Note: Descriptions are shown in the official language in which they were submitted.
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ALUNBNUM-ZINC-MAGNESIUM-COPPER ALLOY EXTRUSION
This invention relates to AI-Zn-Mg-Cu alloys and more particularly it
relates to Al-Zn-Mg-Cu extrusions and the method of making the same for use in
air-
craft applications. Further, the invention relates to AI-Zn-Mg-Cu alloy
extrusion
product having improved fracture toughness.
Existing AI-Zn-Mg-Cu alloys can have relatively high strengths at
moderate corrosion resistance and moderate damage tolerance or fracture
toughness.
Such alloys and methods of obtaining properties are set forth in the patents.
For
example, U.S. Patent 4,863,528 discloses a method for producing an aluminum
alloy
product and the resulting product having improved combinations of strength and
corrosion resistance. The method includes providing an alloy consisting
essentially of
about 6-16% zinc, about 1.5-4.5% magnesium, about 1-3% copper, one or more
elements selected from zirconium, chromium, manganese, titanium, vanadium and
hafnium, the total of said elements not exceeding about 1%, the balance
aluminum and
incidental impurities. The alloy is then solution heat treated; precipitation
hardened to
increase its strength to a level exceeding the as-solution heat treated
strength level by at
least about 30% of the difference between as-solution heat treated strength
and peak
strength; subjected to treatment at a sufficient temperature or temperatures
for
improving its corrosion resistance properties; and again precipitation
hardened to raise
its yield strength and produce a high strength, highly corrosion resistant
alloy product.
U.S. Patent 5,221,377 discloses an alloy product having improved
combinations of strength, density, toughness and corrosion resistance, said
alloy product
consisting essentially of about 7.6 to 8.4% zinc, about 1.8 to 2.2% magnesium,
about 2
to 2.6% copper and at least one element selected from zirconium, vanadium and
hafnium present in a total amount not exceeding about 0.5%, preferably about
0.05 to
0.25% zirconium, the balance aluminum and incidental elements and impurities.
The
alloy product, suitable for aerospace applications, exhibits high yield
strength, at least
about 10% greater yield strength than its 7X50-T6 counterpart, with good
toughness and
corrosion resistance properties typically comparable to or better than those
of its 7X50-
T76 counterpart. Upper wing members made from this alloy typically have a
yield
strength over 84 ksi, good fracture toughness and an EXCO exfoliation
resistance level
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of "EC" or better, typically "EB".
U.S. Patent 4,477,292 discloses a three-step thermal aging method for
improving the strength and corrosion resistance of an article comprising a
solution heat
treated aluminum alloy containing zinc, magnesium, copper and at least one
element
selected from the group consisting of chromium, manganese and zirconium. The
article
is precipitation hardened at about 175 to 325 F., heat treated for from
several minutes to
a few hours at a temperature of about 360 to 390 F. and again precipitation
hardened at
about 175 to 325 F. In a preferred embodiment the article treated comprises
aluminum
alloy 7075 in the T6 condition. The method of the invention is easier to
control and is
suitable for treating articles of greater thickness than other comparable
methods.
U.S. Patent 5,108,520 discloses an aging process for solution-heat-
treated, precipitation hardening metal alloy which includes first underaging
the alloy,
such that a yield strength below peak yield strength is obtained, followed by
higher
aging for improving the corrosion resistance of the alloy, followed by lower
temperature
aging to strength increased over that achieved initially.
U.S. Patent 5,560,789 discloses AA 7000 series alloys having high
mechanical strength and a process for obtaining them. The alloys contain, by
weight, 7
to 13.5%Zn, 1 to 3.8% Mg, 0.6 to 2.7% Cu, 0 to 0.5% Mn, 0 to 0.4% Cr, 0 to
0.2% Zr,
others up to 0.05% each and 0.15% total, and remainder Al. Either wrought or
cast
alloys can be obtained, and the specific energy associated with the DEA
melting signal
of the product is lower than 3 J/g.
U.S. Patent 5,312,498 discloses a method of producing an aluminum
based alloy product having improved exfoliation resistance and fracture
toughness
which comprises providing an aluminum-based alloy composition consisting
essentially
of about 5.5-10.0% by weight of zinc, about 1.75-2.6% by weight of magnesium,
about
1.8-2.75% by weight of copper with the balance aluminum and other elements.
The
aluminum based alloy is worked, heat treated, quenched and aged to produce a
product
having improved corrosion resistance and mechanical properties. The amounts of
zinc,
magnesium and copper are stoichiometrically balanced such that after
precipitation is
essentially complete as a result of the aging process, no excess elements are
present.
The method of producing the aluminum-based alloy product utilizes either a one-
or
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two-step aging process in conjunction with the stoichiometrically balancing of
copper,
magnesium and zinc.
U.S. Patent 4,711,762 discloses an improved aluminum base alloy
product comprising 0 to 3.0 wt. % Cu, 0 to 1.5 wt. % Mn, 0.1 to 4.0 wt. % Mg,
0.8 to
8.5 wt. % Zn, at least 0.005 wt. % Sr, max. 1.0 wt. % Si, max. 0.8 wt. % Fe
and max.
0.45 wt. % Cr, 0 to 0.2 wt. % Zr, the remainder aluminum and incidental
elements and
impurities.
U.S. Patent 1,418,303 discloses an improved aluminum alloy consisting
of copper about 0.1 % to any amount below 3 %, titanium about 0.1 % to about
2%, zinc
about 6% to about 16%, iron (present as an impurity of commercial aluminum)
preferably not exceeding 0.6%, silicon (present as an impurity of commercial
aluminum)
preferably not exceeding 0.4%, other elements (impurities) preferably not
exceeding
0.4%, remainder aluminum.
U.S. Patent 2,290,020 discloses an improved aluminum alloy having the
ternary compound of aluminum, zinc and magnesium present in an amount ranging
from
about 2% to 20%, the preferred range being between about 3% and 15%. At room
temperature the ternary compound goes into solid solution in aluminum alloys
in an
amount of about 2%. The percentage in solid solution increases at high
temperatures
and decreases upon cooling, the excess precipitating out.
U.S. Patent 3,637,441 discloses an aluminum base powder metallurgy
alloy article having an improved combination of high-transverse yield strength
and high-
stress corrosion cracking resistance. The alloy contains the basic
precipitation hardening
elements zinc, magnesium and copper plus dispersion strengthening elements
iron and
nickel. It may additionally contain chromium and/or manganese. The alloy is
prepared
by atomization of a melt of the elements, hot-working, solution heat treating,
quenching
and artificial aging. Components of the alloy in percent by weight are, in
addition to the
aluminum, from at least 6.5 to 13 zinc, 1.75 to 6 magnesium, 0.25 to 2.5
copper, 0.75 to
4.25 iron and 0.75 to 6 nickel, up to 3 manganese and up to 0.75 chromium. The
iron to
nickel ratio is from 0.2:1 to 2.0:1.
U.S. Patent 5,028,393 discloses an Al-based alloy for use as sliding
material, superior in fatigue resistance and anti-seizure property consisting,
by weight,
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of 1-10% Zn, 1-15% Si, 0.1-5% Cu, 0.1-5% Pb, 0.005-0.5% Sr, and the balance
Aland
incidental impurities.
U.S. Patent 6,315,842 discloses a mold for plastics made of a rolled,
extruded or forged AIZnMgCu aluminum alloy product >60 mm thick, and having a
composition including, in weight %: 5.7 <Zn <8.7, 1.7 <Mg < 2.5, 1.2 <Cu <
2.2, Fe
<0.14, Si <0.11, 0.05 <Zr <0.15, Mn <0.02, Cr < 0. 02, with Cu+Mg<4.1 and
Mg>Cu,
other elements <0.05 each and <0.10 in total, the product being treated by
solution heat
treating, quenching and aging to a T6 temper.
In spite of these discloses, there is still a great need for. an improved
alloy
and extrusion fabricated therefrom for aerospace applications having high
levels of
strength, corrosion resistance, fracture toughness and good resistance to
fatigue crack
growth. The subject invention provides such an extrusion.
It is an object of the invention to provide an improved AI-Zn-Mg-Cu
alloy extrusion for use in aircrafts.
It is another object of the invention to provide an AI-Zn-Mg-Cu alloy
extrusion having improved fracture toughness as well as having high strength
levels.
It is yet another object of the invention to provide a method for producing
an AI-Zn-Mg-Cu alloy extrusion having improved strength properties, fracture
toughness and resistance to fatigue crack growth.
It is still another object of the invention to provide a method for
producing an AI-Zn-Mg-Cu alloy product having improved strength properties,
fracture
toughness, good levels of corrosion resistance.
It is another object of this invention to provide aerospace structural
members such as extrusions from the alloy of the invention.
In accordance with these objects, there is provided a method of producing
an aluminum alloy extrusion product having improved fracture toughness, the
method
comprising the steps of providing a molten body of an aluminum base alloy
comprised
of 1.95 to 2.5 wt.% Cu, 1.9 to 2.5 wt.% Mg, 8.2 to 10 wt.% Zn, 0.05 to 0.25
wt.% Zr,
max. 0.15 wt.% Si, max. 0.15 wt.% Fe, max. 0.1 wt.% Mn, the remainder aluminum
and incidental elements and impurities; and casting the molten body of the
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aluminum base alloy to provide a solidified body, the molten aluminum base
alloy being
solidified at a rate between liquidus and solidus temperatures in the range of
600 to
800 K per second to provide a solidified body having a grain size in the range
of 25 to
75 m. Thereafter, the body is homogenized by heating in a first temperature
range of
840 to 860 F followed by heating in a second temperature range of 680 to 880
F to
provide a homogenized body having a uniform distribution of MgZn2 or 11
precipitate.
The homogenized body is then extruded to provide an extrusion, the extruding
being
carried-out in a temperature range of 600 to 850 F and at a rate sufficient
to maintain at
least 80% of said extrusion in a non-recrystallized condition. The extrusion
is solution
heat treated and artificial aged to improve strength properties and to provide
an
extrusion product having improved fracture toughness.
According to an embodiment of the present invention, there is provided a
method
of producing an aluminum alloy extrusion product having improved fracture
toughness,
the method comprising the steps of.
(a) providing a molten body of an aluminum base alloy comprised of 1.95 to 2.5
wt.% Cu, 1.9 to 2.5 wt.% Mg, 8.2 to 10 wt.% Zn, 0.05 to 0.25 wt.% Zr, max.
0.15 wt%
Si, max. 0.15 wt.% Fe, max. 0.1 wt% Mn, and optionally 0.05 to 0.2 wt.% Cr or
0.01 to
0.1 wt.% Sc, or both, with the remainder comprising aluminum and incidental
elements
and impurities;
(b) casting said molten body of said aluminum base alloy to provide a
solidified
body, said molten aluminum base alloy being cast at a rate in the range of
25.4 to 152.4
mm per minute;
(c) homogenizing said body by heating in a first temperature range of 448.9 to
460 C followed by heating in a second temperature range of 460 to 471.1 C to
provide a
homogenized body having uniform distribution of rl precipitate and zirconium
containing
dispersoids;
(d) extruding said homogenized body to provide an extrusion product, said
extruding being carried out in a temperature range of 315.5 to 454.4 C and at
a rate
sufficient to maintain at least 80% of the cross-sectional area of said
extrusion product in
a non-recrystallized condition;
(e) solution heat treating said extrusion product; and
(f) artificial aging said extrusion product to improve strength properties and
facture toughness.
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The improved aluminum base alloy extrusion product can have a fracture
toughness of 8% or greater and a yield strength of 3% or greater than a
similarly sized
7xxx product.
The invention also includes an improved aluminum base alloy wrought
product such as an extrusion product consisting essentially of 1.95 to 2.5
wt.% Cu, 1.9
to 2.5 wt.% Mg, 8.2 to 10 wt.% Zn, 0.05 to 0.25 wt.% Zr, 0.05 to 0.2 wt.% Sc,
max.
0.15 wt. % Si, max. 0.15 wt. % Fe, max. 0.1 wt. % Mn, the remainder aluminum
and
incidental elements and impurities.
Brief Description of the Drawings
Fig. 1 is a flow chart showing steps of the invention.
Fig. 2 illustrates the results of the damage tolerance (normalized denting
speed) of the invention alloy (M703) compared to a high strength 7xxx alloys
(SSLLC).
Referring to Fig. 1, there is shown a flow chart of steps in the invention.
Generally, in the steps a molten body of AI-Zn-Mg-Cu alloy is cast at a
controlled
solidification rate to obtain a specific grain size range in the cast body.
Thereafter, the
cast body is homogenized under controlled conditions to obtain a uniform
distribution of
MgZn2 or rl precipitate. The body is extruded in a specific rate range and
temperature to
obtain an extrusion having a large portion thereof, e.g., at least 80%, in a
non-
recrystallized condition. The extrusion is then solution heat treated and aged
to very
high levels of strength, facture toughness and corrosion resistance.
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The alloy of the invention contains about 8.2 to 10 wt. % Zn, 1.9 to 2.5
wt.% Mg, 1.95 to 2.5 wt.% Cu, 0.05 to 0.25 wt.% Zr, max. 0.15 wt.% Si, max.
0.15
wt. % Fe, max. 0.1 wt. % Mn, the remainder aluminum, incidental elements and
impurities.
Preferably, the alloy contains 1.95 to 2.3 wt.% Cu, 1.9 to 2.3 wt.% Mg.
8.45 to 9.4 wt. % Zn, 0.05 to 0.2 wt. % Cr and 0.05 to 0.15 wt. % Zr. Cr
can range from 0.05 to 0.08 wt.%. For purposes of retarding recrystallization,
the alloys
can contain 0.01 to 0.2 wt.% Sc, preferably 0.01 to 0.1 wt.%. Such alloys when
processed in accordance with the invention possess marked improvements in
fracture
toughness at acceptable or even high levels of strength and corrosion
resistance
compared to conventional 7xxx alloys such as AA7075-T6, for example. The
composition of the AA 7xxx alloys are set forth in The Aluminum Association
publication entitled "Registration Record of Aluminum Association Designations
and
Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum
Alloys", dated December 1993. The term "7xxx" means aluminum alloys containing
zinc as a main alloying ingredient. AA 7075-T6 refers to AA compositional
limits as
registered with The Aluminum Association. A typical T6 aging practice for 7075
is
heating at about 250 F for 24 hours and a typical temperature range is about
175 to
330 F for 3 to 30 hours.
For purposes of the present invention, a molten aluminum alloy of the
invention is cast into a solidified body at a rate which provides a controlled
micro-
structure or grain size. Such molten aluminum alloy typically is cast in the
form of billet
when it is desired to produce extrusion products. Further, typically such
solidified body
is cast at a rate of about 1 to 6 inches per minute, preferably 2 to 4 inches
per minute,
and typically the billet has a diameter in the range of about 1 to 7 inches.
For purposes
of the invention, it is preferred that the solidified body has an average
grain size in the
range of 25 to 100 m, preferably 35 to 75 m. If the alloy of the invention
is cast at
controlled rates and thermally mechanically processed in accordance with the
invention,
very high tensile and compressive strengths, fracture toughness and corrosion
resistance
can be obtained. That is, for purposes of obtaining the desired microstructure
for
thermal mechanical processing in accordance with the invention, the molten
aluminum
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is cast at a controlled solidification rate. It has been discovered that
controlled
solidification rate of the disclosed aluminum alloy in combination with
subsequent
controlled thermal mechanical processing results in extruded products having
superior
properties, i.e., very high tensile strength, good corrosion and dent
resistance.
It should be noted that the strength of the subject aluminum alloys can be
improved by dispersion hardening or by strain hardening. Strain hardening is
the result
of plastic deformation and is dependent on the degree of deformation.
Dispersion
hardening is produced through formation of clusters of atoms (referred to as
Guiner-
Preston or GP zones). In addition, dispersion hardening can result from the
formation of
new phases or precipitates in the alloy which form barriers against
dislocation
movement. This can significantly increase the strength of the alloy. In the Al-
Zn-Mg-
Cu alloys, new strengthening phases include MgZn2, also known as M or t-phase;
Mg3Zn3A12 also as the T-phase; CuMgA12 also known as the S-phase.
Strengthening
resulting from precipitation of new phases is more effective than
strengthening by
formation of GP zones. However, strengthening by precipitation of new phases
can
have an adverse effect on damage tolerance or fracture toughness. Usually, the
greater
the volume fraction in the precipitation phases, the lower is the damage
tolerance. By
comparison, strengthening resulting from GP zone formation does not take place
at the
expense of damage tolerance. Thus, to provide for improved strength and damage
tolerance, the present invention balances the volume fraction of precipitates
and the
volume fraction of GP zones or zinc-rich clusters in the final product while
maintaining
excess zinc in solution. For the purpose of the invention the GP zones size
should be in
the range of 2 to 35 nm and the GP zones density should be in the range of
4x1018 to
5x1018 zones per cm3.
For purposes of producing billet in accordance with the invention, casting
may be accomplished using a mold cooled by an air and liquid coolant to
solidify billet
at a controlled rate which provides the desired grain size or structure. The
grain can
have a size in the range of 35 to 75 m. The air and coolant mixture used with
the
molds are particularly suited for extracting heat from the body of molten
aluminum alloy
to obtain a solidification rate of 5 to 50 C per second for billet having a
diameter of 1 to
6 inches. Molds using the air and coolant mixture which are suitable for
controlling the
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cooling rate for casting molten aluminum alloy of the invention are described
in U.S.
Patent 4,598,763. The coolant for use with these molds for the invention is
comprised
of a gas and a liquid where gas is infused into the liquid as tiny, discrete
undissolved
bubbles and the combination is directed on the surface of the emerging ingot.
The
bubble-entrained coolant operates to cool the metal at an increased rate of
heat
extraction; and if desired, the increased rate of extraction, together with
the discharge
rate of the coolant, can be used to control the rate of cooling at any stage
in the casting
operation, including during the steady state casting stage.
For casting metal, e.g., aluminum alloy to provide a microstructure
suitable for purposes of the present invention, molten metal is introduced to
the cavity of
an annular mold, through one end opening thereof, and while the metal
undergoes partial
solidification in the mold to form a body of the same on a support adjacent
the other end
opening of the cavity, the mold and support are reciprocated in relation to
one another
endwise of the cavity to elongate the body of metal through the latter opening
of the
cavity. Liquid coolant is introduced to an annular flow passage which is
circumposed
about the cavity in the body of the mold and opens into the ambient atmosphere
of the
mold adjacent the aforesaid opposite end opening thereof to discharge the
coolant as a
curtain of the same that impinges on the emerging body of metal for direct
cooling.
Meanwhile, a gas which is substantially insoluble in the coolant liquid is
charged under
pressure into an annular distribution chamber which is disposed about the
passage in the
body of the mold and opens into the passage through an annular slot disposed
upstream
from the discharge opening of the passage at the periphery of the coolant flow
therein.
The body of gas in the chamber is released into the passage through the slot
and is
subdivided into a multiplicity of gas jets as the gas discharges through the
slot. The jets
are released into the coolant flow at a temperature and pressure at which the
gas is
entrained in the flow as a mass of bubbles that tend to remain discrete and
undissolved
in the coolant as the curtain of the same discharges through the opening of
the passage
and impinges on the emerging body of metal. With the mass of bubbles entrained
therein, the curtain has an increased velocity, and this increase can be used
to regulate
the cooling rate of the coolant liquid, since it more than offsets any
reduction in the
thermal conductivity of the coolant. In fact, the high velocity bubble-
entrained curtain
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of coolant appears to have a scrubbing effect on the metal, which breaks up
any film and
reduces the tendency for film boiling to occur at the surface of the metal,
thus allowing
the process to operate at the more desirable level of nucleate boiling, if
desired. The
addition of the bubbles also produces more coolant vapor in the curtain of
coolant, and
the added vapor tends to rise up into the gap normally formed between the body
of metal
and the wall of the mold immediately above the curtain to cool the metal at
that level.
As a result, the metal tends to solidify further up the wall than otherwise
expected, not
only as a result of the higher cooling rate achieved in the manner described
above, but
also as a result of the build-up of coolant vapor in the gap. The higher level
assures that
the metal will solidify on the wall of the mold at a level where lubricating
oil is present;
and together, all of these effects produce a superior, more satin-like, drag-
free surface on
the body of the metal over the entire length of the ingot and is particularly
suited to
thermal transformation.
When the coolant is employed in conjunction with the apparatus and
technique described in U.S. Patent 4,598,763, this casting method has the
further
advantage that any gas and/or vapor released into the gap from the curtain
intermixes
with the annulus of fluid discharged from the cavity of the mold and produces
a more
steady flow of the latter discharge, rather than the discharge occurring as
intermittent
pulses of fluid.
As indicated, the gas should have a low solubility in the liquid; and
where the liquid is water, the gas may be air for cheapness and ready
availability.
During the casting operation, the body of gas in the distribution chamber
may be released into the coolant flow passage through the slot during both the
butt
forming stage and the steady state casting stage. Or, the body of gas may be
released
into the passage through the slot only during the steady state casting stage.
For example,
during the butt-forming stage, the coolant discharge rate may be adjusted to
undercool
the ingot by generating a film boiling effect; and the body of gas may be
released into
the passage through the slot when the temperature of the metal reaches a level
at which
the cooling rate requires increasing to maintain a desired surface temperature
on the
metal. Then, when the surface temperature falls below the foregoing level, the
body of
gas may no longer be released through the slot into the passage, so as to
undercool the
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metal once again. Ultimately, when steady state casting is begun, the body of
gas may
be released into the passage once again, through the slot and on an indefinite
basis until
the casting operation is completed. In the alternative, the coolant discharge
rate may be
adjusted during the butt-forming stage to maintain the temperature of the
metal within a
prescribed range, and the body of gas may not be released into the passage
through the
slot until the coolant discharge rate is increased and the steady state
casting stage is
begun.
The coolant, molds and casting method are further set forth in U.S.
Patents 4,598,763 and 4,693,298.
While the casting procedure for the present invention has been described
in detail for producing billet having the necessary structure for thermal
transformation in
accordance with the present invention, it should be understood that the other
casting
methods can be used to provide the solidification rates that result in the
grain structure
necessary to the invention. As noted earlier, such solidification can be
obtained by belt,
.15 block or roll casting and electromagnetic casting.
A seven inch billet of an alloy containing 8.9 wt.% Zn, 2.1 wt.% Mg, 2.3
wt.% Cu, 0.11 wt.% Zr, the remainder comprising aluminum, cast employing a
mold
using air and water coolant, at a cooling rate of 35 to 50 F per second
provides a
satisfactory grain structure for extruding and thermally mechanically
processing in
accordance with the invention.
While casting has been described with respect to billet, it will be
appreciated that the principles described herein may be applied to ingot or
electro-
magnetic casting of the aluminum alloys.
After the billet is cast, it is subjected to a homogenization treatment.
Preferably, the billet is subjected to two homogenization treatments. In the
first
homogenization treatment, the billet preferably is treated in a temperature
range of 840
to 880 F for a period of 6 to 18 hours. Thereafter, the billet is then
preferably subjected
to a temperature range of 880 to 900 F for a period of 4 to 36 hours.
Subjecting the
billet to a double homogenization treatment as described provides a billet
with a more
uniform distribution of MgZn2 precipitate or M or rl-phase as well as zinc and
chromium
containing dispersoids.
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After homogenization, the billet is extruded to provide an extrusion
member. For purposes of extruding, the billet is heated to a temperature range
of 600
to 850 F and maintained in this temperature range during extruding.
Preferably, the
billet is extruded at a rate in the range of 0.8 to 8 ft/min and preferably at
an extrusion
ratio in the range of 10 to 60. These conditions are important to obtain an
extrusion
wherein at least 80% and preferably 90% of the extrusion is maintained in the
unrecrystallized condition. The extrusion can have an aspect ratio between the
thinnest
and thickest section of 1:4 to 1:18.
After extruding, the product is solution heat treated by heating in a
temperature range of about 845 F to about 900 F, with a preferred temperature
range
being 870 to 890 F. Typical times at these temperatures can range from 5 to
120
minutes. The solution heat treating should be carried out for a time
sufficient to
dissolve a substantial portion of the alloying elements. That is,
substantially all of the
zinc, magnesium and copper is dissolved to provide a solid solution.
After solution heat treating, the extrusion is rapidly cooled or quenched
by immersion or spraying with cold water, for example. After quenching, the
extrusion
may be straightened and/or stretched. That is, the extrusion is straightened
prior to
aging to improve strength properties.
After solution heat treating, the extrusion is treated to improve properties
such as strength, corrosion and fracture toughness.
Thus, the extrusion may be subject to different thermal treatments
depending on the properties desired. For example, the extrusion may be subject
to a
single step thermal treatment to achieve high or peak strength, referred to as
T6 type
tempers. However, such tempers can be susceptible to stress corrosion
cracking. T6
tempers are obtained by aging at a temperature range of 175 to 325 F for 3 to
30 hours.
A two step aging process may be employed wherein a first aging step is carried
out at
175 to 300 F for a period of time of 3 to 30 hours, followed by a second
aging step
carried out at 300 to 360 F for a period of time of 3 to 24 hours. This aging
process
produces an overaged temper referred as T7x temper. This condition improves
stress
corrosion cracking but can decrease strength.
To improve strength and corrosion resistance, the extrusion may be
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subject to a three-step aging process. The aging steps or phases include a low-
high-low
aging sequence. In the first or low aging step, the extrusion is subject to a
temperature
for a period of time which precipitation hardens the extrusion to a point at
or near peak
strength. This can be effected by subjecting the extrusion to precipitation
hardening in a
temperature range of about 150 to 325 F typically for a time between 2 to 30
hours.
Then, the extrusion is subject to a second treatment to improve corrosion
resistance.
The second treatment includes subjecting the extrusion to a temperature range
of 300 to
500 F for 5 minutes to about 3 hours, for example. In the third step, the
extrusion is
subject to another strengthening step. The third thermal treatment includes
subjecting
the extrusion to a temperature of 175 to 325 F for about 2 to 30 hours.
Exfoliation corrosion (EXCO) behavior of the inventive alloy was
compared to 7075 T6511 and 7075 T76511 alloys. The American Society for
Testing
and Materials developed a method (ASTM G34-99) that provides an accelerated
exfoliation corrosion test for 2xxx and 7xxx series aluminum alloys. The
susceptibility
to exfoliation is determined by visual examination, with performance ratings
established
by reference to standard photographs. When tested in accordance with this test
method
the alloy of the invention exhibits a typical EA exfoliation corrosion rating
when aged to
a T76 temper. When aged to a T77 temper the invention alloy exhibits a typical
EB
exfoliation corrosion rating.
While alloy has been described with respect to extrusion products, it can
find use as sheet and plate product and such is contemplated herein.
All ranges set forth herein include all the numbers within the range as if
specifically set forth.
The products or members described herein in accordance with the
invention are particularly suitable for aerospace applications and finds many
uses in
large aircrafts such as commercial and military aircrafts. The products can be
used in
wing components, tail assemblies, fuselage sections or in subassemblies or
other
components comprising the aircraft. That is, the aircraft assemblies can
comprise a
wing assembly or wing subassembly, a center wing box assembly or subassembly,
floor
assembly or subassembly including seat tracks, floor beams, stanchions, cargo
deck
assemblies and subassemblies, floor panels, cargo floor panels, fuselage
assemblies or
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subassemblies, fuselage frames, fuselage stringers and the like. Further, the
products
may be produced as seamless or non-seamless tubes and used in sporting goods
such as
baseball bats.
TABLE
Typical mechanical properties of the inventive alloy (M703) in comparison
to 7075 T651 1 and 7150 T7751 1 for extrusions 0.249 inch thick
Alloy Temper UTS, ksi YS, ksi e,. % T,
7075 T6511 88 82 10 28
M703 T76511 97 93 10 33
M703 T77511 102 100 9 32.5
7150 T77511 93 89 9 27
The table illustrates the mechanical properties of the inventive alloy
when aged to a T76 and a T77 tempers.
The following examples are still further illustrative of the invention.
Example 1
A billet of an alloy containing 8.9 wt. % Zn, 2.1 wt. % Mg, 2.3 wt. % Cu,
0.11 wt. % Zr, incidental elements and impurities, the balance aluminum, was
cast into a
seven inch diameter billet. The billet was cast using casting molds utilizing
air and
liquid coolant (available from Wagstaff Engineering, Inc., Spokane,
Washington). The
air/water coolant was adjusted in order that the body of molten aluminum alloy
was cast
at a rate of 4 inches per minute. The as-cast structure had an average grain
size of 35
m The billet was homogenized for 8 hours at 870 F and then for 24 hours at 890
F.
Thereafter, the billet was brought to a temperature of 725 F and extruded into
a hollow
tube with an outside diameter of 2.65 inch and a wall thickness of 0.080 inch.
The extrusion had a non-recrystallized grain structure. The extrusion was
solution heat treated for 25 minutes at 880 F and quenched in a water-15%
glycol
solution. Thereafter, the quenched extrusion was precipitation hardened for 24
hours at
250 F and then subjected to a temperature of 315 F for 6 hours to improve
corrosion
resistance and yield strength properties. The extrusion was then tested for
tensile
strength and yield strength and compared to AA 7075 T6. The results are
reproduced in
Table 1.
The extrusion was then tested for dent resistance or damage tolerance.
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The dent resistance test was performed by pitching balls of constant size and
weight at
the extruded tube. The number of pitches to the first dent on the extrude tube
represents
the dent resistance. The extrusion was compared to a AA 7055 alloy treated in
a similar
fashion. The alloy of the invention is referred to as M703 and 7055 as SSLLC
(see Fig.
2). Both alloys were aged identically. It will be seen from Fig. 2 that M703
had
superior dent resistance.
Example 2
A billet of an alloy containing 8.9 wt. % Zn, 2.1 wt. % Mg, 2.3 wt. % Cu,
0.11 wt. % Zr, incidental elements and impurities, the balance aluminum, was
cast into a
seven inch diameter billet. The billet was cast using casting molds utilizing
air and
liquid coolant (available from Wagstaff Engineering, Inc., Spokane,
Washington). The
air/water coolant was adjusted in order that the body of molten aluminum alloy
was cast
at a rate of 4 inches per minute. The as-cast structure had an average grain
size of 35
m. The billet was homogenized for 8 hours at 870 F and then for 24 hours at
890 F.
Thereafter, the billet was brought to a temperature of 725 F and extruded into
an aircraft
stringer having a "T" shaped cross section and a wall thickness of 0.245
inches.
The extrusion had a non-recrystallized grain structure. The extrusion was
solution heat treated for 35 minutes at 880 F and quenched in a water-15%
glycol
solution. Thereafter, the quenched extrusion was precipitation hardened for 24
hours at
250 F followed by 25 to 35 minutes at 380 F, then subjected to a temperature
of 250 F
for 24 hours. The extrusion was then tested for tensile strength and yield
strength and
fracture toughness, fatigue crack growth and compared to AA 7075 T6511 and AA
7150
T77511. The results are reproduced in Table 1. It will be seen that the
inventive alloy
has superior strength and fracture toughness when compared to AA 7075 T6511
and AA
7150 T7751 1. Also, the extrusion has a unique combination of tensile
strength,
corrosion resistance, and damage tolerance (i.e., fracture toughness and
fatigue crack
growth).
Having described the presently preferred embodiments, it is to be
understood that the invention may be otherwise embodied within the scope of
the
appended claims.
