EP1026270B1 - AlCuMg alloy product for aircraft body member - Google Patents

Description

Technical area

The invention relates to rolled, spun or forged products made from hardened AlCuMg alloys and tractionned for the manufacture of aircraft structural elements, in particular skin panels and underside stiffeners of wing, and having, for example, 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 to damage and residual stresses. The designation of alloys and metallurgical states corresponds to the nomenclature of Aluminum Association, adopted by European standards EN 515 and EN 573.

State of the art

Large commercial aircraft wings have an upper section (or extrados) consisting of a skin made from thick alloy plates 7150 to the T651 state, or 7055 alloy to the T7751 state or 7449 to the T7951 state, and stiffeners made from profiles of the same alloy, and a lower part (or intrados) consisting of a skin made from thick 2024 alloy state T351 or 2324 in state T39, and stiffeners made from same alloy. Both parts are assembled by longitudinal members and ribs.

The alloy 2024 according to the designation of the Aluminum Association or the 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

Various variants have been developed and filed with the Aluminum Association under the designations 2224, 2324 and 2424, including more limited grades of silicon and iron. The alloy 2324 in the T39 state has been the subject of patent EP 0038605 (= US 4294625) to Boeing, in which the improvement of the elastic limit is obtained by hardening using a pass of cold rolling after quenching. This hardening tends to decrease the tenacity and, to compensate for the decrease in toughness, the contents of Fe, Si, Cu and Mg are reduced. Boeing also developed the alloy 2034 of composition:
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 4336075). He presents, by compared to 2024 in the T351 state, a better specific yield strength due to the increase in the manganese content and the addition of another antirecrystallizer (Zr), as well as improved toughness and fatigue resistance.

EP 0473122 (= US Pat. No. 5,223,639) to Alcoa discloses an alloy, registered at the Aluminum Association as 2524, of composition: Si <0.10 Fe <0.12 Cu: 3.8 - 4.5 Mg: 1.2 - 1.8 Mn: 0.3 - 0.9 may contain possibly another antirecrystallizer (Zr, V, Hf, Cr, Ag or Sc). This alloy is intended more particularly for thin sheets for fuselage and has a tenacity and improved crack propagation resistance over 2024.

Patent Application EP 0731185 of the Applicant relates to an alloy, registered subsequently under No. 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 The thick plates of this alloy have both improved toughness and reduced level of residual stresses, without loss on the other properties.

US Pat. No. 5,863,359 and US Pat. No. 5,865,914 to Alcoa relate respectively to an aircraft wing comprising an alloy intrados 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 _1c (LT)> 38 ksi√inch (42 MPa√m),
and a method of manufacturing a lower surface element having an R _0.2 (LT)> 60 ksi comprising casting an alloy of the above composition, homogenizing at 471 to 482 ° C, a temperature> 399 ° C., dissolution above 488 ° C., quenching, cold working preferably of more than 9% and traction of at least 1%.

Problem

For the construction of new commercial high-capacity 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 leads to 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 US Pat. No. 5,863,359 and US Pat. No. 5,865,914, if it leads to high yield strengths R _0,2 , however, has a certain number of disadvantages for use with other important employment properties in the intended application. Indeed, this results in a plastic gap, that is to say a difference between the breaking strength R _m and the yield strength R _0.2 , very reduced, resulting in a lower cold formability and less good resistance in propagation of fatigue cracks with variable amplitude loading. Indeed, the slowing of crack propagation after partial overload is less important if the plastic gap is reduced.

In addition, larger parts must be machined without distortion in thicker sheets, which implies a better control of the level of constraints residual. However, state T39 has proved to be unfavorable from this point of view.

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

Object of the invention

The subject of the invention is a rolled, spun or forged product made of AlCuMg alloy, treated by homogenization, heat-treated with an outlet temperature greater than 420 ° C., so as to obtain a recrystallization rate at a quarter-thickness less than 20%, dissolving, quenching, cold drawing and aging, intended for the manufacture of aircraft structural elements, composition (% by weight):
Fe <0.15 If <0.15 Cu: 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 L direction at the yield stress in the L direction greater than 1 , 25 (and preferably at 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) K_c > 170 MPa√m et K_co > 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

< 5 10^-4 mm/cycle pour ΔK = 20 MPa√m

It also relates to a laminated product (a sheet) of the same composition of thickness between 6 and 60 mm and having in the quenched and tracted state at least one of the following groups of properties;

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

b) Plastic gap R _m - R _0,2 direction L and TL> 100 MPa

c) Critical intensity factor (LT direction) K _c > 170 MPa√m and K _oc > 120 MPa√m (measured according to ASTM E561 on notched specimens taken at quarter thickness with parameters B = 5 mm, W = 500 and 2a ₀ = 165 mm)

d) Crack propagation velocity (LT) da / dn, measured according to ASTM E 647 on notched specimens 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

<5 10 ^-4 mm / cycle for ΔK = 20 MPa√m

This sheet also has a level of residual stresses such that the arrow f measured in the L and TL directions after machining at mid-thickness of a bar resting on two supports distant by 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 1 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 process for manufacturing a rolled, spun or forged product according to the invention comprising the following steps:

• pouring a plate or a billet of the indicated composition,
• homogenization of this plate or billet between 450 and 500 ° C,
• transformation hot and possibly cold to the desired product,
• solution at a temperature between 480 and 505 ° C,
• quenched with cold water,
• cold draft with at least 1.5% permanent deformation,
• natural aging at ambient temperature.

Description of the invention

The chemical composition of the product differs from that of the usual 2024 by a 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 lower slightly reduced copper content. Compared to the composition of the alloys described in US Pat. No. 5,863,359 and US Pat. No. 5,865,914, the copper content is higher; which makes it possible to compensate, for the mechanical strength, cold working less high after quenching. Surprisingly, this narrow domain of composition (notably with regard to manganese), associated with changes in the range of manufacture, leads, compared to the prior art, to an improvement significant compromise between mechanical strength, elongation and tolerance damage to the operating conditions of a large civil aircraft.

Moreover, and quite unexpectedly, we observe, for thick products, a low residual stress ratio, allowing machining without distortion of parts large size.

The manufacturing method comprises the casting of plates, in the case where the product to be manufactured is a rolled sheet, or billets in the case where it is a spun section or a forged part. The plate or the 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 temperature higher than the temperatures usually used, the outlet temperature being greater than 420 ° C. and preferably 440 ° C. so as to obtain a little recrystallized structure on the treated product, with a rate of recrystallization at a quarter thickness less than 20%, and preferably 10%. The laminated, spun or forged half-product is then dissolved 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 Al ₂ Cu and Al ₂ CuMg precipitates are actually 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 must preferably be less than 2 J / g.

Then the quenching with cold water, and a controlled pulling 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 alloy 2024-T351, currently used for aircraft wing-backs, and only slightly lower than those of 2034-T351. The high plastic gap and elongation of the material result in excellent cold forming ability. The tenacity, measured by the critical stress stress factors K _c and K _oc, is more than 10% greater than that of 2024 and 2034, and the crack propagation rate da / dn is significantly improved by compared to these two alloys, especially for the high values of ΔK, and for variable amplitude loadings. The fatigue life times, measured on notched specimens 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 deflection 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 fibered structure. This arrow, 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 make the products according to the invention particularly well suited to the manufacture of aircraft structural elements, including wing bottoms, but also wing box sections, for sill soles and ribs assembled and skins and stiffeners of fuselage.

Examples

Three plates 1450 mm wide and 446 mm thick were cast in alloy 2024, 2034 and alloy according to the invention respectively. 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

The plates were scalped and then homogenized under the following conditions:

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

For the 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.

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 has been rolled hot up to 15 mm. For the alloy according to the invention, the inlet temperature at hot rolling was 467 ° C, the outlet temperature at 40 mm of 465 ° C and that at 15 mm of 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 in the following conditions:

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

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

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

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

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

Was also measured by the toughness of critical intensity factors K c and K _c0 plane stress (in MPa m) in the LT direction, according to ASTM E 561, on CCT test pieces sampled at a quarter-thickness, width W = 500 mm, thickness B = 5 mm, and an electro-erosion machined center notch 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 in the LT direction (in mm / cycle) was also measured for different values of ΔK (in MPa√m) according to the ASTM E 647 standard. Two CCT test pieces were used for this purpose. width W = 200 mm and thickness B = 5 mm, taken at quarter sheet thickness in the direction LT. The length of the central notch machined by electroerosion is 30 mm, and this notch is enlarged by fatigue test to 40 mm. The crack velocity 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. starting from 40 mm (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 have been carried out on test pieces with a hole length 230 mm, width 50 mm and thickness 7.94 mm, taken at mid-thickness of the L-shaped plate. The diameter of the hole is 7.94 mm.

A mean full test stress of 80 MPa with 4 levels was applied of alternating stresses: 85 MPa, 55 MPa, 45 MPa and 35 MPa for the plates 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 given 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% with respect to alloy 2024. 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

Finally, the arrows f in the direction L and TL were measured, as well as the recrystallization (in%) at the surface, at quarter-thickness and at mid-thickness, determined by image analysis after etching of the sample.

The arrow f is measured as follows. We take from the thick sheet e two bars, one called bar L sense, length b in the direction of the length sheet metal (L-direction), width 25 mm in the width direction of the sheet (TL direction) 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 arrow is measured at mid-length of the bar. This arrow is representative of the level of internal stresses of the sheet and its ability not to deform at machining. The distance 1 between the supports was 180 mm and the length b of the bars 200 mm. Machining is a progressive mechanical machining with passes of about 2 mm. The measurement of the arrow at mid-length is carried out using a comparator with a resolution of one micron. Results for arrows and recrystallization rates are given in Table 6. alloy Thickness f _L (μm) f _TL (μm) Recr. (Surf.)% Recr. (¼ year)% Recr. (½ ep.)% 2024 40 210 120 79 58 30 2034 40 147 129 12 0 0 Invention 40 86 75 46 5 2

Claims (10)

1. Rolled, extruded or forged product made of an AlCuMg alloy processed by homogenisation, hot transformation at an exit temperature above 420°C in order to obtain a recrystallisation rate below 20% (measured at quarter thickness) solution heat treatment, quenching cold stretching and ageing, to be used in the manufacture of aircraft structural elements, with the following composition (% by weight):
      Fe<0.15   Si<0.15   Fe + Si < 0.15   Cu: 4.0-4.3
      Mg:1-1.5   Mn:0.5-0.8   Zr:0.08-0.15
   other elements: < 0.05 each and < 0.15 total with a ratio R_m(L) /R_0.2(L) > 1.25 (and preferably > 1.30).
2. Rolled product from 6 to 60 mm thick according to claim 1, with an ultimate tensile strength R_m(L) in the quenched and stretched temper > 475 MPa and yield stress R_0.2(L) > 370 MPa.
3. Rolled product from 6 to 60 mm thick according to any one of claims 1 or 2, with a plastic range between the ultimate tensile strength R_m and the yield stress R_0.2 in the L and TL directions in the quenched and stretched temper > 100 MPa.
4. Rolled product from 6 to 60 mm thick according to any one of claims 1 to 3, for which the critical intensity factor (L-T direction) K_c in the quenched and stretched temper > 170 MPa√m and K_co > 120 MPa√m measured according to ASTM standard E 561 on notched test pieces sampled at a quarter thickness with parameters W = 500 mm, B = 5 mm and 2_B0 = 165 mm.
5. Rolled product from 6 to 60 mm thick according to any one of claims 1 to 4 characterized in that the crack propagation rate (L-T direction) da/dn in the quenched and stretched temper, measured according to ASTM standard E 647 on notched test pieces sampled at a quarter thickness with parameters W = 200 mm and B = 5 mm) is as follows:

   <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
6. Rolled product according to any one of claims 1 to 5, characterized in that deflection f measured in the L and TL directions after machining a bar supported on two supports separated by a length 1 to its mid-thickness is below (0.14 l²)/e, where f is measured in microns, e is the thickness of the plate and 1 is the length measured in mm.
7. Process according to any one of claims 1 to 6 for manufacturing a product comprising the following steps:

   cast a plate with the indicated composition,

   homogenize this plate between 450 and 500°C,

   hot transformation, and possibly cold transformation by rolling, extrusion or forging, until the required product is obtained,

   solution heat treatment at a temperature of between 480 and 505°C,

   quench in cold water,

   cold stretching to at least 1.5% permanent deformation,

   natural aging under ambient conditions.
8. Process according to claim 7, characterized in that the hot transformation takes place with an exit temperature > 420°C and preferably > 440°C.
9. Use of plates according to one of claims 2 to 6 for manufacturing the skin of an aircraft lower wing.
10. Use of profiles according to claim 1, for manufacturing aircraft lower wings or fuselage stringers.