Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING A HIGH STRENGTH AI-Zn-Mg-Cu ALLOY
The present invention relates to a method for producing a high strength AI-Zn-
Cu-Mg alloy with an improved corrosion resistance while at the same time
maintaining
a high damage tolerance, a plate product of a high strength AI-Zn-Cu-Mg alloy
produced in accordance with the inventive method having a thickness of more
than 50
mm and an aircraft structural member produced from such alloy. More
specifically, the
present invention relates to a high strength AI-Zn-Cu-Mg 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
thick aluminium alloy product having improved combinations of strength,
toughness
and corrosion resistance, particularly a good strength-corrosion balance.
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 for
example
US-6,315,842. Also precipitation-hardened AA7x75 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 AA7050, AA7x5O and AA7x75 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-ageing these 7000-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 some 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 a T74 temper is performed by over-
ageing
the aluminium alloy product at temperatures of 121 C for 6 to 24 hours and
171 C for
about 14 hours.
CONFIRMATION COPY
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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 life time of the aircraft. To meet
these
demands several other AA7000-series alloys have been developed.
US Patent No. 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 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
patent No. 3,881,966 or US Patent No. 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-a-vis the T6 condition.
US Patent No. 4,863,528 therefore discloses a method for producing an
improved aluminium alloy product, the method including providing an alloy
consisting
essentially of, in wt.%:
Zn: 6-16
Cu: 1-3
Mg: 1.5-4.5,
one or more elements selected from Zr, Cr, Mn, Ti, V, or Hf, the total of said
elements
not exceeding 1.0 wt.%, the balance aluminium and incidental impurities. The
aluminium alloy is solution heat-treated after casting, precipitation-hardened
to
increase its strength to a level exceeding the as-solution heat treated
strength level by
about 30% of the difference between as-solution heat-treated strength and peak-
strength and thereafter subjected to a treatment at a sufficient temperature
or
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temperatures for improving its corrosion resistance properties. Thereafter,
the alloy is
again precipitation-hardened to raise its yield strength and produce a
corrosion
resistant alloy product. The ageing temperatures disclosed therein are between
170 C
and 260 C in a range of 0.2 min. to 3 hours. The artificial ageing step is
thereby
preceded and succeeded by a precipitation-hardening step, also known as T77
ageing. Tensile strength values were obtained of between 460 MPa and 486 MPa
and
yield strength of 400 MPa to 434 MPa.
US Patent No. 5,035,754 discloses a heat-treating method for a high strength
aluminium alloy comprising the steps of solution heat-treating an aluminium
alloy
to consisting essentially of, in wt.%:
Zn: 3-9
Cu: 1 -3
Mg: 1 - 6,
at least one element selected from the group consisting of
Cr: 0.1 - 0.5
Zr: 0.1 -0.5
Mn: 0.2-1.0,
the balance being aluminium, heating of the alloy to a temperature of a lower
temperature zone of 100 C to 140 C, optionally maintaining the alloy at a
temperature
within the lower temperature zone for a certain duration of time, re-heating
the alloy to
a temperature of an upper temperature zone of from 160 C to 200 C, optionally
maintaining the alloy at a temperature within the upper temperature zone for a
second
duration of time, cooling of the alloy to a temperature of a lower temperature
zone and
repeating the above mentioned steps at least twice. Such alloy improves the
properties of AA7075 and AA7050 aluminium alloys by obtaining a good corrosion
resistance and a high strength characteristic. Some samples show a tensile
strength
of 57 to 62 kgf/mm2 and values of the exfoliation rating of P or EA. The
threshold
stress value of the SCC-test was more than 50 kgf/mm2.
EP-0377779 discloses a process for producing an alloy for sheet or thin plate
3o 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,
and one or more elements selected from
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Zr: 0.5-0.2
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 AA7x5O counter parts in the T76-temper condition. They
still have at
least about 5% greater strength than their similarly-sized AA7x5O-T77 counter-
part.
US Patent No. 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-step or two-step ageing process in conjunction with the
stoichiometrically balancing of copper, magnesium and zinc. A two-step ageing
sequence is disclosed wherein the alloy is first aged at about 121 C for
about 9 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
that are
used for lower-wing skin applications or fuselage skin.
There is, however, a demand in the fields of aeronautics to provide high
strength
AA7000-series alloys with a cross-sectional thickness of more than 50 mm for
e.g.
spars or bars of wings and upper-wing skin applications with the above
mentioned
specific mechanical properties such as high strength, high toughness and good
corrosion properties such as resistance to stress corrosion or resistance to
exfoliation
corrosion. These parts such as spars of wings for aircraft are typically
manufactured
from a plate product via machining operations wherein the material property is
a
compression yield strength in the L-direction at S/4 of at least 475 MPa, an
ultimate
tensile strength of at least 510 MPa and an ST (short transverse) elongation
at S/2 of
at least 3.0 %.
EP-1158068A1 discloses a heat-treatable aluminium alloy for producing thick
products having a thickness of more than 12 mm, the alloy is an AI-Zn-Cu-Mg
alloy
with the following composition, in wt.%:
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Zn: 4-10
Cu: 1 - 3.5
Mg: 1-4
Cr: < 0.3
Zr: < 0.3
Si: < 0.5
Fe: < 0.5
other elements < 0.05 each and < 0.15 in total, balance aluminium. It is
disclosed that
it was found that for thick products with an only slightly recrystallized
microstructure, a
high as-cast grain size could lead to a specific microstructure of the
transformed and
heat-treated product which has a beneficial effect on the toughness with no
reduction
in strength or other properties. It is therefore described to cast the alloy
in the form of
a rolling, forging or extrusion ingot such that the as-cast grain size is kept
between
300 and 800 pm.
It is therefore the object of the present invention to provide an improved
method
for producing a high strength AI-Zn-Cu-Mg alloy for thick plate products with
an
improved fatigue crack growth resistance and a high damage tolerance which has
the
aforementioned properties of a compression yield strength (in L-direction at
S/4) of at
least 475 MPa, an ultimate tensile strength of at least 510 MPa and an ST
elongation
at S/2 of at least 3.0 %.
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 obtain a thick plate
alloy,
which can be used to produce structural parts of aircraft such as spars of
wings with
high strength levels and good corrosion resistance properties.
The present invention meets these objects by the characterizing features of
claim 1. Further preferred embodiments are described and specified within the
sub-
claims.
According to the invention there is disclosed a method for producing a high
strength AI-Zn-Cu-Mg alloy with an improved fatigue crack growth resistance
and a
high damage tolerance, comprising the steps of:
a) casting an ingot with the following composition (in weight percent):
Zn: 5.5-9.5
Cu: 1.5-3.5
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Mg: 1.5-3.5
Mn: < 0.25
Zr: < 0.25, preferably 0.06 - 0.16
Cr: < 0.10
Fe: < 0.25, preferably < 0.15
Si: < 0.25, preferably < 0.10
Ti: < 0.10
Hf and/or V < 0.25, and
other elements each less than 0.05 and less than 0.15 in total, balance
aluminium,
b) homogenising and/or pre-heating the ingot after casting,
c) hot working the ingot, preferably by means of rolling, and optionally cold
working, preferably by means of rolling, into a worked product of more than 50
mm thickness,
d) solution heat treating,
e) quenching the solution heat treated product, and artificially ageing the
worked
and heat-treated product, wherein the ageing step comprises a first heat
treatment at
a temperature in a range of 105 C to 135 C for more than 2 hours and less than
8
hours and a second heat treatment at a higher temperature than 135 C but below
170 C for more than 5 hours and less than 15 hours to achieve a product with a
compression yield strength in L-direction at S/4 of at least 475 MPa, an
ultimate tensile
strength of at least 510 MPa and an ST elongation at S/2 of at least 3.0%.
The above mentioned combination of chemistry and ageing practice exhibit very
high strength levels, very good exfoliation resistance and high stress
corrosion
resistance for thick plate products with thickness of more than 50 mm.
Specifically, the
two-step ageing practice of the present invention utilizes a first heat
treatment for 2 to
5 hours, at temperatures in the range of 115 C to 125 C, preferably about 4
hours at
120 C and a second heat treatment for 5 to 15 hours, at temperatures in the
range of
155 C to 169 C, preferably for about 13 hours at temperatures between 161 C
to
167 C.
It will be immediately apparent to the skilled person that in the method
according
to this invention, that after quenching of the solution heat treated product
and before
the artificial ageing practice, the product may optionally be stretched or
compressed or
otherwise cold worked to relieve stresses as known in the art.
Preferred amounts (in wt.%) of magnesium are in a range of 1.5 to 2.5,
preferably in a range of 1.6 to 2.3, and more preferably in the range of 1.90
to 2.10.
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Preferred amounts (in wt.%) of copper are in a range of 1.5 to 2.5, preferably
in a range
of 1.6 to 2.3, and more preferably in the range of 1.85 to 2.10. Preferred
amounts (in
wt.%) of zinc are in a range of 5.9 to 6.2 or in a range of 6.8 to 7.1 or in a
range of 7.8 to
8.1.
Copper and magnesium are important elements for adding amongst others
strength to the alloy. The preferred range of copper and magnesium is above
1.6 wt.%
and lower than 2.3 wt.% since too low amounts of magnesium and copper result
in a
decrease of strength white too high amounts of magnesium and copper result in
a lower
corrosion performance and problems with the weldability of the alloy product.
In order to
achieve a compromise in strength, toughness and corrosion performance each of
copper
and magnesium amounts (in weight %) of between 1.6 and 2.3, with preferred
narrower
ranges set out above, have been found to give a good balance for thick alloy
products. If
the amounts of copper and magnesium are chosen too high the properties
relating to
toughness, stress corrosion and elongation will drop, especially for thicker
products.
Furthermore, it has been found that the balance of copper and magnesium to
zinc, especially the balance of magnesium to zinc is of importance. Depending
on the
amount of zinc the amount (in wt.%) of magnesium is preferably in between 2.4-
0.1 and
1.5+0.1 That means that the amount of magnesium depends on the chosen amount
of
zinc. With an amount of approx. 6 wt.% Zn the amount (in wL%) of magnesium is
between 1.8 and 2.1, when Zn is approx. 7 % the amount of magnesium is between
1.7
and 2.2 and if Zn is approx. 8 % the amount of magnesium is between 1.6 and
2.3.
With the method according to the present invention and the chosen balance of
copper, magnesium and zinc it is possible to obtain a homogenised and/or pre-
heated
ingot after casting which is hot-worked and optionally cold-worked into a
worked product
of preferably more than 60 mm thickness, more preferably in a range of 110 mm
to 160
mm and even up to 220 mm thickness with an improved corrosion performance
which is
at least as good as achievable with the T77 ageing method but less complicated
than the
so-called three-step-ageing temper T77.
The alloy of the present invention is preferably selected from the group
consisting
of AA7O1O, AA7x5O, AA7040, AA7020, AA7x75, AA7349 or AA7xS5 or AA7x85,
preferably AA7055, AA7085.
According to the invention there is disclosed a plate product of high strength
aluminium-zinc-copper-magnesium-alloy produced in accordance with a method as
defined above having a thickness of more than 50 mm, preferably 100 mm to 220
mm.
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Such plate product is preferably a part of an aircraft such as a bar or a spar
of a wing.
Most preferably, the plate product according to the present invention is an
upper-wing
member of an aircraft.
EXAMPLES
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.
On an industrial scale 7 different aluminium alloys have been cast into ingots
having the following chemical composition as set out in Table 1.
Table 1. Chemical composition of thick plate alloys, in wt.%, balance
aluminium and
inevitable impurities, Fe = 0.08 and Si = 0.04, and Zr = 0.10, Alloys 1 to 5
with Mn =
0.02 and Alloys 6 and 7 with Mn = 0.08.
Alloy Alloying Element
Cu Mg Zn Zr
1 2.16 2.04 6.18 0.11
2 2.10 2.00 6.10 0.10
3 2.14 2.04 6.12 0.10
4 1.91 2.13 6.86 0.11
5 2.20 2.30 6.90 0.10
6 2.23 2.50 7.80 0.10
7 1.82 2.18 8.04 0.10
Full scale ingots have been sawn from the ingot slices, homogenised for 12
hours at 470 C and for 24 hours at 475 C, pre-heated for 5 hours at 410 C and
hot-
rolled to a thickness of various gauges as identified in Table 2. Thereafter,
the plates
were solution heat treated for 4 hours at 475 C with subsequent quenching and
a two-
step ageing process, first for 4 hours at 120 C and second for 13 hours at 165
C.
The alloys mentioned in Table 1 were examined with regard to various plate
thickness as identified in Table 2.
Table 2. Overview of strength, elongation and exfoliation properties of
different
thickness of the alloys of Table 1 (S/2 = mid-thickness; S/4 = quarter-
thickness);
EXCO testing at S/10 according to ASTM G34, samples shown for EA-ED
classification.
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Plate Alloy Rp-L Rm-L A-(ST) EXCO
thickness (MPa) (MPa) (%)
(mm) S/4 S/4 S/2
63.5 1 553 590 6 EC
110 2 503 553 4 EA
152 3 495 537 5 EA
152 3* 480 528 5 EA
63.5 4 570 604 3 EC
110 5 515 550 2 EA
110 6 510 565 2 EA
152 7 476 529 3 EA
* aged at 120 for 5 hours and subsequently at 165 C at 15 hours.
As shown in Table 2 the alloys of Table 1 show good compression yield strength
("Rp") in the L-direction of more than 476 MPa, most of them more than 500 MPa
while the ultimate tensile strength ("Rm") in the L-direction is above 529 MPa
for all
alloys and thickness, one example even above 600 MPa for 63.5 mm. The ST-
elongation at position S/2 of all but two alloys is 3 % or above, even up to 6
%.
The exfoliation properties are EA or EC. The exfoliation testing was done in
accordance to ASTM G34 at S/10 position. The exfoliation properties are
similar for
similar ageing steps as shown in Table 3 but surprisingly deteriorate if the
first heat
treatment is longer and the second heat treatment is shorter.
Table 3. Exfoliation properties ("EXCO") of selected alloys of Table I
according to
ASTM G34 means not measured).
Alloy Thickness 6h/120 C + 5h/120 C + 4h/120 C +
6h/1550C 12h/1 550C 13h/1 650C
1 63.5 EC - EC
3 110 - EA EA
5 63.5 EC - -
5 110 EC EA EA
6 110 ED EA EA
7 63.5 EC - EA
Alloy 4 has been tested with a plate thickness of 110 mm. The results of
toughness
and elongation are shown in Table 4.
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Table 4. Toughness and elongation properties of selected alloys of Table 1,
all plates
of 110 mm thickness, ageing according to a two-step method, first heat
treatment at
120 C for 4 hours, second heat treatment at 165 C for 13 hours, alloy 5 with a
copper
content of 2.25; Kic measured according to norm ASTM E399-90 C(T) specimens,
thickness of 38.1 mm (1.5") for SL, SL samples taken from mid-thickness (S/2).
Alloy Rp A Kip
(S/2, ST) (S/2, ST) (S/2, SL)
1 465 5 26.9
3 461 5 26.8
4 465 5 27.1
5 453 2 24.1
6 472 1 19.5
7 482 3 26.4
All above mentioned alloys showed an exfoliation rating of EA for the selected
plate
thickness of 110 mm.
Finally, the stress corrosion properties ("SCC") were examined. First, alloys
1
and 4 were tested with thickness of 152 mm. Two different ageing procedures
were
selected in accordance with Table 5. The load level was 172 MPa. The test
direction is
S-L. Samples were taken from the S/2 position. Table 5 shows the number of
days till
failure was given. After 30 days the test was terminated. "NF" means no
failure after
30 days, "30" means failure after 30 days. In total at least three specimens
are tested
per variant. The test was done in accordance with ASTM G47.
Table 5. SCC-properties for thickness of 152 mm for two alloys.
Alloy 5h/120 C + 12h/165 C 4h/120 C+15h/165 C
I NF, NF, NF NF, NF, NF
4 30, NF, NF NF, NF, NF
Finally, 5 other alloys were tested with regard to the stress corrosion
properties
by using plates of a thickness of 125 mm. Samples were taken from the S-L
direction
at a load level of 180 MPa. Table 6 shows the chemistry and the results of
those alloys
with regard to the stress corrosion properties.
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Table 6. SCC-properties of S-L specimens having a thickness of 125 mm, Fe =
0.08,
Si = 0.04, and Zr = 0.10.
Alloy Cu Mg Zn 4h/120 C+13h/165 C
A 1.7 1.8 7.4 NF, NF, NF
B 2.3 1.8 7.5 NF, NF, NF
C 2.25 2.5 7.65 15, NF, NF
D 1.8 2.45 8.0 15, 20, NF
E 2.3 2.4 8.1 20, 25, NF
As can be seen from Table 6 the toughness of the inventive alloy is controlled
by the
copper and magnesium levels while zinc has an influence in particular on the
tensile
properties. The preferred balance of each of copper and magnesium is in
between 1.6
and 2.0 wt.%.
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 scope of the invention as herein described.
