EP1809779B1 - High-strength aluminium alloy products and method for the production thereof - Google Patents

Abstract

Process for manufacturing aluminium alloy products, with high toughness and fatigue resistance comprising: (a) preparing an aluminium alloy bath, (b) adding a refining agent containing particles of AlTiC type phases into the bath, (c) casting an as-cast form such as an extrusion ingot, a forging ingot or a rolling ingot, (d) hot transforming the as-cast form, possibly after scalping, to form a blank or a product with final thickness, (e) optionally cold transforming the blank to a final thickness, (f) applying a solution heat treatment and quenching the product output from (d) or (e), followed by relaxation by controlled stretching with permanent elongation between 0.5 and 5%, and optionally annealing, wherein the quantity of refining agent is chosen such that the average casting grain size of the as-cast form is more than 500 mum. The present invention may be used, for example, to manufacture fuselage sheet or light-gauge plates made with 6056 alloy.

Description

Field of the invention

The invention relates to a new manufacturing method for high tenacity and high fatigue resistant aluminum alloy rolled products, as well as products obtained by this process. This process comprises a particular refining of the liquid metal. These sheets can be used as aircraft fuselage liner.

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, Al-Si-Mg-Cu alloys can be used for fuselage structural members. These elements must have on the one hand a high mechanical strength, and on the other hand good toughness and good resistance to fatigue. Any new opportunity to improve one of these property groups without degrading others would be welcome.

So far, the main efforts have focused on the optimization of the chemical composition of alloys, as well as on the optimization of sheet transformation conditions, that is to say, rolling and heat treatment sequences.

Thus, it is well known that in alloys of the 2xxx and 7xxx series, the reduction of iron and silicon impurities leads to an increase in toughness (see JT Staley, "Microstructure and Toughness of High-Strength Aluminum Alloys" published in the book "Properties Related to Fracture Toughness", ASTM Special Technical Publication 65, 1976, pp. 71-103 ). In some cases, it also tends to increase the resistance to fatigue.

There is little research on the influence of liquid metal refining conditions and the casting of raw forms (such as billets and plates) on the toughness of products obtained from such raw forms.

The patent application EP 1 205 567 A (Alcoa Inc.) teaches that the addition of Ti and B or C to a workout alloy at a rate of 0.003 to 0.010% results in a grit size of less than or equal to 200 μm.

The patent application EP 1 158 068 A (Pechiney Rhenalu ) teaches that the toughness of structural hardening aluminum alloy thick plates in metallurgical states with little recrystallization, that is to say, the fraction of recrystallized grains is less than 35%, is influenced by the casting microstructure: a large Casting size may, in some cases, lead to better toughness than a small grain size. This result is obtained in particular by a careful control of the titanium and boron content, elements which, added in the form of TiB ₂ , refine the grain of the metal during its solidification.

The patent US 5,104,616 (Baeckerud ) is particularly interested in the problems posed by hard boride particles in the beverage or aluminum foil industries and teaches that it may be advantageous to replace a refining containing boron by a refining agent containing carbon. However, the problems encountered in the aluminum packaging industry such as percussion are not comparable to those encountered in the aviation industry.

The object of the present invention is to propose a new process for obtaining highly recrystallized wrought products, preferably laminated products, and in particular 6xxx series alloy thin metal sheets with high mechanical strength which also show excellent toughness and fatigue resistance.

Object of the invention

1. (a) on prépare un bain d'un alliage d'aluminium,
2. (b) on introduit dans ledit bain un affinant,
3. (c) on coule une forme brute, telle qu'une billette de filage, une billette de forge ou une plaque de laminage,
4. (d) on transforme à chaud ladite forme brute, éventuellement après scalpage, pour former une ébauche ou un produit d'épaisseur finale,
5. (e) on transforme optionnellement à froid l'ébauche jusqu'à son épaisseur finale,
6. (f) on soumet le produit issu de l'étape (d) ou (e) à un traitement thermique de mise en solution et trempe, suivi d'un détensionnement par traction contrôlée avec un allongement permanent compris entre 0,5 et 5%, et éventuellement un revenu
   caractérisé en ce que ledit affinant contient des particules de phases de type AlTiC, en ce que la quantité d'affinant est choisie de manière à ce que la taille moyenne de grain de fonderie de ladite forme brute soit supérieure à 500 µm et en ce que ledit alliage est un alliage AA6056 ou un alliage AA6156.

The subject of the invention is a process for manufacturing aluminum alloy products, in particular highly recrystallized products, with high tenacity and fatigue resistance, which comprises the following steps:

1. (a) preparing a bath of an aluminum alloy,
2. (b) introducing into said bath a refining,
3. (c) pouring a raw form, such as a spinning billet, a forge billet or a rolling plate,
4. (d) hot-forming said raw form, optionally after scalping, to form a blank or a product of final thickness,
5. (e) optionally converting the blank to coldness to its final thickness,
6. (f) subjecting the product resulting from step (d) or (e) to a heat treatment for dissolving and quenching, followed by controlled tensile stress relieving with a permanent elongation of between 0.5 and 5% , and possibly an income
   characterized in that said refiner contains AlTiC type phase particles, in that the amount of refining is chosen so that the average size of the foundry grain of said raw form is greater than 500 μm and that said alloy is an AA6056 alloy or an AA6156 alloy.

Another object of the present invention is a rolling plate that can be obtained by the casting process.

Yet another object of the present invention is a sheet capable of being obtained from the process or from the rolling plate according to the invention.

Description of figures

The figure 1 shows the influence of refining and titanium content on the parameter p *. The figure 2 shows the influence of refining and titanium content on the parameter s *. In this two figures, the black triangle represents an alloy refined with TiB ₂ , while the other alloys have been refined with AlTiC.

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. When the concentration is expressed in ppm (parts per million), this indication also refers to a mass concentration.

The designation of the alloys follows the rules of THE ALUMINUM ASSOCIATION. 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 as well as in the publications of THE ALUMINUM ASSOCIATION. These rules, standards and publications are known to those skilled in the art. The term "alloy of the 6xxx series" or "alloy of the Al-Mg-Si type" is understood to mean aluminum alloys (i) whose chemical composition falls into one of the standardized designations of an alloy of the 6xxx series, or (ii) that is derived from an alloy corresponding to such a standardized designation by the addition or deletion of one or more chemical elements other than silicon or magnesium, and / or by the exceedance (to the up or down) of the standardized concentration limit of one or more chemical elements (including silicon and magnesium), it being understood that in both cases (i) and (ii), the application of the Standardized designation should lead to storing this modified alloy in the 6xxx series.

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 growth rate (so-called da / dn test) according to ASTM E 647, and the critical stress intensity factor K _C , Kco 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 "structural element" or "structural element" of a mechanical construction a mechanical part whose failure is likely to endanger the safety of said construction, its users, its users or others. For an aircraft, these structural elements include the elements that make up the fuselage (such as fuselage skin (fuselage skin in English), stiffeners or stringers, bulkheads, fuselage (circumferential frames)), the wings (such as the wing skin), the stiffeners (stringers or stiffeners), the ribs (ribs) and spars) and the empennage composed in particular of horizontal stabilizers and vertical (horizontal or vertical stabilizers), as well as floor beams, seat rails and doors.

b) Detailed description of the invention

The present invention is applicable to wrought alloys AA6056 and AA 6156 It is based on the discovery that the refining of an aluminum alloy using a refining containing AlTiC-type phases added in the right proportion makes it possible to obtain a very particular microstructure of the cast raw form, and in particular a grain size greater than 500 μm and a regular distribution of intermetallic phases, observed by optical microscopy at a magnification Typically 50. After hot transformation according to known processes, optionally followed by a cold conversion and a heat treatment, in particular for highly recrystallized products, wrought products are obtained which show, in a manner surprisingly, a significantly better toughness and a lower crack propagation rate than products made from raw forms obtained by known methods. The term "highly recrystallized product" means a product for which the fraction of recrystallized grains measured between the quarter-thickness and the half-thickness of the finished products is greater than 70%. In an advantageous embodiment of the invention, the products from step (f) are strongly recrystallized. It is known for weakly recrystallized products that the casting microstructure can be reflected up to the properties of the transformed product (for example hot-rolled, cold-rolled and heat-treated), but in the present case the mechanism of this surprising phenomenon does not occur. has not yet been elucidated in terms of structural metallurgy. The product produced by the process according to the invention differs from the products according to the state of the art by the presence of AlTiC type phases. By "AlTiC type phases" we mean any ternary Al-Ti-C phase as well as any Ti-C binary phase in an aluminum matrix; this term notably includes the AlTiC ₂ and TiC phases. These phases are typically added in a refining yarn. Despite the small quantity of these phases, their effect on the microstructure of casting is very clear. Since wire refining containing AlTiC-type phases can substitute for the boron-containing wire refining (such as AT5B) commonly used, the raw form produced by the process according to the invention may contain less than 0, 0001% boron.

The casting microstructure obtained by the process according to the invention is characterized by two parameters, p * (dimension [μm]) and s * (dimension [μm ^-1 ]). These parameters characterize more particularly the smoothness and the uniformity of the microsegrégation. The parameter p * characterizes the average distance between precipitates in the solidification structures, and therefore the average size of the zones without precipitates. The parameter s * characterizes the uniformity of the distribution of these distances. The precise definition of these two parameters as well as the method for their determination are specified in the article "Quantification of Spatial Distribution of As-Cast Microstructural Features" by Ph. Jarry, M. Boehm, and S. Antoine, published in Proceedings of the Light Metals 2001 Conference, Ed. JL Anjier, TMS, p. 903 - 909 . The determination of the parameter p * has been the subject of an inter-laboratory test within the framework of the European project VIRCAST, see the article of Ph. Jarry and A. Johansen "Characterization by the p * method of eutectic aggregates spatial distribution in 5xxx and 3xxx aluminum alloys cast in wedge molds and comparison with SDAS measurements", published in Solidification of Alloys, MG Chu ed., DA Granger and Q. Han, TMS 2004 .

1. a. acquisition de l'image
2. b. seuillage des phases noires et analyse binaire des images présentant des niveaux de gris,
3. c. suppression des phases de très petite taille (pour un grandissement de 50, un groupe de moins de 5 pixels est considéré comme du bruit électronique),
4. d. analyse numérique de l'image à l'aide d'un algorithme de fermeture.

Parameters p * and s * are based on optical microscopy analysis of polished slices of the bulk form at a magnification typically of 50, or any other magnification that achieves a good compromise between a representative sample of the microstructure studied and the resolution necessary. Image acquisition is typically performed by a color CCD camera (charge-coupled device) connected to an image analysis computer. The analysis procedure, described in detail in the aforementioned article by Ph. Jarry, M. Boehm and S. Antoine, comprises the following steps:

1. at. image acquisition
2. b. thresholding of black phases and binary analysis of images with gray levels,
3. vs. suppression of very small phases (for a magnification of 50, a group of less than 5 pixels is considered as electronic noise),
4. d. digital analysis of the image using a closure algorithm.

Digital image analysis is an iterative closing of the image with a step up. The step i which closes the image C _i is defined by i successive dilations of the image of the same object (a dilation consisting of the replacement of each pixel of an image by the maximum value of its neighbors) followed by i successive erosions of the image of the same object (an erosion consisting of the replacement of each pixel of an image by the minimal value of its neighbors) of the image d (note that erosion and expansion operations are not commutative). The surface ratio A, which represents the surface fraction of the objects, is plotted as a function of the number of closing steps i . We obtain a sigmoidal curve, which is then adjusted by a sigmoidal function in order to extract the characteristic parameters p * and s *, knowing that p * is the abscissa of the point of inflection, expressed in units of length, and s * the slope at the point of inflection of the sigmoidal curve.

dans laquelle
A désigne la fraction surfacique d'objets après transformation,
A_min désigne la fraction surfacique initiale de particules intermétalliques après seuillage,
A_max désigne leur fraction surfacique correspondant au remplissage conventionnel auquel on arrêt l'algorithme (en pratique 90%) afin d'éviter les problèmes de convergence lente en fin de remplissage,
i est le nombre de pas de calcul,
et α est un coefficient d'ajustement de la pente de la sigmoïde.The parameter p * is thus defined by the equation $$\mathrm{AT}={\mathrm{AT}}_{\min }+\frac{{\mathrm{AT}}_{\max }-{\mathrm{AT}}_{\min }}{\left(1+\exp \left(\alpha \left(p*-i\right)\right)\right)}\mathrm{.}$$

in which
A denotes the surface fraction of objects after transformation,
A _min denotes the initial surface fraction of intermetallic particles after thresholding,
A _max denotes their surface fraction corresponding to the conventional filling at which the algorithm is stopped (in practice 90%) in order to avoid problems of slow convergence at the end of filling,
i is the number of calculation steps,
and α is an adjustment coefficient of the slope of the sigmoid.

The parameter p * represents the average distance between particles present in the matrix.

Il a été montré que 1/s* est proportionnel à l'écart type de la distribution des distances au premier voisin entre particules. Le paramètre s* est donc une mesure de la régularité de la distribution des phases dans la matrice.The other parameter is s * defined by the equation $$s*=\frac{\alpha \times \left({\mathrm{AT}}_{\max }-{\mathrm{AT}}_{\min }\right)}{4}$$

It has been shown that 1 / s * is proportional to the standard deviation of the distance distribution to the first neighbor between particles. The parameter s * is therefore a measure of the regularity of the phase distribution in the matrix.

The description of the casting structure by the parameters s * and p * therefore takes into account both the smoothness and the uniformity of microsegregation. The Applicant has found that it is no longer significant to describe the regularity of the particle distribution, whereas p * is more significant for describing the smoothness of the particle distribution. their spatial distribution. In a preferred embodiment of the invention, a rolling plate is manufactured according to the method of the invention, so as to obtain a value of s * greater than 0.92 μm ^-1 , and preferably greater than 0.94. ^-1 .mu.m and simultaneously obtain a value of p * of less than 107 .mu.m.

According to the invention, the raw form obtained at the end of the casting, such as a spinning billet, a forge billet or a rolling plate, is transformed hot and optionally cold to its final thickness. The product of final thickness is then subjected to a solution and quenching heat treatment, followed by controlled tensile stress relieving with a permanent elongation of between 0.5 and 5%, and optionally followed by an income. If the permanent elongation obtained during tensioning by controlled tensing is less than 0.5%, the product does not reach sufficient flatness. If the permanent elongation achieved during controlled tensile stress relieving is greater than 5%, the damage tolerance properties may be affected.

The process according to the invention is particularly well suited for producing wrought products made of alloy AA6056, AA6156 or similar alloys. For these two alloys, it is preferred to limit the iron content to 0.15%, and even 0.13%, in order to reduce the tendency to microsegregate during casting. An advantageous embodiment for heat-treated alloys comprises converting the hot-rolling sheet into a sheet having a thickness of between 3 and 12 mm, and heat treating to the T6 state. Applied to alloys AA6056 or AA6156, this process leads to a sheet with a tolerance to damage K _R , determined in TL direction for a crack extension Δa _eff of 20 mm from a curve R measured according to ASTM E561, from minus 115 MPa√m, and preferably at least 116 MPa√m.

It is also possible, using known procedures, to apply a plating on one or both sides of said rolling plate, after scalping or possibly after a first hot rolling sequence; for example, this may be advantageous with alloys AA6056 and AA6156.

A sheet of AA6056 or AA6156 alloys manufactured by the method according to the invention also has in the T6 temper in a thickness between 3 and 12 mm a damage tolerance K _R determined in the TL direction to an extension of cracks .DELTA.a _eff 60 mm obtained from a curve R measured according to ASTM E561, of at least 175 Pa√m.

Moreover, its crack propagation speed da / dn in the TL direction, measured according to ASTM E 561 on a panel of width w = 400 for Δk = 50 NTa√m and R = 0.1, is less than 2 10 ^-2 mm / cycle.

In industrial practice, the improvement of the parameter K _R which results from the method according to the present invention may make it possible to increase the guaranteed minimum value of this parameter for a given constraint, knowing that this parameter, like all the parameters which characterize a product metallurgical, always shows a certain statistical dispersion.

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

Examples: Example 1

An AA6056 alloy was cast into two industrial-size rolling plates, in particular 446 mm thick, at a speed of 55 mm / min and at a temperature of 680 ° C. The chemical composition included (in mass%): If 0.81 Mg 0.70 Cu 0.93 Mn 0.49 Fe 0.09.

Table 1 gives the refining method (AlT3C0,15 or AT5B wire.) The designation A1T3C0,15 corresponds to a composition Al-3% Ti-0,15% C. The designation AT5B corresponds to an Al-5% Ti composition. -1% B, this product is also known under the commercial designation "AlTiB 5: 1"), the Ti content (in ppm mass), the inoculation rate and mean values for the parameters s * and p * as defined above. The parameters s * and p * were determined on samples cut at about 140 mm from the skin and at the third width of the rolling plates. Table 1 Reference Ti [ppm] Inoculation rate refining s * p * [Kg / t] 4032A 180 0.7 AT5B 0.88 110 4032B 180 0.5 AlT3C0,15 0.99 101

From these rolling plates, plated plates having a final thickness of 5 mm in the T6 state were produced using the same transformation range comprising homogenization, hot rolling, dissolution, quenching. , controlled traction stress relief and income. The permanent elongation obtained during controlled tensile stress relaxation was 1.5%. The fraction of recrystallized grains measured between the quarter-thickness and the mid-thickness of the finished products was close to 100%.

The static mechanical characteristics and the damage tolerance of these sheets have been determined. The results are collated in Table 2. The parameter K _{R (20)} refers to a crack extension value Δa _eff of 20 mm.

The crack propagation rate da / dn according to ASTM E 647 was also determined from a sheet of width w = 400 mm in the TL direction, with a ratio R = 0.1. Table 2 Reference / Parameter 4032A 4032B R _{m (L)} [MPa] 369 373 R _{p0.2 (L)} [MPa] 353 355 A _(L) [%] 15.0 14.2 R _{m (TL)} [MPa] 372 375 R _{p0.2 (TL)} [MPa] 340 342 A _(TL) [%] 13.0 12.5 K _{R (20) (TL)} [MPa√m] 113 119 K _{R (40) (TL)} [MPa√m] 148 153 K _{R (60) (TL)} [MPa√m] 172 178 da / dn for Δk = 10 MPa√m [mm / cycle] 1.10 10 ^-4 1.5 10 ^-4 da / dn for Δk = 30 MPa√m [mm / cycle] 3.62 10 ^-3 2.90 10 ^-3 da / dn for Δk = 50 MPa√m [mm / cycle] 2.62 10 ^-2 1.85 10 ^-2

It can be seen that the static mechanical characteristics of the two sheets hardly differ significantly. However, resistance to damage, represented by the _R parameter K, increases significantly when refining molten metal was carried out with a wire containing AlTiC type phases. For the latter product, the crack propagation rate is lower when the stress intensity factor reaches about 30 MPa√m.

Example 2

Other AA6056 alloy rolling plates were cast using the process according to the invention. The refining and casting microstructure parameters are summarized in Table 3. Table 3 Reference Ti [ppm] Inoculation rate [kg / t] refining s * p * 4031A 50 0.5 AlT3C0,15 0.95 106 4031B 50 1 AlT3C0,15 0.98 101 4033A 430 0.5 AlT3C0,15 1.00 99 4033B 430 2 AlT3C0,15 1.04 87 4034A 630 0.5 AlT3C0,15 0.98 97 4034B 630 2 AlT3C0,15 1.01 94 4035A 80 0.5 AlT3C0,15 0.99 95 4035B 80 0.5 AlT3C0,15 0.98 96

On the basis of the data and results of Tables 1 and 3, the figure 1 gives a comparison of the fineness of the casting microstructures (parameter p *) as a function of the Ti content and the type of refining agent. Similarly, figure 2 gives a comparison of the regularity of the casting microstructures (parameter s *).

Commentary on Examples 1 and 2 :

Table 4 summarizes the total Ti content in the alloys of Examples 1 and 2, as well as the size of the foundry grains. Table 4 Reference refining Ti [ppm] Fe [%] Grain size Type Kg / t Average [μm] Standard deviation IC 4031A AlTiC 0.5 50 0.09 902 214 153 4031B AlTiC 1 50 0.09 655 101 72 4032A AT5B 0.7 180 0.08 388 38 27 4032B AlTiC 0.5 180 0.08 713 112 80 4033A AlTiC 0.5 430 0.07 757 143 102 4033B AlTiC 2 430 0.07 664 200 143 4034A AlTiC 0.5 630 0.2 833 201 144 4034B AlTiC 2 630 0.2 644 113 81 4035A AlTiC 0.5 80 0.2 771 171 122 4035B AlTiC 0.5 80 0.2 822 118 84

• Un affinage classique à 0,7 kg/t d'ATB5 introduit environ 7 ppm de B. Un affinage avec 1 kg/t de fil de type AT3C0.15 tel qu'utilisé pour ces essais introduit environ 1,5 ppm de C. Un affinage de 0,5 kg/t du même fil introduit la moitié, soit environ 0,75 ppm de C, alors qu'un affinage de 2 kg/t introduit le double, soit environ 3 ppm. Pour le titane, un affinage de 1 kg/t de AT3C0.15 introduit environ 30 ppm, un affinage de 0,5 kg/t la moitié (environ 15 ppm), et un affinage de 2 kg/t le double (environ 60 ppm).

The content of Ti and C provided by the ripening yarn can be calculated from the inoculation rate and the composition of the yarn:

• A conventional refining at 0.7 kg / t ATB5 introduces about 7 ppm B. A refining with 1 kg / t of AT3C0.15 type wire as used for these tests introduces about 1.5 ppm of C. A refining of 0.5 kg / t of the same thread introduces half, or about 0.75 ppm of C, while a refining of 2 kg / t introduced the double, or about 3 ppm. For titanium, a refining of 1 kg / t of AT3C0.15 introduces about 30 ppm, a refining of 0.5 kg / t half (about 15 ppm), and a refining of 2 kg / t double (about 60 ppm).

Claims (14)

1. Process for manufacturing aluminium alloy products, with high toughness and fatigue resistance that includes the following steps:

   (a) an aluminium alloy bath is prepared,

   (b) a refining agent containing particles of AlTiC type phases is added into the said bath,

   (c) an as-cast form such as an extrusion ingot, a forging ingot or a rolling ingot is cast,

   (d) the said as -cast form is hot transformed, possibly after scalping, to form a blank or a product with the final thickness,

   (e) optionally the blank is cold transformed to its final thickness,

   (f) a solution heat treatment and quenching is applied to the product output from step (d) or (e), followed by relaxation by controlled stretching with permanent elongation between 0.5 and 5%, and possibly annealing,

   characterised in that the quantity of refining agent is chosen such that the average casting grain size of the said as-cast form is more than 500 µm and in that said alloy is an AA6056 or AA6156 alloy.
2. Process according to claim 1, characteri sed in that the quantity of refining agent is chosen such that there is a uniform distribution of intermetallic phases of the said as cast form, observed by an optical microscope with a magnification of 50.
3. Process according to claim 1 or 2, characteris ed in that the recrystallised fraction measured between the quarter thickness and the mid -thickness of products output from step (f) is greater than 70%.
4. Process according to claims 1 to 3, characterised in that said as-cast form contains less than 0.0001% of boron.
5. Process according to any one of claims 1 to 4, in which the iron content does not exceed 0.15%, and preferably does not exceed 0.13%.
6. Process according to any one of claims 1 to 5, in which the said as-cast form is a rolling ingot.
7. Process according to claim 6, in which the said rolling ingot is cladded on one or both sides, after scalping or possibly after a first hot rolling sequence.
8. Rolling ingot made of an alloy AA6056 or AA6156 with an average casting grain size higher than 5 00 µm that can be obtained by a process including steps (a) to (c) of the process according to any one of claims 1 or 4 to 7 characterized by a parameter s* greater than 0.92 µm^-1, and preferably greater than 0.94 µm and by a parameter p* of less than 107 µm,
   wherein parameter p* is defined according to the equation $$A=A_{\min }+\frac{A_{\max }-A_{\min }}{\left(1+\exp \left(\alpha \left(p*-i\right)\right)\right)}$$

   

   
   and wherein parameter s* is defined by the equation $$s*=\frac{\alpha \times \left(A_{\max }-A_{\min }\right)}{4}$$

   

   
   in which:

   A denotes the surface area fraction of objects after transformation,

   A_min denotes the initial surface area fraction of intermetallic particles after thresholding,

   A_max denotes their surface area fraction corresponding to conventional filling at which the algorithm is normally stopped (in practice 90%) in order to avoid slow convergence problems at the end of filling,

   i is the number of calculation steps,

   and α is a sigmoid slope adjustment factor.
9. Rolling ingot according to claim 8 characterized in that the boron content is at least 0,0001%.
10. Rolled sheet that can be obtained star ting from a rolling ingot according to any one of claims 8 to 9
11. Rolled sheet made from an AA6056 or AA6156 alloy according to claim 10, characterised in that it is in the T6 temper with a thickness between 3 and 12 mm, a damage tolerance K _R determined in the T -L direction for a crack extension of Δa_eff equal to 20 mm using an R curve measured according to ASTM E561, equal to at least 115 MPa √m, and preferably at least 116 MPa√M.
12. Rolled sheet made from an AA6056 or AA6156 alloy according to claim 10 or 11, characterised in that it is in the T6 temper with a thickness between 3 and 12 mm, has a damage tolerance K_R determined in the T-L direction for a crack extension Δa_eff equal to 60 mm using an R curve measured according to ASTM E561, equal to at least 175 MPa√m.
13. Rolled sheet made from an AA6056 or AA6156 alloy according to one of claims 10 to 12, characterised in that its crack propagation rate da/dn in the T -L direction, measured according to ASTM E 561 on a panel with width w = 400 for Δk = 50 MPa√m and R = 0.1, is less than 2 x 10^-2 mm/cycle.
14. Rolled sheet made from an AA6056 or AA6156 alloy according to one of claims 10 to 13 characterized in that it is cladded on one or two sides.

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