FR3074191A1 - FLAT ALUMINUM ALLOY PRODUCT HAVING IMPROVED PROPERTIES IN THICKNESS - Google Patents

Abstract

The invention relates to a flat aluminum alloy product with a thickness greater than 12.5 mm having improved properties in the thickness. This product is obtained by adjusting the stirring conditions during casting of the ingot used to obtain the flat product. The casting process is characterized by a melt flow induced by a non-stationary external force field or a melt-induced casting induced by a stationary stationary external force field. The transformation process comprises solution dissolution at a temperature above 450 ° C, quenching and tempering or ripening to obtain a flat product of aluminum alloy. Optionally, a scalping operation may be performed during the transformation process in order to eliminate unacceptable surface defects if a rolling operation is subsequently performed. The invention makes it possible in particular to increase the thickness of the cast product without altering or even improving the elongation in the cross-cutting direction of the final product.

Description

FLAT ALUMINUM ALLOY PRODUCT HAVING IMPROVED PROPERTIES IN THICKNESS

Field of the invention

The technical field of the invention relates to thick flat aluminum alloy products obtained from ingots obtained by vertical casting.

State of the art

Thick flat aluminum alloy products, in particular the structurally hardened aluminum alloys of the 7XXX, 2XXX series, from a thickness greater than 12.5 mm, have differences in properties depending on the positioning in the thickness. These differences in properties are associated, among other things, with the macro-aggregations observed on the cast ingot and which remain on the flat product obtained after a process of transformation of the ingot; the transformation process comprising dissolving, quenching and tempering or maturing.

A macro-aggregation well known to those skilled in the art is the negative central macro-aggregation, resulting from a depletion of eutectic alloy elements, along a part of the median plane parallel to the large faces of the ingot. These macro-aggregations have been described in the work of John Wiley et al "Direct-Chili Casting of light alloys", Editor Wiley, September 2013, pp 158 - 172. It is a continuous macro-aggregation, this term designating the fact that the macro-aggregation takes place continuously over all or part of the height of the ingot, in other words that it is essentially uniform along the axis of casting.

There is also the phenomenon of intermittent macro-aggregation which results in the formation of V-shaped bands on both sides of the negative central macro-aggregation. The publication R.C Dorward et al. "Banded segregation patterns in DC cast AlZnMgCu alloy ingots and their effect on plate properties" Aluminum, 1996, 72. Jahrgang, 4, p.251-259 describes the formation of intermittent segregation bands in a 7000 alloy. V are alternately enriched and depleted (respectively depleted and enriched) in eutectic alloy elements (respectively peritectic). These bands can be observed by X-ray radiographs of vertical slices of ingots, typically in the L / TC plane at half-width, when the segregated elements absorb X-rays in a differentiated way from the atoms of the metal composing the ingot. Other means make it possible to visualize this phenomenon, for example ultrasound or the observation with the naked eye of anodized vertical sections, due to the difference in optical reflectivity between the zones enriched or depleted in alloying elements. Generally, intermittent macro-segregation is most marked in the vicinity of region 172.5, and typically between 173.3 and Ί72.3, region 172 corresponding to the central axis of the ingot.

The thick flat products made of aluminum alloys, in particular the structurally hardened aluminum alloys of the 7XXX, 2XXX, or 6XXX series used in the aeronautical fields present from a thickness of 12.5 mm, more markedly between 40 mm and 200 mm, differences in properties depending on the positioning in the thickness of the product. In particular, this difference is marked between the mid-thickness and the quarter-thickness of the product.

It is known that three factors dominate the difference in properties between 172 and 174.

- The central macro-aggregation lowers the static properties at mid-thickness of the recrystallized sheets.

- The wrought: this deficit of properties at mid thickness is compensated, sometimes beyond, by the wrought effect on the non-recrystallized or partially recrystallized sheets which induces a fiber texture favorable for the properties.

- The quenching speed, especially for thick sheets.

The publication by Chakrabarti et al. "Through thickness property variations in 7050 plate", Materials Science Forum, 1996, Vols. 217-222, p. 1085-1090 indicates that the macro-aggregation dominates the properties at 172 for sheets of thickness less than 40mm which are recrystallized, while the texture induced by wrought dominates the properties for sheets of thickness greater than 40mm because they are not recrystallized .

Those skilled in the art seeking to counter the negative effects of macro-segregation therefore wish to increase the rate of metalwork from starting with thicker foundry plates. However, it is then confronted with intermittent macro-aggregation levels in the vicinity of 172.3 and 173.3 which introduce into the product zones which cannot be redissolved, particularly for the most saturated alloys, since the local solvus of the alloy in intermittent segregations may be beyond the solidus of the alloy of nominal composition. These intermittent macro-aggregations are harmful in particular for the value of elongation at break measured in the short transverse direction. Too low an elongation value in the short cross direction of the product is not desirable during the manufacture of aeronautical structures because of the greater risk of cracking.

The inventors therefore sought to obtain thicker flat products having an advantageous compromise between static mechanical properties and tolerance for damage, this compromise varying little in the thickness of the product. That is to say, the compromise of properties is obtained in the majority of the thickness of the product.

For these products to be selected for typical applications of thick flat products such as aeronautics, the manufacture of molds or vacuum chamber elements, their performance must in particular reach an advantageous property compromise between the properties of static mechanical resistance ( typically elastic limit in tension and compression, resistance to rupture) and the properties of tolerance to damage (typically toughness, resistance to the propagation of cracks in fatigue) and / or elongation these properties being in general contradictory.

Object of the invention

The object of the invention relates to a flat aluminum alloy product extending parallel to a longitudinal axis (L), and whose section perpendicular to the longitudinal axis has a width (TL) and a thickness (TC) , the thickness (TC) being less than the width, the thickness of said flat product being greater than 12.5 mm.

Preferably the flat product has a thickness greater than 40 mm, and even more preferably greater than 70 mm.

Said flat product is obtained from a manufacturing process comprising:

- A casting process with electromagnetic stirring to form an aluminum alloy ingot with a thickness greater than 400 mm.

Preferably, the thickness of the ingot is greater than 450 mm, 500 mm or 600 mm. The casting process is carried out in an ingot mold, the ingot mold defining a parallelepiped, so that the ingot formed extends parallel to a longitudinal axis (Y), along a width (W) and parallel to a transverse axis (X) , along a thickness (T), the thickness of the ingot being less than the width; the ingot defining a median plane extending along the mid thickness (T / 2), parallel to the longitudinal axis (Y). The thickness of the ingot mold is close to the thickness of the ingot formed.

And

a process for transforming said aluminum alloy ingot comprising dissolving at a temperature above 450 ° C., quenching and tempering or maturing in order to obtain a flat aluminum alloy product with optionally an operation of scalping.

The casting process with electromagnetic stirring comprises the following steps: casting of the liquid aluminum alloy in the mold, along a vertical casting axis (Z), the alloy being cooled, during casting, by a runoff '' a coolant (3), so as to form a solid alloy (ls), extending around a liquid alloy (1 £), called swamp, the solid alloy forming a front (10) at the interface with the marsh (1 £), the front being inclined at an angle of inclination (a), relative to the vertical axis, the angle of inclination of the front varying along the transverse axis (X);

displacement of the solid alloy (ls), along the vertical casting axis (Z), according to a casting speed (F);

during casting, application of a sliding magnetic field whose amplitude varies according to a frequency (/), the magnetic field being generated by at least one magnetic field generator (5) disposed at the periphery of the ingot mold, so as to apply an average Lorentz force (F) at different points of the marsh, the average Lorentz force, during a period (P) of the magnetic field, being inclined with respect to the vertical axis (Z) at an angle of inclination (β), said angle of inclination of the Lorentz force, the latter varying along the transverse axis (X); the Lorentz force present during period P a Lorentz force of maximum intensity (F _max );

the marsh has a median zone, extending symmetrically on either side of the median plane (M) whose thickness corresponds to a half-thickness of the ingot, the applied magnetic field propagates along an axis of propagation, so that a maximum amplitude (B ™ ^ax ) of the magnetic field propagates along said propagation axis, by defining a propagation wavelength (λ).

The casting process with electromagnetic stirring is such that it allows the attenuation of intermittent macro-aggregations or their disappearance. According to the invention, the casting method with electromagnetic stirring corresponds either to a non-stationary electromagnetic stirring mode, or to a so-called tacking stationary electromagnetic stirring mode, as defined below.

The non-stationary electromagnetic stirring mode is defined by a magnetic parameter called force, governing the Lorentz force of maximum intensity (F _max ); this magnetic force parameter is variable within a predetermined time interval (Δΐ), said parameter being:

- said maximum amplitude (B ™ ^ax ) of the magnetic field;

- And / or said frequency (f) of the magnetic field;

- And / or the propagation wavelength (λ) of the magnetic field;

so as to obtain a modulation, in said time interval, of said Lorentz force of maximum intensity (F _max ) propagating along the axis of propagation.

The electromagnetic stirring stationary stirring mode is defined by the fact that the sliding magnetic field propagates along an axis of propagation parallel to the vertical axis of casting; the frequency (f) is less than 5 Hz; and the casting speed (L) and the frequency (f) are adapted so that throughout the median zone of the marsh, at the interface between the liquid alloy (1 £) and the front, the angle of inclination of the force (β) is strictly less than the angle of inclination (a) of the forehead.

It has been observed that the flat aluminum alloy product thus obtained has for one element of the aluminum alloy, the weight content of which is greater than 0.5%, or for the sum of several elements of the alloy of which the individual content by weight is greater than 0.5%, a dispersion criterion ε less than 3.3, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5, the dispersion criterion being defined according to the following expressions:

ε = ISCza / ISCzr & Cza = max (Cza) - min (C _ZA ),

ACzr = max (C _ZR ) - min (C _ZR ), where:

max (Cza) and min (C _Z a) denote respectively the maximum and minimum concentrations of the element considered or of the sum of the elements considered measured in an analysis zone (ZA), exhibiting intermittent macro-aggregations, for example between the thicknesses T / 3.3 and T / 2.3 of said flat product;

max (C _Z r) and min (C _Z r) denote respectively the maximum and minimum concentrations of the element considered or of the sum of the elements considered measured in a reference zone (ZR), considered as little affected by intermittent macrosegregations , for example between the thicknesses T / 12 and T / 6 of said flat product, but always excluding the peripheral zones possibly affected by cortical macrosegregation;

said concentrations being measured on at least one profile established at mid-width in a vertical plane L / TC and in the direction TC, said profile being representative of said intermittent macrosegregations of the element considered in the direction TC.

Other advantages and characteristics will emerge more clearly from the description which follows of particular embodiments of the invention, given by way of nonlimiting examples, and represented in the figures listed below.

Description of the figures

Figures 1 and 2 show the main components of the casting device for implementing the invention.

FIGS. 3 and 4 respectively represent a spatial and temporal distribution of the amplitude of a non-stationary sliding magnetic field, according to one of the embodiments of the invention (casting with non-stationary electromagnetic stirring).

FIGS. 5 and 6 respectively present a spatial and temporal distribution of the amplitude of a stationary sliding magnetic field according to another embodiment of the invention (casting with stationary electromagnetic stirring plating).

FIG. 7 represents a curve called resonance of free surface of the marsh, representing values, called critical, of the intensity and the frequency of an induction current at which a resonance of the free surface of the marsh appears, this in implementing an electromagnetic stirring process.

FIGS. 8, 9, 10 respectively represent a so-called resonance curve CR of the free surface of the marsh, obtained by implementing a casting method using stirring by non-stationary magnetic field in the respective examples 1, 3 and 4 according to invention.

FIG. 11 illustrates a Lorentz force applying to a part of the marsh of a casting implementing the invention according to a stationary stirring mixing process.

FIGS. 12A, 12B, 12C and 12D show results of simulations making it possible to obtain the average orientation of a Lorentz force in the marsh, at the level of the front, respectively at different frequencies, for a casting speed of 55 mm. per minute by implementing a casting with stationary electromagnetic stirring.

Figures 12E, 12F, 12G and 12H show results of simulations making it possible to obtain the average orientation of a Lorentz force in the swamp, at the front, respectively at different frequencies, for a casting speed of 35 mm per minute using casting with stationary electromagnetic stirring.

FIGS. 13A and 13B are curves established by considering respectively different frequencies, and representing an evolution of an angle, called differential angle, along the front, the differential angle representing a difference between the angles, relative to the vertical, respectively formed by the Lorentz force and the front. In FIG. 13A, a casting speed of 55 mm per minute has been considered. In FIG. 13B, a casting speed of 35 mm per minute has been considered.

FIG. 14 shows an abacus making it possible to define an operating domain as a function of the speed of casting (abscissa axis) and of the thickness of the casting (ordinate axis), to obtain an orientation of the Lorentz force in a middle zone extending between 172 ± T / 4 according to a casting process with stationary electromagnetic stirring plating.

Description of the invention

In the preamble, we call a flat product a product of parallelepiped shape extending parallel to a longitudinal axis (L), and whose section perpendicular to the longitudinal axis has a width (TL) and a thickness (TC), the thickness (TC) being less than the width. All the aluminum alloys mentioned in the document are designated, unless otherwise indicated, according to the designations defined by 1 "Aluminum Association" in the "Registration Record Sériés" which it publishes regularly.

All information regarding the chemical composition of the alloys is expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 x Si means that the silicon content expressed in% by weight is multiplied by 1.4.

The definitions of metallurgical states are given in European standard EN 515.

The static mechanical characteristics in tension, in other words the tensile strength R _m , the conventional elastic limit at 0.2% elongation R _p o.2, and the elongation at break A%, are determined by a tensile test according to standard NF EN ISO 6892-1 (2016), the sampling and the direction of the test being defined by standard EN 485 (2016).

The stress intensity factor (Kic) is determined according to standard ASTM E 399 (2012). Grain sizes are measured according to ASTM El 12-12.

The reduction in resistance by notch effect is determined by standard E 602-03.

According to a nomenclature known to a person skilled in the art, the term T / n, where n is a positive number, designates a region located at a distance T / n from the surface of a parallelepipedic product where T designates the thickness of the product , said parallelepiped product extends parallel to a longitudinal axis (L), and whose section perpendicular to the longitudinal axis has a width and a thickness, the thickness being less than the width.

The inventors have realized that a flat product obtained from an ingot obtained according to the conditions of the casting process of the invention and then transformed according to a transformation process comprising a working, a dissolution, quenching and tempering or maturing allows to obtain a good compromise between toughness, elastic limit and elongation. In particular, the compromise is obtained for the toughness defined according to the standard E399-12 measured at mid thickness in the direction LT, the elastic limit measured in the longitudinal direction (L) and the elongation at break measured in the short cross direction (TC).

However, the wrought solution is not always possible depending on the thickness of the desired product. This is particularly the case for flat aluminum alloy products, in particular the structurally hardened aluminum alloys of the 7XXX series, used in the fields of mechanical mold machining. These are conventionally used at thicknesses greater than 400 mm, preferably greater than 500 mm, even more preferably greater than 600 mm.

The inventors have realized that flat products obtained from an ingot of thickness greater than 400 mm, preferably greater than 500 mm, even more preferably greater than 600 mm obtained according to the conditions of the casting process of the invention and then transformed according to a transformation process comprising dissolving, quenching and tempering or maturing but not including a plastic deformation step greater than 5% made it possible to obtain a reduced difference in properties in the thickness. In particular, this improvement is observed for the elongation at break measured between the surface and the mid-thickness in the direction TL and the reduction in resistance by notch effect measured at mid-thickness and quarter-thickness in the direction TL.

The invention is based on the observation made by the inventors that the combination of the casting process described in applications PCT / FR2017 / 051195 or FR 1761200 to obtain an aluminum alloy ingot of thickness greater than 400 mm and a process processing comprising dissolving at a temperature above 450 ° C, quenching and tempering or maturing provides a better compromise of properties in the thickness of the processed flat product.

The inventors attribute this benefit to the reduction of intermittent macro-aggregation in the processed flat product.

Applications PCT / FR2017 / 051195 and FR 1761200 both aim to reduce intermittent macro-segregation in the cast ingot by using a sliding magnetic field. The casting conditions described in application PCT / FR2017 / 051195 making it possible to reduce intermittent macro-aggregations will be called casting with non-stationary electromagnetic stirring. We will call casting with stationary electromagnetic stirring plating the conditions of casting described in the application FR 1761200 making it possible to reduce intermittent macrosegregations.

The flat product extends parallel to a longitudinal axis (L), and whose section perpendicular to the longitudinal axis has a width (TL) and a thickness (TC), the thickness (TC) being less than the width, the thickness of said flat product being greater than 12.5 mm.

Preferably the flat product has a thickness greater than 40 mm, and even more preferably greater than 70 mm.

The intermittent macro-aggregations of the flat product can be characterized by performing chemical analyzes according to the thickness of the product, preferably linear profiles along the TC axis in the L-TC plane of the product. These profiles are preferably produced at quarter width or half width. These chemical analyzes can be carried out by microprobe or by any other method with a spatial resolution of at least 0.1 mm.

The profile obtained with a resolution of at least 0.1 mm is the raw profile. A sliding average of 2 mm eliminates microsegregation and provides a smooth profile. Another sliding average of the gross profile over 50 mm makes it possible to get rid of intermittent macro-aggregation, and to obtain the continuous macro-aggregation profile. A continuous macro-aggregation profile is subtracted from the smoothed profile to obtain a so-called corrected profile, corresponding to intermittent macro-aggregation. The corrected profile is mainly representative of intermittent macro-aggregation, and is not or only slightly affected by central continuous macro-aggregation and by micro-aggregation. Such a corrected profile makes it possible to characterize the intermittent macro-aggregation.

We can then calculate a maximum concentration difference in a Za analysis zone located between T / 3.3 and T / 2.3 of the thickness of the flat product, this maximum difference can be expressed according to the following equation:

ACza = max (Cza) - min (Cza) where max (Cza) and min (Cza) denote respectively the maximum and minimum concentrations of the element considered measured between T / 3.3 and T / 2.3.

The element considered is an element whose content by weight in the alloy is greater than or equal to 0.5%. It can preferably be the major element of the alloy, the term major element corresponding to the definition given by The Aluminum Association.

The maximum deviation ACza can be normalized by the nominal concentration Co of the element considered. The products according to the invention preferably have a value of such a standardized ratio of less than 10% and preferably less than 8% or even less than 6%. However, the absolute value of ACza can be influenced by the thickness of the product, the nature of the element considered, in particular its partition coefficient and / or its concentration. It is therefore useful to characterize the products obtained by the method according to the invention to calculate, by way of reference, a maximum deviation in a reference zone Zr not very sensitive to intermittent macro-aggregations, situated between T / 12 and 176, this maximum deviation can be expressed according to the following equation:

AC _Z r = max (Czr) - min (Czr) where max (Czr) and min (Czr) denote respectively the maximum and minimum concentrations of the element considered measured between 1712 and 176.

We thus obtain a dispersion criterion ε allowing to evaluate for the element considered the intermittent macro-aggregation:

ε = ACza / AC _Z r

To overcome local variations in composition, it is advantageous, to determine ACza and ACzr, to calculate an average over at least five concentration profiles at least mm apart.

The lower ε, the less marked the intermittent macro-segregations. The products obtained by the process according to the invention preferably have a dispersion criterion ε less than 3.3, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5.

This dispersion criterion can also be applied to several elements whose individual content is greater than 0.5%. In the case where we consider the sum of several elements, the values to normalize the maximum deviation ACza, and / or the Fourier transform correspond to the sum of the nominal concentrations of the elements considered.

It is also useful to carry out a Fourier transform analysis of the raw composition profile and to normalize it by the nominal composition of the element. Such an analysis makes it possible to identify spatial periods characterizing intermittent macro-aggregation. The inventors have been able to observe that the intermittent macro-segregation has periods between two values defined as a function of the reduction rate R applied to the ingot during the transformation process. Typically intermittent macro-aggregation has periods between 8 / R and 25 / R mm.

The reduction ratio is the ratio between the thickness of the ingot obtained by the casting process and the thickness of the final flat product obtained after working or if a scalcape is carried out the thickness obtained after scalping and the thickness of the final flat product. obtained after wrought. For example, in the case of hot rolling, if the cast ingot has a thickness of 550 mm and it is scalped to 500 mm to remove the cortical layer on the surface or possible defects, if it is rolled at a final thickness of 100 mm, the reduction rate is equal to 5.

When the intermittent macro-segregation is significant, and the ingot does not undergo plastic deformation greater than 5%, the reduction rate is substantially equal to 1. We therefore observe in this case a peak in the amplitude of the Fourier components for spatial periods between 8 and 25 mm.

A dimensionless spectral intensity criterion ζ is determined which corresponds to the maximum amplitude of the Fourier components in a range of spatial period between 8 / R and 25 / R mm, normalized by the nominal concentration Co of the element considered. The products obtained by the process according to the invention preferably have a criterion ζ less than 0.01, preferably less than 0.007 and preferably less than 0.005.

The dispersion ε and spectral intensity ζ criteria are advantageously applied to the major element of the alloy considered, typically Zn for a 7xxx alloy or Cu for a 2xxx alloy. These criteria can also be applied to the sum of several elements whose individual content is greater than 0.5%, for example the sum Zn + Cu in certain alloys 7xxx or the sum Mg + Si in the alloys 6xxx.

In the case where we consider the sum of several elements, the values to normalize the maximum deviation ACza, and / or the Fourier transform correspond to the sum of the nominal concentrations of the elements considered.

Figures 1 and 2 illustrate an ingot mold allowing an implementation of the invention. In this example, an aluminum alloy 1 flows into an ingot mold 2, through an opening 2i. The casting takes place along a vertical Z axis, through the ingot mold. The ingot mold is delimited by a peripheral enclosure the section of which, in a horizontal XY plane, is parallelepiped. The ingot mold defines a frame, parallel to a longitudinal axis Y, along a width W, and, parallel to a transverse axis X, by defining a thickness T. The width W is greater than the thickness T. The thickness T corresponds to a distance between two vertical walls 2p delimiting the ingot mold 2. The casting forms a parallelepipedal ingot. A plane, called median plane M, extends at mid-thickness (T / 2), parallel to the vertical axis Z and the longitudinal axis Y of the ingot. The thickness T is greater than 400 mm, preferably greater than 500 mm and even more preferably greater than 600 mm. The thickness T is preferably between 400 mm and 750 mm.

A cooling fluid 3, for example water, flows against the wall of the solidified ingot. This process is known as semi-continuous casting by direct cooling ("DirectChili Casting"). A false bottom 4 is translated so as to move away from the opening 2i during the casting. The translation speed of the false bottom corresponds to a so-called casting speed V.

Under the effect of cooling, a zone of solid alloy ls is formed, near the cooled enclosure, around a zone of liquid alloy IL, designated by the term "marsh". The interface between the swamp lf and the solid zone ls forms a front 10. After cooling, the ingot is formed. The front 10 has a slope, relative to the vertical, which varies according to the position in the thickness. The angle of the front is called an angle a between the tangent to the front, at a point, and the vertical, that is to say the axis Z. The smaller the angle of the front a, the more the tangent to forehead is oriented vertically. The angle of the front is shown in FIG. 11. The angle of the front varies along the transverse axis X.

In the example shown in Figure 1, the front 10 is stationary: it remains in substantially the same position, while the material moves vertically, at the casting speed.

In the methods according to the prior art, intermittent macro-segregations 11 are formed in the ingot, and in particular in a thickness range between 173.3 and Ί72.3 on either side of the median plane M.

The alloy is an aluminum alloy from the 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX, 7XXX or 8XXX series. Alloys whose mass fraction of alloying elements is greater than 1%, even greater than 3% or even 5% are particularly suitable for a method according to the invention, because the more this mass fraction of these alloying elements is important, the more marked the intermittent segregations. The invention is particularly advantageous for alloy products of the 2XXX, 6XXX or 7XXX type.

There is shown a magnetic field generator 5, capable of generating a magnetic field B intended to be applied to the liquid alloy lf. Such a generator can be a mobile permanent magnet or an electromagnetic inductor, the latter generating a magnetic field when it is traversed by an electric current, called induction current.

The magnetic field B applied to the liquid alloy lf is an alternating field, of amplitude B _o and of frequency f. The effect of this magnetic field is to apply a mixing of the marsh, by means of Lorentz forces applying to the liquid alloy lf. Indeed, the application of a magnetic field B generates, in the alloy, the formation of an electric current J resulting, within the liquid alloy lf subjected to the magnetic field, in the appearance of a force of

Lorentz F such that F oc J x B where x denotes the vector product operator, and oc denotes a relation of proportionality. The Lorentz force has an oscillating component at a frequency twice the frequency f of the magnetic field.

Due to the thickness of the mold, the frequency f is chosen so as to allow sufficient penetration of the magnetic field B into the swamp, so as to obtain efficient mixing of the liquid. The frequency f is lower the higher the thickness of the product. In the case of an ingot with a thickness greater than 400 mm, the frequency is preferably less than 5 Hz, and even more advantageously less than 2 Hz or 1Hz.

The generator 5 is able to generate a sliding magnetic field. The term sliding magnetic field designates an alternating magnetic field, the amplitude B _{o of which} is not constant, and varies between a minimum value and a maximum amplitude B ™ ^ax , the maximum amplitude B ™ ^ax propagating along an axis of spread. By amplitude is meant the maximum value that a periodic quantity takes. The application of a sliding magnetic field results, at a point in the swamp, by a periodic variation in its amplitude. Thus, the amplitude of the magnetic field at a point in the marsh varies as a function of time, between a minimum amplitude B ™ ⁱⁿ and a maximum amplitude B ™ ^ax .

The sliding magnetic field generator 5 can be constituted by several electromagnetic inductors arranged around the peripheral enclosure. In Figure 2, there are shown three pairs 5i, 52 and 5s of electromagnetic inductors. The upper part 5s of the inductors is positioned at the level of the free surface l _SU p of the liquid alloy. Each inductor has a 90 ° phase shift between the upper part 5s and the lower part 5i. In the examples described below, a device was used as described in application WO2014 / 155357. By large face of an ingot is meant a face extending along the longitudinal axis Y and the vertical axis Z. Each inductor comprises one or more coils. In this example, each coil is placed at a distance of 185 mm from the mold. Generally, the distance between a coil of an inductor and the ingot mold can be between 10 mm and 200 mm and preferably for reasons of space between 130 mm and 200 mm.

The sliding magnetic field can also be generated from one or more permanent magnets placed on the periphery of the mold and set in motion relative to the latter. For example, it is possible to generate a sliding magnetic field by rotating a permanent magnet.

The distance λ separating two amplitude maxima of the magnetic field is the wavelength of the sliding magnetic field. FIG. 5 represents an example of distribution of the amplitude Bo of a magnetic field sliding along a propagation axis Δ at an instant t (solid line), and at an instant t + At (dotted line). On the axis of propagation, there is shown a coordinate r corresponding to the position of a point of the marsh. FIG. 5 illustrates a temporal evolution of an alternating magnetic field sliding at this point. This evolution is periodic, and takes place according to a period P. The application of a sliding magnetic field results, at a point in the swamp, by a periodic variation in its amplitude. Thus, the amplitude of the magnetic field at a point in the marsh varies as a function of time, between a minimum amplitude B ™ ⁱⁿ and a maximum amplitude B ™ ^ax .

The Lorentz force, at a point of coordinates r of the marsh, has an oscillating component, modulated according to a frequency 2f double the frequency of the magnetic field. The amplitude Fo of the oscillating Lorentz force density can be explained by the expression:

F ₀ (r) = l / 2ofXB ₀ ² (r) (1), where σ denotes the electrical conductivity.

The amplitude of the Lorentz force, at a point r of the marsh depends on the square of the amplitude of the magnetic field applied at this point. By placing itself in the XYZ coordinate system, linked to the ingot mold 2, the propagation of a maximum value of the amplitude of the magnetic field B ™ ^ax , along a propagation axis, results, simultaneously, in the propagation of a Lorentz force of maximum intensity F _max along the axis of propagation. The combination of the forces propagating along the axis of propagation establishes a movement of the liquid along this axis constituting an electromagnetic stirring.

The inventors have found that it is possible to improve the properties of a flat aluminum alloy product with a thickness greater than 12.5 mm by adjusting the stirring conditions during the casting of the ingot used to obtain the flat product. after transformation of said ingot. The transformation process comprising dissolving at a temperature above 45 CFC, quenching and tempering or maturing to obtain a flat aluminum alloy product.

Optionally, a scalping operation can be carried out during the transformation process in order to eliminate unacceptable surface defects if a working operation, such as rolling, is carried out subsequently.

The inventors have discovered two casting methods making it possible to improve the properties in the thickness of the flat product:

a casting with non-stationary electromagnetic stirring a casting with a stationary electromagnetic stirring plating.

Casting with non-stationary electromagnetic stirring

The casting conditions corresponding to casting with non-stationary electromagnetic stirring are described in application PCT / FR2017 / 051195.

The inventors have found that by modulating, over time, the maximum amplitude of the Lorentz force F _max propagating in the marsh, the intermittent macro-aggregations are attenuated, even disappear, and this particularly on ingots whose thickness is greater than 400 mm.

This temporal modulation can be obtained by a variation of a parameter, called magnetic force parameter, controlling the amplitude of the Lorentz force density explained in equation (1), for example:

the value of the maximum amplitude B ™ ^ax of the magnetic field;

the frequency f of the magnetic field;

the wavelength A of the sliding magnetic field.

The process with non-stationary electromagnetic stirring can include any of the following characteristics, taken individually or in combination:

the frequency of the magnetic field is less than 5 Hz, or 2 Hz or 1 Hz;

the Lorentz force of maximum intensity, propagating along the axis of propagation, varies by at least 30 Nm ^-3 in a time interval between 50 seconds and 10 minutes;

the magnetic field is such that the absolute value of the variation in the density of the maximum Lorentz force is greater than or equal to 0.05 Nm ^-3 .s ¹ during said time interval;

the axis of propagation of the maximum amplitude of the magnetic field belongs to a plane parallel to the direction of casting;

during the casting, the variation of the force parameter is periodic, the period being between 50 s and 20 minutes, or between 1 minute and 15 minutes, or between 2 minutes and 10 minutes;

during casting, the Lorentz force of maximum intensity is not equal to zero.

during the casting, the variation of the force parameter is not obtained by a periodic interruption of the sliding field.

the dimensionless Hartmann number, at at least one point in the liquid part of the alloy, varies at least by a factor of 3, or even by a factor of 5, in said time interval;

the aluminum alloy is chosen from alloys of type 2XXX, 6XXX or 7XXX.

According to one embodiment, the generators are electromagnetic inductors, each electromagnetic inductor being traversed by a current called induction current. The method comprises, during said time interval:

a variation of an intensity of the induction current;

and / or a variation of a frequency of the induction current;

and / or a variation of a distance between an electromagnetic inductor and the mold. and / or a variation in the phase shift of the current between the coils which leads to a variation in the wavelength of the sliding magnetic field.

According to this embodiment, the method may include a variation in the intensity or the frequency of the induction current flowing through an inductor, the method then comprising:

a preliminary step of defining at least one critical value of the intensity and the frequency of the induction current generating, at a free surface of the aluminum alloy flowing in the ingot mold, a wave resonant;

a determination of a range of variation of the intensity or the frequency of the induction current as a function of said previously defined critical value.

The method may include defining a plurality of critical values of the intensity and frequency of the induction current, so as to define a resonance curve, representing the critical values of intensity and frequency generating a resonance of said free surface, the method comprising a determination of a range of variation of the intensity or the frequency of the induction current in a domain delimited by said resonance curve.

Preferably, the method includes a variation in the frequency of the induction current flowing through an inductor.

According to one embodiment, at least one generator is a permanent magnet, the method comprising:

a variation of a distance between the permanent magnet and the mold;

and / or a rotation of the permanent magnet, and a variation in the speed of rotation of the magnet;

and / or a rotation of two permanent magnets.

When the sliding magnetic field is generated by a plurality of electromagnetic inductors arranged at the periphery of the ingot mold, the temporal modulation of the Lorentz force density can be obtained by modifying the pole pitch, i.e. the phase shift between the induction currents flowing in each inductor. Such a modification makes it possible to vary the wavelength A of the sliding magnetic field, that is to say the distance between two maxima propagating along the axis of propagation. The frequency of the induction current flowing in the inductors can be variable, which modifies the frequency f of the magnetic field. The amplitude of the induction current can also be variable, which modifies the value of the maximum amplitude B ™ ^ax of the magnetic field. FIG. 3 shows an embodiment in which the value of the maximum amplitude B ™ ^ax of the magnetic field and the wavelength A of the sliding magnetic field are variable over time. Thus, a spatial distribution of the amplitude B ₀ (t) in the marsh has been represented, at an instant t (solid line), as well as a spatial distribution of the amplitude B ₀ (t + Δί), at a instant t + At (dotted line). During the time interval At, the maximum amplitude B ™ ^ax varies between B ™ ^ax (t) and B ™ ^ax (t + Δί). Likewise, the wavelength A has been modified, going from A (i) to Α (ί + Δί). FIG. 4, which represents a temporal evolution of an alternating magnetic field sliding at a point, an embodiment has been shown in which the value of the maximum amplitude B ™ ^ax of the magnetic field varies, over time, for a constant frequency f and a wavelength A.

Therefore, the maximum amplitude of the Lorentz force, propagating in the swamp, varies between i and ί + Δί, between the values F _max (t) and F _max (t + Δί).

The temporal modulation of a force parameter is implemented during the casting, for a significant duration, preferably greater than 50% or even 80% of the duration of the casting. This time modulation can for example be applied for at least 30 minutes, or even at least 1 hour.

A sliding magnetic field B can in particular be generated from two inductors arranged on the same face of the ingot. The inductors are preferably arranged facing a large face of the ingot, that is to say one of the two faces of the ingot having the largest vertical section. The inductors can be superimposed on each other, so as to generate a so-called vertical phase shift, or arranged side by side, so as to generate a horizontal phase shift. One can use a device described in application WO2014 / 155357, and more precisely three inductors, oriented along the vertical axis Z, are arranged opposite each large face of the ingot. The sliding magnetic field can also be generated from one or more permanent magnets placed on the periphery of the mold and set in motion relative to the latter. For example, it is possible to generate a sliding magnetic field by rotating a permanent magnet.

A variation in the parameters of the sliding magnetic field, whether in terms of its amplitude, frequency or wavelength, makes it possible to apply a non-stationary Uorentz force in the swamp. The inventors have found that this makes it possible to attenuate the appearance of intermittent macro-aggregations or even to make them disappear. Such conditions are likely to affect spontaneous recirculations in the marsh, and reduce their consequences.

Preferably, in the marsh, the speed of variation of the maximum Lorentz force density is greater than 0.05 N.nU.s ¹ , and preferably greater than 0.1 N.nU.s ¹ , and preferably greater than 0.2 N .nU.s ¹ . In one embodiment, the maximum speed of variation of the maximum Lorentz force density during casting is at least 1 Nm ^-3 .s ^-1 and preferably at least 2 N.nU.s ¹ .

Preferably, the variation of one or more force parameters takes place in a time interval less than or equal to the durations characteristic of the recirculations generated by natural convection. These times vary according to the thickness of the ingot and the casting speed. Considering thicknesses e between 400 mm and 700 mm, and casting speeds between 30 mm / min and 80 mm / min, the characteristic durations of recirculations extend between 50 seconds (thickness of 400 mm, casting speed 30 mm / min) and 10 minutes (thickness 700 mm, casting speed 80 mm / min). Thus, the force parameters vary within a time interval Δί determined as a function of these characteristic durations. By variation is meant a significant variation, of at least 10% of the force parameter considered, and preferably of at least 20% or even 30% of the force parameter.

The variation of a force parameter can be periodic, the temporal period of variation being able to be of the order of a characteristic duration of recirculation, that is to say being between 50 seconds and 10 minutes depending on the conditions of dimensions and speed of casting. Preferably, in the marsh, during the temporal period of variation, the maximum Lorentz force density varies at least 30 N.m ' ³ , and advantageously at least 40 Nm ^-3 , and preferably at least 50 Nm ^-3 , and even more preferably at least 60 Nm ^-3 .

The variation of a force parameter can also be monotonous during casting, for example according to an increasing or decreasing function between the start and the end of the casting, the value of the force parameter varying continuously or in successive increments .

Advantageously, during casting, the Lorentz force of maximum intensity is not equal to zero. Typically, it is zero when the current in the inductors or coils is zero. So advantageously, the variation of the force parameter is not obtained by a periodic interruption of the sliding field.

Advantageously, during casting, the Lorentz force of maximum intensity is greater than 80 N / m ³ , preferably greater than 100 N / m ³ , preferably greater than 120 N / m ³ _; even more preferably greater than 140 N / m ³ . The inventors have in fact found that the suppression of intermittent macro-aggregations was not optimum when the force was too weak. The minimum value from which the suppression of intermittent macro-aggregations is improved depends on the set of casting parameters, in particular the mixing mode, the position of the inductors relative to the plate and the composition of the alloy.

According to one embodiment, the frequency f and / or the maximum amplitude B ™ ^ax of the magnetic field are modified respectively by varying the frequency and the amplitude of the induction current flowing in inductors. For this, the method may include a preliminary step of defining an operating domain, that is to say a range of variation of the frequency and / or of the intensity of the induction current. This preliminary step includes the determination of one or more values of frequency / intensity pairs, called critical values, generating, on the free surface l _SU p of the marsh, a resonance, the resonance being expressed by the appearance of significant oscillations. of said free surface l _SU p, the latter being represented in FIG. 7. These significant oscillations are generally observed with the naked eye. By significant oscillation is meant, for example, an oscillation whose amplitude is greater than or equal to 5 mm along the vertical axis Z. For example, the frequency of the current is fixed and the intensity of the induction current is increased up to that a significant oscillation is observed.

By considering different critical values of frequency (or intensity), it is possible to determine experimentally a resonance curve CR, in a frequency / intensity plane corresponding to the different couples (frequency / intensity) to which a resonance is observed on the free surface from the swamp. From this curve CR, a range of variation in intensity and / or frequency is determined, so as to avoid or limit the appearance of a resonance of the free surface of the marsh. Indeed, the resonance curve delimits a zone of stability and a zone of instability, in which the casting can become dangerous. However, the fact of modulating the frequency or the intensity of the induction current, and therefore the frequency f or the maximum amplitude B ™ ^ax of the sliding magnetic field, makes it possible to temporarily approach the resonance curve CR, by example periodically, while remaining in the stability zone. This maximizes the intensity of the Lorentz force, and therefore the mixing of the marsh, while remaining within acceptable safety configurations. Indeed, in the vicinity of the resonance curve, the stirring effect is particularly significant.

Such a resonance curve CR depends on the casting conditions, that is to say on the dimensions of the ingot mold, of the distribution system and in particular on the presence of a floating frame in the liquid metal, of the casting speed. , the configuration of the applied magnetic field, the latter depending on the magnetic field generator, that is to say the inductors or permanent magnet (s) used. Resonance curves CR are shown in FIGS. 8 and 9, the curve of FIG. 8 was obtained by casting an ingot of section 625 mm × 1520 mm, according to the conditions of Example 1, the curve of FIG. was obtained by casting an ingot of section 525 mm x 1650 mm, according to the conditions of Example 3. In this figure, there are also shown abacuses representing a percentage of the intensity of a Lorentz force, called nominal , 100% corresponding to the intensity of the maximum induction current usable in the installation when the frequency is equal to 0.2 Hz. This intensity corresponds to the appearance of a resonance at the frequency of 0.2 Hz. Preferably, the the intensity and the frequency of the induction current lie in a space delimited by the force iso-value curve representing a certain percentage of the intensity of the nominal Lorentz force, for example 10% of this intensity, and the resonance curve.

Preferably, the method includes a variation in the frequency of the induction current flowing through an inductor. The inventors have found it advantageous to vary the frequency because the resulting variation in field penetration allows the force gradient in the thickness and depth of the liquid well to be varied more effectively. In addition, the power electronics make the frequency variation faster than the intensity variation; which gives an additional degree of freedom towards the weaker periods of unsteady forcing. It is in fact advantageous to decouple the hydrodynamic characteristic times from the characteristic solidification times to avoid intermittent macro-segregation.

Casting with stationary electromagnetic stirring plating

The casting conditions corresponding to casting with stationary electromagnetic stirring plating are described in application FR 1761200.

The inventors have found that the appearance of intermittent macro-segregation 11 can be limited by adjusting the electromagnetic stirring when the average Lorentz force applied to the liquid alloy lf flowing at the edge 10 has a certain orientation, and that in a median zone of the marsh, extending symmetrically on both sides of the median plane M, between 172 - 174 and 172 + 174. The thickness of the median zone M corresponds to half of the ingot thickness. By mean Lorentz force is meant an average of the Lorentz force during a period P of the magnetic field. The period P of the magnetic field corresponds to the time interval separating two successive maxima or minima of the magnetic field, as shown in FIG. 4. The period P corresponds to the inverse of the frequency f. The inventors have observed that in the middle zone, at the interface of the marsh lf and the solid alloy ls, at the level of the front 10, the angle β formed by the mean Lorentz force F, with respect to the vertical, must advantageously be less than the angle a of the front, mentioned above, corresponding to the angle between the tangent to the front and the vertical, the angles a and β being oriented in the same direction. In other words, it is advantageous that the direction of the average Lorentz force F is closer to the vertical than the direction of the tangent to the front.

Thus, in the median zone, at the interface between the marsh and the front, the average Lorentz force F is oriented towards the solid alloy ls, and not towards the liquid alloy lf. This condition is illustrated in Figure 11. In this figure, there is shown a section of a flow along an XZ plane. The position of the median plane M corresponds to the thickness 172.

Preferably, the angle of inclination of the average Lorentz force is less, by at least 4 °, than the angle of inclination of the front, so that the average Lorentz force is more inclined, towards the vertical, than the forehead.

Preferably, the frequency is less than 2 Hz or less than 1 Hz. Preferably, the casting speed is less than 45 mm / minute or 40 mm / minute.

According to one embodiment, the casting speed and the frequency are adapted so that in the middle zone of the marsh, in an interface layer between the liquid alloy and the front, the angle of inclination of the force Lorentz average is strictly less than the angle of inclination of the front, the interface layer having a thickness, in a direction perpendicular to the front, is less than 2cm or 1 cm or 5 mm.

The casting process with stationary electromagnetic stirring plating may include, prior to casting, a modeling of the Lorentz force applying at at least one point on the front, so as to define, taking into account the thickness of the mold, a frequency value and / or a casting speed value allowing an average Lorentz force to be obtained, the angle of inclination of which relative to the vertical, is less than the angle, at said point, formed by the front relative to the vertical. Preferably, this modeling is carried out at different points, along the front, along the transverse axis. The modeling can make it possible to define a value of frequency and / or a value of speed of casting making it possible to obtain an average Lorentz force whose angle of inclination, relative to the vertical, is less than 4 ° to the angle of inclination formed by the front relative to the vertical.

The plating effect of the liquid alloy lf against the front 10 is obtained at the interface between the liquid alloy and the front 10. Preferably, this effect is obtained in a layer, called the interface layer, adjacent to the forehead, the thickness of which is less than 2 cm, or 1 cm or 5 mm. The thickness is defined in a direction perpendicular to the front. It is indeed in such a layer that the liquid alloy, in contact with the cold isotherm formed by the forehead, becomes locally denser. A convective boundary layer is then formed along the front, in which the flow of the liquid alloy is accelerated, and can detach from the front, leading to the appearance of vortices. It is mainly in this layer that it is necessary to apply a Lorentz force pressing the liquid alloy against the forehead, in order to maintain the liquid alloy against the forehead, so as to limit the formation of intermittent macro-segregations .

Under these particular conditions, the Lorentz force F tends to press the liquid alloy lf of the marsh against the front 10, which limits the formation of intermittent macro-segregations. The Lorentz force is said to be tackling. It allows the formation of a convective laminar flow along all or part of the front 10, limiting the appearance of intermittent macro-segregations. As described below, the phenomenon of plating of the liquid alloy by the Lorentz force against the front 10 is all the more marked when the casting speed V and the frequency f are low.

A person skilled in the art knows how to model the orientation of an average Lorentz force F, exerted over a period, in the marsh. Calculation codes, for example the AC / DC module of the COMSOL code, allow such modeling, based in particular on the characteristics of the inductors (dimensions, number of ampere-turns, pole pitch, positioning relative to the ingot mold) , the geometry of the ingot mold and operational parameters such as the casting speed or the frequency of the magnetic field. The simulations make it possible to model the electromagnetic stirring of the liquid alloy and to estimate a temporal evolution of the force of Lorentz F, at any point of the marsh, during a period. By evolution, we mean both the evolution of the intensity and the evolution of the direction. It is then possible to determine the orientation and intensity of the average Lorentz force applying at a point in the swamp, during a period P of the magnetic field.

FIGS. 12A, 12B, 12C and 12D show the orientation of the average Lorentz force, obtained by simulation, at different points on a front 10. In these figures, part of a front 10 is shown, according to a plane XZ, extending between the median plane M (abscissa x = 0) and a wall of the ingot mold (abscissa x = 0.26). The abscissa axis represents a position along the transverse axis X and the ordinate axis represents a position along the vertical axis Z. The frequencies considered are respectively 5Hz (Figure 12A), 1 Hz, 0.5 Hz and 0.2 Hz (Figure 12D), the casting speed being 55 mm / min. It is observed that by considering the same position on the front 10, the lower the frequency f, the smaller the angle β of the Lorentz time mean force F. Thus, in the same position on the forehead, the time average Lorentz force F tends to tilt vertically as the frequency decreases.

Furthermore, as previously described, the Lorentz force is tackling when the angle β of the average Lorentz force F is less than the angle a of the front. There is shown, in each figure, a range of thickness Δχ, extending from the median plane M, in which the plating force effect is obtained. This width range is indicated by a double arrow. It is observed that the more the frequency decreases, the more the thickness range Δχ extends, from the median plane M (x = 0), corresponding to the thickness 172, towards the wall of the mold. The technical effect of minimizing intermittent macro-segregation appears in this thickness range Δχ, and it is preferable that it be as wide as possible, preferably including the thickness range 172.3 - 173.3, the latter being generally conducive to the formation of intermittent macrosegregations. In these figures, the thickness range 172.3 - 173.3 corresponds to the interval between x = 0.03 m and 0.1 m. The coordinate 174 corresponds to x = 0.13 m.

Figures 12E, 12F, 12G and 12H respectively show the average orientation of the Lorentz force, obtained by simulation, at different points of a front 10, the frequencies being respectively equal to 5Hz, 1 Hz, 0.5 Hz and 0.2 Hz .

In FIGS. 12A to 12H, so-called normalized forces have been shown, each force being normalized by its intensity, so as to better show the evolution of the orientation of the Lorentz temporal mean force on the front 10, as a function from the position along the forehead. The comparison of FIGS. 12A to 12H shows that the lower the frequency, the greater the thickness range Δχ according to which the Lorentz force becomes tacky. The thickness range Δχ extends from the median plane M towards the wall of the mold, along the transverse axis X. It widens as the frequency decreases. Furthermore, at the same frequency, the lower the casting speed, the greater the thickness range according to which the Lorentz force is tacky. It is therefore advantageous to favor both a low frequency f, preferably less than 2 Hz, or even 1 Hz, and a low casting speed, preferably less than 45 mm / min, or even 40 mm / min.

On the basis of simulations such as illustrated in FIGS. 12A to 12H, the inventors have determined an evolution, along the transverse axis X, of an angle dit, called differential angle, representing a difference between, at the same point, the angle of the front a and the angle of the Lorentz force β, that is θ = α - β. Recall that the angles a and β are oriented in the same direction. When θ> 0, α> β: the Lorentz force is more inclined than the tangent to the front. It is therefore tackling.

FIGS. 13A and 13B show the evolution of the differential angle Θ as a function of a position x on the front 10, along the transverse axis X. The abscissa axis represents the position x, expressed in meters, on the forehead along the transverse axis. As in FIGS. 3A to 3H, the coordinate x = 0 corresponds to position 172, the coordinate x = 0.26 corresponding to the wall 2p of the ingot mold. Figures 13A and 13B were obtained by considering respectively a casting speed of 55 mm / min and 35 mm / min. In each figure, the simulations of the orientation of the average Lorentz force F were carried out by successively considering several frequencies f, between 5 Hz and 0.2 Hz. In each figure, we have shown, in mixed horizontal lines, lines corresponding to the values θ = 0 ° and Θ = 4 ° when the frequency is equal to 1 Hz. The Lorentz force is tackling when θ> 0, but the inventors consider that it is advantageous that Θ> 4 °. We can thus define, on each configuration, a thickness range, in which the Lorentz force is plating, from the median plane M (thickness 172).

FIGS. 13A and 13B show the thickness ranges Δχ (Θ = 0 °) and Δχ (Θ = 4 °) for f = 1 Hz. Similarly to FIGS. 12A at 12H, it is observed that the thickness ranges are all the more important the lower the frequency and the lower the casting speed. Optimal results are obtained for / <2 Hz, even f <1 Hz, and when the casting speed is 35 mm. The lower the frequency, the greater the intensity of the Lorentz force applied to the liquid alloy bordering the front, in the thickness range 172 - T / 4. This strengthens the intensity of the Lorentz force and increases the desired technical effect. In other words, to obtain a significant reduction in intermittent macro-segregation, an orientation of the Lorentz force as described above is necessary. However, its intensity must be sufficient to obtain a plating of the liquid alloy lf against the front 10. This is why it is preferable to modulate the magnetic field at a frequency / relatively low.

Using simulations taking into account different casting thicknesses, the inventors established an abacus, shown in FIG. 14, making it possible to define an operating range for which the Lorentz force is considered to be sufficiently plating, that is to say ie when the differential angle Θ is greater than or equal to 4 °. The abscissa and ordinate axis of the abacus correspond respectively to the casting speed V and to the thickness T of the ingot. The thickness being determined, the abacus makes it possible to define the casting speed and the maximum frequency making it possible to place oneself in the conditions of implementation of the invention.

This chart depends on the number and characteristics of the inductors, their positioning relative to the mold, the dimensions of the latter and the operational parameters of the installation, in particular relating to the magnetic field applied. Those skilled in the art, knowing the characteristics of the installation, can carry out simulations aimed at obtaining the orientation of the average Lorentz force F at different points along the front 10, along the transverse axis X. He can then determine a frequency range and a casting speed range for which we obtain Θ> 0 °, or advantageously Θ> 4 °, so as to implement the invention and obtain the desired technical effect, that is ie a limitation of the intermittent macro-segregation between T / 2 and T / 4, and more particularly between T / 2,3 and T / 3,3.

The inventors have found that by using an ingot of thickness at least greater than 400 mm, preferably greater than 450 mm, even more preferably greater than 500 mm, even more preferably greater than 600 mm, ingot obtained in using the non-stationary electromagnetic stirring casting mode or the stationary electromagnetic stirring casting mode as described, it was possible to obtain a flat product with a thickness at least greater than 12.5 mm, after a transformation process comprising a dissolving at a temperature above 450 ° C, quenching and tempering or maturing. Said flat product thus obtained exhibits a compromise of advantageous properties whatever the position in the thickness.

Optionally, a scalping operation can be carried out during the transformation process in order to eliminate unacceptable surface defects, especially if a rolling operation is carried out subsequently.

Dissolution is carried out at a temperature above 450 ° C. Preferably, the dissolution temperature is lower than T _so iidus, the solidus temperature of the aluminum alloy in order to avoid the appearance of liquid. Preferably, the solution temperature is higher than T _so i _V us - 10 ° C where T _so i _V us is the temperature above which the potentially soluble phases (in general consist only of elements whose content is at least 0.5%) are dissolved. Preferably, the solution temperature is between T _so ivus - 10 ° C and T _so iidus.

The product in solution is soaked. The quenching can be done in air or water horizontally by sprinkling or immersion. Quenching is characterized by a cooling rate expressed in ° C / s. Depending on the composition of the product, there is a critical quenching rate below which the final properties of the product are degraded due to rough precipitation during quenching which tends to deplete the solid solution of the alloy. Preferably the quenching speed is greater than the critical quenching speed of the product. It is however possible that the quenching speed may be less than 0.1 ° C / s, preferably less than 0.05 ° C / s. This type of quenching speed is particularly suitable for 7XXX alloys of type AA7021 or AA7035.

The income corresponds to a heat treatment carried out in one or more stages making it possible to obtain the mechanical characteristics corresponding to a peak income of the T6 or T8 type or an over-income state of the T7 type. The heat treatment is preferably carried out at a temperature typically between 100 ° C and 200 ° C for a period of lh to 70h.

Maturation is obtained at room temperature on a quenched product. By maturation is meant a metallurgical state T4 or T3.

Slightly wrought flat product

The inventors have found that in the case where the transformation process does not include a plastic deformation greater than 5% of any of its dimensions, it was possible to obtain a better homogeneity of properties in the thickness, in particular the elongation and reduction of resistance by notch effect.

Typically, the transformation process does not include a hot rolling step, nor a forging step. However, it may include a step of stress relieving by compression but where the induced plastic deformation is not more than 5%.

The inventors have found that the improvement in properties in the thickness is all the more marked when the thickness of the flat product is greater than 500 mm, even more preferably greater than 600 mm. This result is attributed to the fact that the intermittent macro-aggregations are more marked when the thickness of the cast product increases. In the particular case where there is a small plastic deformation, the intermittent macro-aggregations obtained after casting are substantially identical to those measured on the flat product.

The granular microstructure of the flat product not undergoing plastic deformation greater than 5% is substantially equiaxed.

If we name Gt, Gl, Gs, the grain size respectively in the long transverse direction, the long direction and the short transverse direction of the flat product at mid thickness, the ratio

preferably between 0.5 and 1.3 and even more preferably between 0.8 and 1.2. The grain size Gt, Gl, Gs is determined according to the intercept method of the standard (ASTM El 12-12 §16.3). The grain size Gl can be determined in the longitudinal plane L / TC and corresponds to the notation ft (o °) of ASTM El 12-12 in Figure 7. The grain size Gt can be determined in the longitudinal plane TL / TC and corresponds to the notation tt (0 °) of ASTM El 12-12 in Figure 7. The grain size Gs can be determined in the longitudinal plane L / TC and corresponds to the notation fn (90 °) of ASTM El 12-12 in Figure 7.

To determine the grain size, it is possible to make measurements in optical metallography at mid thickness after Barker attack.

It is advantageous according to the invention to use for this type of flat product a quenching speed of less than 0.1 ° C / s. This range of quenching speed is particularly suitable for aluminum alloys of type AA7021 or AA7035 according to the standards of the Aluminum Association.

The inventors have found that a flat aluminum alloy product quenched at a speed of less than 0.1 ° C / s, typically an alloy of the AA7035 or AA7021 type obtained according to the casting process and the transformation process of the present invention. a difference in elongation between the surface and the mid thickness of less than 3%, preferably less than 2.5%, even more preferably less than 2%.

The elongation measurement is carried out in the TL direction. The surface sampling is carried out in a plane located between 4 and 80 mm relative to the surface of the flat product, typically between T / 200 and T / 10, preferably between T / 150 and T / 80.

If we normalize this difference compared to the mid-thickness elongation value, we obtain a so-called elongation difference. The inventors have found that the elongation difference in this flat product is less than 80%, preferably less than 70%.

The elongation is measured according to standard NF EN ISO 6892-1 (2016) in the TL direction of the flat product.

A flat product of aluminum alloy quenched at a speed of less than 0.1 ° C / s, typically an alloy of the AA7035 or AA7021 type obtained according to the casting process and the transformation process of the invention present between the mid thickness and quarter thickness a difference of less than 50%, preferably less than 25% on the reduction in resistance by the notch effect.

The notch strength reduction is assessed using the Aluminum Association's standardized test E 60203. It consists in measuring the maximum stress of the product in a defined direction obtained on a notched cylindrical test piece. The reduction in resistance by notch effect is defined by the ratio between the maximum stress measured and the value of R0.2 measured in the same direction on a test piece not cut according to standard E8 and B557. This report is also called the “Notch Strength Ratio” or “NSR”.

Furthermore, the inventors have found that the flat product thus obtained exhibited an excellent aptitude for anodization in the sense that, because of intermittent macrosegregations which are not very marked or absent, the anodized product presented a homogeneous visual appearance: the anodic layer does not exhibit differences in colors or reflections associated with differences in composition. Preferably, the flat product is machined and then anodized to be used as parts of injection molds or vacuum chamber elements.

Wrought flat product

The inventors have found that a flat aluminum alloy product obtained from an ingot obtained by casting with non-stationary electromagnetic stirring or by casting with stationary stirring stirring, as described above, exhibited improved properties in thickness. if it was wrought with a reduction rate greater than 2, preferably between 3 and 10, even more preferably between 5 and 7. This wrought step is preferably obtained by hot rolling.

Preferably a scalping of the ingot is carried out. Scalping consists in machining the surface of the cast ingot and removing a certain thickness of material, typically between 20 mm and 100 mm, preferably a thickness less than 50 mm.

The reduction rate R is defined by the ratio between the thickness of the ingot obtained by the casting process and the thickness of the final flat product obtained after working or if a scalping operation is carried out before working, the ratio between the thickness obtained after scalping and the thickness of the final flat product obtained after working.

Preferably, the thickness of the flat aluminum alloy product is between 40 and 200 mm, preferably between 70 and 120 mm.

It is interesting to characterize a spectral intensity criterion (ζ). This is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said spectral intensity criterion being calculated by:

Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macro-segregation of an element whose content by weight is greater than 0.5% or the sum of several elements of the alloy whose individual content is greater at 0.5%, the profile being established in said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 / R and 25 / R mm, normalizing said maximum amplitude by a nominal concentration C0 of said element or by the sum nominal concentrations of the various elements considered.

Preferably, the flat product has a microstructure essentially not between 3 and 10, even more preferably between 4 and 7 where Gt, Gl, Gs, correspond respectively to the size of grains not recrystallized in the long transverse direction, the long direction and the short cross direction of the flat product.

The grain size measurement is made according to the intercept method of the standard (ASTM E112-12 §16.3)

It is possible to distinguish grains which are not recrystallized by metallographic attack, for example by chromic attack.

It is advantageous that the flat aluminum alloy product corresponds to an aeronautical type alloy of the series 7XXX or 2XXX or 6XXX.

The aeronautical type alloys can be within the 7xxx family, the grades 7010, 7040, 7050, 7150, 7250, 7055, 7056, 7068, 7049, 7140, 7149, 7249, 7349, 7449, 7050, 7055, 7056, 7060, 7160, 7075, 7175 and 7475 as defined by the Aluminum Association.

For aeronautical type alloys within the 2XXX family, it is advantageous to distinguish alloys containing lithium and / or silver, alloys not containing any beyond the level of impurity, typically 0.05% in weight. The grades of aeronautical type alloys of the 2XXX family which do not contain lithium and / or silver which are suitable are in particular 2014, 2022, 2023, 2024, 2026, 2027, 2056, 2224, 2324 and 2524 as defined by the Aluminum Association. The aeronautical type alloys of the 2XXX family containing lithium and / or silver which may be suitable may be 2050, 2055, 2060, 2065, 2070, 2076, 2090, 2091, 2094, 2095, 2196, 2296, 2097, 2197, 2297, 2397, 2098, 2198, 2099, 2199, 2029, 2039, 2139 ,, 2297 and 2397, as defined by the Aluminum Association.

The aeronautical aluminum alloys of the 6XXX family can be 6056, 6156, 6061, 6111, as defined by the Aluminum Association.

Preferably, the aluminum alloy has a temperature difference between the solidus temperature and the solvent temperature below 60 ° C, preferably below 40 ° C.

For this type of aluminum alloy having a small temperature difference between the temperature of solvus and of solidus, the inventors have shown that it is possible to obtain an optimized compromise between the toughness, the elastic limit and the elongation at break. In particular, for an AA7050 type alloy of thickness between 70 mm and 120 mm, the toughness defined according to standard E399-12 measured at mid thickness in the direction LT is greater than 40 MPa.m ^1/2 , the limit of elasticity measured in the longitudinal direction (L) is greater than 470 MPa and the elongation at break measured in the short transverse direction (TC) is greater than 6%, preferably greater than 7%.

The products according to the invention can advantageously be used to produce structural elements, preferably aircraft structural elements. Preferred aircraft structural elements are the spars, ribs or fuselage frames. The invention is particularly advantageous for parts of complex shape obtained by integral machining, used in particular for the manufacture of aircraft wings as well as for any other use for which the properties of the products according to the invention are advantageous .

We call here "structural element" or "structural element" of a mechanical construction a mechanical part for which the static and / or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural calculation is usually prescribed or carried out. These are typically elements the failure of which is likely to endanger the safety of the said structure, its users, its users or others. For an aircraft, these structural elements include in particular the elements that make up the fuselage (such as the fuselage skin), the stiffeners or bulkheads, bulkheads, fuselage (circumferential frames), the wings (such as the wing skin), the stiffeners (stringers or stiffeners), the ribs (ribs) and spars (spars)) and the empennage composed in particular of horizontal and vertical stabilizers (horizontal or vertical stabilizers), as well as the floor profiles (floor beams), the seat rails (seat tracks) and the doors.

The inventors have moreover observed that the flat product exhibited an excellent aptitude for anodization in the sense that, due to intermittent macrosegregations which are not very marked or absent, the anodized product exhibited a homogeneous visual appearance: the anodic layer does not show any difference in colors or reflections associated with differences in composition.

Examples

Example 1 - slightly wrought flat product

An ingot of an AA7035 alloy of section 1520 x 625 mm was cast, the composition A of which is indicated in Table 1 below.

Table 1

Yes Fe Cu mn mg Cr Zn Ti Zr AT 0.04 0.13 <0.05 <0.05 1.3 <0.05 5.6 0.03 0.09 B 0.04 0.13 <0.05 <0.05 1.2 <0.05 5.6 0.05 0.08

This ingot was obtained using non-stationary electromagnetic stirring.

The casting speed was 45 mm / min.

The electromagnetic stirring was obtained by placing, opposite each face of the ingot, three coils oriented along the z axis and traversed by an alternating current which was phase-shifted, in the central coil, by 90 ° relative to the current in the extreme coils. The wavelength of the sliding field was therefore 2.4 m. The electromagnetic pump elements thus obtained were arranged in a mirror with respect to the large plane of symmetry of the ingot, the sliding direction being parallel to the transverse-long direction, the sliding produced diverging from the half-width of the ingot. Non-stationary forcing was obtained by imposing a cyclic variation of the alternating electric current flowing through the coils, as illustrated by the double arrow in the frequency vs intensity diagram in Figure 8. The intensity of the force Lorentz maximum volume thus generated by the variation of the frequency typically varied between 290 N / m ³ and 420 N / m ³ over a period of 7 min which corresponds to a speed of variation of approximately 0.30 N / m ³ / s .

The ingot thus obtained was then trimmed to obtain a final dimension of 1520x625x2300 mm Then it underwent a solution treatment of 465 ° C (8h) + 540 ° C (lh), quenched in air and then returned for 10h at 105 ° C followed by a 4h stage at 160 ° C.

The product thus transformed was then characterized in the thickness, the elastic limit Rp0.2, the breaking strength Rm and the elongation A% as defined according to standard NF EN ISO 68921 (2016), the sampling and the direction of the test being defined by standard EN 485 (2016).

They were tested crosswise along the plate (TL) on the surface, T / 8, T / 4 and T / 2.

The surface sample is taken so as to have the axis of the test piece in a plane located 5 mm from the surface. Furthermore, these same properties have also been evaluated in the short cross direction (TC), the tensile test piece having its useful part centered at T / 2. The results are presented in Table 2.

Table 2

Alloy Position Area T / 8 T / 4 T / 2 T / 2 Direction of solicitation Meaning of TL Meaning of TL Meaning of TL Meaning of TL Meaning of TC AT Rp0.2 (MPa) 318 327 330 316 318 Rm (MPa) 355 357 356 340 343 AT% 4.7 3.5 2.9 2.8 3.1 NSR 1.0 0.9 0.9 1.1 1.0

The experimental results obtained by the characterization of this casting are compared with those associated with the conventional production of the same type of alloy AA7035 produced, the composition B of which is indicated in Table 1. These results are presented in Table 3 below. Conventional production is distinguished from the invention by the simple fact that the casting is done without electromagnetic stirring. The other parameters, in particular the size of the ingot and its thickness, the transformation parameters are the same.

Table 3

Alloy Position Area T / 8 T / 4 T / 2 T / 2 Direction of solicitation Meaning of TL Meaning of TL Meaning of TL Meaning of TC B Rp0.2 (MPa) 316 327 309 313 Rm (MPa) 350 356 329 309 AT% 6.0 3.0 1.8 2.3

The static mechanical properties such as R _p o.2 and R _m obtained with electromagnetic stirring are all greater than the average values of the conventional production of the AA7035 alloy.

However, the advantage of the electromagnetic stirring process is particularly pronounced for the elongation value. In fact, with the process according to the invention, the elongation values at mid thickness are 1.5 times greater than in the conventional, unmixed case.

It is observed that the product according to the invention has an elongation difference between the surface and the mid thickness of 1.9% against 4.9% on the conventional product. This difference corresponds to an elongation difference between the surface and the mid-thickness of 68% for the product according to the invention against almost 233% for the conventional product.

The Notch Resistance Abatement or "NSR" is assessed using the Aluminum Association standard test E 602-03. The values measured on the product according to the invention are presented in Table 1.

The measured values are between 0.9 and 1.1. They also happen to be higher than the values obtained on the product obtained according to the conventional process which is usually between 0.5 and 0.8.

Example 2 - Reference

This example is intended to be a reference. Its purpose is to illustrate the properties obtained on a wrought flat product obtained from an ingot with a section of 1700 x 377 mm according to a conventional casting process without electromagnetic stirring.

Table 4 shows the composition C of the AA7050 alloy cast in vertical casting according to a conventional casting process without electromagnetic stirring.

The casting speed was 45mm / min.

Table 4

Yes Fe Cu mn mg Cr Zn Ti Zr VS 0.03 0.06 2.04 0.01 2.22 0.01 6.14 0.02 0.11

The ingot thus obtained undergoes a homogenization for 30 hours at 482 ± 2 ° C, followed by a scalping operation. The ingot then has a section before wrought of 1700 x 337 mm.

The ingot is rolled longitudinally with a reheating temperature between 395 and 430 ° C to reach a final thickness of 80 mm. The reduction rate is 4.2.

At the end of the rolling, we proceed to trim and shore operations in order to reach a final width of 1355 mm and a length of 2990 mm. A solution of 2.5 hours at 479 ± 2 ° C was carried out. An axial traction of 2.3% was applied, followed by tempering with a first stage of 4 hours at 122 ° C and a second stage of 17 hours at 165 ° C.

The mechanical characterizations (traction and toughness) were evaluated at mid thickness.

Test specimens of the CT-40 type were taken at mid-thickness in order to evaluate the toughness according to the plans L-T and T-L and S-L. For the evaluation of the traction characteristics in the short-cross plane, 4d type test pieces with a diameter of 6 mm and useful length 27 mm were taken.

The results are presented in Table 5.

Table 5

T / 2 RO.2 rm 1 I K1C L-T K1C T-L K1C S-L MPa MPa ^% 1 | MPa.ml/2 MPa .ml / 2 MPa .ml / 2 Sense L 474 526 12.1 I VS Meaning of TL 473 533 10.0 I 1 36.2 31.0 35.7 Meaning of TC 444 519 7-5 | 1

The flat product in AA7050 alloy according to a conventional casting process, from an ingot of thickness 377 mm and having undergone a reduction rate of 4.2 has an elastic limit in the direction L of 474 MPa, a tenacity TL of 36.2 MPa.m ^1/2 and a short transverse elongation of 7.5%.

Example 3 - Wrought flat product

In this example, an AA7050 alloy casting was made, the composition D of which is indicated in Table 6. An ingot with a section of 1650x525mm was thus obtained. The refining of the grain is carried out using a wire refining A1TÎC3: 0.15 with a rate of addition of 1.5kg / ton. The casting speed was 45 mm / min.

Table 6

Yes Fe Cu mn mg Cr Zn Ti Zr D 0.03 0.05 2.05 0.02 2.04 0.02 6.09 0.04 0.11

At the start of casting, no electromagnetic stirring is put in place. Then after solidifying a length of about 2 m, the electromagnetic stirring is activated.

By this methodology, it is possible to compare the properties obtained with and without mixing.

Electromagnetic stirring corresponds to non-stationary electromagnetic stirring. Non-stationary electromagnetic stirring was obtained by placing, opposite each face of the ingot, three coils oriented along the z axis and traversed by an alternating current which was phase-shifted in the central coil by 90 ° by compared to the current in the extreme coils. The wavelength of the sliding field was therefore 2.4 m. The electromagnetic pump elements thus obtained were arranged in a mirror with respect to the large plane of symmetry of the ingot, the sliding direction being parallel to the transverse-long direction, the sliding produced diverging from the half-width of the ingot. The unsteady forcing was obtained by imposing a cyclic variation in the frequency of the alternating electric current flowing through the coils, as illustrated by the double arrow in the frequency vs intensity diagram presented in Figure 9. The intensity of the maximum Lorentz density thus generated by the variation of the frequency typically varied between 210 N / m ³ and 275 N / m ³ over a period of 1.33 min which corresponds to a speed of variation of approximately 0.81 N / m ³ / s. It will be noted that the limit stability curve is different from that presented in Example 1 due to both the different casting thickness and the presence of a floating frame which contributes to inhibiting certain resonant modes of the free surface. .

Following a 10h relaxation treatment at 350 ° C, the ingot thus obtained was then trimmed to obtain a dimension of 1150x2240x525 mm. Then it underwent a 30 hour homogenization at 482 ± 2 ° C. Following a scalping operation, the ingot has the final dimensions of 1150x2240x440 mm.

The ingot was then rolled with a reheating temperature between 395-430 ° C to reach a final thickness of 80 mm. The reduction rate is 5.5.

Trimming and shore operations were carried out to reach a final width of 900 mm and a length of 4000 mm. A solution of 2.5 hours at 479 ± 2 ° C was carried out. An axial traction of 2.3% was applied, followed by tempering with a first stage of 4 hours at 122 ° C and a second stage of 17 hours at 165 ° C.

The mechanical characterizations (traction and toughness) were evaluated at mid thickness.

Test specimens of the CT-40 type were taken at mid-thickness in order to evaluate the toughness according to the plans L-T and T-L and S-L. For the evaluation of the traction characteristics in the short-cross plane, 4d type test pieces with a diameter of 6 mm and useful length 27 mm were taken.

The results are presented in Table 7

Table 7

T / 2 R0.2 rm AT K1C L-T K1CT-L K1C S-L MPa MPa % MPa .ml / 2 MPa .ml / 2 MPa .ml / 2 D Invention Sense L 479 532 10.9 43.0 32.8 33.9 Meaning of TL 464 518 10.5 Meaning of TC 453 519 6.4 D Reference Sense L 477 531 11.3 42.0 33.6 32.4 Meaning of TL 463 517 10.4 Meaning of TC 456 517 5.0

The AA7050 alloy flat product obtained according to a conventional casting process, from an ingot 525 mm thick and having undergone a reduction rate of 5.5 has a yield strength in the L direction of 477 MPa, a toughness TL of 42.0 MPa.m ^1/2 and a short transverse elongation of 5.0%.

The AA7050 alloy flat product obtained according to a casting method according to the invention, from an ingot of 525 mm thickness and having undergone a reduction rate of 5.5 has an elastic limit in the L direction of 479 MPa , a TL tenacity of 43.0 MPa.m ^1/2 and a short transverse elongation of 6.4%.

Example 4 - Wrought flat product

In this example, an AA7050 alloy casting was carried out, the composition F of which is indicated in Table 8. An ingot with a cross section of 1650 x 525mm was thus obtained. The refining of the grain is carried out using a wire refining A1TÎC3: 0.15 with a rate of addition of 1.5kg / ton. The casting speed was 45mm / min.

Table 8

Yes Fe Cu mn mg Cr Zn Ti Zr F 0.03 0.05 2.12 0.02 2.15 0.02 6.20 0.04 0.11

At the start of casting, the electromagnetic stirring is set up. Then after having solidified a length of about 2.1 m, the electromagnetic stirring is stopped.

Electromagnetic stirring corresponds to non-stationary electromagnetic stirring. The electromagnetic stirring was obtained by placing, opposite each face of the ingot, three coils oriented along the z axis and traversed by an alternating current which was phase-shifted, in the central coil, by 90 ° relative to the current in the extreme coils. The wavelength of the sliding field was therefore 2.4 m. The electromagnetic pump elements thus obtained were arranged in a mirror with respect to the large plane of symmetry of the ingot, the sliding direction being parallel to the cross-long direction, the sliding produced diverging from the half-width of the ingot. The unsteady forcing was obtained by imposing a cyclic variation of the nominal intensity of the alternating electric current flowing through the coils, as illustrated by the arrows in the frequency vs intensity diagram in Figure 10. The intensity of the maximum Lorentz density thus generated by the variation of the intensity varied over a period of 1.33 min.

Following a 10 h relaxation treatment at 350 ° C., the ingot thus obtained was then trimmed to obtain a dimension of 1145x2365x525 mm. Then it underwent a 30 hour homogenization at 482 ± 2 ° C. Following a scalping operation, the ingot obtained the final dimensions of 1145x2365x455 mm.

The ingot was rolled longitudinally with a reheating temperature of 395-430 ° C to reach a final thickness of 80 mm. The reduction rate is 5.7.

Trimming and shore operations were carried out to reach a final width of 900 mm and a length of 4000 mm. A solution of 2.5 hours at 479 ± 2 ° C was carried out. An axial traction of 2.3% was applied, followed by tempering with a first stage of 4 hours at 122 ° C and a second stage of 17 hours at 165 ° C.

The mechanical characterizations (traction and toughness) were carried out in the same way as in Example 3.

The results are presented in Table 9.

Table 9

T / 2 R0.2 rm AT K1C L-T K1C T-L K1C S-L MPa MPa % MPa .ml / 2 MPa .ml / 2 MPa .ml / 2 F Invention Sense L 483 537 11.2 40.6 31.2 33.4 Meaning of TL 468 522 10.3 Meaning of TC 461 524 7.4 F reference Sense L 479 533 11.1 42.4 32.1 33 Meaning of TL 465 518 10.2 Meaning of TC 456 513 4.0

The AA7050 alloy flat product obtained according to a conventional casting process, from an ingot of 525 mm thickness and having undergone a reduction rate of 5.5 has a yield strength in the L direction of 479 MPa, a toughness TL of 42.4 MPa.m ^1/2 and a short transverse elongation of 4.0%.

The AA7050 alloy flat product obtained according to a casting method according to the invention, from an ingot of 525 mm thickness and having undergone a reduction rate of 5.5 has an elastic limit in the L direction of 483 MPa , a TL tenacity of 40.6 MPa.m ^1/2 and a short transverse elongation of 7.4%.

Claims (19)

1. Flat aluminum alloy product extending parallel to a longitudinal axis (L), the cross section of which is perpendicular to the longitudinal axis has a width (TL) and a thickness (TC), the thickness (TC) being less than the width, the thickness of said flat product being greater than 12.5 mm, said flat product being obtained from a manufacturing process comprising: - A casting process with electromagnetic stirring to form an aluminum alloy ingot of thickness greater than 400 mm, the casting process being carried out in an ingot mold, the ingot mold defining a parallelepiped, so that the ingot formed s 'extends parallel to a longitudinal axis (Y), along a width (W) and parallel to a transverse axis (X), along a thickness (T), the thickness of the ingot being less than the width; the ingot defining a median plane extending along the mid thickness (T / 2), parallel to the longitudinal axis (Y), a process for transforming said aluminum alloy ingot comprising dissolving at a temperature above 450 ° C., quenching and tempering or maturing to obtain a flat aluminum alloy product, characterized in that said casting process comprises the following steps - casting of the liquid aluminum alloy in the mold, along a vertical casting axis (Z), the alloy being cooled, during casting, by a runoff of a coolant (3), so forming a solid alloy (ls), extending around a liquid alloy (1 £), called swamp, the solid alloy forming a front (10) at the interface with the swamp (IC), the front being inclined at an angle of inclination (a), relative to the vertical axis, the angle of inclination of the front varying along the transverse axis (X); - displacement of the solid alloy (1 s), along the vertical casting axis (Z), according to a casting speed (V); - during casting, application of a sliding magnetic field whose amplitude varies according to a frequency (/), the magnetic field being generated by at least one magnetic field generator arranged at the periphery of the mold, so as to apply an average Lorentz force (F) at different points in the marsh, the average Lorentz force, during a period (P) of the magnetic field, being inclined relative to the vertical axis (Z) according to an angle of inclination ( β), said angle of inclination of the Lorentz force, the latter varying along the transverse axis (X); the Lorentz force present during period P a Lorentz force of maximum intensity (F _max ); - the marsh comprising a median zone, extending symmetrically on either side of the median plane (M) whose thickness corresponds to half the thickness of the ingot the applied magnetic field propagates along an axis of propagation, so that a maximum amplitude (B ™ ^ax ) of the magnetic field propagates along said axis of propagation, by defining a propagation wavelength (λ), and such that the casting process with electromagnetic stirring corresponds to a non-stationary electromagnetic stirring mode defined by a magnetic parameter called force, governing the Lorentz force of maximum intensity (F _max ); this magnetic force parameter is variable within a predetermined time interval (Δί), said parameter being: - said maximum amplitude (B ™ ^ax ) of the magnetic field; - And / or said frequency (f) of the magnetic field; - and / or the propagation wavelength (λ) of the magnetic field; so as to obtain a modulation, in said time interval, of said Lorentz force of maximum intensity (F _max ) propagating along the propagation axis or such that the casting process with electromagnetic stirring corresponds to an electromagnetic stirring mode stationary tackle defined by the fact that the sliding magnetic field propagates along an axis of propagation parallel to the vertical axis of casting; the frequency (/) is less than 5 Hz; and the casting speed (V) and the frequency (/) are adapted so that throughout the median zone of the marsh, at the interface between the liquid alloy (1 £) and the front, the angle of inclination of the force (β) is strictly less than the angle of inclination (a) of the forehead. 2. flat aluminum alloy product according to claim 1 characterized in that said flat product has for an element of the aluminum alloy, whose content by weight is greater than 0.5%, or for the sum of several elements of the alloy whose individual content by weight is greater than 0.5%, a dispersion criterion ε less than 3.3, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5 , the dispersion criterion being defined according to the following expressions: ε = ΔΟζαΙ & Czr ACza = max (Cza) - min (Cza), SCzr = max (Czr) - min (C _Z r), where: - max (Cza) and min (Cza) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the elements considered measured in an analysis zone (ZA), exhibiting intermittent macro-aggregations, for example between thicknesses T / 3.3 and 172.3 of said flat product; - max (Czr) and min (Czr) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the elements considered measured in a reference zone (ZR), considered as little affected by intermittent macrosegregations, for example between the thicknesses 1712 and 176 of said flat product; said concentrations being measured on at least one profile established at mid-width in a vertical plane L / TC and in the direction TC, said profile being representative of said intermittent macro-aggregations of the element considered in the direction TC. 3. Flat aluminum alloy product according to any one of claims 1 to 2 characterized in that said transformation process does not include a step inducing plastic deformation greater than 5%, typically the transformation process does not include hot rolling step. 4. Flat product according to claim 3 characterized in that the thickness of said flat product is greater than 500 mm, even more preferably greater than 600 mm. 5. Flat product according to claim 3 or 4 characterized in that said flat product has a microstructure such that the ratio is less than 1.5, preferably between 0.5 and 1.3 and even more preferably between 0.8 and L2 where Gt, Gl, Gs, correspond respectively to the average grain size in the long transverse direction, the long direction and the short transverse direction of the flat product , the grain size being measured at mid thickness according to standard ASTM El 12-12. 6. Flat aluminum alloy product according to any one of claims 3 to 5 characterized in that a spectral intensity criterion (ζ) is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said spectral intensity criterion being calculated by: Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macro-segregation of an element whose content by weight is greater than 0.5% or the sum of several elements of the alloy whose individual content is greater at 0.5%, the profile being established in said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 and 25 mm, normalizing said maximum amplitude by a nominal concentration Co of said element or by the sum of the nominal concentrations of different elements considered. 7. Flat aluminum alloy product according to any one of claims 3 to 6 characterized in that the quenching is carried out at a speed less than 0.1 ° C / s. 8. A flat aluminum alloy product according to any one of claims 3 to 7 characterized in that the aluminum alloy corresponds to a designation of the type AA7021 or AA7035. 9. Flat aluminum alloy product according to claim 7 or 8 characterized in that the elongation at break according to standard NF EN ISO 6892-1 (2016) measured in the direction TL has a difference in elongation between the surface and the mid thickness less than 3%, preferably less than 2.5%, even more preferably less than 2%. 10. Flat aluminum alloy product according to any one of claims 7 to 9 characterized in that the reduction in resistance by notch effect defined according to standard E 602-03 has a difference of less than 50%, preferably less than 25% between the mid thickness and the quarter thickness. 11. Machined and anodized product obtained from the flat product according to any one of claims 3 to 10 characterized in that the flat product is machined and anodized. 12. A flat aluminum alloy product according to claim 1 or 2 characterized in that the transformation process comprises a step of working with a reduction rate R greater than 2, preferably between 3 and 10, even more preferably between 5 and 7, the reduction rate R being defined by the ratio between the thickness of the ingot obtained by the casting process and the thickness of the final flat product obtained after working or if a scalping operation is carried out before working, the ratio between the thickness obtained after scalping and the thickness of the final flat product obtained after working. 13. flat aluminum alloy product according to claim 12 characterized in that the thickness of said flat product is between 40 and 200 mm, preferably between 70 and 120 mm. 14. Flat aluminum alloy product according to claim 12 or 13 characterized in that a spectral intensity criterion (ζ) is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said criterion of spectral intensity being calculated as: Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macro-segregation of an element whose content by weight is greater than 0.5% or the sum of several elements of the alloy whose individual content is greater at 0.5%, the profile being established in said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 / R and 25 / R mm where R is the reduction rate, normalizing said maximum amplitude by a nominal concentration Co of said element or by the sum of the nominal concentrations of the various elements considered. 15. flat aluminum alloy product according to any one of claims 12 to 14 characterized in that said flat product has a microstructure essentially not recrystallized and such that the ratio is greater than 2, preferably between 3 and 10, again more preferably between 5 and 7 where Gt, Gl, Gs, respectively correspond to the average size of grains not recrystallized in the long transverse direction, the long direction and the short transverse direction of the flat product, the grain size being measured according to the standard ASTM El 12-12. 16. Flat aluminum alloy product according to any one of claims 12 to 15 characterized in that the aluminum alloy is an aeronautical type alloy of the series 7XXX or 2XXX or 6XXX. 17. Flat aluminum alloy product according to claim 16 characterized in that the aluminum alloy has a temperature difference between the solidus temperature and the solvent temperature less than 60 ° C, preferably less than 40 ° C . 5 18. flat aluminum alloy product according to claim 17 characterized in that it has an optimized compromise between toughness, yield strength and elongation at break, characterized in that the aluminum alloy is an AA7050 alloy, in that its thickness is between 70 mm and 120 mm, and such that the toughness defined according to standard E399-12 measured at mid thickness in the direction LT is greater than 40 MPa.m ^1/2 , 10 the elastic limit measured in the longitudinal direction (L) is greater than 470 MPa and the elongation at break measured in the short transverse direction (TC) is greater than 6%, preferably greater than 7%. 19. Machined and anodized product obtained from the flat product according to any one of claims 12 to 18 characterized in that the flat product is machined and anodized.