U.S. patent application number 09/324549 was filed with the patent office on 2002-02-07 for aluminum-zinc alloys having ancillary additions of lithium.
Invention is credited to BRAY, GARY H., DENZER, DIANA K., RIOJA, ROBERTO J., STALEY, JAMES T..
Application Number | 20020015658 09/324549 |
Document ID | / |
Family ID | 23264086 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015658 |
Kind Code |
A1 |
RIOJA, ROBERTO J. ; et
al. |
February 7, 2002 |
ALUMINUM-ZINC ALLOYS HAVING ANCILLARY ADDITIONS OF LITHIUM
Abstract
An aluminum-copper-zinc alloy having ancillary additions of
lithium. The alloy composition includes from about 5 to 13 wt %
zinc and from about 0.01 to 1.0 wt % lithium.
Inventors: |
RIOJA, ROBERTO J.;
(MURRYSVILLE, PA) ; BRAY, GARY H.; (MURRYSVILLE,
PA) ; STALEY, JAMES T.; (DURHAM, NC) ; DENZER,
DIANA K.; (LOWER BURRELL, PA) |
Correspondence
Address: |
CHARLES Q BUCKWALTER ESQ
ALCOA INC
ALCOA TECHNICAL CENTER
100 TECHNICAL DRIVE
ALCOA CENTER
PA
150690001
|
Family ID: |
23264086 |
Appl. No.: |
09/324549 |
Filed: |
June 3, 1999 |
Current U.S.
Class: |
420/532 ;
148/439; 420/541; 420/544; 420/545; 420/546 |
Current CPC
Class: |
C22C 21/10 20130101 |
Class at
Publication: |
420/532 ;
420/541; 420/544; 420/545; 420/546; 148/439 |
International
Class: |
C22C 021/10; C22C
021/12 |
Claims
What is claimed is:
1. An aluminum alloy comprising from about 5 to 13 wt % zinc and
from about 0.10 to 0.99 wt % lithium.
2. The aluminum alloy of claim 1, wherein said lithium content is
from about 0.10 to 0.75 wt %.
3. The aluminum alloy of claim 2, wherein said lithium content is
from about 0.15 to 0.50 wt %.
4. The aluminum alloy of claim 1, wherein said lithium content is
about 0.36 wt %.
5. The aluminum alloy of claim 1, wherein said zinc content is from
about 5.5 to 12 wt %.
6. The aluminum alloy of claim 5, wherein said zinc content is from
about 6 to 11 wt %.
7. The aluminum alloy of claim 1, including a dispersoid forming
alloying element selected from the group consisting of chromium,
vanadium, titanium and zirconium and mixtures thereof in the amount
of from about 0.0 to 0.6 wt %.
8. The aluminum alloy of claim 1, including a dispersoid forming
alloying element selected from the group consisting of manganese,
nickel, iron, hafnium, scandium and mixtures thereof in the amount
of from about 0.0 to 1.0 wt %.
9. The aluminum alloy of claim 1 that is a plate.
10. The aluminum alloy of claim 1 that is an extrusion.
11. The aluminum alloy of claim 1 that is a forged product.
12. The aluminum alloy of claim 1 that is an aerospace structural
component selected from the group consisting of a stringer, a wing
spar and an upper wing section.
13. The aluminum alloy of claim 1 that is a recreational product
selected from the group consisting of a baseball bat, a softball
bat, a shaft for an arrow, a golf club shaft and tubing for
bicycles.
14. The aluminum alloy of claim 1 having a yield strength (L) above
about 80 ksi.
15. The aluminum alloy of claim 14 having a yield strength (L)
above about 85 ksi.
16. The aluminum alloy of claim 1 having a fracture toughness of
above about 25 ksi {square root}{square root over (inch)}.
17. The aluminum alloy of claim 16 having a fracture toughness of
above about 33 ksi {square root}{square root over (inch)}.
18. The aluminum alloy of claim 17 having a fracture toughness of
above about 35 ksi {square root}{square root over (inch)}.
19. An aluminum alloy comprising from about 5 to 13 wt % zinc, from
about 1 to 3 wt % copper, from about 1 to 6 wt % magnesium and from
about 0.10 to 0.99 wt % lithium.
20. The aluminum alloy of claim 19, wherein said lithium content is
from about 0.10 to 0.75 wt %.
21. The aluminum alloy of claim 20, wherein said lithium content is
from about 0.15 to 0.50 wt %.
22. The aluminum alloy of claim 16, wherein said lithium content is
about 0.36 wt %.
23. The aluminum alloy of claim 19, wherein said zinc content is
from about 5.5 to 12 wt %.
24. The aluminum alloy of claim 23, wherein said zinc content is
from about 6 to 11 wt %.
25. The aluminum alloy of claim 24, wherein said copper content is
from about 1.5 to 3.0 wt %.
26. The aluminum alloy of claim 25, wherein said copper content is
from about 1.5 to 2.5 wt %.
27. The aluminum alloy of claim 19, wherein said magnesium content
is from about 1.5 to 4 wt %.
28. The aluminum alloy of claim 27, including said magnesium
content is from about 1.5 to 3.5 wt %.
29. The aluminum alloy of claim 19, including a dispersoid forming
alloying element selected from the group consisting of chromium,
vanadium, titanium and zirconium and mixtures thereof in the amount
of from about 0.0 to 0.6 wt %.
30. The aluminum alloy of claim 19, including a dispersoid forming
alloying element selected from the group consisting of manganese,
nickel, iron, hafnium, scandium and mixtures thereof in the amount
of from about 0.0 to 1.0 wt %.
31. The aluminum alloy of claim 19 that is a plate.
32. The aluminum alloy of claim 19 that is an extrusion.
33. The aluminum alloy of claim 19 that is a forged product.
34. The aluminum alloy of claim 19 that is an aerospace structural
component selected from the group consisting of a stringer, a wing
spar and an upper wing section.
35. The aluminum alloy of claim 19 that is a recreational product
selected from the group consisting of a baseball bat, a softball
bat, a shaft for an arrow, a golf club shaft and tubing for
bicycles.
36. The aluminum alloy of claim 19 having a yield strength (L)
above about 80 ksi.
37. The aluminum alloy of claim 36 having a yield strength (L)
above about 85 ksi.
38. The aluminum alloy of claim 19 having a fracture toughness of
above about 25 ksi {square root}{square root over (inch)}.
39. The aluminum alloy of claim 38 having a fracture toughness of
above about 33 ksi {square root}{square root over (inch)}.
40. The aluminum alloy of claim 39 having a fracture toughness of
above about 35 ksi {square root}{square root over (inch)}.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to aluminum-zinc alloys having
ancillary additions of lithium in order to decrease density while
at the same time increasing the strength of the aluminum-zinc alloy
and maintaining the resistance to corrosion.
[0002] In the aircraft industry, it is well known 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. Molten lithium, however, is a highly
reactive and highly aggressive material, which is difficult to
handle and which is also difficult to alloy with the base alloy.
Because of its high reactivity, any moisture in the presence of the
molten aluminum-lithium can cause explosions. In addition, because
of its highly aggressive nature, special refractories must be used
in casting.
[0003] What is still needed is an aluminum alloy useful for, among
other things, aerospace applications which not only has low
density, high strength and good corrosion resistance, but also good
fracture toughness.
SUMMARY OF THE INVENTION
[0004] The aluminum alloy of the invention has met or exceeded the
above-mentioned needs as well as others. The aluminum alloy
comprises from about 5% to 13% zinc and from about 0.10 to 0.99%
lithium. It has been found, quite surprisingly and unexpectedly,
that the ancillary additions of low levels of lithium to
aluminum-zinc alloys provided a high strength, low density material
that exhibits good fracture toughness and corrosion resistance over
aluminum-zinc alloys without lithium additions and those
aluminum-zinc alloys having lithium additions above 1.0 wt %.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] A full understanding of the invention can be gained from the
following detailed description of the invention when read in
conjunction with the accompanying drawings in which:
[0006] FIG. 1 is a chart showing the tensile yield strength of
various specimens made from aluminum-zinc alloys designated Alloy
A, Alloy B, Alloy C and Alloy D after being subjected to different
aging conditions.
[0007] FIG. 2 is a bar graph showing the improvement in specific
strength for some of the specimens shown in FIG. 1.
[0008] FIG. 3 is a plot of strength versus fracture toughness
comparing a conventional aluminum alloy (AA 7150) to the alloys of
the invention.
[0009] FIG. 4 is a table showing the results of the exfoliation
corrosion testing for the alloys mentioned in the EXAMPLE.
[0010] FIGS. 5A and 5B are Dark Fields (DF) Transmission Electron
Micrographs from samples from alloys A and C, respectively.
DETAILED DESCRIPTION
[0011] For the description of alloy compositions that follow, all
references are to weight percentages (wt %) 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. A range of
about 0.01 to 0.99 wt % lithium, for example, would include all
intermediate values of about 0.02, 0.03, 0.04 and 0.1 wt % all the
way up to and including 0.97, 0.98 and 0.9895 wt % lithium. The
same applies to the other elemental ranges set forth below. 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.
[0012] The present invention relates to an aluminum-zinc alloy
having ancillary additions of lithium. In accordance with the
invention, a wrought aluminum-zinc-lithium alloy is provided which
has less density, improved strength and fracture toughness and
comparable corrosion resistance to aluminum-zinc alloys without
ancillary additions of lithium. The alloys of the present invention
can be fabricated into plate, extruded or forged products. From
these products, integrally stiffened structural parts can be
machined. These parts can provide lower manufacturing costs than
built-up structures and also provide greater design flexibility to
the development of aerospace and space structural components.
Examples of aerospace structural components are stringers, wing
spars and upper wing sections, among others. These alloys provide
high strength and low density while at the same time providing
increased toughness and, surprisingly, no degradation in corrosion
resistance over aluminum-zinc alloys having no lithium
additions.
[0013] The alloys of the present invention can also be used for
recreational products, such as baseball and softball bats, arrow
shafts, golf club shafts and tubing for bicycles. The alloys of the
invention have improved specific properties and modulus of
elasticity thus resulting in recreational products having improved
performance.
[0014] In accordance with the invention, the alloy of the invention
has good strength and good fracture toughness. The yield strength
(L) of the alloys of the invention are preferably above about 80
ksi and more preferably above about 85 ksi. The fracture toughness
of the alloys of the invention are preferably above about 25 ksi
{square root}{square root over (inch)}, more preferably above about
33 ksi {square root}{square root over (inch)} and most preferably
above about 35 ksi {square root}{square root over (inch)}. The
alloy of the invention will also preferably have a combination of
(i) good strength and (ii) fracture toughness of preferably (i)
above 80 ksi and (ii) above 30 ksi {square root}{square root over
(inch)}. Finally, the alloy of the invention will have good
corrosion resistance, as measured by the ANCIT test, which will be
explained below.
[0015] The compositional ranges of the main alloying elements
(zinc, copper, magnesium and lithium) of the improved alloy of the
invention are broadly defined as follows: (1) from about 5 to 13 wt
% zinc; (2) from about 1 to 3 wt % copper; (3) from about 1 to 6 wt
% magnesium; and (4) from about 0.10 to 0.99 wt % lithium. The
balance of the aluminum alloy of the invention contains aluminum
and incidental impurities.
[0016] In addition to aluminum, zinc, copper, magnesium and
lithium, the alloys of the present invention can contain alloying
elements which form dispersoids selected from the group consisting
of chromium, vanadium, titanium and zirconium and mixtures thereof
in the range of from about 0.0 to 0.6 wt % and/or other elements
which form dispersoids such as manganese, nickel, iron, hafnium and
scandium and mixtures thereof in the range of 0 to 1 wt %. [Other
alloying elements, such as silver, silicon and indium and mixtures
thereof in amounts up to about 1.0 wt % can also be added.] The
zinc in the alloy is added to increase the strength of the alloy.
High amounts of zinc can be added as this element exhibits a large
solid solubility in aluminum at intermediate temperatures. Care
should be taken not to exceed maximum solid solubility as this can
lead to low fracture toughness and low damage tolerance.
[0017] The copper is added to increase the strength of the aluminum
base alloy and its resistance to stress-corrosion cracking and
exfoliation corrosion. Copper additions beyond maximum solubility
can lead to low fracture toughness and low damage tolerance.
[0018] The 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.
[0019] The lithium is added to reduce density and to increase
strength. Care should be taken, however, in not adding too much
lithium since exceeding the maximum solubility will lead to low
fracture toughness and low damage tolerance. Lithium additions in
amounts of about 1.5 wt % 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.
[0020] 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. These clusters of
solute act as seeds for the nucleation of strengthening
precipitates. This results in a larger number of precipitates in
the alloys containing the lithium additions. Furthermore, a larger
number of nucleation sites leads to smaller precipitate size.
Finally, as the supersaturation from solid solution is released,
the driving force for the heterogeneous nucleation of deleterious
precipitates at the grain boundaries is reduced. Without limiting
the invention, it is believed that this may contribute to the high
fracture toughness of the lithium containing alloys.
[0021] It should be noted that zinc, copper and magnesium in the
compositional ranges set forth above will be soluble only in
appropriate mixtures as defined by the equilibrium phase diagram.
The ancillary additions of lithium reduce the maximum solubility of
all alloying elements. There is no phase diagram data published for
the quinary system Al--Zn--Cu--Mg--Li. We have found that the level
of reduction in solid solubility due to the lithium additions in
decreasing order is copper, magnesium and finally, zinc.
[0022] Referring to Table I below, the broad, preferred and most
preferred ranges for the main alloying elements will be set
forth.
1TABLE I RANGE Zn Cu Mg Li Broad 5.0-13 1.0-3.0 1.0-6.0 .10-.99
Preferred 5.5-12 1.5-3.0 1.5-4.0 .10-.75 More Preferred 6.0-11
1.5-2.5 1.5-3.5 15-.50 Most Preferred 6.0-11 1.5-2.5 1.5-3.5 .4
[0023] The following example sets forth alloys and resulting
wrought products made in accordance with the invention.
EXAMPLE
[0024] An ingot of an aluminum-zinc alloy having the following
composition was cast:
2 INGOT NO. 1 Si Fe Cu Mn Mg Zn Zr 0.02 0.01 2.08 0.01 1.89 6.41
0.11 (Remainder is aluminum and incidental impurities.)
[0025] Material fabricated from this ingot will be designated Alloy
A hereinafter in this Example.
[0026] 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 wt % lithium. A second ingot was
then cast having the following composition:
3 INGOT NO. 2 Li Si Fe Cu Mn Mg Zn Zr 0.25 0.01 -0- 2.09 -0- 1.87
6.73 0.11 (Remainder is aluminum and incidental impurities.)
[0027] Material fabricated from this ingot will be designated Alloy
B hereinafter in this Example.
[0028] Ingot No. 3 was created by re-alloying the remaining molten
metal after casting Ingot No. 2 and then adding another 0.25 wt %
lithium to create a total target addition of 0.50 wt % lithium.
Ingot No. 3 had the following composition:
4 INGOT NO. 3 Li Si Fe Cu Mn Mg Zn Zr 0.36 0.01 0.01 2.09 -0- 1.87
6.65 0.10 (Remainder is aluminum and incidental impurities.)
[0029] Material fabricated from this ingot will be designated Alloy
C hereinafter in this Example.
[0030] Ingot No. 4 was created by re-alloying the remaining molten
metal after casting Ingot No. 3 and then adding another 0.25 wt %
lithium to create a total target addition of 0.75 wt % lithium. A
fourth ingot was cast having the following composition:
5 INGOT NO. 4 Li Si Fe Cu Mn Mg Zn Zr 0.62 0.02 0.01 2.00 -0- 1.80
6.97 0.10 (Remainder is aluminum and incidental impurities.)
[0031] Material fabricated from this ingot will be designated Alloy
D hereinafter in this Example.
[0032] 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 7XXX alloys.
[0033] After the four (4) separate plates were produced, a section
of each of the plates was removed. Each of the four (4) sections
were (a) solution heat treated; (b) quenched; and (c) stretched
1.5%. After this, eight (8) tensile strength test samples were
produced from each of the treated four (4) sections, making a total
of thirty-two (32) tensile strength test samples. One tensile
strength test sample from each group of eight (8) (there being a
total of four (4) plates in each group) was each subject to eight
(8) different aging conditions, as described in the legend of FIG.
1. After this, tensile yield strength tests were performed, with
the results being shown in FIG. 1. It will be seen that the alloys
having lithium additions exhibited greater strength than those
without lithium, while at the same time exhibiting thermal
stability.
[0034] After this, the remainder of three of the four plates (i.e.,
Ingot No. 1 plate, Ingot No. 2 plate and Ingot No. 3 plate) was
each cut into half to form pieces 1 and 2 for each plate, or a
total of 6 pieces. Piece 1 of all three plates were (a) solution
heat treated; (b) quenched; (c) stretched 1 1/2%; and (d) aged to
for 24 hours at 250.degree. F. and then 6 hours at 350.degree. F.
These pieces were designated Alloy A-T6A; Alloy B-T6A; and Alloy
C-T6A. Piece 2 of all three plates were (a) solution heat treated;
(b) quenched; (c) stretched 1 1/2%; and (d) aged for 24 hours at
250.degree. F. and then 14 hours at 325.degree. F. These pieces
were designated Alloy A-T7B; Alloy B-T7B; and Alloy C-T7B.
[0035] Tensile and plane strain fracture toughness tests pursuant
to ASTM E399 were then performed on these pieces. Also, resistance
to exfoliation corrosion was evaluated using the ANCIT test. The
ANCIT (aluminum-nitrate-chloride immersion test) has been developed
as alternative to ASTM G34, the EXCO test, which is the most
commonly accepted accelerated exfoliation corrosion test method.
ANCIT includes an AlCl.sub.3*6H.sub.2O addition to the standard
EXCO solution which buffers the starting pH from 0.3 to a value
just above 3.0. This higher pH prevents the excessive pitting and
intergranular corrosion that often occurs in EXCO and thus gives a
more clear indication of exfoliation corrosion performance. See, S.
Lee and B. W. Lifka, "Modification Of the EXCO Test Method For
Exfoliation Corrosion Susceptibility In 7XXX, 2XXX and
Aluminum-Lithium Alloys", NEW METHODS FOR CORROSION TESTING OF
ALUMINUM ALLOYS, ASTM STP 1134, V. S. Agarwala and G. M. Ugiansky,
Eds., American Society for Testing and Materials, Philadelphia,
1992.
[0036] Referring now to FIG. 2, the specific strength, i.e.,
tensile yield strength divided by density for some of the pieces
produced above is shown. It can be seen that improvements in the
specific strength to density ratio were found for ancillary lithium
additions.
[0037] FIG. 3 shows the strength/toughness relationship for the
alloys from the current invention. It should be noted that as the
strength increases by the lithium additions, the fracture toughness
is decreased. This trade off between strength and toughness, which
is a generally observed characteristic of aluminum alloys, is also
apparent in data from alloy 7150 (designated by the filled
triangles) from U.S. Pat. No. 5,108,520. 7150 is an aluminum-zinc
alloy commonly used in aircraft construction and is representative
of prior art. The dashed line is a linear fit to the 7150 data
showing the trend of decreasing toughness with increasing strength.
The solid line is a linear fit to the data from Alloys A, B and C.
Invention Alloys B and C with ancillary Li additions exhibit
significantly improved combinations of strength and fracture
toughness with respect to Alloy A without ancillary additions and
7150 representing prior art.
[0038] FIG. 4 shows results from exfoliation corrosion testing. The
ANCIT test was conducted. FIG. 4 shows that the ancillary lithium
additions do not reduce the resistance to exfoliation. This is
surprising because it would have been expected that corrosion
resistance would have been reduced due to the higher affinity of
lithium to form corrosion products.
[0039] FIGS. 5A and 5B show Dark Fields (DF) Transmission Electron
Micrographs from samples from alloys A and C, respectively. These
samples were aged to peak strength (T6A temper) . Note that the
amount of precipitates is larger in the alloy with the lithium
addition (Alloy C). In addition, the size of the precipitates is
smaller for the alloy containing the ancillary lithium addition.
This behavior was unexpected and is likely responsible for the
higher strengths observed with lithium additions.
[0040] It will be appreciated that small amounts of lithium have
not been conventionally added to aluminum base alloys since this
typically leads to manufacturing difficulties such as cracking of
plates during rolling or excessive formation of oxides during
casting. Furthermore, lithium additions less than 1% would not
provide a large reduction in density. Therefore, lithium additions
below 1% had not been made. We found, however, that the small
reduction in density coupled with an unexpectedly strong
strengthening potential yields alloys with up to 21% higher
specific strength. In addition, the plate products made from these
alloys exhibited good fracture toughness and the corrosion
resistance was not adversely affected. This was unexpected since
lithium additions are known to reduce both corrosion resistance and
fracture toughness.
[0041] 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.
* * * * *