EP2766503B1 - Improved method for processing sheet metal made of an al-cu-li alloy - 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

Aluminum alloy rolled products are developed to produce high strength parts for the aerospace industry and the aerospace industry in particular.

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. The improvement of the compromise between mechanical resistance and damage tolerance is constantly sought.

Another important property of thin sheets of Al-Cu-Li alloy, particularly those whose thickness is between 0.5 and 12 mm, is the fitness to form. These sheets are used in particular to manufacture aircraft fuselage elements or rocket elements that have a complex overall shape in 3 dimensions. To reduce the cost of manufacture, aircraft manufacturers seek to minimize the number of sheet forming steps, and to use sheets that can be manufactured inexpensively to using short transformation ranges, ie with as few individual steps as possible.

For the manufacture of the fuselage panels, there are currently several possible successions of the transformation steps, which depend in particular on the deformation required during the shaping. For small deformations during shaping, typically less than 4%, it is possible to supply the sheets in a hardened state matured (state "T3" little hardened or "T4"), and to shape the sheets in this state. However, in most cases, the deformation sought is locally at least 5% or 6%. A current practice of aeronautical manufacturers is generally to supply hot-rolled or cold-rolled sheets according to the required thickness, in the raw state of manufacture (state "F" according to EN 515) in the matured tempered state. (state "T3" or "T4"), see in the annealed state ("O" state), subject them to a solution heat treatment followed by quenching and then to form them on fresh quenching (state "W"), before finally subjecting them to natural or artificial aging, so as to obtain the required mechanical characteristics. Generally, after dissolution and quenching, the sheets are in a state characterized by good formability, but this state is unstable (state "W"), and the shaping must occur on fresh quenching, c that is to say within a short time after quenching, of the order of a few tens of minutes to a few hours. If this is not possible for reasons of production management, the sheet must be stored in a cold room at a sufficiently low temperature and for a sufficiently short duration so as to avoid natural ripening. In some cases, it is found that for too short durations after dissolution Lüders lines appear after formatting, which imposes an additional constraint with a minimum waiting time. For large and strongly formed parts, this solution heat treatment requires large furnaces, which makes the operation inconvenient, including with respect to the same operation performed on flat sheet. The possible need for a cold room adds to the costs and disadvantages of the state of the art. In addition, after quenching the sheet may be deformed and cause problems related to this deformation, for example when it comes to the position in the jaws of the drawing-forming tool. For strongly formed parts, this operation must possibly be repeated, if the material does not present, in the metallurgical state in which it is, sufficient formability to achieve the desired form in a single operation.

In another current practice, one starts from a sheet in the state O, see in the state T3, T4 or in the state F, one carries out a first operation of formatting from this state, and a second shaping after dissolution and quenching. This variant is used in particular when the targeted shaping is too important to be carried out in a single operation from a state W, but can however be carried out in two passes from the state O. In addition the plates in the state O being stable in time are easier to transform. However, the manufacture of the sheet in the O state involves a final annealing of the raw rolling sheet, and therefore generally an additional manufacturing step, and also a dissolution and quenching of the product formed which is contrary the aim of simplification aimed at by the present invention.

The shaping of complex structural elements in the T8 state is limited to mild shaping cases because the elongation and the ratio R _m / R _p0 , ₂ are too low in this state. It should be noted that the properties that are optimal in terms of compromise of properties must be obtained once the part has been shaped, in particular as a fuselage element, since it is the shaped part which must in particular have good performances. in damage tolerance to avoid too frequent repair of fuselage elements. It is generally accepted that the large deformations after dissolution and quenching lead to an increase in the mechanical strength but a strong degradation of the tenacity.

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 5,455,003 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 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.

The patent application US 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. This application also describes a process for manufacturing spun products.

The EP Patent 1,966,402 discloses an alloy containing no zirconium for fuselage plates of substantially recrystallized structure comprising (in% by weight) (2.1-2.8) Cu, (1.1-1.7) Li, (0, 2-0.6) Mg, (0.1-0.8) Ag, (0.2-0.6) Mn. The products obtained in the T8 state are not suitable for shaping, with in particular a ratio R _{m /} / R _{p0.2 of} less than 1.2 in the directions L and LT.

The EP Patent 1,891,247 discloses an alloy for fuselage plates comprising (in% by weight) (3.0-3.4) Cu, (0.8-1.2) Li, (0.2-0.6) Mg, ( 0.2-0.5) Ag and at least one of Zr, Mn, Cr, Sc, Hf and Ti, wherein the Cu and Li contents are Cu + 5/3 Li <5.2 . The products obtained in the T8 state are not suitable for shaping, in particular with a ratio R _{m /} / R _{p0.2 of} less than 1.2 in the directions L and LT. It has also been found that the overall fracture energy measured by Kahn test which is related to the toughness decreases with the deformation and more brutally for a deformation of 6%, which poses the problem of obtaining a high tenacity whatever the rate of local deformation during the shaping.

The EP Patent 1045043 describes the process for manufacturing parts formed from AA2024 type alloy, and in particular of highly deformed parts, by the combination of an optimized chemical composition and particular manufacturing processes, making it possible to avoid as much as possible the dissolution in solution on formed sheet.

In the article Al - (4.5-6.3) Cu - 1.3Li - 0.4Ag - 0.4Mg - 0.14Zr Alloy Weldalite 049 from Pickens, JR; Heubaum, FH; Langan, TJ; Kramer, LS published in Aluminum - Lithium Alloys. Flight. III; Williamsburg, Virginia; USA; 27-31 Mar. 1989. (March 27, 1989) ), different heat treatments are described for these alloys with high copper content. WO2006 / 131627 discloses a method of manufacturing an aluminum alloy sheet for the aerospace industry and suitable for use in fuselage applications, the sheet having a high toughness and mechanical strength, wherein: a) produces a bath of liquid metal comprising 2.7 to 3.4% by weight of Cu, 0.8 to 1.4% by weight of Li, 0.1 to 0.8% 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, if it is chosen, being from 0.05 to 0.13% by weight; 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 at 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 Cu (% by weight) + 5/3 Li (% by weight) <5.2; b) pouring a plate from said bath of liquid metal c) said plate is homogenized at a temperature between 490 and 530 ° C for a period of 5 to 60 hours; d) laminating said plate into a sheet having a final thickness of between 0.8 and 12 mm; e) dissolving and quenching said sheet; f) Controllably pulling said sheet with a permanent deformation of 1 to 5%; g) an income of said sheet is obtained by heating at 140 to 170 ° C for 5 to 30 hours. There is a need for aluminum-copper-lithium alloy rolled products having improved properties over those of known products, particularly in terms of the compromise between static strength properties and damage tolerance properties even after a high level of deformation during shaping, while having a low density.

In addition there is a need for a simplified manufacturing process for shaping these products to obtain fuselage elements in particular economically, while obtaining satisfactory mechanical characteristics.

Object of the invention

1. a) on élabore un bain de métal liquide à base d'aluminium comprenant 2,1 à 3,9 % en poids de Cu, 0,7 à 2.0 % en poids de Li, 0,1 à 1,0 % en poids de Mg, 0 à 0,6 % en poids d'Ag, 0 à 1% % en poids de Zn, au plus 0,20 % en poids de Fe + Si, au moins un élément choisi parmi Zr, Mn, Cr, Sc, Hf et Ti, la quantité dudit élément, s'il est choisi, étant 0,05 à 0,18 % en poids pour Zr, 0,1 à 0,6% en poids pour Mn, 0,05 à 0,3 % en poids pour Cr, 0,02 à 0,2 % en poids 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 plaque de laminage à partir dudit bain de métal liquide ;
3. c) optionnellement, on homogénéise ladite plaque de laminage ;
4. d) on lamine à chaud et optionnellement à froid ladite plaque de laminage en une tôle,
5. e) on met en solution ladite tôle et on la trempe;
6. f) on réalise un planage et/ou on tractionne de façon contrôlée ladite tôle avec une déformation cumulée d'au moins 0,5% et inférieure à 3%,
7. g) on réalise un traitement thermique court dans lequel ladite tôle atteint une température comprise entre 130 et 170°C et de préférence entre 150 et 160°C pendant 0,1 à 13 heures et de préférence de 1 à 5 h.

A first object of the invention is a process for manufacturing a laminated product based on aluminum alloy, in particular for the aeronautical industry in which, successively,

1. a) an aluminum-based liquid metal bath comprising 2.1 to 3.9% by weight Cu, 0.7 to 2.0% by weight Li, 0.1 to 1.0% by weight of Mg, 0 to 0.6% by weight of Ag, 0 to 1% by weight of Zn, at most 0.20% by weight of Fe + Si, at least one element selected from Zr, Mn, Cr, Sc , Hf and Ti, the amount of said element, if selected, being 0.05 to 0.18% by weight for Zr, 0.1 to 0.6% by weight for Mn, 0.05 to 0.3% by weight for Cr, 0, 0.2 to 0.2% by weight for Sc, 0.05 to 0.5% by weight for Hf and 0.01 to 0.15% by weight for Ti, the others elements of not more than 0.05% by weight each and 0.15% by weight in total, the balance aluminum;
2. b) casting a rolling plate from said bath of liquid metal;
3. c) optionally, homogenizing said rolling plate;
4. d) the said rolling plate is hot-rolled and optionally cold-rolled into a sheet,
5. e) said sheet is dissolved and quenched;
6. f) planing is carried out and / or the sheet is controlledly tensile with a cumulative deformation of at least 0.5% and less than 3%,
7. g) a short heat treatment is carried out in which said sheet reaches a temperature between 130 and 170 ° C and preferably between 150 and 160 ° C for 0.1 to 13 hours and preferably 1 to 5 hours.

A second object of the invention is a laminated product obtainable by a process according to the invention, having between 0 and 50 days after short thermal treatment, a combination of at least one property chosen from R _{p, 2} ( L) of at least 220 MPa and preferably at least 250 MPa, R _p0.2 (LT) of at least 200 MPa and preferably at least 230 MPa, R _m (L) of at least 340 MPa and preferably at least 380 MPa, R _m (LT) of at least 320 MPa and preferably at least 360 MPa with a property selected from A% (L) at least 14% and preferably at least minus 15%, A% (LT) at least 24% and preferably at least 26%, R _m / R _p0.2 (L) at least 1.40 and preferably at least 1.45, R _m / R _{p0 , 2} (LT) at least 1.45 and preferably at least 1.50.

Another subject of the invention is a product that can be obtained by a process according to the invention, having a yield strength R _p0.2 (L) at least substantially equal and a toughness K _R greater than preferably at least 5%, to those obtained by a similar process not including short heat treatment.

Yet another object of the invention is the use of a product that can be obtained by a method according to the invention for the manufacture of an aircraft fuselage skin.

Description of figures

• Figure 1 R curves in the direction TL obtained on the samples of Example 1
• Figure 2 R _m / R _{ratio P0.2} in the LT direction at the end of the short thermal treatment as a function of time equivalent to 150 ° C for treatment temperatures of 145 ° C, 150 ° C and 155 ° C, as described in example 3.

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 definitions of the metallurgical states are given in the European standard EN 515.
The static mechanical characteristics in tension, in other words the tensile strength R _m , 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 standard NF EN ISO 6892-1, the sampling and the direction of the test being defined by the EN 485-1 standard. The plane stress toughness is determined by a curve of the stress intensity factor as a function of crack extension, known as the R curve, according to ASTM E 561. The critical stress intensity factor K _C in other words, the intensity factor which 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 to the critical load, at the beginning of the monotonous charge. 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.

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), the wings (such as upper or lower wing skin, stringers or stiffeners), ribs and spars) and the composite empennage including horizontal and vertical stabilizers (horizontal or vertical stabilizers), as well as floor beams, seat tracks and doors.

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 423 K. t_i 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.
De manière surprenante, les présents inventeurs ont constaté que les propriétés mécaniques obtenues à l'issue du traitement thermique court sont stables dans le temps, ce qui permet d'utiliser les tôles dans l'état obtenu à l'issue du traitement thermique court à la place de tôle à l'état O ou l'état W pour la mise en forme.According to the invention, after rolling in the form of sheet metal, solution, quenching and leveling and / or pulling is carried out at least one short heat treatment with a duration and a temperature such that the sheet reaches a temperature of between 130 and 170 ° C. C and preferably between 150 and 160 ° C for 0.1 to 13 hours, preferably 0.5 to 9 hours and preferably 1 to 5 hours. Typically, following this short heat treatment, the yield strength R _p0,2 decreases significantly, that is to say at least 20 MPa or even more, whereas the elongation A% is increased it is to say that it is multiplied by a factor of at least 1.1, or even of at least 1.2, or at least 1.3, with respect to the state obtained without short heat treatment, typically T3 or T4. The short heat treatment is therefore not an income with which one would obtain a state T8 but a particular heat treatment which makes it possible to obtain a non-standardized state particularly suitable for shaping. Indeed, a sheet in the T8 state has a yield strength greater than that of a T3 or T4 state while after the short heat treatment according to the invention the elastic limit is on the contrary lower than that of a T3 or T4 state. Advantageously, the short heat treatment is carried out so as to obtain a time equivalent to 150 ° C. from 0.5 h to 6 h and preferably from 1 h to 4 h and preferably from 1 h to 3 h, the equivalent time t _i to 150 h. ° 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 423 K. t _i is expressed in hours, the constant Q / R = 16400 K is derived from the activation energy for Cu diffusion, for which Q = 136100 J / mol was used.
Surprisingly, the present inventors have found that the mechanical properties obtained at the end of the short heat treatment are stable over time, which makes it possible to use the sheets in the state obtained at the end of the short heat treatment. the sheet metal place in the state O or the state W for the shaping.

The present inventors have found that, surprisingly, not only the short heat treatment makes it possible to simplify the manufacturing process of the products by eliminating the shaping on state O or W, but moreover that the compromise between static mechanical resistance and tolerance to damage is at least the same or even improved by the method of the invention in the returned state compared to a method not comprising short heat treatment. In particular for a further cold deformation of at least 5% after short heat treatment, the compromise obtained between static mechanical strength and toughness is improved compared with the state of the art.

The advantage of the process according to the invention is obtained for products having a copper content of between 2.1 and 3.9% by weight. In an advantageous embodiment of the invention, the copper content is at least 2.8% or 3% by weight. A maximum copper content of 3.7 or 3.5% by weight is preferred.
The lithium content is between 0.7% or 0.8% and 2.0% by weight. Advantageously, the lithium content is at least 0.85% by weight. A maximum lithium content of 1.6 or even 1.2% by weight is preferred.

The magnesium content is between 0.1% and 1.0% by weight. Preferably, the magnesium content is at least 0.2% or even 0.25% by weight. In one embodiment of the invention, the maximum magnesium content is 0.6% by weight.
The silver content is between 0% and 0.6% by weight. In an advantageous embodiment of the invention, the silver content is between 0.1 and 0.5% by weight and preferably between 0.15 and 0.4% by weight. The addition of silver contributes to improving the compromise of mechanical properties of the products obtained by the process according to the invention.

The zinc content is between 0% and 1% by weight. Zinc is generally an undesirable impurity, especially because of its contribution to the density of the alloy, however in some cases zinc may be used alone or in combination with silver. Preferably, the zinc content is less than 0.40% by weight, preferably less than 0.2% by weight. In one embodiment of the invention, the zinc content is less than 0.04% by weight. .

The alloy also contains at least one element that can contribute to controlling the grain size selected from Zr, Mn, Cr, Sc, Hf and Ti, the amount of the element, if selected, being 0.05 to 0.18% by weight for Zr, 0.1 to 0.6% by weight for Mn, 0.05 to 0.3% by weight for Cr, 0.02 to 0.2% by weight for Sc, O 0.5 to 0.5% by weight for Hf and 0.01 to 0.15% by weight for Ti. Preferably, it is preferred to add between 0.08 and 0.15% by weight of zirconium and between 0.01 and 0.10% by weight of titanium and the content of Mn, Cr, Sc and Hf to be limited to maximum 0.05% by weight, these elements may have an adverse effect, especially on the density and being added only to further promote the obtaining of a substantially non-recrystallized structure if necessary.
In an advantageous embodiment of the invention, the zirconium content is at least 0.11% by weight.
In another embodiment of the invention, the manganese content is between 0.2 and 0.4% by weight and the zirconium content is less than 0.04% 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 other elements have a content of at most 0.05% by weight each and 0.15% by weight in total, it is inevitable impurities, the rest is aluminum.

The manufacturing method according to the invention comprises the stages of production, casting, rolling, dissolution, quenching, planing and / or pulling and short heat treatment. 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 as a rolling plate.
The rolling plate can then optionally be homogenized so as to reach a temperature between 450 ° C and 550 ° and preferably between 480 ° C and 530 ° C for a period of between 5 and 60 hours. The homogenization treatment can be carried out in one or more stages.

The rolling plate is then hot-rolled and optionally cold-rolled into a sheet. Advantageously, the thickness of said sheet is between 0.5 and 15 mm and preferably between 1 and 8 mm.

The product thus obtained is then put in solution typically by a heat treatment making it possible to reach a temperature of between 490 and 530 ° C. for 15 minutes to 8 hours, and then typically quenched with water at room temperature or, preferably, with water. Cold water.

Then planing is carried out and / or controlled traction said sheet with a cumulative deformation of at least 0.5% and less than 3%. When planing is performed, the deformation performed during planing is not always known precisely but it is estimated at about 0.5%. When it is performed, the controlled traction is implemented with a permanent deformation of between 0.5 to 2.5% and preferably between 0.5 to 1.5%. The combination of controlled traction with a preferred permanent deformation and short heat treatment achieves optimum results in terms of formability and mechanical properties, especially when additional shaping and income are achieved.

The product then undergoes a short heat treatment already described.
After the short heat treatment, the sheet obtained by the process according to the invention preferably has, between 0 and 50 days and preferably between 0 and 200 days after short heat treatment, a combination of at least one property. selected from R _p0.2 (L) of at least 220 MPa and preferably at least 250 MPa, R _p0.2 (LT) of at least 200 MPa and preferably at least 230 MPa, R _m (L) of at least 340 MPa and preferably at least 380 MPa, R _m (LT) of at least 320 MPa and preferably at least 360 MPa with a property selected from A% (L) at less than 14% and preferably at least 15%, A% (LT) at least 24% and preferably at least 26%, R _m / R _p0.2 (L) at least 1.40 and preferably at least 1, 45, R _m / R _p0.2 (LT) at least 1.45 and preferably at least 1.50.

In an advantageous embodiment of the invention after the short heat treatment, the sheet obtained by the process according to the invention has a ratio R _m / R _p0,2 in the direction LT of at least 1.52. or 1.53.

Advantageously, between 0 and 50 days and preferably between 0 and 200 days after the short heat treatment, the sheet obtained by the process according to the invention has a yield strength R _p0.2 (L) of less than 290 MPa and of preferably less than 280 MPa and R _p0.2 (LT) less than 270 MPa and preferably less than 260 MPa.

At the end of the short heat treatment, the sheet is thus ready for additional cold deformation, in particular a 3-dimensional shaping operation. An advantage of the invention is that this additional deformation can locally or generally reach values of 6 to 8% or even up to 10%. To achieve sufficient mechanical properties at the end of the income at the T8 state, a minimum cumulative deformation of 2% between said additional deformation and cumulative deformation by planing and / or controlled traction performed before the short heat treatment is advantageous. Preferably, the additional cold deformation is locally or generally at least 1%, preferably at least 4% and preferably at least 6%.

Finally, an income is produced in which said sheet reaches a temperature between 130 and 170 ° C and preferably between 150 and 160 ° C for 5 to 100 hours and preferably 10 to 70h. The income can be achieved in one or more levels.
Advantageously, the cold deformation is performed by one or more forming processes such as stretching, stretch-forming, stamping, spinning or folding. In an advantageous embodiment, it is a shaping in the three dimensions of the space to obtain a piece of complex shape, preferably by stretch-forming. Thus the product obtained after the short heat treatment can be shaped as a product in the state O or a product in the state W. However, compared to a product in the state O it has the advantage of no longer require dissolution and quenching to achieve the final mechanical properties, a simple income treatment is sufficient. Compared to a product in the W state, it has the advantage of being stable and not requiring a cold room and not to cause problems related to the deformation of this state. The product also has the advantage in general of not generating lines Lüders crippling during formatting. Thus it is possible, for example, to perform the short heat treatment at the sheet metal manufacturer and the shaping at the aeronautical structure manufacturer, directly on the delivered product. The method according to the invention makes it possible to carry out the 3-dimensional shaping of a sheet at the end of the short heat treatment without the sheet being in a state T8, a state O or a state W before this setting shaped in 3 dimensions.

et de préférence d'au moins $\mathrm{170}\mathrm{MPa}\sqrt{\mathrm{m}}$

et/ou K_eff dans le sens T-L d'au moins $200\mathrm{MPa}\sqrt{\mathrm{m}}$

et de préférence d'au moins $220\mathrm{MPa}\sqrt{\mathrm{m}}$

et/ou Δa_eff(max) dans le sens T-L d'au moins 40 mm et de préférence d'au moins 50 mm.Surprisingly, the compromise between the static mechanical properties and the damage-tolerance properties obtained at the end of the income is advantageous compared to that obtained for a similar treatment that does not include short heat treatment. In particular, the inventors have found that the mechanical strength, in particular the tensile yield strength R _p0.2 (L) is high and increases with the additional deformation, but that, contrary to their expectation, the tenacity measured by the curve R ( values of K _R ) does not decrease significantly, in particular up to a crack extension value of 60 mm when increasing the additional deformation, even up to a generalized deformation of 8%. Advantageously, the product that can be obtained by the process comprising the additional deformation and _tempering steps has a tensile yield strength R _p0.2 (L) of at least substantially equal and a higher toughness K _R , preferably of at least 5%, to that obtained by a similar process not including short heat treatment. Typically, the tensile yield strength R _p0.2 (L) is at least 90% or preferably 95% of that obtained by a similar method not comprising short heat treatment. The process according to the invention makes it possible to obtain, in particular, an alloy sheet AA2198 whose thickness is between 0.5 and 15 mm and preferably between 1 and 8 mm having, after thermal treatment of tempering in the T8 state, a combination of at least one static strength property selected from R _p0,2 (L) of at least 500 MPa and preferably at least 510 MPa and / or R _p0,2 (LT) of at least 480 MPa and preferably at least 490 MPa, and at least one toughness property measured on specimens of the CCT760 type (with 2ao = 253 mm) selected from K _app in the TL direction of at least $\mathrm{160}\mathrm{MPa}\sqrt{\mathrm{m}}$

and preferably at least $\mathrm{170}\mathrm{MPa}\sqrt{\mathrm{m}}$

and / or K _eff in the TL direction of at least $200\mathrm{MPa}\sqrt{\mathrm{m}}$

and preferably at least $220\mathrm{MPa}\sqrt{\mathrm{m}}$

and / or Δa _{eff (max)} in the TL direction of at least 40 mm and preferably at least 50 mm.

Thus the products that can be obtained by the process according to the invention are particularly advantageous.

The use of a product that can be obtained by the process according to the invention, comprising the steps of short heat treatment, cold deformation and tempering for the manufacture of a structural element for an aircraft, in particular a skin of fuselage is particularly advantageous ..

Example Example 1

An AA2198 alloy rolling plate was homogenized and then hot rolled to a thickness of 4 mm. The sheets thus obtained were dissolved for 30 minutes at 505 ° C. and then quenched with water.

The sheets were then trimmed in a controlled manner. Controlled traction was achieved with a permanent elongation of 2.2%.

The sheets then underwent a short heat treatment of 2 hours at 150 ° C.

The mechanical properties were measured before the short heat treatment and between two and sixty five days after the treatment. The results are shown in Table 1. It is found that the state obtained after the short heat treatment is remarkably stable over time. Table 1 Rm (L) R _p0.2 (L) A% (L) Rm (LT) R _p0.2 (LT) A% (LT) Before short heat treatment 438 323 13 404 287 23 Duration after short heat treatment (days) 2 396 270 16.8 370 244 27.1 8 396 269 15.3 372 247 28.0 15 398 273 14.5 374 248 27.2 43 397 270 14.9 375 248 27.5 65 398 271 15.0 373 250 27.2 104 398 273 14.3 373 250 26.9 203 401 277 16.1 375 253 26.9 239 402 278 16.7 376 255 27.7

Example 2

An AA2198 alloy rolling plate was homogenized and then hot rolled to a thickness of 4 mm. The sheets thus obtained were dissolved for 30 minutes at 505 ° C. and then quenched with water.

The sheets were then glided and controlled in a controlled manner. Controlled traction was achieved with a permanent elongation of 1%.

The sheets then underwent a short heat treatment of 2 hours at 150 ° C.

The sheets thus obtained then underwent additional cold deformation by controlled traction with a permanent elongation of 2.5%, 4% or 8%. The sheets did not show after deformation of lines Lüders crippling.
The sheets finally had an income of 12h at 155 ° C to obtain a T8 state.
By way of reference, a sheet was subjected directly after quenching to a controlled pull of 2% followed by an income of 14h at 155 ° C. in the T8 state, without intermediate short heat treatment.

The static mechanical properties were characterized at the end of the income and are presented in Table 2 below: samples # 1, # 2 and # 3: according to the invention and sample # 4: reference. Table 2 - Static Mechanical Properties (MPa) Sample N ° Additional cold deformation after short heat treatment Rm (L) R _p0.2 (L) A% (L) Rm (LT) R _p0.2 (LT) A% (LT) # 1 2.5% 511 474 11.0 499 464 11.0 # 2 4% 526 499 10.4 513 485 10.4 # 3 8% 541 518 9.7 516 491 9.7 # 4 No short heat treatment four hundred ninety seven 454 10.2 486 440 12.7

R curves were measured in the TL direction according to E561-05 on CCT760 test specimens, which had a width of 760 mm. The initial crack length was 2ao = 253 mm. The curves R obtained are presented on the figure 1 .

The results of plane strain toughness obtained are shown in Table 3. It is found in particular that even for an additional strain of 8%, the values of K _app and K _eff are high. Thus the decrease in K _app in the TL direction is low, less than 5%, between a controlled pull of 2.5% and a controlled pull of 8%. Table 3 Sample N ° Additional cold deformation after short heat treatment K _app (MPa√m) TL K _eff (MPa√m) TL Δa _{eff max} (mm) # 1 2.5% 182 262 79 # 2 4% 177 265 97 # 3 8% 174 238 68 # 4 No short heat treatment 190 274 60

It is found that even after an additional strain of 8%, the curve R is quite satisfactory: the curve is sufficiently long, greater than 60 mm, and the K _R values are close to those obtained with a smaller deformation ( Figure 1 ).

Example 3

In this example, the conditions of time and temperature of the short heat treatment have been studied. An AA2198 alloy rolling plate was homogenized and then hot rolled to a thickness of 4 mm. The sheets thus obtained were dissolved for 30 minutes at 505 ° C. and then quenched with water.

The sheets were then glided and controlled in a controlled manner. Controlled traction was achieved with a permanent elongation of 1%. The sheets have been aged sufficiently to reach a stabilized T3 state.

The sheets then underwent a short heat treatment at 145 ° C, 150 ° C or 155 ° C. The equivalent time at 150 ° C was calculated taking into account a temperature rise rate of 20 ° C / h. The static mechanical characteristics of the sheets were characterized after the short heat treatment in the TL direction.

The results are presented in Table 4 below and graphically represented on the figure 2 . It is found that the ratio R _m / R _p0,2 highest in the TL direction is obtained for a temperature between 150 and 160 ° C and for a time equivalent to 150 ° C between one and three hours. Table 4 Short heat treatment time (h) Temperature short heat treatment (° C) Time equivalent t _i at 150 ° C. R _p0.2 TL (MPa) Rm TL (MPa) A TL (%) Rm / R _p0.2 (TL) 0 0 0 288.0 407.3 22.6 1.41 2.5 145 1.90 245.7 371.7 29.1 1.51 5 145 3.47 251.3 373.7 27.6 1.49 7 145 4.73 264.3 378.7 27.7 1.43 10 145 6.62 283.3 386.3 25.9 1.36 0.5 150 1.02 240.3 369.3 25.9 1.54 1 150 1.52 237.3 366.0 26.1 1.54 2 150 2.52 240.3 369.3 27.6 1.54 3 150 3.52 246.7 369.3 28.1 1.50 4 150 4.52 253.0 373.3 26.3 1.48 5 150 5.52 259.3 376.7 27.9 1.45 6 150 6.52 264.7 375.7 26.5 1.42 0.5 155 1.63 235.0 364.0 28.1 1.55 1 155 2.41 238.3 367.7 26.4 1.54 2 155 3.98 246.7 369.3 29.2 1.50 3 155 5.55 262.0 380.7 24.8 1.45 4 155 7.12 275.3 382.3 25.5 1.39 5 155 8.70 295.3 392.0 25.1 1.33

Example 4

In this comparative example, the effect of tensile strength on toughness was investigated in a process without short heat treatment. An AA2198 alloy rolling plate was homogenized and then hot rolled to a thickness of 3.2 mm. The sheets thus obtained were dissolved for 30 minutes at 505 ° C. and then quenched with water.
The sheets were then glided and controlled in a controlled manner. Controlled traction was achieved with a permanent elongation of 3% or 5%.
The sheets then had an income of 14h at 155 ° C until the T8 state.

The static mechanical properties were characterized at the end of the income and are presented in Table 5 below. Table 5 Sample Controlled traction Rm (L) R _p0.2 (L) A% (L) Rm (LT) R _p0.2 (LT) A% (LT) # 5 - 3% 3% 525 486 11.1 499 459 14.1 # 6 - 5% 5% 545 519 10.4 518 487 14.0

R curves were measured according to E561-05 on CCT760 test specimens, which had a width of 760 mm in the T-L direction and in the L-T direction. The initial crack length was 2ao = 253 mm.

The tenacity results obtained are shown in Table 6. It can be seen in particular that the decrease in K _app in the TL direction is significant, of the order of 9%, between a controlled pull of 3% and a controlled pull of 5%. %. Table 6 TL LT Sample Thickness [mm] K _app (MPa√m) K _eff (MPa√m) Δa _{eff max} (mm) K _app (MPa√m) K _eff (MPa√m) Δa _{eff max} (mm) # 5 - 3% 3.2 mm 151 178 61 124 152 115 # 6 - 5% 3.2 mm 138 174 67 119 142 55

Claims (16)

1. A method of manufacturing a rolled product made of aluminium alloy in particular for the aerospace industry wherein, successively,

   a) a bath of molten metal based on aluminium is prepared comprising 2.1 to 3.9 % by weight of Cu, 0.7 to 2.0 % by weight of Li, 0.1 to 1.0 % by weight of Mg, 0 to 0.6 % by weight of Ag, 0 to 1% % by weight of Zn, at the most 0.20 % by weight of Fe + Si, at least one element selected from Zr, Mn, Cr, Sc, Hf and Ti, the amount of said element, if selected, being 0.05 to 0.18 % by weight of Zr, 0.1 to 0.6% by weight of Mn, 0.05 à 0.3 % by weight of Cr, 0.02 to 0.2 % by weight of Sc, 0.05 to 0.5 % by weight of Hf and 0.01 to 0.15 % by weight of Ti, other elements at the most 0.05% by weight each and 0.15% by weight in total, the remainder aluminium;

   b) a rolling slab is cast from said liquid metal bath;

   c) optionally said rolling slab is homogenized;

   d) said rolling slab is hot rolled and optionally cold rolled into a sheet,

   e) said sheet undergoes solution heat treatment and quenching;

   f) said sheet is flattened and/or stretched in a controlled manner, with cumulative deformation of at least 0.5 % and less than 3%,

   g) a short heat treatment is performed wherein said sheet reaches a temperature between 130 and 170°C and preferably between 150 and 160°C for 0.1 to 13 hours and preferably from 1 to 5 hours,
   said short heat treatment inducing a decrease of the yield strength R _p0.2 by at least 20 MPa and an increase of the elongation A% such as A% is multiplied by a factor of at least 1.1 in relation to the temper obtained without short heat treatment.
2. Method of claim 1 wherein said short heat treatment is carried out so as to obtain an equivalent time at 150°C of 0.5 h to 6 h, preferably 1h to 4h, the equivalent time t; at 150 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 treatment temperature which changes with time t (in hours), and T_ref is a reference temperature set at 423 K. t; is expressed in hours, the constant Q/R = 16400 K is derived from the activation energy for diffusion of Cu, for which the value Q = 136100 J/mol was used.
3. Method of claim 1 or claim 2 wherein the thickness of said sheet is between 0.5 and 15 mm and preferably between 1 and 8 mm.
4. Method according to any of claims 1 to 3 wherein controlled stretching is carried out in step f with a permanent deformation of between 0.5 and 1.5 %.
5. Method according to any of claims 1 to 4 wherein the copper content is at least 3% and at most 3.5% by weight.
6. Method according to any of claims 1 to 5 wherein the lithium content is at least 0.85% by weight and at most 1.2% by weight.
7. Method according to any of claims 1 to 6 wherein the magnesium content is at least 0.2 % and at most 0.6 % by weight.
8. Method according to any of claims 1 to 7 wherein the silver content is between 0.1 and 0.5% by weight and preferably between 0.15 and 0.4 % by weight and/or the zinc content is less than 0.4 % by weight and preferably less than 0.2 % by weight.
9. Method according to any of claims 1 to 8 wherein the alloy contains between 0.08 and 0.15% zirconium by weight, from 0.01 to 0.10 % titanium by weight and wherein the Mn, Cr, Sc and Hf content is at the most 0.05% by weight.
10. Method according to any of claims 1 to 9 wherein after step g,

    h) further cold deformation of said sheet is carried out so that the additional deformation is less than 10 %,

    i) ageing is performed wherein said sheet reaches a temperature between 130 and 170°C and preferably between 150 and 160°C for 5 to 100 hours and preferably from 10 to 70h.
11. Method of claim 10 wherein said additional cold bending is locally or generally at least 1%, preferably at least 4 % and most preferably at least 6 %.
12. Method of claim 10 or claim 11 wherein cold working is performed by one or more shaping methods such as stretching, stretch-forming, deep drawing, flow forming or bending.
13. Rolled product obtainable by the method according to any one of claims 1 to 9, having, between 0 and 50 days after short heat treatment, a combination of at least one property selected from R_p0.2(L) of at least 220 MPa and preferably at least 250 MPa, R_p0.2(LT) at least 200 MPa and preferably at least 230 MPa, R_m(L) at least 340 MPa and preferably at least 380 MPa, R_m(LT) at least 320 MPa and preferably at least 360 MPa with a property selected from A%(L) at least 15%, A%(LT) at least 24% and preferably at least 26%, R_m /R_p0.2(L) at least 1.40 and preferably at least 1.45, R_m /R_p0,2(LT) at least 1.45 and preferably at least 1.50.
14. Product obtainable by the method according to any one of claims 10 to 12, having a tensile yield stress R_p0.2 (L) at least substantially equal to, and tenacity K_R greater, preferably by at least 5 %, than that obtained by a similar method including no short thermal treatment.
15. Product obtainable by the method according to any one of claims 10 to 12 characterized in that it is an AA2198 alloy sheet having a thickness between 0.5 and 15 mm and preferably between 1 and 8 mm having, after heat ageing treatment in state T8, a combination of at least one static strength property selected from R_p.0.2 (L) of at least 500 MPa and preferably at least 510 MPa and/or R_p0.2 (LT) of at least 480 MPa and preferably at least 490 MPa, and at least one tenacity property measured on test pieces of type CCT760 (with 2ao = 253 mm) chosen among K_app in the T-L direction of at least 160 MPa√m and preferably at least 170 MPa√m and/or K_eff in the T-L direction of at least 200 MPa√m and preferably at least 220 MPa√m and/or Δa_eff(max) in the T-L direction of at least 40 mm and preferably at least 50 mm
16. Use of a product obtainable by the method according to any one of claims 10 to 12 for the manufacture of a structural element for an aircraft, in particular an aircraft fuselage skin.

Images