ALUMINUM-COPPER-LITHIUM ALLOY PRODUCT FOR INTRADOS COMPONENTS WITH IMPROVED PROPERTIES

Abstract

The invention relates to a rolled product with a thickness of between 15 and 50 mm made of aluminum alloy with the following composition, in % by weight: Cu: 2.3 – 2.7; Li: 1.3 – 1.7; Mg: 0.2 – 0.5; Mn: 0.2 – 0.5; Ag: 0 – 0.1; Zn: < 0.20; Ti: 0.01 – 0.15; Zr < 0.07; Fe: < 0.1; Si: < 0.1; other elements < 0.05 each and < 0.15 in total, remaining aluminum in that the phases with an equivalent diameter of 35 to 500 nm have an average equivalent diameter less than or equal to 100 nm. The products according to the invention are obtained by a process in which, in particular, the hot deformation conditions are such that the final hot deformation temperature is at least 400°C and the solution treatment of the product includes a step of at least 15 minutes and less than 8 hours between 540°C and 580°C. Abbreviated figure: 2

Description

Title of the invention: ALUMINUM-COPPER-LITHIUM ALLOY PRODUCT FOR INNER SIDE ELEMENT WITH IMPROVED PROPERTIES Technical field

[0001] The present invention relates generally to aluminum alloy products and, more particularly, to such products, methods of manufacturing them and their use, particularly in the aerospace industry. Prior art

[0002] In aluminum alloys, it is generally accepted that the resistance to fatigue crack propagation under spectrum generally decreases as the strength increases and vice versa.

[0003] Prasad et al. In Aluminum-Lithium alloys - processing properties and applications - Elsevier edition 2014 in chapter 11 pp 341-375 mentions that the improvement of the fatigue of lithium alloys is obtained by solid solution hardening and coalescence of delta prime precipitates. It is also possible to improve the fatigue by thermal and thermomechanical treatments involving tempering (also known as artificial aging) or cold deformation before tempering.

[0004] Reference may be made to US9234566 which discloses a method of manufacturing an aluminum alloy comprising the steps of (i) preparing a first aluminum alloy for artificial aging, and (ii) artificially aging the first aluminum alloy. In one approach, the preparation step (i) comprises (a) solution heat treating an alloy comprising at least 0.1 wt% Li at a temperature of at least 800°F (426°C) and (b) quenching the alloy. The method may optionally include the step of cold working the alloy.The artificial aging step (ii) comprises at least two artificial aging steps, one of which steps comprises (c) aging the first aluminum alloy at a temperature of at least about 250°F (121°C), and the last of which steps (i.e., the final artificial aging step) comprises (d) aging the first aluminum alloy at a temperature of no more than about 225°F (107°C) and for at least about 20 hours.

[0005] US Patent 5,032,359 describes a wide family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, particularly between 0.3 and 0.5 percent by weight, makes it possible to increase the mechanical strength.

[0006] US Patent 5,198,045 describes a family of alloys comprising (in wt%) (2.4-3.5) Cu, (1.35-1.8) Li, (0.25-0.65) Mg, (0.25-0.65) Ag, (0.08-0.25) Zr. The products Wrought irons made with these alloys combine a density of less than 2.64 g / cm3 and an interesting compromise between mechanical strength and toughness.

[0007] US Patent 7,229,509 describes a family of alloys comprising (in wt%) (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, (up to 0.4) Zr or other refining agents such as Cr, Ti, Hf, Sc and V. The examples presented have an improved compromise between mechanical strength and toughness but their density is greater than 2.7 g / cm3.

[0008] Patent EP 1,966,402 describes an alloy not containing zirconium intended for fuselage sheets of essentially 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.

[0009] Patent EP 1,891,247 describes an alloy intended for fuselage sheets 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 element among Zr, Mn, Cr, Sc, Hf and Ti, in which the Cu and Li contents meet the condition Cu + 5 / 3 Li < 5.2.

[0010] US Patent 5,455,003 describes a process for producing aluminum-copper-lithium alloys having improved properties of mechanical strength and toughness at cryogenic temperature. This process applies in particular to an alloy comprising (in % by weight) (2.0-6.5) Cu, (0.2-2.7) Li, (0-4.0) Mg, (0-4.0) Ag, (0-3.0) Zn.

[0011] International application WO 2010 / 055225 describes a manufacturing method in which a liquid metal bath is prepared comprising 2.0 to 3.5% by weight of Cu, 1.4 to 1.8% by weight of Li, 0.1 to 0.5% by weight of Ag, 0.1 to 1.0% by weight of Mg, 0.05 to 0.18% by weight of Zr, 0.2 to 0.6% by weight of Mn and at least one element selected from Cr, Sc, Hf and Ti, the amount of the element, if selected, being 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and 0.01 to 0.15% by weight for Ti, the remainder being aluminum and unavoidable impurities; a raw form is cast from the liquid metal bath and said raw form is homogenized at a temperature between 515°C and 525°C so that the time equivalent to 520°C for homogenization is between 5 and 20 hours.

[0012] International application WO2011 / 141647 relates to an aluminum-based alloy comprising, in % by weight, 2.1 to 2.4% of Cu, 1.3 to 1.6% of Li, 0.1 to 0.5% of Ag, 0.2 to 0.6% of Mg, 0.05 to 0.15% of Zr, 0.1 to 0.5% of Mn, 0.01 to 0.12% of Ti, optionally at least one element chosen from Cr, Sc, and Hf, the amount of the element, if chosen, being 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf, an amount of Fe and Si less than or equal to 0.1 each, and unavoidable impurities at a content less than or equal to 0.05 each and 0.15 in total. The alloy allows the production of extruded, rolled and / or forged products particularly suitable for the manufacture of aircraft wing lower surface elements.

[0013] The article by Lee Chang-Soon et Al “Effect of microstructure and load ratio on fatigue crack growth behavior of advanced Al-Cu-Li-Mg-Ag alloys », Metals and Materials, Vol 3 N°1 (1997) pp 51 - 59 describes the fatigue crack growth behavior of three Al-Cu-Li-Mg-Ag alloys for loading ratios of 0.1 and 0.75.

[0014] EP3077559 discloses a method of manufacturing a rolled or forged material of which the thickness is 14 to 100 mm. Said material consists of an aluminum alloy composed, in % by weight, of: 1.8-2.6 Cu; 1.3-1.8 Li; 0.1-0.5 Mg; 0.1-0.5 Mn; Zr < 0.05, 0-0.5 Ag; Zn < 0.20; 0.01-0.15 Ti; Fe < 0.1; Si < 0.1; other elements < 0.05 each and < 0.15 in total, remainder aluminum whose density is less than 2.670 g / cm3. Said method comprises: homogenization; hot rolling under conditions such that the final temperature is at least 400°C, solution treatment preferably by heat treatment between 490 and 530°C for 15 min to 8 h, then quenching typically with water. The sheet then undergoes controlled traction of 1 to 6%, then undergoes tempering at a temperature between 120 and 170°C for 5 to 100 h. The inventors have found that the product disclosed in EP3077559 makes it possible to obtain satisfactory fatigue resistance under the spectrum but that after aging for 1000 h or 3000 h at 85°C the product saw its toughness decrease..

[0015] In order to lighten aeronautical structures, the fatigue resistance under spectrum must be further improved, as well as the elastic limit-toughness compromise, even after aging at low temperature, preferably aging of 1000 Oh at 85°C.

[0016] The object of this application is to propose a method making it possible to obtain a product improving the fatigue resistance under spectrum and the elastic limit-toughness compromise even after aging of 1000H at 85°C. Statement of the invention

[0017] A first subject of the invention relates to a method for manufacturing a rolled or forged product made of lithium copper aluminum alloy. Said product comprises the following successive steps: (a) a plate is cast from an alloy of composition, in % by weight, Cu: 2.3 - 2.7; Li: 1.3 - 1.7; Mg: 0.2 - 0.5; Mn: 0.2 - 0.5; Ag: 0 - 0.1; Zn: < 0.20; Ti: 0.01 - 0.15; Zr <0.07; Fe: < 0.1; Si: < 0.1; other elements < 0.05 each and < 0.15 in total, remainder aluminum. (b) homogenizing said plate from 480°C to 540°C for 5 to 60 hours. (c) hot deforming said homogenized plate by rolling to obtain an intermediate product having a thickness in the range of 15 mm to 50 mm, the final hot deformation temperature being at least 400°C. (d) said intermediate product is dissolved. (e) quenching said dissolved intermediate product with water. (f) said intermediate product, dissolved and quenched, is pulled in a controlled manner with a permanent deformation of 2 to 5%. (g) said intermediate product thus dissolved, quenched and pulled is tempered by heating from 120 to 170°C for 5 to 100 hours. The process is such that the dissolution carried out in step (d) comprises a step dl) of at least 15 minutes and less than 8 hours during which said intermediate product is at a temperature of between 540°C and 580°C. Advantageously, the dissolution carried out in step d) comprises a step d2) after step dl) of 15 min to 8 h during which said intermediate product is at a temperature of between 480°C and 535°C. Advantageously, the equivalent time t_eq at 155°C of said tempering is 30 to 60 hours, even more preferably 45 to 55 hours. The equivalent time t_eq at 155 °C is defined by the formula: f exu(-11400 / T) dt t eq = --4——--;—— exp ( -114QÜ / Treîj where T (in Kelvin) is the instantaneous treatment temperature, which changes with time t (in hours), and Tref is a reference temperature set at 428 K (155°C). t_eq is expressed in hours.

[0018] Another subject of the invention is a rolled product whose thickness is from 15 mm to 50 mm made of lithium copper aluminum alloy having a composition, in % by weight, Cu: 2.3 - 2.7; Li: 1.3 - 1.7; Mg: 0.2 - 0.5; Mn: 0.2 - 0.5; Ag: 0 - 0.1; Zn: < 0.20; Ti: 0.01 - 0.15; Zr <0.07; Fe: < 0.1; Si: < 0.1; other elements < 0.05 each and < 0.15 in total, remainder aluminum. This product is characterized by the fact that the population of phases having an equivalent diameter of from 35 to 500 nm has an average equivalent diameter less than or equal to 100 nm. Phases are understood to mean a Mn dispersoid or a precipitate. Preferably, this product is characterized by the fact that the population of Mn dispersoids having an equivalent diameter of 35 to 500 nm has an average equivalent diameter of less than or equal to 100 nm. Advantageously, the density of phases whose equivalent diameter is 35 to 500 nm is greater than or equal to 1.3 phases / pm2. Preferably, this product is characterized by the fact that the density of Mn dispersoids whose equivalent diameter is 35 to 500 nm is greater than or equal to 1.3 phases / pm2. Advantageously, the surface fraction of phases whose equivalent diameter is from 35 to 500 nm is less than or equal to 0.8%. Preferably, this product is characterized by the fact that the surface fraction of Mn dispersoids whose equivalent diameter is from 35 to 500 nm is less than or equal to 0.8%. In another preferred embodiment, the equivalent diameter is from 35 to 500 nm and is greater than or equal to 1.0%. Preferably, this product is characterized by the fact that the surface fraction of Mn dispersoids whose equivalent diameter is from 35 to 500 nm is greater than or equal to 1.0%. Advantageously, the texture of the rolled product, measured at mid-thickness, is such that the sum of the volume fractions of the copper, brass and S texture components is less than or equal to 5%. Advantageously, at mid-thickness, the granular structure of the product is essentially recrystallized. Figures

[0019] [Fig.l] represents the compromise between elastic limit and toughness with and without aging according to the conditions of example 1.

[0020] [Fig.2] represents the evolution of the number of fatigue flights under spectrum in function of the solution temperature, according to the conditions of example 2. Detailed description of the invention

[0021] Unless otherwise stated, all statements regarding the chemical composition of alloys are expressed as a percentage by weight based on the total weight of the alloy. Designation of alloys is in accordance with the regulations of The Aluminium Association, known to those skilled in the art. Density depends on the composition and is determined by calculation rather than by a weight measurement method. Values ​​are calculated in accordance with the procedure of The Aluminium Association, which is described on pages 2-12 and 2-13 of "Aluminum Standards and Data". Definitions of metallurgical tempers are given in European Standard EN 515.

[0022] Unless otherwise stated, the static mechanical characteristics, in other words the breaking strength Rm, the tensile strength Rpoj2 and the elongation at break A%, are determined by a tensile test according to standard EN 10002-1 or NF EN ISO 6892-1. The location at which the parts are taken and their direction are defined by standard EN 485-1. Unless otherwise stated, the definitions of standard EN 12258 apply.

[0023] The inventors found that by modifying the chemical composition, the solution conditions and the tempering conditions, it was possible to improve the number of fatigue flights under spectrum and the thermal stability of the product.

[0024] The products according to the invention are obtained by a process comprising the steps of casting, homogenization, hot deformation, solution treatment, quenching, controlled traction and tempering.

[0025] An aluminum alloy plate according to the invention is cast.

[0026] The copper content of the alloy according to the invention is between 2.3% and 2.7%. preferably the copper content is at least 2.4% or 2.5%, preferably at least 2.45% or even more preferably at least 2.50%. The maximum copper content is 2.6% or preferably 2.60 or 2.55% by weight.

[0027] The lithium content is from 1.3 to 1.7% by weight. Advantageously, the lithium content is at least 1.35% and preferably 1.40% by weight. Preferably, the lithium content is at most 1.65% or preferably 1.60% by weight.

[0028] The silver content is from 0 to 0.1% by weight. In one embodiment of the invention, the silver content is between 0.01 and 0.1% by weight. In another embodiment of the invention, which has the advantage of minimizing the density, the silver content is at most 0.05% by weight.

[0029] The magnesium content is from 0.2 to 0.5% by weight. Preferably the magnesium content is at most 0.4% by weight. In an advantageous embodiment of the invention the magnesium content is at least 0.20% by weight.

[0030] The manganese content is from 0.2 to 0.5%, preferably from 0.20 to 0.50%. Preferably, the manganese content is at least 0.25%, or even more preferably at least 0.30%. Preferably, the manganese content is at most 0.45%, or even more preferably at most 0.40%.

[0031] The zirconium content is less than 0.07% by weight. Preferably, the zirconium content is less than or equal to 0.05% by weight, even more preferably less than or equal to 0.04% by weight. Preferably, the zirconium content is at least 0.01%.

[0032] The alloy also contains from 0.01 to 0.15% by weight of Ti and preferably from 0.02 to 0.10% by weight in particular to control the grain size during casting.

[0033] The zinc content is less than 0.20% by weight. Preferably, the zinc content is less than 0.05% by weight, or even 0.04% by weight.

[0034] It is preferable to limit the content of unavoidable impurities in the alloy so as to achieve the most favorable damage tolerance properties. Unavoidable impurities include iron and silicon, these elements having a content of less than 0.1% by weight each, or even 0.08% by weight each. Preferably, the content of iron and silicon is less than 0.06% by weight each. Other impurities have a content of less than 0.05% by weight each and 0.15% by weight in total. The remainder corresponds to aluminum.

[0035] The cast plate is then homogenized. The homogenization treatment is carried out at a temperature of 480°C to 540°C for 5 to 60 hours. Preferably, the homogenization temperature is between 490°C and 510°C.

[0036] After homogenization, the plate is generally cooled to room temperature before being preheated for hot deformation by rolling. preheating aims to reach an initial deformation temperature preferably between 420 and 520°C and preferably in the order of 450°C to 480°C allowing the deformation of the plate.

[0037] Hot deformation is carried out by hot rolling so as to obtain a sheet with a thickness of between 15 mm and 50 mm. The hot rolling conditions are chosen so that the final hot deformation temperature is at least 400°C and preferably at least 405°C, or even 410°C.

[0038] To achieve this hot rolling exit temperature, the skilled person has several technical solutions at his disposal. Examples include the use of reheating and / or cooling boxes to achieve the exit temperature of at least 400°C. Heated rolling cylinders can also be used. It is also possible to adapt the rolling passes, the deformation speed, and the rolling speed.

[0039] The sheet thus rolled is then solution-treated. The inventors have found that it is important to carry out solution-treating comprising a step dl) of at least 15 minutes and less than 8 hours at a temperature of 540°C to 580°C to improve the number of fatigue flights under spectrum while maintaining an excellent compromise R0.2 - direction L and Kapp TL and to obtain excellent thermal stability. Preferably, the solution-treating temperature is at least 550°C.

[0040] According to a preferred embodiment, the solution treatment step may comprise several steps. According to a preferred embodiment of the invention, it is advantageous to carry out a step d2) at the end of step dl) at a temperature of from 480°C to 535°C, preferably from 480°C to 530°C. Preferably, the duration of step d2) is from 15 min to 8 h. The inventors have found that carrying out solution treatment with a step dl) from 540°C to 580°C, followed by a step d2) from 480°C to 535°C makes it possible to improve the elongation in the TL direction of the product as well as its machinability.

[0041] According to another preferred manufacturing method, it may be advantageous not to carry out a second step d2) at the end of step d1). The inventors have found that this makes it possible to obtain a long fatigue life under the spectrum.

[0042] The sheet is then quenched in water, typically water at room temperature, preferably below 40°C.

[0043] The product then undergoes a controlled traction of 2 to 5% and preferably at least 3%, typically around 4%.

[0044] An annealing is then carried out at a temperature of 120 to 170°C for 5 to 100 h, preferably 140 to 160°C for 30 to 90 h. Preferably, the annealing is such that the equivalent time t_eq at 155°C is 30 to 60 hours. Preferably, the equivalent time t_eq at 155°C is at least 32 h, 34 h, 36 h. It may be advantageous to aim for equivalent times t_eq at 155°C of 30 to 40 hours in order to improve the elongation in the TL direction and the tensile strength of the product. In another preferred mode, the equivalent time t_eq at 155 °C is 45 to 55 hours in order to improve the thermal stability of the product. The equivalent time t_eq at 155 °C is defined by the formula: f ex»(-11400 / T) dt * eq = --:—— exp(-11400 / Tretj where T (in Kelvin) is the instantaneous treatment temperature, which changes with time t (in hours), and Tref is a reference temperature set at 428 K (155°C). t_eq is expressed in hours. The constant Q / R = 11400 K is derived from the activation energy for Li diffusion, Q = 95000 J / mol. The formula giving t_eq takes into account the heating and cooling phases.

[0045] The preferred metallurgical states for the sheets are the T8 states, more particularly T84 or T86.

[0046] The inventors found that the product obtained by the process according to the invention made it possible to obtain very good fatigue resistance under spectrum as well as good thermal stability after aging at low temperature from 1000 to 85°C. The material obtained being intended for aeronautical applications, the aging resistance from 1000 to 85°C is carried out to simulate the evolution of the properties of the material in service.

[0047] The inventors attribute this excellent behavior to the microstructure of the product which can be obtained according to the method described above. Without being bound by any theory, the inventors believe that an average equivalent diameter of the phases less than 100 nm makes it possible to improve the fatigue under spectrum, as well as the thermal stability.

[0048] In the present invention, a "phase" is a Mn dispersoid and / or a precipitate. The population of phases considered according to the invention has an equivalent diameter of from 35 nm to 500 nm. It is understood here that each phase considered has an equivalent diameter of from 35 nm to 500 nm. The primary intermetallics formed during casting are excluded. They are distinguished from the phases considered by the invention by their size much greater than 500 nm. The Mn dispersoids contain aluminum and Mn and optionally at least one element chosen from Fe, Cu, Si. The Mn dispersoids can be compounds of the Al-Mn, or Al-Mn-Fe, or Al-Mn-Cu-Fe or Al-Mn-Fe-Si type. The Mn dispersoids are formed during the homogenization step. For example, A1i5 (Mn,Fe)3Si2 can be considered a dispersoid. Precipitates contain a combination of the elements Al, Cu, Li. For example, Al3Li also called ô' phase can be considered a precipitate.The precipitates are formed at the time of tempering. Preferably, the precipitates according to the invention are Al3Li precipitates also called delta prime phase (δ').

[0049] According to the invention, the average equivalent diameter of the phases whose equivalent diameter is from 35 nm to 500 nm is less than or equal to 100 nm, preferably 90 nm. Preferably, the average equivalent diameter of the phases is at least 40 nm. Preferably, the average equivalent diameter of the Mn dispersoids whose equivalent diameter is from 35 nm to 500 nm is less than or equal to 100 nm, preferably 90 nm. If it is known that the presence of δ' (A13Li) precipitates is favorable for fatigue under spectrum, the inventors have found that a reduction in the equivalent diameter of the Mn dispersoids and / or of the precipitates is favorable for fatigue under spectrum.

[0050] Preferably, the density of phases whose equivalent diameter is from 35 nm to 500 nm is greater than or equal to 1.3 phases / pm2, preferably greater than or equal to 1.40 phases / pm2. The phase density corresponds to the number of phases per unit area. Preferably, the density of Mn dispersoids whose equivalent diameter is from 35 nm to 500 nm is greater than or equal to 1.3 Mn dispersoids / pm2, preferably greater than or equal to 1.40 Mn dispersoids / pm2. The inventors have found that it is preferable to combine phases with an average equivalent diameter less than or equal to 100 nm and a phase density greater than or equal to 1.3 phases / pm2 to improve the fatigue under the spectrum.

[0051] The density, the surface fraction and the average equivalent diameter of the phases (also noted as average Dcircle) can be determined using high-resolution techniques such as transmission electron microscopy (TEM) or scanning electron microscopy (SEM). It is possible to combine these techniques with a chemical identification of the phases using, for example, the EDX technique (Energy Dispersive Spectroscopy or Energy Dispersive X-ray Spectroscopy). Image analysis is preferably implemented to obtain automated processing allowing the distribution of the phases to be directly plotted. SEM observations combined with image analysis provide a good method for reporting the phase density, the average equivalent diameter of the phases (average Dcircle) and their surface fraction. Only phases with an equivalent diameter between 35 nm and 500 nm are quantified.This thresholding is performed to ignore intermetallics larger than 500 nm that form during casting or finer objects that do not allow for good resolution and would affect the analysis result. The results are preferably based on at least 200 images taken at high magnification (typical magnification greater than 20,000 X, preferably greater than 30,000 X) covering a total analyzed area of ​​at least 2,000 pm2. This allows for a large area of ​​the product to be covered without the inconvenience of processing large amounts of data.

[0052] The phase density corresponds to the ratio between the total number of phases, which have were identified by image analysis (e.g. by a gray level threshold set to distinguish the aluminum matrix from the phases), and the total surface analyzed.

[0053] The equivalent diameter of the phases or Dcircle corresponds to the equivalent diameter of a phase which would be of circular section and would have the same surface area as the observed phase, if the latter has a more complex section than that of a simple circle. The average Dcircle or average equivalent diameter of the phases corresponds to the average equivalent diameter of the circle having the same surface area as the average surface area of ​​all the phases.

[0054] It is also possible with image analysis to determine the surface fraction of phases. It corresponds to the ratio between the total surface covered by the phases and the total surface analyzed.

[0055] According to one procedure, it is advantageous for the surface fraction of phases whose equivalent diameter is from 35 nm to 500 nm to be less than or equal to 0.8%, preferably less than or equal to 0.7%. This micro structure can be obtained in the case where the solution treatment does not include a second step d2) between 480°C and 535°C. This micro structure is advantageous for improving the fatigue under the spectrum.

[0056] According to another procedure, it is advantageous for the surface fraction of phases whose equivalent diameter is from 35 nm to 500 nm to be greater than or equal to 1.0%, preferably 1.1%. This microstructure can be obtained in the case where the solution treatment comprises a second step d2) of 15 min to 8 h between 480°C and 535°C. This microstructure is advantageous for increasing the elongation in the TL direction.

[0057] The crystallographic texture can be described by a 3-dimensional mathematical function. This function is known in the art as the Orientation Density Function (ODF). It is defined as the volume fraction of the material dV / V having an orientation g to within dg: 6i / ; .... ...... —2— = ? ?i where (q> 1, ¢, q>2) are the Euler angles describing the orientation g.

[0058] The present inventors calculated the FDO of each sheet by the spherical harmonics method from four pole figures measured by X-ray diffraction on a traditional texture goniometer or by EBSD.

[0059] The present inventors used a tolerance of 15° around the orientations “copper”, “brass”, “S” in order to describe the texture obtained. The crystallographic orientations “copper”, “brass”, “S” are known to those skilled in the art and described for example in the reference document by UF Kocks, CN Tomé, and H.-R. Wenk, “Texture and anisotropy: preferred orientations in polycrystals and their effect on materials properties”. Cambridge University Press, 2000.

[0060] The orientations “copper”, “brass”, “S” are reproduced in the table below. Name Indices Bunge (cpi,O>, q>2) Kocks (W,0,0) Copper {112}<1H> 90,35,45 0,35,45 Brass {110} <112> 35,45,0 55,45,0 S {123} <634> 59,37,63 149,37,27

[0061] The inventors have found that the product preferably exhibits better fatigue behavior under spectrum if the sum of the volume fractions of the copper, brass and S texture components, measured at mid-thickness, is less than or equal to 5%.

[0062] Preferably, the product has an essentially recrystallized structure at mid-thickness. By essentially recrystallized structure is meant a structure which has at least 80%, preferably at least 90% of recrystallized grains. Examples

[0063] Example 1

[0064] 7 compositions were cast in plate form (Table 1). All alloys have a composition according to the invention.

[0065] [Table 1] - Chemical composition (% by weight) Reference Si Fe Cu Mn Mg Zn Ti Zr Li Ag A 0.03 0.04 2.51 0.35 0.32 - 0.03 0.02 1.49 0.03 B 0.02 0.04 2.50 0.35 0.34 - 0.03 0.04 1.47 0.08 C 0.02 0.04 2.47 0.33 0.35 - 0.04 0.03 1.5 0.00 D 0.03 0.04 2.51 0.35 0.32 - 0.03 0.02 1.49 0.03 E 0.02 0.04 2.50 0.35 0.34 - 0.03 0.04 1.47 0.08 F 0.03 0.04 2.36 0.35 0.34 - 0.03 0.02 1.48 0.03 G 0.03 0.04 2.48 0.36 0.35 - 0.04 0.03 1.51 0.03

[0066] All plates were homogenized, then reheated, then hot rolled (LAC), then solution treated, then quenched, then stretched and then tempered. The details of the manufacturing conditions and the references of the corresponding thick plates are given in Table 2.

[0067] [Table 2] - Manufacturing conditions Homo Reheating (°C) LAC Start (°C) LAC End (°C) Final Thickness (mm) Solution treatment Tensile Tempering Al Inv. 12h 508°C 492 470 408 31.6 143 min 557°C 4.0% 48h 155°C (teq =49h at 155°C) Bl Inv. 460 440 31.6 150 min 557°C 4.0% Gl Ref 460 426 32.8 92 min 497°C 3.9% A-2 Inv. 12h 508°C 492 470 408 31.6 143' 557°C 4.0% 80h 140°C (t_eq =31 h at 155°C) C-2 Ref 444 410 35 45' 497°C 3.8% D-2 Ref 470 408 31.6 lh37' 497°C 4.23% E-2 Ref 460 440 31.6 47' 497°C 3.93% F-2 Ref 435 368 31.6 51'497°C 4.07%

[0068] Sheets Al, A-2, Bl are thick sheets produced according to the invention. Sheet C-2 is a reference sheet already mentioned in patent EP3077559 (reference 3A in EP3077559).

[0069] The static mechanical characteristics of the sheets were measured at mid-thickness (t / 2) in the L direction and the toughness was measured at mid-thickness (t / 2) on test pieces with a width of 406 mm and a thickness B = 6.35 mm, in the LT direction.

[0070] In addition, the fatigue under spectrum representative of the intrados conditions of a commercial aircraft was measured according to the specification of an aircraft manufacturer on CCT type specimens, 12 mm thick, 700 mm long and 200 mm wide with a 30 mm notch. The fatigue under spectrum characterization specimens were taken so as to stress the LT direction and be centered 11 mm below the surface of the sheet. The fatigue under spectrum results were obtained after pre-cracking by fatigue until the crack reached 40 mm. The result obtained is the number of flights between 50 mm and 130 mm of crack propagation.

[0071] The yield strength and toughness were also characterized after low temperature aging of 1000-85°C to evaluate thermal stability. Only some sheets could be tested.

[0072] [Table 3] - Mechanical properties L LT LT lOOOh 85°C lOOOh 85°C Fatigue Rm (MPa) Rp0.2 (MPa) E% (%) Rp0.2 (MPa) Kapp (MPa^ / m ) Kapp (MPa'Vm) under spectrum number of flights Al Inv. 444 409 11.9 420 121 123 7014 Bl Inv. 441 407 13.2 418 122 122 7955 Gl Ref. 490 453 10.0 469 116 103 5213 A-2 Inv. 419 381 13.5 415 114 120 7393 C-2 Ref. 459 411 10.9 464 117 110 6922 D-2 Ref. 447 408 11.3 6443 E-2 Ref. 479 429 10.0 5490 F-2 Ref. 402 344 14.1 5920

[0073] The sheets produced according to the invention A1, A-2, B1 make it possible to achieve a higher number of fatigue flights under spectrum than the reference sheets. It is found that the sheets produced according to the invention make it possible to improve the elastic limit-toughness compromise after aging by 1000H at 85°C, unlike the reference products which see their toughness decrease after aging ([Fig.l]).

[0074] [Table 4] - Effect of thermal exposure of 1000°C at 85°C Variation of properties after thermal exposure delOOOh 85°C ATYS L t / 2 MPa AKapp LT t / 2 MPa^ / m Al Inv. 11 2 Bl Inv. 12 0 A-2 Inv. 34 6 Gl Ref. 16 -13 C-2 Ref. 53 -7

[0075] A texture measurement by X-ray diffractometry was carried out on each of the Al, A-2, Bl, C-2 and Gl sheets at mid-thickness. The analyzed surface is approximately 50 x 50 mm2. The texture of the D-2, E-2, F-2 sheets was done by EBSD. These two types of techniques give equivalent texture results. The volume fraction in % of the orientations, brass, copper and S, for each of the samples is given in Table 5.

[0076] [Table 5] - Results of texture measurements (volume fraction in %) Copper Cu {112} <111> % Brass L {110} <112> % S {123} <634> % Sum Cu+L+S % Al & A-2 Inv. 1.8 1.8 0.2 3.8 Bl Inv. 0.3 0.8 2.3 3.3 C-2 Ref. 6.3 31.6 32.0 69.9 D-2 Ref. 3.4 16.0 12.2 31.6 E-2 Ref. 5.5 34.7 23.9 64.1 F-2 Ref. 5.7 13.9 9.2 28.8 Gl Ref. 3.4 42.7 38.9 85.0

[0077] The sheets produced according to the invention Al, A-2, Bl are 100% recrystallized at mid-thickness. They have a sum of Copper + Brass + S of less than 5%. The texture of the Al and A-2 sheets are the same. The texture is not affected by the tempering conditions.

[0078] Example 2

[0079] A plate of composition H (Table 6) was rolled according to the conditions of the invention to obtain a sheet of thickness 24 mm. The rolled sheet was then cut and subjected to different solution treatment conditions, quenched, pulled and then tempered. The transformation conditions are indicated in Table 7 below.

[0080] [Table 6] - Chemical Composition (Wt%) Si Fe Cu Mn Mg Zn Ti Zr Li Ag H 0.03 0.05 2.51 0.37 0.34 0.12 0.035 0.015 1.51 0.031

[0081] [Table 7] - Processing Conditions Homo. Reheat. T (°C) LAC Start (°C) LAC End (°C) Solution Setup Traction Return Time equivalent to 155°C H-00 Ref. Inv. 12h 508°C 492 468 410 8h 500°C 2.50% 80h- 147°C 49h H-40 8h 540°C H-60 ​​8h 560°C H-75 8h 575°C H-60 ​​-bi 26h 250°C +297°C 48h 155°C 48h

[0082] Each of the sheets was tested in tension in the L direction and the TL direction at mid-thickness as well as in fatigue under spectrum. The fatigue under spectrum is representative of the intrados conditions of a commercial aircraft according to the specification of an aircraft manufacturer on CCT type specimens, 12 mm thick, 700 mm long and 200 mm wide with a 30 mm notch. This is the same fatigue spectrum as that indicated in Example 1. The fatigue under spectrum characterization specimens were taken at mid-thickness of the sheet. The fatigue under spectrum results were obtained after pre-cracking by fatigue until the crack reached 40 mm. The result obtained is the number of flights between 50 mm and 130 mm of crack propagation. The results are presented in Table 8.

[0083] [Table 8] - Solution setting T (°C) Fatigue under spectrum number of flights Rm (MPa) L Rp0.2 (MPa) LE% (%) L Rm (MPa) TL Rp0.2 (MPa) TL E% (%) TL H-00 Ref. 500 7452 435 398 13.5 451 387 6.5 H-40 Inv. 540 7971 421 377 13.6 445 381 5.3 H-60 ​​560 8588 433 388 11.8 455 395 4.5 H-75 575 9987 430 387 10.9 448 386 3.9 H-60 ​​Bi 560+497 8358 423 384 12.2 440 381 5.5

[0084] These same sheets were characterized by scanning electron microscopy in order to quantify the phase distribution. The microstructural characterization is done in the L-TC plane in such a way as to make the observations at the mid-thickness of the sheet. The quantification is done in such a way as to have approximately 200 images of size 5 x 3.75 pm which covers a total analyzed surface of 3750 pm2. Only phases of sizes between 35 nm and 500 nm were considered. This makes it possible to avoid large intermetallics which could be linked to the casting.

[0085] The phase density corresponds to the ratio between the total number of phases identified by the image analysis (by a gray level threshold set to distinguish the aluminum matrix from the phases), and the total analyzed surface area. The average phase surface fraction, Fs, corresponds to the ratio between the total surface area of ​​the analyzed phases and the analyzed surface area. The average equivalent diameter of the phases corresponds to the average Dcircle. The average Dcircle corresponds to the equivalent diameter of the circle having the same surface area as the average surface area of ​​the analyzed phases.

[0086] Phases are observed in all samples. However, in the samples according to the invention, the phases are smaller in size, with an average Dcircle less than 100 nm. The morphological parameters of the analyzed phases are given in Table 9.

[0087] [Table 9] - Solution Temperature (°C) Fatigue under spectrum number of flights LT Average Dcircle (pm) Density (phase / pm2) Average Fs (%) H-00 Ref. 500 7452 0.124 0.8 1.24 H-40 Inv. 540 7971 0.072 1.44 0.56 H-60 ​​Inv. 560 8588 0.066 1.96 0.49 H-75 Inv. 575 9987 0.054 1.95 0.33 H-60 ​​Bi Inv. 560+497 8358 0.081 1.79 1.16

[0088] A correlation is observed between the number of fatigue flights under spectrum and the solution temperature ([Fig.2]). The inventors noted that the improvement in fatigue under spectrum was inversely correlated with the value of average Dcircle. Similarly, a good correlation is obtained with the phase density.

[0089] Sample H60-bi corresponds to the case of solution treatment according to the invention with a step dl) from 540°C to 580°C followed by a step d2) from 480°C to 535°C. In this case, it is observed that the second stage d2) tends to increase the surface fraction of the phases beyond 1.0% while maintaining an average equivalent diameter of the phases less than 100 nm and / or a density greater than or equal to 1.3 part / pm2. Samples H-40, H-60 ​​and H-75 according to the invention having undergone a single step dl) from 540°C to 580°C have an average surface fraction less than or equal to 0.8%. The sample H60-bi according to the invention with a step dl) from 540°C to 580°C followed by a step d2) from 480°C to 535°C has an average surface fraction greater than or equal to 1.0%.

[0090] The inventors have found that such a microstructure makes it possible to maintain good fatigue under spectrum but also makes it possible to increase the elongation in the LT direction. Indeed, it is found that the value of the elastic limit and the breaking stress are little modified by the solution temperature; however the elongation value in the LT direction decreases with the solution temperature. It seems, however, that the solution treatment comprising two steps dl) and d2) makes it possible to improve the elongation in the LT direction.

Claims

Claims

1. A method of manufacturing a rolled or forged product of lithium copper aluminum alloy in which: (a) an alloy plate of composition, in % by weight, Cu: 2.3 - 2.7 Li: 1.3-1.7 Mg: 0.2 - 0.5 Mn: 0.2 - 0.5 Ag: 0-0.1 Zn: < 0.20 Ti: 0.01-0.15 Zr <0.07 Fe: <0.1 Si: <0.1 other elements < 0.05 each and < 0.15 in total, remainder aluminum, (b) said plate is homogenized from 480°C to 540°C for 5 to 60 hours, (c) said homogenized plate is hot-formed by rolling to obtain an intermediate product having a thickness in the range of 15 mm to 50 mm, the final hot-forming temperature being at least 400°C, (d) said intermediate product is dissolved, (e) said intermediate product dissolved is quenched with water, (f) said intermediate product dissolved and quenched is pulled in a controlled manner with a permanent deformation of 2 to 5%,(g) said intermediate product thus dissolved, quenched and pulled is tempered by heating from 120 to 170°C for 5 to 100 hours, characterized in that the dissolution in step (d) comprises a step dl) of at least 15 minutes and less than 8 hours during which said intermediate product is at a temperature of from 540°C to 580°C.,

2. Manufacturing process according to claim 2 such that the dissolution in step d) comprises a step d2) after step dl) of 15 min to 8 h during which said intermediate product is at a temperature of from 480°C to 535°C.

3. A manufacturing method according to claim 1 or 2 such that the time equivalent t_eq at 155°C of said tempering is 30 to 60 hours, the equivalent time t_eq at 155°C being defined by the formula: f esp(-114ÛO / T) dt t ea = ---:--------— exp(-1140D / Tref) where T (in Kelvin) is the instantaneous treatment temperature, which changes with time t (in hours), and Tref is a reference temperature fixed at 428 K, t_eq is expressed in hours.

4. Manufacturing method according to claim 3 such that the equivalent time t_eq at 155°C is 45 to 55 hours.

5. Rolled product with a thickness of 15 mm to 50 mm made of lithium copper aluminum alloy having a composition, in % by weight, Cu: 2.3 - 2.7 Li: 1.3-1.7 Mg: 0.2 - 0.5 Mn: 0.2 - 0.5 Ag: 0-0.1 Zn: < 0.20 Ti: 0.01-0.15 Zr <0.07 Fe: <0.1 Si: <0.1 other elements < 0.05 each and < 0.15 in total, remainder aluminum, characterized in that the population of phases having an equivalent diameter of 35 to 500 nm has an average equivalent diameter less than or equal to 100 nm.

6. Rolled product according to claim 5 characterized in that the density of phases whose equivalent diameter is 35 to 500 nm is greater than or equal to 1.3 phases / pm2.

7. Rolled product according to claim 5 or 6 characterized in that the surface fraction of phases whose equivalent diameter is 35 to 500 nm is less than or equal to 0.8%.

8. Rolled product according to claim 5 or 6 characterized in that the surface fraction of phases whose equivalent diameter is 35 to 500 nm is greater than or equal to 1.0%.

9. Rolled product according to any one of claims 5 to 8 characterized in that the sum of the volume fractions of the texture components, measured at mid-thickness, copper, brass and S is less than or

10. equal to 5%. Rolled product according to any one of claims 5 to 9 characterized in that at mid-thickness the granular structure of said product is essentially recrystallized.

Images