EP3526358B1 - Thin sheets made of an aluminium-magnesium-scandium alloy for aerospace applications - Google Patents

Description

Field of the invention

The subject of the invention is a process for manufacturing wrought products made of an aluminum and magnesium alloy, also known under the name of aluminum alloy of the 5XXX series according to the Aluminum Association, more particularly products made of Al alloy. -Mg containing Sc having high mechanical strength, high toughness and good formability. A subject of the invention is also products capable of being obtained by said process as well as the use of these products intended for transport and in particular for aeronautical and space construction.

State of the art

Wrought aluminum alloy products are developed in particular to produce structural elements intended for the transport industry, in particular the aeronautics industry and the space industry. For these industries, product performance must constantly be improved and new alloys are developed to present in particular high mechanical strength, low density, high toughness, excellent corrosion resistance and very good processability. form. In particular, the shaping can be carried out hot, for example by creep forming, and the mechanical properties must not decrease at the end of this shaping.

Al-Mg alloys have been extensively studied in the transportation industry, especially road and maritime transport, due to their excellent working properties such as weldability, corrosion resistance and formability, especially in little hardened such as state O and state H111.

However, these alloys have relatively low mechanical strength for the aeronautical industry and the space industry.
The patent US 5,624,632 describes an alloy of composition 3 - 7% by weight of magnesium, 0.03 - 0.2% by weight of zirconium, 0.2 - 1.2% by weight of manganese, up to 0.15% by weight of silicon and 0.05 - 0.5% by weight of a dispersoid-forming element in the group scandium, erbium, yttrium, gadolinium, holmium and hafnium.
The patent US 6,695,935 describes an alloy of composition, in% by weight, Mg 3.5-6.0, Mn 0.4-1.2, Zn 0.4-1.5, Zr 0.25 max., Cr 0.3 max., Ti 0.2 max., Fe 0.5 max., Si 0.5 max. , Cu 0.4 max, one or more elements in the group: Bi 0.005-0.1, Pb 0.005-0.1, Sn 0.01-0.1, Ag 0.01-0.5, Sc 0.01-0.5, Li 0.01-0.5, V 0.01-0.3, Ce 0.01 -0.3, Y 0.01-0.3, and Ni 0.01-0.3.
The patent application WO 01/12869 describes an alloy of composition in% by weight 1.0-8.0% Mg, 0.05-0.6% Sc, 0.05-0.20% Hf and / or 0.05-0.20% Zr, 0.5-2.0% Cu and / or 0.5-2.0% Zn and in addition 0.1-0.8% by weight of Mn.
The patent application WO2007 / 020041 describes an alloy of composition, in% by weight, Mg 3.5 to 6.0, Mn 0.4 to 1.2, Fe <0.5, Si <0.5, Cu <0.15, Zr <0.5, Cr <0.3, Ti 0.03 to 0.2, Sc <0.5, Zn <1.7, Li <0.5, Ag <0.4, optionally one or more elements forming dispersoids in the group erbium, yttrium, hafnium, vanadium, each <0.5% by weight.
The products described in these patents are not sufficient in terms of compromise between mechanical strength, toughness and suitability for hot forming. In particular, it is important that the mechanical properties do not decrease after heat treatment at 300 - 350 ° C, typical temperature of the forming temperature.

There is therefore a need for wrought Al-Mg alloy products exhibiting a low density as well as improved properties with respect to those of known products, in particular in terms of mechanical strength, toughness and heat-forming ability. Such products must also be obtainable according to a reliable, economical and easily adaptable manufacturing process to a conventional manufacturing line.

Object of the invention

1. a) on élabore un bain de métal liquide à base d'aluminium de composition, en % en poids,
   • Mg : 3,8-4,2 ;
   • Mn : 0,3-0,8 ; de préférence 0,5 - 0,7
   • Sc : 0,1-0,3 ;
   • Zn : 0,1-0,4 ;
   • Ti : 0,01 - 0,05 de préférence 0,015-0,030 ;
   • Zr : 0,07 - 0,15 de préférence 0,08-0,12 ;
   • Cr : < 0,01 ;
   • Fe : < 0,15 ;
   • Si < 0,1 ;
   • autres éléments ≤ 0,05 chacun et ≤ 0,15 en association, reste aluminium ;
2. b) on coule une forme brute à partir dudit bain de métal ;
3. c) on homogénéise la dite forme brute à une température comprise entre 370°C et 450 °C, pendant une durée comprise entre 2 et 50 heures telle que le temps équivalent à 400 °C soit compris entre 5 et 100 heures,
   le temps équivalent t(eq) à 400 °C étant défini par la formule : $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{{\displaystyle \int \exp \left(-29122/\mathrm{T}\right)\mathrm{dt}}}{\exp \left(-29122/{\mathrm{T}}_{\mathrm{ref}}\right)}$$
   
   dans laquelle T est la température instantanée exprimée en Kelvin qui évolue avec le temps t (en heures) et Tref est une température de référence de 400 °C (673 K), t(eq) étant exprimé en heures, la constante Q/R = 29122 K étant dérivée de l'énergie d'activation pour la diffusion du Zr, Q = 242000 J/mol,
4. d) on déforme à chaud avec une température initiale comprise entre 350°C et 450 °C et on déforme optionnellement à froid la forme brute ainsi homogénéisée ;
5. e) optionnellement on effectue un planage et/ou un redressage
6. f) optionnellement on réalise un recuit à une température comprise entre 300 °C et 350 °C.

A first object of the invention is a process for manufacturing a wrought aluminum alloy product in which:

1. a) a bath of liquid metal based on aluminum is prepared with the composition, in% by weight,
   • Mg: 3.8-4.2;
   • Mn: 0.3-0.8; preferably 0.5 - 0.7
   • Sc: 0.1-0.3;
   • Zn: 0.1-0.4;
   • Ti: 0.01 - 0.05, preferably 0.015-0.030;
   • Zr: 0.07 - 0.15, preferably 0.08-0.12;
   • Cr: <0.01;
   • Fe: <0.15;
   • If <0.1;
   • other elements ≤ 0.05 each and ≤ 0.15 in combination, remainder aluminum;
2. b) casting a rough form from said metal bath;
3. c) the said crude form is homogenized at a temperature between 370 ° C and 450 ° C, for a period of between 2 and 50 hours such that the time equivalent to 400 ° C is between 5 and 100 hours,
   the equivalent time t (eq) at 400 ° C being defined by the formula: $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{{\displaystyle \int \exp \left(-29122/\mathrm{T}\right)\mathrm{dt}}}{\exp \left(-29122/{\mathrm{T}}_{\mathrm{ref}}\right)}$$
   
   in which T is the instantaneous temperature expressed in Kelvin which evolves with time t (in hours) and Tref is a reference temperature of 400 ° C (673 K), t (eq) being expressed in hours, the constant Q / R = 29122 K being derived from the activation energy for the diffusion of Zr, Q = 242000 J / mol,
4. d) hot deforming with an initial temperature of between 350 ° C and 450 ° C and optionally cold deforming the raw form thus homogenized;
5. e) optionally, planing and / or straightening is carried out
6. f) optionally, annealing is carried out at a temperature between 300 ° C and 350 ° C.

• Mg : 3,8-4,2 ;
• Mn : 0,3 - 0,8 de préférence 0,5-0,7 ;
• Sc : 0,1-0,3 ;
• Zn : 0,1-0,4 ;
• Ti : 0,01 - 0,05 de préférence 0,015-0,030 ;
• Zr : 0,07 - 0,15 de préférence 0,08-0,12 ;
• Cr : < 0,01 ;
• Fe : < 0,15 ;
• Si < 0,1 ;

autres éléments ≤ 0,05 chacun et ≤ 0,15 en association ; reste aluminium.
susceptible d'être obtenu par le procédé selon l'invention.A second object of the invention is a wrought aluminum alloy product of composition, in% by weight,

• Mg: 3.8-4.2;
• Mn: 0.3 - 0.8, preferably 0.5-0.7;
• Sc: 0.1-0.3;
• Zn: 0.1-0.4;
• Ti: 0.01 - 0.05, preferably 0.015-0.030;
• Zr: 0.07 - 0.15, preferably 0.08-0.12;
• Cr: <0.01;
• Fe: <0.15;
• If <0.1;

other elements ≤ 0.05 each and ≤ 0.15 in combination; remains aluminum.
obtainable by the process according to the invention.

Description of the invention

Unless otherwise indicated, all indications concerning the chemical composition of alloys are expressed as a percentage by weight based on the total weight of the alloy. By way of example, the expression 1.4 Cu means that the copper content expressed in% by weight is multiplied by 1.4. The designation of the alloys is made in accordance with the regulations of “The Aluminum Association”, known to those skilled in the art.

• _ le terme « essentiellement non-recristallisé » est utilisé lorsque la structure granulaire ne présente pas ou peu de grains recristallisés, typiquement moins de 20%, préférentiellement moins de 15% et plus préférentiellement encore moins de 10% des grains sont recristallisés;
• _ le terme « recristallisé » est utilisé lorsque la structure granulaire présente une proportion importante de grains recristallisés, typiquement plus de 50%, préférentiellement plus de 60% et plus préférentiellement encore plus de 80% des grains sont recristallisés.

Sauf mention contraire, les définitions de la norme EN 12258-1 (1998) s'appliquent.The definitions of metallurgical states are given in European standard EN 515 (1993). The static mechanical properties in tension, in other words the tensile strength R _m , the conventional yield strength 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 (2009), the sampling and direction of the test being defined by standard EN 485-1 (2016).
The tenacity under plane stress is determined by means of a curve of the stress intensity factor K _R as a function of the effective crack extension Δa _eff known as the curve R, according to standard ASTM E 561 (2010). The critical stress intensity factor Kc, in other words the intensity factor which makes the crack unstable, is calculated from the curve R. The stress intensity factor Kco is also calculated by assigning the length from initial crack to critical load, at the onset of monotonic load. These two values are calculated for a test piece of the required shape. K _app represents the factor K _CO corresponding to the test piece which was used to carry out the curve test R. K _eff represents the factor Kc corresponding to the test piece which was used to carry out the curve test R. K _R 60 corresponds to the value of K _R for an effective crack extension Δa _eff = 60 mm.
In the context of the invention, the granular structure of the samples is characterized in the LxTC plane at mid-thickness, t / 2, and is evaluated quantitatively after a metallographic attack of the anodic oxidation type and under polarized light:

• _ the term “essentially non-recrystallized” is used when the granular structure exhibits little or no recrystallized grains, typically less than 20%, preferably less than 15% and more preferably still less than 10% of the grains are recrystallized;
• _ the term “recrystallized” is used when the granular structure has a large proportion of recrystallized grains, typically more than 50%, preferably more than 60% and more preferably still more than 80% of the grains are recrystallized.

Unless stated otherwise, the definitions of standard EN 12258-1 (1998) apply.

In the context of the present invention, the term “structural element” or “structural element” of a mechanical construction is used to refer to a mechanical part for which the static and / or dynamic mechanical properties are particularly important for the performance of the structure and for which a structural calculation is usually prescribed or performed. These are typically elements whose failure is likely to endanger the safety of said construction, its users, users or others. For an airplane, these structural elements include in particular the elements that make up the fuselage (such as the fuselage skin), the stiffeners or runners of the fuselage (stringers), the watertight bulkheads, the frames of the fuselage (circumferential frames), the wings (such as the upper or lower wing skin), the stiffeners (stringers or stiffeners), the ribs, the spars (spars), the floor (floor beams) and seat tracks (seat tracks)) and the tail unit composed in particular of horizontal and vertical stabilizers (horizontal or vertical stabilizers), as well as the doors.

The present inventors have observed that, for a composition according to the invention, it is possible to obtain, by controlling the homogenization conditions, an advantageous wrought product, the mechanical properties of which exhibit a compromise between mechanical resistance and tenacity useful for aircraft construction and whose properties are stable after a heat treatment corresponding to hot shaping conditions.
According to the invention, a bath of liquid metal based on aluminum is prepared with the composition, in% by weight, Mg: 3.8-4.2; Mn: 0.3 - 0.8, preferably 0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01 - 0.05, preferably 0.015-0.030; Zr: 0.07 - 0.15, preferably 0.08-0.12; Cr: <0.01; Fe: <0.15; If <0.1 other elements ≤0.05 each and ≤0.15 in combination, aluminum remains.

dans laquelle T est la température instantanée exprimée en Kelvin qui évolue avec le temps t (en heures) et Tref est une température de référence de 400 °C (673 K), t(eq) étant exprimé en heures, la constante Q/R = 29122 K étant dérivée de l'énergie d'activation pour la diffusion du Zr, Q = 242000 J/mol.
De préférence la durée d'homogénéisation est comprise entre 5 et 30 heures. De manière avantageuse le temps équivalent à 400 °C est compris entre 6 et 30 heures.The composition according to the invention is remarkable due to a small addition of titanium of 0.01 - 0.05 and preferably from 0.015 to 0.030% by weight and preferably from 0.018 to 0.024% by weight and by weight. absence of addition of chromium, the content of which is less than 0.01% by weight. High static mechanical properties (Rp0.2, Rm) are obtained despite these small additions because the homogenization conditions are carefully controlled. Thus, surprisingly, it is possible to avoid recrystallization during hot forming with small additions of titanium and in the absence of addition of chromium, and simultaneously to achieve high static mechanical properties, which could be obtained in particular by strong additions of Cr and Ti, and a high tenacity.
The addition of Mn, Sc, Zn and Zr is necessary to obtain the desired compromise between mechanical strength, toughness and suitability for hot shaping. The iron content is kept below 0.15% by weight and preferably below 0.1% by weight. The silicon content is kept below 0.1% by weight and preferably below 0.05% by weight. The presence of iron and silicon beyond the indicated maximums has an unfavorable impact, in particular on toughness. The other elements are impurities, that is to say elements whose presence is not intentional, their presence must be limited to 0.05% each and 0.15% in combination and preferably to 0.03% each and 0.10% in combination.
According to the invention, the said raw form is homogenized at a temperature between 370 ° C and 450 ° C, for a period of between 2 and 50 hours such that the time equivalent to 400 ° C is between 5 and 100 hours,
the equivalent time t (eq) at 400 ° C being defined by the formula: $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{{\displaystyle \int \exp \left(-29122/\mathrm{T}\right)\mathrm{dt}}}{\exp \left(-29122/{\mathrm{T}}_{\mathrm{ref}}\right)}$$

in which T is the instantaneous temperature expressed in Kelvin which evolves with time t (in hours) and Tref is a reference temperature of 400 ° C (673 K), t (eq) being expressed in hours, the constant Q / R = 29122 K being derived from the activation energy for the diffusion of Zr, Q = 242000 J / mol.
Preferably, the homogenization time is between 5 and 30 hours. Advantageously, the equivalent time at 400 ° C. is between 6 and 30 hours.

Too low a temperature and / or homogenization time does not make it possible to form dispersoids in order to control recrystallization. Surprisingly, when temperature and / or homogenization time are too high, the properties obtained are not stable at the typical hot forming temperature of 300-350 ° C, in particular because the products recrystallize.
Hot deformation can be carried out directly after homogenization without cooling down to room temperature, the initial hot deformation temperature having to be between 350 and 450 ° C. Alternatively, the raw form can be cooled down to room temperature after homogenization and the raw form can be reheated to an initial hot deformation temperature of between 350 and 450 ° C. In the case of reheating, it should be ensured that the time equivalent to 400 ° C during reheating is low, typically less than 10%, in comparison with the time equivalent to 400 ° C during homogenization.
During hot deformation, the temperature of the metal can in some cases increase, however care should be taken that the time equivalent to 400 ° C during hot deformation is low, typically less than 10%, compared to the time equivalent to 400 ° C during homogenization. In any case, it is preferable that the temperature during hot deformation does not exceed 460 ° C and preferably does not exceed 440 ° C. After hot deformation, a cold deformation can be carried out.

In a first embodiment, the wringing is carried out by rolling to obtain a sheet. According to this first embodiment, the final thickness of the sheet obtained is less than 12 mm.
In a second embodiment, the wringing is carried out by extrusion to obtain a profile.

1. (a) une limite d'élasticité conventionnelle mesurée à 0,2% d'allongement dans le sens TL d'au moins 250 MPa, et de préférence d'au moins 260 MPa et/ou
2. (b) une limite d'élasticité conventionnelle mesurée à 0,2% d'allongement dans le sens L d'au moins 260 MPa, et de préférence d'au moins 270 MPa, ces propriétés étant atteintes même dans le cas où l'étape optionnelle de recuit à une température comprise entre 300 °C et 350 °C est effectuée.

Avantageusement les tôles d'épaisseur inférieure à 4 mm obtenues par le procédé selon l'invention ont une limite d'élasticité conventionnelle mesurée à 0,2% d'allongement dans le sens TL d'au moins 300 MPa, et de préférence d'au moins 320 MPa, ces propriétés étant atteintes même dans le cas où l'étape optionnelle de recuit à une température comprise entre 300 °C et 350 °C est effectuée.In the first embodiment, the hot deformation is typically carried out to a thickness of about 4 mm, then the cold deformation for a thickness of between 0.5 and 4 mm.
After hot and optionally cold deformation, it may be advantageous to perform leveling and / or straightening. During leveling and / or straightening, the permanent set is typically less than 2%, preferably about 1%.
Optionally, annealing is carried out at a temperature between 300 ° C. and 350 ° C. The duration of the annealing is typically between 1 and 4 hours. This annealing mainly has a function of stabilizing the mechanical properties so that they do not change during subsequent shaping at a neighboring temperature. The products according to the invention have the advantage of having very stable mechanical properties at this temperature. Thus for products whose final thickness of 4 to 6 mm is obtained by hot rolling, the variation in static mechanical property is at most 10% and preferably at most 6% after annealing between 300 and 350 ° C. and for products whose final thickness of about 2 mm is obtained by cold rolling, the variation in static mechanical property is at most 40% and preferably at most 30% after annealing between 300 and 350 ° C. . It is therefore possible, within the framework of the process according to the invention, not to carry out stabilization annealing and to proceed directly with the shaping, in particular for products whose final thickness is obtained by hot rolling. Thanks to the process according to the invention, the products according to the invention retain an essentially non-recrystallized granular structure after annealing between 300 and 350 ° C.
The sheets with a thickness of less than 12 mm obtained by the process according to the invention are advantageous, preferably having the following characteristics:

1. (a) a conventional yield strength measured at 0.2% elongation in the TL direction of at least 250 MPa, and preferably at least 260 MPa and / or
2. (b) a conventional elastic limit measured at 0.2% elongation in the L direction of at least 260 MPa, and preferably at least 270 MPa, these properties being achieved even in the case where the optional step of annealing at a temperature between 300 ° C and 350 ° C is carried out.

Advantageously, the sheets of thickness less than 4 mm obtained by the process according to the invention have a conventional elastic limit measured at 0.2% elongation in the TL direction of at least 300 MPa, and preferably of at least 320 MPa, these properties being achieved even in the case where the optional step of annealing at a temperature between 300 ° C and 350 ° C is carried out.

• (c) une ténacité K_R60, mesurée sur des éprouvettes de type CCT760 dans le sens L-T (avec 2ao = 253 mm), pour une extension de fissure effective Δa_eff de 60 mm d'au moins $155\mathrm{MPa}\sqrt{\mathrm{m},}$
  
  et de préférence d'au moins $165\mathrm{MPa}\sqrt{\mathrm{m}}$
  
  et/ou
• (d) une ténacité K_R60, mesurée sur des éprouvettes de type CCT760 dans le sens T-L (avec 2ao = 253 mm), pour une extension de fissure effective Δa_eff de 60 mm d'au moins $160\mathrm{MPa}\sqrt{\mathrm{m}},$
  
  et de préférence d'au moins $170\mathrm{MPa}\sqrt{\mathrm{m}}.$
  

De préférence, pour les produits selon l'invention, la ténacité K_R dans le sens T-L est supérieure à celle dans le sens L-T.
De préférence la ténacité Kapp, mesurée sur des éprouvettes de type CCT760 dans le sens T-L (avec 2ao = 253 mm), est d'au moins 125 MPa , et de préférence d'au moins 130 MPa
Les produits selon l'invention peuvent être mis en forme à une température comprise entre 300 °C et 350 °C pour obtenir des éléments de structure pour avion, de préférence des éléments de fuselage.
Les éléments de fuselage d'aéronef selon l'invention sont avantageux car ils présentent

1. (a) une limite d'élasticité conventionnelle mesurée à 0,2% d'allongement dans le sens TL est d'au moins 250 MPa, et de préférence d'au moins 260 MPa et/ou
2. (b) une limite d'élasticité conventionnelle mesurée à 0,2% d'allongement dans le sens L est d'au moins 260 MPa, et de préférence d'au moins 270 MPa.

The sheets according to the invention preferably exhibit advantageous toughness properties, in particular:

• (c) a toughness K _R 60, measured on specimens of type CCT760 in the LT direction (with 2ao = 253 mm), for an effective crack extension Δa _eff of 60 mm of at least $155\mathrm{MPa}\sqrt{\mathrm{m},}$
  
  and preferably at least $165\mathrm{MPa}\sqrt{\mathrm{m}}$
  
  and or
• (d) a toughness K _R 60, measured on type CCT760 specimens in the TL direction (with 2ao = 253 mm), for an effective crack extension Δa _eff of 60 mm of at least $160\mathrm{MPa}\sqrt{\mathrm{m}},$
  
  and preferably at least $170\mathrm{MPa}\sqrt{\mathrm{m}}.$
  

Preferably, for the products according to the invention, the toughness K _R in the TL direction is greater than that in the LT direction.
Preferably the toughness Kapp, measured on specimens of the CCT760 type in the TL direction (with 2ao = 253 mm), is at least 125 MPa, and preferably at least 130 MPa
The products according to the invention can be shaped at a temperature between 300 ° C and 350 ° C to obtain structural elements for an airplane, preferably fuselage elements.
The aircraft fuselage elements according to the invention are advantageous because they have

1. (a) a conventional yield strength measured at 0.2% elongation in the TL direction is at least 250 MPa, and preferably at least 260 MPa and / or
2. (b) a conventional yield strength measured at 0.2% elongation in the L direction is at least 260 MPa, and preferably at least 270 MPa.

Examples Example 1

Several plates 400 mm thick, the composition of which is given in Table 1, were cast. Table 1: Composition in% by weight (analysis by optical spark emission spectrometer, S-OES). Yes Fe Cr Mn Mg Zn Ti Zr Sc AT 0.02 0.05 <0.01 0.62 4.05 0.28 0.023 0.10 0.19 B 0.02 0.04 <0.01 0.59 3.99 0.29 0.038 0.10 0.19

The alloy A plate was homogenized for 5 hours at 445 ° C while the alloy B plate was homogenized for 15 hours at 515 ° C. The plates thus homogenized were hot rolled directly after homogenization with a hot rolling start temperature of 415 ° C for plate A and 480 ° C for plate B, to obtain sheets having a thickness of 4 mm.

The static mechanical tensile characteristics of the alloy A sheet remained high both in the hot rolled state (LAC) and in the annealed state (annealing treatment of 4 hours at 325 ° C) while those of Alloy B sheet fell off after annealing. Table 2: Static mechanical characteristics obtained for the various sheets in the state such as hot rolled (LAC) and in the annealed state (4h at 325 ° C). Alloy A sheet 4 mm thickness Alloy B sheet 4 mm thick LAKE Annealing LAKE Annealing Rp0.2 L, MPa 303 289 287 233 Rm L, MPa 400 393 364 352 AL,% 14.5 16.2 14.8 17.6 Rp0.2 TL, MPa 311 292 276 238 Rm TL, MPa 396 387 361 349 At TL,% 17.7 19.5 18.2 23.0 K _app MPa√m LT 129.9 129.1 128.5 K _app MPa√m TL 134.9 134.0 125.8 Kr60 MPa√m LT 172.9 171.5 171.2 Kr60MPa√m TL 178.9 177.1 164

The 4mm sheets were cold rolled to a thickness of 2mm in three passes without intermediate heat treatment, and then underwent leveling. Different heat treatments were carried out after cold rolling. The results of the mechanical tensile tests are shown in Table 3. Table 3: Static mechanical characteristics obtained for the various cold-rolled sheets which have undergone annealing under different conditions. Annealing after cold rolling Alloy sheet A thickness 2 mm Alloy B sheet 2 mm thick Rp02 (TL) Rm (TL) A% TL Rp02 (TL) Rm (TL) A% TL - 417 466 9.95 358 422 10.5 2h 275 ° C 349.5 415 19 256 355 18.2 2h 325 ° C 333 405 21.7 168 311 23.0 2h 375 ° C 297.5 393 21.4 156 301 23.1

The granular structure of the sheets was observed after a metallographic attack of the anodic oxidation type and under polarized light after cold rolling (LAF) or after cold rolling and annealing for 2 hours at 325 ° C.

A qualitative assessment of the microstructure was carried out:
Table 4 shows the results of the microstructural observations of the sheets of composition A and B in the as-cold-rolled states and after annealing treatment (2h 325 ° C). Table 4: Microstructure (LxTC plane, at mid-thickness) of the sheets Alloy Reference Microstructure AT LAF Essentially non-recrystallized 2h325 ° C Essentially non-recrystallized B LAF Essentially non-recrystallized 2h325 ° C Recrystallized

Alloy A according to the invention exhibits excellent resistance to recrystallization.

Example 2

In this example, the effect of the homogenization conditions before hot deformation on the mechanical properties was studied. Alloy A blocks of dimension 250 x 180 x 120 mm were hot rolled under different conditions, up to a thickness of 8 or 12 mm. The conditions are described in Table 5 Table 5: Processing conditions for different blocks in alloy A Homogenization temperature (° C) Homogenization time (h) T (eq) at 400 ° C Initial rolling temperature (° C) Final thickness (mm) Final rolling temperature (° C) CD2 450 15 298 440 12 329 CD3 400 15 15 390 12 319 CD4 450 15 298 440 8 325 CF1 450 5 99 440 8 330 CF2 450 5 99 12 327 CF3 400 5 5 405 12 320 CF4 515 17 9341 8 325

The mechanical properties were measured on sheets such as rolled or treated. The results are shown in Table 6 Table 6 Static mechanical characteristics obtained for the various sheets in the state such as hot rolled (LAC) and in the annealed state (4h at 325 ° C). LAKE Annealing 4h 325 ° C block meaning Rp0.2 Rm AT Rp0.2 Rm AT MPa MPa % MPa MPa % CD2 L 251 377 15.4 243 370 16.0 CD3 L 286 398 14.5 278 391 15.4 CD4 L 260 371 13.6 252 366 16.7 CF1 L 275 381 16.1 267 373 17.1 CF2 L 268 390 12.9 262 382 13.8 CF3 L 288 399 14.8 280 392 15.4 CF4 L 223 341 15.7 209 339 17.3

The products obtained by the process according to the invention (CD3, CF1, CF2, CF3) exhibit advantageous mechanical characteristics, in particular Rp0.2 in the L direction of at least 260 MPa after LAC and after annealing for 4 hours at 325.

Claims (9)

1. Method for producing a wrought product made of an aluminium alloy wherein:

   a) a molten metal bath having an aluminium base is produced, composed, in wt%, of

   Mg: 3.8-4.2;

   Mn: 0.3 - 0.8 and preferably 0.5-0.7;

   Sc: 0.1-0.3;

   Zn: 0.1-0.4;

   Ti: 0.01 - 0.05 and preferably 0.015-0.030;

   Zr: 0.07 - 0.15 and preferably 0.08-0.12;

   Cr: ≤ 0.01;

   Fe: ≤ 0.15;

   Si < 0.1;

   other elements ≤ 0.05 each and ≤ 0.15 combined, the remainder being aluminium;

   b) an unwrought product is cast from said metal bath;

   c) said unwrought product is homogenised at a temperature that lies in the range 370°C to 450°C, for a duration that lies in the range 2 to 50 hours such that the equivalent time at 400°C lies in the range 5 to 100 hours,
   the equivalent time t(eq) at 400°C being defined by the formula: $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{{\displaystyle \int \exp \left(-29122/\mathrm{T}\right)\mathrm{dt}}}{\exp \left(-29122/{\mathrm{T}}_{\mathrm{ref}}\right)}$$

   

   where T is the current temperature expressed in Kelvin, which changes over time t (in hours) and Tref is a reference temperature of 400°C (673 K), t(eq) being expressed in hours, the constant Q/R = 29122 K being derived from the activation energy for the diffusion of Zr, Q = 242000 J/mol,

   d) the unwrought product thus homogenised is hot-worked with an initial temperature in the range 350°C to 450°C and is optionally cold-worked;

   e) a flattening and/or straightening process is optionally carried out;

   f) an annealing process is optionally carried out at a temperature that lies in the range 300°C to 350°C.
2. Method according to claim 1, wherein the homogenisation duration lies in the range 5 to 30 hours.
3. Method according to any of claims 1 to 2, wherein working is carried out by rolling in order to obtain a sheet metal and wherein the final thickness of the sheet obtained is less than 12 mm.
4. Method according to any of claims 1 to 2, wherein working is carried out by extrusion in order to obtain a profile.
5. Wrought product made of an aluminium alloy having the composition, in wt %,

   Mg: 3.8-4.2;

   Mn: 0.3 - 0.8 and preferably 0.5-0.7;

   Sc: 0.1-0.3;

   Zn: 0.1-0.4;

   Ti: 0.01 - 0.05 and preferably 0.015-0.030;

   Zr: 0.07 - 0.15 and preferably 0.08-0.12;

   Cr: ≤ 0.01;

   Fe: ≤ 0.15;

   Si < 0.1;

   other elements ≤ 0.05 each and ≤ 0.15 combined, the remainder being aluminium,

   obtainable by the method according to any of claims 1 to 4.
6. Wrought product according to claim 5 in the form of a sheet having a thickness of less than 12 mm, obtainable by the method according to claim 3, characterised in that

   (a) the tensile yield stress thereof measured at 0.2% elongation in the LT direction is at least 250 MPa, and preferably at least 260 MPa and/or

   (b) the tensile yield stress thereof measured at 0.2% elongation in the L direction is at least 260 MPa, and preferably at least 270 MPa.
7. Sheet according to claim 6, characterised in that

   (c) the toughness K_R60 thereof, measured on specimens of type CCT760 in the L-T direction (where 2ao = 253 mm), for an effective crack growth Δa_eff of 60 mm, is at least $155\mathrm{MPa}\sqrt{\mathrm{m}},$

   

   and preferably at least $165\mathrm{MPa}\sqrt{\mathrm{m}}$

   

   and/or

   (d) the toughness K_R60 thereof, measured on specimens of type CCT760 in the T-L direction (where 2ao = 253 mm), for an effective crack growth Δa_eff of 60 mm, is at least $160\mathrm{MPa}\sqrt{\mathrm{m}},$

   

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

   
8. Method according to any of claims 1 to 4, wherein, at the end of step f, forming is carried out at a temperature that lies in the range 300°C to 350°C.
9. Aircraft fuselage element obtainable according to the method according to claim 8, characterised in that

   (a) the tensile yield stress thereof measured at 0.2% elongation in the LT direction is at least 250 MPa, and preferably at least 260 MPa and/or

   (b) the tensile yield stress thereof measured at 0.2% elongation in the L direction is at least 260 MPa, and preferably at least 270 MPa.