Note: Descriptions are shown in the official language in which they were submitted.
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Al-Cu-Mg alloy suitable for aerospace application
Field of invention
The invention relates to an aluminium wrought alloy, in particular an Al-Cu-
Mg type alloy (or AA2000 series aluminium alloy as designated by the Aluminium
Association). More specifically, the present invention relates to an aluminium
alloy
product having high strength, high fracture toughness exhibiting low crack
propagation and high resistance to intergranular corrosion. Products made from
the aluminium alloy according to the invention are very suitable for aerospace
applications but not limited thereto. The alloy can be processed to various
product
forms such as sheet, thin plate or an extruded product, a forged product, or a
welded product. The aluminium alloy product can be uncoated or coated or
plated
with another aluminium alloy in order to improve desired properties even
further.
Background of the invention
Designers and manufacturers in particular in the aerospace industry are
constantly trying to improve fuel efficiency, product performance and
constantly
trying to reduce manufacturing, maintenance and service costs. One way of
achieving these goals is by improving the relevant properties of the used
aluminium alloys so that a structure made from a particular alloy can be
designed
more effectively or will have a better overall performance. By improving the
relevant material properties for a particular application, also the service
costs can
be significantly reduced by longer inspection intervals of the structure such
as an
aeroplane.
The main application of AA2000 series aluminium alloys in aeroplanes is as
fuselage or skin plate, for which purpose typically AA2024 in the T351 temper
is
used or as lower wing plate for which purpose typically AA2024 in the T351
temper
and AA2324 in the T39 temper is used. For these applications high tensile
strength
and high toughness are required. It is known that these properties of a AA2000
series aluminium alloy can be improved by higher levels of alloying elements
such
as Cu, Mg and Ag.
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However, by increasing the concentration of the mentioned alloying
elements, the resistance to corrosion, in particular also to intergranular
corrosion is
decreased to levels which could limit the applicability of the alloy.
Intergranular corrosion of an aluminium alloy not only affects the integrity
of
the structure for which it is used, in which may corroded grain boundaries may
act
as a nucleus for cracks which propagate under the influence of the alternating
load
during operation of the structure. Therefore, the occurrence of intergranular
corrosion sets limits to the use of aluminium alloys of the AA2000 series with
high
levels of the mentioned alloying elements.
The most commonly used aluminium alloys form the AA2000 series for
aerospace application are AA2024, AA2024HDT ("High Damage Tolerant") and
AA2324.
For newly designed aeroplanes, there is a wish for even better properties of
the aluminium alloys than the known alloys have in order to design aeroplanes
which are more manufacturing and operational cost effective. Accordingly, a
need
exists for an aluminium alloy capable of achieving an improved balance of
properties of the aluminium alloy in the relevant form.
Summary of the invention
The present invention is directed to a AA2000 series aluminium alloy having
the capability of achieving a balance of properties in any relevant product
made of
the alloy that is better than the balance of properties of the variety of
commercially
available aluminium alloys of the AA2000 series, nowadays used for such a
product or of AA2000 series aluminium alloys disclosed so far.
One object of the present invention is to provide an aluminium alloy wrought
product, in particular suitable for aerospace application within the AA2000
series
alloys having an improved balance of high strength and fracture toughness and
high resistance to intergranular corrosion.
Another object of the present invention is to provide an aluminium alloy
wrought product as referred to above which shows a high resistance to
exfoliation
corrosion and stress corrosion cracking.
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A further object of the present invention is to provide an aluminium alloy
wrought product as referred to above which is tolerant to the usual variation
in
process parameters during its manufacturing process.
Yet another object of the present invention is to provide an aluminium alloy
wrought product as referred to above which is weldable and suitable for use in
welded constructions.
Yet another object of the present invention is to provide an aluminium
wrought product as referred to above in a form which is suitable for use in an
aerospace structure.
A further object of the present invention is to provide a method of
manufacturing an aluminium alloy wrought product as mentioned hereinabove.
One or more of the objects and other objects and advantages are obtained
with an aluminium alloy wrought product having high strength and high fracture
toughness and high resistance to intergranular corrosion, the aluminium alloy
comprising in weight %:
Cu 4.1 ¨5.5%
Mg 0.30 ¨ 1.6%
Mn 0.15 ¨ 0.8%
Ti 0.03 ¨ 0.4%
Cr 0.05 ¨ 0.4%
Ag <O.7%
Zr <O.2%
Fe <O.20%, preferably < 0.15%, more preferably < 0.1%
Si <0.20%, preferably < 0.15%, more preferably < 0.1%,
and the balance being aluminium and other impurities or incidental
elements each <O.05%, total <0.15%.
Unless stated otherwise herein all percentages are by weight percent
(wt.%).
Brief description of the drawings
In the drawings:
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Fig. 1 shows a Cr-Ti diagram setting out the Cr-Ti range for the invention
together with narrower preferred ranges.
Figs. 2a, 2b show micrographs of a cross section of a sample of an alloy
according to the invention in T3 temper and of a comparative alloy after
corrosion
testing.
Figs. 3a, 3b show micrographs of a cross section of a sample of an alloy
according to the invention in T6 temper and of a comparative alloy after
corrosion
testing.
Detailed description of the preferred embodiments
The present invention provides an aluminium alloy wrought product having
high strength and high fracture toughness and high resistance to intergranular
corrosion, the aluminium alloy comprising in weight %:
Cu 4.1 ¨ 5.5%
Mg 0.30 ¨ 1.6%
Mn 0.15 ¨ 0.8%
Ti 0.03 ¨ 0.4%
Cr 0.05 ¨ 0.4%
Ag <O.7%
Zr <O.2%
Fe <O.20%, preferably < 0.15%, more preferably < 0.1%
Si <O.20%, preferably < 0.15%, more preferably < 0.1%,
and the balance being aluminium and other impurities or incidental
elements each <O.05%, total <O.15%.
Unless stated otherwise herein all percentages are by weight percent
(wt.%).
It was found that the composition of the aluminium alloy according to our
invention leads to an alloy product having a high resistance to intergranular
corrosion while maintaining a higher strength and higher toughness as compared
to the conventional AA2024 alloy. The alloy product of the invention also
exhibits a
high resistance to exfoliation corrosion and stress corrosion cracking.
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Good results are also obtained in a preferred embodiment of the invention
wherein 0.03% < Ti < 0.3%, preferably 0.05% < Ti < 0.2%. According to this
embodiment good properties can also be achieved with a lower concentration of
Ti.
5
Another embodiment has the range wherein 0.05% < Cr < 0.3%, preferably
0.05% < Cr < 0.15%. In this embodiment in particular good intergranular
corrosion
properties are maintained while at the same time the alloy product is less
quench
sensitive.
A further embodiment has the range wherein 0.1% < Ti + Cr < 0.4%. It has
been found that within the given range Ti and Cr can be substituted by each
other
while maintaining good resistance against intergranular corrosion and good
mechanical properties.
Preferably 0.1%< Ti + Cr < 0.3%. In this embodiment of the invention, still
good properties are achieved with reduced addition of the alloying elements Ti
and
Cr.
In a preferred embodiment the Cu level is selected in the range wherein
4.4% < Cu < 5.5%, more preferably 4.7% < Cu <5.3%.
In a further preferred embodiment the Mg level is selected in the range
wherein 0.3% < Mg < 1.2%, more preferably 0.4% < Mg < 0.75%.
Iron can be present in the range of up to 0.20% and preferably is kept to a
maximum of 0.15%, more preferably to a maximum of 0.1%. A typical preferred
iron level would be in the range of 0.03% to 0.08%.
Silicon can be present in a range of up to 0.20% and preferably is kept to a
maximum of 0.15%, more preferably to a maximum of 0.1%. A typical preferred
silicon level would be as low as possible and would typically be for practical
reasons in a range of 0.02% to 0.07%.
Zirconium can be present in the alloy product according to the invention in
an amount of up to 0.20%. A suitable Zr level is a range of 0.04% to 0.15%. A
more preferred upper limit for the Zr level is 0.13%, and even more preferably
not
more than 0.1113/0.
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Manganese can be added alone or in combination with other dispersoid
formers. A preferred maximum for the Mn level is 0.80% and a preferred minimum
level is 0.15%. A preferred range for the Mn level is in the range of wherein
0.2% <
Mn < 0.5%.
In the prior art, it has been proposed to add Ag to an AA2000 alloy to
improve the resistance against intergranular corrosion. However, there are
drawbacks associated with the addition of Ag. One drawback is that Ag is an
expensive element and its addition raises the price of the alloy. A second
drawback is that also in relation to the price of Ag, any scrap of the alloy
should be
carefully handled and recycled to reclaim the Ag.
Therefore, in another preferred embodiment of the aluminium alloy product
according to the invention, the alloy is free of Ag. In practical terms this
would
mean that Ag is present at the level of an impurity or incidental element, so
at a
level of <0.05%. More preferably the alloy is substantially free of Ag. With
"substantially free" is meant that no purposeful addition of Ag was made to
the
chemical composition but that due to impurities and/or leaking from contact
with
manufacturing equipment, trace quantities of Ag may nevertheless find their
way
into the aluminium alloy product.
In a preferred embodiment of the aluminium alloy product according to the
invention, the alloy has a composition consisting of, in wt.%:
Cu 4.1 ¨5.5%
Mg 0.30 ¨ 1.6%
Mn 0.15 ¨ 0.8%
Ti 0.03 ¨ 0.4%
Cr 0.05 ¨ 0.4%
Zr <O.2%
Fe <O.15%, preferably < 0.1%
Si <O.15%, preferably < 0.1%,
and the balance being aluminium and other impurities or incidental
elements each <O.05%, total < 0.15%, and is substantially free of Ag.
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More preferred narrower ranges for the various alloying elements are set out
herein.
Preferably, the aluminium alloy product according to the invention is in the
T3x, T6x or T8x temper. Dependent on the intended field of application of the
alloy
the appropriate temper is selected to give the alloy product desired
properties.
Temper designations are according to the Aluminium Association.
Because of the improved resistance to intergranular corrosion, at high
strength and fatigue levels, the product is preferably provided in the form of
a
sheet, plate, forging or extrusion for use in an aerospace structure.
The aluminium alloy product according to the invention shows an excellent
balance of properties for application as plate over a wide variety of
thickness,
preferably in the form of a plate having a thickness in the range of 0.7 to 80
mm. In
the plate thickness range of 0.6 to 1.5 mm the aluminium alloy product is also
of
particular interest as automotive body sheet.
In the thickness range of up to 40 mm the properties of the aluminium alloy
product will be excellent for fuselage sheet and preferably the thickness is
up to 25
mm.
In the thickness range of 20 to 80 mm, the properties are excellent for wing
plates, e.g. lower wing plate, when tensile strength and fatigue properties
are of
great importance. In this thickness range, the aluminium alloy products can
also be
used for stringers or to form an integral wing panel and stringer for use in
an
aircraft wing structure.
The aluminium alloy product according to the invention can also be used as
tooling plate or mould plate, e.g. for moulds for manufacturing formed plastic
products for example via die-casting or injection moulding. In this
application
higher Fe and Si levels up to 0.4% for each of these elements are acceptable.
The invention is also embodied in a method for the manufacture of an
aluminium alloy product having high strength and high fracture toughness and a
high resistance to intergranular corrosion comprising the steps of:
a. casting an ingot having a composition according to the invention;
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b. homogenising and/or pre-heating the ingot after casting;
c. hot working the ingot into a worked product by one of more methods
selected from the group consisting of rolling, extruding and forging;
d. optional reheating the worked product, and
e. optional further hot working and/or cold working to a desired work piece
form;
f. solution heat treating said formed work piece at a temperature and
time
sufficient to place into solid solutions substantially all soluble
constituents in
the alloy;
g. quenching the solution heat treated work piece by one of spray quenching
or immersion quenching in water or other quenching media;
h. optionally stretching or compressing of the quenched work piece;
naturally or artificially ageing the quenched and optional stretched or
compressed work piece to achieve a desired temper.
The method according to the invention yields aluminium alloy product
having excellent resistance to intergranular corrosion and having high
strength and
excellent fatigue properties.
The alloy products of the present invention are regularly prepared by
melting and alloying an aluminium alloy product and may be direct chill
("D.C.")
cast into ingots or other suitable casting techniques. Homogenisation
treatment is
typically carried out in one or more steps, each step having a temperature
preferably in the range of 460 C to 535 C. The pre-heat temperature involves
heating the ingot to the hot working temperature which is typically in a
temperature
range of 400 C to 480 C. Working the alloy product can be done by one or more
methods selected from the group consisting of rolling, extruding and forging.
For
the present alloy product hot rolling is preferred. Solution heat treatment is
typically
carried out in the same temperature range as used for homogenisation although
somewhat shorter soaking times can be selected.
In an embodiment of the method according to the invention, artificial ageing
preferably comprises an ageing step at a temperature in the range of 135 C to
210 C, preferably for 5 to 20 hours.
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In another embodiment of the invention the natural ageing preferably
comprises a step of ageing at room temperature during 1 to 10 days.
Preferably, the aluminium alloy product is aged to a temper selected from
the group comprising T3, T351, T39, T6, T651, and T87.
In one embodiment the aluminium alloy product is processed to fuselage
sheet, preferably to fuselage sheet having a thickness of less then 30 mm.
In another embodiment the aluminium alloy product is processed to lower
wing plate.
In a further embodiment the aluminium alloy product is processed to upper
wing plate.
In still a further embodiment the aluminium alloy product is processed to an
extruded product.
In yet a further embodiment the aluminium alloy product is processed to a
forged product.
In yet another embodiment the aluminium alloy product is processed to a
thin plate having a thickness in the range of 15 to 40 mm.
In still another embodiment the aluminium alloy product is processed to a
thick plate having a thickness up to 300 mm.
Examples
In the following, the invention will be further illustrated with reference to
the
drawings and the results of laboratory testing.
Fig. 1 shows a Cr-Ti diagram setting out the Cr-Ti range for the invention
together with narrower preferred ranges.
Fig. 1 shows schematically the ranges for the Cr and Ti content for the alloy
according to the invention. The broadest range is identified by a rectangular
box
with angular points A, B, C, D.
Fig. 1 also shows schematically a preferred range of a balanced content of
both Cr and Ti. The broadest range thereof is identified by a quadrangular box
with
angular points E, F, G, H.
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The point P indicates the Cr and Ti content of a sample of an alloy
according to the invention which was used for testing (also referred to as
Alloy 3 in
the following examples).
The letter Q indicates the Cr and Ti content of two comparative alloys, also
5 used
for testing and also referred to as alloys 1 and 2. Alloys 1 and 2 fall
outside
the invention.
Alloy 2 has, with the exception of the Cr and Ti content, the same chemical
composition as Alloy 3 according to the invention. Alloy 1 has a chemical
composition typical for a conventional AA2024 alloy.
On a laboratory scale three ingots were cast and processed to a plate to
proof the principle of the current invention. The alloy compositions of the
three
alloys are listed in Table 1.
Table 1. Composition of the alloys (wt.%), balance aluminium and inevitable
impurities.
Alloy Cu Mg Mn Ti Zr Zn Fe Si Cr
1 (Ref. AA2024) 4.5 1.5 0.6 0.03 <0.01 0.03 <0.06 <0.04 -
2 5.1 0.58 0.30 0.03 0.14 0.08 <0.06 <0.04 -
3 (Inv. Alloy) 5.1 0.58 0.30
0.1 <0.01 0.08 <0.06 <0.04 0.15
The alloys listed in Table 1 were processed as follows:
- Casting an ingot;
- For Alloy
1: homogenising the ingot at a heating rate of 30 C/h to 465
C, soaking at that temperature for 2 hours followed by further heating at
a rate of 15 C/h to 495 C and soaking at that temperature for 24 hours
followed by air cooling to room temperature.
-
For Alloys 2 and 3; homogenising the ingot at a heating rate of 30 C/h
to 525 C, soaking at that temperature for 24 hours, followed by air
cooling to room temperature.
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- Pre-heating to 420 C.
- Hot rolling from 80 mm to 8 mm.
- Cold rolling from 8 mm to 2 mm to a cold rolled plate.
- Solution heat treating of the cold rolled plate
o For Alloy 1 at 495 C for 30 min.
o For Alloys 2 and 3 at 525 C for 30 min.
- Quenching the cold rolled plate by either direct water quenching or
quenching in water after holding 10 sec. in still air.
- Storing the cold rolled plate for 4 hours at room temperature.
- Stretching and ageing the cold rolled plate by either:
o Stretching and natural ageing for 5 days at room temperature to T3x
tempers (viz. T3, T351, and T39); or
o Stretching and artificial ageing for 12 hours at 175 C to T6x and T8x
tempers (viz. T6, T651, T87).
Samples taken from the cold rolled plates processed as described above,
were subjected to an intergranular corrosion test according to ASTM G110.
The results of the corrosion test are shown in Tables 2, 3, 4 and 5.
In the tables (i) indicates only pitting corrosion and no intergranular
corrosion was observed, (ii) indicates pitting corrosion with slight
intergranular
corrosion at the bottom of the pit was observed, and (iii) indicates that
local
intergranular corrosion was observed.
Table 2. Maximum corrosion depth and type of the alloys in T3x temper.
Alloy T3 T351 T39
1 (Ref. AA2024) 151 (i) 172 (iii) 118 (ii)
2 186 (i) 319 (iii) 127 (ii)
3 60 (i) 121 (i) 71 (i)
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Table 2 shows that a balanced addition of Cr and Ti according to an
embodiment of the invention gives rise to outstanding intergranular corrosion
free
properties in the T3x tempers with a markedly lower pit depth compared with
the
other alloys.
Table 3. Maximum corrosion depth and type of the alloys in the T3x temper with
a
quench delay of 10 seconds after solution heat treatment.
Alloy T3 T351 T39
1 (Ref. AA2024) 188 (ii) 181 (iii) 257 (iii)
2 151 (i) 137 (ii) 311 (iii)
3 101 (i) 90 (i) 56 (i)
This table shows that also after a quench delay of up to 10 s the
outstanding corrosion properties are maintained with the alloy according to
the
invention.
Table 4. Maximum corrosion depth and type of the alloys in the T6x temper and
T8x temper.
Alloy T6 T651 T87
1 (Ref. AA2024) 220 (iii) 203 (iii) 151 (iii)
2 263 (iii) 223 (iii) 188 (iii)
3 159 (ii) 124 (ii) 99 (ii)
As can be seen from this table, improved corrosion properties are achieved
by the balanced addition of Cr and Ti. Only pitting with a very slight
intergranular
corrosion at the bottom of the pit was found.
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Table 5. Maximum corrosion depth and type of the alloys in the T6x and T8x
temper with a quench delay of 10 sec. after solution heat treatment.
Alloy T6 T651 T87
1 (Ref. AA2024) 216 (iii) 257 (iii) 235 (iii)
2 218 (iii) 276 (iii) 165 (iii)
3 175 (ii) 180 (ii) 147 (ii)
For long quench delays of up to 10 sec. the intergranular corrosion resistance
decreases slightly, but the performance is still notably superior to the
performance
of the comparative alloys, with or without quench delay.
The results of the corrosion test are also shown in Fig. 2a, 2b, 3a and 3b.
Figs. 2a, 2b show micrographs of a cross section of a sample of an alloy
according to the invention in T3 temper and of a comparative alloy after
corrosion
testing.
In particular, Fig. 2a shows a micrograph of a cross section of a sample of
comparative alloy 1 (Ref. AA2024) in T3 temper after corrosion testing. The
micrograph clearly shows pitting corrosion and intergranular corrosion to a
depth
of more than 150 pm.
Fig. 2b shows a micrograph of a cross section of a sample of an alloy
according to the invention (alloy 3) also in T3 temper after corrosion
testing. The
samples show only slight pitting, with a maximum depth of 60 pm and no
intergranular corrosion.
Figs. 3a, 3b show micrographs of a cross section of sample of an alloy
according to the invention in T6 temper and of a comparative alloy after
corrosion
testing.
In particular, Fig. 3a shows a micrograph of a cross section of a sample of a
comparative alloy 1 (Ref. AA2024) in T6 temper after corrosion testing. The
micrograph clearly shows local intergranular corrosion, extending to a depth
of
about 220 pm.
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Fig. 3b shows a micrograph of a cross section of a sample of an alloy
according to the invention (alloy 3) also in T6 temper after corrosion
testing. The
sample exhibits pitting with only slight intergranular corrosion to a depth of
less
than 160 pm.
In both tempers, T3 and T6, the corrosion performance of the alloy
according to the invention is considerably better than the corrosion
performance of
the comparative alloy Ref. AA2024.
Mechanical properties of the alloys, cast and processed as described
above, were also measured and the results have been collected in Tables 6 and
7.
Table 6. Tensile properties (L direction) of the alloys in T3 temper.
Alloy Rp Rm A
(MPa) (MPa) (%)
1 (Ref. AA2024) 344 465 17.7
2 328 441 21.7
3 334 466 22.6
From Table 6 it can be seen that in the T3 temper, comparable mechanical
properties can be achieved with an alloy according to the invention as for
reference alloys 1 (Ref. AA2024) and 2.
Table 7. Fracture toughness (L-T direction) of the alloys in T3 temper.
Alloy UPE TS/Rp
(kJ m2)
1 (Ref. AA2024) 276 1.68
2 518 1.98
3 410 2.00
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From Table 7 it can be seen that significantly higher toughness is maintained
with an alloy of the present invention as compared to comparative alloy
AA2024.
Although the present invention has been described with reference to some
specific examples, the scope of the claims should not be limited by the
specific
5 examples disclosed herein, but should be given the broadest interpretation
consistent with the description as a whole.
