Note: Descriptions are shown in the official language in which they were submitted.
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High strength, high toughness Al-Zn alloy product and method for producing
such
product
FIELD OF THE INVENTION
The present invention relates to a high-strength high-toughness Al-Zn alloy
wrought
product with elevated amounts of Zn for maintaining good corrosion resistance,
and to a
method for producing such a high-strength high-toughness Al-Zn alloy product
and to a
plate product of such alloy. More specifically, the present invention relates
to a high
strength, high toughness Al-Zn alloy designated by the AA7000-series of the
international
nomenclature of the Aluminum 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 and toughness by maintaining
good
corrosion resistance, which does not need specific ageing or temper
treatments.
BACKGROUND OF THE INVENTION
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. 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 aerospace 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 the cost of strength compared to the T6
temper
condition. A more 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
CONFIRMATION COPY
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toughness. Such 174 temper is performed by over-ageing the aluminium alloy
product at
temperatures of 121 C for 6 to 24 hours and followed by 171 C for about 14
hours.
Depending on the design criteria for a particular aircraft component even
small
improvements in strength, toughness or corrosion resistance result in weight
savings,
which translate amongst others to fuel economy over the life time of the
aircraft. To meet
these demands several other 7000-series alloys have been developed.
For example each of EP-0377779, US-5,221,377 and US-5,496,426 disclose alloy
products and an improved process for producing an 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, about in wt.%: Zn 7.6 to 8.4, Cu 2.2 to 2.6, Mg 1.8
to 2.1 or 2.2,
and one or more elements selected from Zr, Mn V and Hf, the total of the
elements not
exceeding 0.6 wt.%, the balance aluminium plus incidental impurities, solution
heat
treating and quenching the 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
and heating the product to one or more temperatures from 148 C to 174 C. These
products are reported to have an improved exfoliation corrosion resistance of
"EB" or
better with about 15% greater yield strength than similar sized 7x50 counter-
parts in the
176-temper condition. They still have at least about 5% higher strength than
their similar-
sized 7x50-T77 counterpart (7150-T77 will be used herein below as a reference
alloy).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved Al-Zn alloy
preferably
for plate products with high (compressive) strength and high toughness.
Corrosion
resistance should not deteriorate.
More specifically, it is an object of the present invention to provide an
alloy product
which can be used for upper wing applications in aerospace with an improved
compression yield strength and a high unit propagation energy 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 another object of the invention to provide a method of manufacturing the
aluminium alloy product according to this invention.
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The present invention meets one or more of these objects by the characterizing
features of the independent claims. Further preferred embodiments are
described and
specified within the dependent claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As will be appreciated hereinbelow, 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.
One or more of the above mentioned objects of the invention are achieved by
using
an Al-Zn alloy product with an improved combination of high toughness and high
strength
by maintaining good corrosion resistance, said alloy comprising, and
preferably consisting
of, (in weight percent):
Zn 6.0 to 11.0
Cu 1.4 to 2.2
Mg 1.4 to 2.4
Zr 0.05 to 0.15
Ti <0.05,
Hf and/or V < 0.25,
optionally Sc and/or Ce 0.05 to 0.25, and
optionally Mn 0.05 to 0.12,
and inevitable impurities and balance aluminium, preferably other elements
each less than
0.05 and less than 0.50 in total, and wherein the alloy product has a
substantially fully
unrecrystallized microstructure at the position T/10 of the finished product.
Such chemistry window for an AA7000-series alloy exhibits excellent properties
when produced to relatively thin plate products, and which is preferably
useable in
aerospace upper-wing applications having gauges in the range of 20 mm to 60
mm.
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 T77 three-step ageing cycles. The
chemistry
leads to an aluminium product which is more cost effective and is also simpler
to produce
since less processing steps are necessary. Additionally, the chemistry allows
new
manufacturing techniques like age forming or age creep forming which is not
feasible
when a T77-temper alloy is applied. Even better, the chemistry as defined
above can also
be aged to the T77-temper whereby the corrosion resistance further improves.
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According to the 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 and toughness and
maintaining a
good corrosion performance such as exfoliation corrosion resistance and stress
corrosion
cracking resistance.
The present invention uses the chemistry also in combination with a method to
produce a rolled product from such chemistry, as explained herein below, to
obtain a
substantially fully unrecrystallized microstructure at least at the position
T/10 of the
finished product. More preferably the product is unrecystallized across the
whole
thickness. With unrecystallized we mean that more than 80%, preferably more
than 90%
of the gauge of the finished rolled product is sunstantially unrecrystallized.
Hence, the
present invention is disclosing an alloy product which is in particular
suitable for upper
wing skin applications for aircrafts and having a thickness in the range of 20
to 60 mm,
preferably 30 to 50 mm.
It has been found that is not necessary to slowly quench the rolled product or
to
increase the gauge of the rolled product to obtain superior compression yield
strength and
toughness properties.
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 while 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 1.7 and 2.2%, preferably between 1.7 and 2.1% for Mg and 1.8
and
2.1% for Cu have been found to give a good balance for thin plate products.
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 properties
("EXCO") of EB
or better, preferably EA or better.
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The amount (in weight%) of zinc is preferably in a range of 7.4 to 9.6%, more
preferably in a range of 8.0 to 9.6%, most preferably in a range of 8.4 to
8.9%. Testing has
found an optimum zinc level of about 8.6%. Further details are given in the
examples as
described in more details hereinbelow.
5 It has furthermore been shown that, according to a preferred embodiment
of the
present invention, a So-containing alloy is an excellent candidate for
obtaining high
strength versus high notch toughness levels. By adding Sc to an alloy
comprising copper,
magnesium, zinc, zirconium and titanium it has been found that the
microstructure
remains unrecrystallized, thereby showing superior properties with regard to
strength and
toughness. Hence, preferred amounts of Sc (in weight%) are in a range of [Zr]
+ 1.5 [Sc]
<0.15%. Preferred amounts (in weight%) of Sc or Ce are in a range of 0.03 to
0.06%
when the amount of Zn is about 8.70% and Mg and Cu are about 2.10%. The levels
of the
unit propagation energy are considerably good for an alloy with additional Sc,
Ce or Mn
alloying elements.
A preferred method for producing a high strength, high toughness Al-Zn alloy
product with good corrosion resistance according to the present invention
comprises the
steps of
a. casting an ingot with the following composition (in weight percent):
Zn 6.0 to 11.0
Cu 1.4 to 2.2
Mg 1.4 to 2.4
Zr 0.05 to 0.15
Ti <0.05,
Hf and/or V < 0.25,
optionally Sc and/or Ce 0.05 to 0.25, and
optionally Mn 0.05 to 0.12,
and inevitable impurities and balance aluminium, preferably other elements
each
less than 0.05 and less than 0.50 in total,
b. homogenising and/or pre-heating the ingot after casting,
c. hot working the ingot into a pre-worked product,
d. reheating the pre-worked product, and either
dl. hot rolling the reheated product to the final gauge, or
d2 hot rolling and cold rolling the reheated product to the final gauge,
e. solution heat treating and quenching the solution heat treated product,
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f. optionally stretching or compressing of the quenched alloy product or
otherwise cold
worked to relieve stresses, and
g. optionally ageing the quenched and optionally stretched or compressed
product to
achieve a desired temper, and wherein the alloy product has a substanlially
fully
unrecrystallized microstructure at the position T/10 of the finished product.
It has been found that the microstructure of the alloy product remains
substantially
fully unrecrystallized underneath its surface when the inventive method step
of pre-
working the product and hot rolling and/or or cold rolling the pre-worked
product are
applied.
In accordance with an embodiment of the present invention the method includes
a
first hot rolling of the ingot which has been homogenised into a pre-worked
product, hot
rolling the re-heated product to about 150 to 250 (in final-gauge%) and then
cold rolling
the hot rolled product to the final gauge or hot rolling the re-heated product
to about 105 to
140 (in final-gauge%) and then cold rolling the hot rolled product to the
final gauge. "Final-
gauge%" means a percentage in thickness compared to the thickness of the final
product.
200 final-gauge% means a thickness which is twice as much as the thickness of
the finally
worked product. That means that it has been found that it is advantageous to
first hot roll
the pre-heated product to a thickness which is about twice as high as the
thickness of the
final product and then cold rolling the hot rolled product to the final
thickness or to hot roll
the pre-heated product to a thickness which is about 20% higher than the
thickness of the
final product and then cold rolling the product, thereby obtaining another
about 20%
reduction of the gauge of the hot rolled product.
According to another embodiment of the present invention it is advantageous to
hot
roll the re-heated product at low temperatures in the range of 300 C to 420 C
so that the
alloy does not recrystallise. Optionally, it is possible to artificially
ageing the worked and
heat-treated product with a two-step 179 or T76 temper or to use a T77-three
step temper
if SCC performance shall be improved.
The present invention is useful for hot-working the ingot after casting and
optionally
cold-working into a worked product with a gauge in the range of 20 to 60 mm.
The present invention also concerns a plate product of high strength, high
toughness
Al-Zn alloy of the aforementioned composition which plate product is
preferably a thin
aircraft member, even more preferably an elongate structural shape member such
as an
upper-wing member, a thin skin member of an upper-wing or of a stringer of an
aircraft.
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The properties of the claimed alloy may further be enhanced by an artificial
ageing
step comprising 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 then 135 C but below 210 C, preferably
around 155 C
for 4 to 12 hours, preferably 8 to 10 hours.
The foregoing and other features and advantages of the alloys according to the
present invention will become readily apparent from the following detailed
description of
preferred embodiments.
Example 1
On a laboratory scale 14 different aluminium alloys have been cast into
ingots,
homogenised, pre-heated for more than 6 hours at about 410 C and hot rolled to
4 mm
plates. Solution heat treatment was done at 475 C and thereafter water
quenched.
Thereafter, the quenched product was aged by a two-step T76 ageing procedure.
The
chemical compositions are set out in Table 1.
Table 1. Chemical compositions of alloys in thin plate form, in
weight%, balance
aluminium and inevitable impurities, Fe 0.06, Si 0.05, Ti 0.04 and Zr 0.12.
Alloy Cu Mg Zn Others
1 2.0 2.1 8.0 0.08 Mn
2 2.1 2.1 8.1 -
3 1.7 1.75 8.7 -
4 2.1 1.7 8.6 . -
5 2.4 1.7 8.6 -
6 1.7 2.2 8.7 -
7 2.1 2.1 8.6 -
8 2.4 2.1 8.7 -
9 1.7 2.5 8.7 -
10 2.1 2.4 8.6 -
11 2.5 2.5 8.7 -
12 2.1 2.1 9.2 -
13 2.1 2.1 8.7 0.03 Ce
14 2.1 2.1 8.7 0.06 Sc
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The alloys of Table 1 were processed using three processing variants (see step
5):
1. Homogenisation was performed by heating at a temperature rate of 40 C/h
to a
temperature of 460 C, then soaking for 12 hours at 460 C and another increase
with
25 C/h to a temperature of 475 C with another soaking for 24 hours at 475 C,
and
air cooling to room temperature.
2. Pre-heating was done at 420 C for 6 hours with a heating rate of 40 C/h.
3. The lab scale ingots were hot rolled from 80 to 25 mm, thereby reducing
the gauge
by about 6 to 8 mm per pass.
4. The 25 mm thick products were reheated to 420 C for about 30 min.
5. Variant 1: The reheated product was hot rolled to 4.0 mm.
Variant 2: The reheated product was hot rolled to 8.0 mm and thereafter cold
rolled
to 4.0 mm.
Variant 3: The reheated product was hot rolled to 5.0 mm and then cold rolled
to 4.0
mm.
6. Solution heat treatment was done for 1 hour at 475 C, thereafter water
quenched.
7. Stretching was done by 1.5 to 2.0% within about 1 hour after quenching.
8. Thereafter, the stretched products were aged in accordance with a T76
ageing
procedure, thereby raising the temperature to 120 C at a rate of 30 C/h and
maintaining the temperature at 120 C for 5 hours, raising the temperature at a
rate
of 15 C/h to a temperature of 160 C and soaking for 6 hours, and air cooling
the
aged product to room temperature.
Strength was measured using small Euronorm and toughness were measured in
accordance with ASTM B-871(1996). The results of the three above-mentioned
variants
are shown in Table 2a to 2c.
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Table 2a. Strength and toughness properties of the alloys as shown in Table
1 in
MPa and notch toughness (TYR) in accordance with Variant 1.
Alloy Rp UPE 'TYR
1 582 211 1.31
2 564 215 1.48
3 534 243 1.49
4 550 214 1.48 ,
579 208 1.44
6 592 84 1.34
7 595 120 1.32
8 605 98 1.32
9 612 30 1.31
613 54 1.12
11 603 33 1.11
12 - - -
13 597 163 1.27
14 587 121 1.35
Table 2b. Strength and toughness properties of the alloys as shown in Table
1 in
5 MPa and notch toughness (TYR) in accordance with Variant 2.
Alloy Rp UPE TYR
1 599 125 1.30
2 567 268 1.45
3 533 143 1.53
4 587 205 1.38
5 563 178 1.45
6 569 134 1.35
7- - -
8 616 72 1.10
9 - - -
10 601 22 1.00
11 612 5 1.05
12 - - -
13 595 88 1.16
14 626 71 1.26
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Table 2c.
Strength and toughness properties of the alloys as shown in Table 1 in
MPa and notch toughness (TYR) in accordance with Variant 3.
Alloy Rp UPE TYR
1 600 170 1.35
2 575 211 1.47
3 535 232 1.59
4 573 260 1.46
5 604 252 1.39
6 587 185 1.43
7 613 199 1.26
8 627 185 1.18
9
10 607 31 1.09
11 614 26 0.92
12 606 58 1.11
13 601 148 1.26
14 616 122 1.35
5 From
the results presented in Tables 2a to 2c it is clear that a minor degree (10
to
20%) of cold rolling is beneficial for an optimum toughness versus strength
balance. The
purely hot rolled material in accordance with Variant 1 (Table 2a) is close to
the optimum
but in general the Variant 3-alloys are better.
Furthermore, it can be seen that Sc-containing alloy 14 is advantageous if
high
10
strength versus high notch toughness is needed. Small amounts of manganese do
increase the strength but at the cost of some toughness.
Example 2
Additional chemistries have been processed in accordance with the above-
mentioned processing steps 1 to 8, thereby using the variant 3 of step 5 of
example 1
above and a T76 ageing.
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Table 3.
Chemical compositions of thin plate alloys, in weight%, for all alloys
balance aluminium and inevitable impurities, Fe 0.06, Si 0.05.
Alloy Cu Mg Zn Zr Ti Others
1 2.0 2.1 8.0 ' 0.11 0.03 0.08 Mn
2 2.1 2.1 8.1 0.12 0.03 -
3 1.7 2.2 8.7 0.12 0.03 -
4 2.1 2.1 8.6 0.12 0.03 -
2.4 2.1 8.7 0.12 0.03 -
6 2.1 2.1 9.2 0.12 0.03 -
7 2.1 2.1 8.7 0.12 0.04 0.04 Ce
8 2.1 2.1 8.7 0.10 0.04 0.06 Sc
9 1.7 2.1 9.3 0.12 0.03 -
1.6 2.5 9.2 0.12 0.04 -
11 2.1 2.4 9.2 0.12 0.04 -
The properties of the alloys mentioned in Table 3 have been tested in the L-
direction for
5 the strength and in the L-T-direction for the toughness.
Table 4.
Strength and toughness properties of the alloys as shown in Table 3 in
MPa and notch toughness (TS/Rp) in accordance with Variant 3.
Alloy Rp Rm UPE TS/Rp
(MPa) (MPa) (kJ/m2)
1 601 637 177 1.35
2 575 603 221 1.48
3 591 610 194 1.45
4 613 647 199 1.34
5 624 645 178 1.18
6 608 638 63 1.13
7 601 639 163 1.27
8 618 652 132 1.35
9 613 632 75 1.25
10 618 650 5 1.29
11 619 654 26 1.18
10 The
toughness versus tensile yield strength (Rp) shown in Table 4 clearly shows
that the best toughness versus tensile yield strength value is obtained for
alloys having
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around 8.6 to 8.7 weight% zinc. Alloys with lower levels of zinc will show
similar toughness
values but the tensile strength is -generally speaking- lower whereas high
levels of zinc
result in higher strength levels but lower toughness levels. Small amounts of
manganese
do increase the strength at the cost of toughness.
Example 3
Further tests were done with zinc levels of 8.6 and 8.7 thereby varying copper
and
magnesium levels. It can be shown that toughness levels can be elevated at the
same
strength levels. Some additional alloys were processed similar as to the ones
in Example
2, thereby using the processing steps 1 to 8 as described above and Variant 3
of step 5 of
Example 1.
Table 5. Chemical compositions of thin plate alloys, in weight%, for
all alloys
balance aluminium and inevitable impurities, Fe 0.06, Si 0.05.
Alloy Cu Mg Zn Zr Ti Others
3 1.7 2.2 8.7 0.12 0.03 -
4 2.1 2.1 8.6 0.12 0.03 -
5 2.4 2.1 8.7 0.12 0.03 -
12 2.5 2.5 8.7 0.11 0.03 0.08 Mn
13 2.1 2.4 8.6 0.12 0.03 -
14 1.7 2.5 8.7 0.12 0.03 -
1.7 1.7 8.7 0.12 0.03 -
16 2.4 1.7 8.6 0.12 0.03 -
17 2.1 1.7 8.6 0.12 0.04 -
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Table 6. Strength and toughness properties of the alloys as shown in
Table 5 in
MPa and notch toughness (TS/Rp) in accordance with Variant 3.
Alloy Rp UPE TS/Rp
(MPa) (kJ/m2)
=
3 591 194 1.45
4 613 199 1.34
624 178 1.18
12 614 26 0.92
13 607 31 1.09
14 621 55 1.01
535 232 1.59
16 604 252 1.39
17 573 260 1.46
As shown in Table 6 it is advantageous to have magnesium levels of less than
2.4%
5 with an optimum of about 1.7%. When magnesium levels are at about 1.7%,
excellent
toughness properties are obtained but the strength levels decrease. With
magnesium
levels of about 2.1% the best strength levels are obtained. Hence, magnesium
is best in
between 1.7 and 2.1%.
All above mentioned alloys have been tested on exfoliation corrosion according
to
10 ASTM G-34. They all showed a performance of EB or better.
Furthermore, it has been shown that the addition of Ce or Sc enhances the
microstructure of the alloy thereby reducing recovery processes. Since the
recovery within
the alloy material is low, nearly no recrystallization takes place even though
a solution
heat treatment is used in accordance with the standard route. Sc represses
15 recrystallization so that usually more than 90% of the thickness of the
thin plate products
remains unrecrystallized.
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.
