U.S. patent application number 13/328872 was filed with the patent office on 2012-06-21 for aluminum copper lithium alloy with improved resistance under compression and fracture toughness.
This patent application is currently assigned to CONSTELLIUM FRANCE. Invention is credited to Armelle DANIELOU, Gaelle POUGET, Christophe SIGLI, Timothy WARNER.
Application Number | 20120152415 13/328872 |
Document ID | / |
Family ID | 44119503 |
Filed Date | 2012-06-21 |
United States Patent
Application |
20120152415 |
Kind Code |
A1 |
DANIELOU; Armelle ; et
al. |
June 21, 2012 |
ALUMINUM COPPER LITHIUM ALLOY WITH IMPROVED RESISTANCE UNDER
COMPRESSION AND FRACTURE TOUGHNESS
Abstract
The invention relates to a manufacturing process for flat-rolled
products made of an alloy containing aluminum, including the steps
of production, casting, homogenization, rolling at temperature
greater than 400.degree. C., solution heat treating, quenching,
stretching between 2 and 3.5% and aging. The invention also relates
to flat-rolled products obtained by this process, which offer a
favorable compromise of properties between mechanical resistance
under compression and stretching and fracture toughness. The
products according to the invention are useful in particular for
the manufacture of upper wing skins.
Inventors: |
DANIELOU; Armelle; (Les
Echelles, FR) ; POUGET; Gaelle; (Grenoble, FR)
; SIGLI; Christophe; (Grenoble, FR) ; WARNER;
Timothy; (Voreppe, FR) |
Assignee: |
CONSTELLIUM FRANCE
Courbevoie
FR
|
Family ID: |
44119503 |
Appl. No.: |
13/328872 |
Filed: |
December 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61424970 |
Dec 20, 2010 |
|
|
|
Current U.S.
Class: |
148/552 ;
148/439 |
Current CPC
Class: |
C22F 1/057 20130101;
C22C 21/12 20130101; C22C 21/16 20130101 |
Class at
Publication: |
148/552 ;
148/439 |
International
Class: |
C22F 1/057 20060101
C22F001/057; C22C 21/16 20060101 C22C021/16 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2010 |
FR |
1004962 |
Claims
1. A process for manufacturing a flat-rolled product comprising an
aluminum alloy, said process comprising the following performed in
succession, a) producing a molten aluminum metal bath comprising
4.2 to 4.6% Cu by weight, 0.8 to 1.30% Li by weight, 0.3 to 0.8% Mg
by weight, 0.05 to 0.18% Zr by weight, 0.05 to 0.5% Ag by weight,
0.0 to 0.5% Mn by weight, at the most 0.20% Fe+Si by weight, less
than 0.20% of Zn by weight, at least one element chosen from Cr,
Sc, Hf and Ti, the quantity of said element, if it is chosen, being
from 0.05 to 0.3% by weight for Cr and Sc, 0.05 to 0.5% by weight
for Hf and from 0.01 to 0.15% by weight for Ti, other elements at
least 0.05% by weight each and 0.15% by weight in total, remainder
aluminum; b) casting a rolling slab from said molten metal bath;
said rolling slab is homogenized in order to reach a temperature
ranging from 450.degree. C. and 550.degree. and for a period
ranging from 5 to 60 hours; d) hot rolling said rolling slab into a
plate, maintaining a temperature of at least 400.degree. C. e)
allowing said plate to undergo solution heat treatment at a
temperature from 490 to 530.degree. C. for 15 min to 8 hours and
quenching said product; f) allowing said plate to undergo
controlled stretching with a permanent set of 2 to 3.5%, g)
performing aging such that said plate reaches a temperature ranging
from 130 to 170.degree. C. for 5 to 100 hours, with the proviso
that no significant cold working is carried out on said plate,
between said hot rolling d) and said solution heat treatment
e).
2. The process according to claim 1, wherein the Cu content ranges
from 4.3 to 4.4% by weight.
3. The process according to claim 1, wherein the Li content is up
to 1.15% by weight.
4. The process according to claim 1, wherein the Li content ranges
from 1.10 to 1.20% by weight.
5. The process according to claim 1, wherein the Mg content ranges
from 0.50 to 0.70% by weight.
6. The process according to claim 1, wherein the Mn content is not
more than 0.1% by weight.
7. The process according to claim 1, wherein Fe and Si contents are
each at the most 0.08% by weight and/or the Ti content is from 0.01
to 0.10% by weight and the Cr, Sc and Hf content are at the most
0.05% by weight and/or the Zn to content is at most 0.15% by
weight.
8. The process according to claim 1, wherein the permanent set is
accomplished by controlled traction and is selected so as to obtain
a compression yield stress at least equal to tensile yield
stress.
9. The process according to claim 1, wherein controlled stretching
is realized directly after solution treatment and quenching.
10. The process according to claim 1, wherein aging is under-aging
close to peak of compression yield stress.
11. A flat-rolled product of a thickness ranging from 8 to 50 mm
and having a primarily unrecrystallized granular structure
optionally obtained by said process according to claim 1, said
product comprising at mid-thickness at least one of the following
combinations of characteristics: (i) for thicknesses from 8 to 15
mm, at mid-thickness, a tensile yield stress
R.sub.p0.2(L).gtoreq.600 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.620 MPa and fracture toughness such that
K.sub.1C (L-T).gtoreq.28 MPa m and/or K.sub.app (L-T).gtoreq.73 MPa
m, for 300 mm wide and 6.35 mm thick CCT test samples, (ii) for
thicknesses from 8 to 15 mm, at mid-thickness, a tensile yield
stress R.sub.p0.2(L).gtoreq.630 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.640 MPa and fracture toughness such that
K.sub.1C (L-T).gtoreq.26 MPa m and/or K.sub.app (L-T).gtoreq.63 MPa
m, for 300 mm wide and 6.35 mm thick CCT test samples, (iii) for
thicknesses from 15 to 50 mm, at mid-thickness, a tensile yield
stress R.sub.p0.2(L).gtoreq.610 MPa a compression yield stress
R.sub.p0.2(L).gtoreq.620 MPa and fracture toughness K.sub.1C
(L-T).gtoreq.22 MPa m, (iv) for thicknesses from 15 to 50 mm, at
mid-thickness, a tensile yield stress R.sub.p0.2(L).gtoreq.580 MPa
a compression yield stress R.sub.p0.2(L).gtoreq.600 MPa and
fracture toughness K.sub.1C (L-T).gtoreq.24 MPa m.
12. An airplane structural element, optionally an upper wing skin,
said element comprising said product according to claim 11.
13. A product according to claim 11 capable of being used for a
structural element.
14. The product according to claim 11, capable of being used for
aeronautical engineering.
15. The product of claim 13, capable of being used for aeronautical
engineering.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to FR1004962 filed Dec. 20,
2010 and to U.S. Provisional application No. 61/424,970, filed Dec.
20, 2010, the content of which is incorporated herein by reference
in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The invention relates to aluminum-copper-lithium alloy
products, and more particularly such products, their manufacturing
processes and use, designed in particular for aeronautical and
aerospace engineering.
[0004] 2. Description of Related Art
[0005] Hat-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 attain that of alloys in regular use, in
particular in terms of the compromise between static mechanical
resistance 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 contradictory. For certain parts
such as the upper surfaces of wing skins the compression yield
stress is an essential property. These mechanical properties must
moreover be preferably stable over time and have good thermal
stability, i.e. not be significantly modified by thermal exposure
at operating temperature.
[0007] These alloys must also have sufficient corrosion resistance,
be capable of being formed according to usual processes and have
low residual stresses in order to be able to be integrally
machined.
[0008] 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 resistance.
[0009] U.S. Pat. No. 5,455,003 describes a manufacturing process
for Al--Cu--Li alloys which have improved mechanical resistance and
fracture toughness at cryogenic temperature, in particular owing to
appropriate working 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.
[0010] 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 content because
of a reduction in the balance between fracture toughness and
mechanical resistance.
[0011] 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, V.
[0012] US patent application 2009/142222 A1 describes alloys
including (percentage 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 request also describes a manufacturing process for
extruded products.
[0013] There exists a need for flat-rolled products made of
aluminum-copper-lithium alloy having improved properties as
compared to those of known products, in particular in terms of
compromise between static mechanical resistance properties, in
particular tensile yield stress and compression, and damage
tolerance properties, in particular fracture toughness, thermal
stability, corrosion resistance and machinability, while having a
low density.
[0014] In addition there exists a need for a reliable and economic
manufacturing process for these products.
SUMMARY
[0015] A first subject of the invention is a manufacturing process
for a flat-rolled product made of an aluminum alloy in which the
following operations are performed in succession: [0016] a) an
aluminum molten metal bath is prepared comprising 4.2 to 4.6% Cu by
weight, 0.8 to 1.30% Li by weight, 0.3 to 0.8% Mg by weight, 0.05
to 0.18% Zr by weight, 0.05 to 0.5% Ag by weight, 0.0 to 0.5% Mn by
weight, at most 0.20% Fe+Si by weight, less than 0.20% of Zn by
weight, at least one element chosen from Cr, Sc, HT and Ti, the
quantity of said element, if it is chosen, being from 0.05 to 0.3%
by weight for Cr and Sc, 0.05 to 0.5% by weight for Hf and from
0.01 to 0.15% by weight for Ti, other elements at least 0.05% by
weight each and 0.15% by weight in total, the rest aluminum; [0017]
b) a rolling slab is cast from said molten metal bath; [0018] c)
said rolling slab is homogenized in order to reach a temperature
between 450.degree. C. and 550.degree. and preferably between
480.degree. C. and 530.degree. C. for a period between 5 and 60
hours; [0019] d) said rolling slab is hot rolled into a plate,
maintaining a temperature higher than 400.degree. C. and preferably
higher than 420.degree. C., [0020] e) said plate undergoes solution
heat treatment between 490 and 530.degree. C. for 15 min to 8 hours
and said product is quenched; [0021] f) said plate undergoes
controlled stretching with a permanent set of 2 to 15% and
preferably of 2.0 to 3.0%, [0022] g) aging is performed in which
said plate reaches a temperature between 130 and 170.degree. C. and
preferably between 150 and 160.degree. C. for 5 to 100 hours and
preferably from 10 to 70 hours, given that no significant cold
working is carried out on said plate, in particular by cold
rolling, between hot rolling d) and solution heat treatment e).
[0023] A second subject of the invention is a flat-rolled product
of thickness between 8 and 50 mm and of substantially
unrecrystallized granular structure obtainable by the process
according to the invention having at mid-thickness at least one of
the following combinations of characteristics: [0024] (i) for
thicknesses from 8 to 15 mm, at mid-thickness, a tensile yield
stress R.sub.p0.2(L).gtoreq.600 MPa and preferably
R.sub.p0.2(L).gtoreq.610 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.620 MPa and preferably
R.sub.p0.2(L).gtoreq.630 MPa and fracture toughness such that
K.sub.1C (L-T).gtoreq.28 MPa m and preferably K.sub.1C
(L-T).gtoreq.32 MPa m and/or K.sub.app (L-T).gtoreq.73 MPa m and
preferably K.sub.app (L-T).gtoreq.79 MPa m, for 300 mm wide and
6.35 mm thick CCT test samples, [0025] (ii) for thicknesses from 8
to 15 mm, at mid-thickness, a tensile yield stress
R.sub.p0.2(L).gtoreq.630 MPa and preferably
R.sub.p0.2(L).gtoreq.640 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.640 MPa and preferably
R.sub.p0.2(L).gtoreq.650 MPa and fracture toughness such that
K.sub.1C (L-T).gtoreq.26 MPa m and preferably K.sub.1C
(L-T).gtoreq.30 MPa m and/or K.sub.app (L-T).gtoreq.63 MPa m and
preferably K.sub.app(L-T).gtoreq.69 MPa m, for 300 mm wide and 6.35
mm thick CCT test samples, [0026] (iii) for thicknesses from 15 to
50 mm, at mid-thickness, a tensile yield stress
R.sub.p0.2(L).gtoreq.610 MPa and preferably
R.sub.p0.2(L).gtoreq.620 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.620 MPa and preferably
R.sub.p0.2(L).gtoreq.630 MPa and fracture toughness K.sub.1C
(L-T).gtoreq.22 MPa m and preferably K.sub.1C (L-T).gtoreq.24 MPa
m, [0027] (iv) for thicknesses from 15 to 50 mm, at mid-thickness,
a tensile yield stress R.sub.p0.2(L).gtoreq.580 MPa and preferably
R.sub.p0.2(L).gtoreq.590 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.600 MPa and preferably
R.sub.p0.2(L).gtoreq.610 MPa and fracture toughness K.sub.1C
(L-T).gtoreq.24 MPa m and preferably K.sub.1C (L-T).gtoreq.26 MPa
m.
[0028] Another subject of the invention is a structural element for
an airplane, preferably an upper wing skin, including a product
according to the invention.
[0029] Still another subject of the invention is the use of a
product according to the invention or a structural element
according to the invention for aeronautical engineering.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1: Example of an aging curve and determination of the
slope of tangent P.sub.N.
[0031] FIG. 2: Change in the compression yield stress and the
tensile yield stress with the permanent set during controlled
stretching.
[0032] FIG. 3: Property compromise between the compression yield
stress and fracture toughness for K.sub.app for alloys N.degree. 2
to N.degree. 5 in example 2.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0033] 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 states are indicated in European standard EN
515.
[0034] The tensile static mechanical characteristics, in other
words the ultimate tensile strength R.sub.m, the conventional yield
stress at 0.2% of elongation R.sub.p0.2 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.
[0035] The compression yield stress was measured at 0.2% of
compression as per standard ASTM E9.
[0036] The stress intensity factor (K.sub.Q) is given according to
standard ASTM F 399. Standard ASTM E 399 gives the criteria which
make it possible to determine whether K.sub.Q is a valid value of
K.sub.1C. For a given test specimen geometry, the values of K.sub.Q
obtained for various materials are comparable with each other
insofar as the yield stresses of the material are of the same order
of magnitude.
[0037] A plot of the stress intensity versus crack extension, known
as the R curve, is determined according to ASTM standard E561. The
critical stress intensity factor K.sub.C, in other words the
intensity factor that makes the crack unstable, is calculated
starting from the R curve. The stress intensity factor K.sub.CO is
also calculated by assigning the initial crack length to the
critical load, at the beginning of the monotonous load. These two
values are calculated for a test piece of the required shape.
K.sub.app denotes the K.sub.CO factor corresponding to the test
piece that was used to make the R curve test.
[0038] Unless otherwise specified, the definitions of standard EN
12258 apply.
[0039] "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 skin, stringers, bulkheads, circumferential
frames), the wings (such as the upper or lower 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.
[0040] According to the present invention, a selected class of
aluminum alloys which contain specific and critical quantities of
lithium, copper, magnesium, silver and zirconium makes it possible
to prepare, in certain transformation conditions, flat-rolled
products having an improved compromise between fracture toughness,
tensile yield stress and compression yield stress.
[0041] The present inventors noted that, surprisingly, it is
possible to improve the compression yield stress for these alloys
by choosing specific transformation process parameters, in
particular during hot working and stress relieving by controlled
stretching.
[0042] The copper content of the products according to the
invention lies between 4.2 and 4.6% by weight. In an advantageous
embodiment of the invention, the copper content is at least 4.3% by
weight. A maximum copper content of 4.4% by weight is
preferred.
[0043] The lithium content of the products according to the
invention lies between 0.8% or 0.80% and 1.30% by weight and
preferably 1.15% by weight. Advantageously, the lithium content is
at least 0.85% by weight. A maximum lithium content of 0.95% by
weight is preferred.
[0044] The increase in the copper content and, to a lesser extent,
the lithium content contributes to improving static mechanical
resistance; however, as copper has a detrimental effect in
particular on density, it is preferable to limit the copper content
to the preferred maximum value. The increase in the lithium content
has a favorable effect on density; however the present inventors
noted that for alloys according to the invention, the preferred
lithium content ranging between 0.85% and 0.95% by weight in an
embodiment makes for an improved compromise between mechanical
resistance (tensile yield stress and compression) and fracture
toughness and, in addition, the fracture toughness attained for
aging at the peak or close to the peak is higher. In another
embodiment wherein compression yield stress and low density are
favored for a lower toughness, the preferred lithium content ranges
between 1.10% and 1.20% by weight, with preferably a magnesium
content ranging between 0.50% or preferably 0.53% and 0.70% or
preferably 0.65% by weight.
[0045] The magnesium content of the products according to the
invention lies between 0.3% or 0.30% and 0.8 or 0.80% by weight.
Preferably, the magnesium content is at least 0.40% or even 0.45%
by weight, which simultaneously improves static mechanical
resistance and fracture toughness. The present inventors noted that
the combination of a magnesium content ranging between 0.50% or
preferably 0.53% and 0.70% or preferably 0.65% by weight and a
lithium content ranging between 0.85% and 1.15% by weight and
preferably between 0.85% and 0.95% by weight led to a compromise
between mechanical resistance (tensile and compression yield
stress) and particularly advantageous fracture toughness, while
keeping an acceptable failure rate during the transformation, and
thus satisfactory reliability of the manufacturing process.
[0046] The zirconium content lies between 0.05 and 0.18% by weight
and preferably between 0.08 and 0.14% by weight. In an advantageous
embodiment of the invention, the zirconium content is at least
0.11% by weight.
[0047] The manganese content lies between 0.0 and 0.5% by weight.
In an advantageous embodiment of the invention, the manganese
content is between 0.2 and 0.4% by weight. In another embodiment of
the invention, the manganese content is lower than 0.1% by weight
and preferably lower than 0.05% by weight, which makes it possible,
for the products obtained by the process according to the
invention, to decrease the quantity of insoluble metal phases and
to improve damage tolerance still further.
[0048] The silver content lies between 0.05% and 0.5% by weight. In
an advantageous embodiment of the invention, the silver content is
between 0.10 and 0.40% by weight. The addition of silver helps to
improve the compromise of the mechanical properties of the products
obtained by the process according to the invention.
[0049] The sum of the iron content and the silicon content is at
the most 0.20% by weight. Preferably, the iron and silicon contents
are each at the most 0.08% by weight. In an advantageous embodiment
of the invention the iron and silicon contents are at the most
0.06% and 0.04% by weight respectively. A controlled and limited
iron and silicon content helps to improve the compromise between
mechanical resistance and damage tolerance.
[0050] The alloy also contains at least one element which may
contribute to the control of grain size chosen from Cr, Sc, Hf and
Ti, the quantity of the element, if it is chosen, being from 0.05
to 0.3% by weight for Cr and Sc, 0.05 to 0.5% by weight for Hf and
from 0.01 to 0.15% by weight for Ti. Preferably, it is chosen to
add between 0.01 and 0.10% by weight of titanium and to limit the
Cr, Sc and Hf content to 0.05% by weight maximum, as these elements
can have a detrimental effect, in particular on density and are
added only to further help obtain a primarily unrecrystallized
structure if necessary.
[0051] Zinc is an undesirable impurity, in particular because of
its contribution to the density of the alloy. The zinc content is
lower than 0.20% by weight, preferably Zn.ltoreq.0.15% by weight
and preferably still Zn.ltoreq.0.05% by weight. The zinc content is
advantageously lower than 0.04% by weight.
[0052] It is possible to select the content of the alloy elements
so as to minimize density. Preferably, the elements added that
contribute to increasing density such as Cu, Zn, Mn and Ag are
minimized and the elements that contribute to decreasing the
density such as Li and Mg are maximized in order to reach a density
of less than 2.73 g/cm.sup.3 and preferably less than 2.70
g/cm.sup.3.
[0053] The manufacturing process for the products according to the
invention includes the steps of production, casting,
homogenization, rolling at a temperature higher than 400.degree.
C., solution hardening, quenching, stretching between 2 and 3.5%
and aging.
[0054] In the first step, a molten metal bath is produced in order
to obtain an aluminum alloy composed according to the
invention.
[0055] The molten metal bath is then cast in the form of rolling
slab.
[0056] The rolling slab is then homogenized in order to reach a
temperature ranging between 450.degree. C. and 550.degree. and
preferably between 480 and 530.degree. C. for a length of time
ranging between 5 and 60 hours. The homogenization treatment can be
carried out in one or more steps.
[0057] After homogenization, the rolling slab is in general cooled
down to room temperature before being preheated ready for hot
rolling. The aim of the pre-heating is to reach a temperature
making it possible to maintain a temperature of at least
400.degree. C. and preferably of at least 420.degree. C. during hot
rolling. Intermediate reheating is carried out if during hot
rolling the temperature decreases excessively. Hot rolling is
carried out down to a thickness ranging preferably between 8 and 50
mm and preferably between 12 and 40 mm.
[0058] No significant cold working is performed, in particular by
cold rolling, between hot rolling and the solution heat treatment.
Such a cold rolling step would be likely to lead to a
recrystallized structure which is undesirable within the framework
of the invention. Significant cold working is typically a
deformation of at least approximately 5% or 10%.
[0059] The product so obtained is then solution heat treated by
thermal treatment making it possible to reach a temperature ranging
between 490 and 530.degree. C. for 15 min to 8 hours, then quenched
typically with water at room temperature or preferably with cold
water.
[0060] The combination of the chosen composition, in particular the
zirconium content, and the transformation range, in particular the
hot working temperature and the absence of cold working before
solution heat treatment, make it possible to obtain a primarily
unrecrystallized granular structure. "Primarily unrecrystallized
granular structure" is taken to mean an unrecrystallized structure
granular content at mid-thickness greater 70% and preferably
greater than 85%.
[0061] The product then undergoes controlled stretching with a
permanent set of 2 to 3.5% and preferably 2.0% to 3.0%. Controlled
stretching with a maximum permanent set of approximately 2.5% is
preferred. The present inventors noted that, surprisingly, the
compression yield stress decreases with the increasing permanent
sets during controlled stretching while the yield stress under
traction increases in these conditions. There is therefore an
optimal permanent set by controlled stretching making it possible
to obtain a high compression yield stress while maintaining a
sufficient tensile yield stress. Advantageously, the permanent set
by controlled stretching is selected so as to obtain a compression
yield stress at least equal to the tensile yield stress. The
present inventors additionally noted that, surprisingly, the effect
of the rate of permanent set on the compression yield stress is
specific to flat-rolled products; tests on extruded products showed
that such an effect is not observed in this case.
[0062] Known steps such as rolling, flattening, straightening or
shaping may optionally be performed after solution heat treatment
and quenching and before or after controlled stretching. In an
embodiment of the invention a cold rolling step of at least 7% and
preferably at least 9% and at the most 15% is carried out after
solution heat treatment and quenching and before controlled
stretching. But especially given the cost of the additional cold
rolling step, it is advantageous in another embodiment to realize
directly controlled stretching after solution treatment and
quenching.
[0063] Aging is performed in which the product reaches a
temperature ranging between 130 and 170.degree. C. and preferably
between 150 and 160.degree. C. for 5 to 100 hours and preferably
from 10 to 70 hours. Aging may be performed in one or more
steps.
[0064] It is known that for age-hardening alloys such as Al--Cu--Li
alloys the yield stress increases with the duration of aging at a
given temperature up to a maximum value known as the hardening peak
or "peak", then decreases with aging time. Within the framework of
this invention, the aging curve is the change in yield stress
according to the equivalent duration of aging at 155.degree. C. An
example of an aging curve is given in FIG. 1. Within the framework
of this invention, one determines whether a point N of the aging
curve, of duration equivalent to 155.degree. C. t.sub.N and with
yield stress R.sub.p0.2 (N) is close to the peak by determining
slope P.sub.N of the tangent to the aging curve at point N. Within
the framework of this invention, it is considered that the yield
stress of a point N of the aging curve is close to the yield stress
at the peak if the absolute value of slope P.sub.N is at the most 3
MPa/h. As illustrated in FIG. 1, an under-aged state is a state for
which P.sub.N is positive and an over-aged state is a state for
which P.sub.N is negative.
[0065] To obtain an approximate value for P.sub.N, for a point N of
the curve in an under-aged state, one can determine the slope of
the right-hand side passing through point N and through the
previous point N-1 obtained for time t.sub.N-1<t.sub.N and
having a yield stress R.sub.p0.2 (N-1); this gives
P.sub.N.apprxeq.(R.sub.p0.2(N)-R.sub.p0.2
(N-1))/(t.sub.N-t.sub.N-1). In theory, the exact value of P.sub.N
is obtained when t.sub.N-1 tends towards t.sub.N. However, if the
difference t.sub.N-t.sub.N-1 is low, the variation in yield stress
is likely to be insignificant and the value inaccurate. The present
inventors noted that a satisfactory approximation to P.sub.N is in
general obtained when the difference t.sub.N-t.sub.N-1 lies between
2 and 20 hours and is preferably about 3 hours.
[0066] 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=16,400 K is derived from the enablement
energy of the diffusion of Cu for which the value Q=136,100 J/mol
was used.
[0067] The tensile or compression yield stress can be used to
determine whether aging makes it possible to reach a state close to
the peak; the results are, however, not necessarily identical.
Within the framework of the invention, it is preferred to use the
values of compression yield stress to optimize aging.
[0068] In general, for alloys of the Al--Cu--Li type, the clearly
under-aged states correspond to compromises between the static
mechanical resistance (Rp.sub.0.2 R.sub.m) and damage tolerance
(fracture toughness, resistance to spreading of fatigue cracks) of
more interest than at the peak and, a fortiori, beyond the peak.
However, the present inventors noted that a state close to the peak
both provides a good compromise between static mechanical
resistance and damage tolerance and makes it possible to improve
performance in terms of corrosion resistance and thermal
stability.
[0069] In addition, the use of a state close to the peak makes it
possible to improve the robustness of the industrial process: a
variation in the conditions of aging leads to a slight variation in
the properties obtained.
[0070] So it is advantageous to carry out a temper essentially
under-aged close to the peak of the compression yield, i.e. a
temper essentially under-aged with the conditions of time and
temperature equivalent to those of a point N of the aging curve
under compression at 155.degree. C. such that the tangent to the
aging curve at this point has a slope P.sub.N, expressed in MPa/h,
such that -1<P.sub.N.ltoreq.3 and preferably
-0.5<P.sub.N.ltoreq.2.3.
[0071] The flat-rolled products obtained by the process according
to the invention have, for a thickness ranging between 8 and 50 mm,
at mid-thickness at least one of the following combinations of
characteristics: [0072] (i) for thicknesses from 8 to 15 mm, at
mid-thickness, a tensile yield stress R.sub.p0.2(L).gtoreq.600 MPa
and preferably R.sub.p0.2(L).gtoreq.610 MPa, a compression yield
stress R.sub.p0.2(L).gtoreq.620 MPa and preferably
R.sub.p0.2(L).gtoreq.630 MPa and fracture toughness such that
K.sub.1C (L-T).gtoreq.28 MPa m and preferably K.sub.1C
(L-T).gtoreq.32 MPa m and/or K.sub.app (L-T).gtoreq.73 MPa m and
preferably K.sub.app (L-T).gtoreq.79 MPa m, for 300 mm wide and
6.35 mm thick CCT test samples, [0073] (ii) for thicknesses from 8
to 15 mm, at mid-thickness, a tensile yield stress
R.sub.p0.2(L).gtoreq.630 MPa and preferably
R.sub.p0.2(L).gtoreq.640 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.640 MPa and preferably
R.sub.p0.2(L).gtoreq.650 MPa and a fracture toughness such that
K.sub.1C(L-T).gtoreq.26 MPa m and preferably K.sub.1C
(L-T).gtoreq.30 MPa m and/or K.sub.app (L-T).gtoreq.63 MPa m and
preferably K.sub.app (L-T).gtoreq.69 MPa m, for 300 mm wide and
6.35 mm thick CCT test samples, [0074] (iii) for thicknesses from
15 to 50 mm, at mid-thickness, a tensile yield stress
R.sub.p0.2(L).gtoreq.610 MPa and preferably
R.sub.p0.2(L).gtoreq.620 MPa, a compression yield stress
R.sub.p0.2(L-T).gtoreq.620 MPa and preferably
R.sub.p0.2(L).gtoreq.630 MPa and fracture toughness K.sub.1C
(L-T).gtoreq.22 MPa m and preferably K.sub.1C (L-T).gtoreq.24 MPa
m, [0075] (iv) for thicknesses from 15 to 50 mm, at mid-thickness,
a tensile yield stress R.sub.p0.2(L).gtoreq.600 MPa and preferably
R.sub.p0.2(L).gtoreq.610 MPa, a compression yield stress
R.sub.p0.2(L).gtoreq.580 MPa and preferably
R.sub.p0.2(L).gtoreq.590 MPa and fracture toughness K.sub.1C
(L-T).gtoreq.24 MPa m and preferably K.sub.1C (L-T).gtoreq.26 MPa
m.
[0076] Airplane structural elements according to the invention
include products according to the invention. A preferred airplane
structural element is an upper wing skin.
[0077] The use of a structural element incorporating at least one
product according to the invention or manufactured from such a
product is advantageous, in particular for aeronautical
engineering. The products according to the invention are
particularly advantageous for the production of airplane upper wing
skins.
[0078] These aspects, as well as others of the invention are
explained in greater detail using the following illustrative and
non-restrictive examples.
EXAMPLES
Example 1
[0079] In this example, a slab of section 406.times.1520 mm made of
an alloy from the process according to the invention, the
composition of which is given in table 1, was cast.
TABLE-US-00001 TABLE 1 Composition as a percentage by weight and
density of alloy N.sup.o 1 Density Alloy Si Fe Cu Mn Mg Ln Ag Li Zr
Ti (g/cm.sup.3) N.sup.o 1 0.03 0.05 4.56 0.38 0.42 0.02 0.31 1.09
0.13 0.03 2.727
[0080] The slab was homogenized at about 500.degree. C. for about
20 hours. The slab was hot rolled at a temperature greater than
445.degree. C. to obtain plates of thickness 25 mm. The plates were
solution heat treated at approximately 510.degree. C., for 5 hours
and quenched with water at 20.degree. C. The plates were then
stretched with a permanent elongation ranging between 2% and
6%.
[0081] The plates underwent single-step aging of 40 hours at
155.degree. C. for 2 and 3% stretching, 30 hours for 4% and 20
hours for 6%, this aging making it possible to attain a tensile
yield stress and compression at the peak or close to the peak.
Samples were taken at mid-thickness to measure the static
mechanical characteristics under stretching and compression,
together with fracture toughness K.sub.Q. The test specimens used
for fracture toughness measurement were of width W=40 mm and
thickness B=20 mm. The measurements made were valid according to
standard ASTM E399. The results are given in Table 2.
[0082] The structure of the plates obtained was primarily
unrecrystallized. The unrecrystallized granular structure content
at mid-thickness was 90%.
TABLE-US-00002 TABLE 2 Mechanical properties obtained for various
plates. Permanent Rp.sub.0.2 L elongation Rp.sub.0.2 L Com-
K.sub.1C during controlled Stretching pression (MPa m.sup.1/2)
Alloy Aging stretching (Mpa) (Mpa) L-T N.degree.1 40 hrs 2% 621 639
24.2 155.degree. C. 40 hrs 3% 627 633 155.degree. C. 30 hrs 4% 633
629 155.degree. C. 20 hrs 6% 635 622 23.4 155.degree. C.
[0083] FIG. 2 presents the changes in tensile yield stress and
compression as a function of permanent elongation during controlled
stretching. For permanent elongation during stretching ranging
between 2 and 3.5% a favorable compromise is obtained between the
compression yield stress and the tensile yield stress. So under
these conditions, the compression yield stress is higher than the
tensile yield stress, the tensile yield stress remaining higher
than 620 MPa.
Example 2
[0084] In this example, several slabs of section 120.times.80 mm,
the composition of which is given in table 3, were cast.
TABLE-US-00003 TABLE 3 Composition as a percentage by weight and
density of Al--Cu--Li alloys cast in the form of a slab Density
Alloy Si Fe Cu Mn Mg Zn Ag Li Zr Ti (g/cm.sup.3) N.sup.o 2 0.03
0.04 4.34 -- 0.30 -- 0.37 0.91 0.14 0.02 2.717 N.sup.o 3 0.03 0.06
4.37 -- 0.58 -- 0.36 0.89 0.14 0.03 2.715 N.sup.o 4 0.03 0.05 4.31
-- 0.33 -- 0.37 1.14 0.14 0.03 2.698 N.sup.o 5 0.03 0.05 4.37 --
0.58 -- 0.36 1.15 0.13 0.03 2.694
[0085] The slabs were homogenized by a two-step treatment of 8
hours at 500.degree. C. followed by 12 hours at 510.degree. C.,
then surface-machined. After homogenization, the slabs were hot
rolled to obtain plates with a thickness of 9.4 mm, with
intermediate reheating if the temperature decreased to less than
400.degree. C. The plates were solution heat treated for 5 hours at
approximately 510.degree. C., quenched with cold water and
stretched with a permanent elongation of 3%.
[0086] The structure of the plates obtained was primarily
unrecrystallized. The uncrystallized granular structure content at
mid-thickness was 90%.
[0087] The plates underwent aging ranging between 15 and 50 hours
at 155.degree. C. Samples were taken at mid-thickness to measure
the static mechanical characteristics under stretching, under
compression, and fracture toughness K.sub.Q. The test specimens
used for fracture toughness measurement were of width W=25 mm and
thickness B=8 mm. The validity criteria of K.sub.1C were met for
certain samples. Fracture toughness measurements were also obtained
on CCT samples of width 300 mm and thickness 6.35 mm. The results
obtained are given in table 4.
TABLE-US-00004 TABLE 4 Mechanical properties obtained for various
plates. Fracture toughness Stretching properties K.sub.app Aging
Rp.sub.0.2 Compression properties KQ (MPa m.sup.1/2) time at Rm MPa
A Rp.sub.0.2 MPa (MPa m.sup.1/2) L-T Alloy 155.degree. C. MPa
Stretching (%) Compression P.sub.N (Mpa/h) L-T CCT 300 N.sup.o 2 8
582 525 11.8 504 15 625 588 10.3 603 14.2 41.6 20 640 609 10.7 631
5.6 38.6 (K.sub.1C) 30 635 606 9.6 622 -1.0 37.6 50 645 618 9.,7
641 0.9 31.5 (K.sub.1C) 76 N.sup.o 3 8 592 545 10.5 536 15 633 602
9.4 613 11.0 41.9 20 640 613 8.0 625 2.3 39.7 (K.sub.1C) 30 640 613
9.6 623 -0.2 40.9 50 649 626 8.9 647 1.2 35.3 (K.sub.1C) 82 N.sup.o
4 8 619 571 9.7 591 15 657 629 10.0 634 6.1 36.4 (K.sub.1C) 20 668
642 9.7 649 3.0 31.5 30 671 647 8.0 652 0.3 33.6 (K.sub.1C) 66 50
674 653 8.2 668 0.8 28.1 (K.sub.1C) N.sup.o 5 8 622 588 7.7 576 15
645 620 8.3 631 7.8 35.7 20 667 643 9.4 658 5.4 32.6 30 669 650 7.0
654 -0.4 30.9 72 50 665 645 8.6 29.1 (K.sub.1C)
[0088] FIG. 3 illustrates the compromise obtained between the
compression yield stress and fracture toughness K.sub.app.
[0089] The combination of the preferred composition (Alloy
N.degree. 3) with the process according to the invention gives, in
particular for a 50-hour aging at 155.degree. C., the most
favorable aging from the point of view of thermal stability, a
particularly favorable compromise between compression yield stress,
tensile yield stress and fracture toughness.
Example 3
[0090] In this example, a slab of section 406.times.1525 mm made of
an alloy from the process according to the invention, the
composition of which is given in table 1, was cast.
TABLE-US-00005 TABLE 5 Composition as a percentage by weight and
density of alloy N.sup.o 6 Density Alloy Si Fe Cu Mn Mg Zn Ag Li Zr
Ti (g/cm.sup.3) N.sup.o 6 0.02 0.03 4.3 -- 0.58 <0.01 0.34 0.88
0.13 0.04 2.714
[0091] The slab was homogenized at about 500.degree. C. for about
30 hours. The slab was hot rolled at a temperature greater than
400.degree. C. to obtain plates of thickness 25 mm. The plates were
solution heat treated at approximately 510.degree. C. for 5 hours
and quenched with water at 20.degree. C. The plates were then
stretched with a permanent elongation of 2% or 3%.
[0092] The plates underwent single-step aging of 10 h to 30 h at
155.degree. C. Samples were taken at mid-thickness to measure the
static mechanical characteristics under stretching and compression,
together with fracture toughness K.sub.Q. The test specimens used
for fracture toughness measurement were of width W=40 mm and
thickness B=20 mm. The measurements made were valid according to
standard ASTM E399. The results are given in Table 6.
[0093] The structure of the plates obtained was primarily
unrecrystallized. The unrecrystallized granular structure content
at mid-thickness was higher than 90%.
TABLE-US-00006 TABLE 6 Mechanical properties obtained for various
plates. Permanent elongation Properties under Properties under
Toughness during Aging stretching compression K.sub.Q controlled
time at Rm Rp.sub.0.2 A Rp.sub.0.2 MPa (MPa m.sup.1/2) Alloy
stretching 155.degree. C. MPa MPa (%) Compression P.sub.N (MPa/h)
L-T 6 2% 10 h 585 532 12.6 527 52.3 2% 20 h 622 590 10.1 593 6.6
33.4 (K.sub.1C) 2% 30 h 630 604 9.1 610 1.7 28.4 (K.sub.1C) 3% 10 h
604 569 11.7 560 44.4 3% 20 h 630 606 9.9 599 3.9 30.4 (K.sub.1C)
3% 30 h 635 612 9.3 609 1.1 26.4 (K.sub.1C)
* * * * *