Note: Descriptions are shown in the official language in which they were submitted.
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Method for producing a high damage tolerant aluminium alloy
The present invention discloses a method for producing a high damage tolerant
aluminium
rolled alloy having a good toughness and an improved fatigue crack growth
resistance while
maintaining good strength levels and to an aluminium alloy sheet or plate
product having such a
high toughness and an improved fatigue crack growth resistance. Furthermore,
the invention
relates to the use of an alloy product obtained by the method. of this
invention.
It is known in the art to use heat treatable aluminium alloys in a number of
applications
involving relatively high strength such as aircraft fuselages, vehicular
members and other
applications. Aluminium ailoys AA2024, AA2324 and AA2524 are well-known heat
treatable
aluminium alloys which have useful strength and toughness properties in T3,
T39 and T351
tempers. Also aluminium alloys AA6013 and AA6056 are well-known heat treatable
aluminium
alloys which have useful strength and toughness properties as well as a good
fatigue crack
growth resistance in both T4 and T6 tempers.
It is known that the T4 temper condition refers to a solution heat treated and
quenched
condition, naturally aged to a substantially stable property level, whereas T6
tempers refer to a
stronger condition produced by artificially aging.
Several other AA2000 and AA6000 series alloys are generally unsuitable for the
design of
commercial aircraft which require different sets of properties for different
types of structures.
Depending on the design criteria for a particular airplane component even
small improvements
in toughness and crack growth resistance, specifically for high AK-values,
result in weight
savings, which translate to fuel economy over the lifetime of the aircraft
and/or a greater level of
safety. Especially for fuselage skin or lower wing skin it is necessary to
have properties such as
good resistance to crack propagation either in the form of fracture toughness
or fatigue crack
growth resistance. A rolled alloy product either used as a sheet or as a plate
with improved
damage tolerance properties will improve the safety of the passengers, will
reduce the weight of
the aircraft and will result to a longer flight range, lower costs and less
frequent maintenance
intervals.
US-5,213,639 discloses a method for producing an aluminium alloy of the AA2000-
series
with an aluminium base alloy which is hot rolled, heated and again hot rolled,
thereby obtaining
good combinations of strength together with high fracture toughness and a low
fatigue crack
growth rate. It is disclosed to apply an inter-annealed treatment after hot
rolling the casted ingot
with a temperature between 479 C and 524 C and again hot rolling the inter-
annealed alloy.
Such alloy is reported to have a 5% improvement over the conventional AA2024-
series alloys in
CONFIRMATION COPY
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T-L fracture toughness and an improved fatigue crack growth resistance at
certain A K-levels.
It has been reported that the known AA6056 alloy is sensitive to inter-
crystalline
corrosion in the T6 temper condition. In order to overcome this problem US-
5,858,134 provides
a process for the production of rolled or extruded products having a defined
chemical
composition, and whereby the products are brought in an over-aged temper
condition requiring
time and money consuming processing times at the end of the manufacturer of
aerospace
components. Here, it is reported that in order to obtain the improved inter-
crystalline corrosion
resistance it is essential for the process that in the alloy the Mg/Si ratio
is less than 1.
US-4,589,932 discloses an aluminium wrought alloy product for e.g. automotive
and
aerospace constructions, which alloy was subsequently registered under the AA
designation
6013. Such aluminium alloy has been solution heat treated at a temperature in
a range of 449 C
to 582 C, approaching the solidus temperature of the alloy.
EP-A-1 143027 discloses a method for producing an Al-Mg-Si alloy of the AA6000-
series
having a defined chemical composition and wherein the products are subjected
to an artificial
aging procedure to improve the alloy and to meet high damage tolerance ("HDT")
characteristics
similar to those of the AA2024-series which are preferabiy used for
aeronautical applications but
which are not weldable. The aging procedure is being optimised using a
respective function of
the composition.
EP-1170394-A2 discloses an aluminium alloy sheet product with improved fatigue
crack
growth resistance having an anisotropic microstructure defined by grains
having an average
length to width aspect ratio of greater than about 4. Such alloy has an
improvement in
compressive yield strength properties which is achieved by respective sheet
products in
comparison with conventional AA2524-sheet products. Throughout the high an-
isotropical grain
structure the fatigue crack growth resistance could be improved.
WO-97/22724 discloses a method and an apparatus for producing an aluminium
alloy
sheet product, typically for automotive application, with improved yield
strength by continuously
and rapidly heating the hot rolled and cold rolled sheet, which has been
solution heat treated
and quenched, to a pre-aging, temperature prior to the continuous coiling
step. After rapidly
heating, the sheet in coil form is ambiently cooled, the rapid heating and
ambient cooling
improving the paintbake response of the aluminium alloy sheet. It is disclosed
that it is preferred
to rapidly heating the coiled sheet to between 65 C and 121 C and to choose
an ambient
cooling rate and which is preferred to be between 1.1 C/h and 3.3 C/h.
It is the object of the present invention to provide a method for producing an
aluminium
alloy product having an improved toughness and an improved fatigue crack
growth resistance
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thereby maintaining the strength levels of conventional AA2000-, AA6000-,
AA5000- or AA7000-
series alloys. More specifically, it is the object of the present invention to
provide an improved
method for producing high damage tolerant ("HDT") aluminium alloys with
balanced properties
with regard to fatigue crack growth resistance, toughness, corrosion
resistance and strength.
The HDT-properties should preferably be better than those of conventional
manufactured
AA6013-T6, 6056-T6 alloys and preferably better than AA2024-T3 or AA2524-T3
alloys.
More specifically, there is a general requirement for rolled AA6000-series
aluminium alloys
preferably within the range of AA6013 and AA6056-series aluminium alloys, when
used for
aerospace applications, that the fatigue crack growth rate ("FCGR") should not
be greater than a
defined maximum. An FCGR which meets the requirements of high damage tolerant
2024-series
alloy products is for example an FCGR below 0.001 mm/cycles at AK = 20 MPa4m
and 0.01
mm/cycles at AK = 40 MPa4m.
It is yet a further object of the present invention to provide a rolled
aluminium alloy product
for use to construct structural parts in the aircraft industry as well as to
provide an aircraft skin
material produced from such alloy or to provide a vehicle component part.
The present invention solves one or more of the above mentioned objects by the
features
of independent claims.
In one aspect of the present invention provides a method for producing a high
damage
tolerant aluminium alloy having a high toughness and an improved fatigue crack
growth
resistance, comprising the steps of
a.) casting an ingot having a composition selected from the group consisting
of AA2000,
AA5000, AA6000, and AA7000-series alloys;
b.) homogenising and/or pre-heating the ingot after casting;
c.) hot rolling the ingot into a hot rolled product, and optionally further
cold rolling the hot
rolled product into a cold rolled product, characterized in that the hot
rolled product leaves the
hot rolling mill at an hot-mill exit temperature (TEX;t) and cooling the hot
rolled product from said
TEx;t to 150 C with a controlled cooling cycle with a cooling rate falling
within the range defined
by:.
T(t) = 50 - (50 - TExit)ea.t
and wherein T(t) is the temperature ( C) as function in time (expressed in
hours), t is the time
(expressed in hours) and a (expressed in hrs ') is a parameter defining the
cooling rate and is in
the range of -0.09 0.05 (hrs"'), and more preferably in a range of -0.09 0.03
(hrs').
It has been found that below the temperature of 150 C the cooling rate is no
longer relevant to
achieve one or more of the advantages found according to this invention.
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While prior art techniques teach to skiiled person to cast and hot roll an
ingot to obtain a
plate or sheet product, wherein the ingot is optionally preheated or
homogenised before hot
rolling, the hot rolled product lost its elevated temperature fairly fast,
thereby compromising the
performance of the product. It has been found that by maintaining the hot
rolied product at an
elevated temperature for a predetermined time to subject it to a controlled
cooling cycle the
damage tolerance properties such as toughness and crack growth resistance of
such a rolled
product can be improved in accordance with the present invention.
Typical hot-mill exit temperatures in an industrial scale practice are in a
range of 350 to
500 C and are alloy dependent, for example for an AA6xxx the exit temperature
wili be at the
higher end of this range of about 420 to 500 C, whereas for AA2xxx and AA7xxx-
series alloys
this would be at the lower end of this range of about 350 to 425 C.
A further cold rolling of the cooled hot rolled product in coil form is
optional. The cold rolling
can be straight or cross rolling. Further steps of inter-annealing before,
during or after cold rolling
are also optional.
Furthermore, it is possible to subject the hot rolled product to coiling to
obtain a coiled form
and thereby achieving a controlled cooling rate until the product is cooled
down to room
temperature. Then, it is possible to cut the coil into blanks which are then
further cold rolled. The-
material which is produced by this inventive processing route exhibited a
better property balance
than those hot rolled products which were cut into blanks during or after hot
rolling without
coiling (standard plate route) or those products that were coiled after cold
rolling (standard sheet
route).
A second alternative for subjecting the hot rolled product to a controlled
cooling cycle is the
step of continuously moving the alloy through a furnace after hot rolling,
wherein said furnace is
adjustable to apply heat and/or coldness to the alloy while passing to its
cold rolling station or
coiling station.
In a further alternative the rolled product is first hot rolled to a desired
gauge and then
cooled to room temperature using conventional cooling. Thereafter the cooled
hot rolled product
is reheated to a hot-mil exit temperature and then allowed to cool to below
150 C using the
controlled cooling cycle according to the invention and followed by further
processing.
Depending on whether sheets or plates are produced the hot rolled product is
either fed to
said furnace after hot rolling or coiled after hot rolling wherein the further
processing is done on
coils (sheet route). If the product is cut into plates during or after hot
rolling the further
processing is done on thereby produced plates.
The furnace is preferably adjustable to apply various amounts of heat close to
the hot
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rolling station and other amounts of heat at a greater distance from the hot
rolling station,
depending on the cooling rate, thickness and other dimensions of the hot
rolled product leaving
the hot rolling station.
When the hot rolled product is subjected to the controlled cooling cycle by
coiling it is
5 possible to coil the alloy after hot rolling in a respective furnace,
wherein said furnace is then
also preferably adjustable to apply heat to control the cooling cycle.
In an embodiment the hot rolled product has a gauge in a range of up to 12 mm
while
leaving the hot rolling mill at the hot-mill exit temperature, and preferably
in a range of 1 to 10
mm, and most preferably in the range of 4 to 8 mm.
Where to rolled product is being further subjected to a cold rolling
operation, it is preferred
that the total cold roll reduction is in a range of 40 to 70% to further
optimise the mechanical
properties. The final gauge of the rolled alloy product is preferably in a
range of about 2 to 7 mm.
The method in accordance with the present invention may further include one or
more of
the following steps:
d.) solution heat treating of the hot rolled product after being subjected to
the controlled
cooling cycle or of the cold rolled product at a temperature and time
sufficient to place into solid
solution soluble constituents in the alloy;
e.) quenching the solution heat treated alloy product by one of spray
quenching or
immersion quenching in water or other quenching media;
f.) optionally stretching or compressing of the quenched alloy product or
otherwise cold
worked to relieve stresses, for example levelling of sheet products;
g.) optionally ageing the quenched and optionally stretched or compressed
alloy product to
achieve a desired temper, which is dependent of the alloy chemistry, but
includes the tempers
T3, T351, T6, T4, T74, T76, T751, T7451, T7651, T77, T79,
Furthermore, it is possible to anneal and/or reheat a hot rolled ingot after a
first hot rolling
operation and then again hot rolling the product to a final hot-rolled gauge
followed a cooling
according to the invention. It is furthermore possible to inter-anneal the hot
rolled product before
and/or during cold rolling. These techniques, which are known from prior art,
can
advantageously be used in a method according to the present invention.
The average cooling rate when using the controlled cooling cycle according to
the
invention is in a range of 12 to 20 C/hour.
In an embodiment of the present invention the cast ingot for the processing
route of the
method as disclosed herein, comprises the following composition (in weight.%):
Si 0.6 - 1.3, Cu
0.04 - 1.1, Mn 0.1 - 0.9, Mg 0.4 - 1.3, Fe 0.01 - 0.3, Zr < 0.25, Cr < 0.25,
Zn < 0.6, Ti < 0.15, V <
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0.25, Hf < 0.25, other elements, in particular impurities, each less than 0.05
and less than 0.20
in total, balance aluminium. And more preferably alloys within the
compositional range of
AA6013 or AA6056.
Another embodiment of the present invention uses an ingot comprises the
following
composition (in weight.%): Cu 3.8 - 5.2, Mg 0.2 -1.6, Cr < 0.25, Zr < 0.25,
and preferably 0.06 -
0.18, Mn < 0.50 and Mn: > 0, and preferably > 0.15, Fe < 0.15, Si <_ 0.15, and
Mn-containing
dispersoids, and incidental elements and impurities, each less than 0.05 and
less than 0.15 in
total and the balance essentially, and preferably wherein the Mn-containing
dispersoids are at
least partially replaced by Zr-containing dispersoids.
According to another embodiment of the present invention the method uses an
ingot
comprises the following composition (in weight.%): Zn 5.0 - 9.5, Cu 1.0 - 3.0,
Mg 1.0 - 3.0, Mn <
0.35, Zr <0.25, and preferably 0.06 - 0.16, Cr <0.25, Fe <0.25, Si < 0.25, Sc
< 0.35, Ti < 0.10, Hf
and/or V < 0.25, other elements, typically impurities, each less than 0.05 and
less than 0.15 in
total, balance aluminium. Typical examples are alloys within the range of
AA7040, AA7050 and
AA7x75.
According to another aspect of the present invention an aluminium alloy sheet
or plate
product is disclosed which has high toughness and an improved fatigue crack
growth resistance
and which is made of an alloy product which is produced according to a method
which has been
described above and which will be described in more details herein below. More
specifically, the
present invention is most suitable to produce a rolled alloy sheet product
which is a structural
member of an aircraft or an automobile. Such rolled alloy sheet product could
be used for
example as a fuselage skin of an aircraft or a vehicle component part.
The foregoing and other features and advantages of the method and alloy
products
according to the present invention will become readily apparent from the
following detailed
description of preferred embodiments, and figures, in which
Fig. 1 is a typical cooling curve of an aluminium alloy cooled down after hot
rolling using
the method according this invention.
EXAMPLES
Example 1.
In a first preferred embodiment of the present invention, two conventional
alloys (AA6013
and AA6056) were cast and processed to a sheet product. Here, two processing
variants were
used:
Route 1. A normal processing route by lab-casting ingots of conventional
AA6013 and
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AA60156-alloy compositions was used. Blocks of 80x80x100mm were sawn,
homogenised,
preheated and hot rolled to 4.5 mm sheet. After hot rolling the hot rolled
products were
conventionally cooled to ambient temperature by allowing the sheet to cool to
ambient air to
room temperature, fed to the cold rolling station, cold rolled to 2 mm and
heat treated for 20 min
at 550 C, thereafter quenched and aged to a T6-temper for 4 hours at 190 C.
Route 2. Ingots of conventional AA6013 and AA6056-alloy compositions were lab-
cast
and sawn to a size of 80x80x100mm. These blocks were homogenised, pre-heated
and hot
rolled to 4.5 mm. A simulation of the hot coiling in an industrial scale was
incorporated by giving
the hot rolled product a similar temperature history as what a coil in full-
scale production would
have had. The other processing steps were kept similar as with Route 1. After
cold rolling the
cold rolled product was heat treated at 550 C for 20 min, quenched and
consequently aged to a
T6-temper at 190 C for 4 hours. The results are given in Table 1.
Table 1. Overview of strength (Rp, Rm) using small Euronorm, notch toughness
(TS/Rp),
intergranular corrosion (IGC) in depths and type of 6013 and 6056-alloy
compositions processed
in accordance with route I and route 2 as described above, at two different
hot rolling exit
temperature settings.
No. Alloy Route Hot rolling exit Rp Rm TS/Rp IGC IGC
temperature (MPa) (MPa) - Depth ( m) type
( C)
16013 2 490 354 390 1.75 101 P(i)
2 1 490 344 381 1.72 118
3 2 450 345 385 1.73 97
4 1 450 337 377 1.63 108 I
56056 2 490 347 386 1.85 112
6 1 490 349 388 1.79 177 1+
7 2 450 328 372 1.75 103 P(i)
8 1 450 331 375 1.70 143 I
It can be seen from Table 1 that the rolled products exhibited better notch
toughness at
higher hot rolling temperatures by maintaining good tensile yield strength and
ultimate tensile
strength levels. Furthermore, there is an improvement in intergranular
corrosion so that further
testing has been done with regard to the fatigue crack growth resistance
(Table 2).
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Table 2. Overview of the fatigue crack growth resistance ("FCGR") for examples
No. 1, 2 and
5, 6 of Table 1(higher hot rolling temperatures) at two different AK-levels.
Alloy Route Hot rolling exit FCGR FCGR
temperature OK=30MPa4m AK=40MPaqm
( C)
._........ _.._...._....._..._....... . ..... ...... ....... __....... ...__
................. _....._...._._.................. ........... _........... _
.... _..._....._..__.___..... _............... _.._.__.._....... ...... .
......__....__....._...._.....__.._..._.
6013 2 490 1.83E-03 5.26E-03
1 490 1.84E-03 8.88E-03
6056 2 490 1.62E-03 3.32E-03
1 490 1.66E-03 4.89E-03
While the fatigue crack growth resistance of the inventive products is nearly
identical to the
fatigue crack growth resistance of a product produced in accordance with the
standard
processing route at lower AK values, the fatigue crack growth resistance is
improved at higher
AK values.
In accordance with another preferred embodiment of the present invention a low
copper
high damage tolerant AA6000-series alloy composition has been produced in a
full-scale
production trial. The composition is given in Table 3.
Table 3. Composition of high damage tolerant AA6000-series sheet product in
weight-%,
balance aluminium and inevitable impurities.
Si Fe Cu Mg Mn Zn
1.14 0.18 0.32 0.70 0.71 0.08
The alloy has been processed to a sheet product with a hot rolling gauge of
4.5mm. The
following three processing variants were then applied:
Route 1. A standard processing route. (No coiling step after hot rolling).
Route 2. The inventive processing route with coiling after hot rolling and hot
rolling and
cold rolling in the same direction.
Route 3. The inventive processing route with coiling after hot rolling and hot
rolling and
cold rolling in dissimilar directions (cross rolling).
All three above mentioned processing variants were applied to the following
general processing
route:
a. DC-casting of ingots of an alloy composition in accordance with Table 3.
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b. Homogenising the cast ingots.
c. Preheating the homogenised ingots for 6 hours at 510 C and subsequently hot
rolling
the pre-heated ingots resulting that the exit temperature is about 450 C at a
gauge of
4.5 mm.
dl. No coiling (= Route 1).
Q. Coiling, cooling and cutting into plates (= Route 2).
d3. Coiling, cooling and cutting into plates (= Route 3).
el. Cold rolling to a final gauge of 2 mm (Route 1).
e2. Cold rolling in same direction as hot rolling to a final gauge of 2 mm
(Route 2).
e3. Cold rolling in a dissimilar direction as hot rolling (cross rolling) to a
finai gauge of 2
mm (Route 3).
f. Heat treatment of 550 C for 2 hours.
g. Stretching the cold rolled product by 1.5 to 2.5%.
h, Aging to a T6-temper condition at 190 C for 4 hours.
Table 4. Overview of strength (Rp, Rm) using small Euronorm, notch toughness
(TS/Rp) and
intergranular corrosion (IGC) of a finished product with an alloy in
accordance with Table 3 and
using three processing Routes 1, 2 and 3 as described above.
Route Rp Rm Rp Rm TS/Rp IGC
(MPa) (MPa) (MPa) (MPa) - Depth ( m)
L-direction LT-direction T-L
direction
1 334 345 322 344 1.51 62
2 329 344 321 341 1.60 48
3 333 344 326 347 1.58 49
While the strength levels could be maintained the rolled products which were
produced in
accordance with processing Routes 2 and 3 showed a better notch toughness and
a better
intergranular corrosion performance. Hence, the fatigue crack growth
resistance was measured
also and is given in Tables 5 and 6.
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Table 5. Fatigue crack growth resistance in mm/cycle for 5 different AK-values
for the
products produced in accordance with the processing Routes 1, 2 and 3 as
described above.
AK Route I Route 2 Route 3
(MPa4m)
10 1.52E-04 1.71 E-04 1.78E-04
1.43E-03 8.58E-04 1.26E-03
6.14E-03 3.38E-03 5.17E-03
1.70E-02 9.54E-03 --
3.73E-02 1.85E-02 --
5 Table 6. Values of Table 5, relative to standard (Route 1).
AK Route I Route 2 Route 3
(MPaqm)
__....... __.._..______._._..__._.
._.__.._.._._...__._.....___._......._..____. _.._.._.~__.___-__.___..__. _
._._____._..._.__.._.._...___.._ __
10 100% 113% 117%
20 100% 60% 88%
30 100% 55% 84%
40 100% 56% --
50 100% 50% --
The above identified examples show that the damage tolerance properties of
sheet or plate
products can be improved by using the inventive method and that the fatigue
crack growth
resistance can especially be improved for higher AK-values.
Example 2.
Fig. 1 shows a typical continuous cooling down curve for an aluminium AA7050
alloy when
cooled down from a hot-mill exit temperature of 440 C to a temperature below
150 C, whereby
the metal sheet has a gauge of 4.5 mm and being immediately coiled when
leaving the hot-mill
in accordance with an embodiment of the method of this invention. The width of
the coil was 1.4
meter. The temperatures of the coil as function of time is also given in Table
7 for the hottest
spot of a coil (being the centre, and indicated as HotSpt in Fig.1) and the
coldest spot (being the
edge of a coil, and indicated as ColdSpt in Fig.1)). Table 7 provides also the
temperatures in
case a coil having a width of 2.8 meter.
For the shown cooling curve in Fig. I the a is about -0.084 hrs
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In case a sheet with a gauge of about 4.0 to 4.5 mm was allowed to cool down
from the
hot-mill exit temperature to below 150 C using conventional cooling practice,
viz. leaving the
plate to cool in normal stationary air after exit of the hot mill without any
coiling operation or the
like, the a would typically be in the range of -0.5 to -2 hrs -1, and
resulting in the such a plate
would cool down from the hot mill exit temperature to a temperature of 150 C
or less in a time
period of less than 3 hours.
The controlled cooling cycle follows the equation set out above and in the
claims, and the
average cooling rate of the coiled product form from 440 to 150 C is within
the range of 12 to
20 C/hour.
Table 7. Coil temperatures as function of the time when cooled in accordance
with the
invention for an AA7050 alloy having a gauge when being coiled of 4.5 mm.
Time Coil width 1.4 meter Coil width 2.8 meter
(hours)
Coldest spot Hottest spot Coldest spot Hottest spot
( C) ( C) ( C) ( C)
0 431 440 431 440
2 344 372 349 385
6 249 266 262 287
10 187 199 204 222
12 165 175 182 197
14 146 150 163 176
16 130 137 148 159
18 117 123 134 144
