EP2449142B1 - Aluminium-copper-lithium alloy with improved mechanical resistance and toughness - Google Patents

Description

Field of the invention

The invention relates to aluminum-copper-lithium alloy products, more particularly, such products, their manufacturing and use processes, intended in particular for aeronautical and aerospace construction.

State of the art

Products, especially thick rolled products, forged or spun aluminum alloy are developed to produce by cutting, surfacing or mass machining of high strength parts intended especially for the aviation industry, the aerospace industry or mechanical construction.
Aluminum alloys containing lithium are very interesting in this respect, since lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each weight percent of lithium added. For these alloys to be selected in the aircraft, their performance compared to other properties of use must reach that of commonly used alloys, in particular in terms of a compromise between the static mechanical strength properties (yield strength, resistance to rupture) and the properties of damage tolerance (toughness, resistance to the propagation of fatigue cracks), these properties being in general antinomic. For thick products, these properties must in particular be obtained at quarter and at mid-thickness and the products must therefore have a low sensitivity to quenching. A product is said to be quench sensitive if its static mechanical characteristics, such as its yield strength, decrease as quenching speed decreases. The quenching rate is the average cooling rate of the product during quenching.
These mechanical properties must also preferably be stable over time and not be significantly modified by aging at the temperature of use. So, the prolonged use of the products in the context of civil aviation applications requires a good stability of the mechanical properties, this being for example simulated by an aging of 1000 hours at 85 ° C.

These alloys must also have sufficient corrosion resistance, be able to be shaped according to the usual methods and have low residual stresses so that they can be machined integrally.

The US Patent 5,032,359 discloses a large 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.

The US Patent 5,234,662 describes alloys of composition (in% by weight), Cu: 2.60 - 3.30, Mn: 0.0 - 0.50, Li: 1.30 - 1.65, Mg: 0.0 - 1, 8, elements controlling the granular structure selected from Zr and Cr: 0.0 - 1.5.

The US Patent 5,455,003 discloses a process for producing Al-Cu-Li alloys which have improved mechanical strength and toughness at cryogenic temperature, particularly through proper work-hardening and tempering. This patent recommends in particular the composition, in 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. The problem of aging products for civil aviation applications is not mentioned because the products concerned are essentially cryogenic tanks for rocket launchers or space shuttle.

The US Patent 7,438,772 discloses alloys comprising, in weight percent, Cu: 3-5, Mg: 0.5-2, Li: 0.01-0.9 and discourage the use of higher lithium content due to degradation compromise between toughness and mechanical strength.

The US Patent 7,229,509 discloses an alloy comprising (% 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, especially having a toughness K _1C (L)> 37.4 MPa√m for an elastic limit R _p0.2 (L)> 448.2 MPa (products with a thickness greater than 76.2 mm) and in particular a tenacity K _1C (L)> 38.5 MPa√m for an elastic limit R _{p0, 2} (L)> 489.5 MPa (products less than 76.2 mm thick).

The request for US Patent 2009/142222 A1 discloses alloys comprising (in% 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.

AA2050 alloy is also known which comprises (% by weight): (3.2-3.9) Cu, (0.7-1.3) Li, (0.20-0.6) Mg, (0 , 20-0.7) Ag, 0.25max. Zn, (0.20-0.50) Mn, (0.06-0.14) Zr and AA2095 (3.7-4.3) Cu, (0.7-1.5) Li, (0.25-0.8) Mg, (0.25-0.6) Ag, 0.25max. Zn, 0.25 max. Mn, (0.04-0.18) Zr. AA2050 alloy products are known for their quality in terms of static strength and toughness.

There is a need for products, in particular thick products, of aluminum-copper-lithium alloy having improved properties compared to those of the known products, in particular in terms of a compromise between the static mechanical strength properties and the tolerance properties. damage, thermal stability, corrosion resistance and machinability, while having a low density.

Object of the invention

• Cu : 3,2 -3,7 ;
• 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 ;
• Zn ≤ 0,15 ;

au moins un élément parmi

• Ti : 0,01 - 0,15 ;
• Sc : 0,05 - 0,3 ;
• Cr : 0,05 - 0,3 ;
• Hf : 0,05 - 0,5 ;

autres éléments ≤ 0,05 chacun et ≤ 0,15 au total, reste aluminium.A first object of the invention is a wrought product such as a product spun, rolled and / or forged, aluminum alloy comprising, in% by weight,

• Cu: 3.2-3.7;
• 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;
• Zn ≤ 0.15;

at least one of

• 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, remains aluminum.

1. a) on élabore un bain de métal liquide à base d'aluminium comprenant 3,2 à 3,7 % en poids de Cu, 0,8 à 1,3 % en poids de Li, 0,6 à 1,0 % en poids de Mg, 0,05 à 0,18 % en poids de Zr, 0,0 à 0,5 % en poids d'Ag, 0,0 à 0,5% en poids de Mn, au plus 0,20 % en poids de Fe + Si, au plus 0,15 % en poids de Zn, au moins un élément choisi parmi Cr, Sc, Hf et Ti, la quantité dudit élément, s'il est choisi, étant de 0,05 à 0,3 % en poids pour Cr et pour Sc, 0,05 à 0,5 % en poids pour Hf et de 0,01 à 0,15 % en poids pour Ti, les autres éléments au plus 0,05% en poids chacun et 0,15% en poids au total, le reste aluminium ;
2. b) on coule une forme brute à partir dudit bain de métal liquide ;
3. c) on homogénéise ladite forme brute à une température comprise entre 450°C et 550° et de préférence entre 480 °C et 530°C pendant une durée comprise entre 5 et 60 heures ;
4. d) on déforme à chaud et optionnellement à froid ladite forme brute en un produit filé, laminé et/ou forgé ;
5. e) on met en solution entre 490 et 530 °C pendant 15 min à 8 h et on trempe ledit produit ;
6. f) on tractionne de façon contrôlée ledit produit avec une déformation permanente de 1 à 6 % et préférentiellement d'au moins 2% ;
7. g) on réalise un revenu dudit produit comprenant un chauffage à une température comprise entre 130 et 170 °C pendant 5 à 100 heures et de préférence de 10 à 40h de façon à atteindre une limite d'élasticité proche du pic.

A second object of the invention is a process for manufacturing a spun, rolled and / or forged product based on aluminum alloy in which

1. a) an aluminum-based liquid metal bath comprising 3.2 to 3.7 wt.% Cu, 0.8 to 1.3 wt.% Li, 0.6 to 1.0 wt. weight of Mg, 0.05 to 0.18% by weight of Zr, 0.0 to 0.5% by weight of Ag, 0.0 to 0.5% by weight of Mn, at most 0.20% by weight of Fe + Si, at most 0.15% by weight of Zn, at least one element selected from Cr, Sc, Hf and Ti, the amount of said element, if chosen, being from 0.05 to 0 , 3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and 0.01 to 0.15% by weight for Ti, the other elements at most 0.05% by weight each and 0.15% by weight in total, the balance aluminum;
2. b) pouring a raw form from said bath of liquid metal;
3. c) homogenizing said crude form at a temperature between 450 ° C and 550 ° and preferably between 480 ° C and 530 ° C for a period of between 5 and 60 hours;
4. d) hot deformed and optionally cold deformed said raw form into a product spun, rolled and / or forged;
5. e) is dissolved between 490 and 530 ° C for 15 min to 8 h and said product quenched;
6. f) the product is tensed in a controlled manner with a permanent deformation of 1 to 6% and preferably of at least 2%;
7. g) producing an income of said product comprising heating at a temperature between 130 and 170 ° C for 5 to 100 hours and preferably 10 to 40h so as to reach a limit of elasticity close to the peak.

Yet another object of the invention is a structural element comprising a product according to the invention.

Yet another object of the invention is the use of a structural element according to the invention for aeronautical construction.

Description of figures

• Figure 1: Example of income curve and determination of the slope of the tangent P _N.
• FIG. 2: Results of yield strength and toughness obtained for the samples of Example 1.
• 3: Results of yield strength and toughness obtained for the samples of Examples 1 and 2, the elastic limit being close to the peak.
• FIG. 4: Results of yield strength and toughness obtained for the samples of Example 3, the yield strength being close to the peak.

Description of the invention

Unless stated otherwise, all the information concerning the chemical composition of the alloys is 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 in% by weight is multiplied by 1.4. The designation of alloys is in accordance with the regulations of The Aluminum 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 measuring weight. The values are calculated in accordance with the procedure of The Aluminum Association, which is described on pages 2-12 and 2-13 of "Aluminum Standards and Data". The definitions of the metallurgical states are given in the European standard EN 515.
Unless otherwise stated, the static mechanical characteristics, in other words the ultimate tensile strength R _m , the conventional yield stress at 0.2% elongation R _p 0.2 ("yield strength") and the elongation at break A%, are determined by a tensile test according to EN 10002-1, the sampling and the direction of the test being defined by the EN 485-1 standard.
The stress intensity factor (K _Q ) is determined according to ASTM E 399. ASTM E 399 gives the criteria for determining if K _Q is a valid value of K _1C . For a given specimen geometry, the K _Q values obtained for different materials are comparable to each other as long as the elasticity limits of the materials are of the same order of magnitude.
Unless otherwise specified, the definitions of EN 12258 apply. The thickness of the profiles is defined according to EN 2066: 2001: the cross section is divided into elementary rectangles of dimensions A and B; A being always the largest dimension of the elementary rectangle and B can be considered as the thickness of the elementary rectangle. The sole is the elementary rectangle with the largest dimension A.

The MASTMAASIS (Modified ASTM Acetic Acid Salt Intermittent Spray) test is performed according to ASTM G85.

Here, a "structural element" or "structural element" of a mechanical construction is called 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 calculation is usually prescribed or realized. These are typically elements whose failure is likely to endanger the safety of said construction, its users, its users or others. For an aircraft, these structural elements include the elements that make up the fuselage (such as fuselage skin, fuselage skin in English), stiffeners or stringers, bulkheads, fuselage (circumferential frames), wings (such as wing skin), stiffeners (stringers or stiffeners), ribs (ribs) and spars) and the empennage composed in particular of horizontal and vertical stabilizers (horizontal or vertical stabilizers), as well as the floor beams, the seat tracks and the doors.

According to the present invention, it has been discovered that a selected class of aluminum alloys which contain specific and critical amounts of lithium, copper and magnesium and zirconium makes it possible to prepare wrought products having an improved compromise between toughness and mechanical strength, and good resistance to corrosion. In addition, these products, when they undergo an income chosen so as to achieve a yield strength R _p0 , ₂ close to the yield strength R _p0 , ₂ at the peak, exhibit excellent thermal stability.
The present inventors have found that, surprisingly, it is possible to improve the compromise between the static mechanical strength properties and the damage tolerance properties, in particular of thick products made of aluminum-copper-lithium alloys, such as in particular the alloy. AA2050, by increasing the magnesium content. In particular, for thick products having a near-peak income, the choice of copper, magnesium and lithium contents makes it possible to reach a compromise of favorable properties and to obtain a satisfactory thermal stability of the product.

The copper content of the products according to the invention is between 3.2 and 3.7% by weight. When the copper content is too high, the toughness is not sufficient especially for income close to the peak and, moreover, the density of the alloy is not advantageous. When the copper content is too low, the minimum static mechanical characteristics are not reached.
The lithium content of the products according to the invention is between 0.8 and 1.3% by weight. Advantageously, the lithium content is between 0.9% and 1.2% by weight. Preferably, the lithium content is at least 0.93% by weight or even at least 0.94% by weight. When the lithium content is too low, the density reduction associated with the addition of lithium is not sufficient.
The magnesium content of the products according to the invention is between 0.6 and 1.0% by weight and preferably between 0.65 or 0.67 and 1.0% by weight. In a mode of Advantageous embodiment of the invention the magnesium content is at most 0.9% by weight and preferably at most 0.8% by weight. For some applications, it is advantageous that the magnesium content is at least 0.7% by weight.
The zirconium content is between 0.05 and 0.18% by weight and preferably between 0.08 and 0.14% by weight so as to obtain preferably a fibered or slightly recrystallized grain structure.
The manganese content is between 0.0 and 0.5% by weight. In particular for the manufacture of thick plates, a manganese content of between 0.2 and 0.4% by weight makes it possible to improve the toughness without compromising the mechanical strength.
The silver content is between 0.0 and 0.5% by weight. The present inventors have found that, although the presence of silver is advantageous, in the presence of a quantity of magnesium according to the invention a significant amount of silver is not necessary to obtain the desired improvement in the compromise between mechanical resistance and damage tolerance. The limitation of the amount of money is economically very favorable. In an advantageous embodiment of the invention, the silver content is between 0.15 and 0.35% by weight. In one embodiment of the invention, which has the advantage of minimizing the density, the silver content is at most 0.25% by weight.
The sum of the iron content and the silicon content is at most 0.20% by weight. Preferably, the iron and silicon contents are each at most 0.08% by weight. In an advantageous embodiment of the invention, the iron and silicon contents are at most 0.06% and 0.04% by weight, respectively. A controlled and limited iron and silicon content contributes to the improvement of the compromise between mechanical resistance and damage tolerance.
The alloy also contains at least one element that can contribute to controlling the grain size selected from Cr, Sc, Hf and Ti, the amount of the element, if selected, being from 0.05 to 0.3 % by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and from 0.01 to 0.15% by weight for Ti. Preferably, between 0.02 and 0.10% by weight of titanium is chosen. Zinc is an undesirable impurity. The zinc content is Zn ≤ 0.15% by weight and preferably Zn ≤ 0.05% by weight. The zinc content is advantageously less than 0.04% by weight.

1. (i) pour des épaisseurs de 30 à 60 mm, à mi-épaisseur, une limite d'élasticité R_p0,2(L) ≥ 525 MPa et de préférence R_p0,2(L) ≥ 545 MPa et une ténacité K_1C (L-T) ≥ 38 MPa√m et de préférence K_1C (L-T) ≥ 43 MPa√m,
2. (ii) pour des épaisseurs de 60 à 100 mm, à mi-épaisseur, une limite d'élasticité R_p0,2(L) ≥ 515 MPa et de préférence R_p0,2(L) ≥ 535 MPa et une ténacité K_1C (L-T) ≥ 35 MPa√m et de préférence K_1C (L-T) ≥ 40 MPa√m,
3. (iii) pour des épaisseurs de 100 à 130 mm, à mi-épaisseur, une limite d'élasticité R_p0,2(L) ≥ 505 MPa et de préférence R_p0,2(L) ≥ 525 MPa et une ténacité K_1C (L-T) ≥ 32 MPa√m et de préférence K_1C (L-T) ≥ 37 MPa√m,
4. (iv) pour des épaisseurs de 30 à 100 mm, à mi-épaisseur, une limite d'élasticité R_p0,2(L) exprimée en MPa et une ténacité K_1C (L-T) exprimée en MPa√m telles que K_1C (L-T) ≥ - 0.217 R_p0,2(L) + 157 et de préférence K_1C (L-T) ≥ - 0.217 R_p0,2(L) + 163 et supérieure à 35 MPa√m.
5. (v) après vieillissement de 1000 heures à 85 °C, une limite d'élasticité R_p0,2(L) et un allongement à rupture A%(L) présentant une différence avec la limite d'élasticité R_p0,2(L) et l'allongement à rupture A%(L) avant vieillissement inférieure à 10% et de préférence inférieure à 5%..

The density of the products according to the invention is less than 2.72 g / cm ³ . In order to reduce the density of the products, the composition can advantageously be selected to obtain a density of less than 2.71 g / cm ³ and preferably less than 2.70 g / cm ³ .
The alloy according to the invention is particularly intended for the manufacture of thick products, spun, rolled and / or forged. By thick products is meant in the context of the present invention, products whose thickness is at least 30 mm and preferably at least 50 mm. Indeed the alloy according to the invention has a low sensitivity to quenching which is particularly advantageous for thick products.
The rolled products according to the invention preferably have a thickness of between 30 and 200 mm and preferably between 50 and 170 mm.
The thick products according to the invention have a compromise between mechanical strength and particularly advantageous toughness.
A product according to the invention, in a rolled state, dissolved, quenched and tempered so as to reach a limit of elasticity close to the peak, having at least one of the following pairs of characteristics at thicknesses between thicknesses between 30 and 100 mm:

1. (i) for thicknesses of 30 to 60 mm, at mid-thickness, a yield strength R _p0.2 (L) ≥ 525 MPa and preferably R _p0.2 (L) ≥ 545 MPa and a toughness K _1C (LT) ≥ 38 MPa√m and preferably K _1C (LT) ≥ 43 MPa√m,
2. (ii) for thicknesses of 60 to 100 mm, at mid-thickness, a yield strength R _p0.2 (L) ≥ 515 MPa and preferably R _p0.2 (L) ≥ 535 MPa and a toughness K _1C (LT) ≥ 35 MPa√m and preferably K _1C (LT) ≥ 40 MPa√m,
3. (iii) for thicknesses of 100 to 130 mm, at mid-thickness, a yield strength R _p0.2 (L) ≥ 505 MPa and preferably R _p0.2 (L) ≥ 525 MPa and a toughness K _1C (LT) ≥ 32 MPa√m and preferably K _1C (LT) ≥ 37 MPa√m,
4. (iv) for thicknesses of 30 to 100 mm, at mid-thickness, a yield strength R _p0.2 (L) expressed in MPa and a toughness K _1C (LT) expressed in MPa√m such as K _1C ( LT) ≥ 0.217 R _p0.2 (L) + 157 and preferably K _1C (LT) ≥ 0.217 R _p0.2 (L) + 163 and greater than 35 MPa√m.
5. (v) after aging for 1000 hours at 85 ° C, a yield strength R _p0.2 (L) and an elongation at break A% (L) having a difference with the yield strength R _p0.2 (L ) and the elongation at break A% (L) before aging less than 10% and preferably less than 5%.

In another embodiment of the invention, however, thinner products are preferred, the thickness of which is between 10 and 30 mm, typically about 20 mm, because the compromise obtained under these conditions between strength and toughness is particularly advantageous.

1. (i) une limite d'élasticité R_p0,2(L) ≥ 525 MPa et de préférence R_p0,2(L) ≥ 545 MPa et une ténacité K_1C (L-T) ≥ 40 MPa√m et de préférence K_1C (L-T) ≥ 45 MPa√m,
2. (ii) une limite d'élasticité R_p0,2(L) exprimée en MPa et une ténacité K_Q (L-T) exprimée en MPa√m telles que K_1C (L-T) ≥ - 0,4 R_p0,2(L) + 265 et de préférence K_1C (L-T) ≥ - 0,4 R_p0,2(L) + 270 et supérieure à 45 MPa √m,
3. (iii) après vieillissement de 1000 heures à 85 °C, une limite d'élasticité R_p0,2(L) et un allongement à rupture A%(L) présentant une différence avec la limite d'élasticité R_p0,2(L) et l'allongement à rupture A%(L) avant vieillissement inférieure à 10% et de préférence inférieure à 5%.

A product according to the invention, in a rolled state, dissolved, quenched and tempered so as to reach a limit of elasticity close to the peak, having at least one of the following pairs of characteristics at thicknesses between thicknesses between 10 and 30 mm:

1. (i) a yield strength R _p0.2 (L) ≥ 525 MPa and preferably R _p0.2 (L) ≥ 545 MPa and a toughness K _1C (LT) ≥ 40 MPa√m and preferably K _1C ( LT) ≥ 45 MPa√m,
2. (ii) a yield strength R _p0.2 (L) expressed in MPa and a toughness K _Q (LT) expressed in MPa√m such that K _1C (LT) ≥ _-0.4 R _p0.2 (L) + 265 and preferably K _1C (LT) ≥ _-0.4 R _p0.2 (L) + 270 and greater than 45 MPa √m,
3. (iii) after aging for 1000 hours at 85 ° C, a yield strength R _p0.2 (L) and an elongation at break A% (L) having a difference with the yield strength R _p0.2 (L ) and the elongation at break A% (L) before aging less than 10% and preferably less than 5%.

The products according to the invention also have advantageous properties in terms of fatigue behavior both from the point of view of the crack initiation (S / N) and the propagation speed (da / dN).

The corrosion resistance of the products of the invention is generally high; thus, the MASTMAASIS test result (ASTMG85 & G34 standards) is at least EA and preferably P for the products according to the invention.

The method of manufacturing the products according to the invention comprises stages of preparation, casting, wrought, solution, quenching and tempering.
In a first step, a bath of liquid metal is produced so as to obtain an aluminum alloy of composition according to the invention.
The liquid metal bath is then cast in a raw form, such as a billet, a rolling plate or a forging blank.
The crude form is then homogenized at a temperature between 450 ° C and 550 ° and preferably between 480 ° C and 530 ° C for a period between 5 and 60 hours.

où T (en Kelvin) est la température instantanée de traitement du métal , qui évolue avec le temps t (en heures), et T_ref est une température de référence fixée à 428 K. ti est exprimé en heures. La constante Q/R = 16400 K est dérivée de l'énergie d'activation pour la diffusion du Cu, pour laquelle la valeur Q = 136100 J/mol a été utilisée.After homogenization, the raw form is generally cooled to room temperature before being preheated for hot deformation. Preheating aims to reach a temperature preferably between 400 and 500 ° C and preferably of the order of 450 ° C allowing the deformation of the raw form.
Hot deformation and optionally cold deformation is typically performed by spinning, rolling and / or forging to obtain a spun, rolled and / or forged product preferably having a thickness of at least 30 mm. The product thus obtained is then put in solution by heat treatment between 490 and 530 ° C for 15 min to 8 h, then quenched typically with water at room temperature or preferably cold water. The product then undergoes controlled traction with a permanent deformation of 1 to 6% and preferably of at least 2%. The rolled products preferably undergo controlled pulling with a permanent deformation greater than 3%. In an advantageous embodiment of the invention, the controlled traction is carried out with a permanent deformation of between 3 and 5%. A preferred metallurgical state is the T84 state. Known steps such as rolling, planing, straightening shaping may optionally be performed after solution and quenching and before or after controlled pulling. In one embodiment of the invention, a step of cold rolling of at least 7% and preferably at least 9% before performing a controlled pull with a permanent deformation of 1 to 3%.
An income is achieved comprising heating at a temperature between 130 and 170 ° C and preferably between 150 and 160 ° C for 5 to 100 hours and preferably from 10 to 40h so as to achieve a yield strength R _{p0, 2} close to the elastic limit R _p0,2 at the peak.
It is known that for structural hardening alloys such as Al-Cu-Li alloys the yield strength increases with the duration of tempering at a given temperature up to a maximum value called the peak of hardening or "peak" then decreases with the duration of income. In the context of the present invention, the yield curve is defined as the evolution of the elastic limit as a function of the equivalent duration of income at 155 ° C. An example of an income curve is presented in FIG. 1. In the context of the present invention, it is determined whether a point N of the income curve, of duration equivalent to 155 ° C t _N and yield strength R _{p0 , 2} (N) is close to the peak by determining the slope P _N of the tangent to the income curve at point N. It is considered in the context of the present invention that the elastic limit of a point N of the curve of income is close to the yield strength at the peak if the absolute value of the slope P _N is at most 3 MPa / h. As illustrated by FIG. 1, an under-income state is a state for which P _N is positive and an over-income state is a state for which P _N is negative.
To obtain an approximate value of P _N , for a point N of the curve in an under-tempered state, it is possible to determine the slope of the straight line passing through the point N and the preceding point N-1, obtained for a duration t _N-1 <t _N and having a yield strength R _p0,2 (N-1), we thus have P _N ≈ (R _p0,2 (N) -R _p0,2 (N-1)) / ( t _N - t _N-1 ). In theory, the exact value of P _N is obtained when t _N-1 tends to t _N. However, if the difference t _N - t _N - ₁ is small, the variation of elastic limit may be insignificant and the value imprecise. The present inventors have found that a satisfactory approximation of P _N is generally obtained when the difference t _N - t _N-1 is between 2 and 15 hours and preferably is of the order of 3 hours.
The time equivalent t i at 155 ° C is defined by the formula: $${\mathrm{t}}_{\mathit{i}}=\frac{\int \exp \left(-\mathrm{16400}/\mathrm{T}\right)\mathrm{dt}}{\exp \left(-\mathrm{16400}/{\mathrm{T}}_{\mathrm{ref}}\right)}$$

where T (in Kelvin) is the instantaneous metal processing temperature, which changes with time t (in hours), and T _ref is a reference temperature set at 428 K. ti is expressed in hours. The Q / R constant = 16400 K is derived from the activation energy for Cu diffusion, for which Q = 136100 J / mol was used.

The elastic limit close to the yield strength at the peak is typically at least 90%, generally even at least 95% and frequently at least 97% of the elastic limit R _p0,2 at the peak. The yield strength at the peak and the maximum yield strength that can be obtained by varying the parameters of time and temperature of income. The yield strength at the peak is generally satisfactorily evaluated by varying the residence time between 10 and 70h for a temperature of 155 ° C after a pull of 3.5%.

In general, for the Al-Cu-Li type alloys, the clearly underdeveloped states correspond to compromises between the static mechanical resistance (R _p0.2 , R _m ) and the damage tolerance (toughness, resistance to propagation cracks in fatigue) more interesting than peak and a fortiori that beyond the peak. However, the present inventors have found that a state under income but close to the peak makes it possible both to obtain a compromise between static mechanical resistance and damage tolerance of interest, but also to improve the performance in terms of corrosion resistance and thermal stability. In addition, the use of an under-income state close to the peak makes it possible to improve the robustness of the industrial process: a variation in the income conditions leads to a small variation in the properties obtained.
Thus, it is advantageous to achieve an under-income close to the peak, ie an under-income with the conditions of duration and temperature equivalent to those of a point N of the income curve at 155 ° C such that the tangent to the yield curve at this point has a slope P _N , expressed in MPa / h, such that 0 <P _N ≤ 3 and preferably 0.2 <P _N ≤ 2.5.

The products according to the invention can advantageously be used in structural elements, in particular aircraft. 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 construction. The products according to the invention are particularly advantageous for the production of products machined in the mass, such as in particular intrados or extrados elements whose skin and stiffeners come from the same starting material, spars and ribs, as well as any other use where the present properties could be advantageous

These and other aspects of the invention are explained in more detail with the aid of the following illustrative and non-limiting examples.

Examples Example 1

In this example, several plates of dimension 2000 x 380 x 120 mm whose composition is given in Table 1 were cast. Table 1. Composition in% by weight and density of Al-Cu-Li alloys cast in plate form. (Ref: reference; Inv: invention). Yes Fe Cu mn mg Zn Ag Li Zr Density (g / cm ³ ) 1 (Ref) 0.012 0,022 3.54 0.38 0.32 - 0.24 0.89 0.10 2,706 2 (Ref) 0.012 0,023 3.53 0.38 0.32 - - 0.91 0.10 2,699 3 (Inv) 0.012 0,032 3.53 0.38 0.67 - 0.25 0.93 0.10 2,698 4 (Inv) 0,011 0,022 3.5 0.38 0.67 - - 0.94 0.10 2,692 5 (Ref) 0.078 0.088 3.52 0.38 0.34 - 0.25 0.91 0.10 2,705 6 (Ref) 0,015 0,029 3.50 0.39 0.31 0.39 0.24 0.95 0.10 2,707 Ti: target 0.02% by weight for alloys 1 to 6

The plates were homogenized at about 500 ° C for about 12 hours and then cut and scalped to obtain slugs of size 400 x 335 x 90 mm. The slabs were hot rolled to obtain sheets having a thickness of 20 mm. The sheets were dissolved at 505 ± 2 ° C for 1 h, quenched with water at 75 ° C to obtain a cooling rate of about 18 ° C / sec and thus simulate properties obtained at mid-thickness of sheet of thickness 80 mm. The sheets were then trimmed with a permanent elongation of 3.5%.

The sheets were tempered between 10 and 50 hours at 155 ° C. Samples were taken at mid-thickness to measure static mechanical tensile properties as well as toughness K _Q. The test pieces used for the tenacity measurement had a width W = 25 mm and a thickness B = 12.5 mm. In general, the K _Q values obtained from this type of specimen are lower than those obtained from specimens having a greater thickness and width. Two measurements, made from specimens with a width W = 40 mm and a thickness B = 20 mm, confirm this trend. Measurements obtained from even larger specimens to obtain valid K _1C measurements would also be higher than the measurements obtained with specimens of width W = 25 mm and thickness B = 12. 5 mm.
The results obtained are shown in Table 2. Table 2. Mechanical properties obtained for the different sheets. Alloy Duration of income in hours at 155 ° C Rp _0.2 L (Mpa) Rm L (Mpa) AL (%) K _Q (MPa.m ^1/2 ) LT Evaluation of the slope P _N (MPa / h) 1 0 302.6 392.8 15.6 39.4 14 481.4 519.8 13.2 51.2 12.8 18 501.1 538.6 14.3 47.7 4.9 18 48.5 * 23 501.2 536.4 13.9 46.6 0.0 36 509.6 544.8 13.4 45.8 0.6 2 0 300.6 393.6 15.5 30.7 14 442.2 489.9 14.2 44.0 10.1 18 465.7 507.5 13.8 48.4 5.9 23 474.0 513.0 13.0 46.2 1.7 36 486.6 523.7 12.0 47.2 1.0 3 0 358.8 455.8 18.0 - 14 437.0 503.6 15.5 46.1 5.6 18 488.4 532.1 13.2 44.4 12.9 23 502.7 540.7 14.3 48.2 2.8 23 53.6 * 36 534.5 561.7 11.7 45.0 2.4 40 535.5 563.7 12.5 43.6 0.2 4 0 361.6 449.8 14.2 34.1 14 408.7 487.9 15.6 41.3 3.4 18 452.3 506.1 13.3 48.2 10.9 23 469.6 515.2 12.8 45.5 3.5 36 509.2 539.2 10.3 47.2 3.0 5 18 498.3 531.3 10.9 35.8 6 0 310.3 403.9 15.5 36.3 14 512.5 549.2 12.7 41.2 14.4 18 521.3 557.1 12.1 40.9 2.2 23 526.3 561.0 11.7 39.8 1.0 * specimen of width W = 40 mm and thickness B = 20 mm.

FIG. 2 shows the property compromises obtained for the samples having a slope P _N between 0 and 3 and the tenacity measurements made with samples of width W = 25 mm and thickness B = 12.5 mm. The products according to the invention have a significantly improved property compromise compared to the reference products.

Example 2 (Reference)

In this example, several 406 mm thick plates whose composition is given in Table 3 were cast. Table 3. Composition in% by weight and density of Al-Cu-Li alloys cast in plate form. Yes Fe Cu mn mg Zn Ag Li Zr Density (g / cm ³ ) 8 (Ref) 0.03 0.06 3.51 0.41 0.3 0.02 0.37 0.84 0.09 2,713 9 (Ref) 0.03 0.04 4.2 0.4 0.35 1.06 0.11 2,700 10 (Ref) 0.03 0.05 3.87 0.02 0.31 0.01 0.35 1.06 0.11 2,695

The plates were homogenized and then scalped. After homogenization, the plates were hot-rolled to obtain sheets having a thickness of 50 mm. The sheets were dissolved in cold water and triturated with a permanent elongation of between 3.5% and 4.5%

The sheets received an income of between 10 h and 50 h at 155 ° C. Samples were taken at mid-thickness to measure static mechanical tensile properties as well as toughness K _Q. The test pieces used for the tenacity measurement had a width W = 80 mm and a thickness B = 40 mm. K _1C validity criteria were met for some samples. The results obtained are shown in Table 4. Table 4 Mechanical properties obtained for the different sheets duration of income at 155 ° C Rm MPa Rp _{0, 2} MPa AT (%) K _Q (MPa.m ^1/2 ) LT K _Q. (MPa.m ^1/2 ) TL Evaluation of the slope P _N (MPa / h) 8 15 531 494 10.1 46.0 (K _1C ) 37.4 (K _1C ) 18 534 498 10.0 46.1 (K _1C ) 35.7 (K _1C ) 1.2 21 544 510 9.4 44.0 (K _1C ) 35.0 (K _1C ) 4 24 543 508 10.4 44.2 (K _1C ) 35.4 (K _1C ) -0.5 9 20 628 605 7.4 23.4 25 630.5 608.5 7.5 22.3 0.7 30 628 606 6.0 22.9 -0.5 35 626 603 6.5 22.0 -0.6 10 0 410 311 55.5 10 568.5 529.5 36.8 21.8 15 593 562 30.4 6.5 20 594.5 562.5 20.0 0.1 30 587.5 557.5 27.0 -0.5 45 613.5 587.5 24.7 2

In FIG. 3 points 8, 9 and 10 have been added to FIG. 2 (slope P _N between 0 and 3), although they relate to specimens of different geometry for the measurement of K _Q (K _1C ) in order to facilitate the comparison between the invention and the prior art. It is thus confirmed that the products according to the invention have a compromise of significantly improved properties compared to the prior art.

Example 3

In this example, several plates of size 2000 x 380 x 120 mm, the composition of which is given in Table 5, were cast. Table 5. Composition in% by weight and density of Al-Cu-Li alloys cast in plate form. (Ref: reference; Inv: invention). Yes Fe Cu mn mg Zn Ag Li Ti Zr Density (g / cm ³ ) 11 (Ref) 0,035 0.059 3.56 0.35 0.32 - 0.25 0.90 0.03 0.11 2,706 12 (Inv) 0,035 0.058 3.66 0.35 0.68 - 0.25 0.89 0.02 0.12 2,702 13 (Ref) 0,036 0.059 3.57 0.34 1.16 - 0.25 0.86 0.02 0.12 2,697

The plates were homogenized at about 500 ° C for about 12 hours and then cut and scalped to obtain slugs of size 400 x 335 x 90 mm. The slabs were hot rolled to obtain sheets having a thickness of 20 mm. The sheets were dissolved at 505 +/- 2 ° C for 1h and quenched with cold water. The sheets were then trimmed with a permanent elongation of 3.5%.

The sheets received an income between 18 h and 72 h at 155 ° C. Samples were taken at mid-thickness for measuring the static mechanical tensile properties and toughness K _Q. The test pieces used for the tenacity measurement had a width W = 25 mm and a thickness B = 12.5 mm.
The results obtained are shown in Table 6. Table 6. Mechanical properties obtained for the different sheets. Alloy Duration of income in hours at 155 ° C Rp _0.2 L (Mpa) Rm L (Mpa) AL (%) K _Q (MPa.m ^1/2 ) LT Evaluation of the slope P _N (MPa / h) 11 18 512.8 543.2 13.2 54.7 36 521.4 550.4 12.2 50.7 0.5 72 520.4 549.5 11.8 48.5 0.0 12 18 492.0 535.9 13.0 65.9 23 528.8 558.5 11.2 6.7 36 548.1 573.4 11.1 56.9 1.5 40 555.7 579.7 10.8 56.6 1.9 72 566.8 588.1 11.0 49.2 0.3 13 18 409.1 496.7 18.6 61.2 36 427.7 504.1 17.2 60.9 1.0 72 502.2 537.5 13.3 53.4 2.1

FIG. 4 shows the property compromises obtained for the samples having a slope P _N between 0 and 3 and the tenacity measurements made with samples of width W = 25 mm and thickness B = 12.5 mm. The products according to the invention have a significantly improved property compromise compared to the reference samples.

Example 4

In this example, the thermal stability of alloy products 12 was compared according to the income conditions used.

Alloy plates 12 transformed by the method described in Example 3 up to the excluded revenue stage were tempered at 155 ° C or 143 ° C for increasing times indicated in Table 7. After 34h at 143 ° C and 40h at 155 ° C, the aging was 1000 hours at 85 ° C. Samples were taken at mid-thickness to measure static mechanical tensile characteristics before and after aging.

The results obtained are shown in Table 7. The 34 hour yield at 143 ° C., for which the slope P _n is evaluated at 7.1, does not have a satisfactory thermal stability. Thus after aging the yield strength increased by 15% and the elongation decreased by 13%. On the other hand, the 40 hour income at 155 ° C., for which the slope P _n is evaluated at 1.9, has a satisfactory thermal stability, with an evolution of these properties of less than 5%. Table 7. Mechanical properties obtained for the alloy sheets 12 before and after aging from 1000h to 85 ° C. Temperature of income Duration of income in hours Before aging from 1000 h to 85 ° C Evaluation of the slope P _N (MPa / h) After aging for 1000 hours at 85 ° C. Rp _0.2 L (Mpa) Rm L (Mpa) AL (%) Rp _0.2 L (Mpa) Rm L (Mpa) AL (%) 155 ° C 23 528.8 558.5 11.2 6.7 36 548.1 573.4 11.1 1.5 40 555.7 579.7 10.8 1.9 564.3 578.0 10.2 143 ° C 20 368.0 472.7 17.2 24 381.7 479.3 16.1 3.4 34 452.7 516.0 13.5 7.1 521.7 565.3 11.7

Claims (14)

1. Wrought product such as an extruded, rolled and/or forged product, made from aluminium-based alloy comprising, as % by weight,

   Cu : 3.2 - 3.7;

   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;

   Zn ≤ 0.15;

   at least one element from

   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, the remainder aluminium.
2. Product according to claim 1, in which the lithium content is between 0.9% and 1.2% by weight.
3. Product according to either claim 1 or claim 2, in which the magnesium content is between 0.65% and 1.0% by weight and preferably between 0.7% and 0.9% by weight.
4. Product according to any of claims 1 to 3, in which the manganese content is between 0.2% and 0.4% by weight.
5. Product according to any of claims 1 to 4, the thickness of which is at least 30 mm and preferably at least 50 mm.
6. Product according to claim 5, in a rolled state, solution heat treated, quenched and aged so as to achieve a tensile yield strength close to the peak, having at mid-thickness at least one of the following pairs of characteristics for thicknesses of between 30 and 100 mm:

   (i) for thicknesses of 30 to 60 mm, at mid-thickness, a tensile yield strength R_p0.2 (L) ≥ 525 MPa and preferably R_p0.2 (L) ≥ 545 MPa and a toughness K_1C(L-T) ≥ 38 MPa√m and preferably K_1C(L-T) ≥ 43 MPa√m,

   (ii) for thicknesses of 60 to 100 mm, at mid-thickness, a tensile yield strength R_p0.₂(L) ≥ 515 MPa and preferably R_p0.2(L) ≥ 535 MPa and a toughness K_1C(L-T) ≥ 35 MPa√m and preferably K_1C(L-T) ≥ 40 MPa√m,

   (iii) for thicknesses of 100 to 130 mm, at mid-thickness, a tensile yield strength R_p0.₂(L) ≥ 505 MPa and preferably R_p0.2(L) ≥ 525 MPa and a toughness K_1C(L-T) ≥ 32 MPa√m and preferably K_1C(L-T) ≥ 37 MPa√m,

   (iv) for thicknesses of 30 to 100 mm, at mid-thickness, a tensile yield strength R_p0.2(L) expressed in MPa and a toughness K_1C(L-T) expressed in MPa√m such that K_1C(L-T) ≥ - 0.217 R_p0.2(L) + 157 and preferably K_1C(L-T) ≥ - 0.217 R_p0.2(L) + 163 and greater than 35 MPa√m,

   (v) after aging of 1000 hours at 85°C, a tensile yield strength R_p0.2(L) and an elongation at break A% (L) having a difference from the tensile yield strength R_p0.2 (L) and the elongation at break A% (L) before aging of less than 10% and preferably less than 5%.
7. Product according to any of claims 1 to 4, in a rolled state, solution heat treated, quenched and aged so as to achieve a tensile yield strength close to the peak, having at mid-thickness at least one of the following pairs of characteristics for thicknesses of between 10 and 30 mm:

   (i) a tensile yield strength R_p0.2(L) ≥ 525 MPa and preferably R_p0.2(L) ≥ 545 MPa and a toughness K_1C(L-T) ≥ 40 MPa√m and preferably K_1C(L-T) ≥ 45 MPa√m,

   (ii) a tensile yield strength R_p0.2 (L) expressed in MPa and a toughness K_Q(L-T) expressed in MPa√m such that K_1C(L-T) ≥ - 0.4 R_p0.2(L) + 265 and preferably K_1C(L-T) ≥ - 0.4 R_p0.2(L) + 270 and greater than 45 MPa√m,

   (iii) after aging of 1000 hours at 85°C, a tensile yield strength R_p0.2(L) and an elongation at break A%(L) having a difference from the tensile yield strength R_p0.2(L) and the elongation at break A% (L) before aging of less than 10% and preferably less than 5%.
8. Method for manufacturing an extruded, rolled and/or forged product based on aluminium alloy, in which

   a) a liquid metal bath based on aluminium is produced, comprising 3.2 to 3.7% by weight Cu, 0.8 to 1.3% by weight Li, 0.6 to 1.0% by weight Mg, 0.05 to 0.18% by weight Zr, 0.0 to 0.5% by weight Ag, 0.0 to 0.5% by weight Mn, no more than 0.20% by weight Fe + Si, no more than 0.15% by weight Zn, at least one element chosen from Cr, Sc, Hf and Ti, the quantity of said element, if chosen, being from 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and from 0.01 to 0.15% by weight for Ti, the other elements no more than 0.05% by weight each and 0.15% by weight in total, the remainder aluminium;

   b) a rough form is cast from said liquid metal bath;

   c) said rough form is homogenised at a temperature between 450°C and 550°C and preferably between 480°C and 530°C for a period of between 5 and 60 hours.

   d) said rough form is hot worked and optionally cold worked into an extruded, rolled and/or forged product;

   e) solution heat treatment is carried out at between 490°C and 530°C for 15 minutes to 8 hours and said product is quenched;

   f) said product is stretched in a controlled manner with a permanent deformation of 1% to 6% and preferentially at least 2%;

   g) aging of said product is carried out, comprising heating at a temperature of between 130°C and 170°C for 5 to 100 hours and preferably 10 to 40 hours so as to achieve a tensile yield strength close to the peak, the aging being carried out under duration and temperature conditions equivalent to those of a point N on the aging curve at 155°C such that the tangent to the aging curve at this point has a slope P_N, expressed in MPa/h, such that 0 < P_N ≤ 3.
9. Method according to claim 8, in which the hot and optionally cold working is carried out to a thickness of at least 30 mm.
10. Method according to claim 8 or claim 9, in which the controlled stretching is carried with a permanent deformation of between 3% and 5%.
11. Method according to any of claims 8 to 10, in which 0.2 < P_N ≤ 2.5.
12. Structure element comprising a product according to any of claims 1 to 7.
13. Use of a structure element according to claim 12 for aeronautical construction.
14. Use according to claim 13, in which the structure element is an upper wing or lower wing element, the skin and stiffeners of which come from the same starting product, a spar or a rib.