EP1026270A1 - AlCuMg alloy product for aircraft body member - Google Patents

Abstract

The subject of the invention is a rolled, extruded or forged product of AlCuMg alloy, treated by dissolution, quenching and cold traction, intended for the manufacture of aircraft structural elements, of composition (% by weight): Fe <0.15 Si <0.15 Cu: 3.8 - 4.4 Mg: 1 - 1.5 Mn: 0.5 - 0.8 Zr: 0.08 - 0.15 other elements: <0, 05 each and <0.15 in total, aluminum remains, and having an Rm (L) / R0.2 (L) ratio> 1.25. The invention applies more particularly to the manufacture of lower surfaces of wings, having a set of properties (toughness, speed of crack propagation, resistance to fatigue, level of residual stresses) improved compared to the 2024 alloy. .

Description

Technical area

The invention relates to rolled, extruded or forged products of hardened AlCuMg alloy. and towed, intended for the manufacture of aircraft structural elements, in particular skin panels and airfoil stiffeners, and having, by compared to the products of the prior art used for the same application, a improved compromise between the properties of mechanical strength, formability, toughness, tolerance for damage and residual stresses. The designation of alloys and metallurgical states corresponds to the nomenclature of Aluminum Association, taken up by European standards EN 515 and EN 573.

State of the art

The wings of large commercial aircraft have a top (or upper surface) made of a skin made from thick alloy sheets 7150 in state T651, or in alloy 7055 in state T7751 or 7449 in state T7951, and stiffeners made from profiles of the same alloy, and a lower part (or lower surface) made of a skin made from thick sheets of alloy 2024 to state T351 or 2324 in state T39, and stiffeners made from profiles of same alloy. The two parts are assembled by side members and ribs.

Alloy 2024 according to the designation of the Aluminum Association or standard EN 573-3 has the following chemical composition (% by weight):
If <0.5 Fe <0.5 Cu: 3.8 - 4.9 Mg: 1.2 - 1.8 Mn: 0.3 - 0.9 Cr <0.10 Zn <0.25 Ti <0 , 15
Different variants have been developed and deposited with the Aluminum Association under the designations 2224, 2324 and 2424, with in particular more limited contents of silicon and iron. Alloy 2324 in the T39 state was the subject of patent EP 0038605 (= US 4294625) of Boeing, in which the improvement of the elastic limit is obtained by work hardening using a pass of cold rolling after quenching. This work hardening tends to decrease the toughness and, to compensate for the drop in toughness, the Fe, Si, Cu and Mg contents are reduced. Boeing has also developed the composition 2034 alloy:
If <0.10 Fe <0.12 Cu: 4.2 - 4.8 Mg: 1.3 - 1.9
Mn: 0.8 - 1.3 Cr <0.05 Zn <0.20 Ti <0.15 Zr: 0.08 - 0.15
This alloy was the subject of patent EP 0031605 (= US 4,336,075). Compared to 2024 in the T351 state, it has a better specific elastic limit due to the increase in the manganese content and the addition of another anti-recrystallizing agent (Zr), as well as a toughness and improved fatigue resistance.

Alcoa's patent EP 0473122 (= US 5,213,639) describes an alloy, registered at Aluminum Association as 2524, composition: Si <0.10 Fe <0.12 Cu: 3.8 - 4.5 Mg: 1.2 - 1.8 Mn: 0.3 - 0.9 which may contain possibly another anti-recrystallizing agent (Zr, V, Hf, Cr, Ag or Sc). This alloy is intended more particularly for thin sheets for fuselage and has a toughness and improved crack propagation resistance compared to 2024.

The applicant's patent application EP 0731185 relates to an alloy, registered subsequently under n ° 2024A, of composition: Si <0.25 Fe <0.25 Cu: 3.5 - 5 Mg: 1 - 2 Mn <0.55 with the relation: 0 <Mn - 2Fe <0.2 Thick sheets of this alloy have both improved toughness and reduced level of residual stresses, without loss on other properties.

Patents US 5863359 and US 5865914 of Alcoa relate respectively to an aircraft wing comprising an intrados made of an alloy of composition:
Cu: 3.6 - 4 Mg: 1 - 1.6 (pref: 1.15 - 1.5) Mn: 0.3 - 0.7 (pref: 0.5 - 0.6) Zr: 0, 05 - 0.25 and preferably Fe <0.07 and Si <0.05
having both the following properties: R 0.2 (LT)> 60 ksi (414 MPa) and K 1 C (LT)> 38 ksi√inch (42 MPa√m) ,
and a method of manufacturing a lower surface element having a R 0.2 (LT)> 60 ksi comprising the casting of an alloy of the above composition, homogenization between 471 and 482 ° C, hot transformation at a temperature> 399 ° C, solution dissolving above 488 ° C, quenching, work hardening preferably more than 9% cold and at least 1% traction.

Problem

For the construction of new large capacity commercial aircraft, it is certainly imperative to limit the weight, so that the specifications of the manufacturers impose higher typical constraints for the wing panels, which results in higher minimum values for the static mechanical characteristics and the damage tolerance of the aluminum alloy products used. The use of products hardened in the T39 state, such as those recommended in the patents US 5863359 and US 5865914, if it leads to high elastic limits R _0.2 , however presents a certain number of drawbacks for d '' other important properties of use in the intended application. Indeed, this results in a plastic difference, that is to say a difference between the breaking strength R _m and the elastic limit R _0.2 , very reduced, which results in lower cold formability. and poorer resistance to propagation of fatigue cracks with variable amplitude loading. In fact, the slowing down of the crack propagation after partial overload is less important if the plastic gap is reduced.

In addition, larger workpieces must be machined without distortion in thicker sheets, which implies better control of the stress level residual. However, the T39 state was not very favorable from this point of view.

The object of the invention is therefore to provide AlCuMg alloy products in the state cold hardened and deformed, intended for the manufacture of lower surfaces of aircraft wings, and having, compared to similar products of the prior art, a more favorable for all the properties of use: mechanical resistance, speed of crack propagation, toughness, fatigue strength, and stress rate residual.

Subject of the invention

The subject of the invention is a rolled, extruded or forged product of AlCuMg alloy, treated by dissolution, quenching and cold traction, intended for the manufacture of aircraft structural elements, of composition (% by weight):
Fe <0.15 Si <0.15 Cu: 3.8 - 4.4 (pref .: 4.0 - 4.3) Mg: 1.0 - 1.5 Mn: 0.5 - 0.8 Zr : 0.08 - 0.15 other elements: <0.05 each and <0.15 in total, having a ratio R _m (L) / R _0.2 (L) of the breaking strength in the direction L at the elastic limit in direction L, greater than 1.25 (and preferably 1.30).

a) Résistance à la rupture R_m(L) > 475 Mpa et limite d'élasticité R_0,2(L) > 370 MPa

b) Ecart plastique R_m - R_0,2 sens L et TL > 100 MPa

c) Facteur d'intensité critique (sens L-T) Kc > 170 MPa√m et Kco > 120 MPa√m (mesurés selon la norme ASTM E561 sur des éprouvettes entaillées prélevées à quart-épaisseur avec les paramètres B = 5 mm, W = 500 et 2a₀ = 165 mm)

d) Vitesse de propagation de fissures (L-T) da/dn, mesurée selon la norme ASTM E 647 sur des éprouvettes entaillées prélevées à quart-épaisseur avec W = 200 mm et B =5mm : < 10-4 mm/cycle pour ΔK= 10 MPa√m < 2,5 10-4 mm/cycle pour ΔK = 15 MPa√m et < 5 10-4 mm/cycle pour ΔK = 20 MPa√m

Cette tôle présente également un niveau de contraintes résiduelles tel que la flèche f mesurée dans les sens L et TL après usinage à mi-épaisseur d'un barreau reposant sur deux supports distants d'une longueur l est telle que :
   f < (0,14 l²)/e f étant mesurée en microns, l'épaisseur e de la tôle et la longueur l étant exprimées en mm.It also relates to a laminated product (a sheet) of the same composition with a thickness of between 6 and 60 mm and having in the quenched and drawn state at least one of the following groups of properties;

a) Tensile strength R _{m (L)} > 475 Mpa and yield strength R _{0.2 (L)} > 370 MPa

b) Plastic deviation R _m - R _0.2 direction L and TL> 100 MPa

c) Critical intensity factor (LT sense) K vs > 170 MPa√m and K co > 120 MPa√m (measured according to standard ASTM E561 on notched test pieces taken at quarter thickness with parameters B = 5 mm, W = 500 and 2a ₀ = 165 mm)

d) Crack propagation speed (LT) da / dn, measured according to standard ASTM E 647 on notched test pieces taken at quarter thickness with W = 200 mm and B = 5 mm: <10 -4 mm / cycle for ΔK = 10 MPa√m <2.5 10 -4 mm / cycle for ΔK = 15 MPa√m and <5 10 -4 mm / cycle for ΔK = 20 MPa√m

This sheet also has a level of residual stresses such that the deflection f measured in the directions L and TL after machining at mid-thickness of a bar resting on two distant supports of a length l is such that:
f <(0.14 l ² ) / ef being measured in microns, the thickness e of the sheet and the length l being expressed in mm.

• coulée d'une plaque ou d'une billette de la composition indiquée,
• homogénéisation de cette plaque ou billette entre 450 et 500°C,
• transformation à chaud et éventuellement à froid jusqu'au produit désiré,
• mise en solution à une température comprise entre 480 et 505°C,
• trempe à l'eau froide,
• traction à froid avec au moins 1,5% de déformation permanente,
• vieillissement naturel à l'ambiante.

The subject of the invention is also a method of manufacturing a rolled, extruded or forged product comprising the following steps:

• pouring a plate or billet of the indicated composition,
• homogenization of this plate or billet between 450 and 500 ° C,
• hot transformation and possibly cold until the desired product,
• dissolved at a temperature between 480 and 505 ° C,
• cold water quenching,
• cold traction with at least 1.5% permanent deformation,
• natural aging at room temperature.

Description of the invention

The chemical composition of the product differs from that of the usual 2024 by a content reduced in iron and silicon, a higher manganese content and an addition of zirconium. Compared to 2034, we have a lower manganese content and a slightly reduced copper content. Compared to the composition of the alloys described in patents US 5863359 and US 5865914, the copper content is higher, which which compensates, for mechanical strength, cold work hardening less high after quenching. Surprisingly, this narrow area of composition (especially with regard to manganese), associated with modifications of the manufacturing range, leads, compared to the prior art, to an improvement significant of the compromise between mechanical strength, elongation and tolerance damage to the operating conditions of a large civil aircraft.

In addition, and quite unexpectedly, we observe, for thick products, a low residual stress rate, allowing distortion-free machining of parts large.

The manufacturing process involves the casting of plates, in the case where the product to be manufactured is a rolled sheet, or of billets in the case where it is a extruded section or a forged part. The plate or billet is scalped, then homogenized between 450 and 500 ° C. The hot transformation is then carried out by rolling, spinning or forging. This transformation is preferably carried out at a higher temperature than the temperatures usually used, the outlet temperature being greater than 420 ° C. and preferably at 440 ° C. so as to obtain on the treated product a structure which is not very recrystallized, with a rate recrystallization at quarter thickness less than 20%, and preferably 10%. The rolled, extruded or forged semi-finished product is then placed in solution between 480 and 505 ° C., so that this dissolution is as complete as possible, that is to say that the maximum of potentially soluble phases, in particular the precipitates Al ₂ Cu and Al ₂ CuMg, ie effectively in solid solution. The quality of the dissolution can be assessed by differential enthalpy analysis (AED) by measuring the specific energy using the area of the peak on the thermogram. This specific energy should preferably be less than 2 J / g.

Then we proceed to quenching with cold water, and a controlled traction leading to a permanent elongation of at least 1.5%. The product finally undergoes aging natural at room temperature.

The products according to the invention have significantly improved static mechanical characteristics compared to the 2024-T351 alloy, currently used for the lower surfaces of an aircraft wing, and hardly weaker than those of 2034-T351. The high plastic spacing and elongation of the material results in excellent cold formability. The toughness, measured by the critical intensity factors of stress in plane stress K _c and K _co is more than 10% higher than that of 2024 and 2034, and the speed of crack propagation da / dn is clearly improved by compared to these two alloys, in particular for the high values of ΔK, and for loadings with variable amplitude. The fatigue life times, measured on notched test pieces taken at mid-thickness in the L direction, are also improved by more than 20% compared to 2024 and 2034. Finally, the level of residual stresses, measured by the arrow f after machining at mid-thickness of a bar resting on two distant supports of a length l, is rather low, whereas one could have expected on the contrary with a fiber structure. This deflection, measured in microns, is always less than the quotient (0.14 l ² ) / e, the length l and the thickness e of the sheet being expressed in mm.

All of these properties mean that the products according to the invention are particularly well suited to the manufacture of aircraft structural elements, in particular wing lower surfaces, but also profiles for wing box, for assembled spars and ribs soles and skins and stiffeners fuselage.

Examples

We poured 3 plates of width 1450 mm and thickness 446 mm respectively of alloy 2024, 2034 and alloy according to the invention. The chemical compositions (% by weight) of the alloys are given in Table 1: alloy Yes Fe Cu Mg Mn Zr 2024 0.12 0.20 4.06 1.36 0.54 0.002 2034 0.05 0.07 4.30 1.34 0.98 0.104 invention 0.06 0.08 4.14 1.26 0.65 0.102

Pour le 2024, 2h à 495°C puis 5h à 460°C

Pour le 2034, 5 h à 497°C

Pour l'alliage selon l'invention, montée en 12 h et maintien de 6h à 483°C

Une partie des tôles a été ensuite laminée à chaud jusqu'à une épaisseur de 40 mm par passes successives de l'ordre de 20 mm. Une autre partie des tôles a été laminée à chaud jusqu'à 15 mm. Pour l'alliage selon l'invention, la température d'entrée au laminage à chaud était de 467°C, la température de sortie à 40 mm de 465°C et celle à 15 mm de 444°C.The plates were scalped, then homogenized under the following conditions:

For 2024, 2h at 495 ° C then 5h at 460 ° C

For 2034, 5 h at 497 ° C

For the alloy according to the invention, mounted in 12 hours and maintained for 6 hours at 483 ° C

A part of the sheets was then hot rolled to a thickness of 40 mm by successive passes of the order of 20 mm. Another part of the sheets was hot rolled up to 15 mm. For the alloy according to the invention, the inlet temperature for hot rolling was 467 ° C, the outlet temperature at 40 mm from 465 ° C and that at 15 mm from 444 ° C.

3h et 6h à 497°C pour les tôles en 2024 d'épaisseur respective 15 et 40 mm,

2h et 5h à 499°C pour les tôles en 2034 d'épaisseur 15 et 40 mm

9h à 497°C pour les tôles selon l'invention.

The sheets were dissolved under the following conditions:

3h and 6h at 497 ° C for sheets in 2024 of respective thickness 15 and 40 mm,

2h and 5h at 499 ° C for sheets in 2034 of thickness 15 and 40 mm

9h at 497 ° C for the sheets according to the invention.

After quenching in cold water, all the sheets were then subjected to controlled traction at 2% permanent elongation.

The static mechanical characteristics were measured on the sheets in the directions L and TL, namely the breaking strength R _m (in MPa), the conventional elastic limit at 0.2% R _0.2 (in MPa) and the elongation at break A (in%). The results are collated in Table 2: Alloy Thickness Meaning R _m R _0.2 AT 2024 40 L 468 362 20.0 2024 40 TL 469 330 17.4 2024 15 L 462 360 21.2 2024 15 TL 467 325 17.6 2034 40 L 534 416 11.2 2034 40 TL 529 393 12.0 2034 15 L 548 431 13.8 2034 15 TL 531 395 14.6 Invention 40 L 510 384 15.4 Invention 40 TL 475 336 18.9 Invention 15 L 501 390 16.7 Invention 15 TL 491 351 19.1

The toughness was also measured by the critical intensity factors under plane stress Kc and K _c0 (in MPa√m) in the direction LT, according to standard ASTM E 561, on CCT test pieces, taken at quarter thickness, of width W = 500 mm, thickness B = 5 mm, and a central notch machined by EDM 2a ₀ = 165 mm, enlarged by fatigue test up to 170 mm. The results are given in Table 3: Alloy Thickness K _c K _c0 2024 40 143.4 105.2 2034 40 128.8 97.8 Invention 40 179.7 122 2034 15 136.4 103.7 Invention 15 173.6 124.3

The fatigue crack propagation speed da / dn was also measured in the direction LT (in mm / cycle) for different values of ΔK (in MPa√m) according to standard ASTM E 647. Two CCT test pieces are used for this. of width W = 200 mm and of thickness B = 5 mm, taken at quarter sheet thickness in the direction LT. The length of the central notch machined by EDM is 30 mm, and this notch is enlarged by fatigue test to 40 mm. The cracking speed measurement test is carried out on an MTS machine with a stress in R = 0.05 and a stress of 40 MPa, calculated to obtain a value of ΔK of 10 MPa√m for the notch length of 40 mm start (results in Table 4). Alloy Ep. ΔK = 10 ΔK = 12 ΔK = 15 ΔK = 20 ΔK = 25 2024 40 9 10 ^-5 1.5 10 ^-4 3.0 10 ^-4 6 10 ^-4 9 10 ^-3 2034 40 8 10 ^-5 1.5 10 ^-4 3 10 ^-4 5.7 10 ^-4 1.7 10 ^-3 Inv. 40 5.5 10 ^-5 1.7 10 ^-4 2.0 10 ^-4 4.0 10 ^-4 7.8 10 ^-4 2034 15 8 10 ^-5 1.5 10 ^-4 3 10 ^-4 5.2 10 ^-4 2.1 10 ^-3 Inv. 15 4.9 10 ^-5 6.0 10 ^-5 1.3 10 ^-4 2.5 10 ^-4 5.4 10 ^-4

Fatigue tests according to the Airbus AITM 1-0011 specification were carried out on test pieces with hole 230 mm long, 50 mm wide and 7.94 thick mm, taken at mid-thickness of the sheet metal direction L. The diameter of the hole is 7.94 mm.

We applied an average full test stress of 80 MPa with 4 levels of alternating stresses: 85 MPa, 55 MPa, 45 MPa and 35 MPa for sheets of 40 mm, 110, 85, 55 and 45 MPa for 15 mm sheets, with 2 test pieces per level.

The average lifetime values (in number of cycles) are shown in Table 5. It can be seen that, for test pieces with a notch factor K _t = 2.5, the fatigue life is improved by more than 20% compared to the 2024 alloy. alloy Thickness mm 80 ± 85 MPa 80 ± 55 MPa 80 ± 45 MPa 80 ± 35 MPa 2024 40 36044 159721 2034 40 30640 125565 340126 839340 invention 40 42933 219753 392680 1018240 2034 15 41040 204038 352957 invention 15 45841 241932 429895

We finally measured the arrows f in the direction L and TL, as well as the rate of recrystallization (in%) at the surface, quarter-thickness and half-thickness, determined by image analysis after chemical attack on the sample.

The arrow f is measured in the following manner. We take from the sheet of thickness e two bars, one called L direction bar, length b lengthwise sheet metal (direction L), 25 mm wide across the width of the sheet (direction TL) and of thickness e according to the full thickness of the sheet (TC direction), the other, called bar TL direction, having 25 mm in the L direction, b in the TL direction and e in the TC direction.

Each bar is machined to mid-thickness and the deflection is measured at mid-length of the bar. This deflection is representative of the level of internal stresses of the sheet and of its ability not to deform during machining. The distance 1 between the supports was 180 mm and the length b of the bars 200 mm. The machining is a progressive mechanical machining with passes of approximately 2 mm. The measurement of the deflection at mid-length is carried out using a comparator with a resolution of one micron. The results concerning the arrows and the recrystallization rates are given in Table 6. alloy Thickness f _L (µm) f _TL (µm) Recruitment rate (Surf.)% Recruitment rate (¼ th.)% Recruitment rate (½ th.)% 2024 40 210 120 79 58 30 2034 40 147 129 12 0 0 Invention 40 86 75 46 5 2

Claims (13)

1. Rolled, extruded or forged product in AlCuMg alloy, treated by dissolution, quenching and cold traction, intended for the manufacture of aircraft structural elements, of composition (% by weight):
Fe <0.15 Si <0.15 Cu: 3.8 - 4.4 (pref. 4.0 - 4.3) Mg: 1 - 1.5 Mn: 0.5 - 0.8 Zr: 0, 08 - 0.15 other items: <0.05 each and <0.15 in total, and reporting R m (L) / R 0.2 (L)> 1.25 (preferably> 1.30) . 2. Product according to claim 1, characterized in that Fe + Si <0.15%. 3. Rolled product with a thickness of 6 to 60 mm according to one of claims 1 or 2, having in the quenched and drawn state a tensile strength R _{m (L)} > 475 MPa and an elastic limit R _{0 , 2 (L)} > 370 MPa 4. Rolled product of thickness 6 to 60 min according to one of claims 1 to 3, having in the quenched and pulled state a plastic difference between the breaking strength R _m and the elastic limit R _0.2 in the directions L and TL> 100 MPa. 5. Rolled product with a thickness of 6 to 60 mm according to one of claims 1 to 4, having in the quenched and drawn state a critical intensity factor (LT) K vs > 170 MPa√m and K co > 120 MPa√m , measured according to standard ASTM E 561 on notched test pieces taken at quarter thickness with the parameters W = 500 mm, B = 5 mm and 2a ₀ = 165 mm. 6. Rolled product according to one of claims 1 to 5, characterized in that it has a critical intensity factor (direction LT) K _c or K _c0 increased by at least 10% compared to the alloy 2024 under the same conditions. 7. Rolled product with a thickness of 6 to 60 mm according to one of claims 1 to 6, having in the quenched and drawn state a crack propagation speed (LT) da / dn, measured according to standard ASTM E 647 on notched test pieces taken at quarter thickness with W = 200 mm and B = 5 mm: <10 -4 mm / cycle for ΔK = 10 MPa√m <2.5 10 -4 mm / cycle for ΔK = 15 MPa√m and <5 10 -4 mm / cycle for ΔK = 20 MPa√m 8. Laminated product according to one of claims 1 to 7, characterized in that it has an arrow f measured in the directions L and TL after machining at mid-thickness of a bar resting on two distant supports of a length l less than (0.14 l ² ) / e, f being measured in microns, the thickness e of the sheet and the length l being expressed in mm 9. Rolled product according to one of claims 3 to 8, having an average fatigue life, measured on a notched test piece taken at mid-thickness direction L, increased by more than 20% compared to the 2024 alloy. 11. Method for manufacturing a product according to one of claims 1 to 9, comprising the following steps: pouring a plate of the indicated composition homogenization of this plate between 450 and 500 ° C, hot transformation, and possibly cold, by rolling, spinning or forging to the desired product, dissolved at a temperature between 480 and 505 ° C, cold water quenching, cold traction up to more than 1.5% permanent deformation, natural aging at room temperature. 12. Method according to claim 10, characterized in that the hot transformation is carried out with an outlet temperature> 420 ° C, and preferably> 440 ° C. 13. Use of sheets according to one of claims 3 to 9 for the manufacture of skin for aircraft wing soffits. 14. Use of profiles according to one of claims 1 or 2 for the manufacture of airframe or aircraft fuselage stiffeners.