EP1644546A2

EP1644546A2 - Products made from al/zn/mg/cu alloys with improved compromise between static mechanical properties and tolerance to damage

Info

Publication number: EP1644546A2
Application number: EP04767427A
Authority: EP
Inventors: Julien Boselli; Fabrice Heymes; Frank Eberl; Timothy Warner
Original assignee: Alcan Rhenalu SAS; Pechiney Rhenalu SAS
Current assignee: Constellium Issoire SAS
Priority date: 2003-06-24
Filing date: 2004-06-23
Publication date: 2006-04-12
Anticipated expiration: 2024-06-23
Also published as: CA2528614A1; BRPI0411873B1; US20050058568A1; BRPI0411873A; WO2005001149A2; EP1644546B1; US7452429B2; WO2005001149A3; DE04767427T1; CA2528614C

Abstract

The invention relates to a drawn, laminated or forged aluminium alloy product, characterised in comprising (in mass %): Zn 6.7 7.5 %, Cu 2.0 2.8 %, Mg 1.6 2.2 one or several elements selected from the group Zr 0.08 0.20 % ,Cr 0.05 0.25 %, Sc 0.01 0.50%, Hf 0.05 0.20 %, V 0.02 0.20%, Fe +Si < 0.20%, other elements = 0.05 % each and = 0.15 % in total, the residue being aluminium. Said product has an improved compromise between static mechanical resistance and tolerance to damage.

Description

AL-ZN-MG-CU ALLOY PRODUCTS WITH COMPROMISE STATIC MECHANICAL CHARACTERISTICS / IMPROVED DAMAGE TOLERANCE

Technical field of the invention

The present invention relates to alloys of the Al-Zn-Mg-Cu type with compromise static mechanical characteristics - improved damage tolerance, as well as structural elements for aeronautical construction incorporating wrought semi-finished products produced from these alloys.

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 cannot be optimized all at the same time and one independently of the 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 with properties gathered under the term "static mechanical strength" (notably the tensile strength R _m and the elastic limit R _p o. ₂ ) on the one hand, and properties gathered under the term " damage tolerance ”(including toughness and resistance to crack propagation) on the other hand. Certain usage properties such as fatigue resistance, corrosion resistance, formability and elongation at break are linked in a complicated and often unpredictable manner to the properties (or "characteristics"). mechanical. Optimizing all the properties of a material for aeronautical construction therefore very often involves a compromise between several key parameters. Alloys of the Al-Zn-Mg-Cu type (belonging to the family of 7xxx alloys) are commonly used in aeronautical construction, and in particular in the construction of the wings of civil aircraft. For the upper surfaces of the wings, for example, a skin made of heavy plates made of alloys 7150, 7055, 7449, and optionally stiffeners made of profiles made of alloys 7150, 7055 or 7449. The alloys 7150, 7050 and 7349 are also used for the manufacture of fuselage stiffeners. The 7475 alloy is sometimes used for the manufacture of lower airfoil panels, in particular by machining heavy sheets, while the lower airfoil stiffeners are usually made of 2xxx type alloys (eg 2024, 2224, 2027).

Some of these alloys have been known for decades, such as alloys 7075 and 7175 (zinc content between 5.1 and 6.1% by weight), 7475 (zinc content between 5.2 and 6.2%) , 7050 (zinc content between 5.7 and 6.7%), 7150 (zinc content between 5.9 and 6.9%) and 7049 (zinc content between 7.2 and 8.2%). These alloys have different compromises between toughness and elastic limit.

Patent application EP 0 257 167 A1 describes an alloy developed specifically for the production by reverse spinning of pressure-resistant hollow bodies. This alloy has the composition (in percent by mass): Zn 6.25 - 8.0 Mg 1.2 - 2.2 Cu 1.7 - 2.8 Zr≤ 0.05 Fe <0.20 (Fe + Si) <0.40 Cr 0.15 - 0.28 Mn <0.20 Ti <0.05.

These products do not exceed, in the dissolved and tempered state, values of R _m = 530 MPa, values of R _p o _{) 2} = 480 MPa, and A = 15.4%. The increase in the content of zinc (8.0%), Cu (2.2%) and Mg (2.4%) leads to an increase in R _m (up to 570 MPa) and R _p o, ₂ (up to 525 MPa), but these products have poor burst behavior.

Patent application EP 0 589 807 A1 discloses a pressurized gas cylinder with the composition Zn 6.9, Cu 2.3, Mg l, 9, Zr 0.11 which shows in state T73 the following static mechanical characteristics in the sense L: R _pθ , ₂ = 392 MPa, R _m = 459 MPa, A ^≈ 15.2%. US Patent 5,865,911 (Aluminum Company of America) discloses an Al-Zn-Cu-Mg type 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 strong mechanical strength, toughness and resistance to fatigue.

Patent application WO 02/052053 describes three alloys of the Al-Zn-Cu-Mg type with the composition Zn7.3 Cul, 6, Zn6.7 Cu 1.9, Zn7.4 Cul, 9 and each comprising Mg 1.5 Zr 0.11, as well as thermomechanical treatment methods suitable for the manufacture of structural elements for aircraft.

Alloy 7040 is also known, the standardized chemical composition of which is: Zn 5.7 -6.7 Mg 1.7-2.4 Cu 1.5 -2.3 Zr 0.05 -0.12 Si <0 , 10 Fe <0.13 Ti <0.06 Mn <0.04 other elements <0.05 each and <0.15 in total. Alloy 7085 is also known, the standardized chemical composition of which is: Zn 7.0 -8.0 Mg 1.2 -1.8 Cu 1.3 -2.0 Zr 0.08 -0.15 If <0, 06 Fe <0.08 Ti <0.06 Mn <0.04 Cr <0.04 other elements <0.05 each and <0.15 in total.

More recently, the applicant has noted the advantage of reducing the concentration of Cu and Mg compared to a type 7050 alloy (see EP 0876514 Bl). For a heavy plate, the compromise between toughness and mechanical strength is thus improved.

Problem

The problem to which the present invention is trying to respond is to propose a new wrought product of Al-Zn-Mg-Cu type alloy making it possible to achieve very high levels of static mechanical strength while presenting a level sufficient in other properties of use, in particular the toughness, the resistance to corrosion and the resistance to the propagation of fatigue cracks (cracking).

Objects of the invention

A first object of the present invention consists of a spun, rolled or forged product made of aluminum alloy, characterized in that it comprises (in% by mass): Zn 6.7 - 7.5% Cu 2.0 - 2.8% Mg 1.6 - 2.2% one or more elements chosen from the group consisting of: Zr 0.08 - 0.20% Cr 0.05 - 0.25% Se 0.01 - 0.50 % Hf 0.05 - 0.20% N 0.02 - 0.20% Fe + Si <0.20% other elements <0.05% each and <0.15% in total, the rest aluminum.

Another object of the present invention is a manufacturing process for obtaining such a product.

Yet another object of the present invention is an aircraft structural element which incorporates at least one of said products, and in particular a structural element used in the construction of the wing of civil aircraft, such as a stiffener, and in in particular a wing lower stiffener.

Description of the figures Figure 1 shows the section of "I" profiles, the manufacture of which is described in Example 1. 1 = thick branch, 2 = Thickness of the thick branch 1, 3 = sole, 4 = thickness of the sole 3, 5 = long branch, 6 = height, 7 = width. FIG. 2 shows the section of profiles whose manufacture is described in Examples 3 and 5. FIG. 3 shows the section of “inverted T” profiles, the manufacture of which is described in Example 4 (same symbols as in FIG. 1, 8 = reinforcement width).

Description of the invention a) Terminology

Unless otherwise stated, all information relating to the chemical composition of the alloys is expressed in percent by mass. Consequently, in a mathematical expression, "0.4 Zn" means: 0.4 times the zinc content, expressed in percent by mass; this applies mutatis mutandis to other chemical elements. Unless otherwise indicated, all the chemical compositions indicated in the present description and the examples were determined on samples obtained by taking a representative sample of liquid metal during casting, followed by the solidification of the liquid metal taken in a form. which ensures good homogeneity of the concentration of the elements in the solid. The concentration of the chemical elements was determined by solid-state X-ray spectroscopy or by analysis in solution. 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 European standard EN 515. The chemical composition of standardized aluminum alloys is defined for example in standard EN 573-3. Unless otherwise stated, the static mechanical characteristics, that is to say the tensile strength R _m , the elastic limit Rpo. ₂ , and the elongation at break A, are determined by a tensile test according to standard EN 10002-1, the place and direction of the sampling of the test pieces being defined in standard EN 485-1. The elastic limit in compression was measured by a test according to ASTM E9. The tenacity Kic was measured according to standard ASTM E 399. The curve R is determined according to standard ASTM 561-98. From the curve R, one calculates the critical stress intensity factor Kc, ie the intensity factor which causes the instability of the crack. One also calculates the stress intensity factor Kco ₅ ^by assigning to the critical load the initial length of the crack, at the beginning of the monotonous loading. These two values are calculated for a test piece of desired shape. K _app designates the Kco corresponding to the test piece used to perform the R curve test. The resistance to exfoliating corrosion was determined according to the EXCO test described in standard ASTM G34. Unless otherwise stated, the definitions of European standard EN 12258-1 apply. The term "sheet metal" is used here for rolled products of any thickness.

The term "machining" includes any material removal process such as turning, milling, drilling, reaming, tapping, EDM, grinding, polishing. The term "spun product" also includes products which have been drawn after spinning, for example by cold drawing through a die. It also includes drawn products.

The term “structural element” refers to an element used in mechanical construction for which the static and / or dynamic mechanical characteristics are of particular importance for the performance and integrity of the structure, and for which a calculation of the structure is generally prescribed or performed. It is typically a mechanical part, the failure of which is likely to endanger the safety of said construction, of its users, of its users or of others. For an aircraft, these structural elements include in particular the elements that make up the fuselage (such as the fuselage skin), the stiffeners or bulkheads, bulkheads, fuselage (circumferential frames), the wings (such as the wing skin), the stiffeners (stringers or stiffeners), the ribs (ribs) and spars (spars)) and the empennage composed in particular of horizontal and vertical stabilizers (horizontal or vertical stabilizers), as well as the floor profiles (floor beams), the seat rails (seat tracks) and the doors.

The term "monolithic structural element" refers to a structural element which has been obtained from a single piece of rolled, spun, forged or molded semi-finished product, without assembly, such as riveting, welding, bonding, with another room.

In determining the tempering times at a given temperature, the concept of time equivalent to a reference temperature (for example at 160 ° C.) is used. The calculation below gives the formula used:

where TEQ (160 ° C) is the time equivalent to 160 ° C corresponding to an income of a duration of tréei to Tréei (in ° K), where Q is an activation energy of 132000 kj / mol and R = 8.31 kJ / mol / (° K).

b) Detailed Description of the Invention According to the invention, the problem is solved by the combination of a fine adjustment of the content of alloying elements and of the conditions of the heat treatment, in particular of the homogenization of the raw forms, as well as the dissolution and the income of the products obtained by hot transformation.

In the process according to the invention, an alloy of composition Zn 6.7 - 7.5 (preferably: 6.9 - 7.3) is first prepared; Cu 2.0 - 2.8 (preferably 2.2 - 2.6); Mg 1.6-2.2 (preferably 1.8-2.0); one or more elements chosen from the group consisting of Zr 0.08 - 0.20, Cr 0.05 - 0.40, Se 0.01 - 0.50, Hf 0.05 - 0.60, N 0.02 - 0.20; Fe + Si <0.20 and preferably <0.15; other items <0.05 each and <0.15 in total; the rest aluminum.

In the context of the present invention, the content of alloying elements must not significantly exceed their solubility limit, because otherwise, the persistence of intermetallic phases during dissolution which can harm tolerance for damage. For a given magnesium content, the copper content can be brought to a level fairly close to the solubility limit, which depends on the magnesium content. Thus, a composition is preferred in which 3.8 <Cu + Mg <4.8, and preferably 3.9 <Cu + Mg <4.7. In an advantageous embodiment of the invention, 4.0 <Cu + Mg <4.8 is chosen. In another advantageous embodiment, 4.1 <Cu + Mg <4.7 is chosen.

Below a magnesium content of about 1.6%, there is a risk of cracks forming during casting, and a minimum content of about 1.7% or even 1.8% is preferred. The Cu / Mg ratio must be at least 1.0 in order to obtain a good compromise of properties, and in particular a good tolerance for damage, but must not exceed 1.5 to ensure acceptable flowability. It is preferred that it be between 1.1 and 1.5, and even more preferably between 1.1 and 1.4. The Applicant has found that above a magnesium content of approximately 2.2%, we no longer obtain acceptable tenacity properties.

In an advantageous embodiment of the invention, the magnesium and copper content is chosen such that 4.2 <Cu + Mg <4.7 and Cu / Mg between 1.15 and 1.45. The addition of zirconium at 0.08 - 0.20% limits recrystallization. This role can also be fulfilled by other elements, such as chromium (0.05 - 0.40%), scandium (0.01 - 0.50%), hafhium (0.05 - 0, 60%) or vanadium (0.02 - 0.20%). A Zr content of not more than 0.15% is preferred to avoid the formation of primary phases. When several of these anti-recrystallizing elements are added, their sum is limited by the appearance of the same phenomenon. In an advantageous embodiment, only zirconium is added. Chromium is especially suitable for thin products. It is also possible to add up to 0.8% of manganese as an anti-recrystallizing element. In any event, it is preferable that the sum of the anti-recrystallizing elements does not exceed approximately 1%.

This alloy is then cast according to one of the techniques known to a person skilled in the art to obtain a raw form, such as a spinning billet or a rolling plate. This raw form is then homogenized. The purpose of this heat treatment is threefold: (i) dissolve the coarse soluble phases formed on solidification (ii) reduce the concentration gradients in order to facilitate the dissolution step and (iii) precipitate the dispersoids in order to limit / eliminate the recrystallization phenomena during the dissolution step. The Applicant has found that the alloy according to the invention was characterized by a particularly low end-of-solidification temperature compared with alloys of the 7040, 7050 or 7475 type. The same is true of the temperature above which the partial melting of the alloy is observed. thermodynamic equilibrium (so-called solidus temperature). For these reasons, homogenization with a rapid rise to a single temperature creates a risk of burns, and does not give satisfactory dissolution of the particles. Homogenization in at least two steps reduces this risk and improves the result. In a preferred embodiment, the homogenization is carried out in two stages, with a first stage between 452 and 473 ° C, typically for a period of between 4 and 30 hours (preferably between 4 and 15 hours), followed by a second stage between 465 and 484 ° C, and preferably between 467 and 481 ° C, typically for a period of between 4 and 30 hours (preferably between 4 and 16 hours). In a particular embodiment, the first step is carried out between 457 and 463 ° C, and the second between 467 and 474 ° C. In another embodiment, homogenization is carried out in a single step with a linear rise at 40 ° C per hour to a temperature between 467 and 481 ° C, preferably between 471 and 481 ° C, and typically during between 4 and 30 hours. It is also possible to make the homogenization in three stages. Homogenization can also be carried out in a single step, with a temperature rise of less than 200 ° C / h, and preferably between 20 and 50 ° C / h up to a plateau between 465 and 484 ° C, and preferably between 471 and 481 ° C.

The raw form is then transformed hot to form extruded products (in particular bars, tubes or profiles), hot-rolled sheets or forgings. The spinning is preferably done at a die temperature between 380 and 430 ° C, and preferably between 390 and 420 ° C, by one of the methods known to those skilled in the art, such as direct spinning or reverse spinning. It is preferred that the hot transformation by spinning takes place with a block temperature of between 400 and 460 ° C., and preferably between 420 ° C. and 440 ° C. It is thus possible to obtain spun products which nowhere shows a coarse-grained cortical layer with a thickness greater than 3 mm, and preferably limited to 1 mm, in particular in the case of thinner spun products. The hot transformation can optionally be followed by a cold transformation. As an example, it is possible to manufacture spun and drawn tubes. One can also consider, in the case of rolled products, one or more cold rolling passes. This is normally not useful for laminated products with a thickness greater than 10 mm, for which the composition envisaged in the context of the present invention lends itself particularly well.

The products obtained are then placed in solution. In a preferred embodiment of the invention, the temperature is increased continuously for a period of between 2 and 6 hours, and preferably approximately 4 hours, up to a temperature between 470 and 500 ° C (preferably not exceeding 485 ° C), preferably between 474 and 484 ° C, and even more preferably between 477 and 483 ° C, and maintains the product at this temperature for a period of between 1 and 10 hours, and preferably approximately 2 to 4 hours. Then, the products are soaked, preferably in a preferably liquid quenching medium such as water, said liquid preferably having a temperature not exceeding 40 ° C.

Then, the products can be subjected to a controlled traction with a permanent elongation of the order of 1 to 5%, and preferably 1.5 to 3%.

Then, the products are subjected to a tempering treatment, which has a significant influence on the final properties of the product. The Applicant has found that double-tier income works well. However, income can be achieved in three stages, or as “ramp annealing” income. You can even consider income in one step.

For a two-step process, a first step between 110 ° C and 130 ° C is suitable. In an advantageous embodiment of the present invention, the first level is between 115 ° C and 125 ° C. For this preferred temperature range, an equivalent TEQ treatment time (160 ° C.) of between 0J and 2 hours, and preferably between 0.1 and 0.5 hours, can be used. The second level is advantageously between 150 and 170 ° C. According to the Applicant's findings, if we aim to optimize n the compromise between R0.2 and K _app , the equivalent TEQ treatment time (160 ° C) for this second stage is advantageously between 4 and 16 hours, and preferably between 6 and 12 hours. If we aim to optimize the compromise between Ro. ₂ and Kic, a second longer bearing at a temperature between 150 ° C and 170 ° C is preferable, for example an equivalent TEQ treatment time (160 ° C) between 16 and 30 hours. In an advantageous embodiment, the second stage was carried out at a temperature of 160 ° C. for 24 hours.

In a first particular embodiment, the temperature of the second level is between 155 and 165 ° C. The control of the duration of this second stage is particularly important for the final properties of the product. In a particularly advantageous embodiment of this first particular embodiment, the second level is between 157 and 163 ° C, and its duration is between 6 and 10 hours. In another particular embodiment of the invention, the second level is carried out at a slightly lower temperature, between 150 and 160 ° C.

If a single-bearing income is envisaged, a temperature of the order of 115 to 145 ° C. will advantageously be used for a duration of the order of 4 to 50 hours, for example 48 hours at 120 ° C. As an example, an equivalent TEQ treatment time (160 ° C.) of the order of 0.6 hours to 1.20 hours can be used. These single-stage treatments make it possible to obtain products in the T6 state.

In the case of profiles, the static mechanical characteristics are typically measured in the longest leg of the profile. The same applies to samples for corrosion measurements. The samples for assessing tolerance to damage are taken from a flat area of sufficient width which includes, where possible, the longest branch, this area being commonly called the profile sole. In the case of sheets, samples were taken for the measurement of static mechanical characteristics at the depth recommended by standard EN 485-1: 1993 (clause 6.1.3.4.).

The method according to the invention leads to new products which have particularly advantageous characteristics for aeronautical construction. These products can be in the form of sheets, in particular thick sheets, or sections, or forgings. More particularly, the present invention makes it possible to produce thick profiles which can be used as wing stiffeners. These products have an elastic limit R _p o, 2 (L) of at least 550 MPa and preferably at least 580 MPa, and a K _apP (L-τ) measured according to ASTM E 561-98 on a "center-crack tension panel" type test piece (also called "middle-cracked tension panel") of width W = 100 mm of at least 75 MPavm, preferably at least 78 MPa m and even more preferably at least 80 MPa m. Those skilled in the art know that the choice of the width W of the test piece affects the value of K _app obtained.

An important advantage of the product according to the invention is the fact that the value of K _app ( _L -τ), determined as indicated above, is substantially the same at around 20 ° C and at around -50 ° C, knowing that -50 ° C is a typical ambient temperature when flying a civilian jet aircraft. More precisely, this value of K _app (L-τ) does not decrease by more than 3% when going from about 20 ° C to about -50 ° C. In a preferred embodiment of the present invention, it does not decrease at all. We know that in certain alloys of the 7xxx series, the toughness decreases with temperature. As an example, it has been described that 7475 T7651 sheets show a 25% drop in toughness (determined from R curves on panels of thickness B = 6 mm in the LT direction) between approximately 20 ° C and around -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 drop in K _ÏC or K _q 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)). While it is known that the static mechanical characteristics R _p o. ₂ and R _m of the alloys of the 7xxx series tend to increase when the temperature drops from about 20 ° C to about -50 ° C, which provides additional safety of the structure at this temperature, the decrease in the toughness of the alloys of the series 7xxx according to the state of the art (which the applicant has found for example for heavy plates in 7075 T7351, 7475 T7351, 7475 T7651 and 7475 under-income, at a rate of approximately 2 to 10%) taken into account when sizing structural elements. The product according to the invention does not show a significant decrease (that is to say greater than 2%) in the toughness at low temperature.

In an advantageous embodiment of the present invention, the product is an airfoil stiffener, which has the following set of properties (measured at mid-thickness and at a temperature of approximately 20 ° C.):

A breaking strength R _m (D of at least 585 MPa, an elastic limit R _p o, ₂ ( _L ), measured by a tensile test and by a compression test, of at least 555 MPa, an elongation at break A ( _L > of at least 9%, a K _app (L.τ) measured on CCT test pieces with W = 100 mm of at least 88 MPa m, a resistance to fatigue (fatigue crack gro th resistance) ΔK _L - _T of at least 27 MPa m at R = 0.1 and a crack propagation speed of 2.5 10 mm / cycle, a fatigue strength of at least 10 cycles at R = 0 , 1, K _t = 3 and σ _max = 22 ksi (151.7 MPa), an exfoliating corrosion resistance of at least EB (and preferably at least EA), and crack propagation in the direction SL in the middle corrosive (determined by the so-called DCB (double cantilever beam) method according to EN ISO 7539-6) not exceeding 10 ^"8 m / s.

The invention makes it possible to obtain a product which shows at least one set of properties (measured at around 20 ° C.) selected from the group formed by the five sets: (a) an elastic limit R _p o. ₂ ( _L ) of at least 480 MPa (and preferably at least 500 MPa), a breaking strength R _m (L) of at least 530 MPa (and preferably at least 555 MPa) and a K _IC ( _L - _T ) of at least 36 MPa m (and preferably of at least 40 MPaVm and even more preferably of at least 44 MPaVm)

(b) a yield strength R _p o.2 (L) of at least 550 MPa (and preferably at least 580 MPa, and even more preferably at least 600 MPa) and a Ka _P p (- τ) measured with W = 100 mm) of at least 80 MPaVm (and preferably of at least 83 MPa m, and even more preferably of at least 87 MPa m); (c) an elastic limit R _p o.2 ( _L ) of at least 550 MPa (and preferably at least 580 MPa) and a crack propagation speed da / dn not exceeding 3 10 ^"3 mm / cycle (and preferably not exceeding 2.5 10 ^"3 mm / cycle) for Δ K = 27 ;

(d) an elastic limit R _p o. ₂ ( _L ) of at least 550 MPa (and preferably of at least 580 MPa), a breaking strength R _m ( _L ) of at least 580 MPa (and preferably of at least 600 MPa) and a K _apP (L-τ) measured with W = 100 mm of at least 80 MPaVm (and preferably of at least 83 MPa m, and even more preferably of at least 87 MPaVm);

(e) a breaking strength Rm (L) of at least 580 MPa (and preferably of at least 600 MPa and even more preferably of at least 620 MPa) and a K _apP ( _L _-τ ) measured with W = 100 mm of at least 80 MPa m (and preferably at least 83 MPaVm, and even more preferably at least 87 MPaVm).

According to the particular embodiment, such a product can additionally show at least one property selected from the group formed by: (a) an elongation at break A (> of at least 9%, and preferably at least 12% (b) an exfoliating corrosion resistance measured according to ASTM G34 of at least EB.

By way of comparison, the lower surface stiffeners of AA2027 T3511 alloy according to the state of the art typically have the following properties: Rm (L): approximately 545 MPa, Rpθ, 2 ( _L ) in traction: approximately 415 MPa, R _P o, 2 ( _L ) in compression: approximately 400 MPa, Elongation at break A ( _L >: approximately 16%, K _I C ( _L - _T ): approximately 48 MPaVm with a CT test piece with W = 2B, K _app ( LT) (W = 100 mm, B = 6.35 mm): about 75 MPa m, Exfoliating corrosion resistance: EB

It can therefore be seen that the invention makes it possible above all to increase the tensile strength and the elastic limit, while maintaining the other properties of use at an at least comparable level. The reduction in elongation at break is not a drawback for these applications, which normally do not require a particularly high value; if there is in some cases a small drawback associated with this drop, it is very largely offset by the increase in mechanical strength.

The product according to the invention is particularly suitable for the manufacture of structural elements whose effective width to be considered with regard to dimensioning in toughness or in cracking is limited by geometric factors of the structure in which these structural elements must be integrated, for example by a design which effectively limits the width of the panels excluding stiffeners. In such a case, the optimal product according to the invention corresponds to that which offers the maximum static mechanical resistance while ensuring sufficient toughness to ensure that the residual resistance of the part in the presence of a crack is limited by the static resistance of the product, or even a combination of static mechanical strength and toughness, and not by its intrinsic toughness.

A particularly preferred product according to the invention is a wing stiffener, obtained by spinning, for example a lower surface stiffener. Another advantageous product is a fuselage frame. In the context of the present invention, spun products have been produced with a cortical layer (peripheral layer of recrystallized grains) at the center of the long branches which remains a) less than 3.0 mm whatever the section; or b) less than 1.5 mm for sections of width less than equal to 50mm, or c) less than e / 4 mm (where e is the thickness) for sections of width less than or equal to 10 mm. Another advantage of the product according to the invention is the possibility of income forming. We know that aeronautical structural elements must have precise shapes dictated by aerodynamics. These geometries can be obtained by cold forming. In this case, if the alloy requires a tempering treatment, this can be carried out after shaping in order to benefit from a metal that is more ductile and easier to shape. These geometries can also be obtained by shaping during the heat treatment of tempering. In this case, the metal is delivered in an intermediate metallurgical state, typically after a first level of tempering. This advantageous process in terms of cost and reproducibility is only possible with products comprising an income treatment allowing effective shaping. The 2xxx T351x alloys used for the stiffeners and wing panels do not allow this process to be used since they do not undergo any income treatment. On the other hand, the product according to the invention is particularly suitable for the manufacture of structural elements which have to undergo an income forming during the second income level.

The product according to the invention, thanks to its compromise in properties, is very advantageous for applications which require both high mechanical strength and high tolerance with regard to occasional overloads without leading to sudden rupture of the part. Besides the structural elements for aircraft, the products according to the invention have been used for the manufacture of other parts or structural elements which meet high safety requirements. By way of example, the applicant has manufactured by spinning, possibly followed by cold drawing, tubes for the production of frames, forks and handlebars for cycles (bicycles, tricyles, motorcycles, etc.), or baseball bats. For these applications, it has been found to be advantageous to add to the alloy a low content of scandium and / or hafhium, for example between 0.15 and 0.60% of scandium and approximately 0.50% of hafhium. . Preferably, a manufacturing process is chosen which leads to a fiber structure of the tubes.

The invention will be better understood with the aid of the examples, which however are not limiting. Examples

Example 1:

Spinning billets of diameter 291 mm (alloy A) were cast by semi-continuous casting, the composition of which is indicated in Table 1. These billets were homogenized in two stages:

1) 13 hours at 460 ° C

2) 14 hours at 470 ° C. Table 1

The content of Cu, Mg and Zn was determined by chemical analysis after dissolution of part of the sample, while the other elements were determined by X-ray spectroscopy on solid.

Profiles of section “I” were spun (see Figure 1: thickness of the order of 17 mm to 22 mm, width of the order of 160 mm and height of the order of 80 mm) from peeled billets with a diameter of 270 mm, at a plot temperature between 390 and 410 ° C and a container temperature between 400 and 420 ° C, with an exit speed of about 0.5 m / min. The profiles were dissolved by increasing the temperature continuously for 3 hours to 481 ± 3 ° C and keeping them at this temperature for 6 hours, then soaked in water between 22 and 25 ° C and pulled with a permanent deformation of between 1.5 and 3%. An over-income treatment was then carried out to obtain products in the T76 state. The over-tempering was carried out in two stages: first at 120 ° C for 6 hours, then at 160 ° C for a variable duration. The thickness of the coarse-grained recrystallized layer measured at the center of the sole is less than 1 mm. To characterize the products obtained, their static mechanical characteristics (R _m , R _p o, 2, A) were determined according to EN 10001-2, their resistance to exfoliating corrosion according to ASTM G34, (so-called “Exco” test), their resistance to stress corrosion according to ASTM G 47, their crack propagation speed according to ASTM E647 (test called "da dn") in the direction TL or LT for a value ΔK of 50 MPa m and a charge ratio R = 0.1 and their stress intensity factor K _app (parameter known as “apparent K”). The latter was calculated using the maximum load measured during the test according to ASTM E561-98 on test pieces of width W equal to 100 mm, and the initial crack length (at the end of pre-cracking) in the formulas indicated by the cited standard.

Table 2 shows the influence of the duration of the second tempering stage on certain properties measured at the end of the profile; the mechanical characteristics having been measured at 20 ° C. The results of the tensile test were obtained on a test piece of circular section, diameter 10 mm, half-thickness and half-width in the long branch. The KIc toughness results were obtained on specimens taken at half-thickness and half-width in the long branch or the thickest branch. EXCO corrosion results were obtained on specimens taken at half thickness and half width in the branch. The Kapp results were obtained on mid-thickness test pieces and centered in the sole of the profile containing the long branch.

Table 2

We find that a duration of 8 hours or 12 hours gives very good results.

The tenacity K _app (L-τ) at -50 ° C was 87.6 MPa m for an 8 hour tempering, and 83.5 MPaVm for a 24 hour tempering time (on test pieces with B = 6 mm).

For a product having undergone a second tempering step at 160 ° C for 8 hours, the properties in the LT sense were at 20 ° C: R _P o, 2 (LT) = 579 MPa, R _{m (L} τ) = 609 MPa , A (LT) = 12%.

Table 3 shows the crack propagation speed measured in the LT direction with B = 7.61 mm, W = 96.6 mm, R = 0.10, and P _min = 600 N and P _max = 6000 N, for an income time of 6 hours at 120 ° C and 8 hours at 160 ° C. The “Compact-tension panel” type samples were taken at mid-thickness and half-width of the sole at the end of the profile. Table 3:

Corrosion corrosion test specimens were taken at the end of the profile at mid-thickness of the sole in two positions in the section: half-width of the long branch and half-width of the opposite branch in the sole. The results of resistance to corrosion under constant stress in the TL direction with σ = 300, 350 and 400 MPa of imposed stress are presented in Table 4. The monitoring of these tests stopped after 30 days. Table 4:

Corrosion test specimens under stress in a corrosive environment were taken at mid-thickness and half-width of the long branch at the end of the profile. The crack propagation in corrosive medium in the thickness direction (determined by the method called DCB (double cantilever beam) according to standard EN ISO 7539-6) was of the order of 5 10 ^"9 m / s for a second 8 hour tempering stage at 160 ° C.

Example 2:

An alloy was prepared, the composition of which is indicated in Table 5. Spinning billets with a diameter of 410 mm were poured. The homogenization conditions were the same as in Example 1. After peeling, billets with a diameter of 390 mm were obtained. They were spun with a plot temperature of between 410 and 430 ° C and a container temperature of around 420 ° C with an exit speed of 0.65 to 0.8 m / min, in flats of section 279 x 22 mm. Table 5

The products were dissolved with a rise in temperature in 35 min to 479 ± 2 ° C, with a plateau of 4 hours at this temperature. The quenching was carried out in cold water. Then, the flats were pulled with a permanent elongation of between 1.5 and 3%. The tempering was carried out in two stages: 6 hours at 120 ° C + 8 hours at 160 ° C. An ultrasonic check made it possible to verify the absence of internal faults (class AA MIL-STD-2154). The thickness of the coarse-grained recrystallized layer measured at the center of the sole is less than 1 mm.

The results of the tensile and compression test are collated in table 6. The results of the tensile test were obtained on a specimen of circular section, diameter 10 mm, at mid-thickness at the end of the flat and in two positions in the section: mid-width and edge. The results of the compression test were obtained on a test piece of circular section, diameter 10 mm, half-thick at the end of the flat and in two positions in the section: half-width and at the edge.

Table 6

The toughness results ic, K _app and EXCO corrosion were obtained on specimens taken at half-thickness and half-width at the end of the flat. The toughness and corrosion results are collated in Table 7. The test conditions are the same as those presented in Example 1. Table 7

Stress corrosion test specimens were taken at the end of the mid-thickness profile and on either side of the mid-width. The results of resistance to corrosion under constant stress in the TL direction with σ = 300, 350 and 400 Mpa of imposed stress are presented in Table 8. The monitoring of these tests stopped after 40 days. Table 8

Example 3:

Profiles of different geometries were spun from billets of composition A (see example 1). Figure 2 shows the section of these profiles. The manufacturing process was similar to that of Example 1. The thickness of the product is of the millimeter order compared to the preceding products. Nevertheless, a microstructure with a weak cortical layer with large grains could be obtained. Table 9 shows the static mechanical characteristics obtained for different income conditions. The first step of tempering was always 6 hours at 120 ° C.

Table 9

State T6 is close to the point 6 hours at 120 ° C + 1 hour at 160 ° C. Table 10 shows some tenacity compromises - static mechanical characteristics for some points corresponding to T7x states. The test conditions are the same as those presented in Example 1.

Table 10

These sections were used for the manufacture of fuselage frames.

Example 4:

Profiles of inverted 'T' section were spun (see Figure 3: thickness of the sole in the order of 25 mm, width of the reinforcement in the order of 40 mm, width of the sole in the order of 180 mm and height of the order of 70 mm) from billets of composition K (see example 2). The spinning conditions were similar to those of Example 2.

Three sections labeled X, Y and Z have undergone the stages of dissolution, quenching and traction separately. Profiles X and Y underwent a solution similar to Example 2. Profile Z underwent a solution with a rise in temperature between 1 h and 2 h and a maintenance of 3 hours at 480 ± 2 ° C. The three profiles were soaked in cold water and pulled between 1.5% and 3%. The profiles have been rectified to improve their straightness. The tempering was carried out in two stages with a first stage of 6 hours at 120 ° C. An ultrasonic test was carried out to verify the absence of internal faults (class A, MIL-STD-2154). The thickness of the coarse-grained recrystallized layer measured at the center of the sole is less than 1 mm.

Tables 11, 12 and 13 show the influence of the duration of the second tempering stage on certain product properties for the three profiles respectively X, Y and Z; the mechanical characteristics having been measured at 20 ° C. The test conditions are the same as those presented in Example 1. The results of the tensile test were obtained on a test piece of circular section, diameter 10 mm, at mid-thickness and half-width in the long branch . The KIc toughness and EXCO corrosion results were obtained on specimens taken at half-thickness and half-width in the long branch. The Kapp results were obtained on specimens centered in the sole of the profile containing the long branch. Table 11 - Profile X

Table 12 - Profile Y

Table 13 - Profile Z

We note that a duration of 8 hours or 24 hours for the 2nd stage of income gives very good compromises of results.

Corrosion corrosion test specimens were taken at the end of the profile at mid-thickness of the sole in two positions in the section: half-width of the long branch and half-width of the opposite branch in the sole. The results of resistance to corrosion under constant stress in the TL direction with σ = 300, 350 and 400 MPa of imposed stress are presented in table 14. The monitoring of these tests stopped after 40 days.

Table 14

Example 5:

An alloy was developed, the composition of which is shown in Table 15. Spinning billets with a diameter of 525 mm were poured. The homogenization conditions were 15 h between 473 and 481 ° C after a controlled temperature rise to 40 ° C / h. After peeling, billets with a diameter of 498 mm were obtained. They were spun in a container at a temperature between 410 and 430 ° C and a piece between 420 and 440 ° C, with an exit speed between 0.6 m / min and 1.0 m / min in the form of 'a section illustrated in Figure 3 (thickness of the sole of the order of 27 mm, width of the reinforcement of the order of 40 mm, width of the sole of the order of 205 mm and height of the order of 80 mm).

Table 15

The products were dissolved with a temperature rise between 1 h and 2 h to 480 ± 2 ° C, with a plateau of 3 hours at this temperature. The quenching was carried out in cold water between 21 and 22 ° C. Then, the extradited and quenched sections were tensioned with a permanent elongation of between 1.5 and 3%. The profiles have been rectified to improve their straightness. A first income of 6 hours at 120 ° C was carried out. An ultrasonic test was carried out to verify the absence of internal faults (class A, MIL-STD-2154). A second tempering was carried out for 8 hours at 160 ° C. The thickness of the coarse-grained recrystallized layer measured at the center of the sole is less than 1 mm. The results of the tensile test (on a test piece of circular section, diameter 10 mm, taken at the end of the profile, half-thickness and half-width in the long branch) are collated in Table 16. This table also contains the toughness and Kapp results both taken from the sole. The test conditions are the same as those presented in Example 1 except for the thickness B of the CCT specimen for the characterization of the Kapps which is 5 mm.

Table 16

Example 6:

Billet with compositions L, M, N and O were poured with diameters of 200 mm (see table 17). All the compositions underwent the same homogenization between 473 ° C and 481 ° C for 15 hours. After homogenization, the billets were peeled and drilled in the center to allow needle spinning. Seamless tubes were spun. The spinning blanks were cold drawn to produce tubes with a diameter between 20 and 30 mm with a wall thickness between 2 and 5 mm. Cold drawing increases the stored energy which is the main driver of recrystallization. The variation of the transition elements (cf. table 17) made it possible to generate different microstractures. After stretching; the tubes were dissolved at temperatures above 480 ° C for 1 h before quenching in cold water (~ 20 ° C). The tubes were not drawn after quenching. A first stabilization plateau for 6 h at 120 ° C. was carried out before a complete kinetics illustrated in Tables 18 to 20. The mechanical properties were measured on curved test pieces in the spinning direction L. Table 17

Table 19 (composition M)

State T6 is close to the point 6 hours at 120 ° C + 1 hour at 160 ° C. These tubes are used for applications in the sports and leisure market: frames, forks and handlebars, baseball bats.

Claims

1. Spun, rolled or forged product in aluminum alloy, characterized in that it comprises (in% by mass): (a) Zn 6.7 - 7.5% Cu 2.0 - 2.8% Mg 1 , 6 - 2.2%; (b) one or more elements chosen from the group consisting of: Zr 0.08 - 0.20% Cr 0.05 - 0.40% Se 0.01 - 0.50% Hf 0.05 - 0.60% V 0.02 -0.20%; (c) Fe + Si <0.20%; (d) other items <0.05% each and <0.15% in total; (e) the rest of the aluminum.

2. Product according to claim 1, characterized in that its magnesium and copper content is such that 3.8 <(Cu + Mg) <4.8, and preferably 3.9 <(Cu + Mg) <4, 7, and even more preferably 4.1 <(Cu + Mg) <4.7.

3. Product according to claim 1 or 2, characterized in that the Cu / Mg ratio is between 1.0 and 1.5, preferably between 1.1 and 1.5, and even more preferably between 1.1 and 1.4.

4. Product according to any one of claims 1 to 3, characterized in that Zn is between 6.9 and 7.3%.

5. Product according to any one of claims 1 to 4, characterized in that Cu is between 2.2 and 2.6%.

6. Product according to any one of claims 1 to 5, characterized in that Mg is between 1.7 and 2.0%, and preferably between 1.8 and 2.0%.

7. Product according to any one of claims 1 to 6, characterized in that it additionally contains up to 0.8% of manganese.

8. Product according to any one of claims 1 to 6, characterized in that the sum of the content of the elements Zr, Cr, Se, Hf, N does not exceed 1%, and preferably does not exceed 0.8%.

9. Product according to any one of claims 1 to 8, characterized in that Si + Fe does not exceed 0.15%.

10. Product according to any one of claims 1 to 9, characterized in that it has undergone dissolution, quenching and tempering, said tempering comprising a first plateau at a temperature between 110 ° C and 125 ° C, and preferably between 115 and 125 ° C, and a second level at a temperature between 150 and 170 ° C, and preferably between 150 and 165 ° C.

11. Product according to any one of claims 1 to 10, characterized in that it has at least one set of properties (measured at around 20 ° C) selected from the group formed by the five sets: (a) a limit of elasticity R _p o.2 (L) of at least 480 MPa (and preferably at least 500 MPa), a breaking strength R _m ( _L ) of at least 530 MPa (and preferably at least 555 MPa) and a K _I C ( _L - _T ) of at least 36 MPaVm (and preferably of at least 40 MPaVm and even more preferably of at least 44 MPaVm) (b) an elastic limit R _p o. ₂ (L) of at least 550 MPa (and preferably at least 580 MPa, and even more preferably at least 600 MPa) and a K _a pp (L-τ) measured with W = 100 mm) d ' at least 80 MPaVm (and preferably at least 83 MPaVm, and even more preferably at least 87 MPaVm); (c) an elastic limit R _p o.2 (L) of at least 550 MPa (and preferably at least 580 MPa) and a crack propagation speed da / dn not exceeding 3 10 ^"3 mm cycle (and preferably not exceeding 2.5 10 ^"3 mm / cycle) for ΔK = 27 MPaVm; (d) an elastic limit R _p o.2 (L) of at least 550 MPa (and preferably at least 580 MPa), a breaking strength R _m (D of at least 580 MPa (and preferably at least 600 MPa) and a K _app (L-τ) measured with W = 100 mm at least 80 MPaVm (and preferably at least 83 MPaVm, and even more preferably at least 87 MPaVm) ; (e) a breaking strength R _{m (L} ) of at least 580 MPa (and preferably of at least 600 MPa and even more preferably of at least 620 MPa) and a K _a pp (L-τ) measured with W = 100 mm of at least 80 MPaVm (and preferably at least 83 MPaVm, and even more preferably at least 87 MPaVm).

12. Product according to claim 11, characterized in that it additionally shows at least one property selected from the group formed by: (a) an elongation at break A ₍ D of at least 9%, and preferably at least minus 12%; (b) a resistance to exfoliating corrosion measured according to ASTM G34 of at least EB.

13. Product according to any one of claims 1 to 12, characterized in that the value of K _ap p (L-τ) at about -50 ° C is at least 98%, and preferably at least 100%, of the measured value at around 20 ° C.

14. A spun product according to any one of claims 1 to 13, characterized in that the thickness of the peripheral layer of recrystallized grains at the center of the long branches remains d) less than 3.0 mm whatever the section; or e) less than 1.5 mm for sections of width less than equal to 50mm, or f) less than e / 4 mm (where e is the thickness) for sections of width less than or equal to 10 mm.

15. A structure element for aircraft construction, characterized in that it is made from at least one product according to any one of claims 1 to 14.

16. Structural element according to claim 15, characterized in that it is a wing stiffener obtained by spinning.

17. A structure element according to claim 15, characterized in that it is a fuselage frame.

18. Spun tube according to any one of claims 1 to 7 or 9 to 14, characterized in that it contains between 0.15 and 0.60 of scandium and / or up to 0.50% of hafi ium.

19. Use of a tube according to claim 18 for the manufacture of frames, forks or handlebars of cycles, or baseball bats.

20. A method of manufacturing a spun, forged or rolled product comprising the following steps: (a) production of an alloy of composition according to one of claims 1 to 9, (b) casting of a raw form such as rolling plate or a spinning or forging billet, (c) homogenization of said raw form, (d) hot transformation to obtain a first intermediate product, (e) solution of said first intermediate product, (f) hardening (g) possibly controlled traction, (h) tempering.

21. The method of claim 20, characterized in that the homogenization (step a)) is carried out in two stages, with a first level between 452 and 473 ° C, preferably between 457 and 473 ° C, and a second level between 465 and 484 ° C, and preferably between 467 and 481 ° C.

22. The method of claim 20, characterized in that the homogenization (step a)) is carried out in a single step, with a rise in temperature below 200 ° C / h, and preferably between 20 and 50 ° C / h up to a plateau between 465 and 484 ° C, and preferably between 471 and 481 ° C.

23. Method according to one of claims 20 to 22, characterized in that the hot transformation is done by spinning with a slug temperature between 400 and 460 ° C, and preferably between 420 ° C and 440 ° C.

24. Method according to any one of claims 20 to 23, characterized in that the solution temperature does not exceed 500 ° C, and preferably does not exceed 485 ° C.

25. The method of claim 24, characterized in that the dissolution ends with a level between 470 and 485 ° C, preferably between 475 and 484 ° C, and even more preferably between 477 and 483 ° C for a duration between 1 and 10 hours.

26. Method according to any one of claims 20 to 25, characterized in that the controlled traction leads to a permanent elongation of between 1 and 5%, and preferably between 1.5 and 3%.

27. Method according to any one of claims 20 to 26, characterized in that the tempering treatment comprises a) a first stage at a temperature between 110 ° C and 130 ° C, and preferably between 115 - 125 ° C, and preferably, in the latter case, for a period of between 2 and 10 hours, and even more preferably between 5 and 7 hours; b) a second level at a temperature between 150 ° C and 170 ° C, and preferably between 155 and 165 ° C, and even more preferably between 157 and 163 ° C, and preferably for a duration between 4 and 12 hours, and even more preferably between 6 and 10 hours.