Aluminum-copper-lithium alloy product for underwing element having improved properties

Abstract

The present invention relates to a rolled product with a thickness of between 15 and 50 mm which is made of aluminum alloy having 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, the remainder being aluminum, wherein the phases having an equivalent diameter of 35 to 500 nm have a mean equivalent diameter of less than or equal to 100 nm. The products according to the invention are obtained by a method in which, in particular, the hot working conditions are such that the final hot working temperature is at least 400°C and that the solution heat treatment of the product comprises a step of at least 15 minutes and less than 8 hours between 540°C and 580°C.

Description

[0001] Description

[0002] Title of the invention: ALUMINUM-COPPER-LITHIUM ALLOY PRODUCT FOR INNER SIDE ELEMENT WITH IMPROVED PROPERTIES

[0003] Technical field

[0004] 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.

[0005] Prior art

[0006] In aluminum alloys, it is generally accepted that the resistance to fatigue crack propagation under variable amplitude loading (called "spectrum fatigue" in this application) decreases as the yield strength increases and vice versa (Rioja RJ. et al. "The role of crystallographic texture on the performance of flat rolled aluminum products for aerospace applications" LIGHT METALS-WARRENDALE-PROCEEDINGS- 2008, 1065)

[0007] Prasad et al. in Aluminum-Lithium alloys - processing properties and applications - Elsevier edition 2014 in chapter 11 pp 341-375 mentions that fatigue improvement of lithium alloys is achieved by solid solution hardening and coalescence of delta prime precipitates. It is also possible to improve fatigue by thermal and thermomechanical treatments involving tempering (also known as artificial aging) or cold deformation before tempering.

[0008] US9234566 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 not greater than about 225°F (107°C) and for at least about 20 hours. U.S. Patent 5,032,359 describes a broad family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, particularly between 0.3 and 0.5 percent by weight, increases mechanical strength.

[0009] 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. Wrought products made with these alloys combine a density of less than 2.64 g / cm3 and an interesting compromise between mechanical strength and toughness.

[0010] 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.

[0011] 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.

[0012] Patent EP 1,891,247 describes an alloy for fuselage sheets comprising (in wt%) (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 from among Zr, Mn, Cr, Sc, Hf and Ti, in which the Cu and Li contents meet the condition Cu + 5 / 3 Li < 5.2.

[0013] 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 wt%) (2.0-6.5) Cu, (0.2-2.7) Li, (0-4.0) Mg, (0-4.0) Ag, (0-3.0) Zn.

[0014] 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.

[0015] 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 suited to the manufacture of aircraft wing lower surface elements.

[0016] 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 No. 1 (1997) pp 51 - 59 describes the fatigue crack growth behavior of three Al-Cu-Li-Mg-Ag alloys for load ratios of 0.1 and 0.75.

[0017] EP3077559 discloses a method for manufacturing a rolled or forged material having a thickness of 14 to 100 mm. Said material consists of an aluminum alloy composed, in wt%, 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 having a density of 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 heating 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 found that the product disclosed in EP3077559 makes it possible to obtain satisfactory fatigue resistance under the spectrum but that after aging for 1000 hours or 3000 hours at 85°C the product saw its toughness decrease.

[0018] CN 113 718 096 discloses a method for preparing a high-performance aluminum-lithium alloy plate. The method comprises the steps of casting, rolling, solution treatment, quenching, tensile treatment and tempering.

[0019] The article by Fragomeni et al. "Effect of single and duplex aging on precipitation response microstructure and fatigue crack behavior in Al-Li-Cu alloy AF / C-458", Journal of Materials Engineering and Performance - Volume 14 (1) February 2005 pp 18-27 discloses the evolution of the size, distribution, morphology, volume fraction, density and interparticle spacing of the 8' and Tl precipitates as a function of tempering conditions for an AlCuLi alloy of composition AI-1.8 wt. Li - 2.7% Cu - 0.3 Mg-0.5Zr-0.3Mn-0.8Zn, designated AF / C 458. It shows that the fatigue propagation threshold increases when the size of the 8' precipitates is less than 20 nm. This result, however, contradicts other results. One can cite for example the thesis of Rao, KTVenkateswara "Mechanisms of Fatigue Crack Propagation and Fracture Toughness Behavior in Advanced Aluminum-Lithium Alloys" published in 1988 which shows on page 66 that for an AA2091 type alloy, the fatigue propagation threshold is higher in the T8 state than in the T3 state, thus showing that the presence of 8' precipitates is beneficial in this case.

[0020] In order to lighten aeronautical structures, the fatigue resistance under spectrum must be further improved, as well as the yield strength-toughness compromise, even after low temperature aging, preferably aging of 1000H at 85°C.

[0021] The purpose of this application is to propose a method for obtaining a product improving the fatigue resistance under spectrum and the elastic limit-toughness compromise even after aging of 1000H at 85°C.

[0022] Statement of the invention

[0023] 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:

[0024] (a) a plate of alloy composition, in % by weight, is cast: 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:

[0025] < 0.1; other elements < 0.05 each and < 0.15 in total, remainder aluminum.

[0026] (b) homogenizing said plate from 480°C to 540°C for 5 to 60 hours.

[0027] (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.

[0028] (d) said intermediate product is dissolved.

[0029] (e) quenching said dissolved intermediate product with water.

[0030] (f) said intermediate product, dissolved and quenched, is pulled in a controlled manner with a permanent deformation of 2 to 5%.

[0031] (g) said intermediate product thus dissolved, quenched and pulled is tempered by heating from 120 to 170°C for 5 to 100 hours.

[0032] 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 from 540°C to 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 from 480°C to 535°C.

[0033] Advantageously, the equivalent time t_eq at 155°C of said tempering is 30 to 60 hours, even more preferably 45 to 55 hours.

[0034] The equivalent time t_eq at 155 °C is defined by the formula: 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.

[0035] Another subject of the invention is a 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. This product is 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. 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.

[0036] Advantageously, the density of phases whose equivalent diameter is 35 to 500 nm is greater than or equal to 1.3 phases / pm 2. 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 / pm 2 .

[0037] 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%.

[0038] 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%.

[0039] Advantageously, at mid-thickness, the granular structure of the product is essentially recrystallized.

[0040] Figures

[0041] Figure 1 represents the yield strength-toughness trade-off with and without aging under the conditions of Example 1.

[0042] Figure 2 represents the evolution of the number of fatigue flights under spectrum as a function of the solution temperature, according to the conditions of example 2.

[0043] Figure 3 represents the evolution of the number of fatigue flights under spectrum as a function of the average Dcircle value of the phases including the Mn dispersoids and the 8' precipitates according to the conditions of example 2.

[0044] Figure 4 represents the evolution of the number of fatigue flights under spectrum as a function of the value of the area of ​​the first dissolution peak measured by differential scanning calorimetry according to the conditions defined in example 2.

[0045] Detailed description of the invention

[0046] Unless otherwise stated, all information regarding the chemical composition of alloys is expressed as a percentage by weight based on the total weight of the alloy. Alloy designations are made 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.

[0047] Unless otherwise stated, the static mechanical characteristics, in other words the breaking strength R m , the elastic limit in tension R po,2 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. The inventors found that by modifying the chemical composition, the solution treatment conditions and the tempering conditions, it was possible to improve the number of fatigue flights under spectrum and the thermal stability of the product.

[0048] 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.

[0049] An aluminum alloy plate is cast according to the invention.

[0050] The copper content of the alloy according to the invention is from 2.3% to 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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%.

[0055] 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%.

[0056] 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.

[0057] 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. 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 iron and silicon content 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 is aluminum.

[0058] The cast plate is then homogenized. The homogenization treatment is carried out at a temperature between 480°C and 540°C for 5 to 60 hours. Preferably, the homogenization temperature is between 490°C and 510°C.

[0059] After homogenization, the plate is generally cooled to room temperature before being preheated for hot deformation by rolling. The objective of preheating is 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.

[0060] 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.

[0061] 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 an 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 rate, and the rolling speed.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] The sheet is then quenched in water, typically water at room temperature, preferably below 40°C.

[0066] The product then undergoes a controlled traction of 2 to 5% and preferably at least 3%, typically around 4%.

[0067] Tempering is then carried out at a temperature of 120 to 170°C for 5 to 100 hours, preferably 140 to 160°C for 30 to 90 hours. Preferably, the tempering 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 hours, 34 hours, 36 hours. 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 machinability of the product. In another preferred embodiment, the equivalent time t_eq at 155°C is 45 to 55 hours in order to improve the thermal stability of the product.

[0068] The equivalent time t_eq at 155 °C is defined by the formula: f exp(-11400 / T) dt

[0069] * eq = - - : - — exp 11400 / Tret ) where T (in Kelvin) is the instantaneous processing temperature, which changes with time t (in hours), and T re f 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.

[0070] The preferred metallurgical states for sheets are T8 states, more particularly T84 or T86.

[0071] The inventors found that the product obtained by the process according to the invention made it possible to obtain very good fatigue resistance under the spectrum as well as good thermal stability after aging at low temperature from 1000 to 85°C. Since the material obtained is intended for aeronautical applications, the aging resistance of 1000 to 85°C is carried out to simulate the evolution of the properties of the material during service. The inventors attribute this excellent behavior to the microstructure of the product which can be obtained according to the process 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 fatigue under the spectrum, as well as thermal stability.

[0072] 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, Al5(Mn,Fe)sSi2 can be considered as a dispersoid. Precipitates contain a combination of the elements Al, Cu, Li. For example, AhLi also called phase 8' can be considered as a precipitate.The precipitates are formed at the time of tempering. Preferably, the precipitates according to the invention are AhLi precipitates also called delta prime phase (8').

[0073] 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 6' precipitates (Al3 Li) 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.

[0074] Preferably, the density of phases whose equivalent diameter is between 35 nm and 500 nm is greater than or equal to 1.3 phases / pm 2 , preferably greater than or equal to 1.40 phases / pm 2 . The phase density corresponds to the number of phases per unit area. Preferably, the density of Mn dispersoids with an equivalent diameter of 35 nm to 500 nm is greater than or equal to 1.3 Mn dispersoids / pm 2 , preferably greater than or equal to 1.40 dispersoids at Mn / pm 2 The inventors found that it was 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 / pm. 2to improve the fatigue under spectrum. 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 direct plotting of the phase distribution. 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 2000 μm. 2 This allows a large area of ​​the product to be covered without the inconvenience of processing large amounts of data.

[0075] Phase density is the ratio of the total number of phases, which have been identified by image analysis (e.g. by a gray level threshold set to distinguish the aluminum matrix from the phases), to the total surface area analyzed.

[0076] 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 section more complex 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.

[0077] 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.

[0078] 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 microstructure can be obtained in the case where the solution treatment does not include a second step d2) between 480°C and 535°C. This microstructure is advantageous for improving the fatigue under the spectrum. 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 includes 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.

[0079] 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: where (4>1, d>, 4>2) are the Euler angles describing the orientation g.

[0080] 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.

[0081] 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.

[0082] The “copper”, “brass”, “S” orientations are reproduced in the table below.

[0083] 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%.

[0084] Preferably, the product has a structure that is essentially recrystallized at mid-thickness. By essentially recrystallized structure is meant a structure that has at least 80%, preferably at least 90% of recrystallized grains. Examples

[0085] Example 1

[0086] 7 compositions were cast in plate form (Table 1). All alloys have a composition according to the invention. [Table 1] - Chemical composition (% by weight)

[0087] All plates were homogenized, then reheated, then hot rolled (LAC), then solution treated, then quenched, then stretched and then tempered. Details of the manufacturing conditions and the references of the corresponding thick plates are given in Table 2.

[0088] [Table 2] - Manufacturing conditions 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). 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 specimens of width 406 mm and thickness B = 6.35 mm, in the LT direction.

[0089] In addition, the fatigue 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 spectrum characterization specimens were taken so as to load the LT direction and be centered 11 mm below the surface of the sheet. The fatigue 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.

[0090] The yield strength and toughness were also characterized after low temperature aging of 1000-85°C to assess thermal stability. Only some sheets could be tested.

[0091] [Table 3] - Mechanical properties 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 noted that the sheets produced according to the invention make it possible to improve the yield strength-toughness compromise after aging of 1000H at 85°C, unlike the reference products which see their toughness decrease after aging (figure 1). [Table 4] - Effect of thermal exposure of 1000H 85°C

[0092] 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 mm 2 . The texture of sheets D-2, E-2, F-2 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.

[0093] [Table 5] - Results of texture measurements (volume fraction in %)

[0094] 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.

[0095] Example 2

[0096] 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 processing conditions are indicated in Table 7 below. [Table 6] - Chemical composition (in % by weight)

[0097] [Table 7] - Transformation conditions

[0098] Each of the sheets was tested in tension in the L and TL directions at mid-thickness as well as in fatigue spectrum. The fatigue 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 spectrum characterization specimens were taken at mid-thickness of the sheet. The fatigue 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.

[0099] [Table 8] -

[0100] 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 cover a total analyzed surface of 3750 pm 2 . Only phases with an equivalent diameter between 35 nm and 500 nm were considered. This makes it possible to avoid large intermetallics that could be linked to the casting.

[0101] The phase density corresponds to the ratio between the total number of phases identified by 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.

[0102] Phases are observed in all samples. It is observed that 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. The quantification of the phases indicated in Table 9 takes into account the phases having a size of 35 nm to 500 nm without distinguishing whether the phases are Mn dispersoids or §' precipitates.

[0103] [Table 9] -

[0104] A correlation is observed between the number of fatigue flights under spectrum and the solution temperature (Figure 2). The inventors found that the improvement in fatigue under spectrum was inversely correlated with the average Dcircle value (Figure 3). Similarly, a good correlation is obtained with the phase density.

[0105] Sample H60-bi corresponds to the case of a 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 / pm 2 . 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%. 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%.

[0106] The inventors found that such a microstructure allows good fatigue under spectrum to be maintained but also allows the elongation in the LT direction to be increased. Indeed, it is found that the value of the yield strength and the breaking stress are little modified by the solution temperature; however the elongation value in the LT direction decreases with the solution temperature. However, it seems that the solution treatment comprising two stages dl) and d2) allows the elongation in the LT direction to be improved.

[0107] In a second step, the inventors independently analyzed the Mn dispersoids and the §' precipitates. 8' precipitates were observed in all samples. Only samples H-00 and H-60 ​​Bi contain Mn dispersoids larger than 35 nm. Data processing was performed on these two samples to determine the average Dcircle of the Mn dispersoids and the 8' precipitates (Table 10).

[0108] [Table 10]

[0109] The inventors found that the product outside the invention contains a significant quantity of Mn dispersoids, with an average Dcircle greater than 100nm, unlike those of the invention.

[0110] Differential scanning calorimetry (DSC) tests were carried out on the products to quantify the quantity of 8' precipitates. The tests consisted of heating each product at a rate of 20°C / min from room temperature to a temperature of approximately 620°C and recording the change in the differential signal over temperature. At around 90°C, a first dissolution peak is recorded, which is representative of the quantity of 8' precipitates present. It is possible to quantify the proportion of 8' precipitates by the area of ​​the dissolution peak, expressed in J / g. The larger the area value, the greater the quantity of 8' precipitates. [Table 11]

[0111] It is observed that the fatigue resistance under spectrum is not correlated with the quantity of 8' precipitates (Table 11 and Figure 4). The inventors therefore believe that the improvement in fatigue under spectrum observed with the solution temperature is mainly influenced by the Mn dispersoids and that preferably the value of the average Dcircle of the Mn dispersoids is less than 100 nm.

Claims

Claims 1. Process for manufacturing a rolled or forged product made of lithium copper aluminum alloy in which: (a) a plate of alloy of composition, in % by weight, is cast, 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 If: < 0.1 other elements < 0.05 each and < 0.15 in total, aluminum remains, (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 intermediate product in solution 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 between 480°C and 535°C.

3. Manufacturing method according to claim 1 or 2 such that the equivalent time 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: J exp(- 11400 / T) dt exp(- 11400 / Tre0 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 in 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 If: < 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, where the phases are Mn dispersoids and / or precipitates and where the measurements are made by SEM, based on at least 200 images covering a total analyzed surface of at least 2000 pm2.

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 / pm 2 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 equal to 5%.

10. 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, such that the product has at least 80% recrystallized grains.

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