EP1891247B1 - High-strength aluminum-copper-lithium sheet metal for aircraft fuselages - Google Patents

Abstract

The production of an aluminum alloy sheet with elevated tenacity and mechanical strength comprises: (A) preparing a bath of molten metal in which the copper and lithium contents are controlled; (B) casting a plate from the molten metal; (C) homogenizing the plate at 490-530 o>C for 5-60 hours; (D) rolling the plate to form a sheet with a thickness of 0.8-12 mm; (E) putting into solution and tempering the sheet; (F) the controlled drawing of the sheet with a permanent deformation of 1-5 %; and (G) tempering the sheet at 140-170 o>C for 5-30 hours. Independent claims are also included for the following: (1) a rolled, extrudes and/or forged product of this Al alloy; (2) an Al alloy sheet; and (3) structural elements fabricated from the product.

Description

Field of the invention

The present invention generally relates to aluminum alloy products and, in particular, such products useful in the aerospace industry and suitable for use in fuselage applications.

State of the art

In today's civil aviation industry and, in particular, for fuselage applications, there is a strong motivation to reduce both weight and cost. The fuselage of a commercial transport aircraft is subject to a complex set of constraints, depending on the operating phase (takeoff, cruising, maneuvering, landing ...) and environmental conditions (gusts of wind, headwinds ,. ..). In addition, the different parts of the fuselage are subject to different constraints. Despite this complexity, it is possible to distinguish major design guidelines that determine the weight of the structure, some of which have a greater weight impact than others.

For example, compressive and compressive shear strength is an extremely important design guideline, since the heavier fuselage panels suffer this type of constraint. In order for a new material to be able to reduce the weight of these compression-stressed panels, it must have a high modulus of elasticity, a high 0.2% yield strength (to withstand buckling) and a low mass volume.

The second major guideline is the residual resistance of panels longitudinally (in the axis of the fuselage) cracked. Aeronautical certification regulations require the consideration of damage tolerance in design, so it is usual to consider large longitudinal or circumferential cracks in the fuselage panels, to demonstrate that a certain level of stress can be applied. without a catastrophic break. A known property of the materials governing the design here is the toughness under plane stress. All known factors of critical stress intensity, however, only give a limited view of toughness. The R curve test is a widely recognized means for characterizing toughness properties. The curve R represents the evolution of the critical effective stress intensity factor for the crack propagation as a function of the effective crack extension under a monotonic stress. It allows the determination of the critical load for unstable failure for any configuration relevant to cracked aircraft structures. The values of the effective stress intensity factor and the crack extension effective are values defined in ASTM E561. The length of the curve R - namely the maximum crack extension of the curve - is a parameter in itself important for the fuselage design. The classical analysis, generally used, of the tests carried out on panels with central crack gives a factor of intensity of stress apparent to the rupture (K _app ). This value does not vary significantly with the length of the R curve, especially when the slope of the R curve is close to the slope of the stress intensity factor curve applied to the crack length (applied curve) . However, in a real structural element structure such as a panel having fixed stiffeners, when a crack progresses under an unbroken stiffener, the applied curve drops due to the bridging effect of the stiffener. In this case, a local minimum of the applied curve can occur for a crack length greater than the sum of the initial crack length and the crack extension under a monotonic load. In this case, larger stresses prior to unstable fracture are allowed for long R curves. It is thus interesting to have a longer curve R, even for identical critical stress intensity factors, as determined. classically.

For products having identical mechanical properties, a lower density is clearly beneficial for the weight of a structural member. A third major guideline is thus the density of the material. In addition, large parts of the fuselage are not so heavily loaded and the weight of the design is limited by a certain limit generally called "minimum thickness". The minimum thickness concept is the lowest usable thickness for manufacturing (especially panel handling) and repair (repair riveting). The only way to reduce the weight in this case is to use a lower density material.

Other important guidelines are the propagation of fatigue cracks, either under constant amplitude stress, or with variable amplitude (due to maneuvers and gusts of wind, especially in the longitudinal direction, but also around the wing , in all directions).

Today, the fuselages of civil aircraft are, for the most part, made of alloy sheet 2024, 2056, 2524, 6013, 6156 or 7475, plated on each side with an aluminum alloy lightly loaded with alloying elements. , an alloy 1050 or 1070 for example. The purpose of the coating alloy is to impart sufficient corrosion resistance. Light, generalized or pitting corrosion is tolerable but must not be penetrating so as not to attack the core alloy. There is a tendency to try to use non-plated materials for fuselage design, so as to reduce the cost. Corrosion resistance, and in particular intergranular corrosion and corrosion under stress, the fuselage panel is thus an important aspect of its properties.

As stated above, the only way to reduce weight is, in some cases, to reduce the density of materials used in aircraft construction. Aluminum-lithium alloys have long been recognized as an effective solution for reducing weight due to the low density of these alloys. However, the various requirements mentioned above: high modulus of elasticity, high compressive strength, high damage tolerance and high corrosion resistance, were not simultaneously satisfied by the aluminum-lithium alloys of the art prior. Achieving high toughness with these alloys has in particular proved to be a difficult problem to solve. Prasad et al, for example, have recently established (in Sadhana, vol. 28, parts 1 & 2, February / April 2003 pages 209 to 246 ) that "Al-Li alloys are prime candidate materials for replacing traditionally used Al alloys. Despite their many property advantages, low ductility in tension and inadequate toughness, especially in directions across the thickness, militate against their acceptability. " Today, alloys in Al-Li have been limited to very specific military applications such as materials with high resistance to high temperature, materials with improved toughness at cryogenic temperatures for aerospace applications, parts of helicopters, and fuselage parts of military aircraft.

The patent US 5,032,359 (Martin Marietta ) discloses a family of alloys based on aluminum-copper-magnesium-silver alloys to which lithium has been added, in specific ranges and which have high resistance at ambient temperature and at high temperature, high ductility at ambient temperatures and at high temperature, extrusionability, forgeability, and good solderability and natural aging response properties. The examples describe extruded products. No information is provided on toughness, fatigue behavior or corrosion resistance. In a preferred embodiment, the alloy has a composition of 3.0 to 6.5% copper, 0.05 to 2.0% magnesium, 0.05 to 1.2% silver, from 0.2 to 3.1% lithium, from 0.05 to 0.5% of an element chosen from zirconium, chromium, manganese, titanium, boron, hafnium, vanadium, titanium diboride and mixtures thereof.

The document US 5,122,339 (Martin Marietta ) is a continuation of the previous request. It is furthermore described a use of alloys similar as welding alloys or as welded alloys.

The document US 5,211,910 (Martin Marietta ) discloses aluminum-based alloys containing Cu, Li, Zn, Mg and Ag which have favorable properties, such as relatively low density, high modulus, high mechanical strength / ductility combinations , a strong response to natural aging with and without anterior work hardening, and a high modulus after income with or without prior work hardening. The alloys have a composition of 1 to 7% Cu, 0.1 to 4% Li, 0.01 to 4% Zn, 0.05 to 3% Mg, 0.01 to 2% of Ag, from 0.01 to 2% of an element selected from Zr, Cr, Mn, Ti, Hf, V, Nb, B and TiB ₂ , the remainder being Al together with its unavoidable impurities. This invention describes how Zn additions can be used to reduce the Ag content present in the alloys taught in the document. US 5,032,359 in order to reduce the cost.

The document US 5,455,003 (Martin Marietta ) discloses a process for producing aluminum-copper-lithium alloys that exhibit improved strength and toughness at cryogenic temperatures. The improved cryogenic properties are achieved by adjusting the composition of the alloy, along with the processing parameters such as the amount of work hardening and the income. The product is used for cryogenic tanks in space launch vehicles.

The document US 5,389,165 (Reynolds ) discloses an aluminum alloy useful in aircraft and aerospace structures which has low density, high mechanical strength and high toughness and has the formula: Cu _a Li _b Mg _c Ag _d Zr _e Al _bal wherein a, b, c, d, e and bal indicate the amount in% by weight of alloying components, and wherein 2.8 <a <3.8, 0.80 <b <1.3, 0.20 <c <1.00, 0.20 <d <1.00 and 0.08 <e <0.40. Preferably, the copper and lithium components are adjusted so that the combined copper and lithium content is kept below the solubility limit in order to avoid a loss of toughness during high temperature exposure. The relationship between copper and lithium grades must also satisfy the following relationship: $$\mathrm{Cu}\left(\%\mathrm{in\; weight}\right)+1,5\mathrm{Li}(\%\mathrm{in\; weight})<5,4.$$

Special conditions of controlled traction, between 5 and 11%, are applied. The examples are limited to a thickness of 19 mm and a zirconium content greater than or equal to 0.13% by weight.

The document US 2004/0071586 (Alcoa ) discloses an Al-Cu-Mg alloy comprising from 3 to 5% by weight of Cu, from 0.5 to 2% by weight of Mg and from 0.01 to 0.9% by weight of Li. patent application, the toughness of alloys for which an addition of Li between 0.2 and 0.7% by weight is significantly improved over similar alloys containing either no Li or a higher amount of Li.

There is a need for an Al-Li alloy of high mechanical strength, high toughness and in particular high crack extension before unstable fracture, high corrosion resistance, for aeronautical applications and in particular for fuselage sheet applications.

Object of the invention

For these and other reasons, the present inventors have come to the present invention concerning an aluminum-copper-lithium-magnesium-silver alloy, which exhibits high mechanical strength, high toughness and specifically high crack extension prior to fracture. unstable pre-cracked wide panels, and a high resistance to corrosion.

1. a) on élabore un bain de métal liquide comprenant 3,0 à 3,4% en poids de Cu, 0,8 à 1,2 % en poids de Li, 0,2 à 0,5 % en poids d'Ag, 0,2 à 0,6% en poids de Mg et au moins un élément choisi parmi Zr, Mn, Cr, Sc, Hf et Ti, la quantité dudit élément, s'il est choisi, étant de 0,05 à 0,13% en poids pour Zr, 0,05 à 0,8 % en poids pour Mn, 0,05 à 0,3 % en poids pour Cr et pour Sc, 0,05 à 0,5 % en poids pour Hf et de 0,05 à 0,15 % en poids pour Ti,
   le reste étant de l'aluminium et des impuretés inévitables,
   avec la condition supplémentaire que la quantité de Cu et de Li soit telle que $$\mathrm{Cu}(\%\mathrm{en\; poids})+5/3\mathrm{Li}(\%\mathrm{en\; poids})<5,2;$$
   
2. b) on coule une plaque à partir dudit bain de métal liquide
3. c) on homogénéise ladite plaque à une température comprise entre 490 à 530°C pendant une durée de 5 à 60 heures ;
4. d) on lamine ladite plaque en une tôle ayant une épaisseur finale comprise entre 0,8 et 12 mm ;
5. e) on met en solution et on trempe ladite tôle ;
6. f) on tractionne de façon contrôlée ladite tôle avec une déformation permanente de 1 à 5 % ;
7. g) on réalise un revenu de ladite tôle par chauffage à 140 à 170 °C pendant 5 à 30 heures.

An object of the present invention is a method of manufacturing an aluminum alloy sheet having high toughness and strength, wherein:

1. a) a bath of liquid metal comprising 3.0 to 3.4% by weight of Cu, 0.8 to 1.2% by weight of Li, 0.2 to 0.5% by weight of Ag is prepared, 0.2 to 0.6 wt% Mg and at least one member selected from Zr, Mn, Cr, Sc, Hf and Ti, the amount of said element, if selected, being from 0.05 to 0, 13% by weight for Zr, 0.05 to 0.8% by weight for Mn, 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and 0.05 to 0.15% by weight for Ti,
   the rest being aluminum and unavoidable impurities,
   with the additional condition that the quantity of Cu and Li is such that $$\mathrm{Cu}(\%\mathrm{in\; weight})+5/3\mathrm{Li}(\%\mathrm{in\; weight})<5,2;$$
   
2. b) pouring a plate from said liquid metal bath
3. c) said plate is homogenized at a temperature between 490 and 530 ° C for a period of 5 to 60 hours;
4. d) laminating said plate into a sheet having a final thickness of between 0.8 and 12 mm;
5. e) dissolving and quenching said sheet;
6. f) Controllably pulling said sheet with a permanent deformation of 1 to 5%;
7. g) an income of said sheet is obtained by heating at 140 to 170 ° C for 5 to 30 hours.

Another object of the invention is a laminated, extruded and / or forged aluminum alloy product comprising 3.0 to 3.4% by weight of Cu, 0.8 to 1.2% by weight of Li, 0 , 2 to 0.5% by weight of Ag, 0.2 to 0.6% by weight of Mg and at least one element selected from Zr, Mn, Cr, Sc, Hf and Ti, the amount of said element, it is selected from 0.05 to 0.13% by weight for Zr, 0.05 to 0.8% by weight for Mn, 0.05 to Z by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and from 0.05 to 0.15% by weight for Ti, the remainder being aluminum and unavoidable impurities, with the additional condition that the amount of Cu and Li is such that $$\mathrm{Cu}(\%\mathrm{in\; weight})+5/3\mathrm{Li}(\%\mathrm{in\; weight})<5,2.$$

Still other objects of the invention are elements of structures, stiffeners and fuselage panels obtained from said rolled, extruded and / or forged products.

Description of figures

• Figure 1 : Curve R in the TL direction (specimen CCT760).
• Figure 2 : Curve R in direction LT (specimen CCT760).
• Figure 3 : evolution of the cracking velocity in the TL direction when the magnitude of the stress intensity factor varies.
• Figure 4 : evolution of the cracking velocity in the LT direction when the magnitude of the stress intensity factor varies.
• Figure 5 : Curve R in the direction TL (specimen CCT760) samples according to the invention having been obtained with different levels of deformation by traction.

Detailed 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 designation of alloys is in accordance with the regulations of The Aluminum Association, known to those skilled in the art. The definitions of the metallurgical states are given in the European standard EN 515.

Unless stated otherwise, the static mechanical characteristics, in other words the _ultimate tensile strength Rm, the conventional yield stress at 0.2% elongation R _p0.2 and the elongation at break A, are determined by a tensile test according to the EN 10002-1, the sampling and the sense of the test being defined by EN-485-1.

The cracking rate (da / dN) is determined according to the ASTM E 647 standard. A curve of the stress intensity as a function of the crack extension, known as the R curve, is determined according to the ASTM E 561 standard. The critical stress intensity factor K _C , in other words the intensity factor that makes the crack unstable, is calculated from the curve R. The stress intensity factor K _CO is also calculated by assigning the initial crack length at the critical load at the beginning of the monotonic load. These two values are calculated for a specimen of the required form. K _app represents the K _CO factor corresponding to the specimen that was used to perform the R curve test. K _eff represents the K _C factor corresponding to the specimen that was used to perform the R curve test. Δa _{eff (max)} represents the crack extension of the last valid point of the R curve. Unless otherwise stated, the crack size at the end of the pre-fatigue cracking stage is W / 3 for M-type specimens ( T), wherein W is the width of the specimen as defined in ASTM E561. It should be noted that the width of the specimen used in an R curve test can have a substantial influence on the stress intensity measured in the test. The fuselage sheets being large panels, the results of curve R obtained on sufficiently large samples, such as samples having a width greater than or equal to 400 mm, are judged the most significant for the evaluation of toughness. For this reason, CCT760 test specimens, which had a width of 760 mm, were used preferentially for the evaluation of toughness. The initial crack length 2ao = 253 mm.

The toughness was also evaluated in the TL directions using the global energy at break E _g according to the Kahn test. The stress Kahn R _e (in MPa) is equal to the ratio of the maximum load F _max that the specimen can withstand on the section of the specimen (product of the thickness B by the width W). R _e does not make it possible to evaluate the relative toughness of samples whose static mechanical characteristics are different. The overall energy at break E _g is determined as the area under the Force-Displacement curve until the test piece breaks, E _g is directly related to toughness. The test is described in the article Kahn-Type Tear Test and Crack Toughness of Aluminum Alloy Sheet, published in Materials Research & Standards, April 1964, p. 151-155 . The test specimen used for the Kahn toughness test is described, for example, in the "Metals Handbook", 8th Edition, Vol. 1, American Society for Metals, pp. 241-242 .

By "sheet" is meant here a rolled product not exceeding 12 mm thick.

The term "structural element" refers to an element used in mechanical engineering for which the static and / or dynamic mechanical characteristics are of particular importance for the performance and integrity of the structure, and for which a calculation of the structure is usually prescribed or performed. It is typically a mechanical part 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 (stiffeners), ribs (ribs) and spars) and empennage including horizontal stabilizers and vertical stabilizers horizontal or vertical stabilizers, as well as floor beams, seat tracks and doors.

The aluminum-copper-lithium-silver-magnesium alloy according to one embodiment of the invention advantageously has the following composition: <u> Table 1 </ u>: Alloy composition ranges (% by weight, the balance being Al) Cu Li Ag mg Large 2.7 to 3.4 0.8 to 1.4 0.1 to 0.8 0.2 to 0.6 favorite 3.0 to 3.4 0.8 to 1.2 0.2 to 0.5 0.2 to 0.6 Most preferred 3.1 to 3.3 0.9 to 1.1 0.2 to 0.4 0.2 to 0.4

et en assurant une vitesse de refroidissement pendant la trempe suffisamment élevée, par exemple en trempant à l'eau froide.In order to obtain desired results in terms of toughness, it may be advantageous to obtain a almost perfect dissolution during solution heat treatment and also minimize decomposition of the solid solution during quenching. The inventors have determined that this can be achieved, for example, by limiting the total amount of Cu and Li, according to the following relationship between copper and lithium $$\mathrm{Cu}(\%\mathrm{in\; weight})+5/3\mathrm{Li}(\%\mathrm{in\; weight})<5,2$$

and providing a cooling rate during sufficiently high quenching, for example by quenching with cold water.

For the preferred and most preferred compositions of Table 1, the relationship between copper and lithium is preferably: $$\mathrm{Cu}(\%\mathrm{in\; weight})+5/3\mathrm{Li}(\%\mathrm{in\; weight})<5.$$

At least one element such as Zr, Mn, Cr, Sc, Hf, Ti or a combination thereof is included to refine the grain. The additions depend on the element: from 0.05 to 0.13% by weight (preferably from 0.09 to 0.13% by weight) for Zr, from 0.05 to 0.8% by weight for Mn from 0.05 to 0.3% by weight for Cr and Sc, from 0.05 to 0.5% by weight for Hf and from 0.05 to 0.15% by weight for Ti. When several of these anti-recrystallizing elements are added, the sum can be limited by the appearance of primary phases.

In another advantageous embodiment of the invention, grain refining is achieved by the addition of 0.05 to 0.13% by weight of Zr, from 0.02 to 0.3% by weight of Sc and optionally from 0.05 to 0.8% in Mn weight, 0.05 to 0.3% by weight of Cr, 0.05 to 0.5% by weight of Hf and 0.05 to 0.15% by weight of Ti.

In some cases, and particularly for hot-rolled sheets with a thickness of between 4 and 12 mm, it may be advantageous to limit the Mn content to 0.05% by weight and preferably to 0.03% by weight. The inventors have observed that for such thicknesses the presence of Mn makes it more difficult to control the granular structure and may affect both the mechanical properties and the toughness.

Fe and Si generally affect toughness properties. The amount of Fe should preferably be limited to 0.1% by weight and the amount of Si should preferably be limited to 0.1% by weight (preferably to 0.05% by weight). All other elements should also preferably be limited to 0.1% by weight (preferably to 0.05% by weight).

The inventors have found that if the copper content is greater than 3.4% by weight, the toughness properties can in some cases fall rapidly. For certain embodiments of the invention, it is recommended not to exceed a copper content of 3.3% by weight. Preferably, the copper content is greater than 3.0% or even 3.1% by weight.

The present inventors have found that Zr contents greater than 0.13% by weight can, in some cases, lead to a lower toughness performance. Whatever the reason for this drop in toughness, the inventors found that the higher Zr content led to a formation of Al ₃ Zr primary phases. In this case, a temperature of High casting may be used to avoid formation of the primary phases, but this may lead to lower liquid metal quality, in terms of inclusion and gas content. This is why the present inventors consider that the Zr should advantageously not exceed 0.13% by weight.

The inventors have found that if the Li content is less than 0.8% by weight or even 0.9% by weight, the improvement in mechanical strength is too low. In some cases, it may be advantageous if the Li content is> 0.9% by weight. Also, with these low Li content, the decrease in the density of the alloy is too low. For a Li content greater than 1.4% or more than 1.2% by weight or even greater than 1.1% by weight, the toughness is significantly reduced. Also, these high Li levels have several disadvantages related in particular to the thermal stability, flowability and cost of raw materials.

The addition of Ag is an essential feature of the invention. The strength and toughness performances observed by the inventors are not usually achieved for alloys containing no silver. The inventors believe that silver plays a role in the formation of copper-containing hardening phases, formed during natural or artificial aging and in particular allows the formation of finer phases and a more homogeneous distribution of these phases. The advantageous effect of Ag is observed for a content of this element greater than 0.1% by weight and preferably greater than 0.2% by weight. In order to limit the cost associated with the addition of Ag, it may be advantageous not to exceed 0.5% by weight or even 0.4% by weight.

The addition of Mg improves the mechanical strength and decreases the density. An excessive addition of Mg, however, would have a detrimental effect on toughness. In an advantageous embodiment of the invention, the Mg content is limited to 0.4% by weight. The inventors believe that the addition of Mg could also have a role during the formation of copper-containing phases.

The bath of liquid metal having a composition according to the invention is then cast. The present invention makes it possible to obtain a laminated, extruded and / or forged product whose thickness is, advantageously, between 0.8 and 12 mm and preferably between 2 and 12 mm.

According to an advantageous embodiment of the present invention, an alloy having adjusted amounts of alloying elements is cast as a plate. The plate is then homogenized at 490-530 ° C for 5-60 hours. The inventors have observed that homogenization temperatures above 530 ° C can tend to reduce the tenacity performance in some cases.

Before hot rolling, the plates are heated at 490 to 530 ° C for 5 to 30 hours. Hot rolling is carried out to obtain a thickness of between 4 and 12 mm. For a thickness of approximately 4 mm or less, a cold rolling step may be added, if necessary. The sheet obtained at a thickness preferably between 0.8 and 12 mm, and the invention is more advantageous for sheets of 2 to 12 mm thick and even 2 to 9 mm and even more advantageous for sheets of 3 to 7 mm thick. The sheets are then put in solution, for example by heat treatment between 490 and 530 ° C for 15 min to 2 h, and then quenched with water at room temperature or preferably cold water.

The product then undergoes a controlled pull of 1 to 5% and preferably 2.5 to 4%. Such cold hardening levels can also be achieved by cold rolling, planing, forging or a combination of these methods and controlled pulling. Advantageously, the total cold working after quenching is between 2.5 and 4%. In particular, when a leveling operation is carried out between the quenching and the controlled traction and no other cold deformation is performed, it may be advantageous for the controlled tensile deformation to be between 1.7 and 3.5. %. The inventors have observed that the tenacity tends to decrease when the controlled tensile deformation is greater than 5%. In addition, the results of Kahn test, in particular Eg tends to decrease for permanent deformations greater than 5%. It is therefore recommended not to exceed a permanent deformation of 5%. Moreover, if the traction is greater than 5%, there may be industrial difficulties such as high implementation as well as that formatting difficulties, which would increase the cost of the product.

An income is achieved at a temperature between 140 and 170 ° C for 5 to 30 hours, which provides a T8 state. In some cases, and in particular for the preferred and more preferred compositions of Table 1, the reaction is more preferably carried out between 140 and 155 ° C for 10 to 30 hours. Low tempering temperatures generally favor high toughness. In one embodiment of the present invention, the revenue step is divided into two steps: a pre-revenue step prior to a welding operation, and a final heat treatment of a welded structure member. In the context of the present invention, friction stir welding is a preferred welding technique.

The sheets according to the invention have advantageous properties for recrystallized, non-recrystallized or mixed microstructures (that is to say comprising recrystallized zones and non-recrystallized zones). In some cases, the inventors have observed that it could be advantageous to avoid mixed microstructures: for sheets whose thickness is between 4 and 12 mm, it may be advantageous for the microstructure to be completely uncrystallized.

• La limite d'élasticité conventionnelle R_p0,2 dans le sens L est de préférence d'au moins 440 MPa, préférentiellement d'au moins 450 MPa ou même d'au moins 460 MPa.
• La résistance à la rupture Rm dans le sens L est de préférence d'au moins 470 MPa préférentiellement d'au moins 480 MPa ou même d'au moins 490 MPa.
• Les propriétés de ténacité utilisant des éprouvettes CCT760 (avec 2ao = 253 mm) sont telles que :
  • K_app dans la direction T-L est de préférence d'au moins 110 MPa $\sqrt{\mathrm{m}}$
    
    et préférentiellement d'au moins 130 MPa $\sqrt{\mathrm{m}}$
    
    ou même d'au moins 140 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • K_app dans la direction L-T est d'au moins 150 MPa $\sqrt{\mathrm{m}}$
    
    et préférentiellement d'au moins 170 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • K_eff dans la direction T-L est d'au moins 130 MPa $\sqrt{\mathrm{m}}$
    
    et préférentiellement d'au moins 150 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • K_eff dans la direction L-T est d'au moins 170 MPa $\sqrt{\mathrm{m}}$
    
    ou même d'au moins 190 MPa $\sqrt{\mathrm{m}}$
    
    et préférentiellement d'au moins 230 MPA $\sqrt{\mathrm{m}}$
    
    ;
  • Δa_eff(max), l'extension de fissure du dernier point valide de la courbe R dans la direction T-L est de préférence d'au moins 30 mm et préférentiellement d'au moins 40 mm ;
  • Δa_eff(max), l'extension de fissure du dernier point valide de la courbe R dans la direction L-T, est de préférence d'au moins 50 mm.

The characteristics of the sheets obtained according to the invention are in the T8 state:

• The conventional yield strength R _p0,2 in the direction L is preferably at least 440 MPa, preferably at least 450 MPa or even at least 460 MPa.
• The tensile strength Rm in the direction L is preferably at least 470 MPa, preferably at least 480 MPa or even at least 490 MPa.
• The tenacity properties using specimens CCT760 (with 2ao = 253 mm) are such that:
  • K _app in the direction TL is preferably at least 110 MPa $\sqrt{\mathrm{m}}$
    
    and preferably at least 130 MPa $\sqrt{\mathrm{m}}$
    
    or even at least 140 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • K _app in LT direction is at least 150 MPa $\sqrt{\mathrm{m}}$
    
    and preferably at least 170 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • K _eff in the TL direction is at least 130 MPa $\sqrt{\mathrm{m}}$
    
    and preferably at least 150 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • K _eff in the direction LT is at least 170 MPa $\sqrt{\mathrm{m}}$
    
    or even at least 190 MPa $\sqrt{\mathrm{m}}$
    
    and preferably at least 230 MPa $\sqrt{\mathrm{m}}$
    
    ;
  • Δa _{eff (max)} , the crack extension of the last valid point of the curve R in the direction TL is preferably at least 30 mm and preferably at least 40 mm;
  • Δa _{eff (max)} , the crack extension of the last valid point of the curve R in the direction LT, is preferably at least 50 mm.

The forming of the sheet of the invention may advantageously be carried out by deep drawing, stretching, spinning, rolling or folding, these techniques being known to those skilled in the art.

In the assembly of structural parts, all known and possible techniques of riveting and Suitable welding for aluminum alloys can be used, if desired. Said sheet may be attached to stiffeners or frames, for example by riveting or welding. The inventors have found that if welding is chosen, it may be preferable to use low temperature welding techniques, which help to ensure that the thermally affected area is as small as possible. In this respect, laser welding and friction stir welding often give particularly satisfactory results. In the context of the invention, friction stir welding is the preferred welding method. The welded seams of sheets according to the invention advantageously obtained by friction stir welding have a coefficient of effectiveness of the seal greater than 70% and preferably greater than 75%. This advantageous result is obtained that the income is made before or after the welding operation.

A structural element, formed of at least one product according to the invention, in particular a sheet according to the invention and stiffeners or frames, these stiffeners or frames being preferably made of extruded profiles, can be used in particular for the manufacture of aircraft fuselage panels as well as any other use where the present properties could be advantageous.

According to the invention, structural members, stiffeners, and / or fuselage panels can be made from the rolled, extruded, and / or forged products obtained. The inventors have found that the sheet of the invention has mechanical properties Particularly favorable statics and high tenacity. For known products, the high-tenacity sheets generally have low yield strengths and breaking strength. For the sheet of the invention, the high mechanical properties favor an industrial application for aircraft structural parts, the elastic limit and the breaking strength of said sheet being characteristics which are directly taken into account for the calculation. structural dimensioning. Calculations of structural elements and in particular of fuselage panels comprising sheets and / or stiffeners according to the invention have shown a possibility of weight reduction with respect to structural elements of comparable properties comprising only metal sheets. prior art alloy 2024, 2056, 2098, 7475 or 6156. Such weight reductions are generally from 1 to 10% and in some cases even greater weight reductions can be achieved.

By way of example, in a part of given shape and dimensions, the simple substitution of the alloy 2024 with an alloy according to the invention, without resizing the structural element as a function of the improvement of the mechanical characteristics, may allow a weight reduction of the order of 3 to 3.5%.

The high mechanical properties of the alloys according to the invention make it possible to develop products of a lighter size and shape, which makes it possible to reach or even exceed a weight reduction of 10%.

The sheet of the invention does not generally induce any particular problem during subsequent surface treatment operations conventionally used in aircraft construction.

The resistance to intergranular corrosion of the sheet of the invention is generally high; for example, only pits are generally detected when the metal is subjected to a corrosion test. In a preferred embodiment of the invention, the sheet of the invention can be used without plating.

These and other aspects of the invention are explained in more detail with the aid of the following illustrative and nonlimiting example.

examples Example 1

In connection with the present invention, several known materials are presented for comparison purposes (references A to E). They include, respectively, alloys 2024, 2056, 77475, 6156 and 2098. Examples of the invention are marked F to I. The chemical composition of the various alloys tested is given in Table 2. <u> Table 2 </ u>: Chemical composition (% by weight) cast Yes Fe Cu mn mg Cr Zn Zr Li Ag Ti A (2024) 0.12 0.15 4.2 0.5 1.4 0.05 0.2 0.02 - - 0.02 B (2056) 0.06 0.09 4.0 0.4 1.3 - 0.6 - - - 0.02 C (7475) 0.04 0.07 1.6 0.01 2.2 0.2 5.8 0.02 - - 0.02 D (6156) 0.78 0.07 0.9 0.45 0.75 0.14 0.02 - - 0.02 E (2098) 0.03 0.04 3.6 0.01 0.32 0.14 0.02 1.00 0.33 0.02 F 0.02 0.04 3.3 0.01 0.31 0.12 0.96 0.32 0.02 BOY WUT 0.05 0.06 3.2 0.01 0.31 0.11 0.93 0.32 0.03 H 0.05 0.06 3.3 0.02 0.31 0.06 0.11 0.96 0.34 0.02 I 0.05 0.06 3.2 0.01 0.31 0.11 0.94 0.33 0.03 J 0.03 0.04 3.2 - 0.31 - - 0.11 0.98 0.33 0.02 K 0.03 0.04 3.3 0.00 0.31 - - 0.11 0.97 0.34 0.03

The density of the various alloys tested is shown in Table 3. The samples F to I have the lowest density of the various materials tested. <u> Table 3 </ u>: Density of tested alloys Reference Density (g / cm ³ ) A (2024) 2.78 B (2056) 2.78 C (7475) 2.81 D (6156) 2.72 E (2098) 2.70 F, G, H, I, J, K 2.69

The process used for the manufacture of reference samples A to D is the conventional industrial process, these reference samples were plated. The final metallurgical states for A, B, C and D were, respectively, T3, T3, T76 and T6 according to EN573. The process used to make samples E and F is shown in Table 4. In some cases, a planing step was performed between the temple and the controlled pull. For comparison purposes, the E samples were not processed with their most usual conditions, which include a controlled tensile operation with an elongation between 5 and 10%. Sample E # 3 was subjected to annealing before dissolution in order to improve the toughness. The particular process carried out for sample E # 3, including an additional step, would not be favorable for an industrial application because of the cost increase associated with this step. For the other samples made with the alloy E, no annealing step was performed. <u> Table 4 </ u>: Conditions of the consecutive stages of transformation Referenced References F and K References G, H, I and J State T8 T8 T8 Plate relaxation Yes Yes Yes homogenization 8 h at 500 ° C + 36 h at 526 ° C 8 h at 500 ° C + 36 h at 526 ° C 12h at 505 ° C Preheating before hot rolling 20 h at 520 ° C 20 h at 520 ° C 20 h at 520 ° C Hot rolling Thickness> 4 mm Thickness> 4 mm Thickness> 4 mm Cold rolling Thickness <4 mm Thickness <4 mm Thickness <4 mm Dissolution 2 hours at 521 ° C 1 h at 517 ° C 30 minutes 505 ° C temper Cold water Cold water Cold water Controlled traction 1 to 5% permanent deformation 1 to 5% permanent deformation 1 to 5% permanent deformation Returned 14 h at 155 ° C 14 h at 155 ° C 14 h at 155 ° C

For the references G, H, I and J, the precise composition selection allows a complete dissolution while remaining at a dissolution temperature significantly lower than the solidus.

After returning, the samples were cut to the desired size. Table 5 provides the reference of the different samples and their dimensions. <u> Table 5 </ u>: Final dimensions of the samples Sample Thickness [mm] Width [mm] Length [mm] AT 6.0 2,000 3000 B 6.0 2,000 3000 VS 6.3 1,900 4000 D 4.6 2,500 4,500 E # 1 2.0 1,000 2,500 E # 2 3.2 1,000 2,500 E # 3 4.5 1,250 2,500 E # 31 4.5 1,250 2,500 E # 4 6.7 1,250 2,500 F # 1 3.0 1,000 2,500 F # 2 5.0 1,250 2,500 F # 3 6.7 1,250 2,500 G # 1 3.8 2,450 9,600 H # 1 5.0 2,450 9,600 I # 1 5.0 1,500 3000 K # 1 2.0 1,000 2,500

The samples were tested to determine their static mechanical properties as well as their toughness. The yield strength R _p0.2 , the _ultimate strength Rm, and the elongation at break (A) are given in Table 6. <u> Table 6 </ u>: Mechanical properties of the samples Meaning L Meaning of LT Sample Thickness Rm (MPa) R _p0.2 (MPa) AT (%) Rm (MPa) R _p0.2 (MPa) AT (%) AT 6.0 454 367 19.0 448 323 19.3 B 6.0 460 367 20.0 450 325 21.0 VS 6.3 510 450 14.0 506 460 11.5 D 4.6 374 356 12.0 375 339 12.0 E # 1 2.0 532 514 9.9 538 490 10.6 E # 3 4.5 586 570 11.0 568 543 12.0 E # 31 4.5 571 539 10.2 565 522 11.3 E # 4 6.7 560 540 12.0 557 531 11.7 F # 1 3.0 490 469 13.0 512 467 12.5 F # 2 5.0 498 470 12.2 502 453 11.1 F # 3 6.7 514 481 12.2 509 468 11.6 G # 1 3.8 507 470 11.3 494 447 13.8 H # 1 5.0 517 478 11.9 488 444 14.7 I # 1 5.0 493 458 8.7 483 431 11.0 K # 1 2.0 508 481 12.6 496 439 13.0

The static mechanical properties of the samples according to the invention are very high compared to the conventional alloy of the 2XXX range which is tolerant to damage, and of the same order of magnitude as the sample 7475 T76 referenced C. The mechanical strength of the samples according to the invention. The invention considers that the lower copper content and the lower zirconium content of the samples according to the invention have a slight influence on their mechanical strength.

et même supérieure à 130 MPa $\sqrt{\mathrm{m}}$

tandis que pour les échantillons E en alliage 2098 de référence la valeur de K_app dans le sens T-L est inférieure à 110 MPa $\sqrt{\mathrm{m}}$

à part pour l'échantillon E#3 qui a subit une étape spéciale de recuit avant mise en solution. Tableau 7 : Résultats des essais de ténacité T-L (éprouvette de largeur 760 mm) L-T (éprouvette de largeur 760 mm) Echantillon Epaisseur [mm] K_app (MPa $\sqrt{\mathrm{m}}$

) K_eff (MPa $\sqrt{\mathrm{m}}$

) K_app (MPa $\sqrt{\mathrm{m}}$

) K_eff (MPa= $\sqrt{\mathrm{m}}$

) A 6,0 114 160 130 180 B 6,0 140 220 150 236 C 6,3 110 135 150 206 D 4,6 125 178 147 214 E#1 2,0 95 108 114 131 E#2 3,1 104 114 160 200 E#3 4,5 154 174 148 188 E#31 4,5 106 126 143 162 E#4 6,7 103 112 123 143 F#2 5,0 141 171 179 237 F#3 6,7 140 171 155 172 G#1 3,8 162 227 164 213 H#1 5,0 175 277 154 191 I#1 5,0 150 192 K#1 2,0 140 182 158 213 The curves R of certain samples according to the invention and reference samples E are provided on the figures 1 and 2 , for the TL and LT directions, respectively. The figure 1 clearly shows that the crack extension of the last valid point of the curve R (Δa _{eff (max)} ) is much greater for the samples of the invention than for the sample E # 1, E # 3, E # 31 and E # 4. This parameter is at least as critical as the K _app values because, as explained in the description of the prior art, the length of the curve R is an important parameter for the design of the fuselage. The figure 2 shows the same trend, although the LT leadership inherently gives a better result. The curve R of the sample F # 3 could not be measured in the direction LT because the maximum load of the machine was reached. Table 7 summarizes the results of the toughness tests. For the plates according to the invention, the value of K _app in the TL direction is greater than 110 MPa $\sqrt{\mathrm{m}}$

and even greater than 130 MPa $\sqrt{\mathrm{m}}$

while for the reference alloy samples 2098, the value of K _app in the TL direction is less than 110 MPa $\sqrt{\mathrm{m}}$

except for sample E # 3 which has undergone a special annealing step before dissolution. <u> Table 7 </ u>: Results of toughness tests TL (test tube width 760 mm) LT (test specimen width 760 mm) Sample Thickness [mm] K _app (MPa $\sqrt{\mathrm{m}}$

) K _eff (MPa $\sqrt{\mathrm{m}}$

) K _app (MPa $\sqrt{\mathrm{m}}$

) K _eff (MPa = $\sqrt{\mathrm{m}}$

) AT 6.0 114 160 130 180 B 6.0 140 220 150 236 VS 6.3 110 135 150 206 D 4.6 125 178 147 214 E # 1 2.0 95 108 114 131 E # 2 3.1 104 114 160 200 E # 3 4.5 154 174 148 188 E # 31 4.5 106 126 143 162 E # 4 6.7 103 112 123 143 F # 2 5.0 141 171 179 237 F # 3 6.7 140 171 155 172 G # 1 3.8 162 227 164 213 H # 1 5.0 175 277 154 191 I # 1 5.0 150 192 K # 1 2.0 140 182 158 213

) E#1 86 106 E#3 125 161 - - - E#31 97 112 123 E#4 96 F#2 113 141 159 170 178 F#3 104 136 156 168 G#1 115 146 167 184 198 210 221 230 H#1 106 140 166 188 207 225 241 256 I#1 122 147 164 177 188 198 K#1 113 139 156 168 178 186 192 198 K_r (Direction L-T) (MPa= $\sqrt{\mathrm{m}}$

) E#1 96 120 E#3 115 141 159 174 185 E#31 123 152 E#4 102 128 140 F#2 122 159 185 206 225 G#1 123 153 173 189 203 214 224 233 H#1 124 150 168 182 193 203 212 220 K#1 115 149 171 188 201 212 221 228 The results from the curve R are grouped together in Table 8. The crack extension of the last valid point of the curve R is greater for the samples of the invention than for the reference samples. Thus, in the TL direction, all the samples according to the invention reach a crack extension of at least 30 mm and even at least 40 mm while the maximum crack extension is less than 40 mm for the samples of the invention. reference. The inventors consider that several reasons can be proposed to explain this performance, such as the lowest Cu content, and / or the lowest Zr content. <u> Table 8 </ u>: R curve summary data Δa [mm] 10 20 30 40 50 60 70 80 K _r (TL direction) (MPa = $\sqrt{\mathrm{m}}$

) E # 1 86 106 E # 3 125 161 - - - E # 31 97 112 123 E # 4 96 F # 2 113 141 159 170 178 F # 3 104 136 156 168 G # 1 115 146 167 184 198 210 221 230 H # 1 106 140 166 188 207 225 241 256 I # 1 122 147 164 177 188 198 K # 1 113 139 156 168 178 186 192 198 K _r (LT direction) (MPa = $\sqrt{\mathrm{m}}$

) E # 1 96 120 E # 3 115 141 159 174 185 E # 31 123 152 E # 4 102 128 140 F # 2 122 159 185 206 225 G # 1 123 153 173 189 203 214 224 233 H # 1 124 150 168 182 193 203 212 220 K # 1 115 149 171 188 201 212 221 228

The figures 3 and 4 show the evolution of the cracking rate da / dN (in mm / cycle) in the TL and LT orientation, respectively, for different levels of stress intensity factor (ΔK). The width of the sample was 400 mm (CCT 400 specimen) and R = 0.1. No major difference is observed between the samples E and F. The cracking rate of the sample F is in the same range as that typically obtained for alloy 2056 (Sample B) and lower than that obtained for alloy 6156 (Sample D).

The intergranular corrosion resistance was tested according to ASTM G110. For all the samples according to the invention, no intergranular corrosion was detected. No intergranular corrosion was either detected on the 2098 alloy reference samples (E # 1 to E # 4). For Sample B (for which plating had been removed), intergranular corrosion with an average depth of 120 μm was observed and for Sample D (for which plating had been removed) intergranular corrosion was observed. with an average depth of 180 μm. The resistance to intergranular corrosion was thus very high for the samples according to the invention.

Example 2

In this example, the influence of tensile strain rate was studied on laboratory scale samples. Six samples from casting H and converted into sheets 5 mm thick according to the conditions described in Table 4 were deformed by controlled traction with a permanent deformation rate of between 1 and 6% and then had an income of 18h at 155 ° C. The samples were tested to determine their static mechanical properties as well as their toughness. The yield strength R _p0.2 , the _ultimate strength Rm, and the elongation at break (A) are given in Table 9. <u> Table 9 </ u>: Mechanical properties of laboratory samples with different rates of permanent deformation Meaning L Meaning of LT Sample Permanent deformation rate (%) Rm (MPa) R _p0.2 (MPa) AT (%) Rm (MPa) R _p0.2 (MPa) AT (%) H # 11 1 495 436 11.2 469 411 15.1 H # 12 2 515 469 11.1 489 444 13.5 H # 13 3 529 493 10.5 501 464 13.8 H # 14 4 534 501 10.8 501 465 14.2 H # 15 5 542 514 10.8 511 481 13.8 H # 16 6 550 524 10.4 516 485 13.9

Static mechanical characteristics increase with the rate of permanent deformation during controlled traction. Most of the increase is achieved for a permanent deformation rate of 3%. Thus, the increase in R _m (L) is 7% for an increase in the permanent deformation rate of 1 to 3% while it is only 3% for an increase of 4 to 6%. The tenacity was evaluated by the so-called Kahn test method, the results are given in Table 10. <u> Table 10 </ u>: Results of the Kahn test performed on laboratory samples with different rates of permanent deformation Kahn test Sample Permanent deformation rate (%) E _g (J) H # 11 1 30.5 H # 12 2 29.2 H # 13 3 27.8 H # 14 4 25.1 H # 15 5 25.0 H # 16 6 20.6

The relationship between the fracture overall energy E _g and the toughness is direct although the values of E _g can not be used to predict the results of the R curve of large samples because of the different geometries of the tests. It can be noted that E _g slowly decreases to a permanent deformation of 5% and decreases more abruptly for a permanent deformation of 6%.

Example 3

In this example, the influence of the rate of permanent deformation obtained by controlled traction was studied on industrial samples. Two samples from casting J and transformed into sheets of thickness 5 mm according to the conditions indicated in Table 4 have been planed and have undergone a controlled traction with a permanent deformation rate of 1.8% and 3.4%. The samples were tested to determine their static mechanical properties as well as their toughness. The yield strength R _p0.2 , the breaking strength Rm, and the elongation at break (A) are given in Table 11. <u> Table 11 </ u>: Mechanical properties of industrial samples with different rates of permanent deformation Meaning L Meaning of LT Sample Permanent deformation rate (%) Rm (MPa) R _p0.2 (MPa) AT (%) Rm (MPa) R _p0.2 (MPa) AT (%) J # 11 1.8 510.0 465.0 13.1 494.7 444.3 14.5 J # 12 3.4 534.0 499.0 10.7 515.0 475.0 13.7

The curves R obtained for the two samples in the direction TL are presented on the figure 5 . Table 12 summarizes the results of the R-curves. The sample having undergone permanent deformation of 1.8% has a lower mechanical resistance than that of the sample having undergone a permanent deformation of 3.4%. In addition, a very high tenacity was observed for both samples. <u> Table 12 </ u>: Results of toughness tests carried out on industrial samples with different rates of permanent deformation. Ilon Permanent deformation rate (%) TL (760 mm wide specimen) K _app (MPa√m) K _eff (MPa√m) K _r (MPa√m) Δa [mm] 10 20 30 40 50 60 70 80 J # 11 1.8 140 220 118 152 177 198 216 232 246 260 J # 12 3.4 179 259 135 160 181 199 217 234 250 263

Example 4

In this example, the mechanical strength of welded joints between sheets of the invention or between reference sheets has been evaluated. 3.2 mm thick sheets from D (6156), E and I were soldered by friction stir welding. The welding was done on an MTS ISTIR ^® machine. The welding parameters were chosen on the basis of tests carried out during a preliminary study. Parameter selection was performed based on the results of microstructural observations and folding tests. For the sheets from the flows E and I, the assemblies were made with a rotational speed of the tool of 800 rpm (revolutions per minute) and a welding speed of 300 mm / min. For sheets coming from casting D, the assemblies were made with a rotation speed of the tool of 510 rpm (revolutions per minute) and a welding speed of 900 mm / min.

The income was made either before or after assembly by friction stir welding. The results are given in Table 13. The performance of the welded joints obtained with the sheets according to the invention was particularly satisfactory for two aspects. First, the joint efficiency coefficient, which is the ratio between the breaking strength of the welded joint and that of the non-welded sheet, is greater than 70% and even greater than 75% for the sheets of the invention. This coefficient reaches 80% in some cases. This result is better than that obtained with sheets from casting E. Secondly, the results are little influenced by the position of the stage of income (before or after welding), which allows a flexible process. On the contrary, for the sheets obtained from the casting D (6156), a significant influence of the position of the income stage is observed. <u> Table 13 </ u>: Mechanical properties of welded joints cast Position of the income stage Mechanical strength of the joint R _m of reference for non-welded sheets (MPa) Coefficient of effectiveness of joint (%) Rm (MPa) R _p0.2 (MPa) AT (%) D Before welding 264 200 2.8 372 71 D After welding 318 292 1.8 372 86 E Before welding 386 269 4.9 543 71 E After welding 413 309 5.6 543 76 F Before welding 385 309 5.2 483 80 F After welding 377 279 5.9 483 78

Example 5

In this example, the influence of Zr and Mn content on static mechanical characteristics and toughness was evaluated.

Two alloys were cast and transformed into 6 mm thick sheets according to the conditions indicated for samples G, H and I of Table 4. The compositions of these alloys are given in Table 14. <u> Table 14 </ u>: Chemical composition (% by weight) of alloys containing Mn Casting reference Yes Fe Cu mn mg Zr Li Ag Ti The 0.03 0.05 3.3 0.31 0.32 0.05 0.99 0.32 0.02 M 0.03 0.05 3.3 0.30 0.33 0.11 0.98 0.35 0.02

The samples were tested to determine their static mechanical properties as well as their toughness. The yield strength R _p0.2 , the breaking strength Rm, and the elongation at break (A) are given in Table 15 and the results of the toughness tests in Table 16. <u> Table 15 </ u>: Mechanical properties of samples from alloys containing Mn Meaning L Meaning of LT Sample Thickness Rm (MPa) R _p0.2 (MPa) AT (%) Rm (MPa) R _p0.2 (MPa) AT (%) The 6.0 479 447 13.5 477 419 7.8 M 6.0 494 464 13.7 493 448 13.1 Results of toughness tests for alloys containing Mn Sample Thickness (mm) TL (760 mm wide specimen) K _app (MPa√m) K _eff (MPa√m) K _r (MPa√m) Δa [mm] 10 20 30 40 50 60 70 80 The 6.0 140 174 111 137 155 168 178 187 194 200 M 6.0 158 198 123 152 171 186 199 209 219 227

Samples L and M reach the mechanical characteristics according to the invention in the T8 state. Furthermore, the static strength and toughness performances are lower for the sample L, which contains Mn and a low Zr content, than for the other examples according to the invention. The inventors believe that the lower performance of the sample L is related to a less favorable microstructure characterized in particular by the presence of recrystallized zones and non-recrystallized zones (mixed microstructure).

Claims (23)

1. A method for producing an aluminum alloy sheet having high fracture toughness and strength, said method comprising:

   a) elaborating a liquid metal bath comprising 3.0 to 3.4 wt.% Cu, 0.8 to 1.2 wt.% Li, 0.2 to 0.5 wt.% Ag, 0.2 to 0.6 wt.% Mg and at least one element selected from the group consisting of 0.05 to 0.13 wt.% Zr, 0.05 to 0.8 wt.% Mn, 0.05 to 0.3 wt.% Cr, 0.05 to 0.3 wt% Sc, 0.05 to 0.5 wt.% Hf and 0.05 to 0.15 wt.% Ti, remainder aluminum and unavoidable impurities,
   with the additional proviso that the amount of Cu and Li is such that Cu(wt.%) + 5/3 Li (wt.%) < 5.2;

   b) casting an ingot from said liquid metal bath

   c) homogenizing said ingot at 490-530°C for a duration from 5 and 60 hours;

   d) rolling said ingot to a sheet with a final thickness from 0.8 to 12 mm;

   e) solution heat treating and quenching said sheet ;

   f) stretching said sheet with a permanent set from 1 to 5%;

   g) aging said sheet by heating at 140-170°C for 5 to 30 hours.
2. A method according to claim 1 wherein said final thickness is from 2 to 12 mm.
3. A method according to claim 1 or claim 2 wherein said liquid metal bath comprises from 3.1 to 3.3 wt.% copper.
4. A method according to anyone of claims 1 to 3 wherein said liquid metal bath comprises from 0.9 to 1.1 wt.% lithium.
5. A method according to anyone of claims 1 to 4 wherein said liquid metal bath comprises from 0.2 to 0.4 wt.% silver.
6. A method according to anyone of claims 1 to 5 wherein said liquid metal bath comprises less than 0.4 wt.% magnesium.
7. A method according to anyone of claims 1 to 6 wherein said liquid metal bath comprises from 0.05 to 0.13 wt.% zirconium and from 0.02 to 0.3% scandium.
8. A method according to anyone of claims 1 to 7 wherein said liquid metal bath comprises from 0.09 to 0.13 wt.% zirconium
9. A method according to anyone of claims 1 to 8 wherein said liquid metal bath comprises less than 0.05 wt.% Mn.
10. A method according to anyone of claims 1 to 9 wherein the total cold working deformation after quenching is from 2.5 to 4%.
11. A method according to anyone of claims 1 to 10 wherein said stretching permanent set is from 2.5 to 4 %.
12. A method according to anyone of claims 1 to 11 wherein said aging comprises heating at 140-155°C for 10 to 30 hours.
13. A method for producing an aluminum alloy sheet according to claim 1 comprising:

    a) elaborating a liquid metal bath comprising 3.0 to 3.4 wt.% Cu, 0.8 to 1.2 wt.% Li, 0.2 to 0.5 wt.% Ag, 0.2 to 0.6 wt.% Mg and at least one element selected from the group consisting of 0.09 to 0.13 wt.% Zr, 0.05 to 0.8 wt.% Mn, 0.05 to 0.3 wt.% Cr 0.05 to 0.3 wt% Sc, 0.05 to 0.5 wt.% Hf and 0.05 to 0.15 wt.% Ti, remainder aluminum and unavoidable impurities,
    with the additional proviso that the amount of Cu and Li is such that Cu(wt.%) + 5/3 Li (wt.%) < 5.0;

    b) casting an ingot from said liquid metal bath;

    c) homogenizing said ingot at 490-530°C for a duration from 5 to 60 hours;

    d) rolling said ingot to a 2 to 9 mm final gauge sheet ;

    e) solution heat treating said sheet at a temperature from 490 to 530°C for a duration from 15 mn to 2 hours, followed by quenching;

    f) stretching said sheet with a permanent set from 2.5 to 4%;

    g) aging said sheet by heating at 140-155°C for 10 to 30 hours.
14. A rolled, forged and/or extruded product made from an aluminum alloy comprising 3.0 to 3.4 wt.% Cu, 0.8 to 1.2 wt.% Li, 0.2 to 0.5 wt.% Ag, 0.2 to 0.6 wt. Mg and at least one element selected from the group consisting of 0.05 to 0.13 wt.% Zr, 0.05 to 0.8 wt.% Mn, 0.05 to 0.3 wt.% Cr, 0.05 to 0.3 wt% Sc, 0.05 to 0.5 wt.% Hf and 0.05 to 0.15 wt.% Ti,
    remainder aluminum and unavoidable impurities,.
    with the additional proviso that the amount of Cu and Li is such that Cu(wt.%) + 5/3 Li (wt.%) < 5.2.
15. A rolled, forged and/or extruded product according to claim 14, with a thickness from 0.8 to 12 mm and preferably from 2 to 12 mm.
16. A product according to claim 14 or to claim 15 wherein the Zr content, if it is chosen, is higher than 0.09 wt.% and wherein the amount of Cu and Li is such that Cu(wt.%) + 5/3 Li (wt.%) < 5.0
17. An aluminum alloy sheet produced by the method of anyone of claims 1 to 13 comprising in a T8 temper

    (a) a conventional yield strength measured at 0.2 % elongation in the L direction of at least 440 MPa, and

    (b) a toughness K_app, measured on CCT760 (2ao = 253 mm) specimens, of at least 110 MPa√m in the T-L direction, and

    (c) a crack extension of the last valid point of the R-curve Δa_eff(max) in the T-L direction of at least 30 mm.
18. An aluminum alloy sheet produced by the method of anyone of claims 1 to 13 comprising in a T8 temper

    (a) a conventional yield strength measured at 0.2 % elongation in the L direction of at least 460 MPa, and

    (b) a toughness K_app, measured on CCT760 (2ao = 253 mm) specimens, of at least 130 MPa√m in the T-L direction, and

    (c) a crack extension of the last valid point of the R-curve Δa_eff(max) in the T-L direction of at least 40 mm.
19. A structural member comprising at least one product according to anyone of claims 14 to 18 or made of such a product.
20. A structural member according to claim 19 wherein said structural member is an aircraft fuselage panel.
21. A structural member according to claim 19 with reference to claims 14 to 16 wherein said structural member is a stringer.
22. A structural member according to anyone of claims 19 to 21 comprising a welded construction wherein the joint efficiency coefficient is at least 70 %.
23. A structural member of claim 22 wherein said welded construction is welded by friction stir welding.

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