EP1766102B1 - Method for making high-tenacity and high-fatigue strength aluminium alloy products - Google Patents

Abstract

The invention relates to a process for fabrication of worked products made of an aluminium alloy of the Al-Cu, Al-Cu-Mg or Al-Zn-Cu-Mg type with high toughness and resistance to fatigue, which comprises between 0.005 and 0.1% of barium. Such an alloy has better toughness than a similar product without barium.

Description

Field of the invention

The invention relates to a new manufacturing process for rolled, spun or forged products made of high tenacity and high fatigue strength aluminum alloy, especially Al-Zn-Cu-Mg type alloy, and products obtained by this method, including structural elements made from such products and intended for the construction of aircraft. It is based on the introduction of barium in an aluminum-based liquid alloy.

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. On the other hand, certain properties such as fatigue strength, corrosion resistance, formability and elongation at break are complex and often unpredictable to properties (or mechanical characteristics. The optimization of all the properties of a material for mechanical engineering, for example in the aeronautical sector, therefore very often involves a compromise between several key parameters.

For example, in large capacity civil aircraft, type 7xxx alloys are typically used for the wing structure elements. These elements must have high mechanical strength, good toughness and good resistance to fatigue. Any new opportunity to improve one of these property groups without degrading others would be welcome.

With respect to toughness, it is well known that to improve the toughness of structurally hardened aluminum alloys, the residual iron and silicon content must be decreased; this is called in the profession the "seventh golden rule of Staley" ( JT Staley, "Microstructure and Toughness of Aluminum High-Strength Alloys" Properties Related to Toughness Fracture, ASTM STP 605, American Society for Testing and Materials, 1976, p. 71-103 ). This effect is observed in virtually all structurally hardened aluminum alloys, regardless of their toughness level. Iron and silicon are natural impurities of aluminum. Apart from the specific purification processes used for the production of high purity aluminum (eg the segregation process), there is no industrial process for reducing the content of iron and silicon in a liquid aluminum bath. In addition, these elements tend to accumulate during the recycling of aluminum and these alloys. All that can be done to reduce their content is to dilute these impurities with more pure metal, either with electrolysis metal (called "primary aluminum"), whose iron + silicon content is usually around from 0.2 to 0.3%, or with refined metal. In both cases, and especially in the second, the operation entails significant additional costs.

With regard to fatigue resistance, the effect of iron and silicon impurities is also detrimental. A decrease in the residual content of iron and silicon will normally lead to an improvement of the fatigue resistance, if otherwise the usual precautions are taken during the development of the liquid metal and during casting to avoid the formation of inclusions and the incorporation of hydrogen into the metal.

It is well known that the iron and silicon elements form with the aluminum virtually insoluble intermetallic phases, such as Al ₇ Cu ₂ Fe, Al ₆ (Fe _x Mn _1-x ) (with 0 <x <1), Al ₁₂ Fe ₃ Si, Al ₉ Fe ₂ Si ₂ , Mg ₂ Si. When large, these phases are more harmful than when they are small. The possibilities of acting on their size during casting through the physical parameters (especially the rate of solidification) are unfortunately quite limited.

Faced with the difficulty of reducing the intermetallic phases to iron and silicon and modifying their size and morphology by means of physical treatments, it was imagined to modify their size and morphology by adding certain chemical elements. Such an effect, if it is found, will be industrially exploitable only if it does not induce negative effects on other properties of the finished product. Thus, in some Al-Si molding alloys, Na and / or Sr are added to obtain finely fibrous Si phases instead of coarse prismatic. The patent FR 1 507 664 (Metallgesellschaft Aktiengesellschaft) notes that in Al-Si casting alloys with Si content between 5 and 14%, the addition of strontium and / or barium (Ba) at a rate of 0.001 to 2% leads to obtaining a fine eutectic structure; this effect is reinforced by the simultaneous addition of beryllium (Be). The patent EP 1 230 409 B1 (RUAG Components) teaches that the addition of barium (between 0.1 and 0.8%) to aluminum alloys with a silicon content of at least 5% improves their ability to form thixotropically. For structural hardening alloys, the patent US 4,711,762 (Aluminum Company of America) proposes the addition of strontium (Sr), antimony (Sb) and / or calcium (Ca) to an Al-Zn-Cu-Mg type alloy to reduce the size of Al ₇ Cu ₂ Fe phases. , Al ₂ CuMg and Mg ₂ Si.

Aluminum alloys containing barium have been described in other prior art documents. In most cases, its function is to make fluid foundry dross (in English "flux and dross"); on the other hand; its influence on the properties of the product is not described. Thus, the patent GB 505 728 (L'Electrique) mentions an aluminum-based alloy for the production of drawn wire containing Zn 5 - 6.5%, Mg 2 - 3.5%, Si 0.15 - 0.5%, Mn 0 , 25 - 1%, Mo 0.20 - 0.60%, Co 0.20 - 0.60%, K 0 - 0.12%, Ba 0 - 0.25%, Sb 0 - 0.1%, W 0 - 0.50%, Ni 0 - 1%, Ti 0 - 0.40%, in which the barium is introduced in the form of chloride to smooth the dirt; this barium content in the resulting metal product would also have a hardening effect.

The patent GB 596.178 (Tennyson Fraser Bradbury) describes the addition of the Na, K, Ba and / or P elements at a maximum total content of 0.15% to an aluminum alloy containing Cu 5.00 - 9.50 %, Zr, Ni, Ce 0.05 - 1.00 in total, Si 0.02 - 0.40%, Fe 0.02 - 0.50%, Zn 0.00 - 0.25%. It is a casting alloy for pistons. Neither the function nor the mode of introduction of barium are specified.

The patent US 4,631,172 (Nadagawa Corrosion Protection Co.) discloses an aluminum alloy used as a sacrificial anode containing 3.2% Zn, 1.5% magnesium, 0.02% indium, 0.01% d tin and / or calcium and barium, the latter in a content of between 0.002% and 1.0%. Another composition contains Zn 2.5%, Mg 2.5%, In 0.02%, Ca and / or Ba 0.005 - 1.0%, Si 0.004 - 1.0%. The addition of calcium and / or barium increases the current density and ensures uniform wear of the sacrificial anode. The patent application JP 61 096052 A discloses an aluminum alloy sacrificial anode of composition Zn 1 - 10%, Mg 0.1 - 6%, In 0.01 - 0.04%, Sn 0.005 - 0.15%, Si 0.09 - 1%, Ca and / or Ba 0.005 - 0.45%.

CH 328 148 (Wilhelm Neu) discloses the introduction of a barium hydride in a zinc-aluminum alloy with at least 40% zinc.

The patent US 3,310,389 (High Duty Alloys Ltd) mentions the presence of barium, calcium and / or strontium in a total content of up to 0.2% in an aluminum-based alloy containing Cu 2.2 - 2.7%, Mg 1, 3 - 1.7%, Si 0.12 - 0.25%, Fe 0.9 - 1.2%, Ni 0.9 - 1.4%, Ti 0.02 - 0.15%.

The patent RU 2 184 167 (Inventor: IN Fridljander et al) discloses an aluminum-based alloy for structural application in aeronautical construction of composition Cu 3.0 - 3.8%, Li 1.4 - 1.7%, Zr 0.0001 - 0 , 04%, Sc 0.16 - 0.35%, Fe 0.01 - 0.5%, Mg 0.01 - 0.7, Mn 0.05 - 0.5%, Ba 0.001 - 0.2% , Ga 0.001 - 0.08%, Sb 0.00001 - 0.001%.

The patent SU 1,678,080 (Institut khimii im.VI Nikitina) discloses an aluminum-based alloy of composition Cu 5.0 - 5.5%, Cr 0.1 - 0.4%, Mn 0.2 - 0.6%, Zr 0 , 1 - 0.4%, Ti 0.1 - 0.4%, Cd 0.05 - 0.25%, Sr or Ba 0.01 - 0.1%.

Most of these alloys are found to contain unusual elements such as indium, nickel, lithium, cadmium, molybdenum or tungsten, and are therefore, compared to the alloys usually used in aircraft construction, exotic alloys, and this without taking into account the possible addition of barium.

It is an object of the present invention to provide a novel process for modifying the morphology of iron and silicon insoluble phases in Al-Cu-Mg or Al-Zn Cu-Mg structural hardening aluminum alloys, and thus to obtain new products with high mechanical strength which also show excellent toughness and resistance to fatigue.

Object of the invention

The subject of the invention is a process defined in claim 1 for manufacturing wrought products made of aluminum alloy of Al-Cu-Mg or Al-Zn-Cu-Mg type with high tenacity and fatigue resistance, including casting. of a raw form (such as a spinning billet, forge billet or a rolling plate) and the hot deformation of said raw form, said method being characterized in that 0.005 is introduced into said alloy, and 0.1% barium.

The invention also relates to a structural element for aircraft construction, manufactured from a rolled product, spun or forged alloy Al-Cu-Mg or Al-Zn-Cu-Mg which contains between 0.005 and 0 , 1% of barium as defined in claims 8, 9 and 10. Such a product or structural element, obtainable by the method according to the present invention, can be used advantageously in applications that require high tenacity and / or a high resistance to fatigue, such as extrados or intrados (wing skin) wing elements, stiffeners, longitudinal members, or ribs, or bulkhead elements (bulkheads).

Description of figures

• The figure 1 shows the morphology of the Al-Fe-Cu phases in the casting state after selective dissolution of the matrix in a 7449 alloy (scanning electron microscopy micrographs with a field effect gun (FEG-SEM):
  • Alloy 7449 according to the state of the art (magnification: see the bar corresponding to 3 μm at the bottom left of the legend). Sample P4068 # 66.
• The figure 2 shows the morphology of Al-Fe-Cu phases:
  • Alloy 7449 with addition of barium according to the invention (magnification: see the bar corresponding to 10 microns in base left of the legend). Sample P4078-1 # 37.
• The figure 3 shows the morphology of the Al-Fe-Cu phases in a sample that shows both morphologies:
  • Alloy 7449 (with added barium) with coexistence within the same structure of an unmodified form ("without Ba", on the left) and a modified form ("with Ba", on the right) of the AlFeCu phase ( If) (magnification: see the bar corresponding to 10 μm in the left base of the legend).
• The figures 4 and 5 show the morphology of Al-Fe-Cu type phases in a 7449 alloy with added barium. We notice the morphology "in the shape of sea urchins" ( figure 4 ) and "broccoli-shaped" ( figure 5 ) eutectic compounds.
  • Alloy 7449 (with added barium) according to the invention (magnification: see the bar at the bottom left of the figure 4 which represents 1 μm). Sample P4078-1 # 37.
• The figure 6 shows the morphology of platelet-type Al-Fe-Cu phases in a 7449 alloy according to the state of the art. Sample P4013-1- # 66.
• The figure 7 gives a comparison of the toughness K _app measured on a CCT test piece of width 406 mm and thickness 6.35 mm (sampled at a quarter thickness) as a function of R _{0.2 (L)} . Alloy 7449. It is noted that the products according to the invention ("Ba") have a better toughness than the products according to the state of the art ("ref").

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. 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 stated, the static mechanical characteristics, ie the breaking strength R _m , the yield _stress R _p0,2 , and the elongation at break A, are determined by a tensile test according to EN 10002-1 standard, 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, the fatigue crack propagation rate (so-called da / dn test) according to ASTM E 647, and the critical stress intensity factor Kc, K _CO or K _App according to ASTM E 561. The term "spun product" includes so-called "stretched" products, that is 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 "wrought product" a product having undergone a deformation operation after its solidification, this deformation operation may be, without this list being exhaustive, rolling, forging, spinning, drawing and drawing.

Here is called "structural element" or "structural element" of a mechanical construction a mechanical part whose failure is likely to endanger the security of the 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 gives structure elements of larger size and in one piece, without assembly or with a reduced number of assembly points compared to an assembled structure in which sheets, thin or strong depending on the destination of the element of structure (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 is applicable to Al-Cu-Mg or Al-Zn-Cu-Mg structural hardening aluminum based alloys.

The invention applies to alloys of the Al-Cu-Mg type comprising between 3.5 and 5.5% of Cu and between 1 and 2% of Mg, it being understood that the content of iron and silicon must not exceed 0.30% for each of these elements. These alloys may contain other alloying elements and impurities up to about 3% in total. Among these elements are manganese, lithium, zinc. Moreover, and still by way of example, the alloy may also contain the usual additions of zirconium, titanium or chromium. The process according to the invention can advantageously be applied to alloys conventionally used in aeronautical construction: 2024, 2024A, 2056, 2124, 2224, 2324, 2424, 2524 and their variants. On the other hand, the so-called free-cutting alloys which include additions of Pb, Bi or Sb in order to obtain easy splitting chips such as 2004, 2005 and 2030 are excluded here.

The alloys of Al-Zn-Cu-Mg type to which the present invention is applied are the alloys comprising between 4 and 14% of zinc, and more particularly between 7 and 10.5% of zinc, between 1 and 3% of Cu, and more particularly between 1.4 and 2.5% of Cu, and between 1 and 3% of Mg, and more particularly between 1.7 and 2.8% of Mg, it being understood that the content of iron and silicon must not exceed 0.30% for each of these elements. These alloys can contain other alloying elements and impurities up to 2% in total. Among these elements is manganese. Moreover, and still by way of example, the alloy may also contain the usual additions of zirconium, titanium or chromium. The process according to the invention can advantageously be applied to alloys of the 7xxx series, in particular to those conventionally used in aeronautical construction: 7010, 7050, 7055, 7056, 7150, 7040, 7075, 7175, 7475, 7049, 7149, 7249 , 7349, 7449 and their variants.

The method according to the invention comprises casting a raw form, such as a rolling plate, a spinning billet or a forge billet, by any known method. This raw form is then deformed hot, for example by rolling, spinning or forging. The invention is not applicable to products produced by fast solidification, ie with a solidification rate typically greater than 600 ° C / min, which lead to a significantly different microstructure. The process may comprise other stages of thermal or mechanical treatment, the more often homogenization, cold deformation, dissolution, artificial or natural aging, intermediate or final annealing.

The applicant has surprisingly found that the presence of a very small amount of barium partially neutralizes the detrimental effect of iron and silicon for certain properties, as will be explained below. This results in a morphological modification of the intermetallic phases, and in particular that of the intermetallic iron phases (Al-Cu-Fe type). The intermetallic eutectic phases are fragmented ("sea urchin" or "broccoli" morphology, see figure 2 ) whereas without barium, they have a more extensive form (morphology of "petals", "platelets" or "cabbage leaves", see figure 1 ). These eutectic phases may be of Al-Fe-Cu type (in alloys with addition of barium) or Al-Fe-Si-Cu (in alloys without addition of barium). It is observed that in the presence of barium, silicon seems to disappear precipitates.

The properties of the product which are improved by the process according to the invention are in particular the tenacity, the fatigue strength, and the resistance to the propagation of cracks da / dn with a high stress intensity factor ΔK. This effect is particularly pronounced in a non-recrystallized structure.

In a first embodiment, an alloy of barium with silicon is added. An alloy of Si (70%) - Ba (30%) is suitable; this product is commercially available. The silicon content of the alloy can be between 50% and 90%. Other alloys of the same type containing in addition iron up to a content of 20% are also applicable to the invention, the silicon content can then vary between 30% and 90% and the barium content can then vary between 10 and 40%.

In a second embodiment, barium is added in metallic form or, preferably, in the form of an intermetallic compound or alloy with one or more of the constituents of the aluminum alloy in question. For example, an Al-Ba or Zn-Ba type alloy is suitable. These intermetallic compounds or alloys can be obtained directly by reducing the barium oxide BaO with aluminum or zinc according to known methods.

In both embodiments, the amounts of barium used are very small, that is to say less than 0.1% and even more preferably less than 0.05%. A value between 0.005% and 0.03% may be suitable. When introducing a Ba - Si alloy, account must be taken of the relatively low solubility of this alloy in liquid aluminum. The second embodiment is particularly advantageous when it is applied to an aluminum alloy which has a relatively high silicon content, for example of the order of 0.10%. In contrast, the metal barium is expensive. The first embodiment uses a less expensive barium alloy, but leads to the increase of the silicon content and possibly iron in the aluminum alloy. However, it is surprising to note that this increase in the silicon content and possibly iron does not degrade the toughness or the resistance to fatigue. This is due to the fact that silicon and possibly iron are not incorporated in the same way: the morphology of the phases is significantly modified.

The method according to the invention makes it possible to manufacture a semi-finished product or an Al-Zn-Mg-Cu alloy structural element which comprises between 7 and 10.5% of zinc, between 1.4 and 2.5% of copper, and between 1.7 and 2.8% of magnesium, such as an alloy 7049, 7149, 7249, 7349 or 7449, with a tenacity K _{app (LT)} , measured according to the standard ASTM E 561 on a specimen of type CCT with W = 406 mm and B = 6.35 mm taken at mid-thickness, greater than 86 MPa√m. Such a semi- _finished product or structural element has a yield strength R _{p0.2 (L)} greater than 600 MPa.

The Applicant has also observed that the product according to the invention is more resistant to exfoliation corrosion (EXCO test), determined on test pieces taken at mid-thickness, than a corresponding product without barium. The resistance to stress corrosion is also slightly improved.

Due to its remarkable mechanical properties, the product according to the invention can have many possible uses, and it is particularly advantageous to use said product as a structural element in aircraft construction, and especially as an extrados wing element, as part of lower sails, as a sail-skin element, as a stiffener, as a spar, as a rib or as an element for bulkheads.

The method according to the invention has several advantages. The mode of introduction of barium according to the invention avoids the use of hydrides, which increase the residual hydrogen content, may cause pores in the solidified metal. Barium neutralizes the detrimental effect of residual silicon in aluminum-based structural hardening alloys, resulting in improved toughness, including K _IC and K _app . Barium also improves resistance to corrosion, including exfoliating corrosion.

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

Example 1

In this test, the possibility of introducing barium in an aluminum-based liquid alloy by the addition of an Si-Ba type alloy, and casting an Al-Zn-C-type alloy, was investigated. Mg containing barium in the form of industrial sized rolling plates. Two Al-Zn-Cu-Mg aluminum lamination plates were cast under similar conditions, one with the addition of barium in the form of a parent alloy containing about 28% Ba and 72% Si (added to a liquid metal temperature of about 750 ° C), one without adding barium. The liquid metal was treated with an Ar + Cl ₂ mixture. The casting temperature was 665 ° C, casting speed about 65 mm / min. The metal was refined with 0.8 kg of AT5B. The section of the plates was of the order of 2150 x 450 mm. The chemical composition, determined on a solid pion obtained from liquid metal taken from the casting channel, is shown in Table 1. Table 1: Chemical Composition Sample Fe Yes Cu mg Zn Zr Ti Ba P4068 # 66 0.03 0.05 1.76 1.90 7.48 0.11 0.0230 - P4069-2 # 66 0.11 0.12 1.86 2.03 8.40 0.10 0.0200 0.0100

Part of the barium introduced (a few tens of percent compared to the amount used) was found in the dross.

Example 2

An aluminum alloy of type 7449 with the addition of an alloy containing about 52% silicon and 30% barium and 18% iron was developed (reference P4078-1 # 37). Its chemical composition, determined on a solid pion obtained from liquid metal taken from the casting channel, is shown in Table 2.

The alloy was refined with 0.8 kg / t AT5B and cast at 685 ° C with a speed of 65 mm / min in rolling plates. After cooling and scalping, the plates were homogenized at 463 ° C and hot rolled at 420 to 410 ° C. The sheets obtained were dissolved for 6 hours at 120 ° C. and then for 17 hours at 150 ° C. The end product was thus metallurgical T351.

Due to the addition of the Si-Ba alloy, the silicon content of the type 7449 aluminum alloy increases from 0.04% to 0.09% and that of Fe increases from 0.03% to 0.06%

A standard, barium-free 7449 alloy (P4013-1- # 66) was also similarly developed. Its chemical composition, determined on a solid pion obtained from liquid metal taken from the casting channel, is shown in Table 2. Table 2: Chemical Composition Sample Fe Yes Cu mg Zn Zr Ti Ba P4078-1 # 37 (FE02-029 (888887)) 0.06 0.09 1.84 1.94 8.72 0.12 0,019 0,023 P4013-1- # 66 (FE02-028 (888885)) 0.03 0.04 1.83 2.20 8.29 0.12 - -

The microstructure of the sample with added barium shows eutectic compounds "in the shape of sea urchins" ( figure 4 ) or "broccoli-shaped" (see figure 5 ). The microstructure of the barium-free sample added reveals eutectic compounds in the form of platelets ( figure 6 ).

The static mechanical characteristics were measured in the T79 state on a sheet of thickness 40 mm. Toughness K _{app (LT)} was measured on a CCT test specimen with W 406 and B = 6.35 mm Table 3: Mechanical characteristics P4013-1- # 66 P4078-1 # 37 (with barium) (without barium) ¼ ep ½ ep ¼ ep ½ ep Unit R _{p0.2 (L)} 595 622 609 622 MPa R _{p0.2 (LT)} 590 601 611 608 MPa Rm (L) 610 643 628 647 MPa R _{m (LT)} 609 617 636 631 MPa R _{p0,2 (TC)} 573 575 MPa R _{m (TC)} 622 626 MPa A% (L) 11.6 10.4 10.3 9.7 % A% (TL) 10.7 9.9 8.7 8.6 % A% (TC) 4.4 5.7 % K _{IC (TL)} 22.3 20.9 MPa m K _{IC (LT)} 23.3 23 MPa m K _{app (LT)} 69.3 91.6 54.3 83.9 MPa m K _{eff (LT)} 73.6 98.8 58.6 90.3 MPa m

Exfoliation corrosion resistance (EXCO) results obtained on mid-thickness specimens show that the 7449 barium alloy (EXCO: EA performance) is more resistant to exfoliating corrosion than the barium-free reference product (performance EXCO: EB). The resistance to stress corrosion is also slightly improved.

Claims (11)

1. Process for fabrication of worked products made of an aluminium based alloy of the Al-Cu-Mg or Al-Zn-Cu-Mg type with high toughness and resistance to fatigue, comprising:

   (a) production of a liquid aluminium based alloy comprising between 0.005 and 0.1% of barium, said barium being added (aa) in metallic form, or (ab) in the form of an intermetallic compound or of an alloy with one or several constituents of the targeted aluminium alloy or with silicon and/or iron;

   (b) casting of said liquid alloy in the form of an unwrought product (such as an extrusion billet, a forging billet or a rolling slab),

   (c) hot working of said unwrought product,

   wherein said liquid aluminium based alloy comprises between 3.5 and 5.5% copper, between 1 and 2% magnesium, an iron content lower than 0.30%, a silicon content lower than 0.30% and other alloying elements and impurities up to 3% total if it is a Al-Cu-Mg type alloy and between 4 and 14 % zinc, between 1 and 3 % copper, between 1 and 3 % magnesium, an iron content lower than 0.30%, a silicon content lower than 0.30% and other alloying elements and impurities up to 2% total if it is a Al-Cu-Mg type alloy.
2. Process according to claim 1, wherein the barium content of said worked product is between 0.005 and 0.03%.
3. Process according to claim 1 or 2, wherein barium is added in the form of an intermetallic compound or of an alloy with aluminium or zinc.
4. Process according to claim 1 or 2, wherein the barium is added in the form of an Si (70%) - Ba (30%) type alloy.
5. Process for fabrication of worked products made of an aluminium based alloy of Al-Zn-Cu-Mg type according to any one of claims 1 to 4, wherein said aluminium-based liquid alloy comprises between 7 and 10.5% of zinc, between 1.4 and 2.5% of copper and between 1.7 and 2.8% of magnesium.
6. Process for fabrication of worked products made of an aluminium based alloy of Al-Zn-Cu-Mg type according to any one of claims 1 to 5, wherein the aluminium-based liquid alloy to which barium is added is selected in the group consisting of 7010, 7050, 7055, 7056, 7150, 7040, 7075, 7175, 7475, 7049, 7149, 7249, 7349 and 7449 alloys.
7. Process for fabrication of worked products made of an aluminium based alloy of Al-Cu-Mg type according to any one of claims 1 to 4, wherein said aluminium-based liquid alloy to which barium is added is selected in the group consisting of 2024, 2024A, 2124, 2224, 2324, 2424, 2524 alloys.
8. Worked product obtainable with the process according to any one of claims 1 to 7.
9. Worked product made of an aluminium based alloy of the Al-Zn-Cu-Mg type obtainable with the process according to any one of claims 1 to 5, wherein the toughness K_app(L-T) measured according to standard ASTM E 561 on a CCT type test piece taken from mid-thickness and with W=406 mm and B = 6.35 mm, is greater than 86 MPa√m.
10. Worked product obtainable with the process according to any one of claims 5 or 6, wherein the tensile yield stress Rp0.2 (L) is greater than 600 MPa.
11. Use of a worked product according to any one of claims 8 to 10 as a structural element in aeronautical construction, and particularly as a wing upper or lower skin element, as a wing skin element, stiffener, stringer or rib, or element for bulkheads.

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