ES2858125T3 - Method for the manufacture of sheet metal ingots by vertical casting of an aluminum alloy - Google Patents

Abstract

Method for casting an aluminum alloy ingot into a substantially rectangular ingot mold comprising the following steps: - preparing the aluminum alloy; - melting the aluminum alloy in the mold, along a vertical casting axis, the alloy being cooled, during casting, by a runoff of a coolant in contact with the solidified metal; - during casting, application of a magnetic field whose amplitude (B0) is periodically varied according to a frequency (f), said magnetic field being generated by at least one magnetic field generator arranged on the periphery of the mold, in such a way that a Lorentz force (F) is applied at different points of a liquid portion of the alloy in the solidification process; - the applied magnetic field is a moving magnetic field, which propagates along a propagation axis, in such a way that a maximum amplitude of the magnetic field propagates along said propagation axis, defining a wavelength of propagation (λ), said moving magnetic field drives a propagation, along said propagation axis and a Lorentz force of maximum intensity (Fmax); The method is characterized in that a magnetic parameter called the force parameter, which governs the Lorentz force of maximum intensity (Fmax), is variable in a predetermined time interval (Δt), said parameter being: - said maximum amplitude of the magnetic field; - and / or said frequency (f) of the magnetic field; - and / or the propagation wavelength (λ) of the magnetic field; in such a way as to obtain a modulation, in said time interval, of said Lorentz force of maximum intensity (Fmax) that propagates along the axis of propagation.

Description

DESCRIPTION

Method for the manufacture of sheet metal ingots by vertical casting of an aluminum alloy

Technical field

The technical field of the invention is the manufacture of ingots as a result of casting a liquid aluminum alloy.

Description of the invention

During vertical casting, with the intention of forming an ingot, solidification of a metal or metal alloy is achieved by phenomena called macroscopic segregations. During the cooling of the metal, convection currents are formed that cause recirculation vortices, the latter being the source of macroscopic segregations when their useful life is of the same order of magnitude as the characteristic durations of solidification. These phenomena lead, in the solidified ingot, to local impoverishment or local enrichment of chemical species. These macroscopic segregations, or macrosegregations, are the source of heterogeneities in the composition of the ingot.

One type of macrosegregation well known to one of ordinary skill in the art is negative central macrosegregation, which results from a depletion of eutectic alloy elements along a central vertical axis of the ingot. These macrosegregations are described in the work of John Wiley et al. "Direct-Chill Casting of Light Alloys", published by Wiley, September 2013, pages 158-172.

The main mechanisms that are the source of central macrosegregations are

• Thermosolutal convection in the sump caused by temperature and concentration gradients, and the penetration of these flows in the soft zone;

• Transportation of grains to the supercooled zone under the influence of gravity, the Archimedean force and natural or forced convection;

• Flow to the soft zone caused by volumetric contraction during solidification, which can be assisted by metallostatic pressure;

• The flow of the liquid towards the soft zone caused by mechanical deformations;

• Forced flows that can result from spillage, gas injection or release, agitation, vibration, etc. that penetrate the supercooled zone and the soft zone and modify the direction of the convection movements.

It is about continuous macrosegregation, this term designates the fact that macrosegregation occurs continuously over all or part of the height of the ingot, that is, it is essentially uniform along the casting axis.

The phenomenon of intermittent macrosegregation has been less frequently described in the literature and manifests itself by the formation of V-shaped bands on both sides of the negative central macrosegregation. These V-shaped bands are alternately enriched and depleted in eutectic and peritectic alloying elements. These bands can be seen when performing an X-ray radiograph of vertical ingot cuts, typically in the L / CT plane at the mid-width, when the segregated elements absorb the X-rays in a way that differs from the metal atoms they make up. the ingot. Other means make it possible to visualize this phenomenon, for example ultrasound or the observation with the naked eye of the anodized vertical cuts, due to the difference in optical reflectivity between enriched and depleted areas of alloying elements. Generally, intermittent macrosegregation is most marked in the T / 2.5 region of the thickness, with the T / 2 region corresponding to the central axis of the ingot. According to a nomenclature known to one skilled in the art, the term T / n, where n is a positive number, designates a region located at a distance T / n from an edge of the ingot, where T designates the thickness of the ingot.

Periodic intermittent macrosegregations appear very soon after the start of casting, as soon as a sloping front forms between a solid zone and a liquid zone. It is observed in each case of casting of aluminum alloys loaded with aluminum alloys, typically cast according to formats with thicknesses greater than 300 mm, this thickness threshold in itself depends on the casting speed.

RC Dorward et al. Publication “Banded segregation patterns in DC cast AlZnMgCu alloy ingots and their effect on plate properties,” Aluminum, volume 72, number 4, 1996, pages 251-259 describes the formation of intermittent segregation bands in an alloy. of the 7000 series. According to these authors, this phenomenon is due to avalanches of grains periodically triggered by convective oscillations of the sink, that is, the liquid phase of the metal, in relation to an eddy emission mechanism. This article shows in particular that intermittent macrosegregation can be the source of variations in the properties mechanical, for example of hardness, in the metal sheets obtained from the raw cast iron products. Therefore, it is advantageous to find a casting method that suppresses these intermittent macrosegregations. The reduction or suppression of continuous macrosegregations, eg central macrosegregation, has already been described. In particular, it has been shown that the application of a magnetic field, for the purpose of agitating or slowing down the flows, made it possible to limit the appearance of continuous macrosegregations. US5375647 describes, for example, a method for reducing central macrosegregation that occurs during casting of a metal alloy ingot. This method comprises the application, during cooling, of a static magnetic field generated by at least one coil through which a direct current flows.

Document FR2530510 describes an electromagnetic metal casting method in which a stationary magnetic field and a variable frequency magnetic field are caused to act simultaneously to produce both radial vibrations within the metal in the solidification process and to limit agitation. . B. Zhang and others "Effect of low-frequency magnetic field on macrosegregation of continuous casting aluminum alloys" Materials Letters 57 (2003) pages 1707-1711 have applied a variable magnetic field of low frequency (between 10 and 100 Hz) to a billet 200 mm of AA7075 alloy and have observed a beneficial effect on the reduction of central macrosegregation, mainly at a frequency of 30 Hz.

EP 2682201 describes an electromagnetic stirring method using two inductors mounted symmetrically to each other with respect to the vertical plane of symmetry of an ingot mold. These inductors generate two electromagnetic fields of different frequencies that propagate in opposite directions along a vertical axis. At least one of the inductors generates a magnetic field at a resonance frequency of the liquid metal.

Document WO 2014/155357 refers to methods and a device for moving a molten metal, the electromagnetic inductor comprising at least two pairs of electromagnetic poles and a first magnetic field component that is generated between a pole in a first pair of poles and a second pole in a different pair of electromagnetic poles, and a second magnetic field component between two poles in one or more pairs of electromagnetic poles, thus generating the second component of the magnetic field of one or more eddy currents in the molten metal .

Document WO 2009/018810 refers to a method and a device for the electromagnetic stirring of electrically conductive fluids, through the use of an RMF magnetic field rotating in the horizontal plane and a WMF magnetic field that migrates vertically relative to it. . The objective is to avoid asymmetric flow structures in containers filled with molten material, particularly at the beginning and during solidification. Furthermore, efficient mixing of the fluid and / or controlled solidification of the metal alloys must be obtained, while avoiding the formation of segregation zones in the structure in the solidification process. The solution is to connect the rotating magnetic field RMF and the moving magnetic field WMF discontinuously in the form of periodic and adjustable durations over time and alternately, consecutively over time, through associated induction coils.

The inventors have found that the methods described above do not effectively allow the reduction of the occurrence of intermittent macrosegregations. They propose a method that makes it possible to limit the formation of such macrosegregations, or even eliminate them, in order to better control the mechanical properties of the products resulting from casting.

Description of the invention

An object of the invention is a method for casting an aluminum alloy ingot in a substantially rectangular ingot mold that includes the following steps:

- preparation of the aluminum alloy;

- casting the aluminum alloy in the ingot mold, the alloy being cooled, during casting, by a leakage of a cooling liquid in contact with the solidified metal;

- during casting, the application of a magnetic field whose amplitude is periodically varied according to a frequency, said magnetic field being generated by at least one magnetic field generator placed on the periphery of the ingot shell to apply a Lorentz force to different points of a liquid portion of the alloy in the process of solidification;

- the applied magnetic field being a moving magnetic field, propagating along a propagation axis, in such a way that a maximum amplitude of the magnetic field propagates along such propagation axis, defining a propagation wavelength , such a moving magnetic field driving a propagation, along such axis of propagation, of a Lorentz force of maximum intensity; The method is characterized in that a magnetic parameter called the force parameter, which governs the value of the Lorentz force of maximum intensity, is variable in a predetermined time interval, such parameter being:

■ such maximum amplitude of the magnetic field;

■ and / or such frequency of the magnetic field;

■ and / or the propagation wavelength of the magnetic field;

to obtain a modulation, in such a time interval, of such a Lorentz force of maximum intensity propagating along the axis of propagation.

The method can include any of the following characteristics, taken in isolation or in combination:

- the cross section of the ingot mold, in a horizontal plane, defines a thickness and a length, the thickness being less than or equal to the length, the thickness being greater than 300 mm and preferably at least 400 mm;

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

- the Lorentz force of maximum intensity, which propagates along the axis of propagation, varies by at least 30 Nm-3 in a time interval between 20 seconds and 10 minutes;

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

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

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

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

- during casting, the variation of the Lorentz force of maximum intensity is not obtained by a periodic interruption of the displacement field.

- the dimensionless Hartmann number, at a point at least in the liquid portion of the alloy, varies by at least a factor of 3, or even a factor of 5, in such time interval;

- the aluminum alloy is selected from alloys of type 2XXX, 6XXX or 7XXX, the thickness being at least 400 mm or 450 mm.

According to one embodiment, generators are electromagnetic inductors, each inductor has a current that passes through it called an induction current. The method includes, during such a time interval:

- a variation of the intensity of the induction current;

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

- and / or a variation of the distance between an electromagnetic inductor and the mold.

According to this embodiment, the method can include a variation in the intensity or frequency of the induction current passing through an inductor, the method includes:

- a previous step of defining at least a critical value of the intensity and frequency of the induction current generated, on a free surface of the aluminum alloy flowing in the mold, a resonance wave;

- a determination of a range of variation of the intensity or frequency of the induction current as a function of such a 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 that a resonance curve is obtained, representing the critical values of intensity and frequency, generating a resonance of such free surface, the method includes a determination of a range of variation of the intensity or frequency of the induction current in a field delimited by such a resonance curve.

Preferably, the method includes varying the frequency of the induction current passing through an inductor.

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

- a variation in the distance between the permanent magnet and the mold;

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

- and / or a rotation of two permanent magnets.

Another object of the invention is an aluminum alloy ingot obtained by the method as described above and in the description that follows.

The ingot may have, for one element of the alloy, whose content by weight is greater than 0.5%, or the sum of two elements of the alloy whose individual content is greater than 0.5%, a dispersion criterion less than 3.3%, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5, such dispersion criterion being defined according to the following expressions:

£ = AC ^za / AC ^zr (6)

A ^{Cza =} max ^{(Cza) -} min (CZA) (4),

*,

kCzR - max (Czr) - min ( Czr) (5),

where:

- max (Cza) and min (Cza) respectively designate the maximum and minimum concentrations of the element in question or the sum of two elements in question measured in an analysis zone that has intermittent macrosegregations, for example, between T / 2,3 and T / 3.3;

- max (C ^zr ) and min (C ^zr ) respectively designate the maximum and minimum concentrations of the element in question or the sum of the two elements considered in a reference zone considered little affected by intermittent macrosegregations, for example, between T / 6 and T / 12;

such concentrations being measured on at least one profile established in the mean width in a vertical plane L / TC and in the direction TC, such profile being representative of such macrosegregations in such direction T ^c .

The ingot may have a spectral intensity criterion less than 0.01, preferably less than 0.007 and preferably less than 0.005, such spectral intensity criterion being calculated by:

- determine a maximum amplitude of a Fourier transform that represents an intermittent macrosegregation of an element whose content by weight is greater than 0.5% or the sum of two elements of the alloy whose individual content is greater than 0.5%, establishing the profile along said TC direction, said maximum amplitude being determined in a range of spatial amplitudes comprised between 8 and 25 mm,

- normalize such maximum amplitude by a nominal concentration Co of such element or by the sum of the nominal concentrations of the two elements in question.

Other advantages and characteristics will be revealed more clearly from the following description of particular embodiments of the invention, given by means of non-limiting examples, and shown in the Figures listed below.

Figures

Figures 1A to 1E illustrate an example of a device and a method according to the prior art and according to the invention. Figure 1A shows the main components of the device, while Figures 1B and 1C show respectively a spatial and temporal distribution of a moving magnetic field according to the prior art. Figures 1D and 1E respectively show a spatial and temporal distribution of a non-stationary moving magnetic field according to the embodiments of the invention.

Figure 2 shows a curve called the sink free surface resonance curve, showing the values, called critical values, of the intensity and frequency of an induction current at which a sink free surface resonance appears, when an electromagnetic stirring method is implemented. Figure 3 is a radiogram of a vertical section of a product obtained by implementing a first example of a method, representative of the prior art, according to a first example, called Example 1, representative of the prior art.

Figure 4 shows an example of the Zn concentration profile along a horizontal line of the vertical section shown in Figure 3, and the analysis and reference zones.

Figure 5A shows the numerical processing carried out successively on each profile obtained with a resolution of 0.1 mm. Figure 5B shows a profile resulting from the processing performed.

Figures 6A and 6B illustrate characterization profiles of a product obtained by implementing a method according to example 1. Figure 6A shows Zn concentration profiles along several horizontal lines of the vertical section shown in the Figure 3. Figure 6B shows the profiles resulting from the numerical processing performed.

Figure 7 shows the Fourier transforms of the profiles shown in Figure 6B.

Figure 8 shows a curve called the resonance curve of the free surface of the sink, obtained by implementing a method of a second example, called example 2, according to the invention.

Figures 9, 10A, 10B and 11 illustrate a characterization of a product obtained by implementing a method according to this second example. Figure 9 is a radiogram of a vertical section of the product. Figure 10A shows the Zn concentration profiles along several horizontal lines of the vertical section shown in Figure 9. Figure 10B shows the profiles resulting from the numerical processing performed on the profiles illustrated in Figure 9. Figure 11 shows the Fourier transforms of these different profiles.

Detailed description of the invention

Unless otherwise indicated, all indications regarding the chemical composition of alloys are expressed in percent by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed in% by weight is multiplied by 1.4. The designation of the alloys is made in accordance with the rules of the Aluminum Association, known to one skilled in the art.

Figure 1A illustrates an example of a casting method known from the prior art. In this example, an aluminum alloy 1 flows into an ingot mold 2, through an opening 2i. The ingot mold 2 extends along a vertical axis Z. It is delimited by a peripheral enclosure whose cross section, in a horizontal XY plane, is a parallelepiped. A cooling fluid 3, for example water, flows against the wall of the solidified product. This method is known as direct quenching semi-continuous casting ("Direct Cold Casting"). A false bottom 4 can be translated away from the opening 2i during casting. The mold 2 extends, parallel to a first horizontal axis X, with a thickness e and parallel to a second horizontal axis Y, perpendicular to the axis X, of length l. The thickness e is, for example, greater than 300 mm. It is beyond such thickness that intermittent macrosegregations 11 appear markedly.

Under the influence of cooling, a solid zone 1s is formed, in the vicinity of the refrigerated room, around a liquid zone 1l., Designated by the term "sink." The interface between the liquid zone 1l and the solid zone 1s is a front 10, the latter gradually progressing towards the center of the mold as the solidification of the alloy takes place. After cooling is complete, a parallelepiped ingot is formed, also designated by the term "product."

The alloy is an aluminum alloy of the 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX, 7XXX or 8XXX series. Alloys whose mass fraction of the alloying elements is greater than 1%, or greater than 3% or even 5% are particularly suitable for a method according to the invention, because the higher this mass fraction of these elements , more intermittently the macrosegregations are marked. The invention is particularly advantageous for 2XXX, 5XXX, 6XXX or 7XXX alloy products whose thickness is at least equal to 400 mm or 450 mm.

A magnetic field generator 5 is shown, capable of generating a magnetic field B intended to be applied to a liquid zone 1l of the alloy. Such a generator can be a permanent magnet or an electromagnetic inductor, the latter generating a magnetic field when an electric current, called an induction current, passes through it.

The magnetic field B applied to the liquid zone 11 is an alternating field, with amplitude Bo and with a frequency f. The effect of this magnetic field is to apply an agitation to the sump, under the influence of the Lorentz forces applied to the metallic liquid zone 1l. In fact, the application of a magnetic field B causes, in the alloy, the formation of a resulting electric current J resulting, within the liquid zone of the alloy subjected to the effect of the magnetic field, in the appearance of a Lorentz force F so that F oo J x B where x designates the vector product operator and co designates a proportionality relation. This Lorentz force has a component that oscillates at a frequency that is twice the frequency f of the magnetic field.

Due to the thickness of the ingot mold, the frequency f is selected to allow sufficient penetration of the magnetic field B into the sump, in order to obtain effective agitation of the liquid. The frequency f decreases as the thickness of the product increases. In the case of an aluminum alloy with a thickness greater than 300 mm, the frequency is preferably less than 5 Hz, and even more advantageously less than 2 Hz or 1 Hz.

The generator 5 can generate a moving magnetic field. The term moving magnetic field designates an alternating magnetic field, the amplitude 6o of which is not constant and varies between a minimum value and an om kx d max

maximum value D ° , the maximum 0 propagates along a propagation axis A, preferably rectilinear.

By amplitude is understood the maximum value that a periodic quantity assumes. Preferably, the axis of propagation belongs to a plane parallel to the casting direction.

The distance l that separates two maximum amplitude values of the magnetic field is the wavelength of the moving magnetic field. Figure 1B shows an example of the distribution of the amplitude Bo of a magnetic field moving along a propagation axis A at a time t (solid line), and at a time t + At (dotted line). An r coordinate corresponding to the position of a sink point is shown on the axis of propagation. Figure 1C illustrates an evolution over time of a moving magnetic field in this

point. Due to the propagation of the value of the maximum amplitude 0, the amplitude of the magnetic field at this pm in nm max

point, varies between a minimum value D ° and the value 0, the latter does not evolve over time.

A moving magnetic field generator 5 can consist of several electromagnetic inductors arranged around the peripheral enclosure. The Lorentz force, at a point of coordinates r of the sink, includes an oscillating component, modulated according to a frequency 2f that is twice the frequency of the magnetic field. The amplitude Fq of the oscillating Lorentz force can be specified according to the expression: F0 (r) = ^ d / V.Bgír) (1), where a designates the electrical conductivity.

It is possible to define a displacement speed V ^g of the magnetic field VG = f /. (2) in which case the expression F „(r) = ^ oVgBq (r) (3)

(1) can be expressed as follows:

Therefore, the amplitude of the Lorentz force at a point r of the sink depends on the square of the amplitude of the magnetic field applied at this point. The application of a moving magnetic field is manifested, at a point in the sink, through a modulation of its amplitude. Thus, the amplitude of the magnetic field at a point of the nmin nm to sink varies as a function of time, between a minimum amplitude D ° and a maximum amplitude 0. The same occurs with the density of the Lorentz force, the latter having, at a point r of the sink, a maximum value when the amplitude of the magnetic field, at this point, is maximum. When using the XYZ frame of reference, linked to ad max

In ingot mold 2, the propagation of a maximum value of the amplitude of the value of the magnetic field 0 along a propagation axis simultaneously causes the propagation of a Lorentz force of maximum intensity Fmax along the propagation axis A The combination of the forces that propagate along the propagation axis establishes a movement of the liquid along this axis, constituting an electromagnetic pump element.

The inventors have observed that by modulating with time the maximum amplitude of the Lorentz force Fmax When propagating in the sink, intermittent macrosegregations are attenuated, or even disappear, and this particularly in ingots whose thickness is greater than 300 mm.

This modulation with time can be obtained by varying a parameter, called the magnetic force parameter, controlling the amplitude of the Lorentz force density specified in equations (1) and (3), for example:

nm ax

- the value of the maximum amplitude value 0 of the magnetic field;

- the frequency f of the magnetic field;

- the wavelength l of the moving magnetic field.

When the sliding magnetic field is generated by a plurality of electromagnetic inductors arranged on the periphery of the mold, the time modulation of the Lorentz force density can be obtained by modifying the polar pitch, that is, the phase shift between the currents. induction that circulate in each inductor. Such a modification makes it possible to vary the wavelength l of the moving magnetic field, that is, the distance between two maximum values propagating along the axis of propagation. The frequency of the induction current flowing in the inductors can be variable, which modifies the frequency f of the magnetic field. The amplitude of the induction current can also be variable, which modifies the value of the maximum amplitude d max

0 of the magnetic field. Figure 1D shows a mode in which the value of the maximum amplitude

0 of the magnetic field and the wavelength X of the moving magnetic field are variable with time. Thus, a spatial distribution of the amplitude B ^o (t) in the sink is shown, at an instant t (solid line), as well as a spatial distribution of the amplitude Bo (f + Af), at an instant t + At (dash line). During the nm á x D m á x ( A. \ R> m á xfj. I

time interval Af, the maximum amplitude 0 varies between 0 1 ' and and 0 and "r'. Similarly, the wavelength X has been modified, going from 2 (t) to Z ( t At). In Figure 1E, which represents an evolution over time an alternating sliding magnetic field at a point, a modality is shown in which the value of the amplitude p> max

The maximum 0 of the magnetic field varies with time for a constant frequency f and a wavelength X. For this reason, in the examples shown in Figures 1D and 1E, the maximum amplitude of the Lorentz force, which propagates in the sink, it varies between t and t + Dt between the values Fm¿x ( t) and Fm¿x ( t + Dt).

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

A particular moving magnetic field B can be generated by two inductors arranged on the same face of the ingot. The inductors are preferably arranged facing one large face of the ingot, that is, one of the two faces of the ingot that has the largest vertical cross section. The inductors can be superimposed on each other to cause a so-called vertical phase shift, or arranged side by side to cause a horizontal phase shift. In the examples described below, a device described in application WO2014 / 155357 has been used, and more precisely according to the configuration described in relation to Figures 19 and 20A, in which three inductors, oriented along of the vertical axis Z, are arranged facing each other to the long face of the ingot.

The traveling magnetic wave can also be generated by using one or more permanent magnets arranged on the periphery of the mold and moved relative to the latter. For example, it is possible to generate a moving magnetic field by rotating a permanent magnet.

A variation of the parameters of the moving magnetic field, be it its amplitude, its frequency or its wavelength, makes it possible to apply a non-stationary Lorentz force in the sink. The inventors have pointed out that this makes it possible to attenuate the appearance of intermittent macrosegregations or even to make them disappear. These conditions probably influence the recirculation that occurs spontaneously in the sump and reduce its consequences.

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

Preferably, the variation of one or more force parameters takes place in a time interval less than or equal to the characteristic durations of the recirculations generated by natural convection. These durations vary depending on the thickness of the ingot and the casting speed. Considering thicknesses e comprised between 300 mm and 700 mm, and casting speeds comprised between 30 mm / min and 80 mm / min, the characteristic times of the recirculations extend between 20 seconds (thickness 300 mm, casting speed 30 mm / min ) and 10 minutes (thickness 700 mm, casting speed 80 mm / min). Therefore, the force parameters vary over a period of time. Dt determined based on these characteristic durations. What is understood by variation is a significant variation of at least 10% of the force parameter considered, and preferably at least 20% or even 30% of the force parameter.

The variation of a force parameter can be periodic, the period of time of variation being of the order of a characteristic recirculation duration, that is, between 20 seconds and 10 minutes depending on the dimensional conditions and the casting speed. . Preferably, in the sump, during the period of time of variation, the maximum density of the 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 monotonic, for example according to an increasing or decreasing function between the beginning and the end of casting, varying the value of the force parameter continuously or in successive increments.

Advantageously, during casting, the Lorentz force of maximum intensity is not equal to zero. Typically, it is equal to zero when the current in the inductors or coils is equal to zero. Therefore, advantageously, the variation of the force parameter is not obtained by a periodic interruption of the displacement field. Advantageously, during casting, the Lorentz force of maximum intensity is greater than 80 N / m3, preferably greater than 100 N / m3, preferably greater than 120 N / m3, even more preferably greater than 140 N / m3. In fact, the inventors have observed that the suppression of intermittent macrosegregations was not optimal when the force was too weak as shown in example 5 (Figure 20 a to d). The minimum value at which the suppression of intermittent macrosegregations is improved depends on all the casting parameters, in particular the stirring mode, the position of the inductors relative to the plate and the composition of the alloy.

Dma #

According to one embodiment, the frequency f and / or the maximum amplitude 0 of the magnetic field are modified respectively by varying the frequency and the amplitude of the induction current flowing in the inductors. To this end, the method may comprise a previous step of defining an operating range, that is, a range of variation in frequency and / or intensity of the induction current. This previous stage comprises the determination of one or more values of frequency / intensity pairs, called critical values, generating ^{a resonance in the free surface 1 sup} of the sink, manifesting itself by the appearance of significant oscillations of such free surface 1 ^sup , this The latter is shown in Figure 1A. 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, it is observed that the frequency of the current is fixed and the intensity of the induction current increases up to a significant oscillation.

Considering different critical values of frequency (or intensity), it is possible to determine experimentally a resonance curve R, in a frequency / intensity plane corresponding to different pairs (frequency / intensity) in which a resonance is observed on the free surface of the sink . From this R curve, a range of variation of the intensity and / or frequency is determined, to avoid or limit the appearance of a resonance of the free surface of the sink. In fact, the resonance curve delimits a zone of stability and a zone of instability in which casting can become dangerous. However, the fact of modulating the frequency or intensity of the induction current, and therefore the frequency f or the maximum amplitude pm max

0 of the moving magnetic field, makes it possible to temporarily approach the resonance curve R, for example, periodically, while remaining in the stability zone. This allows the intensity of the Lorentz force to be maximized, and therefore the agitation of the sump, while remaining within acceptable safety settings. In fact, in the vicinity of the resonance curve, the stirring effect is particularly great.

A resonance curve R of this type depends on the casting conditions, that is, on the dimensions of the ingot mold, on the casting speed, on the configuration of the applied magnetic field, the latter depending on the magnetic field generator, that is that is, of the inductors or permanent magnets used. In Figure 2 a resonance curve R is shown, obtained this curve by casting a 525 mm x 1650 mm thick ingot, using a casting speed of 45 mm / min, having carried out magnetic stirring by means of the application of a magnetic field by three inductors arranged in front of each large face of the ingot and 90 ° out of phase to form a horizontal electromagnetic pump element, as mentioned above. This Figure also shows templates that show a percentage of the intensity of a Lorentz force, called 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 a frequency of 0.2 Hz. Preferably, the intensity and frequency of the induction current are located in a space delimited by the curve that shows a certain percentage of the intensity of the nominal Lorentz force, for example, 10% of this intensity, and the resonance curve.

Preferably, the method includes varying the frequency of the induction current passing through an inductor. The inventors have found that it is advantageous to vary the frequency because the resulting variation in field penetration allows the force gradient within the thickness and depth of the liquid well to be varied more effectively. In addition, power electronics make frequency variation faster than intensity variation, providing an additional degree of freedom towards smaller periods of non-stationary forcing. In fact, it is advantageous to decouple the hydrodynamic characteristic times from the solidification characteristic times to avoid intermittent macrosegregations.

Another example of a curve is shown in Figure 8 and will be discussed subsequently in connection with the examples. Figures 2 and 8 show the experimentally determined resonance curve R, as well as the curve that represents a Lorentz force whose intensity is equal to 10% of the previously defined nominal Lorentz force.

The variation of one or more force parameters can allow, in particular, alternating periods during which the dimensionless Hartmann number Ha is respectively small, typically less than 1, and high, typically greater than 3, or even than 5. The number Hartmann's dimensionless Ha is a number that is currently used in the field of magnetohydrodynamics. Represents the relationship between the magnetic viscosity and the viscosity of a charged liquid flowing in a magnetic field. The higher this number, the greater the contribution of the Lorentz forces. Preferably, the dimensionless Hartmann number Ha alternates with a ratio between low and high values of at least 3 or at least 5. Such a configuration is preferred, because it allows alternating periods in which the kinetic energy applied by the magnetic field is opposes the natural convection of liquid metal, and periods in which natural convection predominates.

As will be described in connection with the examples presented below, the products obtained by a method according to the invention have limited intermittent macrosegregation, or even no perceptible, compared to the prior art methods. In the examples that follow, the characterization of the products was performed by analyzing horizontal profiles (along the TC axis) of a radiogram performed in the mean width along a vertical L / TC plane, these profiles being sampled to obtain the spatial distribution of heavy alloy elements of the Zn or Cu type. The most absorbent areas enriched with such heavy elements appear in the form of dark spots on the negative of the radiograms produced and therefore white spots on the radiograms presented. An example of how to obtain a Zn concentration profile from a radiogram of an Al-Zn alloy is presented in Figure 4.

The terms L, TL and TC, known to a person skilled in the art, correspond respectively to the dimension of the ingot along the vertical axis, the axis called "long transverse" and the axis called "short transverse." Additionally or alternatively, it is possible to perform chemical analyzes along horizontal profiles to quantify the spatial distribution of such chemical elements along the TC axis. An intermittent macrosegregation can be characterized by a maximum separation in the mass of an alloying element, in this case Zn, in the area most marked by intermittent macrosegregation, that is, in the vicinity of T / 2.5.

To quantify intermittent macrosegregations, concentration profiles, obtained by radiography or by any other method, with a spatial resolution of 0.1 mm, have been processed, as illustrated in Figure 5A. The profile obtained with the resolution of 0.1 is the raw profile called profile A. A moving average greater than 2 mm makes it possible to dispense with microsegregation; The smoothed profile obtained is called profile B. Another moving average of the raw profile greater than 50 mm makes it possible to dispense with intermittent macrosegregations and obtain the continuous macrosegregation profile, the profile obtained being called the base profile, called profile C. Profile C is subtracted from the profile B to obtain a profile called the corrected profile, which corresponds to intermittent macrosegregation, and the corrected profile is called profile D. A profile of this type is shown in Figure 5B. As can be seen in this Figure 5B, the corrected profile is mainly representative of intermittent macrosegregation, and is not or little affected by continuous central macrosegregation and microsegregation. Such a corrected profile makes it possible to characterize intermittent macrosegregation.

Then it is possible to calculate a maximum concentration gap in an analysis zone Za located between T / 2.3 and T / 3.3, and this maximum gap can be expressed according to the following equation:

where max (C ^za ) and min (C ^za ) will designate respectively the maximum and minimum concentrations of the element in question measured between T / 2.3 and T / 3.3.

The element in question is an element whose content by weight in the alloy is greater than or equal to 0.5%. This can preferably be the main element of the alloy, the term main element corresponds to the definition given by The Aluminum Association.

The maximum gap DC ^za can be normalized by the nominal concentration Co of the element in question. The products according to the invention preferably have such a normalized ratio of less than 10% and preferably less than 8% or even less than 6%. However, the absolute value of DC ^za can be influenced by the thickness of the product, the nature of the element in question, particularly by its partition coefficient and / or its concentration. Therefore, it is useful to characterize the products obtained by the method according to the invention to calculate, as a reference, a maximum gap in a Zr zone with little sensitivity to intermittent macrosegregations, located between T / 6 and T / 12 , this maximum gap can be expressed according to the following equation:

where max (C ^zr ) and min (C ^zr ) designate respectively, the maximum and minimum concentrations of the element in question measured between T / 6 and T / 12.

A dispersion criterion e is obtained as follows, which allows evaluating the intermittent macrosegregation for the element in question:

To dispense with local variations in composition, it is advantageous, to determine DC ^za and DC ^zr , to calculate an average over at least 5 concentration profiles separated by at least 10 mm.

The smallest and that is, the least marked are the intermittent macrosegregations. The products obtained by the method according to the invention preferably have a dispersion criterion of less than 3.3, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5.

According to a nomenclature known to those skilled in the art, T / n designates a distance relative to an edge of the ingot, along a horizontal axis, T / 2 corresponding to the center of the ingot.

It is also useful to perform a Fourier transform analysis of the raw composition profile and normalize it to the nominal composition of the element. An analysis of this type makes it possible to identify spatial periods that characterize intermittent macrosegregation. Intermittent macrosegregation has periods between 8 and 25 mm. When the intermittent macrosegregation is large, a peak is therefore observed in the amplitude of the Fourier components for spatial periods between 8 and 25 mm. A dimensionless criterion of spectral intensity Z is determined, which corresponds to the maximum amplitude of the Fourier component in a spatial period interval between 8 and 25 mm, normalized by the nominal concentration Co of the element in question. The products obtained by the method according to the invention preferably have a Z criterion of less than 0.01, preferably less than 0.007 and preferably less than 0.005.

The e-scattering and Z spectral intensity criteria are advantageously applied to the main element of the alloy in question, typically to Zn for a 7xxx alloy or Cu for a 2xxx alloy. It is also possible to apply these criteria to the sum of two elements, for example the sum Zn Cu in certain 7xxx alloys or the sum Mg Si in 6xxx alloys. These criteria can also be applied to an element whose content by weight in the alloy is greater than or equal to 0.5% or the sum of two elements of the alloy, whose individual content is greater than 0.5%.

In the case that the sum of two elements is considered, the values to normalize the maximum gap DC ^za , and / or the Fourier transform correspond to the sum of the nominal concentrations of the elements in question. The ingots of rectangular cross section obtained by the method according to the invention can be used as casts or after working, optionally after solution treatment and cooling and aging for the structural hardening alloys. Advantageously, the rectangular section ingots obtained by the method according to the invention are rolled and / or forged.

Example 1.

A casting of an AA7035 alloy has been made without electromagnetic stirring. The composition of the molten alloy comprises a nominal Zn concentration of 5.6% by weight, a nominal Mg concentration of 1.3% by weight. The format of the ingot was 1650 mm x 525 mm. This example is representative of the prior art. The refining of the grain has been achieved with a concentration of the refining agent AlTiB 5: 1 of 1 Kg / t. The casting speed was 35 mm / min. Figure 3 shows a radiograph of the ingot in the middle width in an L / CT plane, in which negative central macrosegregation and intermittent macrosegregation are clearly identified. In Figure 6A, different raw horizontal profiles of the Zn content are shown, along a TC axis, as well as the smoothed profiles B obtained by means of a moving average greater than 2 mm deduced from Figure 3. The radiogram allows quantify the elements that are the source of a contrast with respect to aluminum, specifically Zn in the present case. This observation applies to Example 2 below. Negative central macrosegregation is clearly observed, which is maximum in T / 2, observing intermittent macrosegregations between T / 2.3 and T / 3.3. In Figure 6B, the different corrected profiles of Zn content (profiles D) are shown, along a TC axis, obtained after subtracting each smoothed profile (profile B) by a base profile (profile C) representative of macrosegregation continues.

The value of the maximum gaps in the Zn content was 0.75% by weight for DC ^za and 0.19% by weight for DC ^zr , the value of the maximum normalized gaps in the analysis zone and in the zone of reference is, therefore, respectively 13.3% and 3.5%. The value of the dispersion criterion e as defined by equation (6) was 3.9. The Fourier transform of each profile was calculated, and is shown in Figure 7, after normalization by the nominal composition of Zn: 5.6% by weight. The abscissa axis represents the spatial period, between 0 and 30 mm. Different predominant peaks are observed, corresponding to different spatial periods distributed between 8 and 25 mm, and more particularly between 10 and 25 mm. The spectral intensity criterion Z, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm, normalized by the nominal concentration Co of Zn, was at least 0.01 for all profiles.

Example 2.

In a second example, a casting of an AA7035 alloy was carried out with electromagnetic stirring. The composition of the coulaitr alloy comprised a nominal zinc concentration of 5.6% by weight and a nominal magnesium concentration of 1.3% by weight. The format of the ingot was 1650 mm x 525 mm. The refining of the grain was carried out with a concentration of the refining agent AlTiB 5: 1 of 1 Kg / t. The casting speed was 35 mm / min. The electromagnetic agitation was obtained by placing, facing each face L / TL of the ingot, (corresponding to a YZ plane in the reference frame indicated by Figure 1A), three inductors oriented along the vertical axis Z, through which an alternating current of frequency 0.25 Hz passes, out of phase with each other at 60 ° and spaced 0.6 m apart, thus constituting an electromagnetic pump element. The distance between the inductors and the ingot was 172 mm. The electromagnetic pump elements on each face were oriented in opposite directions. The inductors generated a magnetic field moving in a horizontal direction, the moving axis being parallel to the TL direction; wavelength l was 3.6 m. The density of the maximum Lorentz force induced in the liquid sump was varied between approximately 180 N / m3 and 240 N / m3 with a variation rate of 2 Nm-3.s-1 modifying the nominal value of the current in the inductors. The resonance curve corresponding to these melting conditions is shown in Figure 8. The variation of the intensity of the induction current is shown in this Figure with a double arrow.

Figure 9 shows a radiogram of the ingot according to an L / TC plane, in which the negative central macrosegregation in T / 2 is identifiable. Figure 10A shows different rough horizontal profiles of Zn content (profile A) and smoothed (profiles B), along a TC axis. Negative central macrosegregation is distinguished, which is a maximum in T / 2. Figure 10B shows the different horizontal profiles of Zn content, along a TC axis, of the type of corrected profile (profile D) obtained after subtracting the profile corresponding to continuous macrosegregation.

The value of the maximum gaps in Zn content was 0.24% by weight for AC ^za and 0.28% by weight for DC ^zr , being the value of the maximum normalized gaps in the analysis zone and in the zone reference 4.3% and 5%, respectively. The value of the dispersion criterion e as defined by equation (6) was 0.9: intermittent macrosegregation was eliminated in the analysis zone between T / 2.3 and T / 3.3. The Fourier transform of each profile was calculated, and is shown in Figure 11, after normalization by the nominal composition of Zn: 5.6% by weight. The abscissa axis represents the spatial period, between 0 and 30 mm. Now there are no predominant peaks. The criterion of spectral intensity Z, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm normalized by the nominal concentration Co of Zn, was less than 0.005 for all profiles.

Example 3

In this example, an AA 7050 alloy has been cast without electromagnetic stirring. The composition of the alloy was 6.3% by weight of Zn, 2.2% by weight of Mg and 2.1% by weight of Cu. The format of the ingot was 1650 x 525 mm. The refining of the grain was carried out by means of an AlTiC3: 0.15 refining wire with an addition rate of 1 kg / ton. The casting speed was 45 mm / min. This constitutes the reference for example 4.

Figure 12 shows a radiogram of the ingot along an L / TC plane, in which the negative central macrosegregation in T / 2 is identifiable. Figure 13a shows the horizontal smoothed profile of the sum of two elements, Zn and Cu (profiles B) along a TC axis, deduced from the radiogram in Figure 12. In fact, the radiography only allows quantifying the elements which are the source of a contrast with respect to aluminum, namely, in the present case Zn and Cu. This observation applies to the following examples 4 and 5. Figure 13b shows the different horizontal profiles of the Zn Cu concentration, along a TC axis, of the corrected profile type (D profiles), obtained after subtract the profile corresponding to continuous macrosegregations. The value of the maximum spaces of the sum Zn Cu was 0.81% by weight for AC ^za and 0.19% for AC ^zr . The value of the dispersion criterion e as defined by equation (6) was 4.4. Figure 14 shows the Fourier transform of each profile, after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The abscissa axis represents the spatial period, between 0 and 30 mm. The criterion of the spectral intensity Z, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm normalized by the sum of the nominal compositions of Zn and Cu, was greater than 0.01 for one of the profiles or greater to 0.007 for all profiles.

Example 4

In this example, a casting of AA 7050 alloy was carried out. The composition of the alloy was 6.3% by weight of Zn, 2.2% by weight of Mg and 2.1% by weight of Cu. The cross section of the ingot was 1650 x 525 mm. The refining of the grain was carried out by means of a refining wire AITiC3: 0.15 with an addition rate of 1 kg / ton. The casting speed was 45 mm / min. The electromagnetic agitation was obtained by placing, in front of each L / TL face of the ingot, (corresponding to a YZ plane in the reference frame indicated in Figure 1A) three coils oriented along the Z axis with an alternating current flowing through them. and that it was out of phase, in the central coil, by 90 ° in relation to the current in the terminal coils. The wavelength of the traveling wave was 2.4 m. The electromagnetic pump elements thus obtained were placed in a mirror arrangement relative to each L / TL face of the ingot, the direction of displacement being parallel to the long transverse direction, the displacement generated diverging from the mean width of the ingot. The non-stationary forcing was obtained by imposing a cyclical variation of the frequency of the alternating electric current that passes through the coils, as illustrated by the double arrow in the frequency vs. intensity diagram of Figure 15. The maximum density of the Lorentz force generated by the variation of the frequency between 0.450 and 0.600 Hz varied between approximately 110 N / m3 and 150 N / m3 over a period of 3 min, which corresponds to a rate of change of approximately 0.22 N / m3 / s.

Figure 16 shows a radiogram of the ingot along an L / TC plane, in which the negative central macrosegregation at T / 2 is identifiable. Intermittent macrosegregations are very strongly attenuated compared to the reference (Figure 12), as shown in Figures 17a and 17b.

Figure 17a shows the smoothed horizontal profile of the sum of Zn Cu elements (B profiles) along a TC axis, deduced from the radiogram in Figure 16. Figure 17b shows the different horizontal profiles of the sum of the two elements Zn and Cu, along a TC axis, of the type of corrected profile (D profiles) obtained after subtracting the profile corresponding to continuous macrosegregation. The value of the maximum Zn Cu gaps was 0.30% by weight for ACza and 0.14% for ACzr. The value of the dispersion criterion e as defined by equation (6) was 2.2. Therefore, intermittent macrosegregation in the analysis zone has been reduced and is shown in Figure 18, after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The abscissa axis represents the spatial period, between 0 and 30 mm. The spectral intensity criterion Z, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm normalized by the sum of the nominal composition of Zn and Cu, was less than 0.005 for all profiles.

Example 5

In this example, an AA7050 alloy casting was carried out. The composition of the alloy was 6.3% by weight of Zn, 2.2% by weight of Mg and 2.1% by weight of Cu, the content of the other elements all being less than 0.05% by weight. . The cross section of the ingot was 1650 x 525 mm. The grain refining was carried out by means of an AITiC3: a 0.15 refining wire with an addition rate of 1 kg / ton. The casting speed was 45 mm / min. The electromagnetic agitation was obtained by placing, facing each L / TL face of the ingot (corresponding to a YZ plane in the reference frame indicated in Figure 1A) three coils oriented along the z axis through which an alternating current passed that it was out of phase, in the central coil, by 90 ° with respect to the current in the terminal coils. The wavelength of the displacement field was 2.4 m. The electromagnetic pump elements obtained in this way were placed in a mirror arrangement relative to each L / TL face of the ingot, the direction of displacement being parallel to the long transverse direction, the displacement generated diverging from the mean width of the ingot.

The non-stationary forcing was obtained by imposing a variation from zero of the intensity of the alternating electric current that passes through the coils, as illustrated by the arrows in the frequency vs. intensity diagram of Figure 19. The intensity of the maximum volume Lorentz force generated by intensity variation typically ranged from 0 N / m3 to 140 N / m3 in 4 min, corresponding to a rate of change of 0.58 N / m3 / s. Subsequently, the intensity of the maximum volume of the Lorentz force was varied between 140 N / m3 and 360 N / m3 in 5 min, which corresponds to a variation rate of 0.73 N / m3 / s.

The results obtained are illustrated by the two vertical X-rayed sections shown in Figure 20a (variation of the intensity between 0 N / m3 up to 140 N / m3 in 4 min) and in Figure 21a (variation of the force between 140 N / m3 up to 360 N / m3 in 5 min) that are continuous from one to the other.

Figure 20b shows the smoothed horizontal profile of the sum of the main Zn Cu elements (B profiles) along a TC axis, deduced from the radiogram of Figure 20a. Figure 20c shows the different horizontal profiles of the sum of the Zn Cu elements, along a TC axis, of the type of corrected profile (Profiles D) obtained after subtracting the profile corresponding to continuous macrosegregation. The value of the maximum gaps of the Zn Cu content was 0.70% by weight for ACza and 0.22% for ACzr. The value of the dispersion criterion e as defined by equation (6) was 3.2. Figure 20d shows the Fourier transform of each profile after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The abscissa axis shows the spatial period, between 0 and 30 mm. The criterion of spectral intensity Z, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm normalized by the sum of the nominal compositions of Zn and Cu, was less than 0.01 for all profiles. However, it is observed that the spectral intensity criterion Z shows values greater than 0.005.

Figure 21b shows the smoothed horizontal profile of the sum of the main Zn Cu elements (B profiles) along a TC axis, deduced from the radiogram of Figure 21a. Figure 21c shows the different horizontal profiles of the sum of the m major elements Zn Cu, along a TC axis, of the type of corrected profile (D profiles) obtained after subtracting the profile corresponding to continuous macrosegregation. The value of the maximum gaps in the Zn Cu content was 0.37% by weight for AC ^za and 0.15% for AC ^zr . The value of the dispersion criterion e as defined by equation (6) was 2.4. Figure 21d shows the Fourier transform of each profile, after normalization by the sum of the nominal compositions of Zn and Cu: 8.3 % by weight. The abscissa axis shows the spatial period, between 0 and 30 mm. The criterion of spectral intensity Z, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm normalized by the sum of the nominal compositions of Zn and Cu, was less than 0.005 for all profiles.

Thus, it is observed that the suppression of intermittent macrosegregations is improved if the force is greater than 140 N / m3. In fact, when the force is too weak, it is observed that the values of the dispersion e, of spectral intensity Z are greater than the preferred values of the invention. Therefore, the inventors assume that a non-stationary force consisting of periodically interrupting the displacement field would not make it possible to advantageously suppress intermittent macrosegregations.

Claims (14)

1. A method for casting an aluminum alloy ingot into a substantially rectangular ingot mold comprising the following steps: - prepare the aluminum alloy; - melting the aluminum alloy in the ingot mold, along a vertical casting axis, the alloy being cooled, during casting, by a runoff of a coolant in contact with the solidified metal; - during casting, application of a magnetic field whose amplitude (Bo) is periodically varied according to a frequency (f), said magnetic field being generated by at least one magnetic field generator arranged on the periphery of the mold, so that a Lorentz force (F) is applied at different points of a liquid portion of the alloy in the solidification process; - the applied magnetic field is a moving magnetic field, which propagates along an axis of propagation, in such a way that a maximum amplitudeâ 0 > of the magnetic field propagates along said axis of propagation, defining a longitude of propagation wave (A), said moving magnetic field drives a propagation along said propagation axis and a Lorentz force of maximum intensity ( Fm¿x); The method is characterized in that a magnetic parameter called the force parameter, which governs the Lorentz force of maximum intensity ( Fm¿x), is variable in a predetermined time interval (At), said parameter being: ( nmax \ ■ said maximum amplitude '0' of the magnetic field; ■ and / or said frequency (f) of the magnetic field; ■ and / or the propagation wavelength (A) of the magnetic field; in such a way that a modulation is obtained, in said time interval, of said Lorentz force of maximum intensity ( Fm¿x) that propagates along the propagation axis. 2. Method according to claim 1, wherein the section of the ingot mold, in a horizontal plane, defines a thickness (e) and a length (l), the thickness being less than or equal to the length, the thickness being greater at 300mm and preferably at least 400mm. 3. Method according to any of the preceding claims, wherein the frequency of the magnetic field is less than 5 Hz, or 2 Hz or 1 Hz. 4. Method according to any of the preceding claims, wherein the Lorentz force of maximum intensity ( Fm¿x), propagating along the axis of propagation, varies by at least 30 Nm-3 in an interval of time (At) between 20 seconds and 10 minutes. 5. Method according to any of the preceding claims, wherein the magnetic field is such that the absolute value of the variation in the density of the maximum Lorentz force is greater than or equal to 0.05 Nm-3.s-1 during said time interval (At). 6. Method according to any of the preceding claims, wherein the axis of propagation of the maximum amplitude of the magnetic field belongs to a plane parallel to the melting direction. 7. Method according to any of the preceding claims, wherein during casting, the variation of the force parameter is periodic, the period being between 20 s and 20 minutes, or between 1 minute and 15 minutes, or between 2 minutes and 10 minutes. 8. Method according to any of the preceding claims, wherein the generators are electromagnetic inductors, each electromagnetic inductor having a current that flows through this so-called induction current, the method comprising, during said time interval: - a variation in the intensity of the induction current; - and / or a variation of the frequency of the induction current; - and / or a variation of the distance between an electromagnetic inductor and the mold. Method according to claim 8, comprising a variation in the intensity or frequency of the induction current flowing through an inductor, the method comprising: - a previous step of defining at least a critical value of the intensity and frequency of the generation of induction current, on a free surface (1 ^sup ) of the aluminum alloy flowing in the mold, a resonant wave; - a determination of a range of the variation in the intensity or in the frequency of the induction current according to such a previously defined critical value. 10. Method according to claim 9 comprising, during said previous step, a definition of a plurality of critical values of the intensity and frequency of the induction current, so as to define a resonance curve (R) , which represents the intensity and frequency values that generate a resonance of said free surface, the method comprising the determination of an interval of variation in the intensity or frequency of the induction current in an interval delimited by said curve resonance. Method according to any of claims 1 to 7, wherein at least one generator is a permanent magnet, the method comprising: - a variation in the distance between the permanent magnet and the mold; - and / or a rotation of the permanent magnet, and a variation in the speed of rotation of the magnet; - and / or a rotation of two permanent magnets. 12. Method according to any of the preceding claims, wherein the aluminum alloy is chosen from alloys of types 2XXX, 5XXX, 6XXX or 7XXX and wherein the thickness is at least 400mm or 450mm. 13. Aluminum alloy ingot, obtained by the object of the method of any of claims 1 to 12 having, for an element of the alloy, whose content by weight is greater than 0.5%, or for the sum of two elements of the alloy whose individual content by weight is greater than 0.5%, a dispersion criterion less than 3.3, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5, defining the dispersion criterion according to the following expressions: £ = ACzV ACzr (6) A ^{Cza -} max (C ^za ) - min (CZ ^a ) (4), h C zn - max (Czr) - min (Czr) (5), where: - max (Cza) and min (Cza) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the two elements considered measured in an analysis zone (ZA), which have intermittent macrosegregations, for example between T / 2 , 3 and T / 3.3; - max (Czr) and min (Czr) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the two elements considered measured in a reference zone (ZR), considered as little affected by intermittent macrosegregations, for example , between T / 6 and T / 12; said concentrations being measured on at least one profile established in the mean width of a vertical plane L / TC and according to the direction TC, said profile being representative of said intermittent macrosegregations of the element considered according to the direction TC. 14. Aluminum alloy ingot, according to claim 13, whose 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 by: - determine a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element whose content by weight is greater than 0.5% or the sum of two elements of the alloy whose individual content is greater than 0.5 %, establishing the profile according to said TC direction, determining said maximum amplitude in an interval of spatial periods between 8 and 25 mm, - standardize said maximum amplitude by means of a nominal concentration C ^or of said element or by the sum of the nominal concentrations of the two elements considered.