Note: Descriptions are shown in the official language in which they were submitted.
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Al-Cu-Li-Mg-Mn-Zn ALLOY WROUGHT PRODUCT
FIELD OF THE INVENTION
The invention relates to an Al-Cu-Li wrought alloy product, more in particular
an Al-Cu-Li-Mg-Mn-Zn type alloy product for structural members. Products made
from this aluminium alloy product are very suitable for aerospace
applications, but
not limited to that. The alloy can be processed to various product forms, e.g.
sheet,
thin plate, thick plate, extruded or forged products.
BACKGROUND TO THE INVENTION
It is generally 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 aluminium
alloys
used in the aircraft construction. This desire led to the addition of lithium,
the lowest
density metal element, to aluminium alloys. Aluminium Association alloys, such
as
AA2090 and AA2091 contain about 2.0% lithium, which translates into about a 7%
weight savings over alloys containing no lithium. Aluminium alloys AA2094 and
AA2095 contain about 1.2% lithium. Another aluminium alloy, AA8090 contains
about 2.5% lithium, which translates into an almost 10% weight savings over
alloys
without lithium.
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 trade-off exists with conventional
alumini-
um-lithium alloys in which fracture toughness decreases with increasing
strength.
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Another important characteristic of aerospace aluminium alloys is fatigue
crack
growth resistance. For example, in damage tolerant applications in aircraft,
in-
creased 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
cata-
strophic propagation. Furthermore, slower crack growth can have an economic
benefit due to the fact that longer inspection intervals can be utilized.
Patent document US-2004/0071586 discloses a broad range for an aluminium
alloy comprising, 3 to 5% of Cu, 0.5% to 2% of Mg, and 0.01% to 0.9% of Li. It
is
disclosed that the Li content should remain at a low level in combination with
having
controlled amounts of Cu and Mg to provide the desired levels of fracture
toughness
and strength. Preferably the Cu and Mg are present in the alloy in a total
amount
below a solubility limit of the alloy. It is known in the art that this patent
document
covers the AA2060 alloy being registered with in Aluminium Association in 2011
and
having a registered alloy composition of:
Cu 3.4 ¨ 4.5
Li 0.6 ¨ 0.9
Mg 0.6 ¨ 1.1
Ag 0.05 ¨ 0.50
Zn 0.30 0.50
Mn 0.10 ¨ 0.50
Zr 0.05 ¨ 0.15.
Patent document WO-2004/106570 discloses an Al-Cu-Li-Mg-Ag-Mn-Zr alloy
for use as a structural member. The alloy has 2.5% to 5.5% Cu, 0.1% to 2.5%
Li,
0.2% to 1% Mg, 0.2% to 0.8% Ag, 0.2% to 0.8% Mn, and up to 0.3% Zr, balance
aluminium. It is known in the art that this patent document covers the AA2050
alloy
being registered with in Aluminium Association in 2004 and having a registered
al-
loy composition of:
Cu 3.2 ¨ 3.9
Li 0.7 ¨ 1.3
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Mg 0.20 - 0.6
Ag 0.20 - 0.7
Mn 0.20 - 0.50
Zr 0.06 - 0.14.
Patent document US-2007/0181229 discloses an aluminium alloy having
2.1% to 2.8% Cu, 1.1% to 1.7% Li, 0.1% to 0.8% Ag, 0.2% to 0.6% Mg, 0.2% to
0.6% Mn, a content of Fe and Si less or equal to 0.1% each, balance impurities
and
aluminium, and wherein the alloy is substantially zirconium free. The low Zr
content
is reported to increase the toughness.
Patent document WO-2009/036953 discloses an Al-Cu-Li-Mg-Ag-Zn-Mn-Zr
alloy for use as an aircraft structural member. The alloy has Cu 3.4% to 5.0%,
Li
0.9% to 1.7%, Mg 0.2% to 0.8%, Ag 0.1% to 0.8%, Mn 0.1% to 0.9%, Zn max.
1.5%, one or more elements selected from the group (Zr, Cr, Ti, Sc, Hf).
Patent document WO-2009/073794 discloses an Al-Cu-Li-Mg-Ag-Zn-Mn-Zr
alloy for use as an aircraft structural member. The alloy has Cu 3.4% to 4.2%,
Li
0.9% to 1.4%, Ag 0.3% to 0.7%, Mg 0.1% to 0.6%, Zn 0.2% to 0.8%, Mn 0.1% to
0.6%, and 0.01`)/0 to 0.6% of a grain structure control element. It is known
in the art
that this patent document covers the AA2050 alloy being registered with in
Alumini-
um Association in 2012 and having a registered alloy composition of:
Cu 3.2 - 4.2
Li 1.0 - 1.3
Mg 0.20 - 0.6
Zn 0.30 - 0.7
Ag 0.20 - 0.7
Mn 0.10 - 0.50
Zr 0.05 - 0.15.
Patent document W02015/082779 discloses an Al-Ci-Li alloy product is the
form of an rolled or forged product having a thickness of 14 to 100 mm, and
wherein
the alloy has 1.8% to 2.6% Cu, 1.3% to 1.8% Li, 0.1% to 0.5% Mg, 0.1% to 0.5%
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Mn with Zr <0.05%, or <0.05% Mn with 0.10% to 0.16% Zr, 0 to 0.5% Ag, <0.20%
Zn, 0.01% to 0.15% Ti, <0.1% Fe, <0.1% Si. The material is in particular
suitable for
manufacturing airplane underwing elements.
A need exists for an aluminium alloy that is useful in aircraft application
which
has an improved thermal stability while providing a good balance in strength
and
fracture toughness.
DESCRIPTION OF THE INVENTION
As will be appreciated herein below, except as otherwise indicated, alloy des-
ignations and temper designations refer to the Aluminium Association
designations
in Aluminium Standards and Data and the Registration Records, as published by
the Aluminium Association in 2015 and known to the skilled person.
For any description of alloy compositions or preferred alloy compositions, all
references to percentages are by weight percent unless otherwise indicated.
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
addi-
tion may vary from the nominal intended amount due to factors such as standard
processing variations as understood by those skilled in the art.
The term "up to" and "up to about", as employed herein, explicitly includes,
but
is not limited to, the possibility of zero weight-percent of the particular
alloying com-
ponent to which it refers. For example, up to 0.07% Fe may include an alloy
having
no Fe.
It is an object of the present invention to provide an improved AlCuLi-type
alloy
wrought product, or at least an alternative product, ideally for structural
members,
having a good balance of high strength and fracture toughness and providing an
increased thermal stability.
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These and other objects and further advantages are met or exceeded by the
present invention in which there is provided an aluminium alloy wrought
product for
structural members having a chemical composition consisting of, in wt.%: Cu
3.2%
to 4.4%, Li 0.8% to 1.4%, Mg 0.20% to 0.90%, Mn 0.10% to 0.8%, Zn 0.20% to
0.80%, one or more elements selected from the group consisting of: (Zr 0.05%
to
0.25%, Cr 0.05% to 0.30%, Ti 0.01% to 0.25%, Sc 0.05% to 0.4%, Hf 0.05% to
0.4%), Ag <0.08%, Fe <0.15%, Si <0.15%, unavoidable impurities and balance al-
uminium.
The alloy wrought product may contain normal and inevitable impurities, typi-
cally each <0.05% and the total <0.15%, and the balance is made by aluminium.
In accordance with the invention it has been found that this compositional
range, and with preferred narrower ranges, offers a good balance of strength,
frac-
ture toughness and corrosion resistance meeting the requirements for
commercial
delivery and also offering a very good thermal stability after being long term
aged or
exposed for 1000 hours at 85 C. These advantages are achieved at least in a T8
condition and by selecting the alloying elements within the defined ranges and
wherein it is an important aspect that the subject alloy has a very low silver
content.
Copper is one of the main alloying elements in the alloy product and is added
to increase the strength of the alloy product. Care must be taken, however, to
not
add too much copper since the corrosion resistance can be reduced. Also,
copper
additions beyond maximum solubility will lead to low fracture toughness and
low
damage tolerance. The upper-limit for the Cu-content is for that reason about
4.4%,
and preferably about 4.2%, and more preferably about 4.10%. A preferred lower-
limit is about 3.6%, and more preferably about 3.75%, and most preferably
about
3.85%.
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Magnesium is another main alloying element in the alloy product and is added
to increase strength and reduce density. Care should be taken, however, to not
add
too much magnesium in combination with copper since additions beyond maximum
solubility will lead to low fracture toughness and low damage tolerance. A
more pre-
ferred lower-limit for the Mg addition is about 0.35%, more preferably 0.38%.
A
more preferred upper-limit is about 0.65%, and more preferably 0.55%. It has
been
found that at a level of above 0.8% the further addition of Mg may result in a
de-
crease in toughness of the alloy product.
Lithium is another important alloying element in the wrought product of this
invention and added together with the copper and magnesium to obtain an im-
proved combination of fracture toughness and strength. A preferred lower-limit
for
the Li addition is 0.9%, and more preferably 1.0%. A preferred upper-limit for
the Li
addition is less than 1.30%. A too high Li content has adverse effect on the
damage
tolerance properties of the alloy product in particular with the relatively
high Cu lev-
els in the alloy product of this invention.
The zinc is purposively added to improve strength and the corrosion resistance
and in addition it has a small effect on the damage tolerance properties of
the alloy
product. In the alloy product the zinc is typically present in a range of
about 0.2% to
0.80%. A preferred lower-limit for the Zn-content is 0.25%. A preferred upper-
limit
for the Zn-content is about 0.70%, and more preferably about 0.65%.
It is an important aspect of the invention that the silver content is less
than
about 0.08% and preferably less than about 0.05%. In an embodiment the silver
content is less than about 0.02%, such that the aluminium alloy is
substantially free
from Ag. With "substantially free" or "essentially free" is meant that no
purposeful
addition was made to the chemical composition but that due to impurities
and/or
leaking from contact with manufacturing equipment, trace quantities of Ag may
nev-
ertheless find their way into the alloy product. For example, less than 0.01%
is an
example of a trace quantity. That the alloy product has a very low Ag content
makes
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the alloy product more cost effective in comparison to the many Al-Cu-Li alloy
known in the art having a purposive addition of Ag, while still offering a
good bal-
ance of engineering properties in combination with a very good thermal
stability.
The manganese addition is to control the grain structure by providing a more
uniform distribution of the main precipitating phases, a reduced grain size
and
thereby further increases strength in particular. The Mn addition should not
exceed
about 0.8% and should be at least about 0.10%. A preferred lower-limit for the
manganese addition is at least about 0.20%, and more preferably at least
0.30%. A
preferred upper-limit for the Mn addition is about 0.6%, and more preferably
about
0.55%. A too high Mn content results in a decrease in both the yield strength
and
fracture toughness.
In addition the alloy product of the present invention contains at least one
ele-
ment selected from the defined group of Zr, Cr, Ti, Sc, and Hf.
It is preferred to add zirconium to the alloy product in a range of 0.05% to
0.25%, and preferably in a range of 0.05% to 0.15%. A too low Zr addition has
an
adverse on the unit propagation energy of the alloy wrought product.
Ti can be added to the alloy product amongst others for grain refiner purposes
during casting of the alloy stock, e.g. ingots or billets. The addition of Ti
should not
exceed 0.25%. A preferred lower limit for the Ti addition is about 0.01%. Ti
can be
added as a sole element or with either boron or carbon serving as a casting
aid, for
grain size control.
The Si content in the alloy product is present as an impurity element of less
than 0.15%, and should be present at the lower-end of this range, e.g. less
than
about 0.10%, and more preferably less than 0.07%, to maintain fracture
toughness
properties at desired levels.
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The Fe content in the alloy product should be less than 0.15%. When the alloy
product is used for aerospace application the lower-end of this range is
preferred,
e.g. less than about 0.1%, and more preferably less than about 0.07% to
maintain in
particular the toughness at a sufficiently high level. Where the alloy product
is used
for non-aerospace applications, such as tooling plate, a higher Fe content can
be
tolerated.
In an embodiment of the alloy product the product is in the form of a rolled,
extruded or forged product, and more preferably the product is in the form of
a
sheet, plate, forging or extrusion as part of an aircraft structural part.
In a preferred embodiment the alloy product is provided in the form of an ex-
truded product.
In a preferred embodiment the alloy product is provided in the form of a plate
product, preferably having a thickness of 12.0 to 175 mm, and preferably of at
least
75 mm. The plate product provides a good balance in engineering properties, in
particular strength and has shown reduced quench sensitivity.
When used as part of an aircraft structural part the part can be for example a
fuselage sheet, upper wing plate, lower wing plate, thick plate for machined
parts,
forging or thin sheet for stringers.
Resistance to intergranular corrosion of the alloy products of the present in-
vention is generally high, for example, typically only pitting is detected
when the
metal is submitted to corrosion testing according to MASTMAASIS (ASTM-69 A2-
85). However, the sheet and light gauge plate may also be clad, with preferred
cladding thickness of from about 1`)/0 to about 8% of the thickness of the
sheet or
plate. The cladding is typically a low composition aluminium alloy.
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In a further aspect of the invention it relates to a method of manufacturing a
wrought aluminium alloy product of an Al-Cu-Li alloy, the method comprising
the
steps of:
a. casting stock of an ingot of an AlCu Li-alloy according to this
invention,
b. preheating and/or homogenizing the cast stock;
c. hot working the stock by one or more methods selected from the group
consisting of rolling, extrusion, and forging;
d. optionally cold working the hot worked stock;
e. solution heat treating ("SHT") of the hot worked and/or optionally cold
worked stock, the SHT is carried out at a temperature and time sufficient to
place
into solid solution the soluble constituents in the aluminium alloy;
f. cooling the SHT stock, preferably by one of spray quenching or immer-
sion quenching in water or other quenching media;
g. optionally stretching or compressing the cooled SHT stock or otherwise
cold working the cooled SHT stock to relieve stresses, for example levelling
or
drawing or cold rolling of the cooled SHT stock; and
h. ageing, preferably artificial ageing, of the cooled and optionally
stretched
or compressed or otherwise cold worked SHT stock to achieve a desired temper.
The aluminium alloy can be provided as an ingot or slab or billet for
fabrication
into a suitable wrought product by casting techniques regular in the art for
cast
products, e.g. DC-casting, EMC-casting, EMS-casting. Slabs resulting from
contin-
uous casting, e.g. belt casters or roll casters, also may be used, which in
particular
may be advantageous when producing thinner gauge end products. Grain refiners
such as those containing titanium and boron, or titanium and carbon, may also
be
used as is known in the art. After casting the alloy stock, the ingot is
commonly
scalped to remove segregation zones near the cast surface of the ingot.
Homogenisation treatment is typically carried out in one or multiple steps,
each
step having a temperature in the range of about 475 C to 535 C. The pre-heat
tem-
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perature involves heating the hot working stock to the hot-working entry
tempera-
ture, which is typically in a temperature range of about 440 C to 490 C.
Following the preheat and/or homogenisation practice the stock can be hot
worked by one or more methods selected from the group consisting of rolling,
extru-
sion, and forging, preferably using regular industry practice. The method of
hot roll-
ing is preferred for the present invention.
The hot working, and hot rolling in particular, may be performed to a final
gauge, e.g. 3 mm or less or alternatively thick gauge products. Alternatively,
the hot
working step can be performed to provide stock at intermediate gauge, typical
sheet
or thin plate. Thereafter, this stock at intermediate gauge can be cold
worked, e.g.
by means of rolling, to a final gauge. Depending on the alloy composition and
the
amount of cold work an intermediate anneal may be used before or during the
cold
working operation.
Solution heat-treatment ("SHT") is typically carried out within the same tem-
perature range as used for homogenisation, although the soaking times that are
chosen can be somewhat shorter. A typical SHT is carried out at a temperature
of
480 C to 525 C for 15 min to about 5 hours. Lower SHT temperatures generally
favour high fracture toughness. Following the SHT the stock is rapidly cooled
or
quenched, preferably by one of spray quenching or immersion quenching in water
or other quenching media.
The SHT and quenched stock may be further cold worked, for example, by
stretching in the range of about 0.5% to 15% of its original length to relieve
residual
stresses therein and to improve the flatness of the product. Preferably the
stretching
is in the range of about 0.5% to 6%, more preferably of about 0.5% to 4%.
After cooling the stock is aged, typically at ambient temperatures, and/or
alter-
natively the stock can be artificially aged.
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The alloy product according to this invention is preferably provided in a
slightly
under-aged T8 condition, in particular a T84 condition, to provide the best
balance
in strength and damage tolerance properties.
A desired structural shape is then machined from these heat treated plate
sections, more often generally after artificial ageing, for example, an
integral wing
spar. SHT, quench, optional stress relief operations and artificial ageing are
also
followed in the manufacture of thick sections made by extrusion and/or forged
pro-
cessing steps.
In one embodiment of the present invention comprising a welding step, the
ageing step can be divided into two steps: a pre-ageing step prior to a
welding op-
eration, and a final heat treatment to form a welded structural member.
The AlCuLi-alloy product according to this invention can be used amongst oth-
ers as in the thickness range of at most 0.5 inch (12.5 mm) the properties
will be
excellent for fuselage sheet. In the thin plate thickness range of 0.7 to 3
inch (17.7
to 76 mm) the properties will be excellent for wing plate, e.g. lower wing
plate. The
thin plate thickness range can be used also for stringers or to form an
integral wing
panel and stringer for use in an aircraft wing structure. When processed to
thicker
gauges of more than 3 inch (75 mm) to about 11 inch (280 mm) excellent
properties
have been obtained for integral part machined from plates, or to form an
integral
spar for use in an aircraft wing structure, or in the form of a rib for use in
an aircraft
wing structure. The alloy products according to the invention can also be
provided in
the form of a stepped extrusion or extruded spar or extruded stiffeners for
use in an
aircraft structure, or in the form of a forged spar for use in an aircraft
wing structure.
When applied in the form of a sheet product the yield strength or proof
strength
of the product should be at least 460 MPa in the L-direction, and preferably
at least
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480 MPa. When applied in the form of an extruded product, e.g. as a stringer,
or in
the form of a plate product the yield strength or proof strength of the
product should
be at least 470 MPa in the L-direction, and more preferably at least 480 MPa.
These
strength levels can be obtained by a selecting the alloy composition within
the
claimed ranges, and preferably within the preferred narrower ranges, in
combination
with the artificial ageing practice to a T8 condition.
In the following, the invention will be explained by the following non-
limitative
example.
Example.
On a laboratory scale 2 alloys have been cast and machined into rolling blocks
of 260x200x80 mm. The alloy compositions are given in Table 1. These were ho-
mogenised for 5h@500 C followed by 10h@510 C. After pre-heating to 480 C the
rolling blocks were hot rolled from 80 mm to a gauge of 30 mm. Then solution
heat-
treated for 30min@500 C followed by a cold water quench and within 30 minutes
thereafter stretched by 2%.
Table 1. Alloy composition (in wt.%) of the alloys processed. Balance is made
by
aluminium and unavoidable impurities and with 0.03% Fe and 0.02% Si.
Alloy Alloying element
Cu Li Mg Mn Zn Zr Ti Ag
A 3.9 1.1 0.4 0.4 0.4 0.11 0.02 0.35
B 3.9 1.1 0.4 0.4 0.4 0.11 0.02 0.0
In order to bring the alloy to a T84 temper the Ag-free alloy was aged for
16h@150 C and the Ag containing alloy for 10.5h@150 C. The difference in
ageing
time to arrive at the T84 temper is due to the difference in silver-content
which has
an effect on the ageing response.
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In order to test the thermal stability the samples in T84 were subsequently
sensitized or aged for 1,000h@85 C.
The materials in the T84 condition and after 1000h@85 C were tested for the
tensile yield strength (TYS) in the L-direction in accordance with ASTM B557M
and
for the fracture toughness (Kc) in the L-T direction in accordance with ASTM
E399.
The results are listed in Table 2 below. The results of Table 2 are also
plotted in Fig.
1.
In addition the samples aged after 1000h@85 C were tested for their corro-
sion resistance in accordance with MASTMAASIS and SCC(ST). All ST-SCC spec-
imens tested at 310 MPa survived without failure for 30 days.
From the results of Table 2 and Fig. 1 it can be seen that the Ag-free alloy B
compared to alloy A provides a significantly lower drop in fracture toughness
after
being sensitized while maintaining a high tensile yield strength in
combination with a
good corrosion resistance. This suggests that alloy B provides an improved
thermal
stability than the similar alloy containing also a purposive addition of
silver.
Table 2. Mechanical properties in T84 condition and
after sensitization for 1,000h@85 C.
Alloy Condition TYS (L) Kic
[MPa] [MPa.Vm]
A T84 537 43.0
A sensitized 591 35.5
B T84 491 41.1
B sensitized 531 40.2
Having now fully described the invention, it will be apparent to one of
ordinary
skill in the art that many changes and modifications can be made without
departing
from the spirit or scope of the invention as herein described.
