EP1544315B1 - Wrought product in the form of a rolled plate and structural part for aircraft in Al-Zn-Cu-Mg alloy - Google Patents

Description

Field of the invention

The invention relates to Al-Zn-Cu-Mg alloy rolled products treated by dissolving, quenching, cold working and tempering, and in particular structural elements made from such products and intended for the construction of aircraft. .

State of the art

It is known that during the manufacture of semi-finished products and structural elements for aeronautical construction, the various desired properties can not be optimized all at the same time and independently of each other. When modifying the chemical composition of the alloy or the parameters of the product development processes, several critical properties can even show antagonistic tendencies. This is sometimes the case of the properties grouped under the term "static mechanical resistance" (in particular the tensile strength R _m and the yield strength R _p0.2 ) on the one hand, and the properties gathered under the term "tolerance". damage "(including toughness and resistance to crack propagation) on the other hand. Some common properties such as fatigue strength, corrosion resistance, formability and elongation to failure are complicated and often unpredictable to the properties (or "characteristics") mechanical. The optimization of all the properties of a material for aircraft construction therefore very often involves a compromise between several key parameters.

Typically, 7xxx alloys are used for the structural elements (except for the undersides) of the wing.

The patent US 5,865,911 (Aluminum Company of America) discloses an Al-Zn-Cu-Mg alloy of composition

Zn 5.9 - 6.7, Mg 1.6 - 1.86, Cu 1.8 - 2.4, Zr 0.08 - 0.15

for the manufacture of structural elements for aircraft. These structural elements are optimized to show high mechanical strength, toughness and fatigue resistance.

The patent application WO 02/052053 discloses three Al-Zn-Cu-Mg type alloys of composition (a) Zn 7.3 + Cu 1.6; (b) Zn 6.7 + Cu 1.9; (c) Zn 7.4 + Cu 1.9; each of these three alloys also containing Mg 1.5 + Zr 0.11. It also describes thermomechanical treatment processes suitable for the manufacture of structural elements for aircraft.

Also known alloy 7040 whose standardized chemical composition is: Zn 5.7 - 6.7 Mg 1.7 - 2.4 Cu 1,5 - 2,3 Zr 0.05 - 0.12 If ≤ 0.10 Fe ≤ 0.13 Ti ≤ 0.06 Mn ≤ 0.04 other elements ≤ 0.05 each and ≤ 0.15 in total.

Alloy 7475 is also known whose standardized chemical composition is: Zn 5.2 - 6.2 Mg 1.9-2.6 Cu 1.2-1.9 Cr 0.18-0.25 If ≤ 0.10 Fe ≤ 0.12 Ti ≤ 0.06 Mn ≤ 0.06 other elements ≤ 0.05 each and ≤ 0.15 in total.

For certain structural elements involved in the construction of the wings of civil aircraft, such as wing bottoms, alloys of the 2xxx series, for example alloy 2324, are commonly used.

The alloys conventionally used for the fuselage structure elements belong to the 2xxx series, for example the alloy 2024.

The object of the present invention is to obtain elements of aircraft structure, and in particular fuselage elements, made of Al-Zn-Cu-Mg alloy, having, compared to the prior art, improved mechanical strength, for comparable damage tolerance and sufficient formability.

It also aims to obtain elements of aircraft structure, including elements for intrados of aircraft wings or for machining integral structures, Al-Zn-Cu-Mg alloy, having, compared to the prior art, an improved compromise between the properties of strength, toughness, and fatigue resistance.

Object of the invention

The subject of the invention is a wrought product as defined in claim 1.

The subject of the invention is also a structural element for aircraft construction, in particular an aircraft fuselage element, or an aircraft wing-bottom element, or an integral aircraft structural element, manufactured from a wrought product, and especially from such a rolled or spun product.

Description of the invention a) Definitions

Unless stated otherwise, all the information relating to the chemical composition of the alloys is expressed in percent by weight. Therefore, in a mathematical expression, "0.4 Zn" means: 0.4 times the zinc content, expressed in mass percent; this applies mutatis mutandis to other chemical elements. When the content of an element is given in ppm (parts per million), it refers to the mass content and not the atomic content. The designation of the alloys follows the rules of THE ALUMINUM ASSOCIATION, known to those skilled in the art. The metallurgical states are defined in the European standard EN 515. The chemical composition of standardized aluminum alloys is defined, for example in the standard EN 573-3. Unless otherwise specified, the static mechanical characteristics, ie the _ultimate tensile strength R _m , the yield strength R _p0,2 and the elongation at break A, are determined by a tensile test according to the standard EN 10002-1, the location and direction of specimen collection being defined in EN 485-1. The fatigue strength is determined by a test according to ASTM E 466, and the fatigue crack growth rate (so-called da / dn test) according to ASTM E 647.

The curve R is determined according to ASTM standard E 561. From the curve R, the critical stress intensity factor K _c is calculated, ie the intensity factor which causes the instability of the crack. . The stress intensity factor K _{CO is} also calculated by assigning to the critical load the initial length of the crack at the beginning of the monotonic loading. These two values are calculated for a specimen of the desired shape. K _app means the Kco corresponding to the test piece used for the R curve test. Unless otherwise stated, the size of the crack at the end of the fatigue pre-cracking step is W / 4 for the test pieces of the type M (T), and W / 2 for CT type specimens, where W is the specimen width as defined in ASTM E 561.

The term "spun product" includes so-called "stretched" products, i.e., products that are made by spinning followed by stretching.

Unless otherwise stated, the definitions of the European standard EN 12258-1 apply.

Here is called "structural element" or "structural element" of a mechanical construction a mechanical part 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.

Here the term "integral structure" refers to the structure of a portion of an aircraft that has been designed to provide as much continuity as possible over as large a dimension as possible to reduce the number of assembly points mechanical. An integral structure can be manufactured either by machining in the mass, or by using shaped parts obtained for example by spinning, forging or molding, or by welding structural elements made of weldable alloys. This results in structure elements of larger size and in one piece, without mechanical assembly or with a reduced number of mechanical assembly points compared to an assembled structure in which sheets, thin or strong depending on the destination of the structural element (for example: fuselage element or wing element), are fixed, usually by riveting, on stiffeners and / or frames (which can be manufactured by machining from spun or rolled products).

b) Detailed description of the invention

The present invention applies to an aluminum alloy containing between 6.7% and 7.3% zinc. The zinc content must be high enough to ensure good mechanical properties, but if it is too high, the sensitivity of the alloy to the quenching increases, which risks, in particular for thick products, to degrade the compromise properties referred. The product is a strong sheet with a thickness greater than 20 mm.

The chemical composition of the Al-Zn-Cu-Mg alloy has been chosen so that the Mg / Cu ratio of the alloy which is the subject of the invention is less than 1. Preferably, this ratio is maintained at a value less than 0.9. A value of less than 0.85 or even about 0.8 is preferred.

The best compromise has been found when the copper is maintained at levels between 2.0 and 2.3%, while the magnesium is set at levels between 1.5 and 1.8%.

The Applicant has found that a zirconium content of between 0.07 and 0.12% gives access for this composition to Al-Zn-Cu-Mg major elements, to a better compromise between R _p0.2 , toughness ( at ambient or cold temperature) and fatigue resistance (in particular speed of propagation of fatigue cracks). Above 0.12%, there is a significant risk of forming Al ₃ Zr type primary phases, unless the cooling is sufficiently rapid; in the case of semi-continuous casting, such a sufficient speed can be achieved, especially when casting billets.

For rolled products, the Zr content must be less than 0.12%, and the best results have been obtained with a content of between 0.07 and 0.09%.

In any case, the silicon and iron contents must be maintained each below 0.15%, and preferably below 0.10%, to have good toughness. In a particularly preferred embodiment of the invention, the iron content does not exceed 0.07%, and the silicon content does not exceed 0.06%.

The alloy according to the invention can be cast according to one of the techniques known to those skilled in the art to obtain a raw form, such as a rolling plate. This raw form, optionally after scalping, is then homogenized, typically for a period of 15 to 16 hours at a temperature between 470 and 485 ° C.

The raw form is then hot processed to form hot-rolled sheets. In a preferred embodiment of the invention, the applicant has found that, surprisingly, the hot rolling of thick products according to the invention can be carried out at a temperature of about 350 ° C, much lower than that traditionally practiced for this type of product (which is about 415 to 440 ° C), without affecting the compromise of properties required for thick products used in aircraft structures.

The hot transformation can optionally be followed by a cold transformation. It is also possible to carry out one or more cold rolling passes.

The products obtained are then dissolved. This dissolution can be done in any suitable oven, such as air oven (horizontal or vertical), or salt bath oven. This dissolution is carried out at a temperature between 470 and 480 ° C, and preferably between 475 and 480 ° C for a period of at least 4 hours.

Then the products are quenched, preferably in a liquid medium such as water, said liquid preferably having a temperature not exceeding 40 ° C.

The products are then generally subjected to controlled traction with a permanent elongation of the order of 1 to 5%, and preferably 1.5 to 3%.

Finally the products are subjected to a treatment of income, which influences in a big way on the final properties of the product. Depending on the desired trade-off, one may prefer a two-tier income or a single-tier income.

The product according to the invention leads to new products which have particularly interesting characteristics for aeronautical construction. These products are in the form of sheets, including fuselage sheets, thick sheets for intrados or integral structures.

1. (a) R_m(L) > 540 MPa ;
2. (b) R_p0.2(L) > 535 MPa ;
3. (c) K_app(L-T) > 100 MPa√m (mesurée à la température ambiante sur éprouvette C(T) avec W=127 mm et B = 7,6 mm) ;
4. (d) ΔK à une vitesse de propagation de fissure de 2,54 µm / cycle > 28 MPa√m ; et
5. (e) K_IC(L-T) > 28 MPa√m.

According to the invention, a sheet with a thickness greater than 20 mm is prepared with the following characteristics:

1. (a) R _m (L)> 540 MPa;
2. (b) R _p0.2 (L)> 535 MPa;
3. (c) K _app (LT)> 100 MPa√m (measured at room temperature on specimen C (T) with W = 127 mm and B = 7.6 mm);
4. (d) ΔK at a crack propagation rate of 2.54 μm / cycle> 28 MPa√m; and
5. (e) K _{IC (LT)} > 28 MPa√m.

Another important advantage of the product according to the invention is the fact that, surprisingly, the value of K _{app (LT)} , as determined above, is stable, or even higher, cold compared to its temperature value. room. More specifically, this value is slightly increased from room temperature when measured at -54 ° C. This is particularly interesting since -54 ° C is about the typical temperature reached by the structural elements during the flight of a civilian jet aircraft. However, it is known that in some alloys of the 7xxx series, toughness decreases with temperature. By way of example, it has been described that 7475 T7651 sheets show a 25% decrease in toughness (determined from curves R on panels of thickness B = 6 mm in the LT direction) between approximately 20 °. C and about -50 ° C (see PR Abelkis et al., Proceedings of "Fatigue at Low Temperatures", Louisville, Kentucky, May 10, 1983, pages 257-273 (ASTM editor )). Under the same conditions, heavy plates in 7050 T7451 show a decrease of K _IC or Kq in the LT or TL sense of at least 5% (see WF Brown et al., Aerospace Materials Handbook, published by CINDAS (USAF CRDA Handbook Operation, Purdue University, 1997). ). The plaintiff has found a decrease in K _IC also for heavy plates in AA7075 T7351, AA7475 T 7351, AA7475 T7651, and AA7475 sub-revenue; this decrease is of the order of 2% to 10%. While it is known that the static mechanical characteristics R _p0.2 and R _m alloys of the series 7xxx tend to increase when the temperature drops from about 20 ° C to about -50 ° C, which provides additional security At this temperature, the drop in the toughness of the 7xxx series alloys according to the state of the art must be taken into account when sizing the structural elements. The product according to the invention does not show a significant (i.e., greater than 2%) decrease in low temperature toughness, and in some cases even shows a slight increase in low temperature toughness.

As structural elements for intrados of aircraft wings, the products according to the invention advantageously replace the alloy structure elements known in alloys 2x24, for example alloy 2024 or 2324. By way of example, laminated products with a thickness greater than about 40 mm can be used for the fabrication of structural elements by integral machining, as described below. Rolled products with a thickness greater than about 60 mm can be used for the manufacture of stiffeners or frames, especially for high capacity aircraft.

The products according to the invention can be plated on at least one face according to the methods and with the alloys usually used for plating the products of Al-Zn-Cu-Mg type alloys. This is particularly interesting for the plates used for the manufacture of aircraft fuselage elements, which must resist corrosion. A useable plating alloy is 7072.

A particularly advantageous use of the products according to the invention is related to the concept of the integral structure in aeronautical construction. A large part of the aircraft structures are dimensioned according to a compromise between the damage tolerance and the resistance to static loads. The requirements of tolerance to damage are specified for example in the article "Damage Tolerance Certification of Commercial Aircraft" by T. Swift, ASM Handbook vol. 19 (1996), pp 566-576 . Sizing under static loadings is explained for example in the book "Airframe Stress Analysis and Sizing" by Niu, Hong Kong Conmilit Press Ltd, 1999, in particular pages 607-654 . From the point of view of the material, it is known that, generally, the damage tolerance of the 7xxx series alloys, and in particular their toughness, decreases when their yield strength increases. This phenomenon leads to a specialization of alloys with a high tolerance to damage - in particular alloys of the 2xxx series - to parts that are very strongly stressed in tension, knowing that certification in tolerance requires the admission of cracks and, conversely , alloys with high yield strength - in particular the alloys of the 7xxx series, towards the parts subjected to very strong compression. In fact, the parts highly stressed in compression such as the upper airfoils and the fuselage boats also undergo tensile loads which, although less strong, require that the material has a certain tolerance to damage. Reciprocally, parts such as wing intrados or fuselage flags, very strongly stressed in traction, need to have a certain compressive strength. Thus, it frequently happens that the damage tolerance is the sizing parameter for a part essentially stressed in compression and vice versa. So, for example, a toughness gain of x% to constant yield strength as with the alloy according to the invention can result in a weight gain of the same level, or even higher if the fact of allowing a higher load on the part in question also allows the lightening of other components. Similarly, a yield strength gain of x%, with a constant damage tolerance, can result in a weight gain of the order of x / 3% to x%. For the products according to the invention, x is typically between 15 and 30%.

In an integral structure, the continuity between the stiffeners and the skin means that the damage tolerance becomes more critical than in a component assembled by riveting. Indeed, given stress, the stress intensity factor increases sharply when the stiffener is crossed by a crack, since it must be admitted that this stiffener will necessarily be cracked. The present inventors have found that products with high tenacity, for a given elastic limit, are particularly well suited to the manufacture of integral structures. In a particularly advantageous embodiment of this aspect of the present invention, fuselage barge panels or wing covers are manufactured by integral machining of products according to one of the preceding embodiments, given that said products, and in particular the plates for machining advantageously have a thickness of at least 40 mm; this value also depends on the type of aircraft, and especially its size. According to the findings of the inventors, it is thus possible to achieve a weight gain which is of the same order of magnitude as the gain in toughness, ie about 10%, with respect to an integral structure made of an AA7475 type alloy depending on the state of the technical. More specifically, the product according to the invention, with a yield strength R _{p0.2 (L)} at half thickness of at least 540 MPa and a K _app toughness _(LT) measured on a specimen of type M (T). ) with a width W of 16 inches (about 406 mm) of at least 140 MPa√m, makes it possible to produce structural elements for aeronautical construction, such as a wing skin element, with a weight gain of at least 10% relative to the same part, of the same shape and size, made of alloy 7475 according to the state of the art and typically having a R _{p0.2 (L)} at mid-thickness of 475 MPa, and a K _{app (LT)} measured on an M (T) type specimen with a width W of 16 inches (about 406 mm) of 125 MPa√m.

The Applicant has found that refining the grain at a reduced level compared to the usual practice during casting provides a compromise of properties, including a level of tenacity, particularly interesting. The use of a TiC refining agent (for example adding A13% Ti0.15% C thread) in controlled doses is particularly beneficial, the solidification germ obtained with this approach having a different germination-growth compromise germs obtained by refining for example with A15% Ti1% B (ie a seed type TiB ₂ ). The level of this refining can be quantified by the amount of C added, since it corresponds indirectly to the amount of added solidification nuclei, as well as by the amount of free Ti (not combined with C) in the alloy. Although the stoichiometry of the seed is not definitively quantified, it can be considered that the seed is composed of TiC, each C atom combining with a Ti atom to form said seeds.

There are different types of Al - x% Ti - y% C affinants, usually Ti being added in excess of C. The amount of sprouts added is proportional to the amount of refining (in kilograms) added per ton. of liquid metal multiplied by y%, that is to say proportional to A (number of kilograms of affine added per tonne of metal) xy%.

By way of example, for the addition of 2 kg / t of Al-3% Ti0.15% C, the addition of seeds can thus be quantified by specifying 3 g / t of added C (2 × 0, 0015 kg / t).

It should be noted that there are other ways than adding Al-3% Ti-0,15% C to add the same amount of germs, for example by adding twice as much of a refining agent with a concentration twice as low as C.

In an advantageous embodiment of the present invention, a refining agent containing titanium and carbon is thus added, so that the amount of carbon added is between 0.4 and 3 g / t of carbon, preferentially between 0, 6 and 2 g / t, and so that the total content of Ti in the final product is between 50 and 500 ppm (by weight), preferably between 150 and 300 ppm.

Other advantageous embodiments are described in the claims.

In the examples which follow, advantageous embodiments of the invention are illustrated by way of illustration. These examples are not limiting in nature.

Examples Example 1 (except invention)

An N-alloy was developed whose chemical composition corresponds to that of a product according to the invention. The liquid metal was first treated in a gas injection holding furnace using an Irma® type rotor, and then in a pocket type Alpur ®, these two brands belonging to the plaintiff. The refining was done online, ie in the channel between the holding oven and the Alpur ® bag, at a rate of 1.1 kg / ton of Al-3% Ti-0.15 yarn. % C (diameter 9.5 mm). An industrial-sized plate was cast. It was relaxed for 10 hours at 350 ° C.

The product thus cast was homogenized after scalping for 15 hours at a temperature of between 471 ° C and 482 ° C (880 ° F to 900 ° F), and then hot rolled to a thickness of 5 mm (0. 2 inches) (thickness outside the invention). The start-up temperature was 450 ° C (840 ° F), and the end-of-rolling temperature was 349 ° C (660 ° F). Sheets of width 178 mm (7 inches) and length 508 mm (20 inches) were taken. These coupons were dissolved in a salt bath oven for 1 hour at 472 ° C, then quenched with water, and triturated until a permanent deformation of 2%. The coupons thus obtained then underwent a two-stage artificial aging treatment, the first bearing being 6 hours at 105 ° C., the second bearing being 18 hours at 155 ° C., in order to reach the peak of mechanical properties.

In a similar process, sheets of thickness 6 mm (thickness outside the invention) and 3.2 mm of Y alloy (thickness outside the invention) were also produced.

Plate alloys 2xxx type (references E and F, outside the invention) were also developed according to the following method:

The alloy was cast by treating the liquid metal first in a gas injection holding furnace using an Irma ® type rotor, and then in an Alpur ® type pouch. The refining was done online, that is to say in the channel between the holding furnace and the Alpur ® pocket, at a rate of 0.7 kg / tonne of AT5B wire (diameter 9.5 mm). The plates were relaxed for 10h at 350 ° C. These plates were homogenized for 12 hours at 500 ° C. and then hot rolled (end of rolling temperature between 230 and 255 ° C.) to a thickness of 6 mm (thickness outside the invention). A solution bath treatment was then carried out in a salt bath oven for 1 hour at 500 ° C on 600 mm by 200 mm coupons. This operation was followed by quenching with cold water at about 20 ° C. and pulling until a permanent deformation of 2% (state T351).

7xxx alloy plates according to the prior art were also cast (reference G) in the same foundry device as the 2xxx alloy sheets described above. A plate was obtained which was then homogenized for 24 hours at 470 ° C and then 24 hours at 495 ° C and then hot rolled (rolling end temperature between 230 and 255 ° C) to a thickness of 6. mm (thickness outside the invention). A solution treatment of 1 hour at 450 ° C. in a salt bath oven was then carried out on a coupon of 600 mm by 200 mm. This operation was followed by quenching with water and traction to obtain a permanent deformation of 2%. The coupon was then subjected to an artificial aging treatment of 5 hours at 100 ° C. and then 6 hours at 155 ° C., in order to reach the peak of mechanical properties (state T6).

An AA7475 alloy plate was also cast (reference H) according to the conventional methods of the prior art. The plate thus obtained was homogenized for 9 hours at 480.degree. C. and then co-laminated at a temperature of about 270.degree. C., with a 7072 sheet, until a thickness of plated plate 4.5 was obtained. mm (thickness outside the invention). The 7072 plating was about 2% of the final thickness. The product thus obtained was dissolved in a salt bath oven for 45 minutes at 478 ° C, then quenched with water at a temperature of about 20 ° C, and then it was pulled to obtain a deformation permanent 2%. He then suffered a double-stage income operation for 4 hours at 120 ° C, then 24 hours at 162 ° C (condition T76).

The chemical compositions of the N, Y, E, F, G and H alloys measured on a spectrometric counter taken from the casting channel are summarized in Table 1: Table 1: Chemical Composition Alloy Yes Fe Cu mn mg Zn Zr Cr NOT 0.05 0.06 2.05 - 1.64 7.08 0.08 - Y 0.04 0.05 2.16 - 1.80 6.76 0.09 - E (2024A) <0.06 0.06 4.12 0.4 1.37 - - - BOY WUT 0.05 0.08 1.47 - 1.56 4.27 0.11 - H (7475) 0.03 0.06 1.5 - 2.22 5.73 - 0.21 Plating (7072) 0.15 0.35 <0.02 <0.05 <0.10 1.05 <0.03 <0.03

The tensile strength R _m (in MPa), the conventional yield stress at 0.2% elongation R _p0.2 (in MPa) and the elongation at break A (in%) were measured by a tensile test according to EN 10002-1.

The results of T6 measurements of the static mechanical characteristics for the N and Y sheets which have a composition corresponding to that of a product according to the invention, and in the T351 state for the E, F and G sheets according to the prior art are shown in Table 2: Table 2: Static mechanical characteristics sheet metal Thickness [mm] Meaning L TL direction R _m [MPa] R _p0.2 [MPa] AT [%] R _m [MPa] R _p0.2 [MPa] AT [%] NOT 5.08 539 508 13.9 541 495 13.9 Y 6 557 530 13.9 555 519 13.6 E 6.35 482 365 22.8 466 319 23.5 BOY WUT 6.35 435 373 15.1 436 366 14.8 H 4.6 475 414 13.3 484 414 12.5

It is found that the sheet whose composition corresponds to that of a product according to the invention has in both directions measured a breaking strength and a yield strength much higher than those of 2xxx alloy sheets. The elongation of the sheet whose composition corresponds to that of a product according to the invention is lower than that of the sheet E, but sufficient compared to the intended applications. Compared to the prior art alloys 7xxx G and H, the alloy whose composition corresponds to that of a product according to the invention has in both directions measured a significantly improved resistance to fracture and yield strength. , for comparable lengthening.

N, E, F, G, and H sheets were evaluated for toughness, as measured by _Koc or K _app stress intensity factors, according to ASTM 561; this determination was made in the TL direction, on specimens C (T) with W = 127 mm (5 inches) and B = 5.5 mm.
The results are reported in Table 3 below. Table 3: K <sub> app measurements </ sub> sheet metal K _app [MPa√m] NOT 107 E (2024 A) 105 BOY WUT 97 H (7475 plated) 87

The sheet Y 6 mm thick exhibited a toughness K _app 150 MPa√m (W = 760 mm) or 134 MPa√m (W = 406 mm) in the LT direction, and 128 MPa√m (W = 760 mm ) or 110 MPa√m (W = 406 mm) in the TL direction.

The sheet whose composition corresponds to that of a product according to the invention has a Kapp much greater than the alloy sheets 7xxx of the prior art, and of the same order of magnitude as 2xxx alloy sheets.

Fatigue behavior according to ASTM E 647 was also tested by measuring the crack growth rate in sheet N in comparison with sheets E, F and G. The test pieces used were of type C (T), with W 76.2 mm (3 inches).

The crack propagation rate da / dN results for a ΔK of 10 MPa√m and then of 30 MPa√m were measured; ΔK for a propagation speed of 100 μinch / cycle (or 2.54 μm / cycle) was measured. The comparative results are shown in Table 4. The sheet Y had a thickness of 6 mm. Table 4: Fatigue Results sheet metal da / dN (10) TL (10 ^-4 mm / cycles) da / dN (30) TL (10 ^-4 mm / cycles) ΔK at 100μ inch / cycle [MPa√m] NOT 1.4 29 27.5 Y 1.3 33 27 E (2024A) 1.4 30 27 BOY WUT 1.1 38 25.9

The sheet whose composition corresponds to that of a product according to the invention behaves at least as well in fatigue as the sheets according to the prior art.

A 3.2 mm thick Y sheet had da / dN (10) TL of 1.7 10 ^-4 mm / cycles, da / dN (30) TL of 10 ^-4 mm / cycles, and ΔK at 100 μ inch. / cycle of 28.3 MPa√m.

• R_m(_L) 548 MPa, R_p0.2(L) 524 MPa, A_(L) 13,6%, R_m(TL) 545 MPa, R_p0.2(TL) 511 MPa, A_(TL) 12,9%, K_app(L-T) 128 MPa√m avec W = 760 mm, K_C(L-T) 154 MPa√m avec W = 760 mm, K_app(T-L) 80 MPa√m, K_C(T-L) 84 MPa√m.

By way of comparison, under the same conditions, a sheet with a thickness of 6 mm made of Zn 9.24 alloy was developed; Mg 1.60; Cu 2.13; Zr 0.06; If 0.03; Fe 0.04 from a rolling plate homogenized at 470 ° C for 48 hours, which was hot rolled, dissolved (465 ° C for 60 minutes), quenched, and triturated (1.5% d permanent accommodation). The following mechanical characteristics were obtained in the T76 state:

• R _m ( _L ) 548 MPa, R _p0.2 (L) 524 MPa, A _(L) 13.6%, R _{m (TL)} 545 MPa, R _{p0.2 (TL)} 511 MPa, A _(TL) 12 , 9%, K _{app (LT)} 128 MPa√m with W = 760 mm, K _{C (LT)} 154 MPa√m with W = 760 mm, K _{app (TL)} 80 MPa√m, K _{C (TL)} 84 MPa m.

Example 2

An alloy M was prepared whose chemical composition was in accordance with that of a product according to the invention. For comparison, an alloy plate 2324 according to the prior art (reference I) was developed according to a conventional casting process. The chemical compositions of alloys M and I, measured on a spectrometric counter taken from the casting channel, are collated in the table: Table 5: Chemical Composition Alloy Yes Fe Cu mn mg Zn Zr M 0.05 0.06 2.05 - 1.64 7.08 0.08 I (AA2324) <0.10 <0.12 3.8-4.4 0.3-0.9 1.2-1.8 <0.20 <0.05

After scalping, the M alloy plates were homogenized for 15 hours at 479 ° C, then slowly cooled to 420-440 ° C and rolled to a thickness of 25.4 mm. The output temperature of the hot rolling mill was 354 ° C, which is significantly lower than that usually used for this type of product.

The sheets thus obtained were then subjected to solution treatment at 479 ° C. for 4 hours (total time, about 1/3 of which was spent at the temperature rise), then quenched, and triturated so that the permanent deformation resulting therefrom that is 2%. The sheets were then artificially treated for 8 hours at 160 ° C.

The tensile strength R _m (in MPa), the conventional yield stress at 0.2% elongation R _p0.2 (in MPa) and the elongation at break A (in%) were measured by a tensile test according to EN 10002-1, for the sheet according to the invention, and for the sheet according to the prior art. The corresponding results are reported in Table 6 below.

The alloy 1 (AA2324) was subjected to a conventional range until an alloy sheet AA 2324, thickness 25.4 mm in the T39 state, that is to say a homogenization step, was obtained. hot rolling, followed by dissolution and quenching, followed by cold working by about 9%, and controlled pulling to obtain a permanent deformation of between 1.5 and 3 %. Table 6: Static mechanical characteristics sheet metal Thickness [mm] Meaning L R _m [MPa] R _p0.2 [MPa] AT [%] M 25.4 570 540 12.3 I 25.4 490 470 14

It is found that the tensile strength and yield strength of the sheet according to the invention are substantially higher than those of the sheet I generally used for these applications, and for quite comparable elongations.

The sheets M and I have also been the subject of an evaluation of the tenacity, measured by the determination of the critical stress intensity factors Kc and K _C0 or K _app , according to the ASTM 561 standard; this determination was made at room temperature in the LT direction, on specimens M (T) with B = 6.35mm (0.25 inches) and W = 406.4mm (16 inches), as well as on specimens C (T). ), with B = 7.6mm (0.3 inches) and W = 127mm (5 inches). K _app was also determined on a specimen C (T) with B = 7.6 mm and W = 127 mm in the LT direction at a temperature of -54 ° C. The results are reported in Table 7 below. Table 7: K <sub> c </ sub> and K <sub> app </ sub> measurements sheet metal K _{C (LT)} test piece M (T) [MPa√m] ambient temperature K _{app (LT)} test tube M (T) [MPa√m] ambient temperature K _{app (LT)} specimen C (T) [MPa√m] ambient temperature K _{app (LT)} specimen C (T) [MPa√m] at -54 ° C K _{app (LT)} specimen M (T) [MPa√m] at -54 ° C M 199 140 118 124 126 I 177 121 96 99 -

It is found that the alloy whose composition corresponds to that of a product according to the invention has under all conditions a toughness better than the conventional alloy 1. Moreover, and surprisingly, the alloy whose composition corresponds to that of a product according to the invention has a _Kapp (LT) at -54 ° C which is of the same order as at room temperature.

1. 1) On utilise une éprouvette dite « à encoche », d'épaisseur 5 mm, de largeur 38,1 mm, et de longueur 254 mm, présentant deux encoches circulaires de rayon 43,2 mm, usinées symétriquement par rapport au centre de l'éprouvette à une distance de 12,7 mm du centre. Le test se fait suivant la norme ASTM E 466, en appliquant une contrainte cyclique telle que la contrainte maximum soit égale à 270 MPa, et la contrainte minimum soit égale à 27 MPa (R=0,1), et ce à une fréquence de 15 Hz.
2. 2) On utilise une éprouvette dite « à double trous », d'épaisseur 2,54 mm, de largeur 25,4 mm et de longueur 209 mm, présentant deux trous circulaires de diamètre 4,8 mm, situés sur la ligne médiane de l'éprouvette, à égale distance du centre de l'éprouvette, dont les centres sont distants de 19 mm. Le test se fait suivant la norme ASTM E 466, en appliquant une contrainte cyclique telle que la contrainte maximum soit égale à 140 MPa, et la contrainte minimum soit égale à 14 MPa (R=0,1 ), et ce à une fréquence de 15Hz.

The sheets M and I have also been tested for fatigue resistance in the direction L, according to the following two protocols derived from the ASTM E 466 standard:

1. 1) Using a so-called "notch" test piece, of thickness 5 mm, width 38.1 mm, and length 254 mm, having two circular notches of radius 43.2 mm, machined symmetrically with respect to the center of the specimen at a distance of 12.7 mm from the center. The test is carried out according to standard ASTM E 466, by applying a cyclic stress such that the maximum stress is equal to 270 MPa, and the minimum stress is equal to 27 MPa (R = 0.1), and this at a frequency of 15 Hz.
2. 2) A so-called "double-hole" specimen, thickness 2.54 mm, width 25.4 mm and length 209 mm, having two circular holes 4.8 mm in diameter, located on the center line of the test piece, equidistant from the center of the test piece, the centers of which are 19 mm apart. The test is carried out according to the standard ASTM E 466, by applying a cyclic stress such that the maximum stress is equal to 140 MPa, and the minimum stress is equal to 14 MPa (R = 0.1), and this at a frequency of 15Hz.

This test was carried out on 5 test pieces for each protocol, and the logarithmic average of the 5 tests was calculated.

The results of these two test protocols on the two sheets M and I are presented in Table 8 below: Table 8: Fatigue Results sheet metal Number of cycles (Log average over 5 tests) Notched test tube Number of cycles (Log average over 5 tests) Double-hole test tube M 299213 330737 I 181402 337730

The variability of this test is generally quite great, but it is found on this test that the sheet according to the invention and the sheet usually used are in the same order of magnitude in terms of fatigue life.

The fatigue behavior according to ASTM E 647 was also tested by measuring the crack growth rate in sheet M compared to sheet I. The test pieces used were of type C (T), with B equal to 9 , 52 mm (0.375 inches), and W equal to 101.6 mm (4 inches).

The crack propagation velocity versus ΔK curve was plotted, and the ΔK at a rate of 2.54 μm / cycle (10 ^-4 inch / cycle) was measured; the comparative results are shown in Table 9: Table 9: Fatigue results Values of ΔK for a given cracking speed sheet metal ΔK at 2.54μm / cycle (10 ^-4 inch / cycle) [MPa√m] M 30.8 I 26.8

Finally, the exfoliating corrosion behavior of the plates of this test was evaluated according to the ASTM G34 standard; this test was made at the surface, and at mid-thickness, under the conditions adapted to the alloys 7xxx for the sheet metal M according to the invention, and under the conditions adapted to the alloys 2xxx for the sheet I. The sample M according to the The invention has been classified EA, both at the surface and at mid-thickness, while the sample I according to the prior art has been classified EA at the surface, and EB at mid-thickness. The sheet according to the invention is therefore at least as powerful, if not more efficient, in resistance to exfoliating corrosion, than the sheet according to the prior art.

It is found that the sheet M is better for each of the following physical parameters: static mechanical characteristics, K _app , fatigue resistance, crack propagation speed.

Example 3 (except invention)

A P alloy similar to the M alloy of Example 2 was produced. From this alloy, using a manufacturing range similar to that of Example 2, hot-rolled plates (FIG. inlet temperature: 420 - 440 ° C) with a thickness of 75 mm.

• Premier palier : montée en température de 30 °C / heure jusqu'à 120°C et un maintien pendant 6 heures à cette température de 120°C.
• Deuxième palier : montée en température de 15°C / heure jusqu'à 160°C et un maintien pendant 5 heures (procédé A), 10 heures (procédé B) ou 15 heures (procédé C) à cette température de 160°C.

Les valeurs de K_app(L-T) ont été déterminées sur des éprouvettes de type C(T) avec W = 127 mm et B = 7,6 mm.After dissolution and quenching as indicated in Example 2, the sheets were subjected to the following two-stage income processes:

• First step: rise in temperature from 30 ° C / hour to 120 ° C and hold for 6 hours at this temperature of 120 ° C.
• Second stage: temperature rise from 15 ° C./hour to 160 ° C. and hold for 5 hours (process A), 10 hours (process B) or 15 hours (process C) at this temperature of 160 ° C.

K _{app (LT)} values were determined on specimens of type C (T) with W = 127 mm and B = 7.6 mm.

Table 10 summarizes the mechanical characteristics obtained: Table 10: Process R _{p0.2 (L)} [MPa] R _{m (L)} [MPa] A _(L) [%] K _{IC (LT)} [MPa√m] K _{app (LT)} [MPa√m] AT 542 561 9.7 30.1 57.1 B 525 549 10.2 32.8 63.2 VS 507 537 11.3 34.6 72.5

Claims (10)

1. Work-hardened product, rolled into the shape of a plate with a thickness of more than 20 mm, in alloy of the type AlZnCuMg,
   characterized in that it comprises (% by weight):

   Zn 6.7 - 7.3% Cu 2.0 - 2.3% Mg 1.5 - 1.8 %

   Zr 0.07-0.12% Fe<0.15% Si<0.15%

   other elements not more than 0.05 % each and 0.15 % total, remainder aluminium,
   said product being treated by solution heat treatment, quenching, cold working and artificial aging and said product having the following characteristics

   (a) R_m(L) > 540 MPa;

   (b) R_P0.2(L) > 535 MPa;

   (c) K_app(L-T) > 100 MPa√m (measured at ambient temperature on a C(T) type test piece with W = 127 mm and B = 7.6 mm);

   (d) ΔK at a crack propagation rate of 2.54 µm / cycle > 28 MPa√m;

   (e) K_IC(L-T) > 28 MPa√m.
2. Product according to claim 1, wherein Zr 0.07 - 0.09%.
3. Product according to anyone of claims 1 to 2, wherein Mg / Cu ≤ 0.80.
4. Product according to anyone of claims 1 to 3, wherein the alloy contains between 50 and 500 ppm titanium, and preferably between 150 and 300 ppm titanium.
5. Product according to anyone of claims 1 to 4, characterized in that it was elaborated from a metal refined with a refining agent which contains Ti and C.
6. Product according to anyone of claims 1 to 5, characterized in that it was elaborated from a liquid metal to which a refining agent containing titanium and carbon was added such that the added carbon quantity is between 0.4 and 3 g/t of carbon, preferably between 0.6 and 2 g/t and such that the total content of Ti in the final product is between 50 and 500 ppm (by weight) preferably between 150 and 300 ppm.
7. Structural member suitable for aircraft manufacturing made from at least one product according to anyone of claims 1 to 6.
8. Integral structure suitable for aircraft manufacturing comprising one or several products according to anyone of claims 1 to 6.
9. Use of a rolled product according to anyone of claims 1 to 6 having a thickness of more than 40 mm for manufacturing by machining aircraft structural members.
10. Use of a rolled product according to anyone of claims 1 to 6 having a thickness of more than 60 mm for manufacturing aircrafts stringers or frames.