U.S. patent application number 14/781097 was filed with the patent office on 2016-03-03 for aluminium-copper-lithium alloy sheets for producing aeroplane fuselages.
The applicant listed for this patent is CONSTELLIUM ISSOIRE. Invention is credited to Bernard BES, Juliette CHEVY, Frank EBERL, Jean-Christophe EHRSTROM.
Application Number | 20160060741 14/781097 |
Document ID | / |
Family ID | 49000974 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160060741 |
Kind Code |
A1 |
CHEVY; Juliette ; et
al. |
March 3, 2016 |
ALUMINIUM-COPPER-LITHIUM ALLOY SHEETS FOR PRODUCING AEROPLANE
FUSELAGES
Abstract
The invention concerns a sheet 0.5 to 8 mm thick made from
aluminium alloy. The sheet can be obtained by a method comprising
casting, homogenising, hot rolling and optionally cold rolling,
solution heat treatment, quenching and tempering, the composition
and the tempering being combined in such a way that the elasticity
limit in the longitudinal direction R.sub.p0.2(L) is between 395
and 435 MPa. A sheet according to the invention is particularly
advantageous for producing aircraft fuselage panels.
Inventors: |
CHEVY; Juliette; (Moirans,
FR) ; BES; Bernard; (Seyssins, FR) ; EBERL;
Frank; (Issoire, FR) ; EHRSTROM; Jean-Christophe;
(Grenoble, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CONSTELLIUM ISSOIRE |
Issoire |
|
FR |
|
|
Family ID: |
49000974 |
Appl. No.: |
14/781097 |
Filed: |
April 1, 2014 |
PCT Filed: |
April 1, 2014 |
PCT NO: |
PCT/FR2014/000069 |
371 Date: |
September 29, 2015 |
Current U.S.
Class: |
148/552 ;
148/417 |
Current CPC
Class: |
B22D 21/007 20130101;
C22C 21/14 20130101; C22C 21/16 20130101; C22F 1/057 20130101 |
International
Class: |
C22F 1/057 20060101
C22F001/057; C22C 21/14 20060101 C22C021/14; B22D 21/00 20060101
B22D021/00; C22C 21/16 20060101 C22C021/16 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2013 |
FR |
13/00763 |
Claims
1. A sheet measuring 0.5 to 8 mm thick made of an aluminum-based
alloy composition comprising 2.6 to 3.0% by weight of Cu, 0.5 to
0.8% by weight of Li, 0.1 to 0.4% by weight of Ag, 0.2 to 0.7% by
weight of Mg, 0.06 to 0.20% by weight of Zr, 0.01 to 0.15% by
weight of Ti, optionally at least one element chosen among Mn, V,
Cr, Sc, and Hf, the quantity of element, if chosen, being from 0.01
to 0.8% by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to
0.3% by weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5%
by weight for Hf, a quantity of Zn less than 0.2% by weight, a
quantity of Fe and Si less than or equal to 0.1% by weight each,
and inevitable impurities having a content less than or equal to
0.05% by weight each and 0.15% by weight in total, said sheet being
obtained by a method comprising casting, homogenization, hot
rolling and optionally cold rolling, solution heat treatment,
quenching and aging, the composition and the aging being combined
in such a way that the yield stress in the longitudinal direction
R.sub.p0.2(L) is between 395 and 435 Mpa.
2. The sheet according to claim 1, the copper content of which lies
between 2.8 and 3.0% by weight and optionally between 2.8 and 2.9%
by weight.
3. The sheet according to claim 1, the lithium content of which
lies between 0.55 and 0.75% by weight and optionally between 0.60
and 0.73% by weight.
4. The sheet according to claim 1, the silver content of which lies
between 0.2 and 0.3% by weight.
5. The sheet according to claim 1, the magnesium content of which
lies between 0.25 and 0.50% by weight and optionally between 0.30
and 0.45% by weight.
6. The sheet according to claim 1 for which the aging is performed
at "peak".
7. The sheet according to claim 1, with thickness between 0.5 and 3
3 mm and having the following properties a fracture toughness in
plane strain K.sub.app, measured on test pieces of type CCT760
(2ao=253 mm), in the L-T direction of at least 120 MPa m and a
fracture toughness in plane strain K.sub.app, measured on test
pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at
least 120 MPa m.
8. The sheet according to claim 7, the granular structure of which
is essentially recrystallized and having the following properties a
fracture toughness in plane strain Kapp, measured on test pieces of
type CCT760 (2ao=253 mm), in the L-T direction of at least 140 MPa
m and a fracture toughness in plane strain Kapp, measured on test
pieces of type CCT1220 (2ao=253 mm), in the L-T direction of at
least 150 MPa m.
9. The sheet according to claim 1, with thickness between 3.4 and 6
mm and having the following properties a fracture toughness in
plane strain K.sub.app, measured on test pieces of type CCT760
(2ao=253 mm), in the L-T direction of at least 150 MPa m and
optionally at least 155 MPa m and a fracture toughness in plane
strain K.sub.app, measured on test pieces of type CCT1220 (2ao=253
mm), in the L-T direction of at least 170 MPa m and optionally at
least 180 MPa m.
10. The sheet according to claim 1, with thickness between 3.4 and
8 mm and optionally between 4 and 8 mm and the granular structure
of which is essentially unrecrystallized.
11. A method of manufacturing a sheet according to claim 1 of
thickness of 0.5 to 8 mm made of an aluminum based alloy
composition, said method comprising, successively a) a molten metal
bath is prepared comprising 2.6 to 3.0% by weight of Cu, 0.5 to
0.8% by weight of Li, 0.1 to 0.4% by weight of Ag, 0.2 to 0.7% by
weight of Mg, 0.06 to 0.20% by weight of Zr, 0.01 to 0.15% by
weight of Ti, optionally at least one element chosen among Mn, V,
Cr, Sc, and Hf, the quantity of element, if chosen, being from 0.01
to 0.8% by weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to
0.3% by weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5%
by weight for Hf, a quantity of Zn less than 0.2% by weight, a
quantity of Fe and Si less than or equal to 0.1% by weight each,
and inevitable impurities having a content less than or equal to
0.05% by weight each and 0.15% by weight in total, b) a slab is
cast from said molten metal bath; c) said slab is homogenized at a
temperature between 450.degree. C. and 535.degree. C.; d) said slab
is hot rolled and optionally cold rolled into a sheet of thickness
between 0 5 mm and 8 mm; e) said sheet is solution heat treated at
a temperature of between 450.degree. C. and 535.degree. C. and
quenched; h) said sheet undergoes controlled stretching with a
permanent deformation of 0.5 to 5%, total cold working after
solution heat treatment and quenching being less than 15%; i) aging
is performed comprising heating to a temperature between
130.degree. C. and 170.degree. C. and optionally between
150.degree. C. and 160.degree. C. for 5 to 100 hours and optionally
from 10 to 40 hours, the composition and aging being combined so
that the yield stress in the longitudinal direction R.sub.p0.2(L)
is between 395 and 435 MPa.
12. A sheet according to claim 1 shaped into an aircraft fuselage
panel.
Description
SCOPE OF THE INVENTION
[0001] The invention relates to aluminum-copper-lithium alloy
rolled products, and more particularly to such products, their
manufacturing processes and use, intended notably for the field of
aeronautical and aerospace construction.
STATE OF THE ART
[0002] Rolled products made of aluminum alloy are developed in
order to produce fuselage components intended notably for the
aeronautical and aerospace industry.
[0003] Aluminum-copper-lithium alloys are particularly beneficial
for the production of this type of product.
[0004] U.S. Pat. No. 5,032,359 describes a vast family of
aluminum-copper-lithium alloys in which the addition of magnesium
and silver, in particular between 0.3 and 0.5 percent by weight,
makes it possible to increase the mechanical strength.
[0005] U.S. Pat. No. 5,455,003 describes a process for
manufacturing Al--Cu--Li alloys that have improved mechanical
strength and fracture toughness at cryogenic temperature, in
particular owing to appropriate strain hardening and aging. This
patent particularly recommends the composition, expressed as a
percentage by weight, Cu=3.0-4.5, Li=0.7-1.1, Ag=0-0.6, Mg=0.3-0.6
and Zn=0-0.75.
[0006] U.S. Pat No. 7,438,772 describes alloys including, expressed
as a percentage by weight, Cu: 3-5, Mg: 0.5-2, Li:0.01-0.9 and
discourages the use of higher lithium contents because of a
reduction in the balance between fracture toughness and mechanical
strength.
[0007] U.S. Pat. No. 7,229,509 describes an alloy including (% by
weight): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag,
(0.2-0.8) Mn, 0.4 max Zr or other grain-refining agents such as Cr,
Ti, Hf, Sc, and V.
[0008] US patent application 2009/142222 A1 describes alloys
including (% by weight), 3.4% to 4.2% Cu, 0.9% to 1.4% Li, 0.3% to
0.7% Ag, 0.1% to 0.6% Mg, 0.2% to 0.8% Zn, 0.1% to 0.6% Mn and
0.01% to 0.6% of at least one element for controlling the granular
structure. This application also describes a process for
manufacturing extruded products.
[0009] US patent application 2011/0247730 describes alloys
including (% by weight), 2.75 to 5.0% Cu, 0.1 to 1.1% Li, 0.3 to
2.0% Ag, 0.2 to 0.8% Mg, 0.50 to 1.5% Zn, and up to 1.0% Mn, with a
Cu/Mg ratio between 6.1 and 17, this alloy being insensitive to
work hardening.
[0010] Patent application CN101967588 describes alloys of
composition (% by weight) Cu 2.8-4.0; Li 0.8-1.9 ; Mn 0.2-0.6 ; Zn
0.20-0.80, Zr 0.04-0.20, Mg 0.20-0.80, Ag 0.1-0.7, Si<0.10.
Fe<0.10, Ti<0.12.
[0011] US patent application 2011/209801 relates to wrought
products such as extruded, rolled and/or forged aluminum
alloy-based products comprising, as a percentage by weight, Cu:
3.0-3.9; Li: 0.8-1.3, Mg: 0.6-1.0; Zr: 0.05-0.18; Ag: 0.0-0.5; Mn:
0.0-0.5; Fe+Si<=0.20; at least one element from a group from
among Ti: 0.01-0.15; Sc: 0.05-0.3; Cr: 0.05-0.3; Hf: 0.05-0.5;
other elements <0.05 each and <0.15 in total, remainder
aluminum, products being particularly useful in the production of
thick aluminum products intended for producing structural elements
in the aeronautical industry.
[0012] The required characteristics for aluminum sheets intended
for fuselage applications are described, for example, in patent EP
1 891 247. It is notably desirable that the sheet has a high yield
stress (to resist buckling) as well as high fracture toughness in
plane strain, notably characterized by a high value of apparent
stress intensity factor at break (K.sub.app) and a long
R-curve.
[0013] Patent EP 1 966 402 discloses an alloy comprising 2.1 to
2.8% by weight of Cu, 1.1 to 1.7% by weight of Li, 0.1 to 0.8% by
weight of Ag, 0.2 to 0.6% by weight of Mg, 0.2 to 0.6% by weight of
Mn, a quantity of Fe and Si less than or equal to 0.1% by weight
each, and inevitable impurities with a content less than or equal
to 0.05% by weight each and 0.15% by weight in total, the alloy
being substantially free of zirconium, particularly suitable for
obtaining recrystallized sheets.
[0014] Damage tolerance dimensioning consists in determining a
limit size for detectable defects, for which it can be guaranteed
that they will not lead to rupture during a defined time interval.
To achieve this dimensioning it is necessary to know the behavior
of cracks subjected to a representative load on panels of
sufficient size. Furthermore in the case of the evaluation of large
damage capability for which the undetected rupture of a stiffener
is assumed, the width of the crack can be large and it is useful to
have accurate fracture toughness data for very long cracks. The
fracture toughness characterizations of sheets are generally
carried out by the R-curve test on panels less than or equal to 760
mm wide. The R-curve test is a widely recognized method to
characterize fracture toughness properties. The R-curve represents
the evolution of the effective stress intensity factor for crack
growth as a function of effective crack extension, under increasing
monotonic loading. The R-curve enables one to determine the
critical load for an unstable fracture for any configuration
relevant to cracked aircraft structures. The values of the stress
intensity factor and crack extension are actual values as defined
in standard ASTM E561. It is generally considered that the width of
the panel must not modify the level of the R-curve, namely the
effective stress intensity factor for a given effective crack
growth, but only the valid length of the curve. However, it has
become apparent within the framework of this invention that this
assumption is not always true, and that in fact the
characterization on wider panels, such as 1220 mm wide panels,
takes into account certain specific properties of the material not
able to be deduced from characterizations performed on less wide
panels. Thus, the state of the art knowledge is unable to predict
which alloys and which thermomechanical treatments will allow the
most advantageous properties to be attained for K.sub.app and for
the level of the R-curve of wide width panels, as properties will
influence the damage tolerance dimensioning.
[0015] Furthermore, for certain fuselage applications, it is
particularly important that the fracture toughness is high in the
L-T direction. Indeed, in some configurations the bending stresses
on the fuselage around the axis of the wings become critical,
notably for the upper part of the fuselage. The cracks on the
sheets, for which the longitudinal direction and also the
longitudinal direction of the fuselage, are strained in the L-T
direction.
[0016] There exists a need for metal sheets measuring 0.5 to 8 mm
thick, made of aluminum-copper-lithium alloy presenting improved
properties as compared with those of known products, particularly
in terms of fracture toughness measured on wide width panels
notably in the L-T direction, static mechanical strength and
corrosion resistance properties, while having low density.
OBJECT OF THE INVENTION
[0017] The object of the invention is an aluminum based alloy sheet
of thickness 0.5 to 8 mm comprising [0018] 2.6 to 3.0% by weight of
Cu, [0019] 0.5 to 0.8% by weight of Li, [0020] 0.1 to 0.4% by
weight of Ag, [0021] 0.2 to 0.7% by weight of Mg, [0022] 0.06 to
0.20% by weight of Zr, [0023] 0.01 to 0.15% by weight of Ti, [0024]
optionally at least one element chosen among Mn, V, Cr, Sc, and Hf,
the quantity of element, if chosen, being from 0.01 to 0.8% by
weight for Mn, 0.05 to 0.2% by weight for V, 0.05 to 0.3% by weight
for Cr, 0.02 to 0.3% by weight for Sc, 0.05 to 0.5% by weight for
Hf, [0025] a quantity of Zn less than 0.2% by weight, a quantity of
Fe and Si less than or equal to 0.1% by weight each, and inevitable
impurities having a content less than or equal to 0.05% by weight
each and 0.15% by weight in total, said sheet being obtained by a
method comprising casting, homogenization, hot rolling and
optionally cold rolling, solution heat treatment, quenching and
aging, the composition and the aging being combined in such a way
that the yield stress in the longitudinal direction R.sub.p0.2(L)
is between 395 and 435 MPa.
[0026] Another object of the invention is the method of
manufacturing a sheet according to the invention of 0.5 to 8 mm in
thickness made of an aluminum-based alloy wherein, successively
[0027] a) a molten metal bath is prepared comprising [0028] 2.6 to
3.0% by weight of Cu, [0029] 0.5 to 0.8% by weight of Li, [0030]
0.1 to 0.4% by weight of Ag, [0031] 0.2 to 0.7% by weight of Mg,
[0032] 0.06 to 0.20% by weight of Zr, [0033] 0.01 to 0.15% by
weight of Ti, [0034] optionally at least one element chosen among
Mn, V, Cr, Sc, and Hf, the quantity of element, if chosen, being
from 0.01 to 0.8% by weight for Mn, 0.05 to 0.2% by weight for V,
0.05 to 0.3% by weight for Cr, 0.02 to 0.3% by weight for Sc, 0.05
to 0.5% by weight for Hf, [0035] a quantity of Zn less than 0.2% by
weight, a quantity of Fe and Si less than or equal to 0.1% by
weight each, and inevitable impurities having a content less than
or equal to 0.05% by weight each and 0.15% by weight in total,
[0036] b) a slab is cast from said molten metal bath; [0037] c)
said slab is homogenized at a temperature between 450.degree. C.
and 535.degree. C.; [0038] d) said slab is hot rolled and
optionally cold rolled into a sheet of thickness between 0.5 mm and
8 mm; [0039] e) said sheet is solution heat treated at a
temperature of between 450.degree. C. and 535.degree. C. and
quenched; [0040] h) said sheet undergoes controlled stretching with
a permanent deformation of 0.5 to 5%, total cold working after
solution heat treatment and quenching being less than 15%; [0041]
i) aging is performed comprising heating to a temperature between
130.degree. C. and 170.degree. C. and preferably between
150.degree. C. and 160.degree. C. for 5 to 100 hours and preferably
from 10 to 40 hours, the composition and aging being combined so
that the yield stress in the longitudinal direction R.sub.p0.2(L)
is between 395 and 435 MPa.
[0042] Yet another object of the invention is the use of a sheet
according to the invention in an aircraft fuselage panel.
DESCRIPTION OF THE FIGURES
[0043] FIG. 1--R-curves obtained in the L-T direction on sheets of
thickness 4 to 5 mm for test pieces measuring 760 mm and 1220 mm
wide.
[0044] FIG. 2--R-curves obtained in the L-T direction on sheets of
thickness 1.5 to 2 5 mm for test pieces measuring 760 mm and 1220
mm wide.
[0045] FIG. 3--R-curves obtained in the L-T direction on E#1 sheets
having undergone various tempering for test pieces measuring 760 mm
and 1220 mm wide.
[0046] FIG. 4--R-curves obtained in the L-T direction on E#2 sheets
having undergone various aging for test pieces measuring 760 mm and
1220 mm wide.
[0047] FIG. 5--Relationship between the yield stress in the
longitudinal direction and the stress intensity factor K.sub.app
L-T measured on test samples measuring 1220 mm wide for sheets 4 to
5 mm thick.
[0048] FIG. 6--Relationship between the yield stress in the
longitudinal direction and the stress intensity factor K.sub.app
L-T measured on test samples measuring 1220 mm wide for sheets 1.5
to 2.5 mm thick.
DESCRIPTION OF THE INVENTION
[0049] Unless otherwise specified, all the indications concerning
the chemical composition of the alloys are expressed as a
percentage by weight based on the total weight of the alloy. The
expression 1.4 Cu means that the copper content, expressed as a
percentage by weight is multiplied by 1.4. The designation of
alloys is compliant with the rules of The Aluminum Association,
known to experts in the field. The density depends on the
composition and is determined by calculation rather than by a
method of weight measurement. The values are calculated in
compliance with the procedures of The Aluminum Association, which
is described on pages 2-12 and 2-13 of "Aluminum Standards and
Data". Unless otherwise specified, the definitions of metallurgical
states listed in European Standard EN 515 apply. The static
mechanical properties under stretching, in other words the ultimate
tensile strength R.sub.m, the conventional yield strength at 0.2%
offset (R.sub.p0.2) and elongation at break A %, are determined by
a tensile test according to standard EN ISO 6892-1, and sampling
and test direction being defined by standard EN 485-1. Within the
framework of the invention, the mechanical properties are measured
in full thickness.
[0050] Within the framework of the present invention, a essentially
unrecrystallized granular structure refers to a granular structure
such that the recrystallization rate at 1/2 thickness is less than
30% and preferably less than 10% and a essentially recrystallized
granular structure refers to a granular structure such that the
recrystallization rate at 1/2 thickness is greater than 70% and
preferably greater than 90%. The recrystallization rate is defined
as the area fraction on a metallographic section occupied by
recrystallized grains.
[0051] A curve giving the effective stress intensity factor as a
function of the effective crack extension, known as the R-curve, is
determined according to standard ASTM E 561. The critical stress
intensity factor K.sub.C, in other words the intensity factor which
makes the crack unstable, is calculated from the R-curve. The
stress intensity factor K.sub.CO is also calculated by allotting
the initial crack length at the beginning of the monotonic load, at
critical load. These two values are calculated for a test piece of
the required shape. K.sub.app represents the K.sub.CO factor
corresponding to the test piece that was used to perform the
R-curve test. K.sub.eff represents the K.sub.C factor corresponding
to the test piece that was used to perform the R-curve test.
.DELTA.a.sub.eff (max) represents the crack extension of the last
point of the R-curve, valid according to standard ASTM E561. The
last point is obtained either at the time of sudden rupture of the
test piece, or possibly when the stress on the uncracked ligament
exceeds the yield stress of the material. Unless otherwise
specified, the crack size at the end of the stage of pre-cracking
by fatigue is W/3 for test pieces of the M(T) type, wherein W is
the width of the test piece as defined in standard ASTM E561.
[0052] Unless otherwise specified, the definitions of standard EN
12258 apply.
[0053] Sheets measuring 0.5 to 8 mm thick made of Al--Cu--Li alloy
according to the composition of the invention allow, when their
yield stress in the longitudinal direction R.sub.p0.2(L) is between
395 and 435 MPa, a particularly advantageous fracture toughness to
be obtained on wide width panels in the L-T direction.
[0054] The present inventors noted that, surprisingly, the fracture
toughness in the L-T direction on 1220 mm wide panels is improved
for a precise range of yield stress values in the longitudinal
direction R.sub.p0.2(L) whereas this effect is not observed when
the measurement is carried out on 760 mm wide panels. Thus, within
the framework of the invention, it was observed that there is an
optimal yield stress value range that is specific to the width 1220
mm, which cannot be interpreted by considerations based on the
plasticization of the uncracked ligament, underlying the limits of
validity of standard ASTM E561. The present inventors have thus
established that sheets obtained by a method comprising casting,
homogenization, hot rolling and optionally cold rolling, solution
heat treatment, quenching and aging possess advantageous properties
when the composition and aging are combined in such a way that the
yield stress in the longitudinal direction R.sub.p0.2(L) is between
395 and 435 MPa.
[0055] For certain compositions according to the invention, the
sheets have advantageous properties when the aging is performed to
"peak". Within the framework of the present invention and for the
sake of simplicity, aging to "peak" refers to an aging process for
which the yield stress in the transverse direction R.sub.p0.2(TL)
has a value of at least 95% of the yield stress in the transverse
direction R.sub.p0.2(TL) obtained for an aging process having an
equivalent time at 155.degree. C. of 48 hours. Within the framework
of the present invention, aging performed to "peak" is preferred.
For other compositions according to the invention, under-aging may
be necessary to reach the desired yield stress. However, if
under-aging is excessive, certain properties of the sheets, notably
the thermal stability, are not satisfactory. Within the framework
of the present invention, thermal stability refers to the stability
of the mechanical properties during an exposure to temperatures
representative of conditions experienced in civil aviation, being
simulated by aging of 1000 hours at 85.degree. C. for example.
Therefore, if necessary, under-aging is performed for which the
yield stress in the transversal direction R.sub.p0.2(TL) has a
value between 88% and 94% and preferably at least 91% of the value
obtained for aging having an equivalent time of 48 hours at
155.degree. C. The copper content of the products according to the
invention lies between 2.6 and 3.0% by weight. In an advantageous
embodiment of the invention, the copper content lies between 2.8
and 3.0% by weight. In an advantageous embodiment of the invention,
the copper content is at most 2.95% by weight and advantageously at
most 2.9% by weight. When the copper content is too high, the yield
stress R.sub.p0.2(L) is too high to be advantageous in the
under-aging conditions according to the invention. When the copper
content is too low, the minimum static mechanical properties are
not achieved, even for aging to peak.
[0056] The lithium content of the products according to the
invention lies between 0.5 and 0.8% by weight. Advantageously, the
lithium content lies between 0.55 and 0.75% by weight. Preferably,
the lithium content lies between 0.60% and 0.73% by weight The
addition of lithium can contribute to increased mechanical strength
and fracture toughness. Lithium content that is too high or too low
does not allow a high value of fracture toughness and/or a
sufficient yield stress to be obtained.
[0057] The magnesium content of the products according to the
invention lies between 0.2 and 0.7% by weight, preferably between
0.25 and 0.50% by weight and most preferably between 0.30 and 0.45%
by weight. In an advantageous embodiment of the invention, the
magnesium content is at most 0.4% by weight.
[0058] The zirconium content lies between 0.06 and 0.20% by weight
and preferably between 0.10 and 0.18% by weight. When a essentially
unrecrystallized granular structure is preferred, the zirconium
content is advantageously between 0.14 and 0.17% by weight.
[0059] The silver content lies between 0.1 and 0.4% by weight. In
an advantageous embodiment of the invention, the silver content
lies between 0.2 and 0.3% by weight. In an embodiment of the
invention, the silver content lies between 0.15 and 0.28% by
weight.
[0060] The titanium content lies between 0.01 and 0.15% by weight.
The addition of titanium helps to control the granular structure,
particularly during casting.
[0061] The alloy can optionally contain at least one element
selected from Mn, V, Cr, Sc, and Hf, the quantity of the element,
if chosen, being from 0.01 to 0.8% by weight for Mn, 0.05 to 0.2%
by weight for V, 0.05 to 0.3% by weight for Cr, 0.02 to 0.3% by
weight for Sc, 0.05 to 0.5% by weight for Hf. These elements can
contribute to controlling the granular structure. In an embodiment
of the invention, Mn, V, Cr or Sc is not added and their content is
less than or equal to 0.05% by weight.
[0062] Preferably, the iron and silicon contents are each at the
most 0.1% by weight. In an advantageous embodiment of the
invention, the iron and silicon contents are at most 0.08% by
weight and preferably at most 0.04% by weight. A controlled and
limited iron and silicon content helps to improve the balance
between mechanical strength and damage tolerance.
[0063] The zinc content is less than 0.2% by weight and preferably
less than 0.1% by weight. The zinc content is advantageously less
than 0.04% by weight.
[0064] The inevitable impurities are kept at a content less than or
equal to 0.05% by weight each and 0.15% by weight in total.
[0065] The sheet manufacturing method according to the invention
comprises steps of preparing, casting, rolling, solution heat
treatment, quenching, controlled stretching and ag ing. In a first
step, a molten metal bath is prepared to obtain an aluminum alloy
of composition according to the invention.
[0066] The molten metal bath is then cast in the form of a rolling
slab.
[0067] The rolling slab is then homogenized at a temperature
between 450.degree. C. and 535.degree. C. and preferably between
480.degree. C. and 530.degree. C. The homogenization time is
preferably between 5 and 60 hours.
[0068] After homogenization, the rolling slab is generally cooled
at room temperature before being preheated ready for hot working.
The aim of preheating is to reach a temperature preferably between
400.degree. C. and 500.degree. C. enabling the deformation by hot
rolling to take place.
[0069] Hot rolling, and optionally cold rolling, is carried out to
obtain to a thickness of 0.5 to 8 mm Intermediate heat treatments
during and/or after the rolling may be carried out in some cases.
Preferably, however, the method does not include intermediate heat
treatment during and/or after the rolling. The sheet thus obtained
is then solution heat treated by thermal treatment between
450.degree. C. and 535.degree. C., preferably for 5 min to 8 hours,
then quenched. It is known to those skilled in the art that the
precise solution heat treatment conditions must be chosen based on
the thickness and the composition so as to place the hardening
elements in a solid solution.
[0070] The sheet then undergoes cold working by controlled
stretching with a permanent deformation of 0.5 to 5% and preferably
of 1 to 3%. Known steps such as rolling, flattening, straightening
or shaping may optionally be performed after heat treatment and
quenching and before or after controlled stretching.
[0071] However, the total cold working after solution heat
treatment and quenching must remain below 15% and preferably less
than 10%. Significant cold working after solution heat treatment
and quenching result in the appearance of numerous shearing bands
through several grains; these shearing bands not being
desirable.
[0072] Aging is carried out at a temperature between 130.degree. C.
and 170.degree. C. and preferably between 150.degree. C. and
160.degree. C. for 5 to 100 hours and preferably from 10 to 40
hours so as to reach a yield stress in the longitudinal direction
R.sub.p0.2(L) between 395 and 435 MPa. In an embodiment of the
invention wherein the granular structure is essentially
recrystallized, a yield stress in the longitudinal direction
R.sub.p0.2(L) between 395 and 415 MPa may be preferred in some
cases. In another embodiment of the invention wherein the granular
structure is essentially unrecrystallized, a yield stress in the
longitudinal direction R.sub.p0.2(L) between 415 and 435 MPa may be
preferred in some cases.
[0073] Advantageously, the composition reaches the desired
longitudinal elasticity limit with an equivalent time at
155.degree. C. less than 48 h and preferably less than 30 h.
Preferably, the final temper is T8.
[0074] The equivalent time t.sub.i, at 155.degree. C. is defined by
the formula:
t i .intg. exp ( - 16400 / T ) t exp ( - 16400 / T ref )
##EQU00001##
where T (in Kelvin) is the instantaneous treatment temperature of
the metal, which changes with time t (in hours), and T.sub.ref is a
reference temperature fixed at 428 K. t.sub.i is expressed in
hours. The constant Q/R=16400 K is derived from the activation
energy of the diffusion of Cu, for which the value Q=136100 J/mol
was used. The present inventors noted in particular that the
preferred range of magnesium content helps limit the aging time
leading to a favorable compromise of properties.
[0075] In an embodiment of the invention, a short thermal treatment
is carried out after controlled stretching and before aging so as
to improve the formability of the sheets. The slabs can thus be
formed by a process such as draw-forming before being aged.
[0076] The most favorable granular structure depends on the
thickness of the products.
[0077] The sheets according to the invention with thickness between
0.5 and 3.3 mm, advantageously have the following properties [0078]
a fracture toughness in plane strain Kapp, measured on test pieces
of type CCT760 (2ao=253 mm), in the L-T direction of at least 120
MPa m and [0079] a fracture toughness in plane strain Kapp,
measured on test pieces of type CCT1220 (2ao=253 mm), in the L-T
direction of at least 120 MPa m.
[0080] The present inventors have further noted that for the sheets
according to the invention with thickness between 0.5 and 3.3 mm
and preferably between 1.0 and 3.0 mm, the fracture toughness in
plane strain Kapp in the L-T direction is higher for sheets for
which the structure is essentially recrystallized. Thus, sheets
with thickness between 0.5 and 3 3 mm and preferably between 1.0
and 3 0 mm and whose granular structure is essentially
recrystallized, advantageously have the following properties:
[0081] a fracture toughness in plane strain Kapp, measured on test
pieces of type CCT760 (2ao=253 mm), in the L-T direction of at
least 140 MPa m and [0082] a fracture toughness in plane strain
Kapp, measured on test pieces of type CCT1220 (2ao=253 mm), in the
L-T direction of at least 150 MPa m. The sheets according to the
invention with thickness between 3.4 and 6 mm, and advantageously
have the following properties [0083] a fracture toughness in plane
strain Kapp, measured on test pieces of type CCT760 (2ao=253 mm),
in the L-T direction of at least 150 MPa m and preferably at least
155 MPa m and [0084] a fracture toughness in plane strain Kapp,
measured on test pieces of type CCT1220 (2ao=253 mm), in the L-T
direction of at least 170 MP am and preferably at least 180 MPa
m.
[0085] Advantageously, the granular structure of sheets with
thickness between 3.4 and 8 mm and preferably between 4 and 8 mm is
essentially unrecrystallized. The intercrystalline corrosion
resistance of the sheets according to the invention is high. In a
preferred embodiment of the invention, the sheet of the invention
can be used without cladding.
[0086] The use of sheets according to the invention in an aircraft
fuselage panel is advantageous. The sheets according to the
invention are also advantageous in aerospace applications such as
the manufacture of rockets.
EXAMPLES
Example 1
[0087] In this example, Al--Cu--Li alloy sheets were prepared.
[0088] Five slabs, the composition of which is given in Table 1,
were cast. Compositions B, C, D and E are according to the
invention.
TABLE-US-00001 TABLE 1 Composition as a percentage by weight
Reference Cu Li Mg Zr Ag Fe Si Ti A 3.2 0.73 0.68 0.14 0.26 0.03
0.04 0.03 B 3.0 0.70 0.64 0.17 0.27 0.02 0.03 0.03 C 3.0 0.73 0.35
0.15 0.27 0.02 0.03 0.03 D 2.7 0.75 0.58 0.14 0.28 0.03 0.02 0.03 E
2.9 0.73 0.45 0.14 0.29 0.04 0.02 0.03
[0089] The slabs were homogenized 12 hours at 505.degree. C. The
slabs were hot rolled to obtain sheets with a thickness of between
4.2 to 6.3 mm Certain sheets where then cold rolled to a thickness
between 1.5 and 2.5 mm. The detail of sheets obtained and the aging
conditions is given in Table 2.
TABLE-US-00002 TABLE 2 detail of sheets obtained and aging
conditions Thickness Thickness Aging after hot after cold time at
Reference rolling (mm) rolling (mm) 155.degree. C. (h) A#1 4.2 --
36 A#2 4.4 1.5 36 B#1 4.6 -- 36 B#2 4.4 1.5 36 C#1 4.3 -- 24 C#2
4.4 1.5 24 D#1 4.3 -- 40 D#2 6.3 2.5 40 E#1 4.3 -- 36 E#2 6.3 2.5
36
[0090] After hot rolling and possibly cold rolling, the sheets were
solution heat treated at 505.degree. C. then smoothed out,
stretched with a permanent elongation of 2% and aging. The aging
conditions are not all identical since the increase in the yield
stress with the aging time differs from one alloy to another. An
attempt was made to obtain a yield stress at "peak" while limiting
aging time. The aging conditions are given in Table 2.
[0091] The granular structure of the test samples was characterized
based on microscopic observation of cross sections after anodic
oxidation under polarized light.
[0092] The granular structure of the sheets was essentially
unrecrystallized for all the sheets except for sheets D#2 and E#2
for which the granular structure was essentially
recrystallized.
[0093] The test samples were mechanically tested to determine their
static mechanical properties as well as their resistance to fatigue
crack propagation. The yield stress under tension, the ultimate
strength and elongation at rupture are given in Table 3.
TABLE-US-00003 TABLE 3 Mechanical properties expressed in MPa
(R.sub.p0.2, R.sub.m) or in percentage (A %) R.sub.p0.2(TL)/
R.sub.p0.2 R.sub.p0.2(TL) (TL) 48 h .sub.Rp0.2 Rm A % R.sub.p0.2
R.sub.m A % 48 h 155.degree. C. Reference (L) (L) (L) (TL) (TL)
(TL) 155.degree. C. (%) A#1 469 513 12.2 439 481 15.8 457 96% A#2
475 522 11.7 441 489 14.0 B#1 431 483 13.5 419 462 16.1 425 99% B#2
431 486 12.9 414 460 17.1 C#1 430 471 13.6 411 455 15.5 434 95% C#2
423 472 12.2 399 451 15.9 D#1 420 462 13.0 384 428 16.3 407 94% D#2
403 437 11.6 371 428 13.9 E#1 453 487 12.5 428 464 15.9 433 99% E#2
433 464 11.4 395 458 11.4
[0094] Table 4 summarizes the fracture toughness test results on
CCT test pieces of width 760 mm for these test samples.
[0095] Table 4 summarizes the results of R-curves for test pieces
of width 760 mm
TABLE-US-00004 Kapp Kr60 .DELTA.aeff max [MPa m] [MPa m] [mm]
Reference T-L L-T T-L L-T T-L L-T A#1 187 161 247 213 166 80 A#2
160 114 210 151 185 103 B#1 180 178 238 238 171 180 B#2 167 124 223
166 152 144 C#1 182 165 242 219 134 151 C#2 154 127 203 162 165 110
D#1 174 150 230 200 238 163 D#2 147 151 196 201 222 210 E#1 181 159
240 213 241 132 E#2 137 164 181 219 161 214
[0096] Table 5 summarizes the fracture toughness test results for
the R-curves obtained with CCT test pieces of width 1220 mm in the
L-T direction.
[0097] Table 5 results of R-curves for test pieces of width 1220 mm
in the L-T direction.
TABLE-US-00005 .DELTA.a.sub.eff Kapp Kr60 max Reference [MPa m]
[MPa m] [mm] A#1 169 202 172 A#2 117 138 247 B#1 176 209 281 B#2
120 145 193 C#1 191 224 237 C#2 120 134 106 D#1 175 206 244 D#2 180
213 244 E#1 159 192 139 E#2 167 196 187
[0098] The R-curves obtained for the sheets with thickness in the
order of 4 mm are shown in FIG. 1. The R-curves obtained for the
sheets with thickness of 1.5 to 2 5 mm are shown in FIG. 2. The
points obtained after the last valid point according to standard
ASTM E561 were represented.
[0099] Surprisingly, it is found that K.sub.app L-T is
substantially identical for 760 mm wide test pieces and 1220 mm
wide test pieces for some sheets, while for other sheets K.sub.app
L-T is lower for 760 mm wide test pieces and for 1220 mm wide test
pieces.
Example 2
[0100] In this example, the effect of the aging conditions was
studied on the fracture toughness of Al--Cu--Li alloy sheets of
composition according to the invention.
[0101] Following treatment identical to that of example 1, except
for aging, sheets made of alloy E underwent aging for 20 h at
155.degree. C. or for 25 h at 155.degree. C.
[0102] The granular structure is not changed by these aging
conditions. The test samples were mechanically tested to determine
their static mechanical properties as well as their resistance to
fatigue crack propagation. The yield stress under tension, the
ultimate strength and elongation at rupture are given in Table
6.
TABLE-US-00006 TABLE 6 Mechanical properties expressed in MPa
(R.sub.p0.2, R.sub.m) or in percentage (A %) R.sub.p0.2(TL)/
R.sub.p0.2(TL) Aging 48 h duration R.sub.p0.2 R.sub.m A %
R.sub.p0.2 R.sub.m A % 155.degree. C. Reference A155.degree. C. (L)
(L) (L) (TL) (TL) (TL) (%) E#1 20 h 422 470 13 390 440 16.5 90% E#2
20 h 411 450 12.4 374 443 12 E#1 25 h 442 483 12.4 415 456 15.7 96%
E#2 25 h 431 466 11.1 391 455 11.7 E#1 36 h 453 487 12.5 428 464
15.9 99% E#2 36 h 433 464 11.4 395 458 11.4
[0103] The R-curves characterized for a test piece of width 760 mm
and 1220 mm in the L-T direction are given in FIGS. 3 (thickness
4.3 mm) and 4 (thickness 2 5 mm) and in Table 7. The points
obtained after the last valid point according to standard ASTM E561
were represented.
TABLE-US-00007 TABLE 7 Results of R-curves for test pieces of width
760 mm and 1220 mm in the L-T direction. Test piece 760 mm Test
piece 1220 mm Aging .DELTA.aeff .DELTA.aeff time at Kapp Kr60 max
Kapp Kr60 max Reference 155.degree. C. [MPa m] [MPa m] [mm] [MPa m]
[MPa m] [mm] E#1 20 h 168 219 173 180 216 208 E#2 20 h 163 216 235
183 216 201 E#1 25 h 160 211 146 170 198 192 E#2 25 h 161 214 193
175 212 205 E#1 36 h 159 213 102 158 190 172 E#2 36 h 164 219 214
167 196 187
[0104] FIGS. 5 and 6 summarize all the results obtained.
* * * * *