U.S. patent application number 13/473303 was filed with the patent office on 2012-11-22 for aluminum magnesium lithium alloy with improved fracture toughness.
This patent application is currently assigned to CONSTELLIUM FRANCE. Invention is credited to Bernard Bes, Frank Eberl.
Application Number | 20120291925 13/473303 |
Document ID | / |
Family ID | 44550865 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120291925 |
Kind Code |
A1 |
Bes; Bernard ; et
al. |
November 22, 2012 |
ALUMINUM MAGNESIUM LITHIUM ALLOY WITH IMPROVED FRACTURE
TOUGHNESS
Abstract
Wrought product made of aluminum alloy composed as follows, as a
percentage by weight Mg: 4.0-5.0; Li: 1.0-1.6; Zr: 0.05-0.15; Ti:
0.01-0.15; Fe: 0.02-0.2; Si: 0.02-0.2; Mn: .ltoreq.0.5;
Cr.ltoreq.0.5; Ag: .ltoreq.0.5; Cu.ltoreq.0.5; Zn.ltoreq.0.5;
Sc.ltoreq.0.01; other elements <0.05; the rest aluminum.
Inventors: |
Bes; Bernard; (Seyssins,
FR) ; Eberl; Frank; (Issoire, FR) |
Assignee: |
CONSTELLIUM FRANCE
Paris
FR
|
Family ID: |
44550865 |
Appl. No.: |
13/473303 |
Filed: |
May 16, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61488196 |
May 20, 2011 |
|
|
|
Current U.S.
Class: |
148/552 ;
148/415; 148/417 |
Current CPC
Class: |
C22C 21/00 20130101;
C22C 21/06 20130101; C22F 1/047 20130101 |
Class at
Publication: |
148/552 ;
148/415; 148/417 |
International
Class: |
C22C 21/08 20060101
C22C021/08; C22F 1/047 20060101 C22F001/047 |
Foreign Application Data
Date |
Code |
Application Number |
May 20, 2011 |
FR |
11/01555 |
Claims
1. A wrought product comprising an aluminum alloy of a composition,
as a percentage by weight, Mg: 4.0-5.0 Li: 1.0-1.6 Zr: 0.05-0.15
Ti: 0.01-0.15 Fe: 0.02-0.2 Si: 0.02-0.2 Mn: .ltoreq.0.5 Cr
.ltoreq.0.5 Ag: .ltoreq.0.5 Cu .ltoreq.0.5 Zn .ltoreq.0.5 Sc
<0.01 other elements <0.05, each remainder aluminum;
2. A wrought product according to claim 1, comprising at least one
element selected from the group consisting of Mn and Cr with the
following contents if chosen, as a percentage by weight Mn:
0.05-0.5 Cr: 0.05-0.3, and an element not chosen from among Mn and
Cr having a content less than 0.05% by weight.
3. The wrought product according to claim 1, comprising at least
one element selected from the group consisting of Cu and Ag with
the following contents if chosen, as a percentage by weight Cu:
0.05-0.3 Ag: 0.05-0.3, and an element not chosen from among Cu and
Ag having a content less than 0.05% by weight.
4. The wrought product according to claim 1,wherein the Li content
is, as a percentage by weight Li: 1.1-1.5.and optionally Li:
1.2-1.4.
5. The wrought product according to claim 1, wherein the Mg content
is, as a percentage by weight Mg: 4.4-4.7.
6. The wrought product according to claim 1, comprising a maximum
Be content of 5 ppm and/or a maximum Na content of 10 ppm and/or a
maximum Ca content of 20 ppm.
7. The wrought product according to claim 1, comprising a Zn
content less than 0.2% by weight and optionally less than 0.05% by
weight.
8. The wrought product according to claim 1, wherein said Fe
content and/or said Si content are, as a percentage by weight Fe:
0.04-0.15 Si: 0.04-0.15.
9. The wrought product according to claim 1, wherein said product
has been worked and the working is carried out by rolling.
10. The wrought product according to claim 9, comprising a
thickness ranging from 0.5 to 15 mm, at mid-thickness having at
least one static mechanical strength property among properties (i)
to (iii) and at least one damage tolerance property among
properties (iv) to (vi) (i) a tensile yield stress
R.sub.p0.2(L).gtoreq.280 MPa and optionally
R.sub.p0.2(L).gtoreq.310 MPa, (ii) a tensile yield stress
R.sub.p0.2(LT).gtoreq.260 MPa and optionally
R.sub.p0.2(LT).gtoreq.290 MPa, (iii) a tensile yield stress
R.sub.p0.2(45.degree.).gtoreq.200 MPa and optionally
R.sub.p0.2(45.degree.).gtoreq.240 MPa, (iv) a fracture toughness
for test-specimens of width W=760 mm K.sub.app (L-T).gtoreq.90 MPa
m for a thickness less than 3 mm and K.sub.app (L-T).gtoreq.110 MPa
m for a thickness of at least 3 mm, (v) a fracture toughness for
test-specimens of width W=760 mm K.sub.app (T-L).gtoreq.100 MPa m
for a thickness less than 3 mm and K.sub.app (T-L).gtoreq.120 MPa m
for a thickness of at least 3 mm, (vi) a crack extension of the
last valid point of R curve for test-specimens of width W=760 mm
.DELTA.a.sub.eff(max) (T-L).gtoreq.80 mm for a thickness of less
than 3 mm and .DELTA.a.sub.eff(max) (T-L).gtoreq.110 mm for a
thickness of at least 3 mm.
11. The manufacturing process for a wrought product according to
claim 1, comprising: preparing a molten metal bath in order to
obtain an aluminum alloy, casting said alloy in a rough shape to
form a cast product, optionally homogenizing of the cast product,
hot and optionally cold working said product, optionally heat
treating the product at a temperature ranging from 300 to
420.degree. C. in at least one step, solution heat-treating the
product so worked, and quenching, optionally cold working the
product that has been solution heat treated and quenched,
conducting artificial aging at a temperature of not more than
150.degree. C.
12. The process according to claim 11, wherein said quenching is
carried out in air.
13. The product according to claim 1, capable of being used to
produce an aircraft structural element, optionally a fuselage skin,
a fuselage framework, a stringer or a rib.
14. The process of claim 11, which is conducted in the order
given.
15. The process of claim 11, wherein working is conducted by
rolling.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to French Application No.
11/01555, filed May 20, 2011, and U.S. Provisional Application No.
61/488,196, filed May 20, 2011, the content of both of which are
incorporated herein by reference in their entireties.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The invention relates to aluminum-magnesium-lithium alloy
products, and more particularly such products, their manufacturing
processes and use, designed in particular for aircraft and
aerospace construction.
[0004] 2. Description of Related Art
[0005] Rolled products made of aluminum alloy are developed to
produce parts of high strength designed in particular for the
aircraft and aerospace industry.
[0006] Aluminum alloys containing lithium (AlLi) are of great
interest in this respect, because lithium can reduce the density of
aluminum by 3% and increase the modulus of elasticity by 6% for
each percent of added lithium weight. For these alloys to be
selected for aircraft, their performance as compared to the other
usual properties must generally attain that of alloys in regular
use, in particular in terms of the balance between static
mechanical strength properties (tensile and compression yield
stress, ultimate tensile strength) and damage tolerance properties
(fracture toughness, resistance to fatigue crack propagation),
these properties being in general in opposition to each other.
[0007] These alloys must also have sufficient corrosion resistance,
allowing them to be formed according to the usual processes and to
have low residual stresses in order to be able to be machined
integrally.
[0008] Aluminum alloys containing magnesium and lithium
simultaneously make it possible to obtain particularly low
densities and have therefore been extensively examined GB patent
1,172,736 discloses an alloy containing 4 to 7% Mg by weight,
1.5-2.6% Li, 0.2-1% Mn and/or 0.05-0.3% Zr, the rest aluminum,
useful for applications requiring high mechanical resistance, good
corrosion resistance, low density and a high modulus of
elasticity.
[0009] International request WO 92/03583 described an alloy useful
for aeronautical structures having low density of general formula
Mg.sub.aLi.sub.bZn.sub.cAg.sub.dAl.sub.bal, in which a ranges
between 0.5 and 10%, b between 0.5 and 3%, c between 0.1 and 5%, d
between 0.1 and 2% and bal indicates that the rest is aluminum.
[0010] U.S. Pat. No. 5,431,876 discloses a ternary group of
aluminum lithium and magnesium or copper alloys, including at least
one additive such as zirconium, chromium and/or manganese.
[0011] U.S. Pat. No. 6,551,424 describes a manufacturing process
for products made of aluminum-magnesium-lithium alloy of
composition (as a percentage by weight) Mg: 3.0-6.0. Li: 0.4-3.0,
Zn up to 2.0, Mn up to 1.0, Ag up to 0.5, Fe up to 0.3, Si up to
0.3, Cu up to 0.3, 0.02-0.5 of an element selected from the group
made up of Sc, Hf, Ti, V, Nd, Zr, Cr, Y, Be, including straight and
cross cold rolling.
[0012] U.S. Pat. No. 6,461,566 describes an alloy composed as
follows (as a percentage by weight), Li: 1.5-1.9, Mg: 4.1-6.0, Zn
0.1-1.5, Zr 0.05-0.3, Mn 0.01-0.8 H, 0.9 10.sup.-5-4.5 10-5 and at
least one element selected from the group Be 0.001-0.2, Y 0.001-0.5
and Sc 0.01-0.3.
[0013] RU patent 2171308 describes an alloy composed as follows (as
a percentage by weight), Li: 1.5-3.0, Mg: 4.5-7.0, Fe 0.01-0.15,
Na: 0.001-0.0015, H, 1.7 10.sup.-5-4.5 10.sup.-5 and at least one
element selected from the group Zr 0.05-0.15, Be 0.005-0.1, and Sc
0.05-0.4 and at least one element selected from the group Mn
0.005-0.3, Cr 0.005-0.2, and Ti 0.005-0.2, the rest aluminum.
[0014] RU patent 2163938 describes an alloy containing (as a
percentage by weight by weight) Mg: 2.0-5.8. Li: 1.3-2.3, Cu:
0.01-0.3, Mn: 0.03-0.5, Be: 0.0001-0.3, and at least one element
from among Zr and Sc: 0.02-0.25 and at least one element from among
Ca and Ba: 0.002-0.1, the rest aluminum.
[0015] Patent application DE 1 558 491 describes in particular an
alloy containing (in weight %) Mg: 4-7, Li: 1.5-2.6, Mn: 0.2-1.0,
Zr 0.05-0.3 et/ou Ti 0.05-0.15ou Cr 0.05-0.3.
[0016] These alloys did not solve certain problems and in
particular their performance in terms of damage tolerance has
prevented them from being used significantly in commercial
aviation. It should also be noted that the manufacture of wrought
products from these alloys has remained difficult and that the
rejection rate is too high.
[0017] There exists a need for wrought products made of
aluminum-magnesium-lithium alloy presenting improved properties as
compared with those of known products, in particular in terms of
the balance between static mechanical strength properties and
damage tolerance properties, in particular fracture toughness and
corrosion resistance while being of low density.
[0018] In addition there exists a need for a reliable and economic
manufacturing process for these products.
SUMMARY
[0019] A first subject of the present invention is a wrought
product made of aluminum alloy composed as follows, as a percentage
by weight, [0020] Mg: 4.0-5.0 [0021] Li: 1.0-1.6 [0022] Zr:
0.05-0.15 [0023] Ti: 0.01-0.15 [0024] Fe: 0.02-0.2 [0025] Si:
0.02-0.2 [0026] Mn: .ltoreq.0.5 [0027] Cr.ltoreq.0.5 [0028] Ag:
.ltoreq.0.5 [0029] Cu.ltoreq.0.5 [0030] Zn.ltoreq.0.5 [0031]
Sc<0.01 [0032] other elements <0.05, each remainder
aluminum;
[0033] Another subject of the present invention is a manufacturing
process for a wrought product according to the invention including,
optionally successively, [0034] preparing a molten metal bath in
order to obtain an aluminum alloy composed according to the
invention, [0035] casting said alloy in a rough shape to form a
cast product, [0036] optionally homogenizing the cast product,
[0037] hot and optionally cold working, [0038] optionally heat
treating at a temperature ranging from 300 to 420.degree. C. in one
or more steps, [0039] solution heat-treating the product so worked,
and quenching, [0040] optionally cold working the product that has
been solution heat treated and quenched, [0041] subjecting the
product to artificial aging at a temperature of not more than
150.degree. C.
[0042] Still another subject of the present invention is the use of
a product of the invention to produce, for example, aircraft
structural elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1: R Curve in direction L-T (test-specimen CCT760).
[0044] FIG. 2: R Curve in direction T-L (test-specimen CCT760).
[0045] FIG. 3: Fracture toughness K.sub.app (L-T) according to the
tensile yield stress R.sub.p0.2(L) for alloys A, C and D.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0046] Unless otherwise stated, 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. Alloys are designated in conformity
with the rules of The Aluminium Association, known to those skilled
in the art. 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
procedure of The Aluminium Association, which is described on pages
2-12 and 2-13 of "Aluminum Standards and Data". The definitions of
the metallurgical tempers are indicated in European standard EN
515.
[0047] The tensile static mechanical properties, in other words the
ultimate tensile strength R.sub.m, the conventional yield stress at
0.2% of elongation Rp.sub.0.2 and elongation at break A %, are
determined by a tensile test according to standard EN ISO 6892-1,
sampling and test direction being defined by standard EN 485-1.
[0048] A curve giving the effective stress intensity factor as a
function of the effective crack extension, known as R curve, is
given 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 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-specimen
of the required shape. K.sub.app represents factor K.sub.CO
corresponding to the test-specimen which was used to carry out the
test of R curve. K.sub.Ceff represents factor K.sub.C corresponding
to the test-specimen which was used to carry out the test of R
curve. .DELTA.a.sub.eff(max) represents the crack extension of the
last valid point of R curve. The length of R curve--namely the
maximum crack extension of the curve--is a parameter that is in
itself important, in particular for fuselage design.
[0049] Unless otherwise specified, the definitions of standard EN
12258 apply.
[0050] "Structural element" of a mechanical construction here
refers to a mechanical part for which the static and/or dynamic
mechanical properties are particularly important for the
performance of the structure, and for which a structural analysis
is usually prescribed or performed. These are typically elements
the failure of which is likely to endanger the safety of said
construction, its users or others. For an aircraft, these
structural elements include the parts which make up the fuselage
(such as the fuselage top skin, stringers, bulkheads,
circumferential frames), the wings (such as the top or bottom wing
skin, stringers or stiffeners, ribs and spars) and the tail unit,
made up of horizontal and vertical stabilizers, as well as floor
beams, seat tracks and doors.
[0051] According to the present invention, a selected grade of
aluminum alloys which contain specific and critical amounts of
magnesium, lithium, zirconium, titanium, iron and silicon makes it
possible to manufacture wrought products having an improved
compromise of properties, in particular between mechanical
resistance and damage tolerance, while having good performance in
terms of corrosion.
[0052] The magnesium content of the products according to the
invention preferably lies from 4.0 to 5.0% by weight. In an
advantageous embodiment of the invention, the magnesium content is
at least 4.3% by weight and preferentially 4.4% by weight. A
maximum content of 4.7% by weight or advantageously 4.6% by weight
of magnesium is preferred.
[0053] The lithium content of the products according to the
invention preferably lies from 1.0 to 1.6% by weight. The present
inventors noted that a limited lithium content, in the presence of
certain additional elements, makes it possible in some embodiments
to very significantly improve fracture toughness and fatigue crack
propagation speed, which largely compensates for the slight
increase in density and the reduction in static mechanical
properties.
[0054] In an advantageous embodiment, the maximum lithium content
is 1.5% by weight and preferably 1.45% by weight or preferentially
1.4% by weight. A minimum lithium content of 1.1% by weight and
preferably of 1.2% of weight is advantageous, in particular in
order to improve resistance to intergranular corrosion.
[0055] The zirconium content of the products according to the
invention preferably lies from 0.05 to 0.15% by weight and the
titanium content lies from 0.01 to 0.15% by weight. The presence of
these elements in conjunction with the working conditions used
advantageously makes it possible in some embodiments to maintain a
substantially unrecrystallized granular structure. In contrast to
certain information disclosed from prior art, the present inventors
noted that it is in some cases not necessary to add scandium to
these alloys to obtain the desired substantially unrecrystallized
granular structure and that the addition of scandium could even
prove to be detrimental by making the alloy particularly fragile
and difficult to cold roll down to thicknesses less than 3 mm, The
scandium content is thus advantageously less than 0.01% by weight.
In an advantageous embodiment of the invention, the titanium
content is from 0.01 to 0.05% by weight. Manganese and/or chromium
may also be added to contribute in particular to control the
granular structure, their content advantageously remaining at a
maximum of 0.5% by weight. In an advantageous embodiment of the
invention, having in particular an improved hot ductility, the
alloy contains at least one element from among Mn and Cr with, as a
percentage by weight Mn: 0.05-0.5 or 0.05-0.3 and Cr: 0.05-0.3, and
an element not chosen from among Mn and Cr having a content lower
than 0.05% by weight. Improvement of hot ductility helps, in
particular hot working, which enables reducing the rejection rate
during transformation.
[0056] Copper and/or silver may also be added to improve the
performances of the wrought products according to the invention,
their content preferably remaining at a maximum of 0.5% by weight
In an advantageous embodiment of the invention, the alloy contains
at least one element from among Ag and Cu with, as a percentage by
weight Cu: 0.05-0.3 and Ag: 0.05-0.3, and an element not chosen
from among Ag and Cu having a content lower than 0.05% by weight.
These elements can contribute, in particular, to the static
mechanical properties. However, in an advantageous embodiment, to
improve resistance to intergranular corrosion, Ag and/or Cu content
are advantageously less than 0.05% by weight.
[0057] The wrought products according to the invention preferably
contain a small quantity of iron and silicon, the content of these
elements ranging from 0.02 to 0.2% by weight. The present inventors
think that the presence of these elements may contribute, by
forming intermetallic phases and/or by contributing to forming
dispersoids in particular when manganese is present, to improving
damage tolerance properties by avoiding the localization of
bending. In an embodiment of the invention, the Fe content and/or
the Si content are as a percentage by weight, Fe: 0.04-0.15, Si:
0.04-0.15. In an embodiment of the invention, the Fe content and/or
the Si content is less than 0.15% by weight and preferably less
than 0.1% by weight.
[0058] The Zn content is preferably a maximum of 0.5% by weight. In
an advantageous embodiment of the invention, the Zn content is less
than 0.2% by weight and preferably lower than 0.05% by weight.
Deliberate Zn addition is typically not desirable because this
element can contribute to deteriorate hot ductility without
providing any advantageous effect on resistance to intergranular
corrosion. Moreover Zn addition contributes to increase the alloy
density, which is often not desirable.
[0059] Other elements have a content less than 0.05% by weight
each. Certain elements can be detrimental for alloys according to
the invention, in particular for reasons of alloy manufacturing
such as toxicity and/or breakages during working and it is
preferable to restrict them to a very low level, i.e. less than
0.05% or even less. In an advantageous embodiment, the products
according to the invention have a maximum Be content of 5 ppm and
preferably 2 ppm of Be and/or a maximum Na content of l Oppm and/or
a maximum Ca content of 20 ppm.
[0060] The wrought products according to the invention are
preferentially extruded products such as sections, rolled products
such as sheets or plates and/or forged products
[0061] A suitable manufacturing process of the products according
to the invention includes the successive steps of preparing a
molten metal bath in order to obtain an aluminum alloy composed
according to the invention, casting said alloy in rough shape,
optionally homogenizing the product so cast, hot and optionally
cold working, solution heat-treating the product so worked, and
quenching, optionally cold working the product that has undergone
solution heat-treatment and has been quenched, and artificial aging
at a temperature of less than 150.degree. C.
[0062] In the first step, a molten metal bath is produced in order
to obtain an aluminum alloy composed according to the
invention.
[0063] The molten metal bath is then cast typically in rough shape,
typically a rolling slab, extrusion billet, or forging stock.
[0064] The rough shape is then optionally homogenized in order to
reach a temperature ranging from 450.degree. C. to 550.degree. and
preferably from 480.degree. C. to 520.degree. C. for a length of
time ranging from 5 to 60 hours. The homogenization treatment can
be carried out in one or more steps. However the present inventors
did not note any significant advantage provided by homogenization
and in a preferred embodiment of the invention, one proceeds
directly to hot working following simple reheating without carrying
out any homogenization.
[0065] Hot working, typically by extrusion, rolling and/or forging,
is carried out preferably with an input temperature greater than
400.degree. C. and advantageously greater than 430.degree. C. or
even 450.degree. C.
[0066] In the case of the manufacture of sheets by rolling, it may
be necessary to perform a cold rolling step for products of which
the thickness is less than 3 mm. It may prove useful to carry out
one or more intermediate heat treatment operations before or during
cold rolling. These intermediate heat treatment operations are
typically carried out at a temperature ranging from 300 to
420.degree. C. in one or more steps.
[0067] The present inventors noted that even by carrying out these
intermediate heat treatment operations, they had not been able to
industrially cold roll reference alloy sheets down to a thickness
of 2 mm, whereas this step proved to be possible with alloy sheets
according to the invention. The sheets according to the invention
have a preferred thickness of at least 0.5 mm and preferably of at
least 0.8 mm or 1 mm.
[0068] After hot and optionally cold working, the product undergoes
solution heat-treatment and is quenched. Before undergoing solution
heat-treatment, it may be advantageous to carry out heat treatment
at a temperature ranging from 300 to 420.degree. C. in one or more
steps, in order to improve control of the substantially
unrecrystallized granular structure. Solution heat-treatment is
preferably carried out, according to the composition of the
product, at a temperature ranging from 370 to 500.degree. C.
Quenching can be carried out, for example, in water and/or in air.
It is advantageous to carry out air quenching because the
intergranular corrosion properties are improved.
[0069] The product that undergoes solution heat-treatment and is
then quenched can optionally be cold worked once more. Flattening
or straightening operations are typically performed at this step
but it is also possible to carry out more thorough working so as to
still further improve the mechanical properties.
[0070] The metallurgical temper obtained for the rolled products is
advantageously a T6 or T6X or T8 or T8X temper and for extruded
products advantageously a T5 or T5X temper in the case of press
quenching or a T6 or T6X or T8 or T8Xtemper.
[0071] The product finally undergoes artificial aging at a
temperature of less than 150.degree. C. Advantageously, artificial
aging is carried out in three steps: a first step at a temperature
ranging from 70 to 100.degree. C., a second step at a temperature
ranging from 100 to 140.degree. C. and a third step at a
temperature ranging from 90 to 110.degree. C., the duration of
these steps being typically from 5 to 50 h.
[0072] The combination of the chosen composition, in particular the
zirconium and titanium content, and the transformation parameters,
in particular the hot working temperature and if necessary the heat
treatment before solution heat-treatment, advantageously makes it
possible to obtain a substantially unrecrystallized granular
structure. "Substantially unrecrystallized granular structure" is
taken to mean an unrecrystallized granular structure content at
mid-thickness greater than 70% and preferably greater than 85%.
[0073] Rolled products according to the invention have particularly
advantageous characteristics. The rolled products preferably have a
thickness ranging from 0.5 mm to 15 mm, but products of thickness
greater than 15 mm, up to 50 mm or even 100 mm or more may have
advantageous properties.
[0074] The rolled products obtained by the process according to the
invention have, for a thickness ranging from 0.5 to 15 mm, at
mid-thickness at least one static mechanical strength property
among properties (i) to (iii) and at least one damage tolerance
property among properties (iv) to (vi) [0075] (i) a tensile yield
stress R.sub.p0.2(L).gtoreq.280 MPa and preferably
R.sub.p0.2(L).gtoreq.310 MPa, [0076] (ii) a tensile yield stress
R.sub.p0.2(LT).gtoreq.260 MPa and preferably
R.sub.p0.2(LT).gtoreq.290 MPa, [0077] (iii) a tensile yield
stress)R.sub.p0.2(45.degree.).gtoreq.200 MPa and
preferably)R.sub.p0.2(45.degree.).gtoreq.240 MPa, [0078] (iv) a
fracture toughness for test-specimens of width W=760 mm K.sub.app
(L-T).gtoreq.90 MPa m for a thickness less than 3 mm and K.sub.app
(L-T).gtoreq.110 MPa m for a thickness of at least 3 mm, [0079] (v)
a fracture toughness for test-specimens of width W=760 mm K.sub.app
(T-L).gtoreq.100 MPa m for a thickness less than 3 mm and K.sub.app
(T-L).gtoreq.120 MPa m for a thickness of at least 3 mm, [0080]
(vi) a crack extension of the last valid point of R curve for
test-specimens of width W=760 mm .DELTA.a.sub.eff(max)
(T-L).gtoreq.80 mm for a thickness of less than 3 mm and
.DELTA.a.sub.eff(.sub.max) (T-L).gtoreq.110 mm for a thickness of
at least 3 mm.
[0081] The rolled products according to the invention typically
have an improved isotropy of mechanical properties, in particular
fracture toughness. The rolled products according to the invention
therefore advantageously have, for test-specimens of width W=760
mm, a difference between K.sub.app (L-T) and K.sub.app (T-L) less
than 20% and/or a difference between .DELTA.a.sub.eff(max) (T-L)
and .DELTA.a.sub.eff(max) (L-T) less than 20% and preferably less
than 15%.
[0082] Moreover rolled products according to the invention such as
those that have been air-quenched have a weight loss less than 20
mg/cm.sup.2 and preferably less than 15 mg/cm.sup.2 after the
intergranular corrosion test NAMLT ("Nitric Acid Mass Loss Test"
ASTM-G67).
[0083] The wrought products according to the invention are
advantageously used to produce structural elements for aircraft, in
particular airplanes. Preferred aircraft structural elements are in
particular a fuselage skin obtained advantageously with sheets of
thickness 0.5 to 12 mm according to the invention, a fuselage
framework a stringer obtained advantageously with sections
according to the invention or a rib.
[0084] These aspects, as well as others of the invention are
explained in greater detail using the following illustrative and
non-restrictive examples.
EXAMPLE 1
[0085] In this example, several Al--Mg--Li alloy plates, the
composition of which is given in table 1, were cast. Alloy D has a
composition according to the invention; alloys A to C are reference
alloys.
TABLE-US-00001 TABLE 1 Composition as a percentage by weight and
density of the Al--Mg--Li alloys used Na Alloy Ag Li Si Fe Cu Ti Mn
Mg Zn Zr (ppm) Sc A 0.1 1.8 0.04 0.04 0.17 0.02 0.13 4.6 0.46 0.07
9 0.08 B 0.1 1.7 0.04 0.04 0.07 0.02 0.13 4.9 0.48 0.13 8 C 0.1 1.7
0.04 0.04 0.17 0.02 0.15 4.8 0.44 0.12 11 D 0.1 1.4 0.05 0.04 0.18
0.02 0.15 4.5 0.12 4
[0086] The plates were heated and hot-rolled to a thickness of
approximately 4 mm. Cold-rolling tests to thickness 2 mm were
carried out after heat treatment made up of two successive steps of
one hour at 340.degree. C. followed by 1 hour at 400.degree. C.
Only the alloy sheets according to the invention could be
cold-rolled successfully to the final thickness, reference alloy
sheets having broken at thickness 2.6 mm After hot and possibly
cold rolling, the sheets underwent solution heat-treatment at
480.degree. C. for 20 min, this treatment being preceded by heat
treatment made up of two successive steps of one hour at
340.degree. C. followed by 1 hour at 400.degree. C. After solution
heat-treatment, the sheets were air-quenched and flattened.
Artificial aging was carried out for 10 hours at 85.degree. C.
followed by 16 hours at 120.degree. C. followed by 10 hours at
100.degree. C.
[0087] The granular structure of all the samples was substantially
unrecrystallized, the rate of recrystallization at mid-thickness
being less than 10%.
[0088] Samples were tested to determine their static mechanical
properties (tensile yield stress R.sub.p0.2, ultimate tensile
strength R.sub.m, and elongation at break (A).
[0089] The results obtained are given in table 2 below.
TABLE-US-00002 TABLE 2 Mechanical properties of sheets obtained.
Direction L Direction TL Direction 45.degree. Th. Rm R0.2 Rm R0.2
Rm R0.2 Alloy (mm) (MPa) (MPa) A % (MPa) (MPa) A % (MPa) (MPa) A %
A 4.5 507 399 4.9 502 355 12.5 436 293 21.8 B 4.5 488 370 6.0 513
354 12.4 423 274 24.7 C 4.2 487 374 5.6 506 349 11.7 444 286 21.0 D
4.2 436 328 8.5 443 304 16.1 394 256 23.1 D 2.1 439 344 5.4 455 327
15.2 379 256 25.8
[0090] The fracture toughness of the sheets was characterized by
the test of R curves as per standard ASTM E561. The tests were
carried out with a full thickness test-specimen CCT (W=760 mm,
2a0=253 mm). All the results are shown in table 3 and table 14 and
are illustrated by the graphs in FIG. 1 and FIG. 2.
TABLE-US-00003 TABLE 3 Summary data of R curve Th. Kr (MPa m) at
.DELTA.a.sub.eff (mm) Alloy (mm) Dir. 10 20 30 40 50 60 70 80 A 4.5
L-T 63 79 91 101 105 107 111 C 4.2 70 91 105 115 122 129 135 142 D
4.2 86 113 131 145 157 166 175 183 D 2.1 79 101 113 120 128 132 137
141 A 4.5 T-L 62 86 95 110 123 135 143 B 4.5 68 87 110 129 147 157
164 174 C 4.2 70 94 110 122 131 134 D 4.2 86 110 128 141 153 164
175 183 D 2.1 84 106 122 133 142 150 157 161
TABLE-US-00004 TABLE 4 Fracture toughness test results Th.
K.sub.app Kc.sub.eff .DELTA.a.sub.effmax Alloy (mm) D MPa m MPa m
Mm A 4.5 L-T 82 102 76 C 4.2 96 132 116 D 4.2 125 177 121 D 2.1 99
122 113 A 4.5 T-L 102 142 72 B 4.5 119 179 102 C 4.2 102 131 63 D
4.2 125 177 134 D 2.1 112 147 103
[0091] FIG. 3 shows the improvement in the compromise between yield
stress and fracture toughness.
[0092] In particular, the improvement in K.sub.app (L-T) is greater
than 25% whereas the reduction in yield stress is less than 15%
compared to alloy C sheet. The length of the R-curve is also
significantly improved, and so .DELTA.a.sub.eff(max) (T-L) is
improved by more than 30%.
[0093] The crack propagation speed was determined as per standard
E647 on CCT test-specimens of width 160 mm.
TABLE-US-00005 Tableau 5 - Crack propagation speed (.sigma..sub.max
= 80 MPa or .sigma..sub.max = 120 MPa (**), R = 0.1-full thickness)
Th.. da/dN (mm/cycles) at .DELTA.K (MPa m) Alloy (mm) Dir. 10 15 20
25 30 35 40 D 4.2 L-T 1.24 10.sup.-04 1.17 10.sup.-04 2.27
10.sup.-04 3.85 10.sup.-04 0.63 10.sup.-03 0.95 10.sup.-03 1.48
10.sup.-03 D 2.1 1.20 10.sup.-04 1.59 10.sup.-04 2.82 10.sup.-04
4.95 10.sup.-04 0.90 10.sup.-03 A 4.5 T-L 1.30 10.sup.-04 2.58
10.sup.-04 7.81 10.sup.-04 35.3 10.sup.-04 14.4 10.sup.-03 B 4.5**
1.37 10.sup.-04 1.89 10.sup.-04 2.73 10.sup.-04 5.63 10.sup.-04
0.98 10.sup.-03 2.20 10.sup.-03 5.30 10.sup.-03 C 4.2** 2.84
10.sup.-04 5.10 10.sup.-04 9.61 10.sup.-04 1.99 10.sup.-03 9.60
10.sup.-03 D 4.2 1.35 10.sup.-04 2.00 10.sup.-04 3.52 10.sup.-04
5.14 10.sup.-04 0.92 10.sup.-03 1.95 10.sup.-03 D 2.1 1.01
10.sup.-04 1.53 10.sup.-04 2.96 10.sup.-04 5.56 10.sup.-04 0.90
10.sup.-03
[0094] The results of the intergranular corrosion test NAMLT
("Nitric Acid Farmhouse Loss Test" ASTM-G67) for various sheets are
summed up in Table 6. Certain sheets underwent solution
heat-treatment and were quenched with water in the laboratory.
TABLE-US-00006 TABLE 6 NAMLT intergranular corrosion test Weight
loss (mg/cm.sup.2) Th. Water-quenched Air-quenched Alloy (mm)
Surface t/10e Surface t/10 A 4.5 24 13 B 4.5 26 16 C 4.2 26 18 D
4.2 26.5 24 16 17 D 2.1 12
[0095] Air-quenched alloy sheets according to the invention have
low sensitivity to intergranular corrosion for a thickness of 4 mm
and are not sensitive to intergranular corrosion for a thickness of
2 mm.
EXAMPLE 2
[0096] In this example, small ingots were cast to evaluate hot
ductility and intergranular corrosion properties of different
alloys. The size of the ingot after machining was in mm
255.times.180.times.28.
[0097] The composition of the alloys is provided in Table 7.
TABLE-US-00007 TABLE 7 Composition as a percentage by weight and
density of the Al--Mg--Li alloys used Alloy Ag Li Si Fe Cu Ti Mn Mg
Zn Zr Cr Sc E -- 1.4 0.03 0.03 -- 0.02 0.40 4.5 -- 0.11 0.18 -- F
-- 1.4 0.03 0.03 -- 0.02 0.16 4.4 -- 0.12 0.19 -- G -- 1.4 0.03
0.03 -- 0.02 0.17 4.4 -- 0.11 -- -- H -- 1.1 0.03 0.03 -- 0.02 0.16
4.5 -- 0.12 -- -- I -- 1.4 0.03 0.03 -- 0.02 0.17 4.5 0.6 0.12 --
--
[0098] Hot ductility was evaluated on tests samples machined from
the small ingots after homogenization of 12 h at 505.degree. C. The
hot ductility test was carried out with a servo hydraulic
instrument provided by Servotest Testing Systems Ltd on specific
test samples having a thickness of 20 mm and at a deformation rate
of 1 s.sup.-1. The test consists in the compression of a sample
containing two holes. Due to the compression,
[0099] the material between the two holes expands at a controlled
deformation rate. The test conditions are described in the journal
article from d'A. Deschamps et al. published in the journal
Materials Science and Engineering A319-321 (2001) 583-586. The
normalized measurement of the reduction in area of the fractured
zone (.DELTA.A/A0) enables to evaluate hot ductility at the
temperature under consideration. Results obtained at 450.degree. C.
and 475.degree. C. are provided in Table 8.
TABLE-US-00008 TABLE 8 Hot ductility (.DELTA.A/A.sub.0) (%) Hot
ductility Deformation (.DELTA.A/A.sub.0) (%) temperature (.degree.
C.) Alloy 450 475 Average E 17 19 18 F 13 19 16 G 12 13 12 H 11 20
15 I 8 12 10
[0100] Alloys E and F which contain Mn and Cr have an advantageous
hot ductility whereas hot ductility of reference alloy I which
contains 0.6 wt. % Zn is the lowest among tested alloys.
[0101] The small ingots were hot rolled to a thickness of 4 mm. The
sheets so obtained were solution heat treated at 480.degree. C.,
this treatment being preceded by a heat treatment made up of two
successive steps of one hour at 345.degree. C. followed by 1 hour
at 400.degree. C. After solution heat treatment, the sheets were
air quenched and flattened by controlled stretching with a 2%
permanent set. Artificial aging was carried out for 10 hours at
85.degree. C. followed by 16 hours at 120.degree. C. followed by 10
hours at 100.degree. C.
[0102] The results of the intergranular corrosion test NAMLT
("Nitric Acid Farmhouse Loss Test" ASTM-G67) are presented in Table
9.
TABLE-US-00009 TABLE 9 NAMLT intergranular corrosion test measured
at the surface Alloy Weight loss (mg/cm.sup.2) E 11 F 11 G 8 H 16 I
8
[0103] Alloy G, which in particular is different from alloy D
through a lower copper content, exhibits a very low weight loss.
Alloy I which contains Zn is not different from alloy G for
resistance to intergranular corrosion. Alloy H, which has a lithium
content lower than that of the other tested alloys, exhibits a
higher weight loss.
* * * * *