U.S. patent application number 12/339611 was filed with the patent office on 2009-06-25 for al-li rolled product for aerospace applications.
This patent application is currently assigned to ALCAN RHENALU. Invention is credited to Armelle Danielou, Jean Christophe Ehrstrom.
Application Number | 20090159159 12/339611 |
Document ID | / |
Family ID | 39262686 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090159159 |
Kind Code |
A1 |
Danielou; Armelle ; et
al. |
June 25, 2009 |
Al-Li ROLLED PRODUCT FOR AEROSPACE APPLICATIONS
Abstract
The present invention is directed to a substantially
unrecrystallized rolled aluminum alloy product, obtained from a
plate with a thickness of at least 30 mm, comprising 2.2 to 3.9 wt.
% Cu, 0.7 to 2.1 wt. % Li, 0.2 to 0.8 wt. % Mg, 0.2 to 0.5 wt. %
Mn, 0.04 to 0.18 wt. % Zr, less than 0.05 wt. % Zn, and optionally
0.1 to 0.5 wt. % Ag, remainder aluminum and unavoidable impurities
having a low propensity to crack branching during L-S a fatigue
test. A product of the invention has a crack deviation angle
.THETA. of at least 20.degree. under a maximum equivalent stress
intensity factor Keff max of 10 MPa m for a S-L cracked test sample
under a mixed mode I and mode II loading wherein the angle .PSI.
between a plane perpendicular to the initial crack direction and
the load direction is 75.degree.
Inventors: |
Danielou; Armelle; (Les
Echelles, FR) ; Ehrstrom; Jean Christophe;
(Echirolles, FR) |
Correspondence
Address: |
Baker Donelson Bearman, Caldwell & Berkowitz, PC
555 Eleventh Street, NW, Sixth Floor
Washington
DC
20004
US
|
Assignee: |
ALCAN RHENALU
Courbevoie
FR
|
Family ID: |
39262686 |
Appl. No.: |
12/339611 |
Filed: |
December 19, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61020038 |
Jan 9, 2008 |
|
|
|
Current U.S.
Class: |
148/502 ;
148/417 |
Current CPC
Class: |
C22C 21/12 20130101;
C22F 1/057 20130101; C22C 21/16 20130101 |
Class at
Publication: |
148/502 ;
148/417 |
International
Class: |
C22C 21/16 20060101
C22C021/16; C21D 11/00 20060101 C21D011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2007 |
FR |
0709069 |
Claims
1. A method for producing a substantially unrecrystallized aluminum
alloy plate with a thickness of at least 30 mm said method
comprising: a) casting an ingot comprising 2.2 to 3.9 wt. % Cu, 0.7
to 2.1 wt. % Li, 0.2 to 0.8 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0.04 to
0.18 wt. % Zr, less than 0.05 wt. % Zn, and optionally 0.1 to 0.5
wt. % Ag, remainder aluminum and unavoidable impurities, b)
homogenizing said ingot at 470-510.degree. C. for a duration from 2
to 30 hours, c) hot rolling said ingot with an exit temperature of
at least 410.degree. C. to a plate with a final thickness of at
least 30 mm, d) solution heat treating by soaking at 490 to
540.degree. C. for 15 min to 4 h, wherein the total equivalent time
of homogenization and solution heat treatment t(eq) t ( eq ) =
.intg. exp ( - 26100 / T ) t exp ( - 26100 / T ref ) ##EQU00005##
does not exceed 30 h, where T (in Kelvin) is the instantaneous
temperature of treatment, which evolves with the time t (in hours),
and T.sub.ref is a reference temperature set at 773 K, e) cold
water quenching, f) stretching said plate with a permanent set from
2 to 5%, g) aging said plate by heating at 130-160.degree. C. for 5
to 60 hours.
2. A method according to claim 1 wherein Li+Cu>4 wt. % and
preferably Li+Cu>4.3 wt. %.
3. A method according to claim 1 wherein Li+0.7 Cu<4.3
4. A method according to claim 1 wherein the lithium content is
from 0.8 to 1.8 wt. %.
5. A method according to claim 1 wherein the lithium content is
from 0.9 to 1.4 wt. %
6. A method according to claim 1 wherein the copper content is from
2.7 to 3.9 wt. %.
7. A method according to claim 6 wherein the copper content is from
3.2 to 3.9 wt. %.
8. A method according to claim 1 wherein the manganese content is
from 0.3 to 0.5 wt. %.
9. A method according to claim 1 wherein said hot rolling exit
temperature is at least 430.degree. C.
10. A method according to claim 1 wherein said aging is done by
heating at 140-160.degree. C. from 12 to 50 hours.
11. A substantially unrecrystallized rolled aluminum alloy product
with a thickness of at least 30 mm having a low propensity to crack
branching obtained from a process according to claim 1.
12. A substantially unrecrystallized rolled aluminum alloy product,
obtained from a plate with a thickness of at least 30 mm, which has
a crack deviation angle .THETA. of at least 20.degree. under a
maximum equivalent stress intensity factor K.sub.eff max of 10 MPa
m for a S-L cracked test sample under a mixed mode I and mode II
loading wherein the angle .PSI. between a plane perpendicular to
the initial crack direction and the load direction is
75.degree..
13. A substantially unrecrystallized rolled aluminum alloy product,
obtained from a plate with a thickness of at least 30 mm, which
exhibit crack branching on not more than 20% L-S test samples after
testing at least four different samples in a fatigue test according
to ASTM E 647 with R=0.1 and .sigma..sub.max=220 MPa.
14. A product according to claim 11 with a thickness of at most 100
mm having in a T8 temper at least one of a1 and a2 and/or at least
one of b1, b2 and b3 where a1, a2, b1, b2 and b3 are the following:
a1: a tensile yield strength at T/4 and T/2 of at least 455 MPa in
the L-direction. a2: an ultimate tensile strength at T/4 and T/2 of
at least 490 MPa, in the L-direction. b1: a fracture toughness
K.sub.1C: in L-T direction at T/4 and T/2 of at least 31 MPa m; b2:
a fracture toughness K.sub.1C: in T-L direction at T/4 and T/2 of
at least 28 MPa m; b3: a fracture toughness K.sub.1C: in S-L
direction at T/4 and T/of at least 25 MPa m.
15. A product according to claim 11 with a thickness higher than
100 mm having in a T8 temper at least one of a4 and a5 and/or at
least one of b4, b5 and b6 where a4, a5, b4, b5 and b6 are the
following: a4: a tensile yield strength at T/4 and T/2 of at least
440 MPa in the L-direction. a5: an ultimate tensile strength at T/4
and T/2 of at least 475 MPa, in the L-direction. b4: a fracture
toughness K.sub.1C: in L-T direction at T/4 and T/2 of at least 26
MPa m; b5: a fracture toughness K.sub.1C: in T-L direction at T/4
and T/2 of at least 25 MPa m; b6: a fracture toughness K.sub.1C: in
S-L direction at T/4 and T/of at least 24 MPa m.
16. A product according to claim 12 with a thickness of at most 100
mm having in a T8 temper at least one of a1 and a2 and/or at least
one of b1, b2 and b3 where a1, a2, b1, b2 and b3 are the following:
a1: a tensile yield strength at T/4 and T/2 of at least 455 MPa in
the L-direction. a2: an ultimate tensile strength at T/4 and T/2 of
at least 490 MPa, in the L-direction. b1: a fracture toughness
K.sub.1C: in L-T direction at T/4 and T/2 of at least 31 MPa m; b2:
a fracture toughness K.sub.1C: in T-L direction at T/4 and T/2 of
at least 28 MPa m; b3: a fracture toughness K.sub.1C: in S-L
direction at T/4 and T/of at least 25 MPa m.
17. A product according to claim 12 with a thickness higher than
100 mm having in a T8 temper at least one of a4 and a5 and/or at
least one of b4, b5 and b6 where a4, a5, b4, b5 and b6 are the
following: a4: a tensile yield strength at T/4 and T/2 of at least
440 MPa in the L-direction. a5: an ultimate tensile strength at T/4
and T/2 of at least 475 MPa, in the L-direction. b4: a fracture
toughness K.sub.1C: in L-T direction at T/4 and T/2 of at least 26
MPa m; b5: a fracture toughness K.sub.1C: in T-L direction at T/4
and T/2 of at least 25 MPa m; b6: a fracture toughness K.sub.1C: in
S-L direction at T/4 and T/of at least 24 MPa m.
18. A product according to claim 13 with a thickness of at most 100
mm having in a T8 temper at least one of a1 and a2 and/or at least
one of b1, b2 and b3 where a1, a2, b1, b2 and b3 are the following:
a1: a tensile yield strength at T/4 and T/2 of at least 455 MPa in
the L-direction. a2: an ultimate tensile strength at T/4 and T/2 of
at least 490 MPa, in the L-direction. b1: a fracture toughness
K.sub.1C: in L-T direction at T/4 and T/2 of at least 31 MPa m; b2:
a fracture toughness K.sub.1C: in T-L direction at T/4 and T/2 of
at least 28 MPa m; b3: a fracture toughness K.sub.1C: in S-L
direction at T/4 and T/of at least 25 MPa m.
19. A product according to claim 13 with a thickness higher than
100 mm having in a T8 temper at least one of a4 and a5 and/or at
least one of b4, b5 and b6 where a4, a5, b4, b5 and b6 are the
following: a4: a tensile yield strength at T/4 and T/2 of at least
440 MPa in the L-direction. a5: an ultimate tensile strength at T/4
and T/2 of at least 475 MPa, in the L-direction. b4: a fracture
toughness K.sub.1C: in L-T direction at T/4 and T/2 of at least 26
MPa m; b5: a fracture toughness K.sub.1C: in T-L direction at T/4
and T/2 of at least 25 MPa m; b6: a fracture toughness K.sub.1C: in
S-L direction at T/4 and T/of at least 24 MPa m.
20. A structural member obtained from a product according to claim
11.
21. A structural member obtained from a product according to claim
12.
22. A structural member obtained from a product according to claim
13.
23. A structural member according to claim 20 comprising a spar, a
rib or a frame suitable for aircraft construction.
24. A structural member according to claim 20 comprising a complex
shaped part made by integral machining which is suitable for
aircraft wing construction.
25. A structural member according to claim 21 comprising a spar, a
rib or a frame suitable for aircraft construction.
26. A structural member according to claim 21 comprising a complex
shaped part made by integral machining which is suitable for
aircraft wing construction.
27. A structural member according to claim 22 comprising a spar, a
rib or a frame suitable for aircraft construction.
28. A structural member according to claim 22 comprising a complex
shaped part made by integral machining which is suitable for
aircraft wing construction.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to aluminum-lithium
alloys, and in particular, to such alloys useful in the aerospace
industry.
[0003] 2. Description of Related Art
[0004] Aluminum-lithium alloys have long been recognized as an
effective solution to reduce weight of structural elements because
of the low density of these alloys. However, the different
requirements of the aircraft industry materials such as, having a
high Young modulus, high compression resistance, high damage
tolerance and high corrosion resistance, have proven difficult to
be obtained simultaneously. Al--Li alloys are particularly
sensitive to crack turning or crack deflection, which is among the
problems related to damage tolerance limiting their use. (Hurtado,
J A; de los Rios, E R; Morris, A. J, <<Crack deflection in
Al--Li alloys for aircraft structures", 18th Symposium of the
International Committee on Aeronautical Fatigue, Melbourne; UNITED
KINGDOM; 3-5 May 1995. pp. 107-136.1995).
[0005] Crack deviation, crack turning or also crack branching are
terms used to express propensity for crack propagation to deviate
from the expected fracture plane perpendicular to the loading
direction during a fatigue or toughness test. Crack deviation
happens on a microscopic scale (<100 .mu.m), on a mesoscopic
scale (100-1000 .mu.m) or on a macroscopic scale (>1 mm) but it
is considered detrimental only if the crack direction remains
stable after deviation (macroscopic scale). The phenomenon is a
particular concern for fatigue trials in L-S direction for
aluminum-lithium alloys. The term crack branching is used herein
for macroscopic deviation of cracks in L-S fatigue or toughness
tests from the S direction towards the L direction which occurs for
rolled products with a thickness of 30 mm or higher. Crack
branching may occur in relation to the rolled product composition
and microstructure and to the test conditions. Rolled products made
of AA7050 can be considered as a reference of products having a low
propensity to crack branching.
[0006] Crack branching has been considered as a major problem by
aircraft manufacturers because it is difficult to take into account
to dimension parts, thereby making impossible the use of
traditional design methods. Thus, crack branching invalidates
conventional, mode I based, materials testing procedure and design
models. The crack branching problem has proven difficult to solve.
Recently it was considered that in the absence of solution for
avoiding crack branching, efforts should be directed to predicting
crack branching behaviors. (M. J. Crill, D. J. Chellman, E. S.
Balmuth, M. Philbrook, K. P. Smith, A. Cho, M. Niedzinski, R.
Muzzolini and J. Feiger, Evaluation of AA 2050-T87 Al--Li Alloy
Crack Turning Behavior, Materials Science Forum, Vol 519-521 (July
2006) pp 1323-1328).
[0007] There is a need for an aluminum lithium alloy rolled product
for aircraft applications and in particular for integrally machined
parts, which has a low propensity to crack branching.
SUMMARY OF THE INVENTION
[0008] For these and other reasons, the present inventors arrived
at the present invention directed to an aluminum copper, lithium
alloy rolled product, that has a low propensity to crack branching
and exhibits high strength, high toughness and high corrosion
resistance.
[0009] In accordance with these and other objects, the present
invention is directed to a substantially unrecrystallized rolled
aluminum alloy plate with a thickness of at least 30 mm, comprising
2.2 to 3.9 wt. % Cu, 0.7 to 2.1 wt. % Li, 0.2 to 0.8 wt. % Mg, 0.2
to 0.5 wt. % Mn, 0.04 to 0.18 wt. % Zr, less than 0.05 wt. % Zn,
and optionally 0.1 to 0.5 wt. % Ag, remainder aluminum and
unavoidable impurities. A plate of the present invention generally
has a crack deviation angle .THETA. of at least 20.degree. under a
maximum equivalent stress intensity factor K.sub.eff max of 10 MPa
m for a S-L cracked test sample under a mixed mode I and mode II
loading wherein the angle .PSI. between a plane perpendicular to
the initial crack direction and the load direction is
75.degree..
[0010] Another object of the invention is directed to a method for
producing a substantially unrecrystallized aluminum alloy plate
with a thickness of at least 30 mm having a low propensity to crack
branching. A suitable method according the present invention
comprises: [0011] a) casting an ingot comprising 2.2 to 3.9 wt. %
Cu, 0.7 to 2.1 wt. % Li, 0.2 to 0.8 wt. % Mg, 0.2 to 0.5 wt. % Mn,
0.04 to 0.18 wt. % Zr, less than 0.05 wt. % Zn, and optionally 0.1
to 0.5 wt. % Ag, remainder aluminum and unavoidable impurities,
[0012] b) homogenizing the ingot at 470-510.degree. C. for a
duration from 2 to 30 hours, [0013] c) hot rolling the ingot with
an exit temperature of at least 410.degree. C. to a plate with a
final thickness of at least 30 mm, [0014] d) solution heat treating
by soaking at 490 to 540.degree. C. for 15 min to 4 h, wherein the
total equivalent time of homogenization and solution heat treatment
t(eq)
[0014] t ( eq ) = .intg. exp ( - 26100 / T ) t exp ( - 26100 / T
ref ) ##EQU00001## does not exceed 30 h and preferably 20 h, where
T (in Kelvin) is the instantaneous temperature of treatment, which
evolves with the time t (in hours), and T.sub.ref is a reference
temperature set at 773 K, [0015] e) cold water quenching, [0016] f)
stretching the plate with a permanent set from 2 to 5%, [0017] g)
aging the plate by heating at 130-160.degree. C. for 5 to 60
hours.
[0018] Yet another object of the invention is a structural member
formed of a plate according to the present invention. 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
[0019] The accompanying drawings, which are incorporated in and
constitute a part 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.
[0020] FIGS. 1-8 are directed to certain aspects of the invention
as described herein. They are illustrative and not intended as
limiting.
[0021] FIG. 1 diagrammatically shows the location of the Sinclair
sample.
[0022] FIG. 2 shows the geometry of the Sinclair sample.
[0023] FIG. 3 diagrammatically shows the mixed-mode testing
conditions of the Sinclair sample.
[0024] FIG. 4 diagrammatically shows the method for determining the
deviation angle .THETA. on a broken Sinclair sample.
[0025] FIG. 5 shows the evolution of deviation angle with the
maximal equivalent stress intensity factor for two different
homogenizing treatment applied on a same alloy and for a reference
7050 plate.
[0026] FIG. 6 shows the test sample used for L-S fatigue
testing.
[0027] FIG. 7 are photographs of samples after L-S fatigue trial
test for two different homogenizing treatment applied on a same
alloy.
[0028] FIG. 8 are photographs of samples of thickness 25 mm or 30
mm after L-S fatigue trial test.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0029] Unless otherwise indicated, all the indications relating to
the chemical composition of the alloys are expressed as a mass
percentage by weight based on the total weight of the alloy. The
expression 1.4 Cu means that the copper content in weight % is
multiplied by 1.4. Alloy designation is in accordance with the
regulations of The Aluminium Association, known to those skilled in
the art. The definitions of tempers are laid down in European
standard EN 515. Definitions according to EN 12258-1 apply unless
mentioned otherwise.
[0030] Unless mentioned otherwise, static mechanical
characteristics, in other words the ultimate tensile strength UTS,
the tensile yield stress TYS and the elongation at fracture A, are
determined by a tensile test according to standard EN 10002-1, the
location at which the pieces are taken and their direction being
defined in standard EN 485-1.
[0031] The fatigue crack propagation rate (using the da/dN test) is
determined according to ASTM E 647. Plane-strain fracture toughness
(K.sub.1C) is determined according to ASTM E 399.
[0032] There are three modes of fracture. Mode I, or the opening
mode, is characterized by a stress normal to the crack faces. Mode
II, the sliding mode or forward shear mode, has a shear stress
normal to the crack front. Finally Mode III is the tearing mode,
with a shear stress parallel to the crack front.
[0033] The propensity to crack branching is usually observed during
a L-S toughness or fatigue test in mode I. A quantitative result is
obtained from a mixed-mode crack growth test carried out on a S-L
sample. Specimens and test conditions for investigating biaxial
fatigue properties have been described by H. A. Richard ("Specimens
for investigating biaxial fracture and fatigue properties", Biaxial
and Multiaxial Fatigue, EGF 3 (Edited by M. W. Brown and K. J.
Miller), 1989, Mechanical Engineering Publications, London pp
217-229). Specimen S9 described by Richard is used herein. The
rational for relating propensity to crack-branching in L-S
toughness or fatigue tests to deviation angles measured in mixed
mode I and mode II loading tests is described by Sinclair and
Gregson ("The effects of mixed mode loading on intergranular
failure in AA7050-T7651", Materials Science Forum, Vol. 242 (1997)
pp 175-180). The objective is to reproduce the local load that
occurs at the tip of the crack after crack branching on an L-S
sample. FIG. 1 schematically shows crack branching on an L-S sample
and the location of the test sample proposed by Sinclair ("the
Sinclair sample"). An L-S sample (1) with elongated grains (3)
under a load (2) with an initial crack in mode I (4) undergoes
crack branching towards the L direction (deviated crack (5)). The
Sinclair test sample (6) is an S-L sample and the initial crack
will correspond to a 90.degree. deviated crack of an L-S sample. If
the crack of "the Sinclair sample" is stable under a mixed mode I
and mode II loading representative of the deviated crack loading,
then the deviated crack would have been stable and the sample has a
high propensity to crack branching. The geometry of "the Sinclair
sample" is provided in FIG. 2. 6 holes (61) are used to fix the
Sinclair sample to a testing device. The sample is pre-cracked
mechanically, the length of the pre-crack is 7 mm.
[0034] The "Sinclair sample" is loaded under a mixed mode I and
mode II according to FIG. 3. Two sample holders (71) and (72) are
used to load the sample under mixed mode I and mode II. The sample
is fixed to the sample holders with the six holes (61) to form an
assembly, which is loaded between the holes (711) and (721). The
load application angle .PSI. between a plane perpendicular to the
initial crack direction and the load direction is 75.degree..
The stress intensity factors K.sub.I and K.sub.II for mode I and
mode II are provided by
K I , II = P .pi. a Wt F I , II ##EQU00002##
where P is the load (N), a is the crack length (mm), W is the
sample width (mm), t is the sample thickness (mm). For a fatigue
test, the maximum load is referred to as P.sub.max and the
corresponding stress intensity factor is referred to as K.sub.max.
Form factors F.sub.I and F.sub.II for mode I and mode II,
respectively, corresponding to the sample geometry are
F I = cos .PSI. 1 - a W 0 , 26 + 2 , 65 ( a W - a ) 1 + 0 , 55 ( a
W - a ) - 0 , 08 ( a W - a ) 2 F II = sin .PSI. 1 - a W - 0 , 23 +
1 , 40 ( a W - a ) 1 - 0 , 67 ( a W - a ) + 2 , 08 ( a W - a ) 2
##EQU00003##
where .PSI. is the angle between a plane perpendicular to the
initial crack direction and the load direction.
[0035] The equivalent stress intensity factor K.sub.eff is
determined according to
K.sub.eff= {square root over
(((1-v.sup.2)K.sub.I.sub.2+(1-v.sup.2)K.sub.II.sup.2+(1+v)K.sub.III.sup.2-
))}{square root over
(((1-v.sup.2)K.sub.I.sub.2+(1-v.sup.2)K.sub.II.sup.2+(1+v)K.sub.III.sup.2-
))}{square root over
(((1-v.sup.2)K.sub.I.sub.2+(1-v.sup.2)K.sub.II.sup.2+(1+v)K.sub.III.sup.2-
))}
For the geometry used in the test K.sub.III=0. K.sub.eff max is the
maximum equivalent stress intensity factor during a fatigue cycle,
it corresponds to the maximum load P.sub.max.
[0036] The deviation angle .THETA. between the initial crack
direction and the deviated crack direction enables a quantitative
evaluation of the propensity to crack branching. It is measured
according to FIG. 4. FIG. 4 is a drawing of a broken Sinclair
sample (61). The profile (65) of the broken specimen is measured
with a profilometer with steps of 0.5 mm. The resulting data points
are smoothed with 3 points moving average. The deviation angle is
measured for each set of three data point. The maximum deviation
angle between the tip of the mechanical crack (69) and a distance
of 32 mm from the sample edge is the .THETA.value.
[0037] A plot of .THETA. vs K.sub.eff max provides a quantitative
measurement which can be related to the crack branching propensity
for a L-S test sample. For a given K.sub.eff max, greater values of
.THETA. indicates less propensity to crack branching. However, for
reasons explained in the above mentioned reference by Sinclair and
Gregson, for a K.sub.eff max value of less than around 5 MPa m or
higher than around 15 MPa m, the .THETA. value typically does not
discriminate among samples. For this reason, the .THETA. value is
particularly significant for K.sub.eff max=10 MPa m. According to
this invention, a substantially unrecrystallized rolled aluminum
alloy product with a thickness of at least 30 mm has a low
propensity to crack branching if .THETA. is maintained at a value
of at least about 20.degree. and preferably at least 30.degree. for
a K.sub.eff max=10 MPa m for a "Sinclair sample" under mixed-mode
load (.PSI.=75.degree.). Sinclair and Gregson clearly show that for
a AA7050 test sample, known to exhibit a low propensity to crack
branching, the condition on .THETA. is met.
[0038] The term "structural member" refers to a component used in
mechanical construction for which the static and/or dynamic
mechanical characteristics are of particular importance with
respect to structure performance, and for which a structure
calculation is usually being prescribed or made. These are
typically components the rupture of which may seriously endanger
the safety of a mechanical construction, its users or third
parties. In the case of an aircraft, structural members can
comprise members of the fuselage (such as fuselage skin),
stringers, bulkheads, circumferential frames, wing components (such
as wing skin, stringers or stiffeners, ribs, spars), empennage
(such as horizontal and vertical stabilisers), floor beams, seat
tracks, doors.
[0039] A substantially unrecrystallized rolled aluminum alloy
product with a thickness of at least 30 mm according to the present
invention has a low propensity to crack branching because of the
combination of a carefully selected composition which has been
treated by specific process steps.
[0040] An aluminum lithium alloy rolled product of the invention
comprises 2.2 to 3.9 wt. % Cu, 0.7 to 2.1 wt. % Li, 0.2 to 0.8 wt.
% Mg, 0.2 to 0.5 wt. % Mn, 0.04 to 0.18 wt. % Zr and less than 0.05
wt. % Zn, optionally 0.1 to 0.5 wt. % Ag, remainder aluminum and
unavoidable impurities. Preferably, the Si and Fe content is at
most 0.15 wt. % each, more preferably 0.10 wt. % and other
unavoidable impurities content is at most 0.05 wt. % each and 0.15
wt. % total. Preferentially, a refining agent comprising titanium
is added during casting. Titanium content is preferentially
comprised from 0.01 to 0.15 wt. % and preferably from 0.01 to 0.04
wt. %. Copper content is preferably at least 2.7 wt. % or even 3.2
wt. %. The copper content can be variously selected in order to
achieve the desired strength characteristics. Lithium content is
preferably at least 0.8 wt. %, or even more preferably at least 0.9
wt. %. The lithium content can be varied as needed in order to
obtain the desired density characteristics. In some embodiments,
the maximum lithium content is not more than 1.8 wt. % or even not
more than 1.4 wt. %, and preferentially in some cases, not more
than about 1.25 wt. %. The present invention is particularly
advantageous for alloys which simultaneously comprise a high Li and
a high Cu content, because these types of alloys exhibit a very
favorable mechanical property balance, but are particularly
sensitive to crack branching. In a preferred embodiment, the Li and
Cu content expressed in weight percent follow Li+Cu>4 and
preferably Li+Cu>4.3. However, if the alloy simultaneously
comprises a very high Li and Cu content, incipient melting may
occur during homogenization. In a preferred embodiment, the Li and
Cu content expressed in weight percent follow Li+0.7 Cu<4.3 and
preferentially Li+0.5 Cu<3.3.
[0041] Manganese is a particularly desirable component of a rolled
product of the invention and the content thereof is carefully
selected, preferably between 0.3 and 0.5 wt. %. A carefully
controlled distribution of manganese dispersoids obtained through a
selected content combined with specific thermo-mechanical treatment
contributes to avoid stress localization and stress at grain
boundaries. Although they are not bound to any specific theory, the
inventors believe that the distribution of manganese containing
dispersoids obtained according to the present invention contributes
to the low propensity to crack branching.
[0042] Performances in strength and toughness observed by the
inventors are usually difficult to reach for silver free alloys, in
particular when the permanent elongation after stretching is less
than 3%. The present inventors believe that silver has a role
during the formation of copper containing strengthening phases
formed during natural or artificial aging and in particular,
enables the production of finer phases and also produces a more
homogeneous distribution of these phases. An advantageous effect of
silver is observed when the silver content is at least 0.1 wt. %
and preferentially at least 0.2 wt. %. Excessive addition of Ag
would likely be economically prohibitive in many cases due to
silver's high cost, and it is thus advantageous not to exceed 0.5
wt. % or even 0.3 wt. %.
[0043] Addition of Mg improves strength and reduces density.
Excessive addition of Mg may however adversely affect toughness. In
an advantageous embodiment, the Mg content is not more than 0.4 wt.
%. The present inventors believe that Mg addition may also have
role during the formation of copper containing phases.
[0044] An alloy with controlled amounts of alloying elements is
cast as an ingot. The ingot is then homogenized at 470-510.degree.
C. for 2 to 30 hours and preferably for 4 to 15 hours. A
homogenization temperature of at least 470.degree. C. or preferably
of at least 490.degree. C. enables simultaneously to form
dispersoids and to prepare for an efficient solution heat
treatment. The present inventors observed that homogenization
temperatures higher than about 510.degree. C. result in a higher
propensity to crack branching. It is believed by the present
inventors that homogenization temperature affects Mn containing
dispersoids size and distribution.
[0045] Hot rolling is carried out, after reheating if necessary, to
produce plates with a thickness of at least 30 mm. A hot-rolling
exit temperature of at least 410.degree. C., preferentially
430.degree. C. or even more preferentially 450.degree. C. is
generally necessary in order to obtain a substantially
unrecrystallized product after solution heat treatment. By
substantially unrecrystallized product it is meant that the
recrystallization rate is not more than 10% (or not more than about
10%) at 1/4 and 1/2 thickness (T/4 and T/2). The plates are then
solution heat treated by soaking at 490 to 540.degree. C. for 15
min to 4h and quenched with cold water. Suitable solution heat
treatment conditions typically depend on product thickness. It is
important to avoid dispersoid coarsening during solution heat
treatment as this would jeopardize the effect obtained with the
carefully controlled homogenization treatment. Thus, the total
equivalent time at 500.degree. C. of homogenization and solution
heat treatment advantageously does not exceed about 30 h and
preferably does not exceed 20 h.
The equivalent time t(eq) at 500.degree. C. is defined by the
formula:
t ( eq ) = .intg. exp ( - 26100 / T ) t exp ( - 26100 / T ref )
##EQU00004##
where T is the instantaneous temperature in Kelvin of treatment
which evolves with the time t (in hours) and T.sub.ref is a
reference temperature selected at 500.degree. C. (773 K). t(eq) is
expressed in hours. The constant Q/R=26100 K is derived from the
diffusion activation energy for Mn, Q=217000 J/mol. The formula
providing t(eq) takes into account the heating and cooling steps.
Cold water quenching is carried out after solution heat treatment.
In an advantageous embodiment, a rapid quench is carried out. By
rapid quench it is meant that the cooling rate is the highest
cooling rate made possible in relation to the plate thickness. In a
preferred embodiment, vertical immersion quenching is used, rather
than horizontal spray quench. The present inventors observed that
products processed with a rapid quench are less prone to crack
branching. The present inventors believe that this effect may be
related to a more limited grain boundary precipitation.
[0046] The product is then preferably stretched from 2 to 5% and
preferentially from 3 to 4%. Aging can be carried out at
130-160.degree. C. for 5 to 60 hours which results in a T8 temper.
In some instances, and particularly for some preferred
compositions, aging is more preferentially carried out at
140-160.degree. C. from 12 to 50 hours. Lower aging temperatures
generally favor high fracture toughness.
[0047] The products of the present invention have a low propensity
to crack branching, which means that samples with a thickness of
preferably at least 30 mm and preferably of at least 60 mm when
tested on a S-L cracked test sample of FIG. 2 under a mixed mode I
and mode II loading (.PSI.=75.degree. and K.sub.eff max=10 MPa m)
the crack deviation angle .THETA. is at least 20.degree. and
preferentially at least 30.degree..
[0048] Low propensity to crack branching is also seen on L-S
fatigue trials. A low propensity to crack branching also means that
products of the present invention exhibit crack branching on not
more than about 20% and preferentially not more than 10% L-S test
samples after testing at least 4 different samples in a fatigue
test according to ASTM E 647 (R=0.1, .sigma..sub.max=220 MPa), with
a test sample according to FIG. 6.
[0049] Some other advantageous characteristics of products of the
present invention from 30 to 100 mm thick include one or more of a1
and a2 and/or one or more of b1, b2 and b3 in a T8 temper,
where:
a1: the tensile yield strength at T/4 and T/2 is at least 455 MPa,
preferably 460 MPa or even better 465 MPa in the L-direction. a2:
the ultimate tensile strength at T/4 and T/2 is at least 490 MPa,
preferably 495 MPa or even better 500 MPa in the L-direction. b1:
the fracture toughness K.sub.1C: in L-T direction at T/4 and T/2 is
at least 31 MPa m and preferentially at least 32 MPa m or even at
least 33 MPa m; b2: the fracture toughness K.sub.1C: in T-L
direction at T/4 and T/2 is at least 28 MPa m and preferentially at
least 29 MPa m or even at least 30 MPa m; b3: the fracture
toughness K.sub.1C: in S-L direction at T/4 and T/2 is at least 25
MPa m and preferentially at least 26 MPa m or even at least 27 MPa
m;
[0050] Some other advantageous characteristics of products of the
present invention of more than 100 mm include one or more of a4 and
a5 and/or one or more of b4, b5 and b6 in a T8 temper:
a4: the tensile yield strength at T/4 and T/2 is at least 440 MPa,
preferably at least 445 MPa or even at least 450 MPa in the
L-direction a5: the ultimate tensile strength at T/4 and T/2 is at
least 475 MPa, preferably at least 480 MPa or even at least 485 MPa
in the L-direction b4: the fracture toughness K.sub.1C: in L-T
direction at T/4 and T/2 is at least 26 MPa m and preferably at
least 27 MPa m or even at least 28 MPa m; b5: the fracture
toughness K.sub.1C: in T-L direction at T/4 and T/2 is at least 25
MPa m and preferably at least 26 MPa m or even at least 27 MPa m;
b6: the fracture toughness K.sub.1C: in S-L direction at T/4 and
T/2 is at least 24 MPa m and preferably at least 25 MPa m or even
at least 26 MPa m.
[0051] A product according to the present invention typically
exhibits a high corrosion resistance. When tested under a
MASTMAASIS (Modified ASTM Acetic Acid Salt Intermittent Spray)
according to ASTM G85 standard, products of the invention can be
capable of reaching the EA rating and preferably the P (pitting
only) rating. When tested for resistance to SCC (Stress Corrosion
Cracking) according to ASTM G47, products of the invention are
capable of reaching more than 30 days for ST samples under a 300
MPa constant strain and preferably under a 350 MPa constant
strain.
[0052] Products of the invention can advantageously be comprised in
structural members. A structural member formed of a plate according
to the present invention can include for example, spars, ribs
and/or frames suitable for aircraft construction. The present
invention is particularly suitable for parts with a complex shape
made by integral machining a plate, and may be used in particular
for aircraft wing construction as well as any other use where the
instant properties could be advantageous.
EXAMPLES
Example 1
[0053] Two AA2050 ingots, reference A and B were cast. Their
composition is provided in Table 1. For comparison purposes, a
AA7050 plate in the T7451 temper was also tested for crack
branching. The composition is also provided in Table 1.
TABLE-US-00001 TABLE 1 Composition (weight %) the different ingots.
Si Fe Cu Mn Mg Ti Zr Li Ag Zn A 0.03 0.04 3.46 0.39 0.4 0.02 0.10
0.88 0.39 0.02 B 0.04 0.05 3.60 0.39 0.4 0.02 0.09 0.91 0.37 0.02
7050 0.04 0.09 2.11 0.01 2.22 0.02 0.11 -- -- 6.18
[0054] Ingot A was homogenized 12 hours at 505.degree. C. (heating
rate:15.degree. C./h, equivalent time at 500.degree. C.:16.7 h),
according to the invention. Ingot B (reference) was homogenized 8
hours at 500.degree. C. followed by 36 hours at 530.degree. C.
(heating rate: 15.degree. C./h, equivalent time at 500.degree. C.:
140 h). Ingot A was hot rolled to a 60 mm thick plate and the hot
rolling exit temperature was 466.degree. C., the resulting plate
was solution heat treated for 2 h at 504.degree. C. (heating
rate:50.degree. C./h, equivalent time at 500.degree. C.: 2.9 h) and
cold water quenched. Ingot B was hot rolled to a 65 mm thick plate
and the hot rolling exit temperature was 494.degree. C., the
resulting plate was solution heat treated for 2 h at 526.degree. C.
(heating rate:50.degree. C./h, equivalent time at 500.degree. C.:6
h) and cold water quenched. Both plates were stretched with a
permanent elongation of 3.5% and aged 18 hours at 155.degree. C.
The plates resulting from ingot A and ingot B are referred to as
plate A-60 and plate B-60, respectively. Total equivalent time at
773 K of homogenization and solution heat treatment t(eq) was thus
19.6 h and 146 h for plates A-60 and B-60, respectively.
The samples were mechanically tested to determine their static
mechanical properties as well as their toughness. Tensile yield
strength (TYS), ultimate tensile strength (UTS), elongation at
fracture (e %) are provided in Table 2 and KIC in Table 3.
TABLE-US-00002 TABLE 2 Tensile properties Sample T/4 T/2 L LT L LT
UTS TYS UTS TYS UTS TYS UTS TYS MPa MPa e (%) MPa MPa e (%) MPa MPa
e (%) MPa MPa e (%) Plate A-60 511 484 13 511 464 10 518 477 10 481
441 13 Plate B-60 531 500 11.2 521 474 8.8 531 489 8.1 490 449
10.9
TABLE-US-00003 TABLE 3 Toughness properties K1c MPa m) Sample T/4
T/2 L-T T-L L-T T-L S-L Plate A-60 44.6 36.7 51.0 39.8 33.2 Plate
B-60 42.5 36.7 40.4 40.8 33.4
[0055] "Sinclair samples" described in FIGS. 1 and 2 and having a
width, W=40 mm, and a thickness of 5 mm, were taken from plates
A-60 and B-60 at T/2 and tested in fatigue (R=0.1). The test
geometry described in FIG. 3 was used. The fatigue tests were
carried out for several K.sub.eff max and the deviation angle
.THETA. was measured on the broken specimens according to the
method of FIG. 4. The results are presented in FIG. 5 and table
4.
TABLE-US-00004 TABLE 4 Deviation angle .THETA. measured after S-L
fatigue trial under mixed mode I and II load. Plate A-60 Plate B-60
7050 Keff max Number Number Load Number (MPa m) Load (N) of cycles
.THETA. (.degree.) Load (N) of cycles .THETA. (.degree.) (N) of
cycles .THETA. (.degree.) 5 2221 800700 57 2216 900 000 49 7.5 3364
336500 50* 3351 297600 51 3317 240 000 42 10 4457 102300 34 4468
44500 4 4423 71 500 31 15 6715 3400 4 6662 2300 11 6648 1 700 4
*failure occurred within the grips
[0056] Plate A-60 exhibits a deviation angle .THETA. higher than
20.degree. for K.sub.eff max of 10 MPa m which shows that it has a
low propensity to crack branching. This result was further
confirmed by L-S fatigue trials. Four L-S samples according to FIG.
6 were taken in plate A-60 and plate B-60 and submitted to a
fatigue trial (.sigma.max=220 MPa, R=0.1) in mode I. FIG. 7a and
FIG. 7b show, respectively, the four samples from plates A and B
after fatigue trial. The results are consistent with those obtained
through mixed mode I and mode II trials on S-L samples: all the
samples from plate B-60 exhibit severe crack branching whereas
samples from plate A-60 show only mode I crack propagation.
Example 2
[0057] Two AA2050 ingots, referenced A' and B and two AA2195 ingots
referenced D and E were cast. Their composition is provided in
Table 5.
TABLE-US-00005 TABLE 5 Composition (wt. %) of the different ingots
Si Fe Cu Mn Mg Ti Zr Li Ag Zn A' 0.03 0.04 3.46 0.39 0.4 0.02 0.10
0.88 0.39 0.02 C 0.02 0.05 3.56 0.41 0.35 0.03 0.09 0.93 0.37 0.02
D 0.03 0.04 4.2 -- 0.4 0.02 0.11 1.06 0.35 0.02 E 0.03 0.06 4.3 0.3
0.4 0.02 0.12 1.17 0.35 0.01
[0058] Ingot A' was homogenized 12 hours at 505.degree. C. (heating
rate: 15.degree. C./h, equivalent time at 500.degree. C.: 16.7 h),
according to the invention. Ingot C (reference) was homogenized 8
hours at 500.degree. C. followed by 36 hours at 530.degree. C.
(heating rate: 15.degree. C./h, equivalent time at 500.degree. C.:
140 h). Ingot A' was hot rolled to a 30 mm thick plate and the hot
rolling exit temperature was 466.degree. C., the resulting plate
was solution heat treated for 2 h at 505.degree. C. (heating
rate:50.degree. C./h, equivalent time at 500.degree. C.: 3.0 h) and
cold water quenched. Ingot C was hot rolled to a 30 mm thick plate
and the hot rolling exit temperature was 474.degree. C., the
resulting plate was solution heat treated for 5 h at 525.degree. C.
(heating rate:50.degree. C./h, equivalent time at 500.degree.
C.:15.7 h) and cold water quenched. Both plates were stretched with
a permanent elongation of 3.5% and aged 18 hours at 155.degree. C.
The plates resulting from ingot A and ingot B are referred to as
plate A'-30 and plate C-30, respectively.
[0059] Ingots D and E were homogenized 15 hours at 492.degree. C.
(heating rate: 15.degree. C./h, equivalent time at 500.degree. C.:
11.5 h). Ingot D was hot rolled to a 25 mm thick plate and the hot
rolling exit temperature was 430.degree. C., the resulting plate
was solution heat treated for 5 h at 510.degree. C. (heating
rate:50.degree. C./h, equivalent time at 500.degree. C.: 8.4 h) and
cold water quenched. Ingot E was hot rolled to a 30 mm thick plate
and the hot rolling exit temperature was 411.degree. C., the
resulting plate was solution heat treated for 4.5 h at 510.degree.
C. (heating rate:50.degree. C./h, equivalent time at 500.degree.
C.:7.6 h) and cold water quenched. Both plates were stretched with
a permanent elongation of 4.3% and aged 24 hours at 150.degree. C.
The plates resulting from ingot D and ingot E are referred to as
plate D-25 and plate E-30, respectively. Total equivalent time at
773 K of homogenization and solution heat treatment t(eq) was thus
19.7 h, 155.7 h, 19.9 h et 19.1 h for plates A'-30, C-30, D-25 et
E-30, respectively.
[0060] Fatigue trials on L-S samples were carried out on test
samples from plates A'-30, C-30, D-25 and E-30. Four L-S samples
according to FIG. 6 were taken in each plate and submitted to a
fatigue trial (.sigma.max=220 MPa, R=0.1) in mode I. FIGS. 8a, 8b,
8c and 8d show, respectively, the four samples from plates A'-30,
C-30, D-25 and E-30 after fatigue trial. Samples form plate A'-30
do not show any crack branching whereas for samples from plates
C-30, D-25 and E-30 at least one sample exhibit severe crack
branching. The process according to the present invention, which
combines a particular composition and defined homogenization and
solution heat treatment conditions enables to obtain a plate free
from crack branching A'-30, whereas this is not reached with plate
C-30 (high homogenizing temperature), and plates D-25 and E-30
(high copper content).
* * * * *