U.S. patent application number 11/771234 was filed with the patent office on 2007-12-13 for alcumg alloys with high damage tolerance suitable for use as structural members in aircrafts.
This patent application is currently assigned to ALCAN RHENALU. Invention is credited to Bes Bernard, Ronan Dif, Timothy Warner.
Application Number | 20070284019 11/771234 |
Document ID | / |
Family ID | 30115694 |
Filed Date | 2007-12-13 |
United States Patent
Application |
20070284019 |
Kind Code |
A1 |
Dif; Ronan ; et al. |
December 13, 2007 |
AlCuMg ALLOYS WITH HIGH DAMAGE TOLERANCE SUITABLE FOR USE AS
STRUCTURAL MEMBERS IN AIRCRAFTS
Abstract
New alloys for potential use in applications such as in lower
wing skins and fuselage skins are disclosed. Specifically, Mn-free
2.times.24 alloys potentially suitable for thick plate and thin
plate and sheet applications are believed to be novel and to
provide unexpectedly superior properties.
Inventors: |
Dif; Ronan; (Saint Etienne
De Saint Geoirs, FR) ; Warner; Timothy; (Voreppe,
FR) ; Bernard; Bes; (Seyssins, FR) |
Correspondence
Address: |
Womble Carlyle Sandridge & Rice, PLLC;Attn: Patent Docketing 32nd Floor
P.O. Box 7037
Atlanta
GA
30357-0037
US
|
Assignee: |
ALCAN RHENALU
Paris
FR
|
Family ID: |
30115694 |
Appl. No.: |
11/771234 |
Filed: |
June 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10614888 |
Jul 9, 2003 |
7252723 |
|
|
11771234 |
Jun 29, 2007 |
|
|
|
60394234 |
Jul 9, 2002 |
|
|
|
Current U.S.
Class: |
148/552 |
Current CPC
Class: |
C22C 21/16 20130101;
C22F 1/053 20130101; C22F 1/057 20130101 |
Class at
Publication: |
148/552 |
International
Class: |
B22D 21/00 20060101
B22D021/00; B22D 25/06 20060101 B22D025/06; C22F 1/057 20060101
C22F001/057 |
Claims
1. A method for obtaining a substantially manganese-free aluminum
alloy rolled product consisting essentially of (in percent by
weight): Cu 3.6-4.5%, Mg 1.0-1.6%, Zr 0.08-0.20%, Sc 0.02-0.05%, Fe
up to 0.08%, Si up to 0.09%, Mn less than 0.05%, remainder aluminum
and incident impurities, wherein said rolled product comprises a
plate, said method comprising: (a) casting a rolling ingot,
followed by optional stress relieving, and scalping; (b)
homogenizing at a temperature between 450 and 510.degree. C.; (c)
hot-rolling on a reversing mill, preferably with an exit
temperature between 350 and 390.degree. C.; (d) optionally, for
plate with a thickness of less than about 30 mm, conducting at
least one intermediate reheating to about 480.degree. C., followed
by one or more hot-rolling passes, the final exit temperature
optionally being between 350 and 370.degree. C.; (e) solution heat
treating at a temperature between 490 and 510.degree. C., followed
by water quenching and natural aging; and (f) cold working by
stretching alone or cold rolling followed by stretching, optionally
followed by artificial aging.
2. The method of claim 1, wherein Zr is present in an amount from
0.08-0.14%.
3. The method of claim 1, wherein Cu is present in an amount from
3.8-4.2%.
4. The method of claim 1, wherein the aluminum alloy rolled product
comprises a recrystallized volume fraction of 5% maximum.
5. The method of claim 1, wherein Mn is present in an amount of
<0.01%.
6. The method of claim 1, wherein the aluminum alloy rolled product
comprises one or more of the following combinations of properties:
a. a tensile strength in the longitudinal direction (TYS.sub.(L))
of more than 400 MPa, and an apparent fracture toughness
K.sub.app(T-L) of more than 110 MPa m, measured according ASTM E
561 in the T-L orientation on a specimen with a width of W=127 mm;
b. an ultimate tensile strength in the longitudinal direction
(UTS.sub.(L)) of more than 450 MPa, and an elongation at fracture
in the longitudinal direction of more than 24%; c. a tensile yield
strength in the longitudinal direction (TYS.sub.(L)) of more than
400 MPa, and a Kahn stress R.sub.e of at least 180 MPa.
7. The method of claim 1, wherein the aluminum alloy rolled product
further comprises at least one of the following combinations of
properties: a. a (UTS.sub.(L)) of more than 500 MPa, preferably
more than 520 MPa, and even more preferably more than 530 MPa, and
a K.sub.app(T-L) of more than 75 MPa m, measured according ASTM E
647 on a 6.35 mm thick C(T) specimen with a width of W=40 mm; b. a
tensile yield strength in the longitudinal direction (TYS.sub.(L))
of more than 450 MPa, and preferably more than 460 MPa, and a
K.sub.app(T-L) of more than 77 MPa m, measured according ASTM E 561
on a 6.35 mm thick C(T) specimen with a width of W=40 mm; c. a
tensile yield strength in the longitudinal direction (TYS.sub.(L))
of more than 350 MPa, preferably more than 400 MPa and even more
preferably more than 450 MPa, and a Kahn stress R.sub.e of at least
190 MPa.
8. The method of claim 1, wherein the aluminum alloy rolled product
further comprises a sheet or thin plate with a thickness below
about 12 mm in T351 temper, having a da/dn in T-L direction which
fulfills at least one of the following conditions: da/dn less than
1.3.times.10.sup.-4 mm/cycles at .DELTA.K=10 MP am; da/dn less than
4.0.times.10.sup.-4 mm/cycles at .DELTA.K=15 MPa m; da/dn less than
8.0.times.10.sup.-4 mm/cycles at .DELTA.K=20 MPa m; da/dn less than
16.times.10.sup.-4 mm/cycles at .DELTA.K=25 MPa m; and da/dn less
than 25.times.10.sup.-4 mm/cycles at .DELTA.K=30 MPa m.
9. The method of claim 1, wherein the aluminum alloy rolled product
further comprises a in T351 temper having a da/dn in T-L direction
which fulfills at least one of the following conditions: da/dn less
than 3.0.times.10.sup.-5 mm/cycles at .DELTA.K=10 MPa m; da/dn less
than 1.0.times.10.sup.-4 mm/cycles at .DELTA.K=15 MPa m; da/dn less
than 1.0.times.10.sup.-3 mm/cycles at .DELTA.K=25 MPa m; and da/dn
less than 3.0.times.10.sup.-3 mm/cycles at .DELTA.K=30 MPa m.
10. The method of claim 1, wherein the aluminum alloy rolled
product exhibits a maximum intergranular attack of less than 80
.mu.m in T39 temper, and/or less than 200 .mu.m in T851 temper,
and/or less than 250 .mu.m in T89 temper, and/or less than 300
.mu.m in T351 temper in a corrosion test according to ASTM G
110.
11. The method of claim 1, wherein the aluminum alloy rolled
product exhibits a maximum intergranular attack of less than 70
.mu.m in T39 temper, and/or less than 180 .mu.m in T851 temper,
and/or less than 220 .mu.m in T89 temper, and/or less than 270
.mu.m in T351 temper in a corrosion test according to ASTM G
110.
12. The method of claim 1, wherein the aluminum alloy rolled plate
product is a lower wing skin structural member.
13. The method of claim 1, wherein the aluminum alloy rolled plate
or sheet product is a fuselage skin member.
14. The method of claim 1, further comprising the step of cladding
the aluminum alloy rolled product.
15. A method for obtaining a substantially manganese-free aluminum
alloy rolled product consisting essentially of (in percent by
weight): Cu 3.6-4.5%, Mg 1.0-1.6%, Zr 0.08-0.20%, Sc 0.02-0.05%, Fe
up to 0.08%, Si up to 0.09%, Mn less than 0.05%, remainder aluminum
and incident impurities, wherein said rolled product comprises a
sheet product, said method comprising: (a) casting a rolling ingot,
followed by optional stress relieving, and scalping; (b)
homogenizing at a temperature between 470 and 530.degree. C.; (c)
hot-rolling down to a thickness of less than 12 mm, and not more
than 200% of final thickness, with a final exit temperature between
230 and 350.degree. C.; (d) optionally cold rolling; (e) solution
heat treating at a temperature between 490 and 510.degree. C.,
followed by water quenching; (f) cold working by stretching alone
or cold rolling followed by stretching, optionally followed by
artificial aging.
16. The method of claim 15, wherein Zr is present in an amount from
0.08-0.14%.
17. The method of claim 15, wherein Cu is present in an amount from
3.8-4.2%.
18. The method of claim 15, wherein the aluminum alloy rolled
product comprises a recrystallized volume fraction of 5%
maximum.
19. The method of claim 15, wherein Mn is present in an amount of
<0.01%.
20. The method of claim 15, wherein the aluminum alloy rolled
product comprises one or more of the following combinations of
properties: a. a tensile strength in the longitudinal direction
(TYS.sub.(L)) of more than 400 MPa, and an apparent fracture
toughness K.sub.app(T-L) of more than 110 MPa m, measured according
ASTM E 561 in the T-L orientation on a specimen with a width of
W=127 mm; b. an ultimate tensile strength in the longitudinal
direction (UTS.sub.(L)) of more than 450 MPa, and an elongation at
fracture in the longitudinal direction of more than 24%; c. a
tensile yield strength in the longitudinal direction (TYS.sub.(L))
of more than 400 MPa, and a Kahn stress R.sub.e of at least 180
MPa.
21. The method of claim 15, wherein the aluminum alloy rolled
product further comprises a sheet comprising at least one of the
following combinations of properties: a. a (UTS.sub.(L)) of more
than 500 MPa, preferably more than 520 MPa, and even more
preferably more than 530 MPa, and a K.sub.app(T-L) of more than 75
MPa m, measured according ASTM E 647 on a 6.35 mm thick C(T)
specimen with a width of W=40 mm; b. a tensile yield strength in
the longitudinal direction (TYS.sub.(L)) of more than 450 MPa, and
preferably more than 460 MPa, and a K.sub.app(T-L) of more than 77
MPa m, measured according ASTM E 561 on a 6.35 mm thick C(T)
specimen with a width of W=40 mm; c. a tensile yield strength in
the longitudinal direction (TYS.sub.(L)) of more than 350 MPa,
preferably more than 400 MPa and even more preferably more than 450
MPa, and a Kahn stress R.sub.e of at least 190 MPa.
22. The method of claim 15, wherein the aluminum alloy rolled
product further comprises a sheet or thin plate with a thickness
below about 12 mm in T351 temper, having a da/dn in T-L direction
which fulfills at least one of the following conditions: da/dn less
than 1.3.times.10.sup.-4 mm/cycles at .DELTA.K=10 MPa m; da/dn less
than 4.0.times.10.sup.-4 mm/cycles at .DELTA.K=15 MPa m; da/dn less
than 8.0.times.10.sup.-4 mm/cycles at .DELTA.K=20 MPa m; da/dn less
than 16.times.10.sup.-4 mm/cycles at .DELTA.K=25 MPa m; and da/dn
less than 25.times.10.sup.-4 mm/cycles at .DELTA.K=30 MPa m.
23. The method of claim 15, wherein the aluminum alloy rolled
product further comprises a in T351 temper having a da/dn in T-L
direction which fulfills at least one of the following conditions:
da/dn less than 3.0.times.10.sup.-5 mm/cycles at .DELTA.K=10 MPa m;
da/dn less than 1.0.times.10.sup.-4 mm/cycles at .DELTA.K=15 MPa m;
da/dn less than 1.0.times.10.sup.-3 mm/cycles at .DELTA.K=25 MPa m;
and da/dn less than 3.0.times.10.sup.-3 mm/cycles at .DELTA.K=30
MPa m.
24. The method of claim 15, wherein the aluminum alloy rolled
product exhibits a maximum intergranular attack of less than 80
.mu.m in T39 temper, and/or less than 200 .mu.m in T851 temper,
and/or less than 250 .mu.m in T89 temper, and/or less than 300
.mu.m in T351 temper in a corrosion test according to ASTM G
110.
25. The method of claim 15, wherein the aluminum alloy rolled
product exhibits a maximum intergranular attack of less than 70
.mu.m in T39 temper, and/or less than 180 .mu.m in T851 temper,
and/or less than 220 .mu.m in T89 temper, and/or less than 270
.mu.m in T351 temper in a corrosion test according to ASTM G
110.
26. The method of claim 15, wherein the aluminum alloy rolled plate
product is a lower wing skin structural member.
27. The method of claim 15, wherein the aluminum alloy rolled plate
or sheet product is a fuselage skin member.
28. The method of claim 15, wherein no reheating is involved
between hot-rolling steps of said hot-rolling.
29. The method of claim 15, wherein there is no additional step of
recrystallization treatment.
30. The method of claim 15, further comprising the step of cladding
the aluminum alloy rolled product.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/614,888, filed Jul. 9, 2003, which claims priority from
Provisional Application Ser. No. 60/394,234, filed Jul. 9, 2002,
the content of which is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to damage tolerant
aluminum alloys, and in particular, to such alloys useful in the
aerospace industry suitable for use in lower wing skin applications
and as fuselage skin.
[0004] 2. Description of the Related Art
[0005] Materials particularly adapted for use in lower wing skin
applications including 2.times.24 alloys are generally known, as
described, for example, in U.S. Pat. Nos. 5,213,639 and 6,444,058,
as well as in the PCT application WO 99/31287, the content of which
is incorporated herein by reference in their entireties. Damage
tolerance of 2.times.24 alloys is of particular importance and
materials that have excellent properties in this regard are highly
desirable. These 2.times.24 alloys, derived from the chemical
composition of the 2024 alloy, usually the case of the 2.times.24
alloys which have been standardized by The Aluminum Association
(AA): 2024, 2024A, 2124, 2224, 2224A, 2324, 2424, 2524.
[0006] European Patent Application EP 1 170 394 A discloses methods
for manufacturing damage tolerant AlCuMg sheet. These methods
involve unusual (hot cross rolling) or otherwise expensive
manufacturing steps (repeated intermediate heat treatment) in order
to obtain a precisely controlled microstructure.
SUMMARY OF THE INVENTION
[0007] According to the present invention, there is provided a
substantially manganese-free aluminum alloy rolled product
consisting essentially of (in percent by weight): [0008] Cu
3.6-4.5%, Mg 1.0-1.6%, Zr 0.08-0.20%, Sc up to 0.06%, [0009] Fe up
to 0.08%, Si up to 0.09% Mn less than 0.05%, [0010] the remainder
aluminum and incident impurities.
[0011] This product, as plate or sheet, presents a good compromise
between fracture toughness and mechanical strength. It can be
provided as plate or sheet, and is suitable for use in applications
that require high damage tolerance, such as in lower wing skins or
fuselage skin.
[0012] As used herein, the term "sheet" includes flat rolled
aluminum products having a thickness form about 0.2 mm to about 12
mm, whereas the term "plate" is limited to products thicker than 12
mm. This definition is different from the one used in European
Standard EN 12258-1.
[0013] Specifically, substantially Mn-free AlCuMg alloys for
applications such as in lower wing skins are believed to be novel
and to provide unexpectedly superior properties. As used herein,
"substantially Mn-free" means up to 0.05% Mn. These alloys were
compared against high damage tolerant material 2024 (Reference DT)
according to prior art. According to embodiments of the present
invention, manganese has been totally replaced by zirconium or by
zirconium+300 .mu.g/g of scandium.
[0014] Sheet or plate according to the present invention may have
one or more of the following combinations of properties: [0015] (a)
a tensile yield strength in the longitudinal direction
(TYS.sub.(L)) of more than 400 MPa, preferably more than 430 MPa
and even more preferably more than 450 MPa, and an apparent
fracture toughness K.sub.app(T-L) of more than 110 MPa m, and
preferably more than 115 MPa m, measured according to ASTM E 561 in
the T-L orientation on a specimen with a width of W=127 mm; [0016]
(b) an ultimate tensile strength in the longitudinal direction
(UTS.sub.(L)) of more than 450 MPa, and preferably more than 460
MPa, and an elongation at fracture in the longitudinal direction of
more than 24%, and preferably more than 26%; [0017] (c) a tensile
yield strength in the longitudinal direction (TYS.sub.(L)) of more
than 400 MPa, preferably more than 430 MPa and even more preferably
more than 450 MPa, and a Kahn stress R.sub.e of at least 180 MPa,
and preferably at least 190 MPa.
[0018] Plate according to the present invention may have one or
more of the following combinations of properties: [0019] (a) a
UTS.sub.(L) of more than 500 MPa, preferably more than 520 MPa, and
even more preferably more than 530 MPa, and a K.sub.app(L-T) of
more than 75 MPa m, and preferably more than 77 MPa m, measured
according to ASTM E 561 on a 6.35 mm thick C(T) specimen with a
width of W=40 mm; [0020] (b) a tensile yield strength in the
longitudinal direction (TYS.sub.(L)) of more than 450 MPa, and
preferably more than 460 MPa, and a K.sub.app(L-T) of more than 77
MPa m, measured according to ASTM E 561 on a 6.35 mm thick C(T)
specimen with a width of W=40 mm; [0021] (c) a tensile yield
strength in the longitudinal direction (TYS.sub.(L)) of more than
350 MPa, preferably more than 400 MPa and even more preferably more
than 450 MPa, and a Kahn stress R.sub.e of at least 190 MPa.
[0022] Another object of the present invention involves providing
methods for manufacturing sheet products and plate products in said
substantially manganese-free alloys. These methods are particularly
simple, especially for production of sheet.
[0023] Additional objects, features and advantages of the invention
will be set forth in the description which follows, and in part,
will be obvious from the description, or may be learned by practice
of the invention. The objects, features and advantages of the
invention may be realized and obtained by means of the
instrumentalities and combination particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The accompanying drawings, which are incorporated in and
constitute apart of the specification, illustrate a presently
preferred embodiment of the invention, and, together with the
general description given above and the detailed description of the
preferred embodiment given below, serve to explain the principles
of the invention.
[0025] FIG. 1 shows optical micrographs according to the present
invention: after chromic etch (FIG. 1a) and after anodic oxidation
(FIG. 1b). The grain structure can be seen.
[0026] FIG. 2 shows the tensile yield strength (TYS) as a function
of cold-work for the different alloys in T3X tempers.
[0027] FIG. 3 shows the ultimate tensile strength (UTS) as a
function of cold-work for the different alloys in T3X tempers.
[0028] FIG. 4 shows the Kahn tear stress in L-T orientation as a
function of TYS for the different alloys in T3X tempers.
[0029] FIG. 5 shows the K.sub.app plane stress fracture toughness
in L-T orientation as a function of TYS for the different alloys in
T3X tempers.
[0030] FIG. 6 shows the K.sub.app plane stress fracture toughness
in T-L orientation as a function of TYS for some of the alloy in
T3X tempers.
[0031] FIG. 7 shows .DELTA.K-da/dN curves for the 2.times.24 type
alloys in the T351 temper.
[0032] FIG. 8 shows .DELTA.K-da/dN curves for the 2.times.24 type
alloys in the T3x temper.
[0033] FIG. 9 shows ageing curves for various 2.times.24 alloys in
the T351 temper.
[0034] FIG. 10 shows ageing curves for various 2.times.24 alloys in
the T39 temper.
[0035] FIG. 11 shows the relationship between TYS in T3X and the
corresponding T8X tempers.
[0036] FIG. 12 shows the TYS-UTS relationship for the different
2.times.24 alloys in T8X tempers.
[0037] FIG. 13 shows the K.sub.app plane stress fracture toughness
in L-T orientation as a function of TYS: summary of all the T3X
(dotted lines, small symbols) and T8X (thick lines, large symbols)
data.
[0038] FIG. 14 shows .DELTA.K-da/dN curves for some of the
2.times.24 alloys (containing Zr+Sc+0% Mn or 0.3% Mn) in T351 and
T851 tempers.
[0039] FIG. 15 shows .DELTA.K-da/dN curves for some of the
2.times.24 alloys (containing Zr+Sc+0% Mn or 0.3% Mn) in T39 and
T89 tempers.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0040] In accordance with the present invention, an attempt has
been made to improve the damage tolerance of 2.times.24 alloys
suitable for lower wing skin applications (in the form of plate of
thickness typically of the order of 12 to 25 mm) and fuselage skin
applications (in the form of sheet of thickness typically of the
order of 3 to 9 mm). Some applications of 2.times.24 alloys
include, for example, lower wing skin structural members and wing
spar members.
[0041] Several alloys were tested: [0042] 2.times.24 without
Manganese and with 0.1% Zirconium [0043] 2.times.24 without
Manganese and with 0.1% Zirconium plus 300 .mu.g/g of Scandium
[0044] 2.times.24 with 0.25% Manganese and with 0.1% Zirconium plus
300 .mu.g/g of Scandium [0045] 2.times.24 with 0.50% Manganese and
with 0.1% Zirconium plus 300 .mu.g/g of Scandium
[0046] A high damage tolerant 2024 with no addition of Scandium and
Zirconium (internal designation DT, composition in agreement with
AA2024A) is taken as the reference material.
[0047] Specifically, Mn-free 2.times.24 alloys for applications
such as in lower wing skins are found to provide unexpectedly
superior properties. As used herein, "Mn-free" means up to 0.05%
Mn. Although a loss of strength is expected in some cases in the
T351 temper, better damage tolerance can be achieved, owing to a
lower volume fraction of A1FeMn-type coarse intermetallics.
[0048] In a preferred embodiment, the Scandium content was chosen
at a level of 300 ppm in order to substantially avoid the
precipitation of coarse (Al, Cu, Sc) primary phases while keeping a
strong anti-recrystallization influence. However, different amounts
of scandium might be possible as well without departing from the
scope of the present invention.
[0049] According to preferred embodiments of the present invention,
there is provided an Al alloy sheet or plate product comprising:
3.6-4.5% Cu, 1.0-1.6% Mg, 0.08-0.20% Zr (preferred 0.08-0.14% Zr),
0.0-0.06% Sc (preferred 0.02-0.05% Sc).
[0050] Al alloy sheet or plate products of the present invention
preferably have a recrystallized volume fraction of 5% maximum
according to some embodiments. In particularly advantageous
embodiments there is provided an aluminum alloy sheet or plate
product comprising 3.7-4.2% Cu (preferred 3.8-4.2%), 1.1-1.5% Mg
(preferred 1.2-1.5%), 0.10-0.14% Zr, and 0-0.05% Sc (preferred
0.02-0.05% Sc). In one embodiment, there is provided an aluminum
alloy sheet or plate product that is substantially Mn-free, which
means here having less than 0.05% Mn. In further embodiments, said
sheet or plate product contains up to 0.01% Mn. Scandium, if
included, is preferably included in an amount from 0.02-0.05%; a
Scandium content of 300 ppm (0.03%) by mass has been used in a
preferred embodiment.
[0051] The products according to the present invention can be
subjected to naturally aged tempers with various degrees of
post-quench cold-working (T351, T37, T39 . . . ) and artificially
aged tempers with various degrees of post-quench cold-working
(T851, T87, T89 . . . ).
[0052] A preferred method for obtaining plate products according to
the present invention comprises: [0053] (a) Casting of a rolling
ingot, followed by optional stress relieving, and scalping, [0054]
(b) Homogenization at a temperature between 450 and 510.degree. C.,
[0055] (c) Hot-rolling on a reversing mill, preferably with an exit
temperature between 350 and 390.degree. C., [0056] (d) Optionally,
for plate with a thickness of less than about 30 mm, one
intermediate reheating to about 480.degree. C., followed by one or
more hot-rolling passes, the final exit temperature preferably
being comprised between 350 and 370.degree. C., [0057] (e) Solution
heat treatment at a temperature between 490 and 510.degree. C.,
followed by a water quench and natural aging, and [0058] (f) Cold
working by stretching alone or cold rolling followed by stretching,
optionally followed by artificial aging.
[0059] A preferred method for obtaining sheet products according to
the present invention comprises: [0060] (a) Casting of a rolling
ingot, followed by optional stress relieving, and scalping, [0061]
(b) Homogenization at a temperature between 450 and 510.degree. C.,
[0062] (c) Hot-rolling down to a thickness of less than 12 mm, and
in any case not more than 200%, and preferably not more than 150%
of final thickness, with a final exit temperature between 230 and
350.degree. C., preferably between 230 and 300.degree. C., and more
preferably between 230 and 255.degree. C., [0063] (d) Optionally
cold rolling, [0064] (e) Solution heat treatment at a temperature
between 490 and 510.degree. C., followed by a water quench, [0065]
(f) Cold working by stretching alone or cold rolling followed by
stretching, optionally followed by artificial aging.
[0066] This preferred method for obtaining sheet is very simple and
does not involve reheating between hot-rolling steps, or
recrystallization treatment.
[0067] The product according to the present invention is
particularly suitable for use as a lower wing skin structural
member. Another advantageous use is the use as fuselage skin sheet.
Both sheet and plate can be clad.
[0068] A preferred sheet or thin plate with a thickness below about
12 mm in T351 temper has a da/dn in T-L direction which fulfils at
least one, and preferably two or more, and even more preferably all
of the following conditions: [0069] da/dn less than 1.3 10.sup.-4
mm/cycles at .DELTA.K=10 MPa m, [0070] da/dn less than 4.0
10.sup.-4 mm/cycles at .DELTA.K=15 MPa m, [0071] da/dn less than
8.0 10.sup.-4 mm/cycles at .DELTA.K=20 MPa m, [0072] da/dn less
than 16 10.sup.4 mm/cycles at .DELTA.K=25 MPa m, [0073] da/dn less
than 25 10.sup.-4 mm/cycles at .DELTA.K=30 MPa m.
[0074] A preferred plate in T351 temper has a da/dn in T-L
direction which fulfils at least one, and preferably two or more,
and even more preferably all of the following conditions: [0075]
da/dn less than 3.0 10.sup.-5 mm/cycles at .DELTA.K=10 MPa m,
[0076] da/dn less than 1.0 10.sup.-4 mm/cycles at .DELTA.K=15 MPa
m, [0077] da/dn less than 1.0 10.sup.-3 mm/cycles at .DELTA.K=25
MPa m, [0078] da/dn less than 3.0 10.sup.-3 mm/cycles at
.DELTA.K=30 MPa m.
[0079] Products according to the present invention exhibit in a
corrosion test according to ASTM G 110 a maximum intergranular
corrosion attack of less than 80 .mu.m in T39 temper, and/or less
than 200 .mu.m in T851 temper, and/or less than 250 1 .mu.m in T89
temper, and/or less than 300 .mu.m in T351 temper. In a preferred
embodiment, they have a maximum intergranular attack of less than
70 .mu.m in T39 temper, and/or less than 180 .mu.m in T851 temper,
and/or less than 220 .mu.m in T89 temper, and/or less than 270
.mu.m in T351 temper.
[0080] It should be noted that according to some embodiments of the
present invention, scandium, although preferred, can optionally be
replaced by one or more of the following chemical elements: Hf, La,
Ti, Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Y, Yb, Cr. Typically, the
concentration of each of these elements should not exceed about
0.1%, and the total of said elements should not exceed about
0.3%.
EXAMPLES
Example 1
Manufacturing and Microstructural Characterization
A) Manufacturing of Alloys/Tempers
[0081] Casting of several ingots was conducted at a laboratory
scale cast house, on (320 mm.times.120 mm) slabs (2t casting unit).
The compositions in weight % are given in Table 1. TABLE-US-00001
TABLE 1 Composition of the alloys (in weight %) Alloy Si Fe Cu Mn
Mg Ti Zr Sc DT <0.06 0.06 4.12 0.40 1.37 0.022 DT + Zr <0.06
0.06 3.81 0.008 1.41 0.022 0.109 DT + Zr + Sc <0.06 0.07 3.81
0.008 1.36 0.024 0.107 0.028 24LoMn <0.06 0.05 4.20 0.24 1.23
0.016 0.11 0.032 24HiMn <0.06 0.06 4.14 0.51 1.24 0.019 0.11
0.032
[0082] Table 1 also gives the alloy designations that will be used
hereinbelow: [0083] -DT stands for reference high damage tolerance
2024 (AA2024A) [0084] -DT+ZR and DT+Zr+Sc respectively designate DT
with manganese totally replaced by zirconium and
zirconium+scandium. [0085] 24LoMn and 24HiMn stand for DT (AA2024A)
based compositions with Zr+Sc and various (respectively 0.25% and
0.50%) Mn levels.
[0086] The detailed conditions of the transformation of the slabs
are provided below: [0087] Homogenization on the slabs scalped down
to 100 mm thick. [0088] Reheating at 480+/-10.degree. C. for at
least 30 minutes. [0089] Rolling on the hot reversing mill, down to
a thickness of 20 mm (intermediate re-heating to 480.degree. C. at
about 27 mm, before the two last passes), aiming at an exit
temperature of 370+/-20.degree. C. [0090] Solution heat treatment
of 600 mm (L).times.60 mm (LT).times.20 mm (ST) specimens, in an
air furnace. [0091] Water quench and 24 h natural aging. [0092]
Cold-Working by stretching or cold-rolling+stretching in order to
obtain T3X tempers that will be characterized, or aged to T8X
tempers (see Table 2).
[0093] The details regarding the actual manufacturing parameters
are given in Table 2. TABLE-US-00002 TABLE 2 Manufacturing
conditions Hot-Rolling Hot-Rolling Lay-on Exit Solution
Cold-Working (%) Temperature Temperature Heat (Bold characters
refer to Alloy Homogenization (.degree. C.) (.degree. C.) Treatment
cold-rolling) DT 12 h at 500.degree. C. 480 +/- 50.degree. C. 370
+/- 20.degree. C. 6 h at T351: 0 + 2 (heat-up of 12 h) 500.degree.
C. T39: 1 + 9.6 + 1 (heat-up for T3x: 1 + 12.3 + 1 2 h) T851: 0 + 2
T89: 1 + 9.8 + 1 DT + Zr 480 +/- 50.degree. C. 370 +/- 20.degree.
C. T351: 0 + 2 T39: 1 + 8.3 + 1 T3x: 1 + 12.6 + 1 T851: 0 + 2 T89:
1 + 9.8 + 1 DT + Zr + Sc 480 + 1-50.degree. C. 370 + 1-20.degree.
C. T351: 0 + 2 T39: 1 + 9.8 + 1 T3x: 1 + 12.8 + 1 T851: 0 + 2 T89:
1 + 9.6 + 1 24LoMn 431.degree. C. 379.degree. C. T351: 0 + 1.7 T39:
7.2 + 0.3 T3x: 12.6 + 0.5 T851: 0 + 2 T89: 8.2 + 0.1 24HiMn
442.degree. C. 386.degree. C. T351: 0 + 2.5 T39: 7.3 + 0.7 T3x:
11.8 + 0.3
b) Microstructural Characterization
[0094] The microstructural characterization program of these alloys
was only conducted in the basic T351 temper. It consisted of
Differential Scanning Calorimetry (DSC) and Optical
micrography.
[0095] Table 3 below gives the main microstructural characteristics
of the alloys in the T351 temper. According to the DSC results, all
these alloys seem to be well solutionized. Detailed micrographs of
some of the alloys are provided in FIG. 1. TABLE-US-00003 TABLE 3
DSC results (before and after solution heat treating, sampled at
half-thickness) and grain structure of the plates (chromic etch and
anodic oxidation) DSC - As-Rolled DSC - T351 Microstructure - T351
Peak Peak ReX Temperature Area Temperature Area rate Alloy
(.degree. C.) (J/g) (.degree. C.) (J/g) (%) Grain structure DT --
-- No Peak 0 >95% Coarse and elongated DT + Zr -- -- No Peak 0
.about.85% Coarse and not very elongated; well-defined sub-grains
DT + Zr + Sc -- -- No Peak 0 <5% Very thin and elongated;
well-defined sub-grains 24LoMn 507.4 1.26 No Peak 0 <5% Very
thin and elongated; well-defined sub-grains 24HiMn 508.7 0.56 No
Peak 0 <5% Very thin and elongated; well-defined sub-grains
Example 2
Mechanical and Corrosion Evaluation IN T3X Tempers
[0096] The alloys manufactured in Example 1 in the various T3X
tempers were characterized as follows: [0097] Static Tensile
Testing at half-thickness in the L and LT directions [0098]
Exfoliation corrosion resistance [0099] Damage tolerance: [0100]
Plane stress fracture toughness at half-thickness by K.sub.app
determination on 6.35 mm (0.25'') thick specimens with W=40 mm
(1.6'') in the L-T orientation (according to ASTM E561). [0101]
Fatigue crack growth rate (FCGR) at half-thickness on 6.35 mm
(0.25'') thick "CT" specimens with W=40 mm (1.6'') in the L-T and
T-L orientation (according to ASTM E647).
[0102] The static tensile properties in the T3X tempers are
summarized in Table 4 and FIGS. 2 and 3.
[0103] The following effects are demonstrated: [0104] A Zr+Sc
addition totally compensates for manganese (compare DT and
DT+Zr+Sc). [0105] Manganese is clearly beneficial for UTS and TYS
tensile properties (compare DT+Zr+Sc", "24LoMn", "24HiMn". [0106]
The evolution of tensile properties with post-quench cold-work is
similar for all the 2 xxx variants studied.
[0107] As for elongation, manganese seems to be the most important
compositional parameter (detrimental influence). TABLE-US-00004
TABLE 4 Static properties in various T3X tempers Cold L orientation
LT orientation Work UTS TYS A UTS TYS A Alloy Process Temper [%]
[MPa] [MPa] [%] [MPa] [MPa] [%] DT 2% T351 2.0 503 390 19.6 488 349
20.4 1% + 10% + 1% T39 11.6 539 468 11.7 518 421 13.6 1% + 13% + 1%
T3x 14.3 536 475 9.3 DT + Zr 2% T351 2.0 463 359 23.0 453 325 23.9
1% + 10% + 1% T39 10.3 500 424 15.4 481 392 15.5 1% + 13% + 1% T3x
14.6 511 451 13.4 DT + Zr + SC 2% T351 2.0 498 379 20.0 465 335
24.1 1% + 10% + 1% T39 11.8 532 462 12.9 495 409 17.2 1% + 13% + 1%
T3x 14.8 542 484 10.5 24LoMn 2.5% T351 1.7 497 388 21.1 471 350
22.9 8% + 2.5% T39 7.5 525 442 15.6 495 397 18.5 12% + 2.5% T3x
13.1 545 483 11.9 521 439 15.8 24HiMn 2.5% T351 2.5 526 411 18.0
482 357 22.0 8% + 2.5% T39 8.0 544 460 13.6 503 403 16.7 12% + 2.5%
T3x 12.1 561 506 9.6 528 448 13.1
[0108] Fracture toughness was evaluated by Kahn tear tests (see
Table 5) and K.sub.app R-curve evaluation (see Table 6).
[0109] Kahn tear maximum stress R.sub.e of initiation energy
E.sub.init (energy spent until the maximum stress is reached) are
indicative of the plane stress fracture toughness performance (the
specimen thickness is about 5 mm).
[0110] The K.sub.app evaluation is conducted on thin (6.35
mm-0.25'') CT specimens (width 40 mm 1.6'') and corresponds to
testing conditions close to the R-curve.
[0111] As for T3X fracture toughness results (FIGS. 4 to 6), the
following comments can be made: [0112] The evolution of toughness
with tensile yield strength is very similar when considering the
Kahn tear test or the K.sub.app determination [0113] Both in L-T
and T-L orientations, the toughness performance seems to be clearly
related to the manganese content: DT+Zr and DT+Zr+Sc with no
manganese perform significantly better than the 0.3% Mn variant
(24LoMn) which in turn represent an improvement over the higher
manganese variants (DT, 24HiMn). [0114] In most of the cases,
fracture toughness increases with the amount of cold-work (i.e. the
TYS-K.sub.app relationship is positive), which is very unexpected
(cold-work decreases the material's intrinsic ductility, hence its
toughness).
[0115] However, in cases where some manganese is present (see
especially 24HiMn), a flat and even decreasing TYS-K.sub.app curve
can be observed. This is especially true in the TL orientation (see
FIG. 6). It can thus be assumed that these alloys with high
dispersoid content could be more sensitive to cold-work, possibly
because of dispersoids fracture. TABLE-US-00005 TABLE 5 Kahn
measurements on T3X tempers Kahn Tear Test Tear Stress Opening
[MPa] Energy [J] Alloy Process Temper L-T T-L L-T T-L DT 2% T351
181.5 174.5 26.7 22.9 1% + 10% + 1% T39 189.0 186.0 20.9 19.9 1% +
13% + 1% T3x 181.3 19.6 DT + Zr 2% T351 189.8 185.5 46.7 43.0 1% +
10% + 1% T39 207.0 197.0 36.9 31.9 1% + 13% + 1% T3x 205.6 32.1 DT
+ Zr + Sc 2% T351 196.3 189.0 54.8 49.1 1% + 10% + 1% T39 198.0
193.0 36.7 30.3 1% + 13% + 1% T3x 210.9 34.4 24LoMn 2.5% T351 190.0
34.0 8% + 2.5% T39 200.0 30.0 12% + 2.5% T3x 200.0 27.0 24HiMn 2.5%
T351 180.0 29.0 8% + 2.5% T39 190.0 24.0 12% + 2.5% T3x 190.0
19.5
[0116] As regards the crack propagation performance of the alloys
in T3X tempers, the following points can be stated (Table 6 and
FIGS. 7 and 8): [0117] At intermediate .DELTA.K levels, manganese
seems to play the major role for 2.times.24-type alloys; the higher
the manganese content, the higher the crack propagation rate. It is
assumed that, since manganese-rich dispersoids entail a
homogenization of deformation, the fracture path is smooth. On the
contrary, in the absence of these incoherent dispersoids, some
crack roughness is developed (owing to localization of deformation
on specific habit planes). Because of crack closure phenomena, this
lowers the effective AK at the crack tip, entailing a slower
propagation rate.
[0118] For the 2.times.24-type alloys, the effect of cold work on
the propagation rate at intermediate .DELTA.K levels is either not
significant (DT+Zr+Sc with 0% Mn or 24LoMn with 0.3% Mn) or
beneficial (24HiMn with 0.5% Mn or the incumbent DT).
TABLE-US-00006 TABLE 6 K.sub.app and da/dN measurements on 0.25''
thick W = 1.6'' CT specimens at T/2, in the L-T and T-L
orientations for T3X tempers K.sub.app test on CT 6.35 mm L-T
FCGR(*) on specimen CT 6.35 mm specimen [MPa m] da/dN in mm/cycle
at .DELTA.K = [MPa m] Alloy Process Temper T-L L-T 10 15 25 30 DT
2% T351 71.1 4.2 10.sup.-5 3.6 10.sup.-4 2.6 10.sup.-3 -- 1% + 10%
+ 1% T39 74.8 2.3 10.sup.-5 1.3 10.sup.-4 1.3 10.sup.-3 T3x 75.8
1.4 10.sup.-5 7.3 10.sup.-5 2.0 10.sup.-3 -- DT + Zr 2% T351 76.6
3.1 10.sup.-5 1.3 10.sup.-4 2.0 10.sup.-3 -- 1% + 10% + 1% T39 86.8
1.5 10.sup.-5 2.1 10.sup.-5 4.5 10.sup.-4 7.5 10.sup.-4 1% + 13% +
1% T3x 88.2 1.4 10.sup.-5 4.0 10.sup.-5 3.7 10.sup.-4 1.8 10.sup.-3
DT + Zr + Sc 2% T351 75.5 2.2 10.sup.-5 3.8 10.sup.-5 7.1 10.sup.-4
2.5 10.sup.-3 1% + 10% + 1% T39 87.0 2.6 10.sup.-5 4.7 10.sup.-5
6.6 10.sup.-4 -- 1% + 13% + 1% T3x 87.8 1.8 10.sup.-5 3.0 10.sup.-5
7.4 10.sup.-4 -- 24LoMn. 2.5% T351 70.0 77.6 1.5 10.sup.-5 3.8
10.sup.-5 -- 8% + 2.5% T39 72.0 79.0 2.7 10.sup.-5 9.6 10.sup.-5
1.3 10.sup.-3 3.0 10.sup.-3 12% + 2.5% T3x 69.6 83.3 1.7 10.sup.-5
4.8 10.sup.-5 5.2 10.sup.-4 -- 24HiMn 2.5% T351 64.1 75.0 1.9
10.sup.-5 2.0 10.sup.-4 1.2 10.sup.-3 4.0 10.sup.-3 8% + 2.5% T39
60.0 75.0 7.8 10.sup.-6 5.1 10.sup.-5 1.9 10.sup.-3 T3x 53.3 70.9
1.2 10.sup.-5 4.3 10.sup.-5 1.4 10.sup.-3 -- (*)FCGR = Fatigue
Crack Growth Rate
[0119] The exfoliation corrosion ratings after the EXCO test (ASTM
G34) are given in Table 7. The alloys containing no manganese seem
to be slightly more sensitive (especially the DT+Zr+Sc variant
which shows a very oriented grain structure). TABLE-US-00007 TABLE
7 EXCO (ASTM G34) rating for the different alloys in different
tempers EXCO Rating (ASTM G34) Alloy Process Temper Surface
Half-thickness DT 2% T351 P EA DT + Zr 2% T351 P EA DT + Zr + Sc 2%
T351 P EB/EC 24LoMn 2% T351 N P 24HiMn 2% T351 N P/EA
Example 3
Mechanical and Corrosion Evaluation in T8X Tempers
[0120] The alloys manufactured in Example 1 (various T3X tempers)
were artificially aged to T8X tempers as explained in Example
1.
[0121] The high manganese variant named 24HiMn was not selected for
the T78X evaluation, due to its relatively poor toughness.
[0122] Prior to the artificial aging treatment, aging kinetics
(using Vickers hardness as a strength indicator) have been
conducted on the various alloys in different T3X conditions. The
results are provided in FIGS. 9 and 10.
[0123] On some of the cases (apparently independent of alloy
chemistry and T3X temper), an initial decrease of hardness is
observed for low ageing times; this is probably due to
retrogression phenomena. Then, hardness increases, owing to
precipitation hardening. A peak in hardness is generally observed,
before hardness slowly decreases by over-ageing.
[0124] Table 8 below gives the aging treatment duration chosen for
the complete characterization program in the T8X tempers.
TABLE-US-00008 TABLE 8 Ageing treatments chosen for the complete
characterization in the T8X tempers Ageing Alloy Process Temper
Cold Work [%] Time at 173.degree. C. DT 2% T851 2.0% 20 h 1% + 10%
T89 11.8% 10 h DT + Zr 2% T851 2.0% 20 h 1% + 10% T89 11.8% 10 h DT
+ Zr + Sc 2% T851 2.0% 20 h 1% + 10% T89 11.6% 10 h 24LoMn 2% T851
2.0% 20 h 8% + 2% T89 8.3% 20 h
[0125] The static tensile properties in the T8X tempers are
summarized in Table 9 and FIGS. 11 and 12. TABLE-US-00009 TABLE 9
Static properties in various T8X tempers L orientation For
Comparision: Cold T8X T3X Work UTS TYS A UTS TYS A Alloy Process
Temper [%] MPa MPa [%] [MPa] [MPa] [%] DT 2% T851 2.0 514 477 10
503 390 19.6 1% + 10% + 1% T89 11.8 547 529 8 539 468 11.7 DT + Zr
2% T851 2.0 499 455 12 463 359 23 1% + 10% + 1% T89 11.8 527 498 11
500 424 15.4 DT + Zr + Sc 2% T851 2.0 510 466 13.6 498 379 20 1% +
10% + 1% T89 11.6 551 525 14 532 462 13 24LoMn 2% T851 2.0 506 454
14 497 388 21 8% + 2% T89 8.3 535 510 12 525 442 15.6
[0126] Regarding the T8X fracture toughness results (Table 10 and
FIG. 13): [0127] First of all, the T8X fracture toughness is almost
always inferior to that of the corresponding T3X temper. This is
frequently observed in alloy products of the 2XXX series and
corresponds to an overall decrease in ductility. [0128] The only
exception to this regards DT+Zr+Sc which shows a slightly higher
toughness in the T851 temper than in the T351 condition. [0129] The
TYS-K.sub.app relationships in the T8X tempers (linked to the
amount of cold-work) are either "slightly positive" (DT+Zr+Sc,
DT+Zr), "flat" (DT) or "negative" (24LoMn). [0130] There is still a
strong detrimental influence of manganese on fracture toughness in
the T8X tempers. [0131] As for the 2.times.24-type alloys, the loss
in fracture toughness from T3X to T8X tempers is much more limited
for the 0% Mn variant containing Zr+Sc (DT+Zr+Sc) than for the
others: standard DT, 0% Mn with no scandium (DT+Zr) and 0.3% Mn
with a Zr+Sc addition (24LoMn).
[0132] As regards the crack propagation performance (FCGR=Fatigue
Crack Growth Rate) of the alloys in T8X tempers (Table 10 and FIGS.
14 and 15): [0133] The crack propagation behavior at low and medium
.DELTA.K levels is strongly degraded in the T8X tempers in
comparison to the T3X performance. The reason is not totally clear,
but could be related to the homogenization of deformation in
artificially aged tempers. [0134] There is very little influence of
the degree of cold-work on the crack growth rate of T8X
tempers.
[0135] When all the alloys are considered in the various T8X
tempers, it is noticeable that their crack propagation performances
are very similar. TABLE-US-00010 TABLE 10 K.sub.app and da/dN
measurements on 0.25'' thick W = 1.6'' CT specimens at T/2, in the
L-T orientation for T8X tempers K.sub.app test on CT 6.35 mm
specimen L-T FCGR on CT 6.35 mm specimen [MPa m] da/dN in mm/cycle
at .DELTA.K = [MPa m] Alloy Process Temper L-T 10 15 20 25 30 DT 2%
T851 65.8 1.0 10.sup.-4 3.5 10.sup.-4 8.6 10.sup.-4 2.3 10.sup.-3
3.4 10.sup.-3 1% + 10% + 1% T89 64.7 3.1 10.sup.-5 2.8 10.sup.-4
1.0 10.sup.-3 2.1 10.sup.-3 DT + Zr 2% T851 75.4 7.4 10.sup.-5 3.1
10.sup.-4 7.1 10.sup.-4 1.5 10.sup.-3 2.4 10.sup.-3 1% + 10% + 1%
T89 76.5 2.6 10.sup.-5 2.1 10.sup.-4 6.1 10.sup.-4 1.2 10.sup.-3
2.1 10.sup.-3 DT + Zr + Sc 2% T851 79.9 1.0 10.sup.-4 3.6 10.sup.-4
8.0 10.sup.-4 1.3 10.sup.-3 2.7 10.sup.-3 1% + 10% + 1% T89 82.1
8.7 10.sup.-5 3.0 10.sup.-4 6.8 10.sup.-4 1.4 10.sup.-3 2.8
10.sup.-3 24LoMn 2% T851 72.9 1.1 10.sup.-4 3.7 10.sup.-4 7.8
10.sup.-4 1.7 10.sup.-3 3.3 10.sup.-3 8% + 2% T89 65.9 9.2
10.sup.-5 3.5 10.sup.-4 7.7 10.sup.-4 1.7 10.sup.-3 3.7
10.sup.-3
[0136] Table 11 below summarizes the EXCO results obtained on the
T8X tempers for the different alloys. The results obtained on the
T351 tempers are recalled. In the T8X tempers, it is noticed that
the corrosion susceptibility decreases from T851 to T89 tempers,
provided that the ageing treatment is the same (20 h at 173.degree.
C.). This is probably due to a more extensive intragranular
precipitation in the case of strongly cold-worked tempers. When
such a strong cold-work is followed by a shorter ageing treatment,
the intragranular precipitation is probably not very different (in
terms of solute content decrease) from that of the T351 temper, and
corrosion susceptibility is similar. TABLE-US-00011 TABLE 11 EXCO
(ASTM G34) rating for the different alloys in different tempers
EXCO Rating (ASTM G34) Alloy Process Temper Surface T/2 DT 2% T351
P EA 2% T851 EB EA/EB 1% + 10% + 1% T89* EB/EC EA/EB DT + Zr 2%
T351 P EA 2% T851 EB EA/EB 1% + 10% + 1% T89* EC EA/EB DT + Zr + Sc
2% T351 P EB/EC 2% T851 EB/EC EB 1% + 10% + 1% T89* EB/EC EB/EC
24LoMn 2% T351 N P 2% T851 EC EB/EC 8% + 2% T89 EB EB *shorter
ageing treatment
Example 4
Fuselage Skin Sheets
[0137] Two alloys N and M with a chemical composition according to
the invention were elaborated. The liquid metal was treated firstly
in the holding furnace by injecting gas using a type of rotor known
under the trade mark IRMA, and then in a type of ladle known under
the trade mark Alpur. Refining was done with AT5B wire (0.7
kg/ton). 3.2 m-long ingots were cast, with a section of 320
mm.times.120 mm. They were relaxed for 10 h at 350.degree. C.
[0138] The ingots were then homogenized at 500.degree. C. for 12
hours and then hot rolled to a thickness of 6 mm. The exit
temperature from the hot rolling mill was between 230.degree. C.
and 255.degree. C. From ingot N, four sheets labeled N1, N2, N3 and
N4 were obtained in this way. They were all solution heat treated
in a salt bath furnace for 1 hour at 500.degree. C., and then water
quenched. Up to this point, the five sheets M, N1, N2, N3 and N4
were elaborated by the same process. [0139] M and N1 were stretched
with a permanent set of 2%; M and N1 correspond thus to a T351
temper. [0140] N2 was cold rolled with a reduction of 7 to 8%, and
then stretched with a peal anent set of 2%; N2 corresponds thus to
a T39 temper. [0141] N3 was stretched with a permanent set of 2%
and then aged at 173.degree. C. during 10 hours; [0142] N3
corresponds thus to a T851 temper.
[0143] N4 was cold rolled with a reduction of 7 to 8%, stretched
with a permanent set of 2%, and finally aged at 173.degree. C.
during 10 hours; N4 corresponds thus to a T89 temper.
[0144] An alloy E according to prior art was elaborated using the
same casting and hot rolling process as for alloy N. Solution heat
treatment was done in a salt bath furnace for 1 hour at 500.degree.
C. on test coupons of size 600 mm.times.200 mm, followed by
quenching in water (about 20.degree. C.) and stretching to a
permanent set of 2% (temper T351).
[0145] The chemical compositions of the alloys N and E alloys
measured on a spectrometry slug taken from the launder, are given
in Table 12: TABLE-US-00012 TABLE 12 Chemical composition Alloy Si
Fe Cu Mn Mg Zr Sc M <0.06 0.06 3.81 0.008 1.41 0.11 -- N
<0.06 0.07 3.81 0.008 1.36 0.11 0.028 E <0.06 0.06 4.12 0.4
1.37 -- --
[0146] No zinc and chromium were detected.
[0147] The ultimate tensile strength (UTS) R.sub.m (in MPa), the
tensile yield stress (TYS) at 0.2% elongation R.sub.p0.2 (in MPa)
and the elongation at failure A (in %) were measured by a tensile
test according to EN 10002-1.
[0148] Table 13 contains the results of measurements of static
mechanical characteristics: TABLE-US-00013 TABLE 13 Static
mechanical characteristics L direction LT direction UTS TYS UTS TYS
R.sub.m R.sub.p0,2 A R.sub.m R.sub.p0,2 A Sheet [MPa] [MPa] [%]
[MPa] [MPa] [%] M 463 348 27.4 453 312 26.7 N1 459 349 23.8 446 313
25.8 E 482 365 22.8 466 319 23.5 N2 478 436 13 473 393 15 N3 472
409 15.4 460 383 17 N4 521 501 11.4 509 469 13.2
[0149] The UTS and TYS of sheets M and N1, according to the
invention, are almost comparable to those of sheet E, according to
prior art, but their elongation is significantly higher. Sheet N2
(T39 temper), N3 (T851 temper) and especially N4 (T89 temper)
exhibit improved mechanical properties compared to sheets M, N1 and
E, as well as elongation values which are deemed sufficient for the
application as fuselage skin sheet.
[0150] Damage tolerance was characterized in the T-L direction
using the maximum stress R.sub.e (in MPa) and the creep energy
E.sub.ec as derived from the Kahn test. The Kahn stress is equal to
the ratio of the maximum load F.sub.max that the test piece can
resist on the cross section of the test piece (product of the
thickness B and the width W). The creep energy is determined as the
area under the Force-Displacement curve as far as the maximum force
F.sub.max resisted by the test piece. The Kahn test, well known to
one skilled in the art, is described in the article "Kahn-Type Tear
Test and Crack Toughness of Aluminum Alloy Sheet" published in the
Materials Research & Standards Journal, April 1964, p. 151-155.
The content of said article is incorporated herein by reference in
its entirety. The test piece used for the Kahn toughness test is
described in the "Metals Handbook", 8.sup.th Edition, vol. 1,
American Society for Metals, pp. 241-242. The results are given in
table 14: TABLE-US-00014 TABLE 14 Results derived from the Kahn
test R.sub.e [MPa] E.sub.e [J] Sheet (T-L) (T-L) M 185 -- N1 184
47.4 E 177 35.1
[0151] The maximum stress to which sheet N1 is capable of resisting
is higher than that of sheet E, for a higher creep energy.
[0152] Fracture toughness was also determined for sheets N1, N2,
N3, N4 and E by a measurement of the plane stress fracture
toughness K.sub.app according to ASTM E 561 in the T-L direction
using C(T) test pieces with W=127 mm. Results are given in table
15. TABLE-US-00015 TABLE 15 K.sub.app results Sheet K.sub.app [MPa
m] M 112 N1 112 N2 113 N3 118 N4 112 E 105
[0153] The sheet according to the present invention, and especially
in T851 temper (sheets N3), show significantly improved K.sub.app
values.
[0154] Fatigue resistance was determined according to ASTM E 647,
by measuring the fatigue crack growth rate using C(T) test pieces
with W=75 mm. The fatigue crack growth rate da/dN (in mm/cycle) for
different levels of AK (expressed in MPa m) was determined. Results
are displayed in table 16. TABLE-US-00016 TABLE 16 Fatigue
resistance da/dN at .DELTA.K (MPa m), T-L direction, (10.sup.-4
mm/cycles) 10 15 20 Sheet MPa m MPa m MPa m 25 MPa m 30 MPa m M
1.21 3.46 7.27 12.9 20.7 N1 (invention) 1.18 3.53 7.68 14 22.9 N2
(invention) 1.1 3.6 8.2 14.4 30.1 N3 (invention) 1.4 4.0 8.4 13.8
23.4 N4 (invention) 1.1 3.4 7.7 11.8 26.3 E (prior art) 1.4 4.3 9.6
17.8 29.6
[0155] All sheets according to the invention have a fatigue crack
growth rate at least as good as sheet E according to prior art,
most are significantly better, and especially sheets M and N1.
[0156] Corrosion resistance was evaluated according ASTM G 110.
After etching and polishing, the maximum depth of corrosion attack
was evaluated. All samples exhibited intergranular corrosion
attack, but the maximum depth of corrosion was only 40 .mu.m for
N2, 165 .mu.m for N3, 180 .mu.m for N4 and 225 .mu.m for N1,
whereas sample E according to prior art exhibited a maximum depth
of 350 .mu.m. Sample N2 also showed pitting, but at maximum depth
not exceeding 60 .mu.m.
[0157] Additional advantages, features and modifications will
readily occur to those skilled in the art. Therefore, the invention
in its broader aspects is not limited to the specific details, and
representative devices, shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
[0158] As used herein and in the following claims, articles such as
"the", "a" and "an" can connote the singular or plural.
[0159] All documents referred to herein are specifically
incorporated herein by reference in their entireties.
* * * * *