METHOD FOR PRODUCING SHEET INGOTS BY VERTICAL CASTING OF AN ALUMINIUM ALLOY

Abstract

A method for casting a metal alloy in an ingot mold extending along a vertical axis, the horizontal section of the ingot mold being parallelepiped in shape. During casting, a travelling alternating magnetic field is applied to a liquid phase of the alloy, the magnetic field having a maximum amplitude propagating along an axis of propagation. Under the effect of the magnetic field, a Lorentz force is applied to the liquid phase of the alloy, such that a Lorentz force of maximum intensity propagates along the axis of propagation. The method includes modulating the maximum intensity of the Lorentz force propagating along the axis of propagation. This modulation is obtained by varying, over time, one or more parameters, referred to as force parameters, governing the Lorentz force. An ingot obtained by the method is also described.

Description

TECHNICAL FIELD

The technical field of the invention is the producing of ingots following a casting of a liquid aluminum alloy

DESCRIPTION OF THE INVENTION

During a vertical casting, aiming to form an ingot, the solidification of a metal or of a metal alloy is affected by phenomena referred to as macroscopic segregations. During the cooling of the metal, convection currents are formed, generating recirculation vortexes, the latter being at the origin of macroscopic segregations when their lifespan is of the same magnitude as the characteristic durations of solidification. These phenomena lead, in the solidified ingot, to a local depletion or to a local enriching with chemical species. These macroscopic segregations, or macrosegregations, are at the origin of heterogeneities in the composition of the ingot.

A macrosegregation well known to those skilled in the art is the negative central macrosegregation, resulting from a depletion in eutectic alloy elements, along a vertical central axis of the ingot. These macrosegregations have been described in the work of John Wiley et al “Direct-Chill Casting of light alloys”, Wiley Publishing, September 2013, pp 158-172.

The main mechanisms at the origin of the central macrosegregation described in this work are

The thermosolutal convection in the sump caused by the temperature and concentration gradients, and the penetration of these convective flows into the pasty zone;

The transport of grains in the supercooled zone under the effect of gravity, the Archimedes buoyancy principle and natural or forced convection;

The flow in the pasty zone solicited by the volumetric shrinkage at solidification, which can be assisted by the metallostatic pressure;

The flow of the liquid in the pasty zone caused by mechanical deformations;

The forced flows that can result from the pouring, injection or from a release of gas, from a stirring, a vibration, etc. that penetrate into the supercooled zone and into the pasty zone and modify the direction of the convection movements.

This is a continuous macrosegregation, this term designating the fact that the macrosegregation takes place continuously over all or a portion of the height of the ingot, in other terms it is substantially uniform according to the casting axis.

The phenomenon of intermittent macrosegregation has not been described as much in literature and results in the formation of V-shaped strips on either side of the negative central macrosegregation. These V-shaped strips are alternatively enriched and depleted with eutectic and peritectic alloy elements. These strips can be observed by carrying out X-ray radiographies of vertical segments of ingots, typically in the plane L/TC at mid-width, when the segregated elements absorb the X-rays in a differentiated manner from the atoms of the metal comprising the ingot. Other means make it possible to view this phenomenon, for example the echography or the observation with the naked eye of anodized vertical segments, due to the difference in optical reflectivity between the enriched or depleted zones with alloy elements. Generally, the intermittent macrosegregation is the most marked on the region T/2.5 of the thickness, the region T/2 corresponding to the central axis of the ingot. According to a nomenclature known to those skilled in the art, the term T/n, where n is a positive number, designates a region located at a distance T/n of an edge of the ingot, where T designates a thickness of the ingot.

The periodic intermittent macrosegregations appear very early after the starting of the casting, as soon as an inclined front is formed between a solid zone and a liquid zone. They are observed in all cases of casting of aluminum alloys loaded with aluminum alloys, cast typically according to formats with a thickness greater than 300 mm, this thickness threshold depending itself of the casting speed.

The publication R. C. Dorward et al. “Banded segregation patterns in DC cast AlZnMgCu alloy ingots and their effect on plate properties” Aluminum, 1996, 72. Jahrgang, 4, p. 251-259 describes the formation of strips of intermittent segregations in an alloy 7000. According to these authors, this phenomenon is due to avalanches of grains triggered periodically by convective oscillations of the sump, i.e. the liquid phase of the metal, in liaison with a mechanism for the emission of swirls. This article shows in particular that the intermittent macrosegregation can be at the origin of variations in mechanical properties, for example in the tenacity, on the sheets obtained from crude casting products. It is therefore advantageous to find a casting method that would suppress these intermittent macrosegregations.

The reduction or the suppression of continuous macrosegregations, for example the central macrosegregation, has already been described. In particular it has been shown that the application of a magnetic field, for the purposes of stirring or braking of the flows, made it possible to limit the appearance of macrosegregations continues. Document US5375647 describes for example a method for reducing central macrosegregation occurring during the casting of a metal alloy ingot. This method comprises the application, during the cooling, of a static magnetic field generated by at least one coil passed through by a direct current.

Document FR2530510 describes a method for the electromagnetic casting of metals wherein a stationary magnetic field and a variable frequency magnetic field are made to act simultaneously, in order to produce both radial vibrations within the metal that is not yet solidified, and limit the stirring.

B. Zhang et al “Effect of low-frequency magnetic field on macrosegregation of continuous casting aluminum alloys” Materials Letters 57 (2003) pp. 1707-1711 applied a variable magnetic field at low frequency (between 10 and 100 Hz) to a billet of 200 mm made of alloy AA7075 and observed a beneficial effect on the decrease in the central macrosegregation, mainly for a frequency of 30 Hz.

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

WO 2014/155357 relates to methods and an apparatus intended to displace a melted metal, the electromagnetic inducer comprising at least two pairs of electromagnetic poles and a first magnetic field component being generated between a pole in a first pair of electromagnetic poles and a second pole in a different pair of electromagnetic poles, and a second magnetic field component being generated between the two poles in one or several pairs of electromagnetic poles, the second magnetic field component as such generating one or several eddy currents in the melted metal.

The inventors have considered that the methods described hereinabove do not make it possible to effectively reduce the appearance of intermittent macrosegregations. They propose a method that makes it possible to limit the formation of such macrosegregations, and even eliminate them, so as to better control the mechanical properties of the products coming from the 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 comprising the following steps:

preparing the aluminum alloy;

casting the aluminum alloy in the ingot mold, along a vertical axis of flow, the alloy being cooled, during the casting, by a runoff of a coolant in contact with the solidified metal;

during the casting, application of a magnetic field of which the amplitude is periodically varied according to a frequency, said magnetic field being generated by at least one magnetic field generator arranged at the periphery of the ingot mold, in such a way as to apply a Lorentz force at different points of a liquid portion of the alloy in the process of solidification;

the magnetic field applied being a traveling magnetic field, propagating along an axis of propagation, in such a way that a maximum amplitude of the magnetic field propagates along said axis of propagation, defining a propagation wavelength, said traveling magnetic field driving a propagation, along said axis of propagation, a Lorentz force of maximum intensity; the method being characterized in that a magnetic parameter referred to as a force parameter, governing a Lorentz force value of maximum intensity, is variable in a predetermined time interval, said parameter being:

said maximum amplitude of the magnetic field;

and/or said frequency of the magnetic field;

and/or the propagation wavelength of the magnetic field; in such a way as to obtain a modulation, in said time interval, of said Lorentz force of maximum intensity propagating along the axis of propagation.

The method can comprise any of the following characteristics, taken individually or in combination:

the 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, propagating along the axis of propagation, varies by at least 30 N.m⁻³ in a time interval between 20 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 N.m⁻³.s⁻¹ during said time interval;

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

during the casting, the variation in the force parameter is periodical, the period being between 20 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 in the force parameter is not obtained via a periodic interruption in the travelling field.

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

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

According to an embodiment, the generators are electromagnetic inducers, each electromagnetic inducer having a current flowing through it referred to as induction current. The method comprises, during said time interval:

a variation in the 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 inducer and the ingot mold.

According to this embodiment, the method can comprise a variation in the intensity or in the frequency du induction current flowing through an inducer, the method then comprising:

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

a determination of a range of variation in the intensity or in the frequency of the induction current according to said critical value defined beforehand.

The method can comprise a definition of a plurality of critical values of the intensity and of the frequency of the induction current, in such a way as to define a resonance curve, representing the critical values of the intensity and of the frequency generating a resonance of said free surface, the method comprising a determination of a range of variation in the intensity or in the frequency of the induction current in a range delimited by said resonance curve.

Preferably, the method comprises a variation in the frequency of the induction current flowing through an inducer.

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

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

and/or a rotation of the permanent magnet, and a variation in 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 such as described hereinabove and in the following description.

The ingot can have, for an element of the alloy, of which the content by weight is greater than 0.5%, or the sum of two elements of the alloy of which the 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, said dispersion criterion being defined according to the following expressions:

ε=ΔC_ZA/ΔC_ZR(6) ΔC_ZA=max (C_ZA)−min (C_ZA) (4), ΔC_ZR=max (C_ZR)−min (C_ZR) (5),

where:

max (C_ZA) and min (C_ZA) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the two elements considered measured in a zone of analysis, having 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 considered or of the sum of the two elements considered in a reference zone considered as little affected by the intermittent macrosegregations, for example between T/6 and T/12;

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

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

determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element of which the content by weight is greater than 0.5% or the sum of two elements of the alloy of which the individual content is greater than 0.5%, the profile being established according to said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 and 25 mm,

standardizing said maximum amplitude by a nominal concentration C₀ of said element or by the sum of the nominal concentrations of the two elements considered.

Other advantages and characteristics shall come 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 hereinbelow.

FIGURES

FIGS. 1A to 1E show an example of the device and of the method according to prior art and according to the invention. FIG. 1A shows the main components of the device while FIGS. 1B and 1C respectively show a spatial and time distribution of the amplitude of a traveling magnetic field according to prior art. FIGS. 1D and 1E respectively show a spatial and time distribution of the amplitude of a non-stationary traveling magnetic field according to embodiments of the invention.

FIG. 2 shows a curve referred to as a resonance curve of the free surface of the sump, showing values, referred to as critical values, of the intensity and of the frequency of an induction current at which a resonance of the free surface of the sump appears, this by implementing a method of electromagnetic stirring.

FIG. 3 is a radiograph of a vertical segment of a product obtained by implementing a first embodiment of the method, representative of prior art, according to a first example, referred to as example 1, representative of prior art.

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

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

FIGS. 6A and 6B show characterization profiles of a product obtained by implementing a method according to the example 1. FIG. 6A shows profiles of concentration of Zn along several horizontal lines of the vertical segment shown in FIG. 3. FIG. 6B shows the profiles resulting from digital processing carried out.

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

FIG. 8 shows a curve referred to as the resonance curve of the free surface of the sump, obtained by implementing a method of a second example, referred to as example 2, according to the invention.

FIGS. 9, 10A, 1013 and 11 show a characterization of a product obtained by implementing a method according to this second example. FIG. 9 is a radiograph of a vertical segment of the product. FIG. 10A shows profiles of concentration of Zn along several horizontal lines of the vertical segment shown in FIG. 9. FIG. 10B shows the profiles resulting from the digital processing carried out on the profiles shown in FIG. 9. FIG. 11 shows Fourier transforms of these various profiles.

DETAILED DESCRIPTION OF THE INVENTION

Unless mentioned otherwise, all of the indications concerning 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 Cu means that the content in copper expressed as a % by weight is multiplied by 1.4. The designation of the alloys is done in compliance with the regulations of The Aluminum Association, known to those skilled in the art.

FIG. 1A shows an example of the casting method known from prior art. In this example, an aluminum alloy 1 flows in an ingot mold 2, through an opening 2i. The ingot mold 2 extends according to a vertical axis Z. It is delimited by a peripheral chamber of which the section, in a horizontal plane XY, is parallelepipedic. A coolant 3, for example water, flows against the wall of the solidified product. This method is known as semi-continuous direct chill casting. A false-bottom 4 can be translated in such a way as to move away from the opening 2i during the casting. The ingot mold 2 extends, parallel to a first horizontal axis X, according to a thickness e and, parallel to a second horizontal axis Y, perpendicular to the X axis, according to a length l. The thickness e is for example greater than 300 mm. It is beyond such a thickness that the intermittent macrosegregations 11 appear in a marked way. Under the effect of the cooling, a solid zone 1s is formed, in the vicinity of the cooled chamber, around a liquid zone 1l, designated by the term “sump”. The interface between the liquid zone 1l and the solid zone 1s is a front 10, with the latter progressing towards the center of the ingot mold as the solidification of the alloy takes place. At the end of the cooling, a parallelepiped ingot, also designated by the term “product”, is formed.

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

A magnetic field generator 5 is shown, able to generate a magnetic field B intended to be applied to the liquid zone 1l of the alloy. Such a generator can be a permanent magnet or an electromagnetic inducer, the latter generating a magnetic field when it is passed through by an electric current, referred to as induction current.

The magnetic field B applied to the liquid zone 1l is an alternating field, of amplitude B₀ and of frequency f. The effect of this magnetic chamber is to apply a stirring of the sump, under the effect of Lorentz forces that are applied on the metal liquid zone 1l. Indeed, the application of a magnetic field B generates, in the alloy, the formation of a resulting electric current J, within the liquid zone of the alloy subjected to the magnetic field, in the appearance of a Lorentz force F such that F ∝J×B where × designates the vector product operator, and ∝ designates a proportional relation. This Lorentz force has a component oscillating at a frequency double the frequency ƒ of the magnetic field.

Due to the thickness of the ingot mold, the frequency ƒ is chosen in such a way as to allow for a sufficient penetration of the magnetic field B in the sump, in such a way as to obtain an effective stirring of the liquid. The frequency ƒ is as low as the thickness of product is high. In the case of an aluminum alloy with a thickness greater than 300 mm, the frequency is more preferably less than 5 Hz, and even more advantageously less than 2 Hz or 1 Hz.

The generator 5 is able to generate a traveling magnetic field. The term traveling magnetic field designates an alternating magnetic field, of which the amplitude B₀ is not constant, and varies between a minimum value and a maximum amplitude B₀^max, the maximum amplitude B₀^max propagating along an axis of propagation Δ, more preferably straight. The term amplitude means the maximum value that a periodic magnitude has. More preferably, the axis of propagation belongs to a plane parallel to the direction of casting.

The distance λ separating two maximas of amplitude of the magnetic field is the wavelength of the traveling magnetic field. FIG. 1B shows an example of the distribution of the amplitude B₀ of un traveling magnetic field along an axis of propagation Δ at an instant t (continuous line), and at an instant t+Δt (dotted line). On the axis of propagation, a coordinate r is shown corresponding to the position of a point of the sump. FIG. 1C shows a time change of a traveling alternating magnetic field at this point. Due to the propagation of the value of the maximum amplitude B₀^max, the amplitude of the magnetic field, at this point, varies between a minimum value B₀^min and the value B₀^max the latter not changing over time.

A traveling magnetic field generator 5 can be formed by several electromagnetic inducers arranged around the peripheral chamber. The Lorentz force, at a coordinate point r of the sump, comprises an oscillating component, modulated according to a frequency 2ƒ double the frequency of the magnetic field. The amplitude F₀ of the density of the oscillating Lorentz force can be expressed according to the expression:

F₀(r)=½σƒλB₀²(r) (1), where σ designates the electrical conductivity.

It is possible to define a travelling speed V_G of the magnetic field V_G=ƒλ(2) in which case the expression (1) can be expressed as follows:

F₀(r)=½σV_GB₀²(r) (3)

As such, the amplitude of the Lorentz force, at a point r of the sump depends on the square of the amplitude of the magnetic field applied at this point. The application of a traveling magnetic field results, at a point of the sump, in a modulation of its amplitude. As such, the amplitude of the magnetic field at a point of the sump varies as a function of time, between a minimum amplitude B₀^min and a maximum amplitude B₀^max. The same applies to the Lorentz force density, the latter having, at a point r of the sump, a maximum value when the amplitude of the magnetic field, at this point, is maximal. In the coordinate system XYZ, linked to the ingot mold 2, the propagation of a maximum value of the amplitude of the magnetic field B₀^max, along an axis of propagation, drives, simultaneously, the propagation of a Lorentz force of maximum intensity F_max according to the axis of propagation Δ. The combination of the forces propagating along the axis of propagation establishes a movement of the liquid according to this axis forming an electromagnetic pump element.

The inventors have observed that by modulating, over time, the maximum amplitude of the Lorentz force F_max propagating in the sump, the intermittent macrosegregations are attenuated, and even disappear, and this particularly on ingots of which the thickness is greater than 300 mm.

This time modulation can be obtained by a variation in a parameter, referred to as the magnetic force parameter, controlling the amplitude of the Lorentz force density explained in the equations (1) and (3), for example:

the value of the maximum amplitude B₀^max of the magnetic field;

de the frequency f of the magnetic field;

the wavelength λ of the traveling magnetic field.

When the traveling magnetic field is generated by a plurality of electromagnetic inducers arranged at the periphery of the ingot mold, the time modulation of the Lorentz force density can be obtained by modifying the pole pitch, i.e. the out of phase between the induction currents flowing in each inducer. Such a modification makes it possible to vary the wavelength λ of the traveling magnetic field, i.e. the distance between two maximas propagating along the axis of propagation. The frequency of the induction current flowing in the inductors can be variable, which modifies the frequency ƒ of the magnetic field. The amplitude of the induction current can also be variable, which modifies the value of the maximum amplitude B₀^max of the magnetic field. FIG. 1D shows an embodiment wherein the value of the maximum amplitude B₀^max of the magnetic field and the wavelength λ of the traveling magnetic field are variable over time. As such, a spatial distribution of the amplitude B₀(t) in the sump is shown, at an instant t (continuous line), as well as a spatial distribution of the amplitude B₀(t+Δt), at an instant t+Δt (dotted line). During the time interval Δt, the maximum amplitude B₀^max varies between B₀^max(t) and B₀^max(t+Δt). Likewise, the wavelength λ was modified, passing from λ(t) to λ(t+Δt). In FIG. 1E, which shows a time change of a traveling alternating magnetic field at a point, an embodiment is shown wherein the value of the maximum amplitude B₀^max of the magnetic field varies, over time, for a frequency ƒ and a constant wavelength λ.

Therefore, in the examples shown in FIGS. 1D and 1E, the maximum amplitude of the Lorentz force, propagating in the sump, varies between t and t+Δt, between the values F_max(t) and F_max(t+Δt).

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

A traveling magnetic field B can in particular be generated from two inducers arranged on the same face of the ingot. The inducers are arranged more preferably facing a large face of the ingot, i.e. one of the two faces of the ingot having the largest vertical section. The inducers can be superimposed upon each other, in such a way as to generate a so-called vertical out of phase, or arranged side by side, in such a way as to generate a so-called horizontal out of phase. In the examples described hereinafter, a device was used described in application WO2014/155357, and more precisely according to the configuration described in liaison with FIGS. 19 and 20A, wherein three inducers, oriented according to the vertical axis Z, are arranged facing each large face of the ingot.

The traveling magnetic field can also be generated from one or several permanent magnets arranged at the periphery of the ingot mold and set into motion in relation to the latter. For example, it is possible to generate a traveling magnetic field by rotating a permanent magnet.

A variation in the parameters of the traveling magnetic field, whether concerning its amplitude, its frequency or its wavelength makes it possible to apply a non-stationary Lorentz force in the sump. The inventors have observed that this makes it possible to attenuate the appearance of intermittent macrosegregations and even make them disappear. Such conditions probably influence the recirculations that are spontaneously produced in the sumps, and reduce the consequences thereof.

Preferably, in the sump, the speed of the variation in the maximum Lorentz force density is greater than 0.05 N.m⁻³.s⁻¹, and more preferably greater than 0.1 N.m⁻³.s⁻¹, and more preferably greater than 0.2 N.m⁻³.s⁻¹. In an embodiment the maximum speed of the variation in the maximum Lorentz force density during the casting is at least 1 N.m⁻³.s⁻¹ and more preferably at least 2 N.m⁻³.s⁻¹.

More preferably, the variation in one or several force parameters takes place in a time interval less than or equal to the characteristic durations of recirculations generated by natural convection. These durations vary according to the thickness of the ingot and the casting speed. Considering thicknesses e between 300 mm and 700 mm, and casting speeds between 30 mm/min and 80 mm/min, the characteristic durations of the recirculations extend between 20 seconds (thickness of 300 mm, casting speed of 30 mm/min) and 10 minutes (thickness of 700 mm, casting speed of 80 mm/min). As such, the force parameters vary in a time interval Δt determined according to these characteristic durations. The term variation means a significant variation, of at least 10% in the force parameter considered, and preferably at least 20% and even 30% of the force parameter.

The variation of a force parameter can be periodical, the time period of variation able to be about a characteristic duration of recirculation, i.e. be between 20 seconds and 10 minutes according to the dimension and speed conditions of the casting. Preferably, in the sump, during the time period of variation, the maximum density of the Lorentz force varies by at least 30 N.m⁻³, and advantageously by at least 40 N.m⁻³, and preferably at least 50 N.m⁻³, and even more preferably by at least 60 N.m⁻³.

The variation in a force parameter can also be monotonous during the casting, for example according to an increasing or decreasing function between the starting and the ending 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 equal to zero when the current in the inducers or the coils is equal to zero. Therefore advantageously, the variation in the force parameter is not obtained via a periodic interruption in the travelling field.

Advantageously, during the casting, the Lorentz force of maximum intensity is greater than 80 N/m³, more preferably greater than 100 N/m³, more preferably greater than 120 N/m³, even more preferably greater than 140 N/m³. The inventors indeed observed that the suppression of the intermittent macrosegregations was not optimal when the force was too weak was shown in the example 5 (FIG. 20a to d). The minimum value starting from which the suppression of the intermittent macrosegregations is improved depends on all of the casting parameters, in particular the stirring method, the position of the inducers in relation to the plate and the composition of the alloy.

According to an embodiment, the frequency ƒ and/or the maximum amplitude B₀^max of the magnetic field are modified respectively by varying the frequency and the amplitude of the induction current flowing in inducers. For this, the method can include a prior step of defining an operating range, i.e. a range in the variation of the frequency and/or of the intensity of the induction current. This prior step comprises the determination of one or of several values of frequency/intensity pairs, referred to as critical values, generating, at the free surface 1_sup of the sump, a resonance, the resonance resulting in the appearance of significant oscillations of said free surface 1_sup, the latter being shown in FIG. 1A. These significant oscillations are generally observed with the naked eye. The term significant oscillation means for example an oscillation of which the amplitude is greater than or equal to 5 mm according to the vertical axis Z. For example, the frequency of the current is fixed and the intensity of the induction current is increased until a significant oscillation is observed.

By considering different critical values of frequency (or of intensity), it is possible to experimentally determine a resonance curve R, in a frequency/intensity plane that corresponds to the various pairs (frequency/intensity) at which a resonance is observed at the free surface of the sump. Using this curve R, a range in the variation of the intensity and/or of the frequency is determined, in such a way as to prevent or limit the appearance of a resonance of the free surface of the sump. Indeed, the resonance curve delimits a zone of stability and a zone of instability, wherein the casting can become dangerous. However, modulating the frequency or the intensity of the induction current, and therefore the frequency f or the maximum amplitude B₀^max of the traveling magnetic field, makes it possible to temporarily approach the resonance curve R, for example periodically, while still remaining in the zone of stability. This makes it possible to maximize the intensity of the Lorentz force, and therefore the stirring of the sump, while still remaining in acceptable safety configurations. Indeed, in the vicinity of the resonance curve, the stirring effect is particularly important.

Such a resonance curve R depends on the casting conditions, i.e. dimensions of the ingot mold, the casting speed, the configuration of the magnetic field applied, the latter depending on the magnetic field generator, i.e. on the inducers or on the permanent magnet or magnets used. A resonance curve R is shown in FIG. 2, this curve having been obtained by casting an ingot of thickness 525 mm×1650 mm, according to a casting speed of 45 mm/min, a magnetic stirring being carried out by the application of a magnetic field by three inducers arranged in front of each large face of the ingot and out of phase by 90° in order to form a horizontal electromagnetic pump element, as mentioned hereinabove. This figure also shows plots representing a percentage of the intensity of a Lorentz force, referred to as the nominal force, 100% corresponding to the maximum intensity of the 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. Preferably, the intensity and the frequency of the induction current is located in a space delimited by the 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 in the frequency of the induction current flowing through an inducer. The inventors found that it was advantageous to vary the frequency because the variation in the penetration in the field that results therefrom makes it possible to more effectively vary the force gradient in the thickness and the depth of the liquid well. Moreover, the power electronics make the variation in frequency faster than the variation in intensity; which gives an additional degree of freedom towards weaker periods of unsteady forcing. It is indeed advantageous to decouple the characteristic hydrodynamic times from the characteristic times of the solidification in order to prevent intermittent macrosegregations.

Another example of a curve is shown in FIG. 8 and will be commented on later in liaison with the examples. FIGS. 2 and 8 show the resonance curve R, determined experimentally, as well as the curve representing a Lorentz force of which the intensity is equal to 10% of the nominal Lorentz force defined beforehand.

The variation in one or several force parameters can in particular make it possible to alternate periods during which the dimensionless Hartmann number Ha is respectively low, typically less than 1, and high, typically greater than 3, and even 5. The dimensionless Hartmann number Ha is a number that is commonly used in the field of magnetohydrodynamics. It represents a ratio between the magnetic viscosity and the viscosity of a loaded liquid flowing in a magnetic field. The higher this number is, the higher the contribution of the Lorentz forces is. More preferably the dimensionless Hartmann number Ha alternates with a ratio between low and high values by at least 3 or by at least 5. Such a configuration is preferred, because it makes it possible to alternate periods during which the kinetic energy applied by the magnetic field opposes the natural convection of the liquid metal, and periods during which the natural convection predominates.

As described in liaison with the examples presented hereinafter, the products obtained by a method according to the invention have a limited intermittent macrosegregation in relation to methods of prior art, and even imperceptible. In the examples that follow, the characterization of the products was carried out by analyzing horizontal profiles (according to the axis TC) of a radiograph carried out at mid-width according to a vertical plane L/TC, these profiles being calibrated in order to obtain the spatial distribution of elements of heavy alloys of the type Zn or Cu. The zones enriched with such heavy elements, more absorbent, appear in the form of dark spots on the negative of the radiographs carried out and therefore light spots on the radiographs presented. An example of obtaining the profile of the concentration in Zn using a radiograph of an alloy Al—Zn is shown in FIG. 4.

The terms L, TL and TC, known to those skilled in the art, correspondent respectively to the dimension of the ingot according to the vertical axis, the axis referred to as “long cross” and according to the axis referred to as “short cross”.

In a complementary or alternative manner, it is possible to conduct chemical analyses according to horizontal profiles, in such a way as to quantify the spatial distribution of said chemical elements according to the axis TC. An intermittent macrosegregation can be characterized by a maximum difference in mass of an alloy element, here Zn, in the zone that is the most marked by the intermittent macrosegregations, i.e. in the vicinity of T/2.5.

In order to quantify the intermittent macrosegregation, the concentration profiles, obtained by radiography or par any other method, with a spatial resolution of 0.1 mm were processed as shown in FIG. 5A. The profile obtained with the resolution of 0.1 mm is the crude profile referenced as profile A. A sliding average over 2 mm makes it possible to overcome the microsegregation, the smoothed profile obtained is referenced as profile B. Another sliding average o the crude profile over 50 mm makes it possible to overcome intermittent macrosegregations, and obtain the continuous macrosegregation profile, the profile obtained being a profile referred to as a basic profile, referenced as profile C. The profile C is subtracted from the profile B in order to obtain a profile referred to as corrected, corresponding to the intermittent macrosegregation, the corrected profile being referenced as profile D. Such a profile is shown in FIG. 5B. As can be seen in this FIG. 5B, the corrected profile is mainly representative of the intermittent macrosegregation, and is not or is little affected by the central continuous macrosegregation and by the microsegregation. Such a corrected profile makes it possible to characterize the intermittent macrosegregation.

It is then possible to calculate a maximum difference in concentration in a zone of analysis Z_A located between T/2.3 and T/3.3, this maximum difference able to be expressed according to the following equation:

ΔC_ZA=max (C_ZA)−min (C_ZA)  (4)

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

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

The maximum difference ΔC_ZA can be standardized by the nominal concentration C_O of the element considered. The products according to the invention preferably have a value of such a standardized ratio less than 10% and more preferably less than 8% or even less than 6%. However the absolute value of ΔC_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 to characterize the products obtained by the method according to the invention to calculate, as a reference, a maximum difference in a zone of reference Z_R that is little sensitive to the intermittent macrosegregations, located between T/6 and T/12, this maximum difference able to be expressed according to the following equation:

ΔC_ZA=max (C_ZR)−min (C_ZR)  (5)

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

A dispersion criterion E is thus obtained that makes it possible to evaluate for the element considered the intermittent macrosegregation:

ε=ΔC_ZA/ΔC_ZR  (6)

In order to overcome local variations in composition, it is advantageous, to determine ΔC_ZA and ΔC_ZR, to calculate an average over at least 5 profiles of concentration that are separated by at least 10 mm.

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

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

It is also useful to carry out an analysis via Fourier transform of the crude profile of composition and to standardize it by the nominal composition of the element. Such an analysis makes it possible to identify spatial periods that characterize the intermittent macrosegregation. The intermittent macrosegregation has periods between 8 and 25 mm. When the intermittent macrosegregation is substantial, a peak in the amplitude of the Fourier components is then observed for spatial periods between 8 and 25 mm. A dimensionless criterion of spectral intensity ζ is determined which corresponds to the maximum amplitude of the Fourier components in a spatial period range between 8 and 25 mm, standardized by the nominal concentration C_o of the element considered. The products obtained by the method according to the invention more preferably have a criterion ζ less than 0.01, preferably less than 0.007 and preferably less than 0.005.

The dispersion criterion E and spectral intensity criterion ζ are advantageously applied to the major element of the alloy considered, typically to the Zn for an alloy 7xxx or to the Cu for an alloy 2xxx. It is also possible to apply these criteria to the sum of two elements, for example the sum Zn+Cu in certain alloys 7xxx or the sum Mg+Si in the alloys 6xxx. These criteria can also be applied to an element of which the content by weight in the alloy is greater than or equal to 0.5% or to the sum of two elements of the alloy of which the individual content is greater than 0.5%,

In the case where the sum of two elements is considered, the values for standardizing the maximum difference ΔC_ZA, and/or the Fourier transform correspond to the sum of the nominal concentrations of the elements considered.

The ingots of rectangular section obtained by the method according to the invention can be used as cast or after working, optionally after solution heat treatment and quenching and aging for age-hardenable alloys. Advantageously the ingots with a rectangular section obtained by the method according to the invention are rolled and/or forged.

Example 1

A casting of an alloy AA7035 was carried out without electromagnetic stirring. The composition of the alloy cast comprising a nominal concentration of Zn of 5.6% by weight, a nominal concentration of Mg of 1.3% by weight. The format of the ingot was 1650 mm×525 mm. This example is representative of prior art. The grain refining was carried out with a concentration of refining agent AlTiB 5:1 of 1 Kg/t. The casting speed was 35 mm/min. FIG. 3 shows a radiograph of the ingot at mid-width along a plane L/TC, whereon the negative central macrosegregation and the intermittent macrosegregations are clearly identifiable. FIG. 6A shows different horizontal crude profiles of the content of Zn, along an axis TC, as well as smoothed profiles B are obtained by a sliding average over 2 mm deduced from FIG. 3. The radiograph makes it possible to quantify that the elements at the origin of a contrast with respect to the aluminum, namely in this case the Zn. This remark applies to the following example 2. The negative central macrosegregation is clearly observed, maximum at T/2, the intermittent macrosegregations being observed between T/2.3 and T/3.3. FIG. 6B shows the different profiles corrected for the content of Zn (profiles D), along an axis TC, obtained after subtraction of each smoothed profile (profile B) by a basic profile (profile C) representative of the continuous macrosegregation.

The value of the maximum differences of the content of Zn was 0.75% by weight for ΔC_ZA and 0.19% by weight for ≢C_ZR, the value of the maximum standardized differences in the analysis zone and in the reference zone being as such respectively 13.3% and 3.5%. The value of the dispersion criterion ε such as defined by the equation (6) was 3.9. The Fourier transform of each profile was calculated, and is shown in FIG. 7, after standardization by the nominal composition of Zn: 5.6% by weight. The axis of the abscissa 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 mm and 25 mm. The spectral intensity criterion , which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm, standardized by the nominal concentration C_o of the Zn, was for all of the profiles at least 0.01.

Example 2

During a second example, a casting of an alloy AA7035 was carried out with an electromagnetic stirring. The composition of the cast alloy had a nominal concentration of Zinc of 5.6% by weight and a nominal concentration of Magnesium of 1.3% by weight. The format of the ingot was 1650 mm×525 mm. The grain refining was carried out with a concentration of refining agent AlTiB 5:1 de 1 Kg/t. The casting speed was 35 mm/min. The electromagnetic stirring was obtained by setting up, opposite each face L/TL of the ingot, (corresponding to a plane YZ in the coordinate system indicated in FIG. 1A), three inducers oriented along the vertical axis Z, passed through by an alternating current, of a frequency 0.25 Hz, out of phase with respect to one another by 60° and spaced apart from one another by 0.6 m, as such forming 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 the opposite direction. The inducers generated a traveling magnetic field along a horizontal plane, the traveling axis being parallel to the direction TL, the wavelength λ was 3.6 m. The maximum density of the Lorentz force induced in the liquid sump was varied between about 180 N/m³ and 240 N/m³with a variation speed of 2 N.m⁻³.s⁻¹ by modifying the nominal value of the current in the inducers. The resonance curve, corresponding to these casting conditions, is shown in FIG. 8. The variation in the intensity of the induction current is represented, in this figure by a double arrow.

FIG. 9 shows a radiography of the ingot according to a plane L/TC, whereon the negative central macrosegregation at T/2 can be identified. FIG. 10A shows different horizontal crude profiles of the content of Zn (profile A) and smoothed (profiles B), along an axis TC. The negative central macrosegregation is distinguished, maximum at T/2. FIG. 10B shows the different horizontal profiles of the content of Zn, along an axis TC, of the corrected profile type (profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation.

The value of the maximum differences of the content of Zn was 0.24% by weight for ΔC_ZA and 0.28% by weight for ΔC_ZR, the value of the standardized maximum differences in the zone of analysis and in the zone of reference being respectively 4.3% and 5%. The value of the dispersion criterion ε such as defined by the equation (6) was 0.9: the intermittent macrosegregation in the zone of analysis between T/2.3 and T/3.3 was removed. The Fourier transform of each profile was calculated, and is shown in FIG. 11, after standardization by the nominal composition of Zn: 5.6% by weight. The axis of the abscissa represents the spatial period, between 0 and 30 mm. Predominant peaks are no longer observed. The spectral intensity criterion ζ, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm standardized by the nominal concentration C_o of the Zn, was for all of the profiles less than 0.005.

Example 3

In this example, a casting of an alloy AA 7050 was carried out 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 1650×525 mm. The grain refining is carried out using a grain refining rod AlTiC3:0.15 with an addition ratio of 1 kg/ton. The casting speed was 45 mm/min. It forms the reference of the example 4.

FIG. 12 shows a radiography of the ingot according to a plane L/TC, whereon the negative central macrosegregation at T/2 can be identified. FIG. 13a shows the smoothed horizontal profile of the sum of two elements Zn and Cu (profiles B) along an axis TC, deduced from the radiograph of FIG. 12. Indeed, the radiograph makes it possible to quantify only the elements at the origin of a contrast in relation to the aluminum, namely in this case the Zn and the Cu. This remark applies to the following examples 4 and 5. FIG. 13b shows the different horizontal profiles of the concentration of Zn+Cu, along an axis TC, of the corrected profile type (Profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum differences of the sum Zn+Cu was 0.81% by weight for ΔC_ZA and 0.19% for ΔC_ZR. The value of the dispersion criterion ε such as defined by the equation (6) was 4.4. FIG. 14 shows the Fourier transform of each profile, after standardization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The axis of the abscissa represents the spatial period, between 0 and 30 mm. The spectral intensity criterion ζ, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm standardized by the sum of the nominal compositions of Zn and Cu, was for one of the profiles greater than 0.01 or for all of the profiles greater than 0.007.

Example 4

In this example, a casting of an alloy AA 7050 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 section of the ingot was 1650×525 mm. The grain refining is carried out using a grain refining rod AlTiC3:0.15 with an addition ratio of 1 kg/ton. The casting speed was 45 mm/min. The electromagnetic stirring was obtained by setting up, opposite each face L/TL of the ingot, (corresponding to a plane YZ in the coordinate system indicated in FIG. 1A) three coils oriented according to the axis z and passed through by an alternating current which was out of phase, in the central coil, by 90° in relation to the current in the end coils. The wavelength of the traveling field was 2.4 m. The electromagnetic pump elements obtained as such were arranged as a mirror in relation to each face L/TL of the ingot, the traveling direction being parallel to the long-cross direction, the traveling generated diverging from the mid-width of the ingot. The unsteady forcing was obtained by the imposing of a cyclical variation in the frequency of the alternating electric current that passed through the coils, such as shown by the double arrow in the frequency vs. intensity diagram of FIG. 15. The maximum density of the Lorentz force generated as such by the variation in the frequency between 0.450 and 0.600 Hz was varied between about 110 N/m³ and 150 N/m³ over a period of 3 min which corresponds to a speed of variation of about 0.22 N/m³/s.

FIG. 16 shows a radiograph of the ingot according to a plane L/TC, whereon the negative central macrosegregation at T/2 can be identified. The intermittent macrosegregations are greatly attenuated in relation to the reference (FIG. 12), as shown in FIGS. 17a and 17b.

FIG. 17a shows the smoothed horizontal profile of the sum of the elements of Zn+Cu (profiles B) along an axis TC, deduced from the radiograph of FIG. 16. FIG. 17b shows the different horizontal profiles of the sum of the two elements Zn and Cu, along an axis TC, of the corrected profile type (Profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum differences of the content of Zn+Cu was 0.30% by weight for ΔC_ZA and 0.14% for ΔC_ZR. The value of the dispersion criterion ε such as defined by the equation (6) was 2.2. The intermittent macrosegregation in the zone of analysis was therefore reduced and is shown in FIG. 18, after standardization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The axis of the abscissa represents the spatial period, between 0 and 30 mm. The spectral intensity criterion which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm standardized by the sum of the nominal compositions of Zn and Cu, was for all of the profiles less than 0.005.

Example 5

In this example, a casting of an alloy AA7050 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 contents of the other elements were all less than 0.5% by weight. The section of the ingot was 1650×525 mm. The grain refining is carried out using a grain refining rod AlTiC3:0.15 with an addition ratio of 1 kg/ton. The casting speed was 45 mm/min. The electromagnetic stirring was obtained by setting up, opposite each face L/TL of the ingot, (corresponding to a plane YZ in the coordinate system indicated in FIG. 1A) three coils oriented according to the axis z and passed through by an alternating current which was out of phase, in the central coil, by 90° in relation to the current in the end coils. The wavelength of the traveling field was 2.4 m. The electromagnetic pump elements obtained as such were arranged as a mirror in relation to each face L/TL of the ingot, the traveling direction being parallel to the long-cross direction, the traveling generated diverging from the mid-width of the ingot.

The unsteady forcing was obtained by the imposition of a variation starting from zero of the intensity of the alternating electric current that flowed through the coils, such as shown by the arrows in the frequency vs. intensity diagram of FIG. 19. The intensity of the maximum volume Lorentz force generated as such by the variation in the intensity varied typically from 0 N/m³ to 140 N/m³ in 4 min which corresponds to a speed of variation of 0.58 N/m³/s. In what follows, the intensity of the maximum volume Lorentz force was made to vary between 140 N/m³ and 360 N/m³ in 5 min which corresponds to a speed of variation of 0.73 N/m³/s.

The results obtained are illustrated by the two radiographed vertical segments shown in FIG. 20a (variation in the intensity between 0 N/m³ to 140 N/m³ in 4 min) and FIG. 21a (variation of the force between 140 N/m³ to 360 N/m³ in 5 min) which are in continuity with one another.

FIG. 20b shows the smoothed horizontal profile of the sum of the major elements Zn+Cu (profiles B) along an axis TC, deduced from the radiograph of FIG. 20a. FIG. 20c shows the different horizontal profiles of the sum of the elements Zn+Cu, along an axis TC, of the corrected profile type (Profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum differences of the content of Zn+Cu was 0.70% by weight for ≢C_ZA and 0.22% for ΔC_ZR. The value of the dispersion criterion ε such as defined by the equation (6) was 3.2. FIG. 20d shows the Fourier transform of each profile, after standardization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The axis of the abscissa represents the spatial period, between 0 and 30 mm. The spectral intensity criterion ζ, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm standardized by the sum of the nominal compositions of Zn and Cu, was for all of the profiles less than 0.01. Note however that the spectral intensity criterion ζ shows values greater than 0.005.

FIG. 21b shows the smoothed horizontal profile of the sum of the major elements Zn+Cu (profiles B) along an axis TC, deduced from the radiograph of FIG. 21a. FIG. 21c shows the different horizontal profiles of the sum of the major elements Zn+Cu, along an axis TC, of the corrected profile type (Profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum differences of the content of Zn+Cu was 0.37% by weight for ΔC_ZA and 0.15% for ΔC_ZR. The value of the dispersion criterion ε such as defined by the equation (6) was 2.4. FIG. 21d shows the Fourier transform of each profile, after standardization by the sum of the nominal compositions of Zn and Cu:8.3% by weight. The axis of the abscissa represents the spatial period, between 0 and 30 mm. The spectral intensity criterion ζ, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm standardized by the sum of the nominal compositions of Zn and Cu, was for all of the profiles less than 0.005.

It is as such observed that the suppression of the intermittent macrosegregations is improved if the force is greater than 140 N/m³. Indeed, when the force is too weak, it is observed that the value of the dispersion criterion ε of spectral intensity ζ are greater than the preferred values of the invention. The inventors suppose as such that an unsteady forcing that would consist in periodically interrupting the traveling field would not make it possible to advantageously suppress the intermittent macrosegregations.

Claims

1. A method for casting an aluminum alloy ingot in a substantially rectangular ingot mold comprising the following steps:

preparing the aluminum alloy;

casting the aluminum alloy in the ingot mold, along a vertical casting axis, the alloy being cooled, during the casting, by a runoff of a coolant in contact with the solidified metal;

during the casting, applying a magnetic field of which the amplitude (B0) is periodically varied according to a frequency (ƒ), said magnetic field being generated by at least one magnetic field generator arranged at the periphery of the ingot mold, in such a way as to apply a Lorentz force (F) at different points of a liquid portion of the alloy in the process of solidification;

the magnetic field applied being a traveling magnetic field, propagating along an axis of propagation, in such a way that a maximum amplitude (B0max) of the magnetic field propagates along said axis of propagation, defining a propagation wavelength (λ), said traveling magnetic field driving a propagation, along said axis of propagation, a Lorentz force of maximum intensity;

wherein the force parameter, governing the Lorentz force of the maximum intensity, is variable in a predetermined time interval, said parameter being:

said maximum amplitude of the magnetic field;

and/or said frequency of the magnetic field;

and/or the propagation wavelength of the magnetic field;

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

2. The method according to claim 1, wherein the 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.

3. The method according to claim 1, wherein the frequency of the magnetic field is less than 5 Hz, or 2 Hz or 1 Hz.

4. The method according to claim 1, wherein the Lorentz force of the maximum intensity, propagating along the axis of propagation, varies by at least 30 N.m−3 in the predetermined time interval between 20 seconds and 10 minutes.

5. The method according to claim 1, wherein 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 N.m−3.s−1 during said predetermined time interval.

6. The method according to claim 1, wherein the axis of propagation of the maximum amplitude of the magnetic field belongs to a plane parallel to the direction of casting.

7. The method according to claim 1, wherein during the casting, the variation in the force parameter is periodical, the period being between 20 s and 20 minutes, or between 1 minute and 15 minutes, or between 2 minutes and 10 minutes.

8. The method according to claim 1, wherein the generators are electromagnetic inducers, each electromagnetic inducer having a current flowing through it referred to as induction current, the method comprising, during said time interval:

a variation in the 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 inducer and the ingot mold.

9. The method according to claim 8, comprising a variation in the intensity or in the frequency of the induction current flowing through an inducer, the method comprising:

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

a determination of a range of variation in the intensity or in the frequency of the induction current according to said critical value defined beforehand.

10. The method according to claim 9 comprising, during said prior step, a definition of a plurality of critical values of the intensity and of the frequency of the induction current, in such a way as to define a resonance curve, representing the values of intensity and of frequency generating a resonance of said free surface, the method comprising a determination of a range of variation in the intensity or in the frequency of the induction current in a range delimited by said resonance curve.

11. The method according to claim 1, wherein at least one generator is a permanent magnet, the method comprising:

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

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

and/or a rotation of two permanent magnets.

12. The method according to claim 1, wherein the aluminum alloy is chosen from alloys of types 2XXX, 5XXX, 6XXX or 7XXX and wherein the thickness is at least 400 mm or 450 mm.

13. The method according to claim 1, wherein the dimensionless Hartmann number, at at least one point of the liquid portion of the alloy, varies at least by a factor of 3, even by a factor of 5, in said predetermined time interval.

14. An aluminum alloy ingot obtained by the method according to claim 1.

15. The aluminum alloy ingot according to claim 14 having, for an element of the alloy, of which the content by weight is greater than 0.5%, or for the sum of two elements of the alloy of which the 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, the dispersion criterion being defined according to the following expressions:

ε=ΔCZA/ΔCZR

ΔCZA=max (CZA)−min (CZA), ΔCZR=max (CZR)−min (CZR), 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 a zone of analysis, having 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, considered as little affected by the intermittent macrosegregations, for example between T/6 and T/12;

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

16. The aluminum alloy ingot according to claim 14, wherein a spectral intensity criterion is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said spectral intensity criterion being calculated by:

determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element of which the content by weight is greater than 0.5% or the sum of two elements of the alloy of which the individual content is greater than 0.5%, the profile being established according to said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 and 25 mm,

standardizing said maximum amplitude by a nominal concentration of said element or by the sum of the nominal concentrations of the two elements considered.