may produce strength and ductility values
higher or lower, depending on the speed, but not at high temperature.
Temperatures below room temperature increase strength and hardness,
with some loss of ductility and a decrease in anisotropy.
Correspondingly,
exposure to temperatures above room temperature eventually results in a
decrease in strength and hardness with a decided increase in
elongation. Heat treatment has a substantial effect: if the alloys are
quenched from high temperature and only naturally aged, exposure to
temperatures in the range up to 500-600 K may produce a temporary
increase in hardness and strength due to artificial aging. Eventually
this increase disappears, the faster the higher the temperature, and
the normal decline sets in, as in alloys already aged to peak hardness.
Prolonged heating (for up to 2 years) results in appreciable softening
at all temperatures. For intermediate exposure times this softening is
less if the materials are thermo-mechanically treated. In short-time
tests fast heating to test temperature increases the strength.
Impact
resistance is low, as for all aluminum alloys: in the Charpy test
values range from a minimum of 2-3 x 104N/m
for cast alloys with 7% Cu to a maximum of 30-40 x 104N/m
for wrought products in the naturally aged temper. Notch sensitivity is
usually low, especially in the wrought alloys, or in the cast alloys
heat treated to maximum ductility. The plane strain fracture toughness
ranges from 85 to 100% of the yield strength, depending on a variety of
factors. Both impact resistance and notch toughness increase with
increasing temperature, but the decrease with subzero temperatures is
limited. In the softer alloys at 70 K the difference is within error of
testing; only for the higher-strength alloys is the decrease
appreciable.
Shear
strength is of the order of 70-75% of tensile strength, even at high
temperature; bearing strength is approximately 1.5 of tensile;
compressive yield strength is 10-15% higher or lower than ultimate
tensile strength.
Most
alloying elements raise the modulus of elasticity of aluminum, but the
increase is not substantial: for the aluminum-copper alloys the modulus
of elasticity at room temperature is of the order of 70-75 GPa and
practically the same in tension and in compression. It changes
regularly with temperature from a value of 76-78 GPa at 70 K to a value
of the order of 60 GPa at 500 K. The change during aging is negligible
for practical purposes. The Poisson ratio is slightly lower and of the
order of 0.32-0.34, and so is the compressibility. The Poisson ratio
increases with increasing temperature.
Many of
the cast alloys and of the aluminum-copper-nickel alloys are used for
high-temperature applications, where creep resistance is important.
Resistance is the same whether the load is tensile or compressive.
Wear
resistance is favored by high hardness and the presence of hard
constituents. Alloys with 10-15% Cu or
treated to maximum hardness have very high wear resistance.
Silicon increases
the strength in cast alloys, mainly by increasing the castability and
thus the soundness of the castings, but with some loss of ductility and
fatigue resistance, especially when it changes the iron-bearing
compounds from FeM2
Magnesium increases
the strength and hardness of the alloys, but, especially in castings,
with a decided decrease in ductility and impact resistance.
Iron has some
beneficial strengthening effect, especially at high temperature and at
the lower contents (< 0.7% Fe).
Nickel has a
strengthening effect, similar to that of manganese, although more
limited because it only acts to reduce the embrittling effect of iron.
Manganese and nickel together decrease the room-temperature properties
because they combine in aluminum-manganese-nickel compounds and reduce
the beneficial effects of each other. The main effect of-nickel is the
increase in high-temperature strength, fatigue and creep resistance.
Titanium is added
as grain refiner and it is very effective in reducing the grain size.
If this results in a better dispersion of insoluble constituents,
porosity and nonmetallic inclusions, a decided improvement in
mechanical properties results.
Lithium has an
effect very similar to that of magnesium: it increases strength,
especially after heat treatment and at high temperatures, and there is
a corresponding decrease in ductility. Zinc increases the strength
but reduces ductility.
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