Low internal stress Al-Zn-Cu-Mg plates

Abstract

The present invention relates to a method for producing Al--Zn--Cu--Mg type alloy plates comprising between 4 and 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, and the remainder aluminum. The method comprising hot rolling, solution heat-treatment, quenching, controlled stretching with permanent elongation greater than 0.5% and aging, wherein the elapsed time D between the end of quenching and the start of controlled stretching is less than 2 hours. The invention further relates to plates and products produced or capable of being produced using such methods.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to French Application No. 04/13204 filed Dec. 13, 2004, the content of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to a method to relieve the level of residual stress throughout 7xxx series aluminum alloy plates subjected to stretching with permanent elongation.

[0004] 2. Description of Related Art

[0005] It is generally known that in 7xxx series aluminum alloys, natural aging starts immediately after quenching. The underlying microstructural mechanism is associated with the formation of Guinier-Preston zones by nucleation, and the formation of metastable phases which precipitate from a supersaturated aluminum matrix. The nucleation and growth of these precipitates results in a rapid increase in the yield stress, as these precipitates impede the movement of dislocations in the crystalline network. The degree of hardening by these mechanisms at a given point in a thick plate will typically depend on the chemical composition, the quenching rate, the metal grain and sub-grain structure, as well as its crystallographic texture.

[0006] 7xxx series alloy plates (that is, Al--Zn--Mg type alloys with or without copper) generally must be quenched rapidly after a solution heat-treatment thereof to be able to display, after artificial aging, high mechanical properties throughout their thickness. The presence of high thermal gradients close to the plate surface at the time of quenching causes non-homogeneous plastic strain. As a result, when the plate has completely cooled, it contains residual stress (internal stress). More specifically, compression stress is located in the vicinity of the surface, and stretching stress in the center. The extent of this stress depends on the alloy and the structure of the material, along with the solution heat-treatment and quenching method; the order of magnitude is 200 MPa. A detailed description of the residual stress in 7xxx type alloys can be found, for example, in the following articles: J. C. Chevrier, F. Moreaux, G. Beck, J. Bouvaist, "Contribution a l'etude des contraintes thermiques de trempe. Application aux alliages d'aluminum." Memoires Scientifiques--Revue de Metallurgie vol 72, No. 1, p. 83-94 (1975); P. Jeanmart, J. Bouvaist, "Finite element calculation and measurement of thermal stresses in quenched plate of high-strength 7075 aluminum alloy", Materials Science and Technology Vol. 1, No. 10, p. 765-769 (1985); D. Godard, Doctoral thesis, Institut National Polytechnique de Lorraine, Nancy 1999, particularly pages 285-290 and 209-250, all of which are incorporated herein by reference in their entirety.

[0007] Many of the most common methods to relieve residual stress in 7xxx series alloy plates make use of plastic strain, either by stretching in the L direction or by compression in the ST direction. The advantage of these methods is that they generally do not affect the hardening potential of the material significantly during a subsequent artificial aging step. Stretching is considered to be more effective than compression, as it generally results in more homogeneous plastic strain.

[0008] U.S. Pat. Nos. 6,159,315 and 6,406,567 (Corus Aluminum Walzprodukte GmbH) disclose a method of stress relieving solution heat-treated and quenched plates, comprising a first cold stretch step in the L direction, followed by a cold-compression step in the ST direction.

[0009] Patent application WO 2004/053180 (Pechiney Rhenalu) discloses a method of relieving the residual stress in a plate by means of edge compression. However, although it makes it possible to obtain plates with low residual energies, this compression method is difficult to implement.

[0010] Plastic strain typically makes it possible to relieve residual stress by a factor of approximately 10. This is illustrated in FIG. 2. However, in practice, the residual stress in thick semi-finished products considered as identical may vary significantly. This may be associated with the variation in their chemical composition, but also, and above all in many cases, with the variation in the production process parameters, such as casting, rolling, quenching, stretching and artificial aging; the influence of these process parameters on the level of residual stress in the finished product is still not clearly understood. Some changes to the process indeed result in a relief in the level of residual stress (such as the choice of slower quenching or a higher artificial aging temperature), but they also change the compromise between some properties which are important for structural applications, such as, typically, the mechanical strength, damage tolerance and corrosion resistance. The following articles discuss such issues and are incorporated herein by reference in their entireties: R. Habachou, M. Boivin, "Numerical predictions of quenching and relieving by stretching of aluminum alloys cylindrical bars", Journal de Mecanique Theorique et Appliquee, Vol 4, pp. 701-723, 1985; J. C. Boyer and M. Boivin, "Numerical calculations of residual stress relaxation in quenched plates", Materials Science and Technology Vol. 1 1985 pp. 786-792; R. Vignaud, P. Jeanmart, J. Bouvaist, B. Dubost (1990), "Detensionnement par deformation plastique", Physique et mecanique de la mise en forme des metaux, Ecole d'ete d'Oleron, directed by F. Moussy and P. Franciosi, published by Presses du CNRS, 1990, pp. 632-642.

[0011] The critical influence of residual stress on distortion during machining has been described extensively in the literature. In the aeronautical industry, complex components are frequently machined from thick aluminum alloy plates; this frequently results in more than 80% scrap. Excessive distortion during machining must often be compensated for by complex and costly measures, such as: (a) mechanical straightening, (b) shot peening, (c) optimisation of the location of the target component in the thickness of the plate, i.e. with respect to the residual stress depth profile, or (d) modification of the shape of the component with a view to minimising its strain (it being understood that the permanent set of the machine part is low if its shape is similar to a symmetric shape with respect to the longitudinal axis of the plate in which said component is machined). As a result, aircraft manufacturers typically prefer plates wherein the residual stress is not only lower, but also controlled, i.e. displaying a small variation for a given type of products (alloy, thickness, temper).

[0012] EP 0 731 185 and U.S. Pat. No. 6,077,363 disclose a method for relieving residual stress in 2024 alloy plates. Optimizing the manganese content and the hot rolling outlet temperature makes it possible to obtain a recrystallization rate of over 50% throughout the thickness. Such a plate displays improved mechanical property homogeneity as a function of the thickness, and a reduced level of residual stress after stretching.

[0013] For 7xxx plates, it is generally preferred to retain a largely non-recrystallized microstructure, particularly for applications which require high toughness, such as structural components for aircraft. This is disclosed in the article by F. Heymes, B. Commet, B. Dubost, P. Lassince, P. Lequeu, and G. M. Raynaud, "Development of new Al alloys for distortion free machined aluminum aircraft components", published in 1st International Non-Ferrous Processing and Technology Conference, St. Louis, Mo., 1997, 249-255, and incorporated herein by reference.

[0014] The residual stress in plates can be determined by means of the successive machining method disclosed in the article by Heymes, Commet et al., referred to above. A method based on this article is described in detail below, and this article is incorporated herein by reference.

SUMMARY OF THE INVENTION

[0015] A purpose of the present invention was to propose a method to obtain 7xxx series aluminum alloy plates which display, in a stretched temper, a naturally aged temper or in any artificially aged temper, a lower level of residual stress, without degrading the mechanical strength and damage tolerance. More specifically, it was desired to obtain plates which do not distort during machining, which is observed when the total elastic energy stored in the plate, W, is less than about 2 kJ/m.sup.3 and preferentially less than about 1 kJ/m.sup.3.

[0016] The invention relates to a method for producing Al--Zn--Cu--Mg alloy plates comprising from 4 to 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, and the remainder aluminum. The method comprises hot rolling, solution heat-treatment, quenching, controlled stretching with permanent elongation greater than 0.5% and aging, wherein the elapsed time D between the end of quenching and the start of controlled stretching is less than about 2 hours, and preferentially less than about 1 hour.

[0017] The present invention also relates to an Al--Zn--Cu--Mg alloy thick plate comprising from 4 to 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, and the remainder aluminum, which is hot rolled, solution treated, quenched, stretched with a permanent elongation greater than 0.5%, aged, wherein its total elastic energy is less than or equal to W [kJ/m.sup.3]=0.54+0.013(R.sub.p0.2(L) [MPa]-400).

[0018] The invention also relates to an inspection lot or a heat treatment batch of Al--Zn--Cu--Mg alloy plates comprising from 4 to 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, the remainder aluminum, in a solution-treated, quenched, stretched and aged temper, wherein the total elastic energy W (expressed in kJ/m.sup.3) of the plates displays a standard deviation less than or equal to 0.20+0.0030(R.sub.p0.2(L) [MPa]-400) around an average value.

[0019] Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a schematic representation of the definition of the three main directions in a plate.

[0021] FIG. 2 is a schematic representation of a stretching curve. Curve 2 represents the stress condition in the plate core. Curve 1 represents the surface stress condition. This figure shows the controlled stretching stress relieving principle: before the controlled stretching, the difference in the stress between the surface and the core is defined by x and -x. Controlled stretching reduces this difference (defined by y and -y) typically by a factor of 10.

[0022] FIG. 3 represents the definition of the parameters h, l and w of a plate. At the bottom, the strain gauge (with its connection wire) can be seen schematically.

[0023] FIG. 4 is a schematic representation of the sequences of the measurement and calculations to determine a residual stress profile in the plate thickness using the successive layer removal method.

[0024] FIG. 5 is a schematic representation of the critical part of the method according to the invention. D refers to the time interval between the end of quenching and the start of controlled stretching.

[0025] FIG. 6 shows the natural aging kinetics of 7010 and 7050 alloy plates for two different quenching rates. The X-axis shows the yield stress in the L direction, the Y-axis the natural aging time.

[0026] FIG. 7 shows the effect of increasing the variation in yield stress values on residual stress profiles after quenching.

[0027] FIG. 8 shows the total elastic energy as a function of the thickness for batches of 7xxx alloy plates according to the invention (where D.ltoreq.1 hour) (unfilled dots) and according to the prior art (where D.gtoreq.8 hours) (black squares).

DETAILED DESCRIPTION OF A PREFERRED EMOBIDMENT

a) Terminology

[0028] Unless specified otherwise, all the indications relating to the chemical composition are expressed as a percentage by weight. References to alloys herein observe the rules of The Aluminum Association, known to those skilled in the art. The expression "Al--Zn--Cu--Mg alloy" refers to an aluminum-based alloy containing zinc, copper and magnesium alloy elements; such an alloy may also contain other alloy elements along with other elements, the presence of which may be intentional or not, e.g. impurities.

[0029] The tempers 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 specified otherwise, the static mechanical properties, i.e. the ultimate tensile strength UTS or R.sub.m, the tensile yield stress TYS or R.sub.p0.2, and the elongation at rupture A, are determined by means of a tensile test according to the standard EN 10002-1, the position and direction of test piece sampling being defined in the standard EN 485-1. The toughness K.sub.IC was measured according to the standard ASTM E 399.

[0030] Unless specified otherwise, the definitions of the European standard EN 12258-1 are applied.

[0031] Within the scope of the present invention, the term "thick plate" refers to a plate of a thickness greater than or equal to 6 mm.

[0032] The term "inspection lot" is defined in the standard EN 12258-1; it refers to a shipment or part of a shipment, submitted for inspection, and which comprises products of the same grade or alloy, of the same form, temper, size, shape, thickness or cross-section, and processed in the same manner.

[0033] The term "heat treatment batch" refers to a quantity of products of the same grade or same alloy, of the same form, thickness or cross-section, and which were produced in the same way, wherein the heat treatment or solution heat-treatment followed by quenching were performed in one furnace load. More than one solution-treatment batch can be included in one precipitation furnace load.

[0034] The "aging" comprises natural aging at ambient temperature and any artificial aging.

[0035] The term "machining" comprises any material removal method such as turning, free machining, milling, drilling, boring, tapping, electroerosion, straightening, polishing, and chemical machining.

[0036] The term "structural element" refers to an element used in mechanical construction for which the static and/or dynamic mechanical properties are particularly important for the performance and integrity of the structure, and for which a calculation of the structure is generally specified or performed. It typically consists of a mechanical component, the failure of which is liable to endanger the safety of said construction, its users or other parties. For an aircraft, these structural elements particularly comprise the elements making up the fuselage (such as the fuselage skin, stringers, bulkheads, circumferential frames), wings (such as the wing skin, stringers or stiffeners, ribs and spars) and the tails particularly consisting of horizontal or vertical stabilisers and floor beams, seat tracks and doors.

[0037] The term "monolithic structural element" refers to a structural element which has been obtained most frequently by machining, from a single piece of rolled, extruded, forged or cast semi-finished product, with no assembly, such as riveting, welding, bonding, with another piece.

[0038] The L (Length), LT (Long Transverse) and ST (Short Transverse) directions in a rolled product refer to the direction of rolling corresponding to the L direction. These three directions are defined in FIG. 1.

[0039] The term "about" or "approximately" refers to any value within 10% of a stated given value, particularly preferably within 5% of the stated value.

b) Determination of Residual Stress

[0040] Within the scope of the present invention, the residual stress was determined using the method based on the successive removal of layers described in the article "Development of New Alloy for Distortion Free Machined Aluminum Aircraft Components", F. Heymes, B. Commet, B. Dubost, P. Lassince, P. Lequeu, G M. Raynaud, in 1st International Non-Ferrous Processing & Technology Conference, 10-12 Mar. 1997--Adams's Mark Hotel, St Louis, Mo. The content of this article is incorporated herein by reference in its entirety.

[0041] This method mostly applies to stretched plates, wherein the stress state can be considered as biaxial with its two main components located in the L and LT directions, and therefore no component in the ST direction. This method is based on the determination of the residual stress in the L and LT direction as measured in full thickness rectangular bars, which are cut from the plate along these directions. These bars are machined down the ST direction step by step and at each step, the stress and/or deflection is measured, as well as the thickness of the bar. A most preferred way is to measure the strain by using a strain gauge bound to the surface opposite the machined surface at half-length of the bar. Then the two residual stress profiles in the L and LT directions can be calculated. The bar must be sufficiently long to avoid edge effects. The recommended dimensions as a function of the plate thickness are given in table 1. TABLE-US-00001 TABLE 1 Dimensions [mm] used for the successive layer removal method Plate thickness (h) Width (w) Length (1) 20 < h .ltoreq. 100 24 .+-. 1 5h .+-. 1 h > 100 30 .+-. 1 5h .+-. 1

[0042] The one-way strain gauges with thermal expansion compensation are bonded to the lower surface of the bar (see FIG. 3), according to the manufacturer's instructions. They are then coated with an insulating lacquer. The value read on each of these gauges are then set to zero.

[0043] A measurement is performed after each machining pass. Between 18 and 25 passes are typically taken to obtain a sufficient number of points to calculate the stress profile. The machining depth must not be less than 1 mm, so as to obtain a good cutting quality; for very thick plates, it may be up to 10 mm. Chemical machining may also be used to remove a very thin layer of metal. The machining interval should be the same for both samples (i.e. in the L direction and in the LT direction).

[0044] After each machining pass, the bar is detached from the vice and the temperature is allowed to stabilise before the strain is measured. At each step i, the thickness h(i) and the strain .epsilon.(i) are recorded. The diagram in FIG. 4 shows how these data are collected.

[0045] These data allow the calculation of the initial stress profile in each bar in the form of a curve u(i), corresponding to the average stress in the layer removed during the machining step i, given by the following formulas: For .times. .times. i = 1 .times. .times. to .times. .times. N - 1 .times. : ##EQU1## u .function. ( i ) = - E .times. ( .times. ( i + 1 ) - .function. ( i ) ) .times. h .function. ( i + 1 ) 2 [ h .function. ( i ) - h .function. ( i + 1 ) ] .function. [ 3 .times. h .function. ( i ) - h .function. ( i + 1 ) ] - S .function. ( i ) ##EQU1.2## where .times. : ##EQU1.3## S .function. ( i ) = E .times. k = 1 .times. i - 1 .times. ( .function. ( k + 1 ) - .function. ( k ) ) .function. [ 1 - 3 .times. h .function. ( k ) .times. ( h .function. ( i ) + h .function. ( i + 1 ) ) ( 3 .times. h .function. ( k ) - h .function. ( k + 1 ) ) .times. h .function. ( k + 1 ) ] ##EQU1.4##

[0046] where E is the Young's modulus of the thick plate. This gives two profiles: u(i).sub.L and u(i).sub.LT corresponding to rectangular section bars in the L and LT directions. The stress profiles in the plate are obtained using the following equations: For .times. .times. i = 1 .times. .times. to .times. .times. N - 1 ##EQU2## .sigma. .function. ( i ) L = u .function. ( i ) L + vu .function. ( i ) LT 1 - v 2 ##EQU2.2## .sigma. .function. ( i ) LT = u .function. ( i ) LT + vu .function. ( i ) L 1 - v 2 ##EQU2.3##

[0047] where v is the Poisson coefficient of the plate. It is then possible to calculate the energy stored in the plate (W.sub.L, W.sub.LT and W) using the equations: W L = 500 Eh .times. i = 1 N - 1 .times. .sigma. .function. ( i ) L .function. [ .sigma. .function. ( i ) L - v .times. .times. .sigma. .function. ( i ) LT ] .times. dh .function. ( i ) ##EQU3## W LT = 500 Eh .times. i = 1 N - 1 .times. .sigma. .function. ( i ) LT .function. [ .sigma. .function. ( i ) LT - v .times. .times. .sigma. .function. ( i ) L ] .times. dh .function. ( i ) ##EQU3.2## W = W L + W LT ##EQU3.3##

[0048] where W.sub.L represents the stored elastic energy resulting from the residual stress profile in the L direction, and W.sub.LT represents the stored energy resulting from the residual stress profile in the LT direction. W is the total elastic energy stored in the plate (expressed in kJ or kJ/m.sup.3). The method used to measure the stress and to obtain the stored elastic energies is described above specifically, giving, for example, the bar dimensions used in practice. It should be noted that these dimensions are not compulsory and do not restrict the method. The length of the bar does not affect the result. The length of two times h plus three times the gauge length is sufficient for measurements using strain gauges. The dimensions given are based on practical experience and have been adapted to the machining and measurement means used. Those skilled in the art will easily be capable of selecting other dimensions without altering the results.

[0049] Similarly, other techniques may be used to measure the stress gradient in the plate thickness. After obtaining the stress profiles .sigma..sub.L and .sigma..sub.LT in the thickness, the same formulas of the above incremental sums are used to calculate the stored energies W.sub.L and W.sub.LT. Therefore, it is possible to obtain the stored energies using any technique enabling stress measurements in the thickness.

c) Detailed Description of the Invention

[0050] The present invention applies to 7xxx series aluminum alloy plates, particularly plates, wherein the chemical composition meets the following criteria:

[0051] 4<Zn<12; Mg<4; Cu<4;

[0052] other elements.ltoreq.0.5 each

[0053] the remainder aluminum,

[0054] and which are treated by means of solution heat-treatment, quenching and controlled stretching.

[0055] According to the invention, a problem can be solved by modifying the production process so that the natural aging between the end of quenching and the start of controlled stretching is minimized such that the total elastic energy (W) in the artificially aged state remains below a specific limit value. This limit value represents a preferred maximum value to retain the machining strain at an acceptable level; for most applications, this limit value is about 2 kJ/m.sup.3 for a plate between 60 mm and 100 mm thick, and preferentially about 1.5 kJ/m.sup.3. For particularly complex components, it should be about 1 kJ/m.sup.3.

[0056] FIG. 5 shows a diagram of the heat treatment process applied to a plate after rolling. The solution heat-treatment can be performed, for example, in a single plateau, in several plateaus or in a ramp with or without a clearly defined plateau. The same applies for artificial aging. An important phase within the scope of the present invention is the elapsed time D between the end of quenching and the start of controlled stretching. The inventors found that a long elapsed time D results in greater heterogeneity of the mechanical properties between the zones near the surface and the zones near the mid-thickness of the material. This heterogeneity may essentially be attributed to the differences in the cooling rate in the plate thickness. FIG. 6 shows the progression of the yield stress in the L direction, determined close to the surface and at mid-thickness, as a function of the natural aging time for very high-strength AA7010 and AA7050 alloy plates and for different nominal quenching rates. These quenching rates were obtained on stretching test pieces but they are representative of the differences in quenching rates observed between the surface and core of a thick plate. It can be seen that the difference between the levels of mechanical strength is accentuated over time.

[0057] The inventors observed that the variation in residual stress through the thickness of 7xxx alloy plates depends on (i) the variation in the cooling and plastic strain rates during quenching, (ii) heterogeneities in the microstructure, granular structure and texture generated during rolling, and (iii) local variations in the chemical composition resulting from the casting process (including solidification and homogenization). Between the end of quenching and the start of stretching, natural aging is observed throughout the plate thickness, but the rate of this natural aging depends on the thickness: the yield stress increases more rapidly in the vicinity of a surface than at mid-thickness. This is probably due to the precipitation kinetics: firstly, the potentially hardening element content of the supersaturated solid solution is greater near the surface than at mid-thickness (as the semi-continuous casting process results in macro-segregation such that the concentration of eutectic elements, such as Cu, Mn and Zn, is higher close to the surface and the cooling rate during casting is also higher), and, secondly, close to the surface, a greater density of heterogeneous sites (gaps, dislocations, etc.) can be found, facilitating precipitation and resulting from the higher cooling rate and the higher plasticity during quenching.

[0058] The inventors found, by means of a calculation based on a finite element model, that an increase in the heterogeneity of the mechanical properties (i.e. of the yield stress or the strain hardening coefficients) results in an increase in the residual stress after stretching. FIG. 7 shows the effect of the increase in the variation in the yield stress values on the residual stress profiles after quenching.

[0059] However, this attempt to find a metallurgical explanation for the method according to the invention does not imply any limitation of the present invention to the underlying phenomena. Moreover, the inventors observed that the effect is in fact greater than the values obtained using the mathematical model.

[0060] Finally, a change to the production process resulting in an improvement in the homogeneity of the yield stress (R.sub.p02) in the thickness of the plate after quenching would relieve the residual stress after controlled stretching or after any stress relieving by means of plastic strain.

[0061] A method according to the present invention may not give the same level of improved results for other structural hardening alloys, such as 2xxx and 6xxx series alloys. For highly concentrated alloys, i.e. with contents consists of Zn>12%, Mg>4% and Cu>4%, the stored energy is very high and the improvement obtained with a method according to the invention may be as significant. In addition, these alloys may not respond well to solution heat-treatment.

[0062] A method according to the present invention makes it possible to produce plates having a total elastic energy which is preferably less than or equal to W [kJ/m.sup.3]=0.54+0.013(R.sub.p0.2(L) [MPa]-400).

[0063] In this equation, R.sub.p0.2(L) refers to the yield stress of the finished plate measured according to the standards EN 10002-1 and EN 485-1. The influence of the thickness on the level of residual stress and the total elastic energy is expressed here in terms of the yield stress, measured as recommended by the standard EN 485-1. The method according to the invention may be applied advantageously to the manufacture of a plurality of plates wherein the thickness is between approximately 10 mm and approximately 250 mm, and more advantageously to plates wherein the thickness is greater than 25 mm, but these values are not restrictive.

[0064] A method according to the present invention also makes it possible to reduce the dispersion between the values of W for a plurality of plates belonging to the same inspection lot or heat treatment batch, such that all the plates have a standard deviation of the total elastic energy W of the different plates around an average value that is preferably less than or equal to 0.20+0.0086(R.sub.p0.2(L) [MPa]-400)

[0065] and preferentially and advantageously less than or equal to 0.20+0.0030(R.sub.p0.2(L) [MPa]-400).

[0066] In this equation, R.sub.p0.2(L) refers to the average R.sub.p0.2(L) measurement performed according to the standard for each of the finished plates in the batch, according to the standards EN10002-1 and EN485-1.

[0067] The standard deviation between the measurements of the total elastic energy W of the different plates in a batch may depend on the number of plates contained in the batch. In particular, a standard deviation obtained on two measurements is not significant and may be very high or very low. From 3 plates, the standard deviation of the measurements may be considered, but preferentially, the quality control or heat treatment batches used within the scope of the present invention contain at least 5 plates.

[0068] The use of a method according to the present invention enables the manufacturer to guarantee that a particular inspection lot or heat treatment batch comprises plates wherein the average total elastic energy is preferably less than about 3 kJ/m.sup.3. Preferentially, this average value is less than about 2 kJ/m.sup.3, and a value less than about 1 kJ/m.sup.3 is preferred, which requires excellent control of the critical processes and very rigorous management of production schedule at the solution heat-treatment, quenching and stretching stages. In fact, the implementation of a method according to the instant invention may require an adaptation of the metal flows within the plant, because if the producer wishes to produce plates within an elapsed time D of less than a few hours, it may potentially be necessary to synchronize the quenching furnace with the stretching bench. In practice, this involves limiting the intermediate stock to a minimum between these two machines; this particularly applies to the particularly preferred embodiments where D<1 hour or D<30 minutes. EP 1 231 290 A1 describes in example 1 thereof, a 38 mm thick 7449 alloy plate for which controlled stretching was performed 1 hour after quenching; however, this document does not provide any information on the benefit of this short time. A method according to the present invention made it possible to produce inspection lots or heat treatment batches for which the elapsed time D between the end of quenching and the start of controlled stretching is systematically less than 2 hours, which made it possible to minimize the average and standard deviation of the total elastic energy W of the plates in these batches. However, the industrial production of such an inspection lot requires a reorganization of the product flows around the machines required for the implementation of the method according to the invention.

[0069] In another embodiment of the present invention, natural aging is performed at a low temperature, i.e. at a temperature below about 10.degree. C. and preferentially at a temperature below about 5.degree. C., which makes it possible to obtain similar results in terms of total elastic energy W for times D between 2 hrs and 3 hrs.

[0070] Other preferred embodiments of the invention include those specified in the dependent claims. The invention is particularly advantageous for AA7010, 7050, 7056, 7449, 7075, 7475, 7150, 7175 alloy thick plates.

[0071] One advantage of a method according to the invention is the overall relief in the level of stress in plates. This induces an overall reduction in the machining strain.

[0072] A further advantage of the method according to the invention is that the monitoring of the time elapsed between the end of quenching and the start of stretching also makes it possible to reduce the dispersion of the stress level observed between different nominally identical plates, even within the same production batch or heat treatment batch. This enables improved standardization of the machining processes for a given product series and reduces the number of incidents during the production of machine components in the machining workshop.

[0073] In the examples below, advantageous embodiments of the invention are given for illustration purposes. These examples are not restrictive.

EXAMPLES

Example 1

[0074] Three AA7010 alloy rolling ingots were cast by means of semi-continuous casting. After homogenisation, they were hot rolled to a thickness of 100 mm. At the hot rolling mill outlet, they underwent quenching followed by controlled stretching and finally an artificial aging treatment. The temper of the three products A1, A2 and A3 obtained in this way was T7651. For these three products, all the production parameters were nominally identical and well controlled. The only difference was the elapsed time D between the end of quenching and the start of stress relieving by means of stretching.

[0075] Using a similar method, three rolling ingots made of AA7050 alloy were processed by means of homogenisation, hot rolling to a thickness of 100 mm, quenching, controlled stretching and artificial aging. The temper of the three products B1, B2 and B3 obtained in this way was T7451. For these three products, all the production parameters were nominally identical and well controlled, and the only difference was the elapsed time D between the end of quenching and the start of stress relieving by means of stretching.

[0076] Table 2 shows the stored elastic energy in the different plates obtained, determined in the final temper. When the elapsed time D between the end of quenching and the start of stress relieving by means of stretching is reduced, a reduction in the overall stress level as measured by W.sub.L, W.sub.LT and W is observed. TABLE-US-00002 TABLE 2 Stored elastic energy (final temper) as a function of the natural aging time for three 7010 and 7050 alloy plates. Natural aging time W W.sub.L W.sub.LT Plate Alloy/temper D [h] [kJ/m.sup.3] [kJ/m.sup.3] [kJ/m.sup.3] A1 7010 T7651 1.17 1.02 0.8 0.22 A2 7010 T7651 9 1.76 1.37 0.4 A3 7010 T7651 48.92 2.37 1.74 0.63 B1 7050 T7451 1.25 1.22 0.84 0.38 B2 7050 T7451 8.83 2.28 1.57 0.71 B3 7050 T7451 49.08 3.15 2.02 1.12

[0077] The static mechanical properties were measured in the L, LT and ST directions at 1/4, 1/2 and 3/4 thickness, in final temper. The results are compiled in tables 3, 4 and 5. It is observed that the natural aging time D does not have a significant influence on the static mechanical characteristics. TABLE-US-00003 TABLE 3 Static mechanical properties (L direction) in final temper as a function of the natural aging time D for 7010 and 7050 alloy plates Natural Alloy/ aging R.sub.m(L) R.sub.po.2(L) A.sub.(L) Plate temper time D [h] Location [MPa] [MPa] [%] A1 7010 T7651 1.17 1/4 thickness 524 479 14.0 1/2 thickness 519 468 12.7 3/4 thickness 533 471 11.0 A2 7010 T7651 9 1/4 thickness 529 480 14.4 1/2 thickness 523 477 11.5 3/4 thickness 539 480 9.6 A3 7010 T7651 48.92 1/4 thickness 521 472 12.6 1/2 thickness 516 466 9.2 3/4 thickness 528 472 8.2 B1 7050 T7451 1.25 1/4 thickness 536 482 13.0 1/2 thickness 519 465 10.4 3/4 thickness 531 470 9.6 B2 7050 T7451 8.83 1/4 thickness 534 479 14.2 1/2 thickness 519 461 10.8 3/4 thickness 533 469 8.7 B3 7050 T7451 49.08 1/4 thickness 534 478 14.2 1/2 thickness 519 459 10.5 3/4 thickness 531 463 9.4

[0078] TABLE-US-00004 TABLE 4 Static mechanical properties (LT direction) in final temper as a function of the natural aging time D for 7010 and 7050 alloy plates Natural Alloy/ aging R.sub.m(LT) R.sub.po.2(LT) A.sub.(LT) Plate temper time D [h] Location [MPa] [MPa] [%] A1 7010 T7651 1.17 1/4 thickness 529 470 10.4 1/2 thickness 527 464 9.4 3/4 thickness 513 446 9.2 A2 7010 T7651 9 1/4 thickness 536 475 11.0 1/2 thickness 534 478 8.4 3/4 thickness 521 463 8.1 A3 7010 T7651 48.92 1/4 thickness 527 461 10.1 1/2 thickness 526 463 7.8 3/4 thickness 511 452 8.0 B1 7050 T7541 1.25 1/4 thickness 541 461 10.6 1/2 thickness 526 456 6.6 3/4 thickness 516 443 6.7 B2 7050 T7541 8.83 1/4 thickness 541 464 9.6 1/2 thickness 528 464 6.9 3/4 thickness 519 447 7.2 B3 7050 T7451 49.08 1/4 thickness 538 467 10.8 1/2 thickness 527 451 7.8 3/4 thickness 513 440 6.4

[0079] TABLE-US-00005 TABLE 5 Static mechanical properties (ST direction) in final temper as a function of the natural aging time D for 7010 and 7050 alloy plates Natural Alloy/ aging R.sub.m(ST) R.sub.po.2(ST) A.sub.(ST) Plate temper time D [h] Location [MPa] [MPa] [%] A1 7010 T7651 1.17 1/4 thickness 517 449 6.5 1/2 thickness 508 432 7.7 3/4 thickness 518 455 6.3 A2 7010 T7651 9 1/4 thickness 521 455 5.7 1/2 thickness 520 438 5.3 3/4 thickness 515 442 7.6 A3 7010 T7651 48.92 1/4 thickness 514 451 5.7 1/2 thickness 514 449 5.0 3/4 thickness 509 440 7.4 B1 7050 T7451 1.25 1/4 thickness 507 445 3.4 1/2 thickness 519 470 4.6 3/4 thickness 507 428 5.6 B2 7050 T7451 8.83 1/4 thickness 513 446 4.2 1/2 thickness 513 438 3.9 3/4 thickness 511 413 5.9 B3 7050 T7451 49.08 1/4 thickness 514 423 4.6 1/2 thickness 505 420 4.8 3/4 thickness 513 442 3.7

[0080] The toughness K.sub.IC was also measured in the L-T and T-L directions at 1/4 thickness. The results, compiled in table 6, demonstrate that natural aging does not have a significant influence on toughness. TABLE-US-00006 TABLE 6 Toughness (in final temper) as a function of the natural aging time D for 7010 and 7050 alloy plates Natural Alloy/ aging K.sub.IC(L-T) K.sub.IC(T-L) Plate temper time D [h] Location (MPa m) (MPa m) A1 7010 1.17 1/4 thickness 33.6 28.0 T7651 A2 7010 9 1/4 thickness 32.7 26.0 T7651 A3 7010 48.92 1/4 thickness 32.9 27.7 T7651 B1 7050 1.25 1/4 thickness 32.2 26.1 T7451 B3 7050 49.08 1/4 thickness 32.3 27.7 T7451

Example 2

[0081] Three rolling ingots made of AA7475 alloy were processed by means of homogenisation, hot rolling to a thickness of 46 mm, quenching and controlled stretching. The temper of the three products C1, C2 and C3 obtained in this way was W51. For these three products, all the production parameters were nominally identical and well controlled and the only difference was the elapsed time D between the end of quenching and the start of stress relieving by means of stretching.

[0082] Table 7 shows the stored elastic energy in the different plates obtained, determined in the final temperature (i.e. after controlled stretching). When the elapsed time D between the end of quenching and the start of stress relieving by means of stretching is reduced, a reduction in the overall stress level W.sub.L, W.sub.LT and W is observed. TABLE-US-00007 TABLE 7 Stored elastic energy as a function of the natural aging time D for 7475 alloy W51 plates Alloy/ Natural aging time W W.sub.L W.sub.LT Plate temper D [h] [kJ/m.sup.3] [kJ/m.sup.3] [kJ/m.sup.3] C1 7475 W51 1.75 2.24 1.6 0.64 C2 7475 W51 22.5 4.51 3.61 0.9 C3 7475 W51 48 5.18 3.97 1.21

Example 3

[0083] Two rolling ingots made of AA7449 alloy were processed by means of homogenisation, hot rolling to a thickness between 16.5 and 21.5 mm, quenching and controlled stretching, followed by artificial aging. The temper of the two products D1 and D2 obtained in this way was T651. For these two products, all the production parameters were nominally identical and well controlled and the only difference was the elapsed time D between the end of quenching and the start of stress relieving by means of stretching.

[0084] Table 8 shows the stored elastic energy in the different plates obtained, determined in the final temperature (i.e. after controlled stretching). When the elapsed time D between the end of quenching and the start of stress relieving by means of stretching is reduced, a reduction in the overall stress level W.sub.L, W.sub.LT and W is observed. The slight difference between the thicknesses of both products does not, as such, result in a significant difference between their stress levels. TABLE-US-00008 TABLE 8 Stored elastic energy as a function of the natural aging time D for 7449 alloy T651 plates Natural aging Alloy/ Thickness time D W W.sub.L W.sub.LT Plate temper [mm] [h] [kJ/m.sup.3] [kJ/m.sup.3] [kJ/m.sup.3] D1 7449 16.5 10.5 6.3 5.56 0.74 T651 D2 7449 21.5 3 4.17 3.66 0.51 T651

[0085] This result confirms that, even for a high zinc content Al--Zn--Mg alloy such as 7449, it is possible to reduce the total elastic energy very significantly by reducing the natural aging time D.

Example 4

[0086] Using industrial processes wherein the only differences lay in the waiting time, quality control plate lots according to the invention were prepared. The stored energy was measured. A mathematic model was then developed which is used to calculate this stored energy as a function of the critical parameters of the production process. The values of the stored energy measured for plates according to the invention were used to validate this mathematical model. The same mathematical model was then applied to batches of Al--Zn--Mg alloy plates obtained using methods according to the prior art. FIG. 8 shows the values of the stored energy in the plates according to the invention (where D.ltoreq.1 hour) (unfilled dots) ("Optimized") and according to the prior art (where D.gtoreq.8 hours) (black squares).

[0087] It is observed that, for a thickness between approximately 60 mm and approximately 100 mm, the stored energy is maximal. The method according to the invention results in, for a given thickness, firstly, a relief in the overall level of residual stress (i.e. the stored energy W.sub.total) of approximately 50%, and, secondly, a significant reduction in the statistical dispersion of this value. The effect of the invention on the overall level of residual stress is particularly remarkable for thicknesses between 40 and 150 mm and it is even more marked for thicknesses between 50 and 100 or even 80 mm.

[0088] Additional advantages, features and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices, shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

[0089] All documents referred to herein are specifically incorporated herein by reference in their entireties.

[0090] As used herein and in the following claims, articles such as "the", "a" and "an" can connote the singular or plural.

Claims

1. A method for producing an Al--Zn--Cu--Mg alloy plate comprising between 4 and 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, and the remainder aluminum, said method comprising hot rolling, solution heat-treatment, quenching, controlled stretching with permanent elongation greater than 0.5% and aging, wherein the elapsed time D between the end of quenching and the start of controlled stretching is less than 2 hours.

2. Method according to claim 1, wherein the elapsed time D is less than or equal to 1 hour and preferentially less than 30 minutes.

3. Method according to claim 1, wherein said alloy is selected from the group consisting of the alloys AA7010, 7050, 7056, 7449, 7075, 7475, 7150, and 7175.

4. Method according to claim 1, wherein the thickness of said plate is greater than 40 mm.

5. Method according to claim 1, wherein the thickness of said plate is between 40 and 80 mm.

6. Method according to claim 1, wherein the thickness of said plate is between 40 and 150 mm.

7. Method according to claim 1, wherein said plate has a total elastic energy less than or equal to W [kJ/m.sup.3]=0.54+0.013(R.sub.p0.2(L) [MPa]-400).

8. Al--Zn--Cu--Mg alloy thick plate comprising between 4 and 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, and the remainder aluminum, which is hot rolled, solution treated, quenched, stretched with a permanent elongation greater than 0.5%, aged, wherein the total elastic energy of said plate is less than or equal to W [kJ/m.sup.3]=0.54+0.013(R.sub.p0.2(L) [MPa]-400).

9. Plate according to claim 8, having a thickness between 60 and 100 mm and a total elastic energy less than 2 kJ/m.sup.3.

10. Plate according to claim 9, wherein said total elastic energy is less than 1.5 kJ/m.sup.3.

11. Plate according to claim 8, having a thickness greater than 100 mm and a total elastic energy less than 1.5 kJ/m.sup.3.

12. Thick plate obtainable by the method according to claim 7.

13. Inspection lot or heat treatment batch of Al--Zn--Cu--Mg alloy plates comprising between 4 and 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, the remainder aluminum, in a solution-treated, quenched, stretched and aged temper, wherein the total elastic energy W in kJ/m of the plates displays a standard deviation less than or equal to 0.20+0.0030(R.sub.p0.2(L) [MPa]-400) around an average value of said total elastic energy.

14. Inspection lot or heat treatment batch of thick plates according to claim 13, wherein said average total elastic energy value is less than W [kJ/m.sup.3]=0.54+0.013(R.sub.p0.2(L) [MPa]-400).

15. Inspection lot or heat treatment batch of thick plates according to claim 13, characterised in that said average total elastic energy value is less than 3 kJ/m.sup.3.

16. Inspection lot or heat treatment batch according to claim 13 wherein said average total elastic energy value is less than 2 kJ/m.sup.3.

17. Inspection lot or heat treatment batch according to claim 13 wherein a nominal thickness of the plates is between 40 and 100 mm.

18. Inspection lot or heat treatment batch according to claim 13 wherein the plates comprise an alloy selected from the group consisting of AA7010, 7050, 7056, 7449, 7075, 7475, 7150, and 7175.

19. Inspection lot or heat treatment batch according to claim 13 comprising at least 3 plates.

20. Inspection lot or heat treatment batch according to claim 13 wherein said plates are obtained by a method comprising hot rolling, solution heat-treatment, quenching, controlled stretching with permanent elongation greater than 0.5% within a time after quenching of less than 2 hours.

21. A method for the production of machine components comprising obtaining a plate according to claim 8 and using said plate for the production of machine components.

22. A method for reducing the dispersion of the stress level between different nominally identical plates comprising subjecting said plates to hot rolling, solution heat-treatment, quenching, and after quenching, controlling the time period for a predetermined time before subjecting said plates to controlled stretching with permanent elongation greater than 0.5%.

23. A method of claim 22 wherein said predetermined time is less than 2 hours.

24. A method of claim 22, wherein said plates are in the same production or heat treatment batch.

25. A method of claim 22 wherein said plates comprise an Al--Zn--Cu--Mg alloy plate comprising between 4 and 12% zinc, less than 4% magnesium and less than 4% copper, other elements.ltoreq.0.5% each, and the remainder aluminum

26. A plate prepared by a method of claim 1.

27. An inspection lot or heat treatment batch of claim 13, wherein said standard deviation is calculated based on at least 3 samples.

28. An inspection lot or heat treatment batch of claim 13, wherein said standard deviation is calculated based on at least 5 samples.

29. Method according to claim 4, wherein said plate has a total elastic energy less than or equal to W [kJ/m.sup.3]=0.54+0.013(R.sub.p0.2(L) [MPa]-400).