U.S. patent application number 12/211515 was filed with the patent office on 2009-01-08 for aluminum-copper-magnesium alloys having ancillary additions of lithium.
This patent application is currently assigned to Alcoa Inc.. Invention is credited to Gary H. Bray, Paul E. Magnusen, Roberto J. Rioja.
Application Number | 20090010798 12/211515 |
Document ID | / |
Family ID | 34435362 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090010798 |
Kind Code |
A1 |
Rioja; Roberto J. ; et
al. |
January 8, 2009 |
ALUMINUM-COPPER-MAGNESIUM ALLOYS HAVING ANCILLARY ADDITIONS OF
LITHIUM
Abstract
An aluminum-copper-magnesium alloy having ancillary additions of
lithium. The alloy composition includes from about 3 to about 5
weight percent Cu, from about 0.5 to about 2 weight percent Mg, and
from about 0.01 to about 0.9 weight percent Li. The combined amount
of Cu and Mg is maintained below a solubility limit of the aluminum
alloy. The alloys possess improved combinations of fracture
toughness and strength, and also exhibit good fatigue crack growth
resistance.
Inventors: |
Rioja; Roberto J.;
(Murrysville, PA) ; Bray; Gary H.; (Murrysville,
PA) ; Magnusen; Paul E.; (Pittsburgh, PA) |
Correspondence
Address: |
INTELLECTUAL PROPERTY
ALCOA TECHNICAL CENTER, BUILDING C, 100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Assignee: |
Alcoa Inc.
Pittsburgh
PA
|
Family ID: |
34435362 |
Appl. No.: |
12/211515 |
Filed: |
September 16, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10678290 |
Oct 3, 2003 |
7438772 |
|
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12211515 |
|
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09104123 |
Jun 24, 1998 |
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10678290 |
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Current U.S.
Class: |
420/532 ;
420/533; 420/534 |
Current CPC
Class: |
C22C 21/16 20130101 |
Class at
Publication: |
420/532 ;
420/533; 420/534 |
International
Class: |
C22C 21/10 20060101
C22C021/10; C22C 21/14 20060101 C22C021/14; C22C 21/16 20060101
C22C021/16; C22C 21/18 20060101 C22C021/18 |
Claims
1-24. (canceled)
25. A wrought aluminum alloy consisting essentially of: 3 to 4.5
wt. % Cu; 0.6 to 2 wt. % Mg; 0.01 to 0.8 wt. % Li; optionally up to
2 wt. % Zn; optionally up to 2 wt. % Ag; optionally up to 2 wt. %
Si; optionally up to 1 wt. % of a dispersoid-forming element; the
balance being aluminum and incidental elements and impurities;
wherein the wrought aluminum alloy possesses at least one of: (i)
higher fracture toughness and equivalent or higher strength in at
least one temper in comparison to a similar alloy having no lithium
or greater than 0.9 wt. % lithium; and (ii) higher strength and
equivalent or higher fracture toughness in at least one temper in
comparison to a similar alloy having no lithium or greater than 0.9
wt. % lithium.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 09/104,123 filed Jun. 24, 1998, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to aluminum alloys useful in
aerospace applications, and more particularly relates to
aluminum-copper-magnesium alloys having ancillary additions of
lithium which possess improved combinations of fracture toughness
and strength, as well as improved fatigue crack growth
resistance.
BACKGROUND OF THE INVENTION
[0003] It is generally well known in the aerospace industry that
one of the most effective ways to reduce the weight of an aircraft
is to reduce the density of aluminum alloys used in aircraft
construction. This desire led to the addition of lithium, the
lowest density metal element, to aluminum alloys. Aluminum
Association alloys, such as 2090 and 2091 contain about 2.0 weight
percent lithium, which translates into about a 7 percent weight
savings over alloys containing no lithium. Aluminum alloys 2094 and
2095 contain about 1.2 weight percent lithium. Another aluminum
alloy, 8090 contains about 2.5 weight percent lithium, which
translates into an almost 10 percent weight savings over alloys
without lithium.
[0004] However, casting of such conventional alloys containing
relatively high amounts of lithium is difficult. Furthermore, the
combined strength and fracture toughness of such alloys is not
optimal. A tradeoff exists with conventional aluminum-lithium
alloys in which fracture toughness decreases with increasing
strength.
[0005] Another important characteristic of aerospace aluminum
alloys is fatigue crack growth resistance. For example, in damage
tolerant applications in aircraft, increased fatigue crack growth
resistance is desirable. Better fatigue crack growth resistance
means that cracks will grow slower, thus making airplanes much
safer because small cracks can be detected before they achieve
critical size for catastrophic propagation. Furthermore, slower
crack growth can have an economic benefit due to the fact that
longer inspection intervals can be utilized.
[0006] A need therefore exists for an aluminum alloy that is useful
in aircraft application which has high fracture toughness, high
strength and excellent fatigue crack growth resistance.
SUMMARY OF THE INVENTION
[0007] The present invention provides aluminum alloys comprising
from about 3to about 5 weight percent copper; from about 0.5 to
about 2 weight percent magnesium; and from about 0.01 to about 0.9
weight percent lithium. It has been found that ancillary additions
of low levels of lithium to aluminum alloys having controlled
amounts of copper and magnesium provide a high fracture toughness
and high strength material which also exhibits equivalent or
improved fatigue crack growth resistance over prior art
aluminum-copper-magnesium alloys. An aspect of the present
invention is to provide an aluminum alloy comprising from about 3
to about 5 weight percent Cu, from about 0.5 to about 2 weight
percent Mg, and from about 0.01 to about 0.9 weight percent Li,
wherein the Cu and Mg are present in the alloy in a total amount
below a solubility limit of the alloy.
[0008] This and other aspects of the present invention will be more
apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a graph of Mg content versus Cu content,
illustrating maximum limits of those elements for Al--Cu--Mg--Li
alloys in accordance with embodiments of the present invention.
[0010] FIG. 2 is a graph of fracture toughness (K.sub.Q) and
elongation properties versus lithium content for Al--Cu--Mg based
alloys in the form of plate products having varying amounts of
Li.
[0011] FIG. 3 is a graph of fracture toughness (K.sub.Q) and
tensile yield strength properties versus lithium content for
Al--Cu--Mg based alloys in the form of plate products having
varying amounts of Li.
[0012] FIG. 4 is a graph of fracture toughness (K.sub.c and
K.sub.app) and tensile yield strength properties versus lithium
content for Al--Cu--Mg based alloys in the form of sheet products
having varying amounts of Li.
[0013] FIG. 5 is a plot of the fracture toughness and tensile yield
strength values shown in FIG. 4 in comparison with plant typical
and minimum fracture toughness and yield strength values for
conventional alloy 2524 sheet.
[0014] FIG. 6 is a chart showing the tensile yield strength of
various specimens made from Al--Cu--Mg alloys with various amounts
of Li designated Alloy A, Alloy B, Alloy C, and Alloy D after being
subjected to different aging conditions.
[0015] FIG. 7 is a bar graph showing the improvement in specific
strength for some of the specimens shown in FIG. 6.
[0016] FIG. 8 is a graph showing the typical representation of
fatigue crack growth rate, da/dN (in/cycle) and how it changes.
[0017] FIG. 9 is a graph showing the fatigue crack growth curves
for Alloy A-T3 plate; Alloy C-T3 plate; and Alloy D-T3 plate.
[0018] FIG. 10 is a graph showing the fatigue crack growth curves
for Alloy A-T39 plate; Alloy C-T39 plate; and Alloy D-T39
plate.
[0019] FIG. 11 is a graph showing the fatigue growth curves for
Alloy A-T8 plate; Alloy C-T8 plate; and Alloy D-T8 plate.
[0020] FIG. 12 is a bar graph showing the percentage change in
da/dN at .DELTA.K=10 Ksi (in).sup.1/2.
[0021] FIG. 13 is a graph showing the fracture toughness R-curves
of Alloy A-T3 and Alloy C-T3.
[0022] FIG. 14 is a graph showing the fracture toughness R-curves
for Alloy A-T39, Alloy C-T39 and Alloy D-T39 plate.
DETAILED DESCRIPTION
[0023] For the description of alloy compositions herein, all
references are to weight percentages unless otherwise indicated.
When referring to any numerical range of values, such ranges are to
be understood to include each and every number and/or fraction
between the stated range minimum and maximum.
[0024] As used herein, the term "about" when used to describe a
compositional range or amount of an alloying addition means that
the actual amount of the alloying addition may vary from the
nominal intended amount due to factors such as standard processing
variations as understood by those skilled in the art.
[0025] The term "substantially free" means having no significant
amount of that component purposely added to the alloy composition,
it being understood that trace amounts of incidental elements
and/or impurities may find their way into a desired end
product.
[0026] The term "solubility limit" means the maximum amount of
alloying additions that can be made to the aluminum alloy while
remaining as a solid solution in the alloy at a given temperature.
For example, the solubility limit for the combined amount of Cu and
Mg is the point at which the Cu and/or Mg no longer remain as a
solid solution in the aluminum alloy at a given temperature. The
temperature may be chosen to represent a practical compromise
between thermodynamic phase diagram data and furnace controls in a
manufacturing environment.
[0027] The term "improved combination of fracture toughness and
strength" means that the present alloys either possess higher
fracture toughness and equivalent or higher strength, or possess
higher strength and equivalent or higher fracture toughness, in at
least one temper in comparison with similar alloys having no
lithium or greater amounts of lithium.
[0028] As used herein, the term "damage tolerant aircraft part"
means any aircraft or aerospace part which is designed to ensure
that its crack growth life is greater than any accumulation of
service loads which could drive a crack to a critical size
resulting in catastrophic failure. Damage tolerance design is used
for most of the primary structure in a transport category airframe,
including but not limited to fuselage panels, wings, wing boxes,
horizontal and vertical stabilizers, pressure bulkheads, and door
and window frames. In inspectable areas, damage tolerance is
typically achieved by redundant designs for which the inspection
intervals are set to provide at least two inspections per number of
flights or flight hours it would take a visually detectable crack
to grow to its critical size.
[0029] The present invention relates to aluminum-copper-magnesium
alloys having ancillary additions of lithium. In accordance with
the invention, wrought aluminum-copper-magnesium alloys are
provided which have improved combinations of fracture toughness and
strength over prior art aluminum-copper-magnesium alloys. The
present alloys also possess improved fatigue crack growth
resistance. The alloys of the present invention are especially
useful for aircraft parts requiring high damage tolerance, such as
lower wing components including thin plate for skins and extrusions
for stringers for use in built-up structure, or thicker plate or
extrusions for stiffened panels for use in integral structure;
fuselage components including sheet and thin plate for skins,
extrusions for stringers and frames, for use in built-up, integral
or welded designs. They may also be useful for spar and rib
components including thin and thick plate and extrusions for
built-up or integral design or for empennage components including
those from sheet, plate and extrusion, as well as aircraft
components made from forgings including aircraft wheels, spars and
landing gear components. The strength capabilities of the alloys
are such that they may also be useful for upper wing components and
other applications where aluminum-copper-magnesium-zinc alloys are
typically employed. The addition of low levels of lithium avoids
problems associated with higher (i.e., over 1.5 weight percent
lithium) additions of lithium, such as explosions of the molten
metal during the casting of ingots.
[0030] In accordance with embodiments of the present invention, the
aluminum alloy may be provided in the form of sheet or plate. Sheet
products include rolled aluminum products having thicknesses of
from about 0.006 to about 0.25 inch. The thickness of the sheet is
preferably from about 0.025 to about 0.25 inch, more preferably
from about 0.05 to about 0.25 inch. For many applications such as
some aircraft fuselages, the sheet is preferably from about 0.05 to
about 0.25 inch thick, more preferably from about 0.05 to about 0.2
inch. Plate products include rolled aluminum products having
thicknesses of from about 0.25 to about 8 inch. For wing
applications, the plate is typically from about 0.50 to about 4
inch. In addition, light gauge plate ranging from 0.25 to 0.50 inch
is also used in fuselage applications. The sheet and light gauge
plate may be unclad or clad, with preferred cladding layer
thicknesses of from about 1 to about 5 percent of the thickness of
the sheet or plate. In addition to sheet and plate products, the
present alloys may be fabricated as other types of wrought
products, such as extrusion and forgings by conventional
techniques.
[0031] The compositional ranges of the main alloying elements
(copper, magnesium and lithium) of the improved alloys of the
invention are listed in Table 1.
TABLE-US-00001 TABLE 1 Copper, Magnesium and Lithium Compositional
Ranges Cu Mg Li Al Typical 3-5 0.5-2 0.01-0.9 balance Preferred
3.5-4.5 0.6-1.5 0.1-0.8 balance More Preferred 3.6-4.4 0.7-1
0.2-0.7 balance
[0032] Copper is added to increase the strength of the aluminum
base alloy. Care must be taken, however, to not add too much copper
since the corrosion resistance can be reduced. Also, copper
additions beyond maximum solubility can lead to low fracture
toughness and low damage tolerance.
[0033] Magnesium is added to provide strength and reduce density.
Care should be taken, however, to not add too much magnesium since
magnesium additions beyond maximum solubility will lead to low
fracture toughness and low damage tolerance.
[0034] In accordance with the present invention, the total amount
of Cu and Mg added to the alloy is kept below the solubility limits
shown in FIG. 1. In FIG. 1, the typical Cu and Mg compositional
ranges listed in Table 1 are shown with a first solubility limit
(1), and a second solubility limit (2), for the combination of Cu
and Mg contained in the alloy. The solubility limit may decrease,
e.g., from the first (1) to the second (2) solubility limit, as the
amount of other alloying additions is increased. For example,
additions of Li, Ag and/or Zn may tend to lower the solubility
limit of Cu and Mg.
[0035] In order to remain below the solubility limit, the amount of
Cu and Mg should conform to the formula: Cu.ltoreq.2-0.676(Mg-6).
Preferably, the amount of Cu and Mg conforms to the formula:
Cu.ltoreq.1.5-0.556(Mg-6) when about 0.8 wt % Li is added.
[0036] The amounts of copper and magnesium are thus controlled such
that they are soluble in the alloy. This is important in that atoms
of the alloying elements in solid solution or which form clusters
of atoms of solute may translate to increased fatigue crack growth
resistance. Furthermore, the combination of copper, magnesium and
lithium needs to be controlled as to not exceed maximum
solubility.
[0037] Within the controlled copper and magnesium ranges, the range
of the lithium content may be from about 0.01 to 0.9 weight
percent, preferably from about 0.1 or 0.2 weight percent up to
about 0.7 or 0.8 weight percent. In accordance with the present
invention, relatively small amounts of lithium have been found to
significantly increase fracture toughness and strength of the
alloys as well as provided increased fatigue crack growth
resistance and decreased density. However, at lithium levels above
the present levels, fracture toughness decreases significantly.
Furthermore, care should be taken in not adding too much lithium
since exceeding the maximum solubility will lead to low fracture
toughness and low damage tolerances. Lithium additions in amounts
of about 1.5 weight percent and above result in the formation of
the .delta.' ("delta prime") phase with composition of Al.sub.3Li.
The presence of this phase, Al.sub.3Li, is to be avoided in the
alloys of the present invention.
[0038] While not intending to be bound by any particular theory,
the interaction of lithium atoms in supersaturated solid solution,
with atoms of magnesium and/or copper appear to give rise to the
formation of clusters of atoms of solute in a W or T3 tempers. This
behavior is observed by the appearance of diffuse scatter in
electron diffraction images. This behavior may be a contributor for
the improvements in fatigue performance of the alloys of the
invention.
[0039] In addition to aluminum, copper, magnesium and lithium, the
alloys of the present invention can contain at least one
dispersoid-forming element selected from chromium, vanadium,
titanium, zirconium, manganese, nickel, iron, hafnium, scandium and
rare earths in a total amount of from about 0.05 to about 1 weight
percent. For example, manganese may be present in a preferred
amount of from about 0.2 to about 0.7 weight percent.
[0040] Other alloying elements, such as zinc, silver and/or silicon
in amounts up to about 2 weight percent may optionally be added.
For example, zinc in an amount of from about 0.05 to about 2 weight
percent may be added, typically from about 0.2 to about 1 weight
percent. As a particular example, zinc in an amount of 0.5 weight
percent may be added. When zinc is added to the alloy, it may serve
as a partial or total replacement for magnesium.
[0041] Silver in an amount of from about 0.01 to about 2 weight
percent may be added, typically from about 0.05 to about 0.6 weight
percent. For example, silver in an amount of from about 0.1 to
about 0.4 weight percent may be added.
[0042] Silicon in an amount of from about 0.1 to about 2 weight
percent may be added, typically from about 0.3 to about 1 weight
percent.
[0043] In accordance with embodiments of the present invention,
certain elements may be excluded from the alloy compositions, i.e.,
the elements are not purposefully added to the alloys, but may be
present as unintentional or unavoidable impurities. Thus, the
alloys may be substantially free of elements such as Sc, Ag and/or
Zn, if desired.
[0044] It has been found that the combination of lower copper
levels, higher magnesium levels and lower levels of lithium produce
an aluminum alloy that has increased fracture toughness and
strength, superior fatigue crack growth resistance and relatively
low density. Fracture toughness and strength are critical
properties for aluminum alloys used in aircraft applications.
Fatigue crack growth resistance is also a critical property for
damage tolerant aircraft parts, such as fuselage sections and lower
wing sections. As is known, these parts of an aircraft are subject
to cyclical stresses, such as the fuselage skin which is expanded
and contracted upon pressurization and depressurization of the
aircraft cabin and the lower wing skin which experiences tensile
stresses in flight and compressive stresses while the aircraft is
on the ground. Improved fatigue crack growth resistance means
cracks will grow and reach their critical dimension more slowly.
This allows longer inspection intervals to be used, thus reducing
aircraft operating cost. Alternatively, the applied stress could be
raised while keeping the same inspection interval, thereby reducing
aircraft weight.
[0045] The following examples illustrate various aspects of the
invention and are not intended to limit the scope of the
invention.
EXAMPLE 1
[0046] Five Al--Cu--Mg based alloys with varying amounts of Li
having compositions as listed in Table 2 were cast as ingots.
TABLE-US-00002 TABLE 2 Measured Compositions of Cast Ingots Alloy
No. Cu Mg Li Ag Mn Zr Si Fe 1 4.0 0.76 -- 0.49 0.3 0.11 0.06 0.04 2
3.9 0.74 0.19 0.49 0.3 0.11 0.02 0.03 3 4.0 0.79 0.49 0.50 0.3 0.11
0.02 0.03 4 4.1 0.75 0.70 0.50 0.3 0.11 0.02 0.03 5 4.1 0.78 1.20
0.50 0.3 0.11 0.02 0.03
[0047] The ingots listed in Table 2 were then fabricated into plate
and sheet. Based on calorimetric analyses, the ingots were
homogenized as follows. For alloys 1, 2 and 3: the ingots were
heated at 50.degree. F./hr to 905.degree. F. (16 hours), then
soaked at 905.degree. F. for 4 hours, then heated in 2 hours to
970.degree. F. and soaked for 24 hours. Finally, the ingots were
air cooled to room temperature. For alloys 4 and 5: the ingots were
heated at 50.degree. F./hour to 905.degree. F. (16 hours), soaked
at 905.degree. F. for 8 hours, then heated in 2 hours to
940.degree. F. and soaked for 48 hours prior to air cooled to room
temperature.
[0048] All ingots were the heated to 940.degree. F., and hot rolled
at about 900.degree. F. Re-heats at 940.degree. F. were provided to
keep the metal temperature above 750.degree. F. Rolling parameters
were controlled to provide about a 0.5 inch bite reductions. Plate
product with 0.7 inch and 0.5 inch gauges was fabricated. In
addition, sheet product was hot rolled to a 0.10 inch gauge.
[0049] For alloys 1, 2 and 3, samples were solution heat treated
(SHT) at a temperature of 970 F. Plate pieces were SHT for 2 hours.
Sheet samples got a soak of only 1 hour. For alloys 4 and 5,
samples were solution heat treated at a temperature of 940.degree.
F. Plate pieces were SHT for 2 hours. Sheet samples got a soak of
only 1 hour.
[0050] All samples were quenched in water at room temperature and
stretched 4% prior to aging to reach a T3 temper. All samples were
aged at 310.degree. F. for 24 hours to reach a T8-type temper.
[0051] Fracture toughness (K.sub.Ic or K.sub.Q), ultimate tensile
strength, tensile yield strength and elongation (4D) of the 0.5
inch gauge plate were measured. Tensile tests were performed in the
longitudinal direction in accordance with ASTM B 557 "Standard Test
Methods of Tension Testing of Wrought and Cast Aluminum and
Magnesium-Alloy Products" on round specimens 0.350 inch in
diameter. Fracture toughness was measured in the L-T orientation in
accordance with ASTM E399-90 "Standard Test Method for Plane Strain
Fracture Toughness of Metallic Materials" supplemented by ASTM
B645-02 "Standard Practice for Plane Strain Fracture Toughness of
Aluminum Alloys." The test specimens used were of full plate
thickness and the W dimension was 1.0 inch. The results are listed
in Table 3 and shown in FIGS. 2 and 3. Only the test results from
Alloy 5 satisfied the validity requirements in ASTM E399-90 for a
valid K.sub.Ic. The test results from Alloys 1-4 failed to meet the
following validity criteria: (1)
B.gtoreq.2.5(K.sub.Q/.sigma..sub.ys).sup.2; (2) a
.gtoreq.2.5(K.sub.Q/.sigma..sub.ys).sup.2; and (3)
P.sub.max/P.sub.Q.ltoreq.1.1, where B, K.sub.Q, .sigma..sub.ys,
P.sub.max, and P.sub.Q are as defined in ASTM E399-90. The
remaining validity criteria were all met. Test results not meeting
the validity criteria are designated K.sub.Q, the designation
K.sub.Ic being reserved for test results meeting all the validity
criteria. Failure to satisfy the above three criteria indicates
that the specimen thickness was insufficient to achieve
linear-elastic, plane-strain conditions as defined in ASTM E399.
Those skilled in the art will appreciate that the higher the
toughness or the lower the yield strength of the product the
greater the thickness and width required to satisfy the above three
criteria and achieve a valid result, K.sub.Ic. The specimen
thickness in these tests was necessarily limited by the plate
thickness. A valid K.sub.Ic is generally considered a material
property relatively independent of specimen size and geometry.
Those skilled in the art will appreciate that K.sub.Q values, while
they may provide a useful measure of material fracture toughness as
in this case, can vary significantly with specimen size and
geometry. Therefore, in comparing K.sub.Q values from different
alloys it is imperative that the comparison be made on the basis of
a common specimen size as was done in these tests. K.sub.Q values
from specimens of insufficient thickness and width to meet the
above validity criteria are typically lower than a valid K.sub.Ic
coming from a larger specimen.
TABLE-US-00003 TABLE 3 Measured Properties from Plate Li amount TYS
UTS Elongation Toughness-K.sub.Q Alloy No. (wt %) (ksi) (ksi) (%)
(ksi in) 1 0 66.1 70.3 15.7 37 1 0 65.9 70.1 16.4 37.4 2 0.19 68.6
72.4 17.1 42.3 2 0.19 68.4 72.4 17.1 41.3 3 0.49 76.4 79.6 15 40.3
3 0.49 76.8 79.7 14.3 39.8 4 0.70 80.6 84.5 12.9 39 4 0.70 80.6
84.4 12.9 40.6 5 1.20 85.9 90 8.6 26.5 (K.sub.Ic) 5 1.20 85.7 89.9
8.6 25.6
[0052] Fracture toughness (K.sub.c and K.sub.app) in the L-T
orientation and tensile yield strength in the L orientation were
measured for 0.150 inch gauge sheet. The tests were performed in
accordance with ASTM E561-98 "Standard Practice for R-Curve
Determination" supplemented by ASTM B646-97 "Standard Practice for
Fracture Toughness Testing of Aluminum Alloys". The test specimen
was a middle-cracked tension M(T) specimen of full sheet thickness
having a width of 16 inches, an overall length of 44 inches with
approximately 38 inches between the grips, and an initial crack
length, 2a.sub.o, of 4 inches. K.sub.c was calculated in accordance
with ASTM B646 and K.sub.app in accordance with Mil-Hdbk-5J,
"Metallic Materials and Elements for Aerospace Structural
Vehicles." The results are shown in Table 4 and FIG. 4. It is
recognized in the art that K.sub.app and K.sub.c, for alloys having
high fracture toughness, typically increases as specimen width
increases or specimen thickness decreases. K.sub.app and K.sub.c
are also influenced by initial crack length, 2a.sub.o, and specimen
geometry. Thus K.sub.app and K.sub.c values from different alloys
can only be reliably compared from test specimens of equivalent
geometry, width, thickness and initial crack length as was done in
these tests. While the toughness improvements observed in the
invention alloys (Alloys 2-4) correspond to a test specimen of the
type and dimensions noted, it is expected that similar improvements
will be observed in other types and sizes of test specimens,
although the values of K.sub.app and K.sub.c and the absolute
magnitude of the numerical differences may vary for the reasons
just stated.
TABLE-US-00004 TABLE 4 Measured Properties from Sheet: L
orientation Li Amount TYS Toughness-K.sub.app Toughness-K.sub.c
Alloy No. (wt %) (ksi) (ksi in) (ksi in) 1 0 63 122 172 2 0.19 69
128 184 3 0.49 77 131 183 4 0.70 80 131 185 5 1.20 90 87 97
[0053] FIG. 5 is a graph plotting the fracture toughness and
longitudinal tensile yield strength values shown in FIG. 4 against
plant typical and minimum values for conventional alloy 2524 sheet
under similar conditions.
[0054] As shown in FIGS. 2-5, the Al--Cu--Mg based alloys of the
present invention having Li additions of from 0.2 to 0.7 weight
percent possess significantly improved fracture toughness in
comparison with similar alloys containing either no Li or a greater
amount of Li. In addition, the alloys of the present invention
having relatively low levels of lithium achieve significantly
improved combinations of fracture toughness and strength.
EXAMPLE 2
[0055] An ingot of an aluminum-copper-magnesium alloy having the
following composition was cast (remainder is aluminum and
incidental impurities):
TABLE-US-00005 INGOT NO. 1 Si Fe Cu Mn Mg Zn Zr 0.03 0.03 3.24 0.58
1.32 0 0.11
Material fabricated from this ingot is designated Alloy A.
[0056] After this, the remaining molten metal was re-alloyed (i.e.,
alloying again an alloy already made) by adding 0.25% lithium to
create a target addition of 0.25 weight percent lithium. A second
ingot was then cast having the following composition (remainder is
aluminum and incidental impurities):
TABLE-US-00006 INGOT NO. 2 Li Si Fe Cu Mn Mg Zn Zr 0.19 0.03 0.04
3.41 0.61 1.28 0 0.1
Material fabricated from this ingot will be designated Alloy B
hereinafter in this example.
[0057] Ingot No. 3 was created by re-alloying the remaining molten
metal after casting Ingot No. 2 and then adding another 0.25 weight
percent lithium to create a total target addition of 0.50 weight
percent lithium. Ingot No. 3 had the following composition
(remainder is aluminum and incidental impurities):
TABLE-US-00007 INGOT NO. 3 Li Si Fe Cu Mn Mg Zn Zr 0.35 0.04 0.04
3.37 0.6 1.2 0 0.11
Material fabricated from this ingot will be designated Alloy C
hereinafter in this example.
[0058] Ingot No. 4 was created by re-alloying the remaining molten
metal after casting Ingot No. 3 and then adding another 0.26 weight
percent lithium to create a total target addition of 0.75 weight
percent lithium. A fourth ingot was cast having the following
composition (remainder is aluminum and incidental impurities):
TABLE-US-00008 INGOT NO. 4 Li Si Fe Cu Mn Mg Zn Zr 0.74 0.02 0.03
3.34 0.56 1.35 0.01 0.12
Material fabricated from this ingot will be designated Alloy D
hereinafter in this example.
[0059] The four ingots were stress relieved and homogenized. The
ingots were then subjected to a standard presoak treatment after
which the ingots were machine scalped. The scalped ingots were then
hot rolled into four (4) separate 0.7 inch gauge plates using hot
rolling practices typical of 2XXX alloys.
[0060] After the four separate plates were produced, a section of
each of the plates was removed. Each of the four sections were (a)
solution heat treated; (b) quenched; and (c) stretched 1.5%. After
this, eight tensile strength test samples were produced from each
of the treated four (4) sections, making a total of thirty-two
tensile strength test samples. One tensile strength test sample
from each group of eight (there being a total of four plates in
each group) was each subject to eight different aging conditions,
as described in the legend of FIG. 6. After this, tensile yield
strength tests were performed, with the results being shown in FIG.
6. It will be seen that the alloys having lithium additions
exhibited greater strength than those without lithium, which at the
same time exhibiting thermal stability.
[0061] After this, the remainder of three of the four plates (i.e.,
Ingot No. 1 plate, Ingot No. 3 plate and Ingot No. 4 plate) was
each cut into thirds, to form pieces 1, 2 and 3 for each plate, or
a total of 9 pieces. Piece 1 of all three plates were (a) solution
heat treated; (b) quenched; (c) stretched 11/2%; and (d) aged to T8
temper by aging it 24 @ 350.degree. F. These pieces were designated
Alloy A-T8, Alloy C-T8; and Alloy D-T8. Piece 2 of all three plates
were (a) solution heat treated; (b) quenched; (c) stretched 11/2%;
and (d) naturally aged to T3 temper. These pieces were designated
Alloy A-T3; Alloy C-T3; and Alloy D-T3. Finally, Piece 3 of all
three plates were (a) solution heat treated; (b) quenched; (c) cold
rolled 9%; (d) stretched 11/2%; and (e) naturally aged. These
pieces were designated Alloy A-T39; Alloy C-T39; and Alloy D-T39.
It was these pieces which provided the material for all of the
further testing which will be reported herein.
[0062] Referring now to FIG. 7, the tensile yield strength divided
by density for a testing portion of each of the nine pieces
produced above is shown. It can be seen that improvements in the
tensile yield strength to density ratio were found for ancillary
lithium additions.
[0063] Referring now to FIGS. 8-12, the key property of fatigue
crack growth resistance will now be discussed. FIG. 8 is a graph
showing the typical representation of fatigue crack growth
performance and how improvements therein can be shown. The x-axis
of the graph shows the applied driving force for fatigue crack
propagation in terms of the stress intensity factor range,
.DELTA.K, which is a function of applied stress, crack length and
part geometry. The y-axis of the graph shows the material's
resistance to the applied driving force and is given in terms of
the rate at which a crack propagates, da/dN in inch/cycle. Both
.DELTA.K and da/dN are presented on logarithmic scales as is
customary. Each curve represents a different alloy with the alloy
having the curve to the right exhibiting improved fatigue crack
growth resistance with respect to the alloy having the curve to the
left. This is because the alloy having the curve to the right
exhibits a slower crack propagation rate for a given .DELTA.K which
represents the driving force for crack propagation. Fatigue crack
growth testing of all alloys in the L-T orientation was performed
in accordance with ASTM E647-95a "Standard Test Method for
Measurement of Fatigue Crack Growth Rates". The test specimen was a
middle-cracked tension M(T) specimen having a width of 4 inches and
a thickness of 0.25 inch. The tests were performed in controlled
high humidity air having a relative humidity greater than 90% at a
frequency of 25 Hz. The initial value of the stress intensity
factor range, .DELTA.K, in these tests was about 6 ksi in and the
tests were terminated at a .DELTA.K of about 20 ksi in.
[0064] Turning to FIGS. 9-11, it can be seen, that based on the
criteria discussed with respect to FIG. 8, the addition of lithium
substantially increases the fatigue crack growth resistance in the
respective alloys in the T3 and T39 conditions. The fatigue crack
rates for crack driving forces of .DELTA.K equal to 10 ksi in are
summarized in FIG. 12. The percentage improvement in fatigue crack
growth resistance (i.e., percentage reduction in fatigue crack
growth rates) is given at the top of the graph. Alloy C-T3 and
Alloy D-T3 show improvements of 27% and 26%, respectively over
Alloy A-T3 (no lithium additions). The percentage improvements in
fatigue crack growth resistance of Alloy C-T39 and Alloy C-T39 over
Alloy A-T39 (no lithium additions) was 67% and 47%, respectively.
Those skilled in the art will appreciate that fatigue crack growth
rates may be significantly influenced by humidity level and
frequency in moist air environments as a result of an environmental
contribution to fatigue crack growth. Thus, while the fatigue crack
growth improvements exhibited by the invention alloys correspond to
the specific humidity and frequency noted, it is expected that
similar improvements will be observed under other testing
conditions.
[0065] With regard to the T8 alloys, it can be seen that the
lithium additions do not improve the fatigue crack growth
resistance. In the case of artificially aged alloys, aged to peak
strength, the only advantage of lithium additions is in terms of
additional strength and lower density.
[0066] FIGS. 13 and 14 show the fracture toughness R-curves for the
T3 and T39 tempers, respectively, in the T-L orientation. The
R-curve is a measure of resistance to fracture (K.sub.R) versus
stable crack extension (.DELTA.aeff). In addition, Table 5 shows
single-point measurements of fracture toughness for Alloys A, C and
D in the T3, T39 and T8 tempers in terms of K.sub.R25, which is the
crack extension of resistance, K.sub.R, on the R-curve
corresponding to the 25% secant offset of the test record of load
versus crack-opening displacement (COD), and K.sub.Q, which is the
crack extension resistance correspondence to the 5% secant offset
of the test record of load versus COD. K.sub.R25 is an appropriate
measure of fracture toughness for moderate strength, high toughness
alloy/tempers such as T3 and T39, which K.sub.Q is appropriate for
higher strength, lower toughness alloy/tempers such as T8. The
R-curve tests were performed in accordance with ASTM E561-98
"Standard Practice for R-Curve Determination" The test specimen was
a compact-tension C(T) specimen having a W dimension of 6 inches, a
thickness of 0.3 inches and an initial crack length, a of 2.1
inches. The K.sub.R25 value was determined from these same tests in
accordance with ASTM B646-94 "Standard Practice for Fracture
Toughness Testing of Aluminum Alloys". Those skilled in the art
will appreciate that K.sub.R25 values, like K.sub.c and K.sub.app,
depend on specimen width, thickness and initial crack length and
that reliable comparisons between alloys can only be made on test
specimens of equivalent dimensions. Plane strain fracture toughness
testing was performed in the L-T orientation in accordance with
ASTM E399-90 supplemented by ASTM B645-95. The test specimens used
had a thickness of 0.65 inch and the W dimension was 1.5 inches.
The results did not satisfy one or more of the following validity
criteria: B.gtoreq.2.5(K.sub.Q/.sigma..sub.ys).sup.2; (2)
a.gtoreq.2.5(K.sub.Q/.sigma..sub.ys).sup.2; and (3)
P.sub.max/P.sub.Q.ltoreq.1.1, where B, K.sub.Q, .sigma..sub.ys,
P.sub.max, and P.sub.Q are as defined in ASTM E399-90. The previous
discussion regarding K.sub.Q values which are invalid by the above
criteria is also applicable to these results.
TABLE-US-00009 TABLE 5 Strength and Toughness Measurements (Tensile
Longitudinal Properties - Toughness Orientation L-T or T-L) TYS UTS
Elongation K.sub.Q, L-T K.sub.R25, T-L Alloy/Temper (ksi) (ksi) (%)
(ksi in) (ksi in) Alloy A-T3 47.7 65.6 18.6 -- 97.9 Alloy C-T3 51.4
69.8 17.1 -- 107.8 Alloy D-T3 51.1 70.6 17.5 -- not tested Alloy
A-T39 61.2 67.3 11.4 -- 88.8 Alloy C-T39 63.3 70.7 9.3 -- 91.5
Alloy D-T39 65.7 70.5 9.9 -- 97.5 Alloy A-T8 63.7 69.7 12.1 32.4 --
Alloy C-T8 65.9 71.9 11.7 38.7 -- Alloy D-T8 67.8 73.8 10.7 38.9
--
[0067] It will be appreciated that fracture toughness is
significantly improved by the low levels of lithium additions in
accordance with the present invention, in comparison with similar
alloys having either no lithium or greater amounts of lithium.
Furthermore, the lithium additions of the present invention yield
improved toughness at higher strength levels. Therefore, the
combination of fracture toughness and strength is significantly
improved. This is unexpected because lithium additions are known to
decrease fracture toughness in conventional
aluminum-copper-magnesium-lithium alloys.
[0068] While specific embodiments of the invention have been
disclosed, it will be appreciated by those skilled in the art that
various modifications and alterations to those details could be
developed in light of the overall teachings of the disclosure.
Accordingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of the invention
which is to be given the full breadth of the appended claims and
any and all equivalents thereof.
* * * * *