US20070243097A1

US20070243097A1 - Process for Fabrication of Products Made of an Aluminium Alloy With High Toughness and High Fatigue Resistance

Info

Publication number: US20070243097A1
Application number: US11/571,189
Authority: US
Inventors: Emmanuelle Sarrazin; Jarry Philippe
Original assignee: Alcan Rhenalu SAS
Current assignee: Constellium Issoire SAS
Priority date: 2004-06-25
Filing date: 2005-06-22
Publication date: 2007-10-18
Also published as: ATE417136T1; CA2570618A1; BRPI0512590A; FR2872172B1; EP1766102A1; FR2872172A1; DE602005011619D1; CN100564571C; EP1766102B1; CN1977063A; WO2006010817A1

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

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 National Stage Application of International Application No. PCT/FR05/001572 filed Jun. 22, 2005, which claims priority to French Application No. 0406957 filed Jun. 25, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a new fabrication process for rolled, extruded, or forged products made of an aluminium alloy with high toughness and high fatigue resistance, particularly an Al—Zn—Cu—Mg type alloy, and products obtained using this process, and particularly structural elements made from such products intended for aircraft construction. It is based on the introduction of barium into an aluminium based liquid alloy.
2. Description of Related Art
It is known that the various required properties cannot all be optimised at the same time and independently of each other during the fabrication of partly finished products and structural elements for aeronautical construction. When the chemical composition of the alloy or the parameters of product manufacturing processes are modified, several critical properties may tend to change in opposing directions. This is sometimes the case firstly for properties referred to collectively as “static mechanical strength” (particularly the ultimate tensile stress R_mand the tensile yield stress R_p0.2), and secondly for properties referred to collectively as “damage tolerance” (particularly the toughness and the resistance to crack propagation. Moreover, some working properties such as resistance to fatigue, resistance to corrosion, formability and elongation at failure are related to the mechanical properties (or “characteristics”) in a complex and often unpredictable manner. Therefore, optimisation of all properties of a material for mechanical construction, for example in the aeronautical sector, often requires a compromise between several key parameters.
For example, type 7xxx alloys are typically used for wing structural elements for high capacity civil aircraft. These elements must have high mechanical strength, good toughness and good resistance to fatigue. Any new means of improving one of these groups of properties without degrading the others would be very useful.
Concerning the toughness, it is well known that to increase the toughness of structurally hardened aluminium alloys, the residual content of iron and silicon have to be reduced; this is called “Staley's seventh golden rule” in the business (J. T. Staley, “Microstructure and Toughness of High-Strength Aluminum Alloys” Properties Related to Fracture Toughness, ASTM STP 605, American Society for Testing and Materials, 1976, p. 71-103). In practice, this effect is observed in almost all structurally hardened aluminium alloys, regardless of their degree of toughness. Iron and silicon are natural impurities of aluminium. Apart from specific purification processes used for the production of high purity aluminium (for example the segregation process), there is no standard industrial process for reducing the iron and silicon content in a bath of liquid aluminium. Furthermore, these elements tend to accumulate when aluminium and these alloys are recycled. All that can be done to reduce the content of these impurities is to dilute them with purer metal, either using electrolysis metal (called “primary aluminium”), for which the iron+silicon content is typically about 0.2 to 0.3%, or using refined metal. The operation significantly increases the cost in both cases, and particularly in the second case.
Iron and silicon impurities also have a negative effect on the resistance to fatigue. A drop in the residual content of iron and silicon will normally cause an increase in the resistance to fatigue, provided that normal precautions are taken during production 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 practically insoluble inter-metallic phases with aluminium, such as Al₇Cu₂Fe, Al₆(Fe_xMn_1-x) (where 0<x<1), Al₁₂Fe₃Si, Al₉Fe₂Si₂and Mg₂Si. These phases are more harmful when they are large than when they are small. Unfortunately, there are few means of acting upon their size by varying physical parameters during casting (particularly the solidification rate).
Faced with the difficulty of reducing inter-metallic phases with iron and silicon and modifying their size and morphology by means of physical treatments, it was thought that it might be possible to modify their size and morphology by adding specific chemical elements. If such an effect is observed, it will only be useable industrially if it does not have any negative effects on other properties of the finished product. Thus, Na and/or Sr are added to some Al—Si type casting alloys to obtain finely formed fibrous Si phases instead of coarsely formed prismatic phases. Patent FR 1 507 664 (Metallgesellschaft Aktiengesellschaft) states that the addition of 0.001 to 2% of strontium and/or barium (Ba) into Al—Si type casting alloys with an Si content of between 5 and 14% leads to a fine eutectic structure; this effect is reinforced by the simultaneous addition of beryllium (Be). Patent EP 1 230 409 B1 (RUAG Components) discloses that the addition of barium (between 0.1 and 0.8%) to aluminium alloys with a silicon content of at least 5% improves their thixotropic formability. For work hardening alloys with structural hardening, patent U.S. Pat. No. 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 the Al₇Cu₂Fe, Al₂CuMg and Mg₂Si phases.
Aluminium based alloys containing barium have been described in other state of the art documents. In most cases, its function is to make the flux and dross more fluid; on the other hand, its influence on the properties of the product is not described. Thus, patent GB 505 728 (L'Eléctrique) describes an aluminium based alloy intended for the manufacture of drawn wire and 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 barium is added in chloride form so as to make the flux and dross more fluid; this barium content in the metallic product would also have a hardening effect.
Patent GB 596,178 (Tennyson Fraser Bradbury) describes the addition of the Na, K, Ba and/or P elements with a maximum total content of 0.15% to an aluminium based alloy containing Cu 5.00-9.50%, Zr, Ni, Ce 0.05-1.00 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 method of adding barium are mentioned.
U.S. Pat. No. 4,631,172 (Nadagawa Corrosion Protection Co.) describes an aluminium based alloy used as a sacrificial anode containing 3.2% of Zn, 1.5% of magnesium, 0.02% of indium, 0.01% of tin and/or calcium and barium, the barium content varying 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 assures uniform wear of the sacrificial anode. Patent application JP 61 096052 A describes an aluminium-based alloy sacrificial anode with 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%.
Patent CH 328 148 (Wilhelm Neu) describes the introduction of a barium hydride into a zinc-aluminium type alloy containing not less than 40% of zinc.
U.S. Pat. No. 3,310,389 (High Duty Alloys Ltd) mentions the presence of barium, calcium and/or strontium with a total content of up to 0.2% in an aluminium-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%.
Patent RU 2 184 167 (inventor I. N. Fridljander et al) describes an aluminium-based alloy for structural applications in aeronautical construction with 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%.
Patent RU 1 678 080 (Institut khimii im. V. I. Nikitina) describes an aluminium-based alloy with 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%.
It is found that most of these alloys contain unusual elements such as indium, nickel, lithium, cadmium, molybdenum or tungsten, and therefore that they are exotic alloys compared with alloys usually used in aeronautical construction, without taking account of the possible addition of barium.

SUMMARY OF THE INVENTION

The purpose of this invention is to propose a new process to modify the morphology of insoluble iron and silicon phases in work hardening alloys with structural hardening of the Al—Cu—Mg or Al—Zn—Cu—Mg type, and thus obtaining new products with a high mechanical strength that also have excellent toughness and resistance to fatigue.
The purpose of the invention is 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, including casting of a unwrought product (such as a extrusion billet, a forging billet or a rolling slab) and hot deformation of said unwrought product, said process being characterised in that between 0.005 and 0.1% of barium is added into said alloy.
Another purpose of the invention is a structural element for aeronautical construction, made from a rolled, extruded or forged product made of an Al—Cu, Al—Cu—Mg or Al—Zn—Cu—Mg type alloy that contains between 0.005 and 0.1% of barium. Such a product or structural element obtainable by the process according to this invention, can advantageously be used in applications that require high toughness and/or resistance to fatigue, for example such as wing upper or lower surface elements (wing skin), stiffeners, stringers or ribs, or elements for sealed partitions (bulkheads).

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the morphology of Al—Fe—Cu type phases in the rough as-cast state after selective dissolution of the matrix in a 7449 alloy (micrographs obtained by a field effect gun scanning electron microscope (FEG-SEM):

- Alloy 7449 according to the state of the art (magnification: see the 3 μm bar at the bottom left of the legend). Sample P4068#66.

FIG. 2 shows the morphology of Al—Fe—Cu type phases:

- Alloy 7449 with added barium according to the invention (magnification: see the 10 μm bar at the bottom left of the legend). Sample P4078-1#37.

FIG. 3 shows the morphology of Al—Fe—Cu type phases in a sample that has both morphologies at the same time:

- Alloy 7449 (with added barium) with the coexistence of an unmodified form (“without Ba”, at the left) and a modified form (“with Ba”, at the right) of the AlFeCu phase (Si) in the same structure (magnification: see the 10 μm bar at the bottom left of the legend).

FIGS. 4 and 5 show the morphology of Al—Fe—Cu type phases in a 7449 type alloy with added barium. Note the “sea urchin shaped” morphology (FIG. 4) and “broccoli shaped” morphology (FIG. 5) of the eutectic compounds.
Alloy 7449 (with added barium) according to the invention (magnification: see the bar at the bottom left of FIG. 4 that represents 1 μm). Sample P4078-1#37.
FIG. 6 shows the morphology of Al—Fe—Cu type phases in the form of platelets in a 7449 alloy according to the state of the art. Sample P4013-1-#66.
FIG. 7 gives a comparison of the toughness Kapp measured on a 406 mm wide and 6.35 mm thick CCT type test piece (taken at one quarter of the thickness) as a function of R _p0.2(L)7449 alloy. Note that the products according to the invention (“Ba”) have better toughness than the products according to the state of the art (“ref”).

DESCRIPTION OF THE A PREFERRED EMBODIMENT

a) Definitions
Unless mentioned otherwise, all information about the chemical composition of alloys is expressed as a percent by mass. Consequently, the mathematical expression “0.4 Zn” means 0.4 times the zinc content, expressed as a percent by mass; the same is true mutatis mutandis for other chemical elements. The designation of alloys follows the rules of The Aluminum Association and is known to a person skilled in the art. Metallurgical states are defined in European standard EN 515. For example, the chemical composition of normalised aluminium alloys is defined in standard EN 573-3. Unless mentioned otherwise, the static mechanical characteristics, in other words, the ultimate tensile strength R_m, the tensile yield stress R_p0.2, and the elongation at failure A, are determined using a tensile test according to standard EN 10002-1, the location and direction from which test pieces are taken being defined in standard EN 485-1. The resistance to fatigue is determined by a test according to ASTM E 466, the fatigue crack propagation speed (so-called da/dn test) according to ASTM E 647, and the critical stress intensity factor K_C, K_COor K_appaccording to ASTM E 561. The term “extruded product” comprises so-called “drawn” products, in other words products fabricated by extrusion followed by drawing.
Unless mentioned otherwise, the definitions in European standard EN 12258-1 are applicable.
In this description, a “worked product” means a product on which a deformation operation has been carried out after its solidification, this deformation operation possibly being rolling, forging, extruding, drawing or stamping, although this list is not limitative.
In this description, a “structure element” or “structural element” of a mechanical construction means a mechanical part that, if it fails, would endanger said construction, its users, passengers or others. For an aircraft, these structural elements include particularly elements making up the fuselage (such as the fuselage skin), stringers, bulkheads, circumferential frames, wings (such as wing skin), stringers or stiffeners, ribs and spars and the tail fin composed particularly of horizontal and vertical stabilisers, and floor beams, seat tracks and doors.
In this description “integral structure” means the structure of a part of an aircraft that was designed to maximise continuity of the material over the largest possible dimension so as to minimise mechanical assembly points. An integral structure can be fabricated either by in-body machining or by the use of shaped parts obtained for example by extrusion, forging or casting, or by welding of structural elements made from weldable alloys. Thus, larger elements made of a single part can be obtained without assembly or with a smaller number of assembly points than for a structure in which thin or thick plates (depending on the destination of the structural element, for example fuselage element or wing element) are fixed to stiffeners and/or frames (that may be fabricated by machining from extruded or rolled products), usually by riveting.
b) Description of the Invention
This invention can be applied to all alloys based on structurally hardened work hardening aluminium of the Al—Cu, Al—Cu—Mg or Al—Zn—Cu—Mg type. More particularly, Al—Cu type alloys to which this invention could be applied are alloys containing between 1 and 7% of Cu, and more particularly between 3 and 5.5% of Cu. The invention can be applied to Al—Cu—Mg type alloys containing between 1 and 7% of Cu and between 0.2 and 2% of Mg, and more particularly 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% each. These alloys may contain other alloying elements and impurities up to about 3% in total. These elements include manganese, lithium, and zinc. Furthermore, and still as an example, the alloy may also contain normal additions of zirconium, titanium or chromium. The process according to the invention can advantageously be applied to Al—Mg—Cu type alloys or to alloys in the 2xxx series, particularly alloys conventionally used in aeronautical construction, namely 2024, 2024A, 2056, 2022, 2023, 2139, 2124, 2224, 2324, 2424, 2524 and their variants. On the other hand, this invention excludes so-called free-machining alloys such as 2004, 2005 and 2030 that include additions of Pb, Bi or Sb, so as to obtain discontinuous chips.
Alloys of the Al—Zn—Cu—Mg type to which this invention can be applied are alloys containing 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 iron and silicon contents shall not exceed 0.30% each. These alloys may contain other alloying elements and impurities, up to 2% in total. These elements include manganese. Furthermore, and still as an example, the alloy may also contain normal additions of zirconium, titanium and chromium. The process according to the invention can advantageously be applied to alloys in the 7xxx series, and particularly to alloys conventionally used in aeronautical construction, namely 7010, 7050, 7055, 7056, 7150, 7040, 7075, 7175, 7475, 7049, 7149, 7249, 7349 and 7449, and their variants.
The process according to the invention comprises casting of an unwrought product such as a rolling slab, an extrusion billet or a forging billet using any known process. This unwrought product is then hot worked, for example by rolling, extrusion or forging. The invention is not applicable to products produced by fast solidification, i.e. with a solidification rate typically greater than 600° C./min that result in a significantly different microstructure. The process may also include other heat treatment or mechanical treatment steps, usually homogenisation, cold working, dissolution, artificial or natural ageing, intermediate or final annealing.
The applicant surprisingly found that the presence of a very small quantity of barium partially neutralises the harmful effect of iron and silicon for some properties, as will be explained below. This causes a morphological modification to inter-metallic phases, and particularly to iron inter-metallic phases (of the Al—Cu—Fe type). Eutectic inter-metallic phases are fragmented (“sea urchin” or “broccoli”, morphology, see FIG. 2), whereas their shapes without barium would be more extensive (“petal”, “platelet” or “cabbage leaf” morphologies, see FIG. 1). These eutectic phases may be of the Al—Fe—Cu type (in alloys with added barium) or the Al—Fe—Si—Cu type (in alloys without added barium). It can be seen that silicon apparently disappears from precipitates in the presence of barium.
The principal properties of the product that are improved by the process according to the invention are particularly the toughness, resistance to fatigue, and resistance to crack propagation da/dn with a high stress intensity factor ΔK. This effect is particularly marked in an unrecristallised structure.
In a first embodiment, a barium and silicon alloy is added. An Si (70%)-Ba (30%) type alloy is suitable; this product is available on the market. The silicon content of the alloy may vary between 50% and 90%. Other alloys of the same type also containing up to 20% of iron can also be used within the invention, the silicon content of the alloy then possibly varying between 30% and 90% and the barium content then varying between 10 and 40%.
In a second embodiment, barium is added in metallic form, preferably in the form of an inter-metallic compound or an alloy with one or several constituents of the target aluminium alloy. For example, an Al—Ba or Zn—Ba type alloy is suitable. These inter-metallic compounds or alloys can be obtained directly by reduction of barium oxide BaO with aluminium or zinc using known processes.
In both embodiments, the barium quantities used are very low, preferably less than 0.1% and even more preferably less than 0.05%. A value between 0.005% and 0.03% might be suitable. When a Ba—Si alloy is added, the relatively low solubility of this alloy in liquid aluminium has to be allowed for. The second embodiment is particularly interesting when it is applied to an aluminium alloy that has a fairly high silicon content, for example of the order of 0.10%. On the other hand, metallic barium is expensive. The first embodiment uses a less expensive barium alloy but increases the silicon content and possibly the iron content in the aluminium alloy. However, it is surprising to realise that this increase in the content of silicon and possibly of iron does not deteriorate the toughness or the resistance to fatigue. This is related to the fact that silicon and possibly iron are not incorporated in the same way; the phase morphology is significantly modified.
The process according to the invention can be used to make a partly finished product or a structural element made of an Al—Zn—Mg—Cu type alloy that 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 a 7049, 7149, 7249, 7349 or 7449 alloy with toughness K_app(L-T), as 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, greater than 86 MPa√m. The yield stress R_p0.2(L)of such a partly finished product or structural element is greater than 600 MPa.
The applicant also observed that the product according to the invention has better resistance to exfoliation corrosion (EXCO test), determined on test pieces taken from the mid-thickness, than a corresponding product without barium. The resistance to stress corrosion is also slightly improved.
Due to its remarkable mechanical properties, there can be many possible applications for the product according to the invention, and it is particularly advantageous to use said product as a structural element in aeronautical construction, and particularly as an upper wing element or a lower wing element, such as a wing skin element, stiffener, stringer, rib or a bulkhead element.
The process according to the invention has several advantages. The method of adding barium according to the invention prevents the use of hydrides that would increase the residual hydrogen content that could cause pores in the solidified metal. Barium neutralises the harmful effect of residual silicon in aluminium-based alloys with structural hardening, which results in better toughness, particularly K_ICand K_app. Barium also improves the resistance to corrosion and particularly the resistance to exfoliation corrosion.
The following examples describe advantageous embodiments of the invention, for illustrative and non-limitative purposes.

EXAMPLE 1

This test explored the possibility of introducing barium into a liquid aluminium based alloy by the addition of an Si—Ba type alloy, and casting an Al—Zn—Cu—Mg type alloy containing barium in the form of industrially sized rolling slabs. Two rolling slabs made of Al—Zn—Cu—Mg type aluminium were cast under similar conditions, one with barium added in the form of a mother alloy containing about 28% of Ba and 72% of Si (added at a liquid metal temperature of about 750° C.), and one without any added barium. The liquid metal was treated with an Ar+Cl₂mix. The casting

TABLE 1

Chemical composition

Sample Fe Si Cu Mg Zn Zr Ti Ba

P4068#66 0.03 0.05 1.76 1.90 7.48 0.11 0.0230 —

P40692#66 0.11 0.12 1.86 2.03 8.40 0.10 0.0200 0.0100

temperature was 665° C., and the casting rate was about 65 mm/min. The metal was refined with 0.8 kg of AT5B. The cross-section of the slabs was of the order of 2150×450 mm. The chemical composition, determined on a solid slug obtained from liquid metal taken from the runner, is given in table 1.
Part of the barium added (a few tens of percent of the quantity used) was found in the dross.

EXAMPLE 2

A type 7449 aluminium alloy was produced with an added alloy containing about 52% of silicon and 30% of barium and 18% of iron (reference P4078-1#37). Table 2 shows its chemical composition determined on a solid slug obtained from liquid metal taken from the runner.
The alloy was refined with 0.8 kg/t of AT5B and cast into rolling slabs at 685° C. at a rate of 65 mm/min. After cooling and scalping, the slabs were homogenized at 463° C. and hot rolled at a temperature of between 420 and 410° C. The plates obtained were put into solution for 6 hours at 120° C. and then for 17 hours at 150° C. Consequently, the final product was in the T351 metallurgical temper.
Due to the addition of the Si—Ba alloy, the silicon content of the type 7449 type aluminium alloy increases from 0.04% to 0.09% and the content of Fe increases from 0.03% to 0.06%
Similarly, a standard 7449 alloy without any barium (P4013-1-#66) was also produced. Its chemical composition, determined on a solid slug obtained from liquid metal taken from the runner, is shown in table 2.

TABLEAU 2

Chemical composition

Sample Fe Si Cu Mg Zn Zr Ti Ba

P4078-1#37 0.06 0.09 1.84 1.94 8.72 0.12 0.019 0.023

(FE02-029 (888887))

P4013-1-#66 0.03 0.04 1.83 2.20 8.29 0.12 — —

(FE02-028 (888885))
The microstructure of the sample with added barium shows “sea urchin shaped” eutectic compounds (FIG. 4) or “broccoli shaped” eutectic compounds (see FIG. 5). The microstructure of the sample without any added barium comprises eutectic compounds in the form of platelets (FIG. 6).

The static mechanical characteristics were measured in the T79 temper on a 40 mm thick plate. The toughness K_app(L-T)was measured on a CCT type test piece with W 406 and B=6.35 mm

TABLE 3


Mechanical characteristics

	P4013-1-#66		P4078-1#37
	(with barium)		(w/o barium)

	¼ t	½ t	¼ t	½ t t	Unit

	R_{p0.2 (L)}	595	622	609	622	MPa
	R
_{p0.2 (LT)}	590	601	611	608	MPa
	R
_m(L)	610	643	628	647	MPa
	R_m(LT)	609	617	636	631	MPa
	R_{p0.2 (ST)}		573		575	MPa
	R_{m (ST)}		622		626	MPa
	A % (L)	11.6	10.4	10.3	9.7	%
	A % (LT)	10.7	9.9	8.7	8.6	%
	A % (ST)		4.4		5.7	%
	K_{1C (T-L)}		22.3		20.9	MPa√m
	K_{1C (L-T)}		23.3		23	MPa√m
	K_{app (L-T)}	69.3	91.6	54.3	83.9	MPa√m
	K_{eff (L-T)}	73.6	98.8	58.6	90.3	MPa√m

The resistance to exfoliation corrosion results (EXCO) determined on test pieces taken at mid-thickness show that the 7449 alloy with barium (EXCO performance: EA) has better resistance to exfoliation corrosion than the reference product without barium (EXCO performance: EB). The resistance to stress corrosion is also slightly improved.

Claims

1. 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, comprising:

(a) production of a liquid aluminium 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.

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, 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, wherein the barium is added in the form of an Si (70%)-Ba (30%) type alloy.

5. Process according to claim 1, wherein said aluminium-based liquid alloy comprises between 4 and 14% of zinc, between 1 and 3% of copper and between 1 and 3% of magnesium.

6. Process according to claim 5, 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.

7. Process according to claim 5, wherein the aluminium-based liquid alloy to which barium is added is selected from the group consisting of 7010, 7050, 7055, 7056, 7150, 7040, 7075, 7175, 7475, 7049, 7149, 7249, 7349 and 7449 alloys.

8. Process according to claim 1, wherein said aluminium-based liquid alloy comprises between 1 and 7% of copper.

9. Process according to claim 8, wherein said aluminium-based liquid alloy also comprises additionally between 0.2 and 2% of magnesium.

10. Process according to claim 8, wherein said aluminium-based liquid alloy comprises between 3.5 and 5.5% of copper and between 1 and 2% of magnesium.

11. Process according to claim 8, wherein said aluminium-based liquid alloy to which barium is added is selected from the group consisting of 2024, 2024A, 2056, 2022, 2023, 2139, 2124, 2224, 2324, 2424, and 2524 alloys.

12. Worked product obtainable with the process according to claim 11.

13. Worked product obtainable with the process according to claim 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√on.

14. Worked product obtainable with the process according to claim 6, wherein the tensile yield stress Rp0.2 (L) is greater than 600 MPa.

15. A method for making a structural element in an aeronautical construction comprising employing a worked product according to claim 12.

16. A wing skin element comprising a worked product of claim 12.

17. A stiffener comprising a worked product of claim 12.

18. A stringer comprising a worked product of claim 12.

19. A rib comprising a worked product of claim 12.

20. An element for bulkheads comprising a worked product of claim 12.