WO2019106254A1

WO2019106254A1 - Aluminium alloy flat product having improved thickness properties

Info

Publication number: WO2019106254A1
Application number: PCT/FR2018/052862
Authority: WO
Inventors: Philippe Jarry; Fabio TAINA; Jean-Louis ACHARD; Marc Bertherat; Pierre-Yves Menet; Mircea CABLEA; Timothy Warner
Original assignee: Constellium Issoire
Priority date: 2017-11-29
Filing date: 2018-11-16
Publication date: 2019-06-06
Also published as: FR3074191B1; EP3717146A1; FR3074191A1; EP3717146B1

Abstract

The invention relates to a flat product made of aluminium alloy with a thickness of greater than 12.5 mm that has improved thickness properties. This product is obtained by adjusting the mixing conditions during the casting of the ingot used for obtaining the flat product. The casting process is characterized by casting with mixing induced by a non-stationary field of external forces or casting with mixing induced by a pressing stationary field of external forces. The transformation process comprises a solution heat treatment at a temperature above 450°C, a quenching and a tempering or ageing in order to obtain a flat product made of aluminium alloy. Optionally, a scalping operation may be carried out during the transformation process in order to eliminate unacceptable surface defects if a rolling operation is carried out subsequently. The invention makes it possible in particular to increase the thickness of the cast product without degrading, or even while improving, the elongation in the short transverse 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 products of aluminum alloy from ingots obtained by vertical casting.

State of the art

Thick aluminum alloy flat products, in particular 7XXX, 2XXX series of hardened aluminum alloys, show differences in properties depending on the thickness in the case of thickness greater than 12.5 mm. These differences in properties are associated inter alia with the macrosegregations observed on the cast ingot and which remain on the flat product obtained after an ingot transformation process; the transformation process comprising dissolution, quenching and tempering or ripening. A macrosegregation well known to those skilled in the art is the negative central macrosegregation, resulting from a depletion of eutectic alloying elements, along a portion of the median plane parallel to the large faces of the ingot. These macrosegregations have been described in John Wiley et al, "Direct-Chili Casting of Light Alloys," Wiley Publisher, September 2013, pp. 158-172. This is a continuous macrosegregation, which refers to the fact that that the macrosegregation takes place continuously on all or part of the height of the ingot, in other words that it is substantially uniform along the casting axis.

There is also the phenomenon of intermittent macrosegregation which results in the formation of V-shaped bands on either side of the negative central macrosegregation. The publication RC Dorward et al. Aluminum, 1996, 72. Jahrgang, 4, p.251-259 describes the formation of intermittent segregation bands in a 7000 alloy. V are alternatively enriched and depleted (respectively depleted and enriched) in eutectic alloy elements (respectively peritectic). These bands are observable by X-ray radiographs of vertical slices of ingots, typically in the L / TC plane at mid-width, when the segregated elements absorb X-rays in a manner differentiated from the atoms of the metal composing the ingot. Other means are to visualize this phenomenon, for example ultrasound or the observation with the naked eye of anodized vertical slices, because the difference in optical reflectivity between the zones enriched or depleted in alloying elements. Generally, the intermittent macrosegregation is most marked in the vicinity of the T / 2.5 region, and typically between T / 3.3 and T / 2.3, the T / 2 region corresponding to the central axis of the ingot.

Thick aluminum alloy flat products, in particular the 7XXX, 2XXX, or 6XXX series of 7XXX, or 6XXX hardened aluminum alloys used in the aerospace industry, have a thickness greater than 12.5 mm. 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 T / 2 and T / 4.

- The central macrosegregation lowers the static properties at half thickness of the recrystallized sheets.

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

- Quenching speed, especially for thick plates.

The publication of Chakrabarti et al. "Thickness variations in 7050 plate", Materials Science Forum, 1996, Vols. 217-222, p. 1085-1090 indicates that the macrosegregation dominates the properties at T / 2 for sheets less than 40 mm thick which are recrystallized, whereas the texture induced by the wrought iron dominates the properties for the sheets of thickness greater than 40 mm because they are not recrystallized.

Those skilled in the art seeking to counter the negative effects of macrosegregation therefore wish to increase the rate of sheet metal work from thicker foundry plates. But it is then confronted with levels of intermittent macrosegregations in the vicinity of T / 2.3 and T / 3.3 which introduce into the product areas that can not be put back in solution, especially for the most saturated alloys, because the local solvus of the alloy in the intermittent segregations may lie beyond the solidus of the alloy of nominal composition. These intermittent macrosegregations are detrimental especially for the break elongation value measured in the short cross direction. An elongation value that is too low in the short cross direction of the product is undesirable during the manufacture of the aeronautical structures because of the risk of greater cracking.

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

In order for these products to be selected for typical thick flat products applications such as aeronautics, the manufacture of molds or vacuum chamber elements, their performance must notably achieve a compromise of advantageous property between the static mechanical strength properties ( typically yield strength in tension and compression, tensile strength) and the properties of damage tolerance (typically toughness, resistance to the propagation of fatigue cracks) and / or elongation these properties are generally antinomic.

Object of the invention

The object of the invention relates to a flat product made of aluminum alloy extending parallel to a longitudinal axis (L), and whose section perpendicular to the longitudinal axis has a width (TL) and a thickness (TC). 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 shaped ingot extends parallel to a longitudinal axis (Y), along a width (W) and parallel to a transverse axis (X) according to 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 mold is close to the thickness of the ingot formed.

And

a process for converting said aluminum alloy ingot comprising dissolving at a temperature above 450 ° C., quenching and tempering or maturation in order to obtain a flat product made of aluminum alloy with optionally an operation of scalping. The casting method with electromagnetic stirring comprises the following steps casting the liquid aluminum alloy in the mold, along a vertical casting axis (Z), the alloy being cooled, during casting, by a runoff of a cooling liquid (3), so as to form a solid alloy (ls), extending around a liquid alloy (1b), said swamp, the solid alloy forming a front (10) at the interface with the marsh (1b), the front being inclined at an angle of inclination (a), with respect to the vertical axis, the angle of inclination of the front varying along the transverse axis (X);

moving the solid alloy (ls), along the vertical casting axis (Z), according to a casting speed (V);

during the 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 mold, of to apply a mean Lorentz force (F) at different points of the marsh, the average Lorentz force, during a period (P) of the magnetic field, being inclined relative to the vertical axis (Z) at an angle of inclination (b), said angle of inclination of the Lorentz force, the latter varying along the transverse axis (X); the Lorentz force has during the period P a Lorentz force of maximum intensity (F _max );

the marsh comprises a median zone, extending symmetrically on either side of the median plane (M), the thickness of which corresponds to half a 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, defining a propagation wavelength (1).

The casting process with electromagnetic mixing is such as to allow the attenuation of intermittent macrosegregations or their disappearance. According to the invention, the casting method with electromagnetic stirring corresponds to either a non-stationary electromagnetic stirring mode, or to a stationary electromagnetic stirring said plating, 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 in a predetermined time interval (At), 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 (1) of the magnetic field;

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

The stationary electromagnetic stirring electroplating mode is defined by the fact that the sliding magnetic field propagates along a propagation axis parallel to the vertical casting axis; the frequency (/) is less than 5 Hz; and the casting velocity (V) and the frequency (/) are adapted such that throughout the middle zone of the marsh, at the interface between the liquid alloy (1b) and the front, the angle of inclination the force (b) is strictly less than the angle of inclination (a) of the front.

It has been observed that the aluminum alloy flat product thus obtained has, for one element of the aluminum alloy, whose weight content 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 e of less than 3.3, preferably less than 3, more preferably 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:

e = C _ZA / AC _ZR

ACZA = max (C _ZA ) - min (C _ZA ),

AC _ZR = max (C _ZR ) - min (C _ZR ),

or :

max (C _ZA ) and min (C _ZA ) denote respectively the maximum and minimum concentrations of the element considered or the sum of the considered elements measured in an analysis zone (ZA), exhibiting intermittent macrosegregations, 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 the sum of the considered elements 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 still excluding the peripheral zones possibly affected by cortical macrosegregation; said concentrations being measured on at least one profile set at half width in a vertical plane L / TC and in the direction TC, said profile being representative of said intermittent macrosegregations of the element under consideration in the direction TC.

Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention, given by way of non-limiting examples, and shown in the figures listed below.

Description of 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).

Figures 5 and 6 respectively show 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 electromagnetic stirring).

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

FIGS. 8, 9 and 10 respectively represent a so-called CR free surface resonance curve of the marsh, obtained by implementing a casting method using non-stationary magnetic field stirring in the respective examples 1, 3 and 4 according to FIG. 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 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 forehead, respectively at different frequencies, for a casting speed of 55 mm per minute by implementing a casting with stationary electromagnetic stirring.

FIGS. 12E, 12F, 12G and 12H show simulation results making it possible to obtain the average orientation of a Lorentz force in the marsh, at the forehead, respectively at different frequencies, for a casting speed of mm per minute using a casting with stationary electromagnetic stirring. FIGS. 13A and 13B are curves established by respectively considering different frequencies, and representing an evolution of an angle, called a differential angle, along the front, the differential angle representing a difference between the angles, with respect to the vertical, respectively formed by the Lorentz force and the forehead. In Figure 13A, a casting speed of 55 mm per minute was considered. In Fig. 13B, a casting speed of 35 mm per minute was considered.

FIG. 14 shows an abacus making it possible to define an operating range as a function of the casting speed (abscissa axis) and the casting thickness (ordinate axis), in order to obtain an orientation of the Lorentz force in a median zone extending between T / 2 ± T / 4 according to a casting process with stationary electromagnetic stirring.

Description of the invention

In the preamble, a flat product is a product of parallelepipedal 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, in accordance with the designations defined by the "Aluminum Association" in the "Registration Record Sériés" which it publishes regularly.

All indications regarding the chemical composition of the alloys are 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 the metallurgical states are given in the European standard EN 515.

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

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

The notch resistance allowance is determined by E 602-03.

According to a nomenclature known to those skilled in the art, the term T / n, where n is a positive number, denotes a region located at a distance T / n from the surface of a parallelepiped product where T designates the thickness of the product said parallelepipedal 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 found that a flat product resulting from an ingot obtained according to the casting process conditions of the invention and then converted according to a conversion process comprising a roughing, a dissolution, quenching and tempering or ripening allowed to obtain a good compromise between toughness, yield strength and elongation. In particular, the compromise is obtained for the toughness defined according to the E399-12 standard measured at mid-thickness in the LT direction, the elastic limit measured in the longitudinal direction (L) and the elongation at break measured in the short transverse direction. (TC).

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

The inventors have found 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 casting process conditions of the invention and then transformed according to a transformation process comprising dissolution, quenching and tempering or maturing but not including a plastic deformation step greater than 5% allowed to obtain a reduced property gap in the thickness. In particular, this improvement is observed for the measured fracture elongation between the surface and the mid-thickness in the TL direction and the notch resistance reduction measured at half thickness and quarter thickness in the TL direction.

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

The inventors attribute this benefit to the reduction of intermittent macrosegregation in the processed flat product.

The contents of applications PCT / FR2017 / 051195 and FR 1761200 are hereby incorporated by reference. Applications PCT / FR2017 / 051195 and FR 1761200 both seek to reduce intermittent macrosegregations in the cast ingot using a sliding magnetic field. Casting with non-stationary electromagnetic stirring will be called casting conditions described in application PCT / FR2017 / 051195 for reducing intermittent macrosegregations. We will call casting with stationary electromagnetic stirring plating conditions described in the application FR 1761200 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.

The intermittent macrosegregations 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 made to 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 slippery average of the 50 mm gross profile makes it possible to get rid of intermittent macrosegregations and to obtain the continuous macrosegregation profile. The continuous macrosegregation profile is subtracted from the smoothed profile to obtain a so-called corrected profile, corresponding to the intermittent macrosegregation. The corrected profile is primarily representative of intermittent macrosegregation, and is unaffected or unaffected by central continuous macrosegregation and microsegregation. Such a corrected profile makes it possible to characterize the intermittent macrosegregation.

It is then possible to calculate a maximum concentration difference in an analysis zone ZA situated between T / 3.3 and T / 2.3 of the thickness of the flat product, this maximum difference being able to be expressed according to the following equation:

AC / A = max (CZA) - min (CZA)

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

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

The maximum deviation AC _{/ A} can be normalized by the nominal concentration Co of the element under consideration. The products according to the invention preferably have a value of such a ratio standardized less than 10% and preferably less than 8% or even less than 6%. However, the absolute value of AC _ZA 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, in order to characterize the products obtained by the process according to the invention, to calculate, by way of reference, a maximum difference in a reference zone ZR which is not very sensitive to intermittent macrosegregations, located between T / 12 and T / 6, this maximum deviation that can be expressed according to the following equation:

AC / R = max (CZR) - min (CZR)

where max (CZR) and min (CZR) denote respectively the maximum and minimum concentrations of the considered element measured between T / 12 and T / 6.

This gives a dispersion criterion e which makes it possible to evaluate intermittent macrosegregation for the element in question:

S = ACZA / AC _ZR

In order to overcome local variations in composition, it is advantageous, in order to determine ACZA and AC _ZR , to calculate an average over at least five concentration profiles at least distant from each other.

10 mm.

The lower e is, the less intermittent macrosegregations are marked. The products obtained by the process according to the invention preferably have a dispersion criterion e of less than 3.3, preferably less than 3, more preferably less than 2.5, still more preferably 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 the sum of several elements is considered, the values for normalizing the maximum deviation AC _{/ A,} and / or the Fourier transform correspond to the sum of the nominal concentrations of the considered elements.

It is also useful to perform 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 the intermittent macrosegregation. The inventors have found that the intermittent macrosegregation has periods between two values defined according to the reduction rate R applied to the ingot during the transformation process. Typically the intermittent macrosegregation has periods of 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 grinding or if a scalping is performed 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 to a final thickness of 100 mm, the reduction rate is equal to 5.

When the intermittent macrosegregation is important, and the ingot does not undergo plastic deformation greater than 5%, the reduction ratio is substantially equal to 1. There is therefore in this case a peak of the amplitude of the Fourier components for spatial periods between 8 and 25 mm.

An adimensional spectral intensity criterion z 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 considered element. The products obtained by the process according to the invention preferably have a z-criterion less than 0.01, preferably less than 0.007 and preferably less than 0.005.

The dispersion criteria e and spectral intensity z are advantageously applied to the major element of the alloy in question, 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 some alloys 7xxx or the sum Mg + Si in alloys 6xxx.

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

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 is carried out along a vertical axis Z, through the ingot mold. The mold is delimited by a peripheral enclosure whose section, in a horizontal plane XY, is parallelepipedic. The mold defines a frame, parallel to a longitudinal axis Y, along a width W, and, parallel to a transverse axis X, 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 defining the ingot mold 2. The casting forms a parallelepiped ingot. A plane, said 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 ("Direct- Chili Casting "). A false bottom 4 is translated so as to move away from the opening 2i during the casting. The translational speed of the false bottom corresponds to a so-called casting speed V. Under the effect of cooling, a solid alloy zone ls forms, near the cooled enclosure, around an alloy zone liquid lf, referred to as "swamp". The interface between the marsh l and the solid zone ls forms a front 10. At the end of cooling, the ingot is formed. The front 10 has a slope, relative to the vertical, variable depending on 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 Z axis. The lower the angle of the front a, the more the tangent to the forehead is oriented vertically. The angle of the front is shown in Figure 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 substantially at the same position, while the material moves vertically at the casting speed.

In the processes according to the prior art, intermittent macro-segregations 11 are formed in the ingot, and in particular in a thickness range between T / 3.3 and T / 2.3 on either side of the median plane M.

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

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 may be a moving permanent magnet or an electromagnetic inductor, the latter generating a magnetic field when traversed by an electric current, said induction current.

The magnetic field B applied to the liquid alloy lf is an alternating field of amplitude B ₀ and frequency /. 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 a resulting electric current J, 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 proportionality relation. The Lorentz force has an oscillating component at a frequency twice the frequency / magnetic field.

Due to the thickness of the mold, the frequency / is chosen so as to allow sufficient penetration of the magnetic field B in the marsh, so as to obtain a stirring effective liquid. The frequency / is all the lower as the thickness of the product is high. 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 1 Hz.

The generator 5 is able to generate a sliding magnetic field. The term "sliding magnetic field" designates an alternating magnetic field, whose amplitude B ₀ 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, we mean the maximum value taken by a periodic quantity. The application of a sliding magnetic field is translated, at a point of the marsh, by a periodic variation of 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 ™ ^ul and a maximum amplitude B ™ ^ax .

The sliding magnetic field generator 5 may consist of several electromagnetic inductors arranged around the peripheral enclosure. FIG. 2 shows three pairs 5 ₁ , 5 ₂ and 5 ₃ of electromagnetic inductors. The upper part 5s of the inductors is positioned at the level of the free surface l _sup of the liquid alloy. Each inductor has a phase shift of 90 ° between the upper part 5s and the lower part 5i. In the examples described below, a device as described in patent application WO2014 / 155357 was used. By large face of an ingot, it is understood 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 disposed at a distance of 185 mm from the mold. In general, the distance between a coil of an inductor and the mold may 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 disposed at the periphery of the mold and set in motion relative thereto. For example, it is possible to generate a sliding magnetic field by rotating a permanent magnet.

The distance l between 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 D at a time t (continuous line), and at a time t + Dί (dashed line). On the axis of propagation, there is shown a coordinate r corresponding to the position of a marsh point. Figure 5 illustrates a time evolution of an alternating magnetic field sliding at this point. This evolution is periodic, and is carried out according to a period P. The application of a sliding magnetic field is translated, in a point of the swamp, by a periodic variation of its amplitude. Thus, the amplitude the magnetic field at a point in the marsh varies with time, between a minimum amplitude B ™ ^ul and a maximum amplitude B ™ ^ax .

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

F ₀ (r) = 1/2 sfl B ₀ ² (r) (1), where s 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 reference XYZ, linked to the mold 2, the propagation of a maximum value of the amplitude of the magnetic field B ™ ^ax , along an axis of propagation, causes, simultaneously, the propagation of a Lorentz force of maximum intensity F _max along the axis of propagation. The combination of 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 greater than 12.5 mm thick by adjusting the stirring conditions during the casting of the ingot used to obtain the flat product. after transformation of said ingot. The process of transformation comprising dissolution at a temperature above 450 ° C, quenching and tempering or ripening to obtain a flat product of aluminum alloy.

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

The inventors have discovered two casting modes for improving the properties in the thickness of the flat product:

a casting with non-stationary electromagnetic stirring

a casting with stationary electromagnetic stirring. 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, in time, the maximum amplitude of the Lorentz force F _max propagating in the marsh, the intermittent macrosegregations are attenuated or 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 / magnetic field;

the wavelength l of the sliding magnetic field.

The non-stationary electromagnetic stirring process may include any of the following characteristics, taken alone 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 of between 50 seconds and 10 minutes; the magnetic field is such that the absolute value of the variation of 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 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 the 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 adimensional number of Hartmann, in at least one point of the liquid part of the alloy, varies by at least a factor of 3, or even a factor of 5, in said time interval; the aluminum alloy is chosen from alloys of types 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 of the phase shift of the current between the coils which leads to a variation of the wavelength of the sliding magnetic field.

According to this embodiment, the method may comprise a variation of the intensity or 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 mold, a wave resonant;

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

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

Preferably, the method comprises a variation of 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 of the rotational speed 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 mold, the temporal modulation of the Lorentz force density can be obtained by modifying the polar pitch, that is to say the phase shift between the induction currents flowing in each inductor. Such a modification makes it possible to vary the wavelength 1 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 / 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 l of the sliding magnetic field are variable over time. Thus, a spatial distribution of the amplitude B ₀ (t) in the marsh, at a time t (continuous line), as well as a spatial distribution of the amplitude B ₀ (t + At), has been represented at a moment t + At (dashed line). During the time interval At, the maximum amplitude B ™ ^ax varies between B ™ ^ax (t) and B ™ ^ax (t + At). Similarly, the wavelength has been changed from A (t) to A (t + At). FIG. 4, which represents a temporal evolution of an alternating magnetic field sliding at a point, shows an embodiment in which the value of the maximum amplitude B ™ ^ax of the magnetic field varies, over time, for a constant frequency / wavelength l.

As a result, the maximum amplitude of the Lorentz force, propagating in the marsh, varies between t and t + At, between the values F _max (t) and F _max (t + At).

The temporal modulation of a force parameter is implemented during casting, during a significant period of time, preferably greater than 50% or even 80% of the duration of the casting. This temporal modulation may 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 disposed 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 one another, so as to generate a so-called vertical phase shift, or arranged side by side, so as to generate a horizontal phase shift. It is possible to use a device described in the application WO2014 / 155357, and more precisely three inductors, oriented along the vertical axis Z, are arranged facing each large face of the ingot.

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

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

Preferably, the variation of one or more force parameters takes place in a time interval that is less than or equal to the characteristic durations 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 of between 30 mm / min and 80 mm / min, the characteristic durations of the recirculations extend between 50 seconds (thickness of 400 mm, casting speed 30 mm / min) and 10 minutes (thickness of 700 mm, casting speed of 80 mm / min). Thus, the force parameters vary in a time interval At determined according to these characteristic times. 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 may be periodic, the time period of variation may be of the order of a characteristic recirculation time, that is to say be between 50 seconds and 10 minutes depending on the conditions of dimensions and speed of the casting. Preferably, in the marsh, during the temporal period of variation, the maximum density of Lorentz force varies by at least 30 Nm ³ , and advantageously by at least 40 Nm ³ , and preferably by 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 the casting, for example according to an increasing or decreasing function between the beginning and the end of the casting, the value of the force parameter varying continuously or in successive increments .

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

Advantageously, during the 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 indeed found that the suppression of intermittent macrosegregations was not optimum when the force was too weak. The minimum value from which the suppression of intermittent macrosegregations is improved depends on the set of casting parameters, in particular the mixing mode, the position of the inductors with respect to the plate and the composition of the alloy.

According to one embodiment, the frequency / and / or the maximum amplitude B ™ ^ax of the magnetic field are modified respectively by varying the frequency and amplitude of the induction current flowing in inductors. For this, the method may comprise a preliminary step of defining an operating range, that is to say a range of variation of the frequency and / or intensity of the induction current. This preliminary step comprises the determination of one or more values of frequency / intensity pairs, called critical values, generating, on the free surface of the _sup of the marsh, a resonance, the resonance resulting in the appearance of significant oscillations of said free surface l _sup , the latter being shown in Figure 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 a significant oscillation is observed.

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 pairs (frequency / intensity) at which a resonance is observed at the free surface marsh. From this curve CR, a range of variation of the intensity and / or the 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 defines 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 / or the maximum amplitude B ™ ^ax of the sliding magnetic field, makes it possible to approach temporarily 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 in acceptable safety configurations. Indeed, in the vicinity of the resonance curve, the brewing effect is particularly important.

Such a resonance curve CR depends on the casting conditions, that is to say the dimensions of the mold, the distribution system and in particular the presence of a floating frame in the liquid metal, 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 the permanent magnet or 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 a ingot section 525 mm x 1650 mm, according to the conditions of Example 3. In this figure, there is also represented abacuses representing a percentage of the intensity of a Lorentz force, said nominal , 100% corresponding to the intensity of the maximum induction current that can be used 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. intensity and frequency of the induction current lie in a space delimited by the iso-force 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 comprises a variation of the frequency of the induction current flowing through an inductor. The inventors have found that it is advantageous to vary the frequency because the resulting change in field penetration allows the force gradient to be varied more effectively in the thickness and depth of the liquid well. On the other hand, the power electronics cause the frequency variation to be faster than the intensity variation; which gives an additional degree of freedom to the weaker periods of unsteady forcing. It is indeed advantageous to decouple the hydrodynamic characteristic times characteristic times of solidification to avoid intermittent macrosegregations.

Casting with stationary electromagnetic stirring

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

The inventors have found that it is possible to limit the appearance of intermittent macro-segregations 11 by adjusting the electromagnetic stirring when the average Lorentz force applying to the liquid alloy 1i flowing at the level of the front 10 has some degree of orientation, and this in a central zone of the marsh, extending symmetrically on either side of the median plane M, between T / 2 - T / 4 and T / 2 + T / 4. The thickness of the central zone M corresponds to half the thickness of the ingot. Average Lorentz force means 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 represented in FIG. 4. The period P corresponds to the inverse of the frequency /. The inventors have observed that in the median zone, at the interface of the marsh 1f and the solid alloy 1s, at the level of the front 10, the angle b formed by the average Lorentz force F, relative to the vertical , should be advantageously less than the angle a forehead, previously mentioned, corresponding to the angle between the tangent to the front and the vertical, the angles a and b 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 II. This condition is illustrated In this figure, there is shown a section of a casting in a plane XZ. The position of the median plane M corresponds to the thickness T / 2.

Preferably, the angle of inclination of the average Lorentz force is at least 4 ° lower than the angle of inclination of the forehead, so that the average Lorentz force is more inclined towards the vertical, as 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 mean Lorentz 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 plating stationary electromagnetic stirring casting process may comprise, prior to casting, a Lorentz force modeling application at at least one point of the front, so as to define, given the thickness of the mold, a frequency value and / or a casting speed value for obtaining an average Lorentz force, whose angle of inclination with respect to the vertical, is less than the angle, at said point, formed by the front in relation to the vertical. Preferably, this modeling is performed at different points along the front along the transverse axis. The modeling can make it possible to define a frequency value and / or a value of casting velocity making it possible to obtain an average Lorentz force whose angle of inclination, with respect 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 1 f 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 adjacent interface layer. on the forehead, whose thickness 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 front, 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, plating the liquid alloy against the front, in order to maintain the liquid alloy against the front, so as to limit the formation of intermittent macro-segregations. .

In these particular conditions, the Lorentz force F tends to press the liquid alloy 1 f of the swamp against the front 10, which limits the formation of intermittent macro-segregations. The Lorentz's strength is called plating. 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 the liquid alloy by the Lorentz force against the front 10 is more marked than the casting speed V and the frequency / are low.

The skilled person can model the orientation of an average Lorentz force F, exercising during a period, in the marsh. Calculation codes, for example the AC / DC module of the COMSOL code, allow such modeling, based notably on the characteristics of the inductors (dimensions, number of ampere-turns, polar pitch, positioning relative to the mold) , the geometry of the 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 Lorentz F force, at any point of the marsh, during a period. By evolution, one understands as well the evolution of the intensity as the evolution of the direction. It is then possible to determine the orientation and the intensity of the average Lorentz force applying at a point of the marsh 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 of a front 10. In these figures, a part of a front 10 is represented, according to a XZ plane, extending between the median plane M (abscissa x = 0) and a wall of the 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 equal to 5 Hz (FIG. 12A), 1 Hz, 0.5 Hz and 0.2 Hz (FIG. 12D), the casting speed being 55 mm / min. It is observed that considering the same position on the front 10, the lower the frequency / is, the lower the angle b of the mean temporal Lorentz force F is low. Thus, in the same position on the front, the average temporal Lorentz force F tends to tilt vertically as the frequency decreases.

Moreover, as previously described, the Lorentz force is plating when the angle b of the average Lorentz force F is less than the angle a of the front. In each figure, there is shown a thickness range Dc, extending from the median plane M, in which the plating force effect is obtained. This range of width is materialized by a double arrow. It is observed that as the frequency decreases, the thickness range Dc extends from the median plane M (x = 0), corresponding to the thickness T / 2, towards the wall of the mold. The technical effect of minimizing intermittent macro-segregations appears in this thickness range Dc, and it is preferable that it be as wide as possible, preferably including the range. thickness T / 2,3 - T / 3.3, the latter being generally conducive to the formation of intermittent macro segregations. In these figures, the thickness range T / 2.3 - T / 3.3 corresponds to the interval between x = 0.03 m and 0.1 m. The T / 4 coordinate corresponds to x = 0.13 m.

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

FIGS. 12A to 12H show so-called normalized forces, each force being normalized by its intensity, so as to better show the evolution of the Lorentz average time force orientation on the front 10, depending on from the position along the front. The comparison of FIGS. 12A to 12H shows that the lower the frequency, the greater the thickness range Dc according to which the Lorentz force becomes plating. The thickness range Dc extends from the median plane M towards the wall of the mold, along the transverse axis X. It widens as the frequency decreases. Moreover, at the same frequency, the lower the casting speed, the greater the thickness range in which the Lorentz force is plating. It is therefore advantageous to favor both a frequency / low, 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 as illustrated in FIGS. 12A to 12H, the inventors have determined an evolution, along the transverse axis X, of an angle Q, called the differential angle, representing a difference between, at the same point, the angle of the front a and the angle of the Lorentz force b, that is q = a - b. It is recalled that the angles a and b are oriented in the same direction. When Q> 0, a> b: the Lorentz force is more inclined than the tangent to the front. She is therefore plastering.

FIGS. 13A and 13B show the evolution of the differential angle Q as a function of a position x on the front 10, along the transverse axis X. The abscissa 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 the position T / 2, the coordinate x = 0.26 corresponding to the wall 2p of the mold. FIGS. 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 have been carried out by successively considering several frequencies /, between 5 Hz and 0.2 Hz. In each figure, there is shown, in mixed horizontal lines, lines corresponding to the values Q = 0 ° and Q = 4 ° when the frequency is equal to 1 Hz. The Lorentz force is plaquante when Q> 0, but the inventors consider that it is advantageous that Q> 4 °. It is thus possible to define, on each configuration, a thickness range, in which the Lorentz force is plating, from the median plane M (thickness T / 2).

FIGS. 13A and 13B show thickness ranges Dc (0 = 0 °) and Dc (0 = 4 °) for / = 1 Hz. In a similar manner to FIGS. 12A to 12H, it is observed that the Thickness ranges are all the more important as the frequency is low and the casting speed is low. The optimal results are obtained for f <2 Hz, even / <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 T / 2 - T / 4, is important. This reinforces the strength of the Lorentz force and increases the desired technical effect. In other words, to obtain a significant reduction in intermittent macro segregations, an orientation of the Lorentz force as previously described 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.

With the aid of simulations taking into account different casting thicknesses, the inventors have established an abacus, shown in FIG. 14, making it possible to define an operating range for which the Lorentz force is considered sufficiently plating, that is, that is, when the differential angle Q is greater than or equal to 4 °. The axis of abscissa and ordinate of the abacus respectively corresponds to the casting speed V and the thickness T of the ingot. As the thickness is 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 applied magnetic field. The person 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. then determining a frequency range and a range of casting speed for which Q> 0 °, or advantageously Q> 4 °, is obtained so as to implement the invention and obtain the desired technical effect, that is, that is, 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 melt casting mode stationary electromagnetic electroplating as described, it was possible to obtain a flat product with a thickness of at least greater than 12.5 mm, after a conversion process comprising dissolution at a temperature above 450 ° C., quenching and tempering or a ripening. Said flat product thus obtained has a compromise of advantageous properties irrespective of the position in the thickness.

Optionally a scalping operation can be performed during the transformation process to eliminate surface defects unacceptable especially if a rolling operation is performed subsequently.

The dissolution is carried out at a temperature above 450 ° C. Preferably, the solution treatment temperature is less than T _so _iidus, solidus temperature of the aluminum alloy in order to avoid the appearance of liquid. Preferably, the solution treatment temperature is greater than T _n IVUS - l0 ° C where T _n IVUS is the temperature above which potentially soluble phases (generally consist only of elements whose content is at least 0.5%) are dissolved. Preferably, the solution annealing temperature is between T _n IVUS - 10 ° C and T _n iidus.

The solution product is quenched. The quenching can be done in air or water horizontally by spraying 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 coarse precipitation during quenching tending to deplete the solid solution of the alloy. Preferably the quenching rate is higher than the critical quenching speed of the product. It is possible, however, that the quench rate may be less than 0. 1 ° C / sec, preferably less than 0.05 ° C / sec. This type of quenching speed is particularly suitable for 7XXX alloys of AA7021 or AA7035 type.

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 type T6 or T8 or an over-income state of type T7. The heat treatment is preferably carried out at a temperature typically between 100 ° C and 200 ° C for a period of lh to 70h.

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

Flat product weakly wrought

The inventors have found that in the case where the transformation process does not include 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 the reduction of resistance by notch effect.

Typically, the transformation process does not include a hot rolling step or forging. It may, however, include a step of compressive stress relief but where the induced plastic deformation is not greater than 5%.

The inventors have found that the improvement in properties in the thickness is even more marked than 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 macrosegregations are more marked when the thickness of the cast product increases. In the particular case where there is a small plastic deformation, the intermittent macrosegregations 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 call G _T , G _L , G _S , the grain size respectively in the long cross direction, the long direction and the short cross direction of the flat product at half thickness, the ratio is less than 1.5,

preferably between 0.5 and 1.3 and even more preferably between 0.8 and 1.2. The grain size G _T , G _L , G _S is determined according to the intercepts method of the standard (ASTM El 12-12 §16.3). The grain size G _L can be determined in the longitudinal plane L / TC and corresponds to the notation g ₍ o ° ₎ of ASTM El 12-12 in Figure 7. The grain size G _T can be determined in the longitudinal plane TL / TC and corresponds to the notation t _{t (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 ig ₍ 90 ₎ of ASTM El 12-12 in Figure 7.

To determine the grain size, it is possible to perform mid-thickness optical metallography measurements 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.degree. This quenching speed range is particularly suited to aluminum alloys of the AA7021 or AA7035 type according to the standards of the Aluminum Association. The inventors have found that an aluminum alloy flat product hardened at a speed below 0.degree. C./s, typically an AA7035 or AA7021 type alloy obtained by the casting process and the process for converting it. The invention has a difference in elongation between the surface and the thickness of less than 3%, preferably less than 2.5%, more preferably less than 2%.

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

If this difference is normalized with respect to the elongation value at half thickness, a so-called elongation gap is obtained. 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 aluminum alloy product hardened at a speed below 0.1 ° C./s, typically an alloy of the AA7035 or AA7021 type obtained according to the casting process and the conversion process of the invention present between half thickness and quarter thickness a difference less than 50%, preferably less than 25% on the reduction of resistance by notch effect.

The notch resistance allowance is evaluated using the Aluminum Association Standard Test E 602-03. It consists of measuring the maximum stress of the product in a defined direction obtained on a notched cylindrical specimen. The cut resistance is defined by the ratio of the measured maximum stress to the R0.2 value measured in the same direction on a non-notched specimen according to E8 and B557. This report is also called "Notch Strength Ratio" or "NSR".

Furthermore, the inventors have found that the flat product thus obtained has an excellent aptitude for anodization in the sense that because of the intermittent macrosegregations little marked or absent, the anodized product had a homogeneous visual appearance: the anodic layer does not present differences in color or reflections associated with compositional differences. Preferably, the flat product is machined and then anodized for use as injection mold parts or vacuum chamber elements.

Wrought flat product

The inventors have found that a flat product made of aluminum alloy obtained from an ingot obtained by a casting with non-stationary electromagnetic stirring or by casting with Plating stationary stirring, as described above, had improved properties in the 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 performed. Scalping involves machining the surface of the cast ingot and removing a certain thickness of material, typically between 20 mm and 100 mm, preferably less than 50 mm thick.

The reduction ratio 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 wrought or if a scalping operation is performed before wrought, the ratio between the thickness obtained after scalping and the thickness of the final flat product obtained after wrought.

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

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

Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element whose weight content 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 of the nominal concentrations of the different elements considered.

Preferably, the flat product has a substantially non-recrystallized microstructure and such that the ratio is greater than 2, preferably comprised

between 3 and 10, more preferably between 4 and 7 where GT, GL, GS respectively correspond to the size of non-recrystallized grains in the long cross direction, the long direction and the short cross direction of the flat product. The grain size measurement is made according to the intercepts method of the standard (ASTM El 12-12 § 16.3)

It is possible to distinguish the non-recrystallized grains by a metallographic attack, for example by chromic attack.

It is advantageous for the flat aluminum alloy product to correspond to an aerospace type alloy of the 7XXX or 2XXX or 6XXX series.

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 them above the impurity level, typically 0, 05% by weight. The aerospace-grade alloy grades of the 2XXX family containing no suitable lithium and / or silver are, for example, 2014, 2022, 2023, 2024, 2026, 2027, 2056, 2224, 2324 and 2524 as defined. by the Aluminum Association. Suitable aerospace alloys of the 2XXX family containing lithium and / or silver 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 aerospace type 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 solvus temperature below 60 ° C, preferably below 40 ° C.

For this type of aluminum alloy having a small temperature difference between the solvus and solidus temperature, the inventors have shown that it is possible to obtain an optimized compromise between the tenacity, the yield strength and the elongation at break. In particular, for an AA7050 type alloy with a thickness of between 70 mm and 120 mm, the tenacity defined according to the E399-12 standard measured at mid thickness in the LT direction 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 break elongation measured in the short cross 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 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 and for any other use for which the properties of the products according to the invention are advantageous.

Here, a "structural element" or "structural element" of a mechanical construction is called 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 realized. These are typically elements whose failure is likely to endanger the safety of said construction, its users, its users or others. For an aircraft, these structural elements include the elements that make up the fuselage (such as fuselage skin (fuselage skin in English), stiffeners or stringers, bulkheads, fuselage (circumferential frarnes), wings (such as wing skin), stiffeners (stringers or stiffeners), ribs (ribs) and spars) and empennage including horizontal stabilizers and vertical stabilizers horizontal or vertical stabilizers, as well as floor beams, seat tracks and doors.

The inventors have furthermore found that the flat product has an excellent aptitude for anodization in the sense that, because of the intermittent macrosegregations which are scarcely marked or absent, the anodized product has a homogeneous visual appearance: the anodic layer does not exhibit differences in colors or reflections associated with compositional differences.

Examples

Example 1 - flat product weakly wrought

An ingot of an AA7035 alloy of section 1520 x 625 mm was cast whose composition A is shown in Table 1 below.

Table 1

This ingot was obtained using a non-stationary electromagnetic stirring.

The casting speed was 45 mm / min.

The electromagnetic stirring was obtained by placing, facing each face of the ingot, three coils oriented along the axis z and traversed by an alternating current which was out of phase, in the central coil, by 90 ° with respect 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 direction of sliding being parallel to the cross-long direction, the generated slips diverging from the mid-width of the ingot. The nonstationary forcing was obtained by imposing a cyclic variation of the alternating electric current that ran through the coils, as illustrated by the double arrow in the frequency vs intensity diagram of Figure 8. The strength of the force The maximum Lorentz volumic 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 rate 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 was dissolved at 465 ° C (8h) + 540 ° C (lh), quenched in air and then returned for 10 h at 105 ° C followed by a step of 4h at 160 ° C .

The product thus transformed was then characterized in the thickness, the yield strength Rp0.2, the breaking strength Rm and the elongation A% as defined according to the standard NF EN 1SO 6892-1 (2016), the sampling and the meaning of the test being defined by EN 485 (2016).

They were tested in the long cross-sectional area (TL), T / 8, T / 4 and T / 2.

The surface sampling is carried out so as to have the specimen tax in a plane located 5 mm from the surface. Moreover, these same properties have also been evaluated in the sense short traverse (TC), the traction test piece having its useful part centered at T / 2. The results are shown in Table 2.

Table 2

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, whose composition B is indicated in Table 1. These results are presented in Table 3 below. . Conventional production differs from the invention by the simple fact that the casting is done without electromagnetic stirring. The other parameters in particular the ingot format and its thickness, the transformation parameters are the same.

Table 3

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

But the advantage of the electromagnetic stirring process is particularly pronounced for the elongation value. Indeed, with the process according to the invention, the elongation values at half thickness are 1.5 times greater than in the non-stirred case, conventional.

It is observed that the product according to the invention has an elongation gap between the surface and the middle thickness of 1.9% against 4.9% on the conventional product. This difference corresponds to a difference in elongation between the surface and the middle thickness of 68% for the product according to the invention against nearly 233% for the conventional product.

The Notch Resistance Rating or "NSR" is evaluated 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 are also 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. It aims to illustrate the properties obtained on a wrought flat product obtained from an ingot section 1700 x 377 mm according to a conventional casting process without electromagnetic stirring.

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

The casting speed was 45mm / min.

Table 4

The ingot thus obtained undergoes homogenization for 30 hours at 482 ± 2 ° C., followed by a scalping operation. The ingot then has a front section 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 rolling, we proceed to trim and shore operations to reach a final width of 1355 mm and a length of 2990 mm. A dissolution of 2.5 hours at 479 ± 2 ° C was performed. An axial traction of 2.3% was applied, followed by an income with a first step of 4 hours at 222 ° C. and a second step of 17 hours at 165 ° C.

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

Specimens of the CT-40 type were taken at mid-thickness to evaluate the tenacity along the L-T and T-L and S-L planes. With regard to the evaluation of the tensile characteristics in the cross-court plane, specimens of the type 4d with a diameter of 6 mm and a useful length of 27 mm were taken.

The results are shown in Table 5. Table 5

The AA7050 alloy flat product according to a conventional casting process, from an ingot of thickness 377 mm and having undergone a reduction rate of 4.2 has an L yield strength of 474 MPa, a TL toughness of 36.2 MPa.m ^{1 2} and a short through elongation of 7.5%.

Example 3 - Wrought Flat Product

In this example, a casting of alloy AA7050 was made, the composition D of which is indicated in Table 6. An ingot with a section of 1650x525mm was thus obtained. The grain is refined using a refining yarn A1TiC3: 0.15 with an addition rate of 1.5 kg / ton. The casting speed was 45 mm / min.

Table 6

At the beginning of the 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 is a non-stationary electromagnetic stirring. The non-stationary electromagnetic stirring was obtained by placing, facing each face of the ingot, three coils oriented along the z axis and traversed by an alternating current which was out of phase, in the central coil, by 90 ° by relative to the current in the extreme coils. The wavelength of the sliding field was 2.4m. 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 generated slips diverging from the mid-width of the ingot. The unsteady forcing was obtained by imposing a cyclic variation of 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 FIG. the Lorentz maximum volume force 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 variation speed of approximately 0.81 N / m ³ / s. It will be noted that the stability limit curve is different from that presented in Example 1 because of both the different casting thickness and the presence of a floating frame which contributes to inhibiting certain resonant modes of the free surface. .

After a relaxation treatment of 1 Oh at 350 ° C, the ingot thus obtained was then trimmed to obtain a dimension of 1150x2240x525 mm. Then it was homogenized for 30 hours at 482 ± 2 ° C. Following a scalping operation, the ingot has the final dimensions of 1150x2240x440 mm.

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

Trimming and shore operations were carried out in order to reach a final width of 900 mm and a length of 4000 mm. A dissolution of 2.5 hours at 479 ± 2 ° C was performed. An axial traction of 2.3% was applied, followed by an income with a first step of 4 hours at 222 ° C. and a second step of 17 hours at 165 ° C.

The results are shown in Table 7

Table 7

The AA7050 alloy flat product obtained according to a conventional casting process, from an ingot of thickness 525 mm and having undergone a reduction ratio of 5.5 has an elasticity limit in the L direction of 477 MPa, a tenacity TL of 42.0 MPa.m ^{1 2} and a stretch through short of 5.0%.

The flat product AA7050 alloy obtained according to a casting process according to the invention, from an ingot of thickness 525 mm and having undergone a reduction rate of 5.5 has an elastic limit in the direction L of 479 MPa a TL toughness of 43.0 MPa.m ¹⁷² and a short through elongation of 6.4%.

Example 4 - Wrought Flat Product

In this example, an AA7050 alloy casting was made, the composition F of which is indicated in Table 8. An ingot with a section of 1650 × 525 mm was thus obtained. Grain refining is carried out using a refining yarn A1TiC3: 0.15 with an addition rate of l.5kg / ton. The casting speed was 45mm / min.

Table 8

At the beginning of the casting, the electromagnetic stirring is put place. Then after solidifying a length of about 2.1 m, the electromagnetic stirring is stopped.

Electromagnetic stirring is a non-stationary electromagnetic stirring. The electromagnetic stirring was obtained by placing, facing each face of the ingot, three coils oriented along the axis z and traversed by an alternating current which was out of phase, in the central coil, by 90 ° with respect to the current in the extreme coils. The wavelength of the sliding field was 2.4m. The electromagnetic pump elements thus obtained were arranged in a mirror with respect to the large plane of symmetry of the ingot, the direction of sliding being parallel to the cross-long direction, the generated slips diverging from the mid-width of the ingot. Unsteady forcing was achieved by imposing a cyclic variation in the nominal AC current current flowing through the coils, as illustrated by the arrows in the frequency vs intensity diagram in Figure 10. The intensity the maximum volume force of Lorentz thus generated by the change in intensity varied over a period of 1.33 min. Following an expansion treatment of 1 Oh at 350 ° C, the ingot thus obtained was then trimmed to obtain a dimension of 1145x2365x525 mm. Then it was homogenized for 30 hours at 482 ± 2 ° C. Following a scalping operation, the ingot obtained the final dimensions of 1145x2365x455 mm.

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

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

The results are shown in Table 9.

Table 9

The AA7050 alloy flat product obtained according to a conventional casting process, from an ingot of thickness 525 mm and having undergone a reduction ratio of 5.5 has an elastic limit in the direction L of 479 MPa, a tenacity TL of 42.4 MPa.m ^{1 2} and a short transversal elongation of 4.0%.

The flat product AA7050 alloy obtained according to a casting process according to the invention, from an ingot of thickness 525 mm and having undergone a reduction rate of 5.5 has an elasticity limit in the direction L of 483 MPa , a TL tenacity of 40.6 MPa.m ¹⁷² and a short through elongation of 7.4%.

Claims

claims

1. 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, 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 mold forming 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), according to 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 converting said aluminum alloy ingot comprising dissolving at a temperature above 450 ° C., quenching and tempering or maturation to obtain a flat product made of aluminum alloy,

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 the casting, by a trickling of a cooling liquid (3), forming a solid alloy (ls), extending around a liquid alloy (lf), said swamp, the solid alloy forming a front (10) at the interface with the marsh (lf), the front being inclined at an angle of inclination (a), with respect to the vertical axis, the inclination angle of the front varying along the transverse axis (X);

- Moving the solid alloy (ls) along the vertical axis of casting (Z), according to a casting speed (L);

during the 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 disposed at the periphery of the mold, so as to applying a mean 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) at an angle of inclination ( b), said angle of inclination of the Lorentz force, the latter varying along the transverse axis (X); the Lorentz force has during the period P a Lorentz force of maximum intensity (F _max );

- the marsh having a median zone, extending symmetrically on either side of the median plane (M) whose thickness corresponds to half a 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 (1), and as the electromagnetic stirring casting method 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 in a predetermined time interval (At), 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 (1) of the magnetic field; so as to obtain a modulation, in said time interval, of said maximum intensity Lorentz force (F _max ) propagating along the axis of propagation or such that the electromagnetic stirring casting process corresponds to an electromagnetic stirring mode flattening stationary 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 velocity (V) and the frequency (/) are adapted so that throughout the middle zone of the marsh, at the interface between the liquid alloy (1) and the front, the angle of inclination the force (b) is strictly less than the angle of inclination (a) of the front.

2. Flat aluminum alloy product according to claim 1 characterized in that said flat product has an aluminum alloy element whose weight content is greater than 0.5%, or for the sum of several elements. of the alloy whose individual weight content is greater than 0.5%, a dispersion criterion e of less than 3.3, preferably less than 3, more preferably 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: e = ACZA! ACZR

ACZA = max (CZA) - min (CZA), ACZR max (CZR) - min (CZR),

or :

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

- max (CZR) and min (CZR) denote respectively the maximum and minimum concentrations of the considered element or the sum of the considered elements measured in a reference zone (ZR), considered as being little affected by intermittent macrosegregations, for example between the thicknesses T / 12 and T / 6 of said flat product;

said concentrations being measured on at least one profile set at half width in a vertical plane L / TC and in the direction TC, said profile being representative of said intermittent macrosegregations of the element under consideration in the direction TC.

3. Flat aluminum alloy product according to any one of claims 1 to 2 characterized in that said transformation method does not include a step inducing a 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 1.2 where G _T , G _L, G _S, correspond respectively to the mean grain size in the long cross direction, the long direction and the short cross direction of the product. flat, the grain size being measured at mid thickness according to 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 (z) is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said spectral intensity criterion being calculated in: Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element whose weight content 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 the various elements considered.

7. flat aluminum alloy product according to any one of claims 3 to 6 characterized in that the quenching is performed at a speed less than 0.1 ° C / s.

8. flat aluminum alloy product according to any one of claims 3 to 7 characterized in that the aluminum alloy corresponds to a type designation AA7021 or AA7035.

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 TL direction 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%.

Flat aluminum alloy product according to any one of Claims 7 to 9, characterized in that the notch resistance reduction defined in accordance with the standard E 602-03 has a difference of less than 50%, preferably less than 50%. at 25% between mid-thickness and quarter-thickness.

11. Flat aluminum alloy product according to claim 1 or 2 characterized in that the transformation process comprises a wrought step having a reduction ratio R greater than 2, preferably between 3 and 10, even more preferably between 5 and 10. and 7, the reduction ratio 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 wrought or if a scalping operation is performed before wrought, the ratio between the thickness obtained after scalping and the thickness of the final flat product obtained after grinding.

12. The flat product of aluminum alloy according to claim 11 characterized in that the thickness of said flat product is between 40 and 200 mm, preferably between 70 and 120 mm.

13. Flat aluminum alloy product according to claim 11 or 12 characterized in that a spectral intensity criterion (z) is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said criterion of spectral intensity being calculated in:

Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element whose weight content 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 set 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 ratio, normalizing said maximum amplitude by a nominal concentration Co of said element or by the sum of the nominal concentrations of the different elements considered.

14. Flat aluminum alloy product according to any one of claims 11 to 13 characterized in that said flat product has a substantially non-recrystallized microstructure and such that the ratio is greater than 2, preferably

between 3 and 10, more preferably between 5 and 7 where GT, GL, GS respectively correspond to the average size of non-recrystallized grains in the long transverse direction, the long direction and the short cross direction of the flat product, the size of grain being measured according to ASTM standard El 12-12.

15. Flat aluminum alloy product according to any one of claims 11 to 14 characterized in that the aluminum alloy is an aerospace type alloy of the 7XXX or 2XXX or 6XXX series.

16. Flat aluminum alloy product according to claim 15 characterized in that the aluminum alloy has a temperature difference between the solidus temperature and the solvus temperature below 60 ° C, preferably below 40 ° C .

17. Flat aluminum alloy product according to claim 16 characterized in that it has an optimized compromise between the tenacity, the elastic limit and the elongation at break, characterized in that the aluminum alloy is an alloy AA7050, in that its thickness is between 70 mm and 120 mm, and such that the tenacity defined according to the standard E399-12 measured at mid thickness in the direction LT is greater than 40 MPa.m ^{1 2} , the limited elasticity measured in the longitudinal direction (L) is greater than 470 MPa and the break elongation measured in the short cross direction (TC) is greater than 6%, preferably greater than 7%.