Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINUM-LITHIUM ALLOY
Back~round of the Invention
The present invention relates to
aluminum-lithium alloys and more particularly to an
aluminum-lithium alloy composition with good ~racture
toughness and high strength.
It has been estimated that current large
commercial transport aircraft may be able to save from 15
10 to 20 gallons of fuel per year for every pound of weight
that can be saved when building the aircraft. Over the
projected 20 year life of an airplane, this savings
amounts to 300 to 400 gallons of fuel. At current fuel
costs, a significant investment to reduce the structure
15 weight of the aircraft can be made to improve overall
economic efficiency of the aircraft.
The need for improved performance in aircraft of
various types can be satisfied by the use of improved
20 engines, improved airframe design, and improved or new
structural materials in the aircraft. The development of
new and improved structural materials has recently
received increased attention and is expected to yield
significant gains in performance.
Materials have always played an important role
in dictating aircraft structural concepts. In the early
part of this century, aircraft structure was composed of
wood, primarily spruce, and fabric. Because shortages of
30 spruce developed in the early part of the century,
lightweight metal alloys began to be used as aircraft
structural materials. At about the same time,
improvements in design brought about the development of
the all metal cantilevered wing. It was not until the
35 1930's, however, that the metal skin wing design became
standard, and firmly established metals, primarily
aluminum alloys, as the major airframe structural
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material. Since that time, aixcraft structural materials
have remained remarkably consistent with aluminum
structural materials being used primarily in the wing,
body and empennage, and with steel comprising the material
for the landing gear and certain other speciality
applications requiring very high strength materials.
Several new materials are currently being
developed for incorporation into aircraft structure.
10 These include new metallic materials, metal matrix
composites and resin matrix composites. It is believed
that improved aluminum alloys and carbon fiber composites
will dominate aircraft structural materials in the coming
decades. Nhile composites will be used in increasecl
15 percentages as aircraft structural materials, new
low-density aluminum alloys, and especially
aluminum-lithium alloys show great promise for extending
the use of aluminum alloys in aerospace structures.
Heretofore, aluminum-lithium alloys have been
used only sparsely in aircraft structure. The relatively
low use has been caused by casting difficulties associated
with aluminum-lithium alloys and by their relatively low
fracture toughness compared to other more conventional
25 aluminum alloys. Lithium additions to aluminum alloys,
however, provide a substantial lowering of the density as
compared to conventional aluminum alloys, which has been
found to be very important in decreasing the overall
structural weight of aircraft. Lithium additions are also
30 effective in achieving a relatively high strength to
weight ratio. While substantial strides have been made in
improving the aluminum-lithium processing technology, a
major challenge still outstanding is an ability to obtain
a good blend of fracture toughness and high strength in an
35 aluminum-lithium alloy.
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Summary of the Invention
The present invention provides a novel aluminum
alloy composition that can be worked and heat treated so
as to provide an aluminum-lithium alloy with high
strength, good fracture toughness, and relatively low
density compared to conventional aluminum alloys such as
7XXX and 2XX~ series alloys that it is intended to
replace. An alloy prepared in accordance with the present
10 invention has a nominal composition on the order of 2.2
weight percent lithium, 0.6 percent magnesium, 2.5 percent
copper, and 0.12 percent zirconium. Artificial aging of
the alloy at a temperature in the range of 250 to 350F
to a near-peak age condition results in high strengths
lS comparable to those of current 7XX~-T6 alloys in
combination with good toughness and resistance to stress
corrosion cracking. By underaging the alloy, strength and
fracture toughness levels equivalen~ to or better than
those of existing 2XXX-T3 type alloys are obtained.
20 Considerable improvement in the already excellent
combination of fracture toughness and high strength is
achieved for both 7XXX and 2XXX applications by underaging
the alloy within the relatively low temperature range of
250 to 300F.
Detailed Description of the Invention
An aluminum-lithium alloy formulated in accordance
with the present invention can contain from about 2.0 to
30 about 2.4 percent lithium, 0.3 to 0.9 percent magnesium,
2.1 to 2.9 percent copper, and from about 0.08 to a
maximum of 0.15 percent zirconium as a grain refiner.
Preferably from about 0.09 to 0.14 percent zirconium is
incorporated. All percentages herein are by weight
35 percent based on the total weight of the alloy unless
otherwise indicated. The magnesium is included to
increase strength without increasing density. Preferred
amounts of magnesium range from about 0.4 to 0.8 percent,
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with 0.6 percent being most preferred. The copper adds
strength to the alloy.
Iron and silicon can each be present in maximums
up to a total of 0.3 percent. It is preferred that these
impurities be present only in trace amounts, limiting the
iron to a maximum of 0.15 percent and the silicon to a
maximum of 0.12 percent, and preferably to maximums of
0.10 and 0.10 percent, respectively. The element zinc may
10 be present in amounts up to but not exceeding 0.25 percent
of the total. Titanium and chromium should not exceed
0.15 percent and 0.10 percent, respectively. Other
elements such as manganese must each be held to levels of
0.05 percent or below, and the total amount of such other
15 trace elements must be held to a maximum 0.15 percentage.
If the foregoing maximums are exceeded the desired
properties of the aluminum-lithium alloy will tend to
deteriorate. The trace elements sodium and hydrogen are
also thought to be harmful to the properties (fracture
20 toughness in particular) of aluminum-lithium alloys and
should be held to the lowest levels practically
attainable, for example on the order of 15 to 30 ppm
(0.0015-0.0030 wt. %) or less for the sodium and less than
15 ppm (0.0015 wt. %) and preferably less than 1.0 ppm
25 (0.0001 wt. %) for the hydrogen. The balance of the
alloy, of course, comprises aluminum.
An aluminum-lithium alloy formulated in the
proportions set forth in the foregoing two paragraphs is
30 processed into an article utilizing known techniques. The
alloy is formulated in molten form and cast into an
ingot. The ingot is then homogenized at temperatures
ranging from 925 to 1010F or higher. Thereafter, the
alloy is converted into a usable article by conventional
35 mechanical formation techniques such as rolling,
extrusion, or the like. Once an article is formed, the
alloy is normally subjected to a solution treatment at
temperatures ranging from 950 to 1010F, followed by
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5~
quenching in a quenching medium such as water that is
maintained at a ~emperature on the order of 70 to 15QF.
If the alloy has been rolled or extruded, it is generally
stretched on the order of 1 to 3 percent of its original
length to relieve internal stresses, and to provide
improved age hardening response.
The aluminum alloy can then be further worked
and formed by secondary operations into the various shapes
10 for its final application. Additional heat treatments
such as solution heat treatment and/or aging can be
employed if desired after such forming operations. For
example, sheet products after stretch forming to the
desired shapes may be re-solution heat treated at a
15 temperature on the order of 995F for 10 minutes to one
hour. The article is normally then quenched in a
quenching medium held at temperatures ranging from ahout
70 to 150F.
Thereafter, in accordance with the present
invention, the alloy is subjected to an aging treatment at
moderately low temperatures on the order of from 250 to
350F.
When this alloy is intended to replace
conventional 7XXX series type alloys, the alloy can be
aged for a period of time that will allow it to achieve
near peak strength, preferably about 95 percent, and most
preferably about 95 to 97 percent, of its peak strength.
30 Preferred aging temperatures for this purpose range from
275 to 325F. Within these temperature ranges, 95 to 97
percent peak age can be achieved by aging for about 4 to
120 hours, and preferably for about 24 to 96 hours.
When this alloy is intended to replace
conventional 2XXX series alloys, the alloy can be aged to
achieve moderately high strength in conjunction with high
fracture toughness. Preferred aging temperatures for this
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purpose range from about 250 to 300F, more preferably
from about 250 to 275F, and most preferably on the order
of 25nF. Within these temperature ranges, moderately
high strength in conjunction with high fracture toughness
can be achieved by aging for about 4 to 48 hours, and
preferably about 4 to 24 hours.
An alternative way to achieve 2XXX series alloy
properties is to naturally age the alloy, after quenching,
10 for periods of four to seven days. Ultimate strength
values of approximately 65 ksi are developed in plate
products together with yield strengths of about 55 ksi.
In addition, outstanding fracture toughness and ductility
properties are developed in both longitudinal and
15 transverse grain directions.
Example
The following example is presented to illustrate
20 the significantly improved and unexpected characteristics
of an aluminum-lithium alloy formulated and manufactured
in accordance with the parameters of the present
invention. Moreover, it is intended to illustrate the
superior characteristics of this aluminum-lithium alloy
25 when aged in accordance with the present invention and to
assist one of ordinary skill in making and using the
present invention. The following example is not intended
in any way to otherwise limit the scope of this disclosure
or the protection granted by Letters Patent hereon.
An aluminum alloy containing 2.2 percent
lithium, 0.62 percent magnesium, 2.5 percent copper, 0.09
percent zirconium with the balance being aluminum was
formulated. The trace elements present in the formulation
35 constituted less than 0.25 percent of the total. The iron
and silicon present in the formulation constituted 0.10
percent each. The alloy was cast and homogenized at about
975F. Thereafter, the alloy was extruded to a thickness
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of 0.75 inches. The resulting extrusion was then solution
treated at about 975F for about 90 minutes. The
extrusion was then quenched in water maintained at about
70F. Thereafter, the extrusion was subjected to a
stretch Gf about 1 1/2 percent of its initial length. The
material was then cut into specimens for fracture
toughness (precracked Charpy impact test) and tensile
strength testing. The precracked Charpy impact test
specimens were machined to a final specimen size of 0.394
10 X 0.394 X 2.16 inches. The specimens prepared for tensile
strength tests were standard round specimens having a
gauge section diameter of 0.25 inches. A plurality of
specimens were then artificially aged for up to 72 hours
at 300F to a near-peak age condition. In addition, a
15 second set of tensile and Charpy specimens were aged at
275F for eight hours to a considerably underaged
condition. The specimens were then subjected to tensile
strength and precracked Charpy impact tests in accordance
with standard A~TM and industry testing procedures. The
20 specimens aged at 300F to a near-peak age condition
exhibited ultimate strengths ranging on the order of 82 to
92 ksi with fracture toughness values on the order of 220
to 350 in-lbs/in2. By comparison, the specimens underaged
at 275 developed a very high fracture toughness, on the
25 order of 650 to 850 in-lbs/in2, in combination with
ultimate strengths in the range of 70 to 75 ksi.
The present invention has been described in
relation to various embodiments, including the preferred
30 formulation and processing parameters. One of ordinary
skill after reading the foregoing specification will be
able to effect various changes, substitutions of
equivalents, and other alterations without departing from
the broad concepts set forth herein. It is therefore
35 intended that the scope of the Letters Patent granted
hereon will be limited only by the definition contained in
the appended claims and equivalents thereof.
