EP2235224B1 - MANUFACTURING METHOD OF AN Al-Li ROLLED PRODUCT FOR AERONAUTICAL APPLICATIONS - Google Patents

Description

Field of the invention

The present invention generally relates to aluminum-lithium alloys and, in particular, such products useful in the aeronautical industry.

State of the art

Aluminum and lithium (Al-Li) alloys have long been recognized as an effective solution for reducing the weight of structural elements due to their low density. However, the different properties required for materials used in the aerospace industry, such as high yield strength, high compressive strength, high damage tolerance, and high corrosion resistance, have proved difficult. to obtain simultaneously. Al-Li alloys are particularly sensitive to the crack bifurcation which is part of the problems related to the damage tolerance limiting the use of Al-Li alloys, ( Hurtado, JA; de los Rios, ER; Morris, A.J., "Crack deflection in Al-Li alloys for aircraft structures," 18th Symposium of the International Committee on Aeronautical Fatigue, Melbourne, UNITED KINGDOM, 3-5 May 1995, pp. 107-136. ).

Crack bifurcation, crack deflection, crack rotation, or crack branching are terms used to express the propensity for the propagation of a crack to deviate from the expected plane of fracture perpendicular to the load applied during a stress test. fatigue or tenacity. The crack bifurcation occurs at the microscopic (<100 μm), mesoscopic (100-1000 μm) or macroscopic scale (> 1 mm) scale, but it is considered harmful only if the direction of the crack remains stable after bifurcation (macroscopic scale). This phenomenon is of particular concern for fatigue tests in the LS direction for aluminum-lithium alloys. The term crack bifurcation is used here for the macroscopic crack bifurcation during fatigue or toughness testing in the LS direction, from the S direction to the L direction which occurs for rolled products whose thickness is from less than 30 mm. The crack bifurcation may occur in relation to the composition of the rolled product, its microstructure and the test conditions. Rolled products in AA7050 alloy can be considered as a product reference with a low tendency to crack bifurcation.
Bifurcation of cracks has been considered a major problem by aircraft manufacturers because it is difficult to take into account for sizing of elements, which makes it impossible to use traditional design methods. Thus, crack bifurcation invalidates material testing procedures and traditional design methods, based on mode I propagation. The problem of crack bifurcation has proved difficult to solve. Recently it has been envisaged that in the absence of a solution to avoid the bifurcation of cracks, the efforts should be oriented on the prediction of crack bifurcation behaviors. ( MJ Crill, DJ Chellman, Balmuth ES, Philbrook M, KP Smith, Cho A., Niedzinski M., Muzzolini R. and Feiger J., Evaluation of AA 2050-T87 Al-Alloy Crack Turning Behavior, Materials Science Forum, Flight 519-521 (July 2006) pp 1323 - 1328 ).
There is a need for a laminated aluminum lithium alloy product for aeronautical applications, particularly for fully machined parts having a low tendency to crack bifurcation.

Object of the invention

1. a) la coulée d'une plaque comprenant 2,2 à 3,9 % en poids de Cu, 0,7 à 2,1 % en poids de Li, 0,2 à 0,8 % en poids de Mg, 0,2 à 0,5 % en poids de Mn, 0,04 à 0,18 % en poids de Zr, moins de 0,05 % en poids de Zn, et optionnellement 0,1 à 0,5 % en poids de Ag, reste aluminium et impuretés inévitables,
2. b) l'homogénéisation de ladite plaque entre 470 °C et 510 °C pour une durée de 2 à 30 heures,
3. c) le laminage à chaud de ladite plaque pour obtenir une tôle d'au moins 30 mm d'épaisseur, avec une température de sortie d'au moins 410 °C,
4. d) la mise en solution entre 490 °C et 540 °C pendant 15 mn à 4h, de façon à ce que le temps équivalent total pour l'homogénéisation et la mise en solution t(eq) $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{\int \exp \left(-\mathrm{26100}/\mathrm{T}\right)\mathrm{dt}}{\exp \left(-\mathrm{26100}/{\mathrm{T}}_{\mathrm{ref}}\right)}$$
   
   ne dépasse pas 30h et de manière préférée 20h, où T (en Kelvin) est la température instantanée de traitement, qui évolue avec le temps t (en heures), et T_ref est une température de référence fixée à 773 K,
5. e) la trempe à l'eau froide,
6. f) la traction contrôlée de la dite tôle avec une déformation permanente de 2 à 5%,
7. g) le revenu de ladite tôle par chauffage entre 130°C et 160 °C pendant 5 à 60 heures.

The object of the invention is a method of manufacturing a substantially unrequistallized sheet having a thickness of at least 30 mm having a low propensity for crack bifurcation, the method comprising:

1. a) casting a plate comprising 2.2 to 3.9% by weight of Cu, 0.7 to 2.1% by weight of Li, 0.2 to 0.8% by weight of Mg, O, 2 to 0.5% by weight of Mn, 0.04 to 0.18% by weight of Zr, less than 0.05% by weight of Zn, and optionally 0.1 to 0.5% by weight of Ag, remains aluminum and unavoidable impurities,
2. b) homogenizing said plate between 470 ° C and 510 ° C for a period of 2 to 30 hours,
3. c) hot rolling said plate to obtain a sheet of at least 30 mm thickness, with an outlet temperature of at least 410 ° C,
4. d) dissolving between 490 ° C and 540 ° C for 15 minutes to 4 hours, so that the total equivalent time for homogenization and dissolution t (eq) $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{\int \exp \left(-\mathrm{26100}/\mathrm{T}\right)\mathrm{dt}}{\exp \left(-\mathrm{26100}/{\mathrm{T}}_{\mathrm{ref}}\right)}$$
   
   does not exceed 30h and preferably 20h, where T (in Kelvin) is the instantaneous treatment temperature, which changes with time t (in hours), and T _ref is a reference temperature set at 773 K,
5. e) quenching with cold water,
6. f) the controlled traction of said sheet with a permanent deformation of 2 to 5%,
7. g) the income of said sheet by heating between 130 ° C and 160 ° C for 5 to 60 hours.

Description of figures

• Figure 1: Schematic representation of the location of the Sinclair sample.
• Figure 2 : geometry of the Sinclair sample.
• Figure 3 : schematic representation of the mixed mode test conditions I and II used on the Sinclair sample.
• Figure 4 : Schematic representation of the method of determining the deflection angle on a fractured Sinclair sample.
• Figure 5 : evolution of the deflection angle with the maximum equivalent stress intensity factor for two homogenization treatments applied to the same alloy and for a standard AA7050 alloy sheet.
• Figure 6 : geometry of the sample used for fatigue tests in the LS direction.
• Figure 7 : photographs of samples after an LS fatigue test.
• Figure 8 : photographs of 25 or 30 mm thick samples after LS fatigue test

Detailed description of the invention

Unless stated otherwise, all the information concerning the chemical composition of the alloys is expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed in% by weight is multiplied by 1.4. The designation of alloys is in accordance with the regulations of The Aluminum Association, known to those skilled in the art. The definitions of the metallurgical states are given in the European standard EN 515.

Unless otherwise stated, the static mechanical characteristics, in other words the _ultimate tensile strength R _m , the conventional yield stress at 0.2% elongation R _p0.2 and the elongation at break A, are determined by a tensile test according to EN 10002-1, the sampling and the direction of the test being defined by EN 485-1. Unless otherwise stated, the definitions of EN 12258-1 apply.

The cracking rate (da / dN) is determined according to ASTM E 647.

The stress intensity factor (K _1C ) is determined according to ASTM E 399.

There are three modes of rupture. The mode I, or mode by opening, is characterized in that one exerts a stress perpendicular to the faces of the crack. Mode II, where plane bias mode, has a shear stress perpendicular to the crack front. Finally, the mode III, or anti-plane biasing mode, is a mode in which the shear stress is parallel to the crack front.

The propensity for crack bifurcation is generally observed during a fatigue or LS test. A quantitative result is obtained with a crack propagation test carried out in mixed mode I and II on a sample SL. Samples and test conditions for studying bi-axial fatigue properties have been described by HA Richard ("Specimens for Investigating Biaxial Fracture and Fatigue Properties," Biaxial and Multiaxial Fatigue, EGF 3 (Edited by MW Brown and KJ Miller), 1989, Mechanical Engineering Publications, London pp 217-229 ). S9 samples described by Richard are used in the context of the present invention. The reasoning for linking crack crack propensity in LS fatigue or toughness tests to deflection angles measured for mixed mode I and II tests is described by Sinclair and Gregson ("The effects of mixed mode loading on intergranular failure in AA7050-T7651", Materials Science Forum, Vol.242 (1997) pp 175-180 ). The objective is to reproduce the local stress occurring at the crack end of a LS sample after bifurcation. Figure 1 schematically shows a crack bifurcation on an LS sample and the location of the sample proposed by Sinclair ("the Sinclair sample"). An LS (1) sample with elongated grains (3) stressed (2) with an initial crack in mode I (4) undergoes a crack bifurcation towards the L direction (deviated crack (5)). The Sinclair sample (6) is a SL sample and the initial crack corresponds to a 90 ° bifurcated crack in an LS sample. If the crack of the Sinclair sample is stable when subjected to a mixed mode I and II stress representative of the stress experienced by the bifurcated crack, then the bifurcated crack would have been stable and the sample has a high propensity to crack bifurcation. The geometry of the Sinclair sample is given Figure 2 . Six orifices (61) are used to attach the Sinclair sample to the test device. The sample is mechanically pre-milled, the length of the pre-crack is 7 mm.

The Sinclair sample is subjected to mixed mode I and II stress in accordance with the Figure 3 . Two sample holders (71) and (72) are used to subject the sample to mixed mode I and II stress. The samples are attached to the sample holders by the six orifices (61) to form an assembly that is stressed between the orifices (711) and (721). The angle Ψ of application of the load between a plane perpendicular to the initial direction of crack and the direction of the stress is 75 °. It may be noted that the angle Ψ is the angle complementary to the angle of inclination of the crack with respect to the axis of stress.

où P est la charge (N), a est la longueur de fissure (mm), W est la largeur de l'échantillon (mm), t est l'épaisseur de l'échantillon (mm). Pour des tests de fatigue, la charge maximale est référencée P_max et le facteur d'intensité de contrainte correspondant est référencé K_max. Les facteurs de forme F_I et F_II, qui correspondent aux mode I et au mode II, respectivement, sont pour la géométrie de l'échantillon donnés par $$F_I=\frac{\mathrm{cos\Psi }}{1-\frac{a}{W}}\sqrt{\frac{0,26+2,65\left(\frac{a}{W-a}\right)}{1+0,55\left(\frac{a}{W-a}\right)-0,08{\left(\frac{a}{W-a}\right)}^2}}$$

$$F_{\mathit{II}}=\frac{\mathrm{sin\Psi }}{1-\frac{a}{W}}\sqrt{\frac{-0,23+1,40\left(\frac{a}{W-a}\right)}{1-0,67\left(\frac{a}{W-a}\right)+2,08{\left(\frac{a}{W-a}\right)}^2}}$$

où Ψ est l'angle entre un plan perpendiculaire à la direction initiale de fissure et la direction de la contrainte.The stress intensity factors K _I and K _II are obtained according to $$K_{I,\mathit{II}}=\frac{P\sqrt{\pi \mathrm{.}\mathrm{at}}}{\mathit{Wt}}{F}_{I,\mathit{II}}$$

where P is the load (N), a is the crack length (mm), W is the width of the sample (mm), t is the thickness of the sample (mm). For fatigue tests, the maximum load is referenced P _max and the corresponding stress intensity factor is referenced K _max . The form factors F _I and F _II , which correspond to the modes I and II, respectively, are for the geometry of the sample given by $$F_I=\frac{\mathrm{cos\Psi }}{1-\frac{\mathrm{at}}{\mathrm{<figure-callout\; id="0"\; label="W"\; filenames="imgf0005.png"\; state="\{\{state\}\}">W</figure-callout>}}}\sqrt{\frac{0,26+2,65\left(\frac{\mathrm{at}}{W-\mathrm{at}}\right)}{1+0,55\left(\frac{\mathrm{at}}{W-\mathrm{at}}\right)-0,08{\left(\frac{\mathrm{at}}{W-\mathrm{at}}\right)}^2}}$$

$$F_{\mathit{II}}=\frac{\mathrm{sin\Psi }}{1-\frac{\mathrm{at}}{W}}\sqrt{\frac{-0,23+1,40\left(\frac{\mathrm{at}}{W-\mathrm{at}}\right)}{1-0,67\left(\frac{\mathrm{at}}{W-\mathrm{at}}\right)+2,08{\left(\frac{\mathrm{at}}{W-\mathrm{at}}\right)}^2}}$$

where Ψ is the angle between a plane perpendicular to the initial crack direction and the direction of the stress.

The equivalent stress intensity factor K _eff is obtained according to $$K_{\mathit{eff}}=\sqrt{\left(1-v^2\right)K_I^2+\left(1-v^2\right){\mathrm{<figure-callout\; id="2"\; label="K\; II"\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">K\; </figure-callout>}}_{\mathit{II}}^{\mathit{}}+\left(1+\mathrm{<figure-callout\; id="2"\; label="v\; K\; III"\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">v\; </figure-callout>}\right)K_{\mathit{III}}^{}{\mathit{}}_2^{}}$$

For the geometry used in the test K _III = 0. K _{eff max} is the maximum stress intensity factor during a fatigue cycle, it corresponds to the maximum load P _max .

The deflection angle Θ between the initial crack direction and the direction of the deflected crack allows a quantitative evaluation of the propensity for crack bifurcation. It is measured as described in Figure 4 . The Figure 4 is a representation of a broken Sinclair sample (61). The profile (65) of the broken sample is measured using a profilometer with steps of 0.5 mm. The data obtained is smoothed by a sliding average over three points. The deflection angle is measured for each set of three points. The maximum deflection angle between the end of the mechanical crack (69) and a distance of 32 mm from the edge of the sample is the value of Θ.

A plot of Θ versus K _{eff max} provides a quantitative measure that can be related to the propensity to crack bifurcation for an LS sample. For a given value of K _{eff max,} higher values of Θ indicate a lower propensity for crack bifurcation. However, for reasons explained in the article by Sinclair and Gregson already mentioned, for values of K _{eff max} less than about 5 MPa √m or greater than about 15 MPa √m, the value of Θ is not discriminating between the samples. For this reason, the value of Θ is particularly significant for K _{eff max} = 10 MPa √m.

According to the invention, a substantially non-recrystallized laminate having a thickness of at least 30 mm has a low propensity for crack bifurcation if the crack deflection angle Θ is at least 20 ° and preferably at least 20 °. 30 ° under a maximum equivalent stress intensity factor K _{eff max} of 10 MPa √m for a cracked SL test specimen subjected to mixed mode stress I and II, (Ψ = 75 °). The article by Sinclair and Gregson clearly shows that for an AA7050 alloy sample, known to have a low propensity for crack bifurcation, the condition on the angle Θ is reached.

Here, a "structural element" or "structural element" of a mechanical construction is called a mechanical part for which the static and / or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural calculation is usually prescribed or realized. These are typically elements whose failure is likely to endanger the safety of said construction, its users, its users or others. For an aircraft, these structural elements include the elements that make up the fuselage (such as fuselage skin (fuselage skin in English), stiffeners or stringers, bulkheads, fuselage (circumferential frames), wings (such as wing skin), stiffeners (stiffeners), ribs (ribs) and spars) and empennage including horizontal stabilizers and vertical stabilizers horizontal or vertical stabilizers, as well as floor beams, seat tracks and doors.

A substantially uncrystallized laminate product having a thickness of at least 30 mm according to the invention has a low propensity for crack bifurcation due to the combination of a carefully selected composition and specific steps of the manufacturing process.

The laminated aluminum-lithium alloy product according to the invention comprises 2.2 to 3.9% by weight of Cu, 0.7 to 2.1% by weight of Li, 0.2 to 0.8% by weight of Mg, 0.2 to 0.5 wt% Mn, 0.04 to 0.18 wt% Zr, less than 0.05 wt% Zn, and optionally 0.1 to 0.5 wt%. weight of Ag, remains aluminum and unavoidable impurities. Preferably, the content of iron and silicon is at most 0.15% by weight each or preferably 0.10% by weight and the content of the other unavoidable impurities is at most 0.05% by weight each and 0 15% by weight in total. Preferably, a refining agent containing titanium is added during casting. The titanium content is preferably between 0.01 and 0.15% by weight and preferably between 0.01 and 0.04% by weight. The copper content is preferably at least 2.7% by weight or even at least 3.2% by weight so as to achieve sufficient strength. The lithium content is preferably at least 0.8% by weight and even more preferably at least 0.9% by weight, so as to obtain a low density. In some embodiments of the invention, the maximum lithium content is limited to 1.8 wt% or even 1.4 wt% and more preferably 1.25 wt%. The invention is particularly advantageous for alloys which simultaneously contain a high lithium content and a high copper content, because these alloys have a very favorable compromise of mechanical properties but are particularly sensitive to the bifurcation of cracks. In one embodiment Advantageously, the content of Li and Cu, expressed in% by weight, are in accordance with Li + Cu> 4 and preferably Li + Cu> 4.3. However, if the alloy simultaneously contains a very high content of Li and Cu, burn phenomena can occur during homogenization. In a preferred embodiment of the invention, the contents of Li and Cu, expressed in% by weight are in accordance with Li + 0.7 Cu <4.3 and preferably Li + 0.5 Cu <3.3.

Manganese is an essential component of the laminated product according to the invention and its content is carefully selected, preferably between 0.3 and 0.5% by weight. Carefully controlled distribution of manganese dispersoids obtained through the combination of selected content and thermo-mechanical processing conditions helps to avoid stress localization and grain boundary constraints. Although not related to any specific theory, the inventors believe that the distribution of the manganese-containing dispersoids obtained according to the invention contributes to the low propensity for crack bifurcation.

The performance in terms of strength and toughness observed by the inventors are generally difficult to achieve for alloys containing no silver, especially when the permanent deformation after controlled pulling is less than 3%. The present inventors believe that silver plays a role during the formation of the copper-containing hardening phases formed during natural or artificial aging, and, in particular, allows the formation of finer phases and also allows a more homogeneous distribution of these phases. . The advantageous effect of silver is observed when the silver content is at least 0.1% by weight and preferably at least 0.2% by weight. An excessive addition of silver would probably be prohibitively expensive in many cases because of the high price of silver, and it is advantageous not to exceed a content of 0.5% by weight and preferably 0.3% by weight. in weight.

The addition of magnesium improves the mechanical strength and decreases the density. Too high an addition of Mg can, however, be detrimental to toughness. In an advantageous embodiment of the invention, the Mg content is at most 0.4% by weight. The present inventors believe that the addition of Mg may also play a role in the formation of copper-containing phases.

An alloy containing controlled amounts of alloying elements is cast as a plate.

The plate is homogenized at a temperature between 470 ° C and 510 ° C for 2 to 30 hours. A homogenization temperature of at least 470 ° C and preferably at least 490 ° C simultaneously makes it possible to form the dispersoids and to prepare an effective solution solution. The present inventors have found that a homogenization temperature greater than about 510 ° C causes a higher propensity for crack bifurcation. The present inventors believe that high homogenization temperatures affect the size and distribution of manganese-containing dispersoids.

A hot rolling step is performed after reheating if necessary to obtain sheets having a thickness of at least 30 mm. A hot rolling exit temperature of at least 410 ° C, preferably at least 430 ° C, and preferably at least 450 ° C, is necessary to obtain a substantially non-recrystallized product after dissolution. . The term product essentially not recrystallized a product whose recrystallization rate is less than 10% to quarter and half thickness (T / 4 and T / 2). The sheets are dissolved by heating between 490 and 540 ° C for 15 minutes to 4 hours and quenched with cold water. The dissolution parameters depend on the thickness of the product. It is important to avoid the coalescence of dispersoids during dissolution, as this may compromise the effect of the carefully controlled homogenization treatment. Thus, the total equivalent time for homogenization and dissolution t (eq) does not exceed 30h and preferably 20h.

dans laquelle T est la température instantanée exprimée en Kelvin qui évolue avec le temps t (en heures) et T_ref est une température de référence de 500 °C (773 K). t(eq) est exprimé en heures. La constante Q/R = 26100 K est dérivée de l'énergie d'activation pour la diffusion du Mn, Q = 217000 J/mol. La formule donnant t(eq) tient compte des phases de chauffage et de refroidissement.The equivalent time t (eq) at 500 ° C is defined by the formula: $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{\int \exp \left(-\mathrm{26100}/\mathrm{T}\right)\mathrm{dt}}{\exp \left(-\mathrm{26100}/{\mathrm{T}}_{\mathrm{ref}}\right)}$$

where T is the instantaneous temperature expressed in Kelvin which changes with time t (in hours) and T _ref is a reference temperature of 500 ° C (773 K). t (eq) is expressed in hours. The constant Q / R = 26100 K is derived from the activation energy for the diffusion of Mn, Q = 217000 J / mol. The formula giving t (eq) takes into account the heating and cooling phases.

Quenching with cold water is carried out after dissolution. In an advantageous embodiment of the invention, rapid quenching is performed. Fast quenching means that the cooling rate is as high as possible given the thickness of the sheet. In an advantageous embodiment of the invention, vertical immersion quenching is preferably carried out by horizontal spraying quenching. The present inventors have observed that fast quenched products have a lower propensity to crack bifurcation. The present inventors believe that this effect could be related to a lower precipitation at the grain boundaries.

The product then undergoes a controlled pull with a permanent deformation of between 2% and 5% and preferably between 3% and 4%. The income is made at a temperature between 130 ° C and 160 ° C for a period of 5 to 60 hours, resulting in a T8 state. In some cases, and particularly for some preferred compositions, the yield is preferably between 140 and 160 ° C for 12 to 50 hours. Lower tempering temperatures generally favor higher toughness.

The products according to the invention have a low propensity for crack bifurcation, which means that when a cracked sample SL having a thickness of at least 30 mm and preferably at least 60 mm is tested under a mixed mode I and II (Ψ = 75 ° and K _{eff max} = 10 MPa √m) the crack deflection angle Θ is at least 20 ° and preferably at least 30 °.

• a1 : la limite élastique R_P0,2 à T/4 et T/2 est au moins de 455 MPa, préférentiellement au moins 460 MPa ou même au moins 465 MPa dans le sens L.
• a2 : la résistance à rupture R_m à T/4 et T/2 est au moins 490 MPa, préférentiellement au moins 495 MPa ou même au moins 500 MPa dans le sens L.
• b1 : la ténacité K1C: dans le sens L-T à T/4 et T/2 est au moins 31 MPa√m, préférentiellement au moins 32 MPa√m ou même au moins 33 MPa√m.
• b2 : la ténacité K1C: dans le sens T-L à T/4 et T/2 est au moins 28 MPa√m et préférentiellement au moins 29 MPa√m ou même au moins 30 MPa√m.
• b3 : la ténacité K1C: dans le sens S-L à T/4 et T/2 est au moins 25 MPa√m et préférentiellement au moins 26 MPa√m ou même au moins 27 MPa√m.

The propensity for crack bifurcation is also observed for fatigue tests in the LS direction. A low propensity for crack bifurcation also means that for the products according to the invention, a crack bifurcation is observed on less than 20% and preferably less than 10% of the samples of a batch of at least 4 LS samples. hole according to the figure 6 , fatigue tested according to ASTM E 647 (R = 0.1, σ _max = 220 MPa). Other advantageous properties of the products according to the invention whose thickness is between 30 and 100 mm include at least one of the characteristics a1 and a2 and at least one of the characteristics b1, b2 and b3 in the T8 state, where the characteristics a1, a2, b1, b2 and b3 are defined by:

• a1: the elastic limit R _P0.2 to T / 4 and T / 2 is at least 455 MPa, preferably at least 460 MPa or even at least 465 MPa in the L direction.
• a2: the breaking strength R _m at T / 4 and T / 2 is at least 490 MPa, preferably at least 495 MPa or even at least 500 MPa in the L direction.
• b1: the toughness K1C: in the direction LT to T / 4 and T / 2 is at least 31 MPa√m, preferably at least 32 MPa√m or even at least 33 MPa√m.
• b2: the tenacity K1C: in the direction TL at T / 4 and T / 2 is at least 28 MPa√m and preferably at least 29 MPa√m or even at least 30 MPa√m.
• b3: the tenacity K1C: in the direction SL to T / 4 and T / 2 is at least 25 MPa√m and preferably at least 26 MPa√m or even at least 27 MPa√m.

• a4 : la limite élastique R_P0,2 à T/4 et T/2 est au moins de 440 MPa, préférentiellement au moins 445 MPa ou même au moins 450 MPa dans le sens L.
• a5 : la résistance à rupture R_m à T/4 et T/2 est au moins 475 MPa, préférentiellement au moins 480 MPa ou même au moins 485 MPa dans le sens L.
• b4 : la ténacité K1C: dans le sens L-T à T/4 et T/2 est au moins 26 MPa√m, préférentiellement au moins 27 MPa√m ou même au moins 28 MPa√m.
• b5 : la ténacité K1C: dans le sens T-L à T/4 et T/2 est au moins 25 MPa√m et préférentiellement au moins 26 MPa√m ou même 27 MPa√m.
• b6 : la ténacité K1C: dans le sens S-L à T/4 et T/2 est au moins 24 MPa√m et préférentiellement au moins 25 MPa√m ou même au moins 26 MPa√m.

Other advantageous properties of the products according to the invention whose thickness is greater than 100 mm include at least one of characteristics a4 and a5 and at least one of characteristics b4, b5 and b6 in the T8 state, where the characteristics a4, a5, b4, b5 and b6 are defined by:

• a4: the elastic limit R _P0.2 at T / 4 and T / 2 is at least 440 MPa, preferably at least 445 MPa or even at least 450 MPa in the direction L.
• a5: the breaking strength R _m at T / 4 and T / 2 is at least 475 MPa, preferably at least 480 MPa or even at least 485 MPa in the L direction.
• b4: toughness K1C: in the direction LT to T / 4 and T / 2 is at least 26 MPa√m, preferably at least 27 MPa√m or even at least 28 MPa√m.
• b5: the tenacity K1C: in the direction TL at T / 4 and T / 2 is at least 25 MPa√m and preferably at least 26 MPa√m or even 27 MPa√m.
• b6: the tenacity K1C: in the direction SL to T / 4 and T / 2 is at least 24 MPa√m and preferably at least 25 MPa√m or even at least 26 MPa√m.

The products according to the invention have a high resistance to corrosion. The products according to the invention tested under the MASTMAASIS (Modified ASTM Acetic Acid Salt Intermittent Spray) conditions according to the ASTM G85 standard reach the EA level and, preferably, the P level (pitting alone). The stress corrosion resistance according to the ASTM G47 standard of the products according to the invention reaches a 30-day hold for ST samples subjected to a stress of 300 MPa and preferably at a stress of 350 MPa.

The products according to the invention can advantageously be used in structural elements. A structural member made of a laminate according to the present invention may typically include a spar, rib or frame for aircraft construction in a preferred manner. The invention is particularly advantageous for parts of complex shape obtained by integral machining, used in particular for the manufacture of aircraft wings and for any other use for which the properties of the products according to the invention are advantageous. .

EXAMPLES Example 1

Two AA2050 alloy plates, referenced A and B, were cast. Their composition is given in Table 1. For comparison purposes, an alloy plate AA7050 in the state T7451 has also been tested for crack bifurcation. Its composition is also given in Table 1. Table 1. Composition (% by weight) of the different plates. Yes Fe Cu mn mg Ti Zr Li Ag Zn AT 0.03 0.04 3.46 0.39 0.4 0.02 0.10 0.88 0.39 0.02 B 0.04 0.05 3.60 0.39 0.4 0.02 0.09 0.91 0.37 0.02 7050 0.04 0.09 2.11 0.01 2.22 0.02 0.11 - - 6.18

Plate A was homogenized according to the invention for 12 hours at 500 ° C. (rise speed: 15 ° C./h, time equivalent to 500 ° C.:16.7h). Plate B (reference) was homogenized for 8 hours at 500 ° C. and then for 36 hours at 530 ° (rise rate: 15 ° C./hr, time equivalent to 500 ° C. 140 h). Plate A was hot rolled to a 60 mm thick sheet and the hot roll output temperature was 466 ° C. The sheet thus obtained was dissolved for 2 hours at 504 ° C. (rise rate: 50 ° C./h, time equivalent to 500 ° C./2.9 hours) and quenched with cold water. Plate B was hot rolled to a 65 mm thick sheet and the hot roll output temperature was 494 ° C. The sheet thus obtained was dissolved for 2 hours at 526 ° C (rise rate: 50 ° C / h, time equivalent to 500 ° C: 6h) and quenched with cold water. Both sheets were controlled in a controlled manner, with a permanent elongation of 3.5% and were 18 hours at 155 ° C. The sheets from plates A and B are referenced sheet A-60 and sheet B-60, respectively. The total equivalent time at 773 K for homogenization and dissolution t (eq) was therefore 19.6 h and 146 h for sheets A-60 and B-60, respectively.

The samples were mechanically tested to determine their static mechanical properties and toughness. The tensile strength R _m , the conventional yield stress at 0.2% elongation R _p0.2 and elongation at break A are given in Table 2 and the toughness K _1C is given in the table. 3. Table 2. Static mechanical properties. Sample T / 4 T / 2 The LT The LT R _m MPa R _p0.2 MPa AT (%) R _m MPa R _p0.2 MPa AT (%) R _m MPa R _p0.2 MPa AT (%) R _m MPa R _p0.2 MPa AT (%) Sheet A-60 511 484 13 511 464 10 518 477 10 481 441 13 Sheet B-60 531 500 11.2 521 474 8.8 531 489 8.1 490 449 10.9 K1c MPa√lm) Sample T / 4 T / 2 LT TL LT TL SL Sheet A-60 44.6 36.7 51.0 39.8 33.2 Sheet B-60 42.5 36.7 40.4 40.8 33.4

"Sinclair samples" as described on the Figures 1 and 2 and having the characteristics width W = 40 mm and thickness 5 mm, were taken from sheets A-60 and B-60 to T / 2 and tested in fatigue (R = 0.1). The test geometry described in Figure 3 was used. The fatigue tests were carried out for several values of K _{eff max} and the deflection angle Θ was measured on the broken samples according to the method described on the Figure 4 . The results obtained are presented on the Figure 5 and in Table 4. Table 4. Deviation angle Θ measured after an SL fatigue test under a mixed mode I and II stress Sheet A-60 Sheet B-60 7050 Keff max (MPa √m) Charge (N) Number of cycles Θ (°) Charge (N) Number of cycles Θ (°) Charge (N) Number of cycles Θ (°) 5 2221 800700 57 2216 900,000 49 7.5 3364 336500 * 50 3351 297600 51 3317 240,000 42 10 4457 102300 34 4468 44500 4 4423 71500 31 15 6715 3400 4 6662 2300 11 6648 1,700 4 * the break occurred in the jaws

Sheet A-60 has a deflection angle Θ greater than 20 ° for a K _max value of 10 MPa √m, which demonstrates a low propensity for crack bifurcation. This result was confirmed by fatigue tests on LS specimens. Four LS samples according to Figure 6 were taken from sheet A-60 and sheet B-60 and subjected to a fatigue test (σmax = 220 MPa, R = 0.1) in mode I. Figures 7a and 7b show, respectively, the four samples from sheets A-60 and B-60 after the fatigue test. The results are consistent with those obtained in tests on SL samples under mixed mode I and II stress: all samples from sheet B-60 show a severe crack bifurcation while the samples from sheet A-60 show only mode I crack propagation.

Example 2

Two AA2050 alloy plates referenced A 'and C and two AA2195 alloy reference plates, referenced D and E, were cast. Their composition is given in Table 5. Table 5. Composition (% by weight) of the different plates. Yes Fe Cu mn mg Ti Zr Li Ag Zn AT' 0.03 0.04 3.46 0.39 0.4 0.02 0.10 0.88 0.39 0.02 VS 0.02 0.05 3.56 0.41 0.35 0.03 0.09 0.93 0.37 0.02 D 0.03 0.04 4.2 - 0.4 0.02 0.11 1.06 0.35 0.02 E 0.03 0.06 4.3 0.3 0.4 0.02 0.12 1.17 0.35 0.01

The plate A 'was homogenized according to the invention for 12 hours at 500 ° C. (rise speed: 15 ° C./h, time equivalent to 500 ° C.:16.7h). Plate C (reference) was homogenized for 8 hours at 500 ° C and then for 36 hours at 530 ° (rise rate: 15 ° C / h, time equivalent to 500 ° C: 140h). Plate A 'was hot rolled to a 30 mm thick sheet and the hot roll output temperature was 466 ° C. The sheet thus obtained was dissolved for 2 hours at 505 ° C. (rise rate: 50 ° C./h, time equivalent to 500 ° C. for 3.0 hours) and quenched with cold water. Plate C was hot rolled to a 30 mm thick sheet and the hot roll output temperature was 474 ° C. The sheet thus obtained was dissolved for 5 hours at 525 ° C. (rise rate: 50 ° C./h, time equivalent to 500 ° C. for 15.7 hours) and quenched with cold water. Both sheets were controlled in a controlled manner, with a permanent elongation of 3.5% and were 18 hours at 155 ° C. The sheets from the plates A 'and C are referenced sheet A'-30 and sheet C-30, respectively.

The plates D and E were homogenized for 15 hours at 492 ° C. (rise rate: 15 ° C./hr, time equivalent to 500 ° C. 11.5h). Plate D was hot rolled to a 25 mm thick sheet and the hot roll output temperature was 430 ° C. The sheet thus obtained was dissolved for 5 hours at 510 ° C. (rise speed: 50 ° C./h, time equivalent to 500 ° C: 8.4h) and quenched with cold water. Plate E was hot rolled to a 30 mm thick sheet and the hot rolling exit temperature was 411 ° C. The sheet thus obtained was dissolved for 4.5 h at 510 ° C. (rise rate: 50 ° C./h, time equivalent to 500 ° C.: 7.6 h) and quenched with cold water. The two sheets were controlled in a controlled manner, with a permanent elongation of 4.3% and a 24 hour income at 150 ° C. The sheets from the plates D and E are referenced sheet D-25 and sheet E-30, respectively. The total equivalent time at 773 K for homogenization and dissolution in solution t (eq) was therefore 19.7h, 155.7h, 19.9h and 19.1h for the sheets A'-30, C-30, D-25 and E-30, respectively.

Fatigue tests on LS specimens were performed on samples from sheets A'-30, C-30, D-25 and E-30. Four LS samples according to Figure 6 were taken from each of the sheets and subjected to a fatigue test (σmax = 220 MPa, R = 0.1) in mode I. Figures 8a, 8b, 8c and 8d show, respectively, the four samples from plates A'-30, C-30, D-25 and E-30 after the fatigue test. Only the samples from the sheet A'-30 do not have crack bifurcation while the samples from sheet C-30, D-25 and E-30 have in at least one case a severe crack bifurcation. The method according to the invention which combines a particular composition and defined homogenization and dissolution conditions makes it possible to obtain a sheet free from crack bifurcation A'-30, while the C-30 plates (temperature of 30.degree. high homogenization) and D-25 and E-30 (high copper content) plates do not allow this.

Claims (8)

1. Method of manufacturing an essentially unrecrystallised plate with a thickness of at least 30 mm with low propensity to crack bifurcation, comprising the following steps:

   a) casting a slab containing 2.2 to 3.9% by weight of Cu, 0.7 to 2.1% by weight of Li, 0.2 to 0.8% by weight of Mg, 0.2 to 0.5% by weight of Mn, 0.04 to 0.18% by weight of Zr, less than 0.05% by weight of Zn and optionally 0.1 to 0.5% by weight of Ag, the remainder being aluminium and inevitable impurities,

   b) homogenising said slab at between 470°C and 510°C for a duration of 2 to 30 hours,

   c) hot rolling said slab to obtain a plate at least 30 mm thick, with an exit temperature of at least 410°C

   d) solution heat treating the plate at between 490°C and 540°C for 15 minutes to 4h, such that the total equivalent time for homogenising and solution heat treating t(eq) $$\mathrm{t}\left(\mathrm{eq}\right)=\frac{\int \exp \left(-\mathrm{26100}/\mathrm{T}\right)\mathrm{dt}}{\exp \left(-\mathrm{26100}/{\mathrm{T}}_{\mathrm{ref}}\right)}$$

   

   does not exceed 30h and preferably 20h, in which T (in Kelvin) is the instantaneous treatment temperature, that varies with the time t (in hours), and T_ref is a reference temperature fixed at 773 K,

   e) quenchng in cold water

   f) stretching in a controlled manner of said plate with a permanent deformation of 2 to 5%

   g) annealing of said plate by heating to between 130°C and 160°C for 5 to 60 hours.
2. Method according to claim 1, in which the lithium and copper contents expressed as a % by weight satisfy the relation Li + Cu > 4 and preferably Li + Cu > 4.3
3. Method according to claim 1 or claim 2, in which the lithium and copper contents expressed as a % by weight satisfy the relation Li + 0.7 Cu < 4.3 and preferably Li + 0.5 Cu < 3.3.
4. Method according to claim 3, in which the lithium content is between 0.9 and 1.4% by weight and is preferably between 0.9 and 1.25% by weight.
5. Method according to any one of claims 1 to 4, in which the copper content is between 2.7 and 3.9% by weight and is preferably between 3.2 and 3.9% by weight.
6. Method according to any one of claims 1 to 5, in which the manganese content is between 0.3 and 0.5% by weight.
7. Method according to any one of claims 1 to 6, in which said temperature at the exit from hot rolling is at least 430°C and is preferably at least 450°C.
8. Method according to any one of claims 1 to 7, in which said annealing is done by heating to between 140°C and 160°C for 12 to 50 hours.

Images