Note: Descriptions are shown in the official language in which they were submitted.
1 338007
ALUMINUM-LITHIUM ALLOYS
This invention relates to aluminum base
alloys, and more particularly, it relates to improved
lithium containing aluminum base alloys, products made
therefrom and methods of producing the same.
In the aircraft industry, it has been
generally recognized that one of the most effective ways
to reduce the weight of an aircraft is to reduce the
density of aluminum alloys used in the aircraft
construction. For purposes of reducing the alloy
density, lithium additions have been made. However, the
addition of lithium to aluminum alloys is not without
problems. For example, the addition of lithium to
aluminum alloys often results in a decrease in ductility
and fracture toughness. Where the use is in aircraft
parts, it is imperative that the lithium containing
alloy have both improved fracture toughness and strength
properties.
It will be appreciated that both high strength
and high fracture toughness appear to be quite difficult
to obtain when viewed in light of conventional alloys
such as AA (Aluminum Association) 2024-T3X and 7050-TX
normally used in aircraft applications. For example, a
paper by J. T. Staley entitled "Microstructure and
Toughness of High-Strength Aluminum Alloys", Properties
Related to Fracture Toughness, ASTM STP605, ~n rican
1 338007
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Society for Testing and Materials, 1976, pp. 71-103,
shows generally that for AA2024 sheet, toughness
decreases as strength increases. Also, in the same
paper, it will be observed that the same is true of
AA7050 plate. More desirable alloys would permit
increased strength with only minimal or no decrease in
toughness or would permit processing steps wherein the
toughness was controlled as the strength was increased
in order to provide a more desirable combination of
strength and toughness. Additionally, in more desirable
alloys, the combination of strength and toughness would
be attainable in an aluminum-lithium alloy having
density reductions in the order of 5 to 15%. Such
alloys would find widespread use in the aerospace
industry where low weight and high strength and
toughness translate to high fuel savings. Thus, it will
be appreciated that obtaining qualities such as high
strength at little or no sacrifice in toughness, or
where toughness can be controlled as the strength is
increased would result in a remarkably unique aluminum-
lithium alloy product.
U.S. Patent 4,626,409 discloses aluminum base
alloy consisting of, by wt.% 2.3 to 2.9 Li, 0.5 to 1.0
Mg, 1.6 to 2.4 Cu, 0.05 to 0.25 Zr, 0 to 0.5 Ti, 0 to
0.5 Mn, 0 to 0.5 Ni, 0 to 0.5 Cr and 0 to 2.0 Zn and a
method of producing sheet or strip therefrom. In
addition, U.S. Patent 4,582,544 discloses a method of
superplastically deforming an aluminum alloy having a
composition similar to that of U.S. Patent 4,626,409.
European Patent Application 210112 discloses an aluminum
alloy product containing 1 to 3.5 wt.% Li, up to 4 wt.%
Cu, up to 5 wt.% Mg, up to 3 wt.% Zn and Mn, Cr and/or
Zr additions. The alloy product is recrystallized and
has a grain size less than 300 micrometers. U.S. Patent
4,648,913 discloses aluminum base alloy wrought product
having improved strength and fracture toughness
combinations when stretched, for example, an amount
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greater than 3%.
The present invention provides an improved
lithium containing aluminum base alloys which permit
products having improved strength characteristics while
S retaining high toughness properties.
The present invention provides an improved
lithium containing aluminum base alloy product which can
be processed to improve strength characteristics while
retaining high toughness properties or which can be
processed to provide a desired strength at a controlled
level of toughness.
Figure 1 shows a strength and fracture
toughness plot of alloys in accordance with the
invention.
Figure 2 shows strength plotted against aging
time of an alloy in accordance with the invention.
Figure 3 illustrates different toughness yield
strength relationships where shifts in the upward
direction and to the right represent improved
combinations of these properties.
The alloy of the present invention can contain
0.2 to 5.0 wt.% Li, 0.5 to 6.0 wt.% Mg, at least 2.45
wt.% Cu, 0.05 to 12 wt.% Zn, 0.01 to 0.14 wt.% Zr, 0.5
wt.% max. Fe, 0.5 wt.% max. Si, the balance aluminum and
incidental impurities. The impurities are preferably
limited to about 0.05 wt.% each, and the combination of
impurities preferably should not exceed 0.35 wt.%.
Within these limits, it is preferred that the sum total
of all impurities does not exceed 0.15 wt.%.
A preferred alloy in accordance with the
present invention can contain 1.5 to 3.0 wt.% Li, 2.5 to
2.95 wt.~ Cu, 0.2 to 2.5 wt.% Mg, 0.2 to 11 wt.% Zn,
0.08 to 0.12 wt.% Zr, the balance aluminum and
impurities as specified above. A typical alloy
composition would contain 1.8 to 2.5 wt.% Li, 2.55 to
2.9 wt.% Cu, 0.2 to 2.0 wt.% Mg, 0.2 to 2.0 wt.% Zn,
greater than 0.1 to less than 0.16 wt.% Zr, and max. 0.1
1 338007
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wt.% of each of Fe and Si.
A suitable alloy composition would contain 1.9
to 2.4 wt.% Li, 2.55 to 2.9 wt.% Cu, 0.1 to 0.6 wt.% Mg,
0.5 to 1.0 wt.% Zn, 0.08 to 0.12 wt.% Zr, max. 0.1 wt.%
of each of Fe and Si, the remainder aluminum.
In an embodiment of the invention, an aluminum
base alloy wrought product having improved combinations
of strength, fracture toughness and corrosion resistance
is provided. The product can be provided in a condition
suitable for aging and has the ability to develop
improved strength in response to aging treatments
without substantially impairing fracture toughness
properties or corrosion resistance. The product
comprises 0.2 to 5.0 wt.% Li, 0.05 to 6.0 wt.% Mg, at
least 2.45 wt.% Cu, 0.05 to 0.16 wt.% Zr, 0.05 to 12
wt.% Zn, 0.5 wt.% max. Fe, 0.5 wt.% max. Si, the balance
aluminum and incidental impurities. The product is
capable of having imparted thereto a working effect
equivalent to stretching so that the product has
combinations of improved strength and fracture toughness
after aging. In the method of making an aluminum base
alloy product having improved combinations of strength,
fracture toughness and corrosion resistance, a body of a
lithium containing aluminum base alloy is provided and
may be worked to produce a wrought aluminum product.
The wrought product may be first solution heat treated
and then stretched or otherwise worked amount equivalent
to stretching. The degree of working as by stretching,
for example, is normally greater than that used for
relief of residual internal quenching stresses.
The alloy of the present invention can contain
0.2 to 5.0 wt.% Li, 0 to 5.0 wt.% Mg, up to 5.0 wt.% Cu,
0 to 1.0 wt.% Zr, 0 to 2.0 wt.% Mn, 0.05 to 12.0 wt.%
Zn, 0.5 wt.% max. Fe, 0.5 wt.% max. Si, the balance
aluminum and incidental impurities. The impurities are
preferably limited to about 0.05 wt.% each, and the
combination of impurities preferably should not exceed
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0.15 wt.%. Within these limits, it is preferred that
the sum total of all impurities does not exceed
0.35 wt.%.
A preferred alloy in accordance with the
present invention can contain 0.2 to 5.0 wt.% Li, at
least 2.45 wt.% Cu, 0.05 to 5.0 wt.% Mg, 0.05 to 0.16
wt.% Zr, 0.05 to 12.0 wt.% Zn, the balance aluminum and
impurities as specified above. A typical alloy
composition would contain 1.5 to 3.0 wt.% Li, 2.55 to
2.90 wt.% Cu, 0.2 to 2.5 wt.% Mg, 0.2 to 11.0 wt.% Zn,
0.08 to 0.12 wt.% Zr, 0 to 1.0 wt.% Mn and max. 0.1 wt.%
of each of Fe and Si. In a preferred typical alloy, Zn
may be in the range of 0.2 to 2.0 and Mg 0.2 to
2.0 wt.%.
In the present invention, lithium is very
important not only because it permits a significant
decrease in density but also because it improves tensile
and yield strengths markedly as well as improving
elastic modulus. Additionally, the presence of lithium
improves fatigue resistance. Most significantly though,
the presence of lithium in combination with other
controlled amounts of alloying elements permits aluminum
alloy products which can be worked to provide unique
combinations of strength and fracture toughness while
maintaining meaningful reductions in density.
It must be recognized that to obtain a high
level of corrosion resistance in addition to the unique
combinations of strength and fracture toughness as well
as reductions in density requires careful selection of
all the alloying elements. For example, for every 1
wt.% Li added, the density of the alloy is decreased
about 2.4%. Thus, if density is the only consideration,
then the amount of Li would be maximized. However, if
it is desired to increase toughness at a given strength
level, then Cu should be added. However, for every 1
wt.% Cu added to the alloy, the density is increased by
0.87% and resistance to corrosion and stress corrosion
1 338007
-- 6
cracking is reduced. Likewise, for every 1 wt.% Mn
added, the density is increased about 0.85%. Thus, care
must be taken to avoid losing the benefits of lithium by
the addition of alloying elements such as Cu and Mn, for
example. Accordingly, while lithium is the most
important element for saving weight, the other elements
are important in order to provide the proper levels of
strength, fracture toughness, corrosion and stress
corrosion cracking resistance.
With respect to copper, particularly in the
ranges set forth hereinabove for use in accordance with
the present invention, its presence enhances the
properties of the alloy product by reducing the loss in
fracture toughness at higher strength levels. That is,
as compared to lithium, for example, in the present
invention copper has the capability of providing higher
combinations of toughness and strength. Thus, in the
present invention when selecting an alloy, it is
important in making the selection to balance both the
toughness and strength desired, since both elements work
together to provide toughness and strength uniquely in
accordance with the present invention. It is important
that the ranges referred to hereinabove, be adhered to,
particularly with respect to the limits of copper, since
excessive amounts, for example, can lead to the
undesirable formation of intermetallics which can
interfere with fracture toughness. Typically, copper
should be less than 3.0 wt.%; however, in a less
preferred embodiment, copper can be increased to less
than 4.0 wt.% and preferably less than 3.5 wt.%. The
combination of lithium and copper should not exceed 5.5
wt.% with lithium being at least 1.5 wt.% with greater
amounts of lithium being preferred.
Thus, in accordance with this invention, it
has been discovered that adhering to the ranges set
forth above for copper, fracture toughness, strength,
corrosion and stress corrosion cracking can be
1 338007
maximized.
Magnesium is added or provided in this class
of aluminum alloys mainly for purposes of increasing
strength although it does decrease density slightly and
is advantageous from that standpoint. It is important
to adhere to the limits set forth for magnesium because
excess magnesium, for example, can also lead to
interference with fracture toughness, particularly
through the formation of undesirable phases at grain
boundaries.
Zirconium is the preferred material for grain
structure control; however, other grain structure
control materials can include Cr, V, Hf, Mn, Ti,
typically in the range of 0.05 to 0.2 wt.% with Hf and
Mn up to typically 0.6 wt.%. The level of Zr used
depends on whether a recrystallized or unrecrystallized
structure is desired. The use of zinc results in
increased levels of strength, particularly in
combination with magnesium. However, excessive amounts
of zinc can impair toughness through the formation of
intermetallic phases.
Zinc is important because, in this combination
with magnesium, it results in an improved level of
strength which is accompanied by high levels of
corrosion resistance when compared to alloys which are
zinc free. Particularly effective amounts of Zn are in
the range of 0.1 to 1.0 wt.% when the magnesium is in
the range of 0.05 to 0.5 wt.%, as presently understood.
It is important to keep the Mg and Zn in a ratio in the
range of about 0.1 to less than 1.0 when Mg is in the
range of 0.1 to 1 wt.% with a preferred ratio being in
the range of 0.2 to 0.9 and a typical ratio being in the
range of about 0.3 to 0.8. The ratio of Mg to Zn can
range from 1 to 6 when the wt.% of Mg is 1 to 4.0 and Zn
is controlled to 0.2 to 2.0 wt.%, preferably in the
range of 0.2 to O.9 wt.%.
Working with the Mg/Zn ratio of less than one
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is important in that it aids in the worked product being
less anisotropic or being more isotropic in nature,
i.e., properties more uniform in all directions. That
is, working with the Mg/Zn ratio in the range of 0.2 to
0.8 can result in the end product having greatly reduced
hot worked texture, resulting from rolling, for example,
to provide improved properties, for example in the 45
direction.
Toughness or fracture toughness as used herein
refers to the resistance of a body, e.g. extrusions,
forgings, sheet or plate, to the unstable growth of
cracks or other flaws.
The Mg/Zn ràtio less than one is important for
another reason. That is, keeping the Mg/Zn ratio less
than one, e.g., 0.5, results not only in greatly
improved strength and fracture toughness but in greatly
improved corrosion resistance. For example, when the Mg
and Zn content is 0.5 wt.% each, the resistance to
corrosion is greatly lowered. However, when the Mg
content is about 0.3 wt.% and the Zn is 0.5 wt.%, the
alloys have a high level of resistance to corrosion.
While the inventors do not wish to be held to
any theory of invention, it is believed that the
resistance to exfoliation and the resistance to crack
propagation under an applied stress increases as Zn is
added. It is believed that this behavior is due to the
fact that Zn stimulates the desaturation of Cu from the
matrix solid solution by enhancing the precipitation of
Cu-rich precipitates. This effect is believed to change
the solution potential to higher electronegative values.
It is also believed that Zn forms Mg-Zn bearing phases
at the grain boundaries that interact with propagating
cracks and blunt the crack tip or deflect the advancing
crack and thereby improves the resistance to crack
propagation under an applied load.
Improved combinations of strength and
toughness is a shift in the normal inverse relationship
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between strength and toughness towards higher toughness
values at given levels of strength of towards higher
strength values at given levels of toughness. For
example, in Fig. 3, going from point A to point D
represents the loss in toughness usually associated with
increasing the strength of an alloy. In contrast, going
from point A to point B results in an increase in
strength at the same toughness level. Thus, point B is
an improved combination of strength and toughness.
Also, going from point A to point C results in an
increase in strength while toughness is decreased, but
the combination of strength and toughness is improved
relative to point A. However, relative to point D, at
point C, toughness is improved and strength remains
about the same, and the combination of strength and
toughness is considered to be improved. Also, taking
point B relative to point D, toughness is improved and
strength has decreased yet the combination of strength
and toughness are again considered to be improved.
As well as providing the alloy product with
controlled amounts of alloying elements as described
hereinabove, it is preferred that the alloy be prepared
according to specific method steps in order to provide
the most desirable characteristics of both strength and
fracture toughness. Thus, the alloy as described herein
can be provided as an ingot or billet for fabrication
into a suitable wrought product by casting techniques
currently employed in the art for cast products, with
continuous casting being preferred. Further, the alloy
may be roll cast or slab cast to thicknesses from about
0.25 to 2 or 3 inches or more depending on the end
product desired. It should be noted that the alloy may
also be provided in billet form consolidated from fine
particulate such as powdered aluminum alloy having the
compositions in the ranges set forth hereinabove. The
powder or particulate material can be produced by
processes such as atomization, mechanical alloying and
., ~ ,3~, .
- lo - 1 3 3 8 0 0 7
melt spinning. The ingot or billet may be preliminarily
worked or shaped to provide suitable stock for
subsequent working operations. Prior to the principal
working operation, the alloy stock is preferably
S subjected to homogenization, and preferably at metal
temperatures in the range of 900 to 1050F. for a period
of time of at least one hour to dissolve soluble
elements such as Li, Cu, Zn and Mg and to homogenize the
internal structure of the metal. A preferred time
period is about 20 hours or more in the homogenization
temperature range. Normally, the heat up and
homogenizing treatment does not have to extend for more
than 40 hours; however, longer times are not normally
detrimental. A time of 20 to 40 hours at the
homogenization temperature has been found quite
suitable. In addition to dissolving constituent to
promote workability, this homogenization treatment is
important in that it is believed to precipitate the Mn
and Zr-bearing dispersoids which help to control final
grain structure.
After the homogenizing treatment, the metal
can be rolled or extruded or otherwise subjected to
working operations to produce stock such as sheet, plate
or extrusions or other stock suitable for shaping into
the end product. To produce a sheet or plate-type
product, a body of the alloy is preferably hot rolled to
a thickness ranging from 0.1 to 0.25 inch for sheet and
0.25 to 6.0 inches for plate. For hot rolling purposes,
the temperature should be in the range of 1000F. down
to 750F. Preferably, the metal temperature initially
is in the range of 850 to 975F.
When the intended use of a plate product is
for wing spars where thicker sections are used, normally
operations other than hot rolling are unnecessary.
Where the intended use is wing or body panels requiring
a thinner gauge, further reductions as by cold rolling
can be provided. Such reductions can be to a sheet
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thickness ranging, for example, from 0.010 to 0.249 inch
and usually from 0.030 to 0.10 inch.
After working a body of the alloy to the
desired thickness, the sheet or plate or other worked
article is subjected to a solution heat treatment to
dissolve soluble elements. The solution heat treatment
is preferably accomplished at a temperature in the range
of 900 to 1050F. and preferably produces an
unrecrystallized grain structure.
Solution heat treatment can be performed in
batches or continuously, and the time for treatment can
vary from hours for batch operations down to as little
as a few seconds for continuous operations. Basically,
solutionizing of the alloy into a single phase field can
occur fairly rapidly, for instance in as little as 30 to
60 seconds, once the metal has reached a solution
temperature of about 1000 to 1050F. However, heating
the metal to that temperature can involve substantial
amounts of time depending on the type of operation
involved. In batch treating a sheet product in a
production plant, the sheet is treated in a furnace load
and an amount of time can be required to bring the
entire load to solution temperature, and accordingly,
solution heat treating can consume one or more hours,
for instance one or two hours or more in batch solution
treating. In continuous treating, the sheet is passed
continuously as a single web through an elongated
furnace which greatly increases the heat-up rate. The
continuous approach is favored in practicing the
invention, especially for sheet products, since a
relatively rapid heat up and short dwell time at
solution temperature is obtained. Accordingly, the
inventors contemplate solution heat treating in as
little as about 1.0 minute. As a further aid to
achieving a short heat-up time, a furnace temperature or
a furnace zone temperature significantly above the
desired metal temperature provides a greater temperature
- 12 - 1 3 3 8 0 0 7
head useful in reducing heat-up times.
To further provide for the desired strength
and fracture toughness, as well as corrosion resistance,
necessary to the final product and to the operations in
forming that product, the product should be quenched to
prevent or minimize uncontrolled precipitation of
strengthening phases referred to herein later.
After the alloy product of the present
invention has been solution heat treated and quenched,
it may be artificially aged to provide the combination
of fracture toughness and strength which are so highly
desired in aircraft members. This can be accomplished
by subjecting the sheet or plate or shaped product to a
temperature in the range of 150 to 400F. for a
sufficient period of time to further increase the yield
strength. Some compositions of the product are capable
of being artificially aged to a yield strength as high
as 95 ksi. However, the useful strengths are in the
range of 50 to 85 ksi and corresponding fracture
toughnesses for plate products are in the range of 25 to
75 ksi ~ . Preferably, artificial aging is
accomplished by subjecting the alloy product to a
temperature in the range of 275 to 375F. for a period
of at least 30 minutes. A suitable aging practice
contemplate a treatment of about 8 to 24 hours at a
temperature of about 325F. Further, it will be noted
that the alloy product in accordance with the present
invention may be subjected to any of the typical
underaging treatments well known in the art, including
natural aging and multi-step agings. Also, while
reference has been made herein to single aging steps,
multiple aging steps, such as two or three aging steps,
are contemplated and stretching or its equivalent
working may be used prior to or even after part of such
multiple aging steps.
After solution heat treatment and quenching as
noted herein, the improved sheet, plate or extrusion and
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- 13 -
other wrought products can have a range of yield
strength from about 25 to 50 ksi and a level of fracture
toughness in the range of about 50 to 150 ksi ~ .
However, with the use of artificial aging to improve
strength, fracture toughness can drop considerably. To
minimize the loss in fracture toughness associated in
the past with improvement in strength, it has been
discovered that the solution heat treated and quenched
alloy product, particularly sheet, plate or extrusion,
must be stretched, preferably at room temperature, an
amount greater than 1%, e.g. about 2 to 6% or greater,
of its original length or otherwise worked or deformed
to impart to the product a working effect equivalent to
stretching greater than 1% of its original length. The
working effect referred to is meant to include rolling
and forging as well as other working operations. It has
been discovered that the strength of sheet or plate, for
example, of the subject alloy can be increased
substantially by stretching prior to artificial aging,
and such stretching causes little or no decrease in
fracture toughness. It will be appreciated that in
comparable high strength alloys, stretching can produce
a significant drop in fracture toughness. Stretching
AA7050 reduces both toughness and strength, as shown by
the reference by J.T. Staley, mentioned previously. For
AA2024, stretching 2% increases the combination of
toughness and strength over that obtained without
stretching; however, further stretching does not provide
any substantial increases in toughness. Therefore, when
considering the toughness-strength relationship, it is
of little benefit to stretch AA2024 more than 2%, and it
is detrimental to stretch AA7050. In contrast, when
stretching or its equivalent is combined with artificial
aging, an alloy product in accordance with the present
invention can be obtained having significantly increased
combinations of fracture toughness and strength.
While the inventors do not necessarily wish to
- 1 338007
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be bound by any theory of invention, it is believed that
deformation or working, such as stretching, applied
after solution heat treating and quenching, results in a
more uniform distribution of lithium-containing
metastable precipitates after artificial aging. These
metastable precipitates are believed to occur as a
result of the introduction of a high density of defects
(dislocations, vacancies, vacancy clusters, etc.) which
can act as preferential nucleation sites for these
precipitating phases (such as T1', a precursor of the
Al2CuLi phase) throughout each grain. Additionally, it
is believed that this practice inhibits nucleation of
both metastable and equilibrium phases such as A13Li,
AlLi, Al2CuLi and Al5CuLi3 at grain and sub-grain
boundaries. Also, it is believed that the combination
of enhanced uniform precipitation throughout each grain
and decreased grain boundary precipitation results in
the observed higher combination of strength and fracture
toughness in aluminum-lithium alloys worked or deformed
as by stretching, for example, prior to final aging.
In the case of sheet or plate, for example, it
is preferred that stretching or equivalent working is
greater than 1%, e.g. about 2% or greater, and less than
14%. Further, it is preferred that stretching be in the
range of about 2 to 10%, e.g., 3.7 to 9% increase over
the original length with typical increases being in the
range of 5 to 8%.
When the ingot of the alloy is roll cast or
slab cast, the cast material may be subjected to
stretching or the equivalent thereof without the
intermediate steps or with only some of the intermediate
steps to obtain strength and fracture toughness in
accordance with the invention.
After the alloy product of the present
invention has been worked, it may be artificially aged
to provide the combination of fracture toughness and
strength which are so highly desired in aircraft
- 15 ~ l 3 3 8 0 0 7
members.
Specific strength, as used herein, is the
tensile yield strength divided by the density of the
alloy. Plate products, for example, made from alloys in
accordance with the invention, have a specific strength
of at least 0.75 x 106 ksi in3/lb and preferably at
least 0.80 x 106 ksi in3/lb. The alloys have the
capability of producing specific strengths as high as
1.00 x 106 ksi in3/lb.
The wrought product in accordance with the
invention can be provided either in a recrystallized
grain structure form or an unrecrystallized grain
structure form, depending on the type of
thermomechanical processing used. When it is desired to
have an unrecrystallized grain structure plate product,
the alloy is hot rolled and solution heat treated, as
mentioned earlier. If it is desired to provide a
recrystallized plate product, then the Zr is kept to a
very low level, e.g., less than 0.05 wt.%, and the
thermomechanical processing is carried out at rolling
temperatures of about 800 to 850F. with the solution
heat treatment as noted above. For unrecrystallized
grain structure, Zr should be above 0.10 wt.~ and the
thermomechanical processing is as above except a heat-up
rate of not greater than 5F./min and preferably less
than 1F./min is used in solution heat treatment.
If recrystallized sheet is desired having low
Zr, e.g., less than 0.1 wt.%, typically in the range of
0.05 to 0.08 Zr, the ingot is first hot rolled to slab
gauge of about 2 to 5 inches as above. Thereafter, it
is reheated to between 700 to 850F. then hot rolled to
sheet gauge. This is followed by an anneal at between
500 to 850F. for 1 to 12 hours. The material is then
cold rolled to provide at least a 25~ reduction in
thickness to provide a sheet product. The sheet is then
solution heat treated, quenched stretched and aged as
noted earlier. Where the Zr content is fairly
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substantial, such as about 0.12 wt.%, a recrystallized
grain structure can be obtained if desired. Here, the
ingot is hot rolled at a temperature in the range of
800 to 1000F. and then annealed at a temperature of
5 about 800 to 850F. for about 4 to 16 hours.
Thereafter, it is cold rolled to achieve a reduction of
at least 25% in gauge. The sheet is then solution heat
treated at a temperature in the range of 950 to 1020F.
using heat-up rates of not slower than about 10F./min
with typical heat-up rates being as fast as 200F./min
with faster heat-up rates giving finer recrystallized
grain structure. The sheet may then be quenched,
stretched and aged.
Wrought products, e.g., sheet, plate and
forgings, in accordance with the present invention
develop a solid state precipitate along the (100)
family of planes. The precipitate is plate like and
has a diameter in the range of about 50 to 100
Angstroms and a thickness of 4 to 20 Angstroms. The
precipitate is primarily copper or copper-magnesium
containing; that is, it is copper or copper-magnesium
rich. These precipitates are generally referred to as
GP zones and are referred to in a paper entitled "The
Early Stages of GP Zone Formation in Naturally Aged Al-
4 Wt Pct Cu Alloys" by R. J. Rioja and D. E. Laughlin,Metallurgical Transactions A, Vol. 8A, August 1977, pp.
1257-61. It is believed that the precipitation of GP
zones results from the addition of Mg and Zn which is
believed to reduce solubility of Cu in the Al matrix.
Further, it is believed that the Mg and Zn stimulate
nucleation of this metastable strengthen;ng
precipitate. The number density of precipitates on
the (1 0 0) planes per cubic centimeter ranges from 1 x
1015 to 1 x 1017 with a preferred range being higher
35 than 1 x 1015 and typically as high as 5 x 1016.
These precipitates aid in producing a high level of
strength without losing fracture toughness,
.~;'
6039B-11623
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particularly if short aging times, e.g., 15 hours at
350F., are used for unstretched products.
The alloy of the present invention is useful
also for extrusions and forgings with improved levels of
mechanical properties, as shown in Figure 2, for
example. Extrusions and forgings are typically prepared
by hot working at temperatures in the range of 600 to
1000F., depending to some extent on the properties and
microstructures desired.
The following examples are further
illustrative of the invention:
Example 1
The alloys of the invention (Table 1) in this
Example were cast into ingot suitable for rolling.
Alloy A corresponds to AA2090, Alloy B corresponds to
AA2090 plus 0.3 wt.% Mg, and Alloy C corresponds to
AA2090 plus 0.6 wt.% Mg. Alloys A, B and C were
provided for comparative purposes. The ingots were then
homogenized at 950F. for 8 hours followed by 24 hours
at 1000F., hot rolled to 1 inch thick plate and
solution heat treated for one hour at 1020F. The
specimens were quenched and aged. Other specimens were
stretched 2% and 6% of their original length at room
temperature and then artificially aged. Unstretched
samples were aged at 350F. Samples stretched 2% and 6%
were aged at 325F. Table 2 shows the highest attained
specific strengths. Stretched and unstretched samples
were also aged to measure corrosion performance. EXCO
(ASTM G34) is a total immersion test designed to
determine the exfoliation corrosion resistance of high
strength 2XXX and 7XXX aluminum alloys. Table 3 shows
that Alloys E, F and G, which had ratios of Mg to Zn
less than l, performed better in the four day
accelerated test than Alloys A, B, C and D which either
contained no Zn (A, B, C) or had an Mg to Zn ratio of 1
(alloy D). Alloys A, B, C and D received many ratings
of EC (severe exfoliation corrosion) or ED (very severe
1 338007
- 18 -
exfoliation). Alloy C suffered especially severe
attack; all four samples received ED ratings after four
days exposure to EXCO. Conversely, Alloys E, F and G
received ratings that were predominantly EA (mild
exfoliation) or EB (moderated exfoliation). Only one
specimen from these three alloys was rated worse than
EB. This was the 2% stretch 25 hour aging of Alloy E
which was rated ED. This data indicates that Al-Cu-Li
alloys with Mg to Zn ratios of less than 1 have improved
resistance to exfoliation corrosion.
Tables 5, 6 and 7 list the strength and
toughness exhibited by these alloys at 0, 2 and 6%
stretch, respectively. Figure 1 shows the properties of
alloys E, F and G which exhibit improved combinations of
corrosion resistance, strength and toughness.
Table 1
Composition of the Seven Alloys in Weight Percent
Alloy Cu Li Mg Zn Zr Si Fe Al
A 2.5 2.2 0 0 0.12 0.04 0.07 Balance
B 2.5 2.2 0.3 0 0.12 0.04 0.07 Balance
C 2.5 2.1 0.6 0 0.12 0.04 0.07 Balance
D 2.6 2.2 0.6 0.6 0.12 0.04 0.07 Balance
E 2.5 2.2 0.5 1 0.12 0.04 0.07 Balance
F 2.6 2.1 0.3 0.5 0.12 0.04 0.07 Balance
G 2.6 2.2 0.3 0.9 0.12 0.04 0.07 Balance
Table 2
Specific Tensile Yield Strengths
(x106 KSI in3/lb)
Calculated
Alloy 0% Stretch 2% Stretch 6% Stretch Density
A 0.71 0.81 0.82 0.0909
B 0.80 0.82 0.88 0.0908
C 0.81 0.84 0.93 0.0910
D 0.79 0.89 0.93 0.0915
E 0.83 0.87 0.90 0.0913
F 0.81 0.85 0.92 0.0910
G 0.90 0.90 0.93 0.0912
1 338007
-- 19 --
Example 2
The alloys of the invention in this example
are the same as those from Example 1 except they were
hot rolled to 1.5 inch thick plate rather than to 1 inch
plate before they were solution heat treated for one
hour at 1020F. The specimens were quenched and
artificially aged at 350F. for 20 and 30 hours. Alloys
E, F and G, which had ratios of Mg to Zn of less than 1,
had better resistance to stress corrosion cracking (SCC)
than Alloys A, B, C and D which either contained no Zn
(A, B, C) or had a Zn to Mg ratio of 1 (Alloy D). The
stress corrosion cracking test results are listed in
Table 4 which also contains a description of the test
procedures.
Alternate immersion testing in 3.5 wt.% NaCl
solution (ASTM G44) is commonly used to evaluate the
stress corrosion cracking performance of high strength
aluminum alloys, per ASTM G47. It can be seen in the
table that Alloys E, F and G have superior SCC
resistance to the other four alloys since specimens from
Alloys E, F and G have all survived 30 days in alternate
immersion at 40,000 psi. One difference between the
groups is the Mg to Zn ratio which is less than 1 (based
on weight) and achieves high resistance to stress
corrosion.
- 20 - l 338007
Table 3
EXCO Ratings of Several Al-Li Alloys
1.0 Inch Thick Plate in T8 (Cold Work Prior to Aging) Temper
Tensile
Yield
Strength
Stretch Age (Longitudinal)
Alloy (%)* (hr/F.) ksi 2 Day 4 Day
A 2 25/325 66.8 EC ED
A 2 35/325 71.5 EC EC
A 6 15/325 68.4 EA EB
A 6 20/325 72.4 EA EB
B 2 25/325 73.7 EB EC
B 2 35/325 73.5 EB EB
B 6 15/325 75.7 EC EC
B 6 20/325 78.0 EC EC
C 2 25/325 73.9 EC ED
C 2 35/325 77.6 ED ED
C 6 15/325 78.0 EC ED
C 6 20/325 81.5 EC ED
D 2 25/325 77.8 EB EB
D 2 35/325 73.5 EB EB
D 6 15/325 75.8 EC ED
D 6 20/325 76.7 EC EC
E 2 25/325 77.4 EC EC
E 2 35/325 79.5 EB EB
E 6 15/325 79.2 EB EB
E 6 20/325 84.1 EB EB
F 2 25/325 83.1 EA EA
F 2 35/325 78.4 EA EA
F 6 15/325 81.8 EB EB
F 6 20/325 84.8 EB EB
G 2 25/325 80.3 EB EB
G 2 35/325 80.8 EB EB
G 6 15/325 77.8 EB EB
G 6 20/325 89.5 EB EB
EXC0 testing conducted per ASTM G34.
*In the unstretched condition, the alloys had a rating of EC
or ED after four days.
EA=Mild Exfoliation EC=Severe Exfoliation
EB=Moderate Exfoliation ED=Very Severe Exfoliation
- 21 - l 338007
Table 4
Stress Corrosion Cracking Performance
of Several Al-Li Alloy Specimens
1.5 Inch Thick Plate In T6 Condition
(No Cold Work Prior to Aqinq)
Age 25 KSI* 40 KSI*
Alloy (hr/F) F/N** Days*** F/N** DaYs***
A 20/350 1/33,11,11 3/31,2,2
A 30/350 1/39,11 3/32,3,6
B 20/350 2/38,15 3/31,2,2
B 30/350 0/3 - 2/31,6,7
C 20/350 3/31,1,1 2/21,1
C 30/350 2/21,1 1/1
D 20/350 1/3 2 3/31,3,3
D 30/350 1/3 3 2/36,2
E 20/350 0/3 - 0/3
E 30/350 0/3 - 0/3
F 20/350 0/3 - 0/3
F 30/350 0/3 - 0/3
G 20/350 0/3 - 0/3
G 30/350 0/3 - 0/3
One eighth inch diameter smooth tensile bars tested in 3.5
wt.% NaCl solution by alternate immersion for 30 days, per
ASTM G44.
*Ksi=Thousand pounds per square inch.
**F/N=Number of specimens that failed/Number of specimens
in test.
***Days=Days to failure.
Example 3
This sample illustrates that forgings made
from alloys of the present invention have improved
combinations of corrosion resistance, strength and
fracture toughness. The alloys in this Example are the
same as those in Example 1 and the ingots were prepared
also as in Example 1. Specimens were prepared from
these ingots by hot extruding and forging.
The forged specimens were solution heat
treated for one hour at 1020F. then artificially aged
- 22 - 1 338007
at 350F. for 20 and 40 hours. Alloys E, F and G, which
had ratios of Mg to Zn of less than 1, had better
resistance to stress corrosion cracking (SCC) than
Alloys A, B, C and D which either contained no Zn (A, B,
C) or had an Mg to Zn ratio of 1 (Alloy D). Alloys E, F
and G all survived 20 days in alternate immersion at
40,000 psi. The stress corrosion cracking results are
listed in Table 8. The strength and fracture toughness
are shown in Table 9.
1 338007
-- 23 --
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1 338007
-- 24 --
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~ - 26 - 1 338007
Table 8
Stress Corrosion Cracking Results for Die Forgings
Short Transverse Properties
Age 25 ksi* 40 Ksi*
5 Alloy (hr/F.) F/N** Days*** F/N** Days***
A 20/350 3/31,1,4 3/31,2,2
A 40/350 3/34,7,12 3/32,3,4
B 20/350 2/37,15 3/34,11,11
B 40/350 3/31,3,3 3/31,1,1
C 20/350 3/31,3,2 3/31,1,1
C 40/350 3/31,3,3 3/31,1,1
D 20/350 0/3 -- 3/31,2,7
D 40/350 0/3 -- 1/3 6
E 20/350 0/3 -- 0/3 --
E 40/350 0/3 -- 1/3 25
F 20/350 0/3 -- 0/3 --
F 40/350 0/3 -- 0/3 --
G 20/350 0/3 -- 0/3 --
G 40/350 0/3 -- 0/3 --
One eighth inch diameter smooth tensile bars tested in
3.5 wt.~ NaCl solution by alternate immersion for 30
days, per ASTM G44.
*Ksi=Thousand pounds per square inch.
**F/N=Number of specimens that failed/Number of
specimens in test.
***Days=Days to failure.
- - 27 - 1 338007
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1 338007
- 28 -
While the invention has been described in
terms of preferred embodiments, the claims appended
hereto are intended to encompass other embodiments which
fall within the spirit of the invention.
