Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINUM ALLOY PRODUCT
This invention relates to an aluminum alloy
product, and more particularly to aluminum alloy
products developed for aerospace applications.
Nearly all commercial airplanes have fuselage
skins made of Alclad 2024-T3. The base metal, 2024-T3
sheet, has the necessary strength and damage tolerance
for aerospace applications, but suffers from
susceptibility to pitting and/or intergranular
corrosion attack. To compensate for that problem, the
base metal is effectively isolated from the environment
by a cladding layer, a paint or coating system or a
combination of both.
An alcladding process involves combining a
thin layer of an aluminum alloy anodic relative to
2024-T3 on both sides of 2024-T3 sheet. These layers
act as a barrier and also afford galvanic protection to
the 2024-T3 in case the cladding is damaged. In cases
where these layers are intentionally removed by
machining or chemical milling to save weight, 2024-T3
sheet may be protected w3.t:h coatings and/or by
anodization.
While the above protection systems are
generally effective, they have some notable
disadvantages. The Alclad layer contributes little
with respect to strength, adds weight to the sheet and
can act to initiate fatigiie cracks. Other coating
systems may also add weight and, if damaged, fail to
protect 2024-T3 base metal. Surfaces that are anodized
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are brittle and can act to initiate cracks. Another
disadvantage of 2024-T3 sheet is its relatively high
density (0.101 lb/in3).
It is a principal interest of this invention
to provide a damage tolerant aluminum alloy product
useful for airplane application including fuselage
skin, the lower wing sections, stringers and/or
pressure bulkheads. The alloys of this invention have
a relatively low density, good corrosion resistance and
a good combination of strength and toughness so as to
obviate cladding, painting and/or other base metal
protection systems.
It is another main interest of this invention
to provide an aluminum alloy product for damage
tolerant applications, such as fuselage skins, that has
sufficient strength primarily generated through strain
hardening of a generally uniform matrix composition, as
opposed to precipitating particles that are
electrochemically different from the matrix as in 2024-
T3 aluminum.
It is still a further interest of this
invention to provide a lower density alloy than 2024-T3
aluminum for potential weight savings in commercial
aircraft. With a lower density alloy, increased fuel
efficiency and/or increased payload capacity will
result. It is yet another object to provide an
aluminum alloy system that retains superior performance
over the long (generally 20 to 40 year) life of
commercial aircraft. It is also an interest of this
invention to provide such a material with improved
resistance to fatigue crack initiation.
One embodiment of the present invention
pertains to an aluminum alloy product comprising an
alloy composition which includes about 3-7 wt %
magnesium, about 0.03-0.20 wt % zirconium, about 0.2-
1.2 wt % manganese, up to 0.15 wt % silicon and about
0.05-0.5 wt % of a dispersoid-forming element selected
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from the group consisting of: scandium, erbium,
yttrium, gadolinium, holmium and hafnium, the balance
being aluminum and incidezital elements and impurities.
It is preferred that the ciispersoid-forming element is
scandium. This alloy composition is also preferably
zinc-free and lithium-freEa.
For the descript:ion of alloy compositions
that follows, all references are to weight percentages
(wt %) unless otherwise ir.Ldicated. When referring to
any numerical range of values, such ranges are
understood to include each and every number and/or
fraction between the stated range minimum and maximum.
A range of about 0.05-0.5 wt % scandium, for example,
would include all intermediate values of about 0.06,
0.07, 0.08 and 0.1 wt % all the way up to and including
about 0.48, 0.49 and 0.4995 wt % scandium. 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 eleaLents and/or impurities may
find their way into a desired end product.
The alloys of the invention are based on the
Al-Mg-Sc system and are of sufficient corrosion
resistance so as to obviate cladding or other
protection systems. Strength in these alloys is
primarily generated throug=h,strain hardening of a metal
matrix which is generally uniform in composition.
Combinations of strength and damage tolerance
properties sufficient for fuselage skin applications
can be obtained by an appx=opriate selection of
composition, deformation processing and subsequent
stabilization treatments.
It has been found that the Al-Mg-Sc alloy
materials of this invention display adequate tensile
strength properties and toughness indicators together
with excellent resistance to intergranular (or grain
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boundary) corrosion. These materials, also demonstrate
good resistance to exfoliation attack and excellent
stress corrosion cracking ("SCC") resistance during
alternate immersion in an NaCl solution tested
according to ASTM G-47.
A principal alloy embodiment of this
invention comprises an alloy composition which includes
about 3-7 wt % magnesium, about 0.03-0.2 wt %
zirconium, about 0.2-1.2 wt % manganese, up to 0.15
wt % silicon, and about 0.05-0.5 wt % of a dispersoid-
forming element selected from the group consisting of:
scandium, erbium, yttrium, gadolinium, holmium and
hafnium, the balance being aluminum and incidental
elements and impurities. On a more preferred basis,
the aluminum alloy composition contains about 3.5-6
wt % magnesium; about 0.06-0.12 wt % zirconium; about
0.4-1 wt % manganese, up to 0.08 wt % silicon and about
0.16-0.34 wt % scandium. Most preferably, the aluminum
alloy composition consists essentially of about 3.8-5.2
wt % magnesium; about 0.09-0.12 wt % zirconium, about
0.5-0.7 wt % manganese, up to 0.05 wt % silicon and
about 0.2-0.3 wt % scandium. Preferred embodiments of
this aluminum alloy are also substantially zinc-free
and lithium-free.
While not being limited to any particular
theory, it is believed that this invention manages to
impart significantly higher strengths and greater
corrosion resistance to fuselage skin sheet stock
through the addition of certain rare earths or rare
earth "act-alikes", such as scandium, by causing rare
earth-rich precipitates to form. These precipitates
have the ability to store and resist loss of strength
arising from plastic deformation. Because of the
relatively small size and fine distribution of these
particles, recovery and recrystallization of the
resulting alloy are also inhibited.
The invention alloy is more temperature
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resistant than the same alloy devoid of scandium or
acandium-like additives. By "temperature resistant",
it is meant that a large portion of the strength and
structure imparted by working this alloy is retained in
the fuselage skin sheet erid product, even after
exposure to one or more higher temperatures, typically
above about 450 F., such as during subsequent rolling
operations or the like.
When referring to the main alloying
components of this invention, it is understood that a
remainder of substantially aluminum may include some
incidental, yet intentionally added elements which may
affect collateral properties of the invention, or
unintentionally added impurities, neither of which
should change the essential characteristics of this
alloy. With respect to the main alloying elements of
this invention, it is believed that magnesium
contributes to strain hardening and strength.
Zirconium additions are believed to improve the
resistance of scandium precipitates to rapid growth.
Scandium and zirconium sezve yet another purpose. When
added to aluminum-magnesium alloys of the type
described herein, scandium is believed to precipitate
to form a dispersion of fine, intermetallic particles
(referred to as "dispersoids"), typically of an A13X
stoichiometry, with X being either Sc, Zr or both Sc
and Zr. A13(Sc, Zr) dispe:rpoids impart some strength
benefit as a precipitation-hardening compound,.but more
importantly, such dispersoids efficiently retard or
impede the process of recovery and recrystallization by
a phenomenon sometimes called the "Zener Drag" effect.
[See generally, C.S. Smith, TMS-AIMF, J_U, 15(1948).)
It is believed to result as follows: Scandium
dispersoids are very small. in size, but also large in
number. They generally act as "pinning" points for
migrating grain boundariesi and dislocations which must
bypass them for metal to soften. Recrystallization and
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recovery are the principal metallurgical processes by
which such strain hardenable alloys soften. In order
to "soften" an alloy having a large population of
A13(Sc, Zr) particles, it is necessary to heat the
material to higher temperatures than would be required
for an alloy not having such particles. Put another
way, when strain-hardened and annealed under identical
conditions, a sheet product that contains A13(Sc, Zr)
dispersoids will have higher strength levels than a
comparable alloy to which no scandium was added.
For fuselage skin sheet stock and other
aerospace applications, this invention exhibits an
ability to resist softening during the high temperature
thermal exposures usually needed to roll sheet
products. In so doing, the invention alloy will retain
some of the strength acquired through rolling. Other
scandium-free alloys would tend to retain less strength
through rolling, thus yielding a lower strength final
product. An added benefit of zirconium is its ability
to limit the growth of these A13X particles to assure
that such dispersoids remain small, closely spaced and
capable of producing a Zener Drag effect.
Although it is preferred to limit silicon in
the aluminum alloy, it is inevitable that silicon from
the refractory will be included. in commercial
practice, over 80% of an alloy is obtained from scrap,
thus adding to the presence.of silicon. The alloy of
this invention may contain up to 0.15 wt % silicon with
up to 0.08 wt % being preferred and 0.05 wt % or less
being most preferred.
In a similar manner, while copper is not an
intentional elemental additive, it is a mildly soluble
element with respect to this invention. As such, the
alloy products described herein may accommodate up to
about 0.25 wt % copper or preferably about 0.15 wt % Cu
or less.
The aluminum alloy product of this invention
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is especially suited for applications where damage
tolerance is required. Specifically, such damage
tolerant aluminum products are used for aerospace
applications, particularly fuselage skin, and the lower
wing sections, stringers or pressure bulkheads of many
airplanes.
The following example is provided to further
illustrate the objectives and advantages of this
invention. It is not intE:nded to limit the scope of
this invention in any manrier, however.
E;KAMPLE
This example refers to the following main
additions to an aluminum based alloy of the present
invention:
Mg Mn Sc Zr
Alloy A 4.0 --- 0.23 0.10
Alloy B 4.1 0.62 0.23 0.09
Alloy C 6.5 --- 0.23 0.09
with the balance of each alloy being aluminum,
incidental elements and impurities.
All of the aforementioned alloys were direct
chill (or "DC") cast as 2-1/2 x 12 inch ingots and the
rolling surfaces scalped therefrom. Alloy A was not
homogenized. Alloy B was homogenized for 5 hours at
550 F. followed by 5 hours at 800 F. Alloy C was
homogenized for 5 hours at: 500 F., then for 6 more
hours at 750 F. The scalped ingots were heated to
550 F. for 30 minutes and cross rolled approximately
50% to a nominal thicknessi of 1 inch. Alloys A and B
were then reheated to 550 F. and rolled to a final
nominal thickness of 0.1 inch. Mechanical properties
for each alloy were then e:valuated after a
stabilization treatment of'. 5 hours at 550 F. The ingot
of Alloy C was heated to 700 F. and cross rolled to
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approximately 1 inch thick. This slab was then
reheated to 530 F. and rolled to 0.5 inch thickness.
The resulting plate from Alloy C was then aged for 15
hours at 500 F. until the electrical conductivity
increased to 28.0% of the International Annealed Copper
Standard (or "IACS"). Alloy C plate was then heated
again to 500 F. and warm rolled to a final thickness of
0.1 inch before being subjected to its final heat
treatment of 2 hours at 500 F.
Table I reports the physical, mechanical
property and corrosion data available for the foregoing
samples of Alloys A, B and C, then compares them with
typical values for 2024-T3 aluminum, 6013-T6 aluminum
and another potential fuselage skin material known
coamercially as Alcoa's C-188 product as manufactured
in accordance with V.S. Patent No. 5,213,639.
The materials of this invention display
adequate tensile strength properties. The toughness
indicators of Alloy A and B, per center notch toughness
and fatigue crack growth (or "FCG") data also strongly
indicate that these materials will exhibit good
inherent toughnesses as well. The resistance to grain
boundary corrosion attack of the present invention is
also noteworthy. A standard test for measuring such
attacks in Al-Mg base alloys is the ASSET (or ASTM G-
66) test after a "sensitization" treatment at 212 F.
The subject materials demonstrated good resistance to
exfoliation attack in that test with only Alloy B
showing any evidence of exfoliation, and even then to
just an EA level. By comparison, other materials
showed some pitting attack (P) with minimal blistering.
The invention materials also showed excellent SCC
resistance during alternate iaimersion testing using an
NaCl solution.
TABLE 1
O
glclad Alclad
2024-T3 C-188 6013-T6
Property Typicals Typicals Typicals Alloy A Alloy B Alloy C
Strength (ksi)
UTS L 66 66 57 56 61.4 63.7
LT 65 57 57 55 60.4 64.6
45 >68.5 --- --- 51 55.6 60.0
+( ,
E N
TYS L 55 55 53 48 48.2 51.8
LT 45 45 51 49 48.9 53.0
\o
45 >50.4 - -- -- - 45 45 49.5 ~O ~
= o
Elong. L 14 14 11 11.0 12.0
f o
LT 18 18 11 16 16.2 12.0
45 >21 --- --- 22 18.8 12.0
Density (lb/cu in) 0.101 0.100 0.098 0.0958 0.0963 0.0943
Toughness (ksifin) 6" panel/16" 6" panel 6" panel
Kc T-L --- --- 108/147 91.4 97.2
Kapp T-L --- --- 62/94 60.5 62.8
lA clad Alclad
O
2024-T3 C-188 6013-T6 w
Go
Property Typicals Typicals Typicals Alloy A Alloy B Alloy C 'I~i
Fatigue 00
Life at 25 ksi
(Kt=3; R=0.1) --- --- 3 x 104 3 x 104 2 x 104
DR at 10(-4) T-L 20 24 --- 23/24 21 15
Modulus (Msi)
Tension 10.6 10.6 9.9 10.1 10.4 10 0
~
I
~..
Corrosion (after lwk at 212 F.)
ASSET (24 hrs) ASTM G-66 EC EC PA EA p Exco (96 hrs) ASTM G-34 ED ED N --- N
MASTMASSIS (4 wks) ASTM G-85 ED ED N --- EA
SWAAT (2 wks) ASTM G-85 --- --- --- EC ---
SCC1 ASTM G-47
(180 day exposure) --- --- 0/3 0/3 0/3
NOTE:
-+
1. SCC: (# failures/# samples) Transverse Orientation, 75% Y.S. (after 1 wk at
212 F.)
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It will be appreciated that an improved
aluminum alloy for aerospace applications has been
disclosed. This aluminum alloy has low density, good
corrosion resistance and a good combination of strength
and toughness by comparison to conventional fuselage
skin materials. While specific embodiments of the
invention have been disclosed, those skilled in the art
will appreciate that various modifications and
alterations to these details could be developed in
light of the overall teachings of this disclosure.
Accordingly, the particular arrangements disclose 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 equivalents
thereof.
