Note: Descriptions are shown in the official language in which they were submitted.
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HIGH STRENGTH AI-Zn ALLOY AND METHOD FOR PRODUCING SUCH AN ALLOY PRODUCT
This application is a divisional of Canadian Patent Application No. 2,519,387
filed April
9, 2004 for "HIGH STRENGTH Al-Zn ALLOY AND METHOD FOR PRODUCING SUCH AN
ALLOY PRODUCT".
The present invention relates to a wrought high strength Al-Zn alloy with an
improved
combination= of corrosion resistance and toughness, a method for producing a
wrought high
strength Al-Zn alloy with an improved combination of corrosion resistance and
toughness and
a plate product of such alloy, optionally produced in accordance with the
method. More
specifically, the present invention relates to a wrought high strength Al-Zn
alloy designated by
the 7000-series of the international nomenclature of the Aluminium Association
for structural
aeronautical applications. Even more specifically, the present invention
relates to a new
chemistry window for an Al-Zn alloy having improved combinations of strength,
toughness and
corrosion resistance, which does not need specific ageing or temper
treatments.
It is known in the art to use heat treatable aluminium alloys in a number of
applications
involving relatively high strength, high toughness and corrosion resistance
such as aircraft
fuselages, vehicular members and other applications. Aluminium alloys AA7050
and AA7150
exhibit high strength in T6-type tempers, see e.g. US-6,315,842. Also
precipitation-hardened
AA7x75, AA7x55 alloy products exhibit high strength values in the T6 temper.
The T6 temper
is known to enhance the strength of the alloy, wherein the aforementioned
AA7x50, AA7x75
and AA7x55 alloy products which contain high amounts of zinc, copper and
magnesium are
known for their high strength-to-weight ratios and, therefore, find
application in particular in the
aircraft industry. However, these applications result in exposure to a wide
variety of climatic
conditions necessitating careful control of working and ageing conditions to
provide adequate
strength and resistance to corrosion, including both stress corrosion and
exfoliation.
In order to enhance resistance against stress corrosion and exfoliation as
well as
fracture toughness it is known to artificially over-age these AA7000-series
alloys. When
artificially aged to a T79, T76, T74 or T73-type temper their resistance to
stress corrosion,
exfoliation corrosion and fracture toughness improve in the order stated (T73
being best and
T79 being close to T6) but at cost to strength compared to the T6 temper
condition. An
acceptable temper condition is the T74-type temper, which is a limited over-
aged condition,
between T73 and T76, in order to obtain an acceptable level of tensile
strength, stress
corrosion resistance, exfoliation corrosion resistance and fracture toughness.
Such T74
temper is performed by
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over-ageing the aluminium alloy product at temperatures of 121 C for 6 to 24
hours
and 171 C for about 14 hours.
Depending on the design criteria for a particular airplane component even
small improvements in strength, toughness or corrosion resistance result in
weight
savings, which translate to fuel economy over the lifetime of the aircraft. To
meet
these demands several other 7000-serles type alloys have been developed:
EP-0377779 discloses an improved process for producing a 7055 alloy for
sheet or thin plate applications in the field of aerospace such as upper-wing
members with high toughness and good corrosion properties which comprises the
steps of working a body having a composition consisting of, in wt.%:
Zn: 7.6 - 8.4
Cu: 2.2 - 2.6
Mg: 1.8 - 2.1,
one or more elements selected from
Zr: 0.5 7 0.2
Mn: 0.05 - 0.4
V: 0.03 - 0.2
Hf: 0.03 - 0.5,
the total of said elements not exceeding 0.6 wt.%, the balance aluminium plus
incidental impurities, solution heat treating and quenching said product and
artificially ageing the product by either heating the product three times in a
row to
one or more temperatures from 79 C to 163 C or heating such product first to
one
or more temperatures from 79 C to 141 C for two hours or more or heating the
product to one or more temperatures from 148 C to 174 C. These products show
an improved exfoliation corrosion resistance of "EB" or better with about 15%
greater yield strength than similar sized AA7x50 counter- parts in the '76-
temper
condition. They still have at least about 5% greater strength than their
similarly-
sized 7x50-T77 counterpart (AA7150-T77 will be used herein below as a
reference
alloy).
US-5,312,498 discloses another method for producing an aluminium-based
alloy product having Improved exfoliation resistance and fracture toughness
with
balanced zinc, copper and magnesium levels such that there is no excess of
copper
and magnesium. The method of producing the aluminium-based alloy product
utilizes either a one- or two-step ageing process in conjunction with the
stochiometrically balancing of copper, magnesium and zinc. A two-step ageing
sequence is disclosed wherein the alloy is first aged at approx. 121 C for
about 9
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hours followed by a second ageing step at about 157 C for about 10 to 16 hours
followed by air-cooling. Such ageing method is directed to thin plate or sheet
products, which are used for lower-wing, skin applications or fuselage skin.
US-4,954,188 discloses a method for providing a high strength aluminium
alloy characterised by improved resistance to exfoliation using an alloy
consisting of
the following alloying elements, in wt.%:
Zn: 5.9 - 8.2
Cu: 1.5 - 3.0
Mg: 1.5 - 4.0
Cr: <0.04,
other elements such as zirconium, manganese, iron, silicon and titanium in
total
less than 0.5, the balance aluminium, working the alloy into a product of a
pre-
determined shape, solution heat treating the reshaped product, quenching, and
ageing the heat treated and quenched product to a temperature of from 132 C to
140 C for a period of from 6 to 30 hours. The desired properties of having
high
strength, high toughness and high corrosion .resistance were achieved in this
alloy
by lowering the ageing temperature rather than raising the temperature as
taught
previously from e.g. US-3,881,966 or US-3,794,531.
It has been reported that the known precipitation-hardened aluminium alloys
AA7075 and other AA7000-series alloys, in the T6 temper condition, have not
given
sufficient resistance to corrosion under certain conditions. The T7-type
tempers
which improve the resistance of the alloys to stress-corrosion cracking
however
decrease strength significantly vis-à-vis the T6 condition.
US-5,221,377 therefore discloses an alloy product consisting essentially of
about 7.6 to 8.4 wt.% Zn, about 1.8 to 2.2 wt.% Mg and about 2.0 to 2.6 wt.%
Cu.
Such alloy product exhibits a yield strength, which is about 10 % greater than
its
7x50-T6 counterpart with good toughness and corrosion resistance. The yield
strength was reported to be over 579 MPa with an exfoliation resistance (EXCO)
level of "EC" or better.
US-5,496,426 discloses an alloy as disclosed in US-5,221,377 and a process
including hot rolling, annealing and cold rolling within a preferred cold
reduction
range of 20 % to 70 % which, in turn, is preferably followed by controlled
annealing
thereby displaying characteristics which are better than AA7075-T6
characteristics.
While the AA7075-T6 failed the stress corrosion resistance test (SCC
resistance 40
days in the 35% NaCl alternate immersion test) at 138 MPa the disclosed
processed alloy had a SCC resistance of 241 MPa.
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US-5,108,520 and US-4,477,292 disclose an ageing process for solution-
heat-treated, precipitation hardening metal alloy including three steps of
ageing,
comprising (1) ageing the alloy at one or more temperatures substantially
above
room temperature but below 163 C to substantially below peak yield strength,
(2)
subsequently ageing the alloy at one or more temperatures at about 190 C for
increasing the resistance of the alloy to corrosion and thereafter, (3) ageing
the
alloy at one or more temperatures substantially above room temperature but
below
about 163 C for increasing yield strength. The resultant product displayed
good
strength' properties and a good corrosion performance. However, the three step
ageing procedure is cumbersome and difficult to perform so that the costs for
producing such alloy increase.
It is therefore the object of the present invention to provide an improved Al-
Zn
alloy preferably for plate products with high strength and an improved balance
of
toughness and corrosion performance. More specifically, it is the object of
the
present invention to provide an alloy, which can be used for upper wing
applications
In aerospace with an improved compression yield strength with properties,
which
are better than the properties of a conventional AA7055-alloy in the T77
temper.
It is another object of the invention to obtain an AA7000-series aluminium
alloy, which exhibits strength in the range of T6-type tempers and toughness
and
corrosion resistance properties in the range of T73-type tempers.
It is furthermore an object of the present invention to provide an alloy that
can
be used in an age-creep forming process, which is an alloy, which does not
need a
complicated or cumbersome ageing process.
The present Invention has a number of preferred objects.
As will be appreciated herein below, except otherwise indicated, alloy
designations and temper designations refer to the Aluminum Association
designations in Aluminum Standards and Data and the Registration Records, all
published by the US Aluminum Association. All percentages are in weight
percents,
unless otherwise indicated.
The above mentioned objects of the invention are achieved by using a high
strength Al-Zn allay product with an improved combination of corrosion
resistance
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and toughness, the alloy comprising essentially (in wt.%):
Zn about 6.0 to 9.5
Cu about 1.3 to 2.4
Mg about 1.5 to 2.6
Mn <0.12
Zr <0.20, and preferably 0.05 - 0.15
Cr <0.10
Fe <0.25? and preferably <0.12
Si <0.25, and preferably <0.12
Ti <0.10
Hf and/or V < 0.25, and
optionally Ce and/or Sc < 0.20, especially in a range of 0.05 to 0.15,
other elements each less than 0.05 and less than 0.25 in total, balance
aluminium,
wherein (in weight percent):
0.1 [Cu] + 1.3 < [Mg] < 0.2[Cul 4- 2.15,
and preferably 0.2[Cu] + 1.3 < [Mg] < 0.1[Cu] + 2.15.
Such chemistry window for an AA7000-series alloy exhibits excellent properties
when produced to thin plate products which is preferably useable in aerospace
upper-wing applications.
The above-defined chemistry has properties, which are comparable or better
than existing alloys of the AA7x50 or AA7x55 series in the T77-temper, without
using the above-described cumbersome and complicated 177 ageing cycles. The
chemistry leads to an aluminium product which is not only superior with regard
to
the question of costs but also simpler to produce since less processing steps
are
necessary. Additionally, the chemistry allows new manufacturing techniques
like
age creep forming which is not feasible when a 177-temper alloy is applied.
Even
better, the chemistry as defined above can also be aged to the T77-temper
wherein
the corrosion resistance further improves as compared to the two-step ageing
procedure, which is described herein below, wherein especially the exfoliation
corrosion performance is enhanced.
By way of this invention it has been found.that a selected range of elements,
using a higher amount of Zn and a specific combination of a particular range
of Mg
and Cu, exhibit substantially better combinations of strength, toughness and
corrosion performance such as exfoliation corrosion resistance and stress
corrosion
cracking resistance.
While it has been reported that copper contents should be maintained higher,
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preferably above about 2.2 wt.% in order to improve the exfoliation and stress
corrosion cracking performance, better combinations of strength and density
were
reported to be achievable with relatively low zinc contents.
In this invention, however, it has been found that elevated amounts of zinc
together with an optimised relation of magnesium to copper results in better
strength while maintaining a good corrosion performance and a toughness which
is
better than conventional T77-temper alloys. It is therefore advantageous to
have a
combined zinc, magnesium and copper content in a range of between about 11.50
and 12.50 (in wt.%) without any manganese and below 11.00 in the presence of
manganese, which is preferably between 0.06 and 0.12 (in wt.%).
A preferred amount of magnesium is in a range of 0.2[Cu] + 1.3 < [Mg] <
0.1[Cu] + 2.15, most preferably in a range of 0.2[Cu] + 1.4 < [Mg] < 0.1[Cu] +
1.9.
Copper is in a range of about 1.5 to 2.1, more preferably in a range of 1.5 to
less
than 2Ø The balance of magnesium and copper is important for the inventive
chemistry.
Copper and magnesium are important elements for adding strength to the
alloy. Too low amounts of magnesium and copper result in a decrease of
strength
while too high amounts of magnesium and copper result in a lower corrosion
performance and problems with the weldability of the alloy product. Prior art
techniques used special ageing procedures to ameliorate the strength and low
amounts of magnesium and copper are used in order to achieve a good corrosion
performance. In order to achieve a compromise in strength, toughness and
corrosion performance copper and magnesium amounts (In wt.%) of between about
1.5 and 2.3 have been found to give a good balance for thick alloy products.
However, the corrosion performance is the vital parameter for thin alloy
products so
that less amounts of copper and magnesium must be used, thereby resulting in a
lower strength. Throughout the claimed chemistry of the present invention it
is now
possible to achieve strength levels in the region of a T6-temper alloy while
maintaining corrosion performance characteristics similar to those of T74-
temper
alloys.
Apart from the amounts of magnesium and copper the invention discloses a
balance of magnesium and copper amounts to zinc, especially the balance of
= magnesium to zinc, which gives the alloy these performance
characteristics. The
improved corrosion resistance of the alloy according to the Invention has
exfoliation
resistance properties ("EXCO") of EB or better, preferably EA or better.
These exfoliation properties are measured in accordance with the standards
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for resistance to stress corrosion cracking (''SCC") and exfoliation
resistance
("EXCO") currently required for AA7075, AA7050 and AA7150-products aged to the
T73, T74 and 176, along with typical. performance of T6, tempers. To determine
whether commercial alloys meet the SCC standards, a given test specimen is
subjected to predefined test conditions. Bar-shaped specimens are exposed to
cycles of immersing in a 3.5% NaCI aqueous solution for 10 minutes, followed
by
50 minutes of air drying while being pulled from both ends under a constant
strain
(stress level). Such testing is usually carried out for a minimum of 20 days
(or for
less time should the specimen fail or crack before 20 days have passed). This
test
is the ASTM standard G47 (G47-98) test.
Another preferred SCC-test, conducted in accordance with ASTM standard
G47, (G38-73) is used for extruded alloy products that include thin plate
products.
This test consists of compressing the opposite ends of a C-shaped ring using
constant strain levels and alternate immersion conditions substantially
similar to
those as described above. While an AA7075, AA7050 or AA7150-T6 tempered
alloy fails the SCC test in less than 20 days and while the exfoliation
properties are
EC or ED, the corrosion resistance performance increases with tempers T76-,
T74-,
T73, The exfoliation prope, ties of T73 are EA or better. Specific examples
are
described herein below.
The inventive alloy has a chemistry with a preferred amount of magnesium
and =copper of about 1.93 when the amount (in wt.%) of zinc is about 8.1.
However,
the amount (in wt.%) of zinc is in a range of 6.1 to 8.3, more preferably in a
range
of 6.1 to 7.0 if manganese is lower than 0.05, and preferably lower than 0.02.
Some
preferred embodiments of the present invention are described within the
examples
herein below.
The amount of manganese (in wt.%) is preferably in a range of about 0.06 to
0.12 when the amount of zinc is above 7.6. Manganese contributes to or aids in
grain size control during operations that can cause the alloy microstructure
to
recrystallize. The preferred levels of manganese are lower than in
conventional
AA7000-Series alloys but may be raised when zinc is raised.
The amount of the additional alloying elements Ce and/or Sc is smaller than
0.20, preferably in a range of 0.05 to 0.15, most preferably around 0.10.
A preferred method for producing a wrought high strength Al-Zn alloy product
with an improved combination of corrosion resistance and toughness comprises
the
steps of
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a) casting an ingot with the following composition (in weight percent):
Zn about 6.0 to 9.5
Cu about 1.3 to 2.4
Mg about 1.5 to 2.6
Mn <0.12
Zr <0.20, preferably 0,05 - 0.15
, Cr <0.10
Fe <0.25
Si <0.25
Ti <0.10
Hf and/or V < 0.25, optionally Ce and/or Sc < 0.20,
other elements each less than 0.05 and less than 0.25 in total, balance
aluminium, and wherein (in weight percent):
0.1[Cu] + 1.3 < [Mg] < 0.2[C u] + 2.15,
b) homogenising and/or pre-heating the ingot after casting,
c) hot working the ingot and optionally cold working into a worked product,
d) solution heat treating at a temperature and time sufficient to place
into solid
solution essentially all soluble constituents in the alloy, and
e) quenching the solution heat-treated product by one of spray quenching or
immersion quenching in water or other quenching media.
The properties of the invention may be further achieved throughout a
preferred method which includes artificially ageing the worked and solution
heat-
treated product, wherein the ageing step comprises a first heat treatment at a
temperature in a range of 105 C to 135 C, preferably around 120 C for 2 to 20
hours, preferably around 8 hours, and a second heat treatment at a higher
temperature than 135 C but below 210 C, preferably around 155 C for 4 to 12
hours, preferably 8 to 10 hours.
Throughout such two-step aging treatment a corrosion performance is
achieved which is similar to the corrosion performance of a T76-temper alloy.
However, it is also possible to artificially ageing the worked and heat
treated
product wherein the ageing step comprises a third heat treatment at a
temperature
in a range of 105 C to 135 C for more than 20 hours and less than 30 hours.
This
T77-temper ageing procedure is known and even increases the performance
characteristics as compared to the two-step ageing procedure. However, the two-
step ageing procedure results in thin aluminium alloy products, which are
partially
comparable and partially better than T77-temper products.
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It is furthermore possible to artificially ageing the worked and heat-treated
product with a two-step ageing procedure to a T79- or T76-temper. After
homogenizing and/or pre-heating the ingot after casting it is preferably
advisable to
hot working the ingot and optionally cold working the hot worked products into
a
worked product of 15 mm to 45 mm, thereby obtaining a thin plate.
Such plate product of high strength Al-Zn alloy may be obtained by an alloy
having a composition as described above or being produced in accordance with a
method as described above. Such plate product is preferably useable as thin
aircraft member, more preferably as an elongated structural shape member. Even
to more preferred is a plate product for use as an upper-wing member,
preferably a
thin skin member of an upper-wing or of a stringer of an aircraft.
The foregoing and other features and advantages of the alloys according to
the invention will become readily apparent from the following detailed
description of
preferred embodiments.
Example 1
Tests were performed comparing the performance of the alloy according to
the =present invention and AA7150-T77 alloys. It has been found that the
examples
of the alloy of the present invention show an improvement over conventional
AA7150-T77-temper alloys,
On an industrial scale four different aluminium alloys have been cast into
ingots, homogenized, preheated for more than 6 hours at 410 C and hot rolled
to
mm plates. Thereafter, the plates were solution heat treated at 475 C and
water
quenched. Thereafter, the quenched product was aged by a two-step 179-176
25 ageing procedure. The chemical compositions are set out in Table 1.
Table 1
Chemical composition of thin plate alloys, in wt.%, balance aluminium and
inevitable
impurities, Alloys 1 to 4 with Mn :5. 0.02:
Si Fe Cu Mn Mg Cr Zn Ti Zr
Alloy 1 0.03 0.06 2.23 0.00 2.08 0.00 6.24 0.03
0.10
(7050)
Alloy 2 0.05 0.08 2.05 0.01 2.04 0.01 6.18 0.04
0.11
Alloy 3 0.05 0.09 2.20 0.01 2.30 0.01 = 7.03 0.04
0.10
Alloy 4 0.04 0.07 1.91 0.02 2.13 0.00 6.94
0.03 = 0.11
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The aged alloys were thereafter tested in accordance with the following test
conditions:
Tensile yield strength was measured according to EN 10.002, exfoliation
resistance properties ("EXCO") were measured according to ASTM G-34-97, stress
corrosion cracking ("SCC") was measured according to ASTM G-47-98, all in ST-
direction, Kahn-Tear (toughness) was measured according to ASTM E-399 and the
compression yield strength ("CYS") was measured according to ASTM E-9.
The results of the 179-T76 aged plate products of the four alloys as shown in
Table 1 are shown in Table 2a when compared with conventional AA7150-T77
to tempered alloys, and in Table 2b when compared with conventional AA7150-
T76a74/T6 tempered alloys:
Table 2a
Overview of strength and toughness of the alloys of Table 1 (30 mm plates)
compared to three reference alloys (AA7150-177); alloys 1 to 4 aged to T79-
176:
Rp-L CYS-LT EXCO Kic-LT
(MPa) (MPa) (MPa4m)
Alloy 1 555 565 EC 35.1
Alloy 2 561 604 EA/B 34.5
Alloy 3 565 590 EB 29.1
Alloy 4 591 632 EB 28.9
AA7150-T77 586 EB 28.6
AA7150-T77 579 EB 29.2
AA7150-T77 537 EA 33.2
NF = no failure after 40 days.
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Table 2b
Overview of corrosion performance of the alloys of Table 1 (30 mm plates)
compared to three reference alloys (AA7150-T76, AA7150-T74, AA7150-T6); alloys
1-4 aged to T79-176:
SCC threshold
Alloy 1 NF at 172 MPa
Alloy 2 NF at 240 MPa
Alloy 3 NF at 240 MPa
Alloy 4 NF at 240 MPa
AA7150-T76 117 - 172 MPa
AA7150-T74 240 MPa
AA7150-T6 < 48 MPa
NF = no failure after 40 days.
As can be seen from Table 2a,b alloys 1, 2 and 4 show better
strength/toughness combinations. Alloys 2, 3 and 4 all have an acceptable EXCO
performance wherein alloys 2, 3 and 4 have a significant higher compression
yield
strength than alloy No, 1 (AA7050-alloy). Alloys 2 and 4 exhibit a property
balance
that makes them very suitable for upper-wing applications in aerospace thereby
showing a balance of properties, which is better than those of conventional
7150-
T77 alloys. However, it is still possible to use a T77-temper for the
inventive alloys
as shown in Table 3.
Table 3
Alloys 2 and 4 tempered according to T77 temper conditions, overview of
strength,
toughness and corrosion performance.
Rp-L CYS-LT EXCO- Kic-LT SCC threshold
(MPa) (MPa) (MPaqm)
Alloy 2 585 613 EA 32.2 NF at 240 MPa
Alloy 4 607 641 EA 26.4 NF at 240 MPa
Further SCC testing was performed on the promising alloy No. 4 wherein alloy
4-samples were prepared according to the procedure described in ASTM G-47-98
(standard test methods for determining susceptibility to stress corrosion
cracking of
AA7000-series aluminium alloy products) and exposed to the corrosive
atmosphere
according to ASTM G-44-94 (alternate immersion in accordance with the standard
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practice for evaluating stress corrosion cracking resistance of metals and
alloys by
alternate immersion in 3.5% NaCI solution).
Four different stress levels were chosen for samples of alloy 4 as shown in
Table 4. For each stress level three samples were exposed to the test
environment
(ASTM G-44). One was taken out after 1 week while the other two were exposed
for 40 days. When no cracking had occurred during the exposure the tensile
properties were determined as shown in Table 4.
Table 4
Overview of tensile strength properties of alloy 4 after exposure to four
different stress levels, pre-stress was acting in LT direction.
Alloy 4 Pre-stress Tensile strength [MPa]
[MPe] 1 week 40 days
300 524.3 428.0
340 513.1 = 416.9
380 503.1 424.5
420 515.5 425.1
As can be seen from Table 4, no decrease in residual strength was measured
with increasing load, which means that no measurable stress corrosion appeared
after 40 days as far as tensile strength properties are concerned.
Example 2
When higher strength levels are required and toughness properties are less
important conventional AA7055-T77 alloys are preferred instead of AA7150-T77
alloys as an alloy for upper wing applications. The present invention
therefore
discloses optimised copper and magnesium windows, which show properties equal
or better than conventional AA7055-777 alloys.
11 different aluminium alloys were cast into ingots having the following
chemical composition as set out in Table 5.
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Table 5
Chemical composition of 11 alloys, in wt.%, balance aluminium and inevitable
impurities, Zr = 0.08, Si .= 0.05, Fe = 0.08.
Alloy Cu Mg Zn Mn
1 2.40 2.20 8.2 0.00
2 1.94 233 8.2 0.00
3 1.26 2.32 8.1 0.00
7
4 2.36 ' 1.94 ' 8.1 0.00
' 1.94 1.92 8.1 0.00 -
6 1.30 2.09 8.2 0.00
7 1.92 1.54 8.1 0.00
8 ' 1.27 1.57 8.1 0.00
9 2.34 2.25 8.1 0.07
2.38 2.09 8.1 0.00
,
11 = 2.35 1.53 ' 8.2 0.00
5 Strength and toughness properties were measured after pre-heating the
cast
alloys for 6 hours at 410 C and then hot rolling the alloys to a gauge of 28
mm.
Thereafter, solution heat treating was applied at 475 C and water quenching.
Ageing was done for 8 hours at 120 C and 8 to 10 hours at 155 C (T79-176-
temper). The results are shown in Table 6.
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Table 6
Overview of strength and toughness of 11 alloys according to Table 5 in the
identified directions.
Rp Rm Kq
Alloy L LT L LT L - T
1 628 596 651 633 28.9
2 614 561 642 604 29.3
3 566 544 596 582 39.0
4 614 568 638 604 33.0
595 556 620 590 37.1
6 562 513 590 552 38.6
7 549 509 573 542 41.7
8 530 484 556 522 41.9
9 628 584 644 618 26.6
614 575 631 606 = 28.1
11 568 529 594 568 36.6
5 While alloys 3 to
8 and 11 displayed good toughness properties alloys 1 to 5
and 9 and 10 displayed good strength properties. Hence, alloys 3, 4 and 5 show
a
good balance of strength and toughness so that it is clear to have a copper
content
of above 1.3 and a magnesium content of above 1.6 (in wt.%) when zinc is
present
in an amount of 8.1. Such amounts are lower limits for the copper and
magnesium
10 windows. As can be
seen from Table 6 the toughness will drop to un-acceptable
low-levels when copper and magnesium levels are too high (alloys 1, 2, 9 and
10).
Example 3
The influence of manganese was investigated on the properties of the
inventive alloy. An optimum manganese level was found between 0.05 and 0.12 in
alloys with a high amount of zinc. The results are shown in Tables 7 and 8.
Ail not
mentioned chemistry properties and processing parameters are similar to those
of
Example 2.
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Table 7
Chemical composition of three alloys (Mn-O, Mn-1 and Mn-2), in wt.%, balance
aluminium and inevitable impurities, Zr = 0.08, SI = 0.05, Fe = 0.08.
Alloy Cu Mg Zn Mn
Mn-O 1.94 2.33 8.2 0.00
Mn-1 1.94 = 2.27 8.1 0.06
Mn-2 1.96 2.29 8.2 0.12
Table 8
Overview of strength and toughness of three alloys according to Table 7 in the
identified directions.
Alloy Rp Rm Kq
LT L LT L - T
Mn-O 614 561 " 642 604 29.3
Mn-1 612 562 635 602 31.9
Mn-2 612 560 639 596 29.9
As shown in Table 8 the toughness properties decrease while strength
properties increase. For alloys with high amounts of zinc an optimised
manganese
level is between 0.05 and 0.12.
Example 4
When higher strength levels are required and toughness properties are less
important conventional AA7055-177 alloys are preferred instead of AA7150-T77
alloys as an alloy for upper wing applications. The present invention
therefore
discloses optimised copper and magnesium windows, which show properties equal
or better to conventional AA7055-T77 alloys.
Two different aluminium alloys were cast into ingots having the following
chemical composition as set out In Table 9.
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Table 9
Chemical composition of three alloys, in wt.%, balance aluminium and
inevitable
Impurities, Zr = 0.08, SI = 0.05, Fe = 0.08; (Ref = AA7055 alloy).
Alloy Si Fe Cu Mn Mg Cr Zn Ti Zr
1 0.05 0.09 2.24 0.01 2.37 0.01 7.89 0.04 0.10
2 0.04 0.07 1.82 0.08 2.18 0.00 8.04 003 0.10
Ref. 2.1- 1.8- 7.6-
2.6 2.2 8.4
= Alloys 1 and 2 were tested with regard to their strength properties.
These
properties are shown in Table 10. Alloy 2 has been tempered in accordance with
two temper conditions (T79-T76 and T77). Reference alloy AA7055 has been
measured in T77 temper (M-Ref) while the technical data of an AA7055 reference
alloy in a T77 temper are given as well (as identified by Ref).
Table 10
Overview of strength of the two inventive alloys of Table 9, alloy No. 2 in
two temper
conditions, reference alloy (AA7055) measured (M-Ref) and tech sheet (Ref).
Alloy Temper Rp-L Rp-LT Rp-ST Rm-L Rm-LT Rm-ST
1 T79-T76 604 593 559 634 631 613
2 T79-T76 612 598 571 645 634 618
2 T77 619 606 569 640 631 = 610
Ref 1-77 614 614 634 641 -
M-Ref T77 621 611 537 638 634 599
The toughness properties in LT and TL direction as well as the compression
yield strength properties in L and LT direction as well as the corrosion
performance
characteristics are shown In Table 11.
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= Table 11
Toughness and CYS properties of the two inventive alloys of Table 9 in
different
temper conditions and different test directions, NF = no fallize after 40 days
at
designated stress levels, otherwise days are stated after which the specimen
failed.
Alloy Temper Kw CYS-L CYS-LT EXCO SCC
(L-T) (T-L)
1 T79-T76 21.0 596 621 EC 23,8
2 T79-T76 28.9 27.1 630 660 EB NF at
172MPa
2 - T77 28.8 26.5 628 656 EA NF at
210MPa
Ref 177 28.6 26,4 621 648 EB NF at
103M Pa
M-Ref T77 EB NF at
103MPa
The inventive alloy has similar tensile properties as a conventional AA7055-
T77 alloy. However, the properties in the ST direction are better than those
of the
conventional AA7055-T77 alloy. Also the stress corrosion performance is better
than of an AA055-177 alloy. The inventive alloy can therefore be used as an
inexpensive substitute for AÄ7055-T77 tempered alloys which is also useable
for
age-creep forming, thereby showing a superior compression yield strength and
corrosion resistance.
The scope of the claims should not be limited by the specific embodiments
set forth herein, but should be given the broadest interpretation consistent
with the
description as a whole.
