EP3463716B1 - Method for producing sheet ingots by vertical casting of an aluminium alloy - Google Patents

Description

Technical area

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

Disclosure of the invention

During a vertical casting, aimed at forming an ingot, the solidification of a metal or a metal alloy is affected by so-called macroscopic segregation phenomena. When the metal cools, convection currents are formed, generating recirculation vortices, the latter being the source of macroscopic segregation when their lifetime is of the same order of magnitude as the characteristic solidification times. These phenomena lead, in the solidified ingot, to a local impoverishment or to a local enrichment in chemical species. These macroscopic segregations, or macrosegregations, are the source of heterogeneities in the composition of the ingot.

A macrosegregation well known to those skilled in the art is the central negative macrosegregation, resulting from a depletion of eutectic alloying 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”, Publisher Wiley, September 2013, pp 158 - 172 .

• La convection thermosolutale dans le marais causée par les gradients de température et de concentration, et la pénétration de ces écoulements convectifs dans la zone pâteuse ;
• Le transport de grains dans la zone en surfusion sous l'effet de la gravité, de la force d'Archimède et de la convection naturelle ou forcée ;
• L'écoulement dans la zone pâteuse suscité par le retrait volumétrique à solidification, qui peut être assisté par la pression métallostatique ;
• L'écoulement du liquide dans la zone pâteuse causé par des déformations mécaniques ;
• Les écoulements forcés qui peuvent résulter de la verse, de l'injection ou d'un dégagement de gaz, d'un brassage, d'une vibration, etc. qui pénètrent dans la zone en surfusion et dans la zone pâteuse et modifient la direction des mouvements de convection.

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

• Thermosolutal convection in the marsh caused by temperature and concentration gradients, and the penetration of these convective flows into the pasty zone;
• Transport of grains in the supercooled zone under the effect of gravity, Archimedean force and natural or forced convection;
• The flow in the pasty zone caused by the volumetric shrinkage at solidification, which can be assisted by the metallostatic pressure;
• The flow of liquid in the pasty area caused by mechanical deformations;
• Forced flows which may result from the pouring, injection or evolution of gas, stirring, vibration, etc. which penetrate the supercooled zone and the pasty zone and modify the direction of the convection movements.

This is a continuous macro-segregation, this term designating the fact that the macro-segregation takes place continuously over all or part of the height of the ingot, in other words that it is essentially uniform along the casting axis. .

The phenomenon of intermittent macrosegregation has been less often described in the literature and results in the formation of V-shaped bands on either side of the central negative macrosegregation. These V-shaped bands are alternately enriched and depleted in eutectic and peritectic alloy elements. These bands are observable by performing X-ray radiographs of vertical slices of ingots, typically in the L / TC plane at mid-width, when the segregated elements absorb the X-rays in a manner different from the atoms of the metal composing the ingot. Other means make it possible to visualize this phenomenon, for example ultrasound or observation with the naked eye of anodized vertical slices, due to the difference in optical reflectivity between the zones enriched or depleted in alloying elements. Generally, intermittent macrosegregation is most marked at the level of the T / 2.5 region of the thickness, the T / 2 region 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 from an edge of the ingot, where T denotes a thickness of the ingot.

Periodic intermittent macrosegregations appear very early after the start of 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, typically cast in formats with a thickness greater than 300 mm, this thickness threshold itself depending on the casting speed.

The publication RC Dorward et al. “Banded segregation patterns in DC cast AIZnMgCu alloy ingots and their effect on plate properties” Aluminum, 1996, 72. Jahrgang, 4, p.251-259 describes the formation of bands of intermittent segregation in a 7000 alloy. According to these authors, this phenomenon is due to avalanches of grains periodically triggered by convective oscillations of the marsh, that is to say the liquid phase of the metal, in connection with a whirlpool emission mechanism. This article shows in particular that intermittent macrosegregation can be at the origin of variations in mechanical properties, for example of toughness, on the sheets obtained from the as-cast products. It is therefore advantageous to find a casting process which would eliminate these intermittent macrosegregations.

The reduction or elimination of continuous macro-segregations, for example central macro-segregation, has already been described. In particular, it has been shown that the application of a magnetic field, for purposes of mixing or braking the flows, makes it possible to limit the appearance of continuous macrosegregations. The document US5375647 describes, for example, a process for reducing central macrosegregation occurring during the casting of a metal alloy ingot. This method comprises the application, during cooling, of a static magnetic field generated by at least one coil carrying a direct current.

The document FR2530510 describes a process for electromagnetic casting of metals in which a stationary magnetic field and a magnetic field of variable frequency are simultaneously made to act, both to produce radial vibrations within the metal not yet solidified, and to limit the stirring.

B. Zhang et al "Effect of low-frequency magnetic field on macrosegregation of continuous casting aluminum alloys" Materials Letters 57 (2003) pp1707-1711 applied a variable magnetic field at low frequency (between 10 and 100 Hz) to a 200 mm billet of AA7075 alloy and found a beneficial effect on the decrease in central macrosegregation, mainly for a frequency of 30 Hz.

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

WO 2014/155357 relates to methods and apparatus for moving molten metal, the electromagnetic inductor 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 more pairs of electromagnetic poles, the second magnetic field component thereby generating one or more eddy currents in the molten metal.

WO 2009/018810 relates to a method and a device for electromagnetic stirring of electrically conductive fluids, using an RMF magnetic field rotating in the horizontal plane and a WMF magnetic field migrating vertically with respect thereto. The aim is to avoid non-symmetrical flow structures in vessels filled with molten material, especially at the start and during solidification. In addition, efficient mixing of the fluid and / or controlled solidification of metal alloys should be achieved, while avoiding the formation of segregation zones in the structure being solidified. The solution is that the rotating magnetic field RMF and the migrating magnetic field WMF are connected discontinuously in the form of periodic and adjustable durations in time and alternately, consecutively in time. via associated induction coils.

The inventors considered that the methods described above do not make it possible to effectively reduce the appearance of intermittent macrosegregations. They propose a process making it possible to limit the formation of such macrosegregations, or even to eliminate them, so as to better control the mechanical properties of the products resulting from the casting.

Disclosure of the invention

• préparation de l'alliage d'aluminium ;
• coulée de l'alliage d'aluminium dans la lingotière, selon un axe vertical d'écoulement, l'alliage étant refroidi, au cours de la coulée, par un ruissellement d'un liquide refroidisseur au contact avec le métal solidifié;
• au cours de la coulée, application d'un champ magnétique dont l'amplitude est variée périodiquement selon une fréquence, ledit champ magnétique étant généré par au moins un générateur de champ magnétique disposé à la périphérie de la lingotière, de façon à appliquer une force de Lorentz en différents points d'une partie liquide de l'alliage en cours de solidification ;
• le champ magnétique appliqué étant un champ magnétique glissant, se propageant selon un axe de propagation, de telle sorte qu'une amplitude maximale du champ magnétique se propage selon ledit axe de propagation, en définissant une longueur d'onde de propagation, ledit champ magnétique glissant entraînant une propagation, selon ledit axe de propagation, d'une force de Lorentz d'intensité maximale ;

le procédé étant caractérisé en ce qu'un paramètre magnétique dit de force, régissant une valeur force de Lorentz d'intensité maximale, est variable dans un intervalle temporel prédéterminé, ledit paramètre étant :

• ▪ ladite amplitude maximale du champ magnétique ;
• ▪ et/ou ladite fréquence du champ magnétique ;
• ▪ et/ou la longueur d'onde de propagation du champ magnétique ;

de façon à obtenir une modulation, dans ledit intervalle temporel, de ladite force de Lorentz d'intensité maximale se propageant selon l'axe de propagation.An object of the invention is a method for casting an aluminum alloy ingot in a substantially rectangular ingot mold comprising the following steps:

• preparation of aluminum alloy;
• casting of the aluminum alloy in the mold, along a vertical flow axis, the alloy being cooled, during the casting, by a flow of a cooling liquid in contact with the solidified metal;
• during casting, application of a magnetic field the amplitude of which 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 mold, so as to apply a force of Lorentz at different points of a liquid part of the alloy during solidification;
• the applied magnetic field being a sliding magnetic field, propagating along a propagation axis, such that a maximum amplitude of the magnetic field propagates along said propagation axis, by defining a propagation wavelength, said magnetic field sliding causing propagation, along said propagation axis, of a Lorentz force of maximum intensity;

the method being characterized in that a so-called force magnetic parameter, governing a Lorentz force value of maximum intensity, is variable over 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;

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

• la section de la lingotière, dans un plan horizontal, définit une épaisseur et une longueur, l'épaisseur étant inférieure ou égale à la longueur, l'épaisseur étant supérieure à 300 mm et de préférence d'au moins 400 mm ;
• la fréquence du champ magnétique est inférieure à 5Hz, ou 2 Hz ou 1 Hz ;
• la force de Lorentz d'intensité maximale, se propageant selon l'axe de propagation, varie d'au moins 30 N.m^-3 dans un intervalle temporel compris entre 20 secondes et 10 minutes ;
• le champ magnétique est tel que la valeur absolue de la variation de la densité de la force de Lorentz maximale est supérieure ou égale à 0.05 N.m^-3.s^-1 durant ledit intervalle temporel ;
• l'axe de propagation de l'amplitude maximale du champ magnétique appartient à un plan parallèle à la direction de coulée ;
• au cours de la coulée, la variation du paramètre de force est périodique, la période étant comprise entre 20s et 20 minutes, ou entre 1 minute et 15 minutes, ou entre 2 minutes et 10 minutes ;
• au cours de la coulée, la force de Lorentz d'intensité maximale n'est pas égale à zéro.
• au cours de la coulée, la variation du paramètre de force n'est pas obtenue par une interruption périodique du champ glissant.
• le nombre adimensionnel de Hartmann, en au moins un point de la partie liquide de l'alliage, varie au moins d'un facteur 3, voire d'un facteur 5, dans ledit intervalle temporel ;
• l'alliage d'aluminium est choisi parmi les alliages de types 2XXX, 6XXX ou 7XXX, l'épaisseur étant au moins 400 mm ou 450 mm.

The process may include any of the following features, taken alone or in combination:

• the section of the 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 5Hz, or 2Hz or 1Hz;
• the Lorentz force of maximum intensity, propagating along the axis of propagation, varies by at least 30 Nm ^-3 over a time interval of 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 ^-3 .s ^-1 during said time interval;
• the axis of propagation of the maximum amplitude of the magnetic field belongs to a plane parallel to the direction of casting;
• during casting, the variation of the force parameter is periodic, the period being between 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 zero.
• during the casting, the variation of the force parameter is not obtained by a periodic interruption of the sliding field.
• the dimensionless Hartmann number, at at least one point of the liquid part of the alloy, varies at least by a factor of 3, or even by a factor of 5, in said time interval;
• the aluminum alloy is chosen from alloys of types 2XXX, 6XXX or 7XXX, the thickness being at least 400 mm or 450 mm.

• une variation d'une intensité du courant d'induction ;
• et/ou une variation d'une fréquence du courant d'induction ;
• et/ou une variation d'une distance entre un inducteur électromagnétique et la lingotière.

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

• a variation of an intensity of the induction current;
• and / or a variation of a frequency of the induction current;
• and / or a variation of a distance between an electromagnetic inductor and the mold.

• une étape préalable de définition d'au moins une valeur critique de l'intensité et de la fréquence du courant d'induction générant, au niveau d'une surface libre de l'alliage d'aluminium s'écoulant dans la lingotière, une onde de résonance ;
• une détermination d'une plage de variation de l'intensité ou de la fréquence du courant d'induction en fonction de ladite valeur critique préalablement définie.

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

• a preliminary step of defining at least one critical value of the intensity and the frequency of the induction current generating, at a free surface of the aluminum alloy flowing in the mold, a wave resonance;
• a determination of a range of variation of the intensity or of the frequency of the induction current as a function of said previously defined critical value.

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

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

• une variation d'une distance entre l'aimant permanent et la lingotière ;
• et/ou une rotation de l'aimant permanent, et une variation de la vitesse de rotation de l'aimant ;
• et/ou une rotation de deux aimants permanents.

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

• a variation of a distance between the permanent magnet and the mold;
• and / or a rotation of the permanent magnet, and a variation of the speed of rotation of the magnet;
• and / or a rotation of two permanent magnets.

Another subject of the invention is an aluminum alloy ingot, obtained by the process as described above and in the description which follows.

$$\mathrm{\Delta }{C}_{\mathit{ZA}}=\max \left({\mathrm{C}}_{\mathrm{ZA}}\right)-\min \left({\mathrm{C}}_{\mathrm{ZA}}\right)$$

$$\mathrm{\Delta }{C}_{\mathit{ZR}}=\max \left({\mathrm{C}}_{\mathrm{ZR}}\right)-\min \left({\mathrm{C}}_{\mathrm{ZR}}\right)$$

où :

• max (C_ZA) et min (C_ZA) désignent respectivement les concentrations maximale et minimale de l'élément considéré ou de la somme des deux éléments considérés mesurées dans une zone d'analyse, présentant des macroségrégations intermittentes, par exemple entre T/2.3 et T/3.3 ;
• max (C_ZR) et min (C_ZR) désignent respectivement les concentrations maximale et minimale de l'élément considéré ou de la somme des deux éléments considérés dans une zone de référence considérée comme peu affectée par les macroségrégations intermittentes, par exemple entre T/6 et T/12 ;

lesdites concentrations étant mesurées sur au moins un profil établi à mi-largeur dans un plan vertical L/TC et selon la direction TC, ledit profil étant représentatif desdites macroségrégations intermittentes selon ladite direction TC.The ingot may have, for an element of the alloy, the content of which 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, said dispersion criterion being defined according to the expressions following: $$\mathrm{\epsilon }=\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZA}}/\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZR}}$$

$$\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZA}}=\max \left({\mathrm{VS}}_{\mathrm{ZA}}\right)-\min \left({\mathrm{VS}}_{\mathrm{ZA}}\right)$$

$$\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZR}}=\max \left({\mathrm{VS}}_{\mathrm{ZR}}\right)-\min \left({\mathrm{VS}}_{\mathrm{ZR}}\right)$$

or :

• max (C _ZA ) and min (C _ZA ) respectively denote the maximum and minimum concentrations of the element considered or of the sum of the two elements considered measured in an analysis zone, presenting intermittent macrosegregations, for example between T / 2.3 and T / 3.3;
• max (C _ZR ) and min (C _ZR ) respectively denote 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 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 along the TC direction, said profile being representative of said intermittent macrosegregations along said TC direction.

• déterminant une amplitude maximale d'une transformée de Fourier d'un profil représentatif d'une macroségrégation intermittente d'un élément dont la teneur en poids est supérieure à 0.5% ou la somme de deux éléments de l'alliage dont la teneur individuelle est supérieure à 0.5%, le profil étant établi selon ladite direction TC, ladite amplitude maximale étant déterminée dans une plage de périodes spatiales comprise entre 8 et 25 mm,
• normalisant ladite amplitude maximale par une concentration nominale C₀ dudit élément ou par la somme des concentrations nominales des deux éléments considérés.

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 whose content by weight is greater than 0.5% or the sum of two elements of the alloy whose individual content is greater at 0.5%, the profile being established in said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 and 25 mm,
• normalizing said maximum amplitude by a nominal concentration C _{0 of} said element or by the sum of the nominal concentrations of the two elements considered.

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

Figures

• The figures 1A to 1E illustrate an example of a device and method according to the prior art and according to the invention. The figure 1A presents the main components of the device while the figures 1B and 1C respectively represent a spatial and temporal distribution of the amplitude of a sliding magnetic field according to the prior art. The figures 1D and 1E respectively exhibit a spatial and temporal distribution of the amplitude of an unsteady sliding magnetic field according to embodiments of the invention.
• The figure 2 represents a curve known as the resonance of the free surface of the marsh, representing values, known as critical, of the intensity and the frequency of an induction current at which a resonance of the free surface of the marsh appears, this by implementing an electromagnetic mixing process.
• The figure 3 is an X-ray of a vertical slice of a product obtained by implementing a first example of a process, representative of the prior art, according to a first example, known as Example 1, representative of the prior art.
• The figure 4 shows an example of a Zn concentration profile along a horizontal line of the vertical slice shown in the figure 3 and the analysis and reference areas.
• The figure 5A shows the digital processing successively carried out on each profile obtained with a resolution of 0.1 mm. The figure 5B shows a profile resulting from the treatments performed.
• The figures 6A and 6B illustrate characterization profiles of a product obtained by implementing a process according to Example 1. The figure 6A shows Zn concentration profiles along several horizontal lines of the vertical slice shown on the figure 3 . The figure 6B shows the profiles resulting from the digital processing performed.
• The figure 7 shows Fourier transforms of the profiles represented on the figure 6B .
• The figure 8 represents a curve known as the free surface resonance of the marsh, obtained by implementing a method of a second example, called example 2, according to the invention.
• The figures 9 , 10A , 10B and 11 illustrate a characterization of a product obtained by implementing a process according to this second example. The figure 9 is an x-ray of a vertical slice of the product. The figure 10A shows Zn concentration profiles along several horizontal lines of the vertical slice shown on the figure 9 . The figure 10B show them profiles resulting from digital processing carried out on the profiles illustrated on the figure 9 . The figure 11 shows Fourier transforms of these different profiles.

Detailed description of the invention

Unless otherwise indicated, all indications concerning the chemical composition of alloys are expressed as a percentage 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 regulations of The Aluminum Association, known to those skilled in the art.

The figure 1A illustrates an example of a casting process known from the prior art. In this example, an aluminum alloy 1 flows into an ingot mold 2, through an opening 2i. The mold 2 extends along a vertical Z axis. It is delimited by a peripheral enclosure whose section, in a horizontal XY plane, is parallelepiped. A cooling fluid 3, for example water, flows against the wall of the solidified product. This process is known as semi-continuous casting by direct cooling ("Direct-Chill Casting"). A false bottom 4 can be translated so as to move away from the opening 2i during the casting. The mold 2 extends, parallel to a first horizontal axis X, along a thickness e and, parallel to a second horizontal axis Y, perpendicular to the axis X, along 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 markedly.

Under the effect of cooling, a solid zone 1s forms, near the cooled enclosure, around a liquid zone 1ℓ, designated by the term "marsh". The interface between the liquid zone 1ℓ and the solid zone 1s is a front 10, the latter progressing towards the center of the mold as the solidification of the alloy takes place. After cooling, a parallelepipedal ingot, also designated by the term “product”, is formed.

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

There is shown a magnetic field generator 5, capable of generating 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 inductor, the latter generating a magnetic field when it is traversed by an electric current, called an induction current.

The magnetic field B applied to the liquid zone 1ℓ is an alternating field, of amplitude B ₀ and of frequency f . The effect of this magnetic field is to apply a mixing of the marsh, under the effect of Lorentz forces applying to the metallic liquid zone 1ℓ. Indeed, the application of a magnetic field B generates, in the alloy, the formation of an electric current J resulting, 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 × denotes the vector product operator, and ∝ denotes a relation of proportionality. This Lorentz force exhibits an oscillating component at a frequency twice the frequency f of the magnetic field.

Due to the thickness of the mold, the frequency f is chosen so as to allow sufficient penetration of the magnetic field B in the swamp, so as to obtain efficient mixing of the liquid. The frequency f is all the lower the greater the thickness of the product. 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.

l'amplitude maximale $B_0^{\mathit{max}}$

se propageant selon un axe de propagation Δ, de préférence rectiligne. Par amplitude, on entend la valeur maximale que prend une grandeur périodique. De préférence, l'axe de propagation appartient à un plan parallèle à la direction de coulée.The generator 5 is able to generate a sliding magnetic field. The term sliding magnetic field designates an alternating magnetic field, the amplitude B _{0 of which} is not constant, and varies between a minimum value and a maximum amplitude $B_{}^{}$${}_0^{\mathit{max}},$

maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

propagating along a propagation axis Δ, preferably rectilinear. By amplitude, we mean the maximum value that a periodic quantity takes. Preferably, the axis of propagation belongs to a plane parallel to the direction of casting.

l'amplitude du champ magnétique, en ce point, varie entre une valeur minimale $B_0^{\mathit{min}}$

et la valeur $B_0^{\mathit{max}}$

cette dernière n'évoluant pas dans le temps.The distance λ separating two amplitude maxima of the magnetic field is the wavelength of the sliding magnetic field. The figure 1B represents an example of the distribution of the amplitude B ₀ of a magnetic field sliding along a propagation axis Δ at an instant t (solid line), and at an instant t + Δt (dotted line). On the axis of propagation, there is represented a coordinate r corresponding to the position of a point of the marsh. The figure 1C illustrates a temporal evolution of an alternating magnetic field sliding at this point. Due to the propagation of the maximum amplitude value $B_{}^{}$${}_0^{\mathit{max}},$

the amplitude of the magnetic field, at this point, varies between a minimum value $B_{}^{}$${}_0^{\mathit{min}}$

and the value $B_{}^{}$${}_0^{\mathit{max}}$

the latter not changing over time.

où σ désigne la conductivité électrique.A sliding magnetic field generator 5 can be formed by several electromagnetic inductors arranged around the peripheral enclosure. The Lorentz force, at a point of coordinates r of the marsh, comprises an oscillating component, modulated according to a frequency 2 f double the frequency of the magnetic field. The amplitude F ₀ of the oscillating Lorentz force density can be explained according to the expression: $F_0\left(r\right)=\frac{1}{2}{\mathit{<figure-callout\; id="0"\; label="\sigma f\lambda B"\; filenames="imgf0006.png,imgf0009.png"\; state="\{\{state\}\}">\sigma f\lambda B</figure-callout>}}_0^2\left(r\right)$

where σ denotes the electrical conductivity.

We can define a sliding speed V _G of the magnetic field V _G = fλ (2) in which case the expression (1) can be expressed as follows: $F_0\left(r\right)=\frac{1}{2}{\mathit{<figure-callout\; id="0"\; label="\sigma V\; G\; B"\; filenames="imgf0006.png,imgf0009.png"\; state="\{\{state\}\}">\sigma V</figure-callout>}}_G{B}_{}^{}{}_0^2\left(r\right)$

et une amplitude maximale $B_0^{\mathit{max}}.$

Il en est de même de la densité de force de Lorentz, cette dernière ayant, en un point r du marais, une valeur maximale lorsque l'amplitude du champ magnétique, en ce point, est maximale. En se plaçant dans le repère XYZ, lié à la lingotière 2, la propagation d'une valeur maximale de l'amplitude du champ magnétique $B_0^{\mathit{max}},$

le long d'un axe de propagation, entraîne, simultanément, la propagation d'une force de Lorentz d'intensité maximale F_max selon l'axe de propagation Δ. La combinaison des forces se propageant le long de l'axe de propagation établit un mouvement du liquide selon cet axe constituant un élément de pompe électromagnétique.Thus, the amplitude of the Lorentz force at a point r of the marsh depends on the square of the amplitude of the magnetic field applied at this point. The application of a sliding magnetic field results, at a point in the marsh, by a modulation of its amplitude. Thus, the amplitude of the magnetic field at a point in the marsh varies as a function of time, between a minimum amplitude $B_{}^{}$${}_0^{\mathit{min}}$

and a maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}.$

The same is true of the Lorentz force density, the latter having, at a point r of the marsh, a maximum value when the amplitude of the magnetic field, at this point, is maximum. By placing itself in the XYZ reference, linked to the mold 2, the propagation of a maximum value of the amplitude of the magnetic field $B_{}^{}$${}_0^{\mathit{max}},$

along a propagation axis, simultaneously causes the propagation of a Lorentz force of maximum intensity F _max along the propagation axis Δ. The combination of forces propagating along the axis of propagation establishes a movement of the liquid along this axis constituting 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 marsh, the intermittent macrosegregations are attenuated, or even disappear, and this particularly on ingots whose thickness is greater than 300 mm.

• la valeur de l'amplitude maximale ${\mathrm{<figure-callout\; id="0"\; label="B"\; filenames="imgf0006.png,imgf0009.png"\; state="\{\{state\}\}">B</figure-callout>}}_0^{\mathit{max}}$
  
  du champ magnétique ;
• de la fréquence f du champ magnétique ;
• la longueur d'onde λ du champ magnétique glissant.

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

• the value of the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$
  
  the magnetic field;
• the frequency f of the magnetic field;
• the wavelength λ of the sliding magnetic field.

du champ magnétique. Sur la figure 1D, on a représenté un mode de réalisation dans lequel la valeur de l'amplitude maximale $B_0^{\mathit{max}}$

du champ magnétique et la longueur d'onde λ du champ magnétique glissant sont variables au cours du temps. Ainsi, on a représenté une distribution spatiale de l'amplitude B ₀(t) dans le marais, à un instant t (trait continu), ainsi qu'une distribution spatiale de l'amplitude B ₀(t + Δt), à un instant t + Δt (trait pointillé). Durant l'intervalle temporel Δt, l'amplitude maximale $B_0^{\mathit{max}}$

varie entre $B_0^{\mathit{max}}\left(t\right)$

et $B_0^{\mathit{max}}\left(t+\mathrm{\Delta }t\right).$

De même, la longueur d'onde λ a été modifiée, passant de λ(t) à λ(t + Δt). Sur la figure 1E, qui représente une évolution temporelle d'un champ magnétique alternatif glissant en un point, on a représenté un mode de réalisation dans lequel la valeur de l'amplitude maximale $B_0^{\mathit{max}}$

du champ magnétique varie, au cours du temps, pour une fréquence f et une longueur d'onde λ constantes.When the sliding magnetic field is generated by a plurality of electromagnetic inductors arranged at the periphery of the mold, the temporal modulation of the Lorentz force density can be obtained by modifying the pole pitch, that is to say the phase shift between the induction currents flowing in each inductor. Such a modification makes it possible to vary the wavelength λ of the sliding magnetic field, that is to say the distance between two maxima propagating along the axis of propagation. The frequency of the induction current flowing in the inductors can be variable, which modifies the frequency f of the magnetic field. The amplitude of the induction current can also be variable, which changes the value of the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

of the magnetic field. On the figure 1D , there is shown an embodiment in which the value of the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

of the magnetic field and the wavelength λ of the sliding magnetic field are variable over time. Thus, we have shown a spatial distribution of the amplitude B ₀ ( t ) in the marsh, at an instant t (solid line), as well as a spatial distribution of the amplitude B ₀ ( t + Δ t ), at an instant t + Δ t (dotted line). During the time interval Δ t , the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

varies between ${\mathrm{<figure-callout\; id="0"\; label="B"\; filenames="imgf0006.png,imgf0009.png"\; state="\{\{state\}\}">B</figure-callout>}}_0^{\mathit{max}}\left(t\right)$

and ${\mathrm{<figure-callout\; id="0"\; label="B"\; filenames="imgf0006.png,imgf0009.png"\; state="\{\{state\}\}">B</figure-callout>}}_0^{\mathit{max}}\left(t+\mathrm{\Delta }t\right).$

Likewise, the wavelength λ has been changed from λ ( t ) to λ ( t + Δ t ). On the figure 1E , which represents a temporal evolution of an alternating magnetic field sliding at a point, an embodiment has been shown in which the value of the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

of the magnetic field varies, over time, for a constant frequency f and a wavelength λ .

Therefore, in the examples shown on the figures 1D and 1E , the maximum amplitude of the Lorentz force, propagating in the marsh, varies between t and t + Δ t , between the values F _max ( t ) and F _max ( t + Δ t ).

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

A sliding magnetic field B can in particular be generated from two inductors arranged on the same face of the ingot. The inductors are preferably placed facing a large face of the ingot, that is to say one of the two sides of the ingot having the largest vertical section. The inductors can be superimposed on one another, so as to generate a so-called vertical phase shift, or arranged side by side, so as to generate a horizontal phase shift. In the examples described below, a device described in the application was used. WO2014 / 155357 , and more precisely according to the configuration described in connection with the figures 19 and 20A , in which three inductors, oriented along the vertical axis Z, are arranged facing each major face of the ingot.

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

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

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

Preferably, the variation of one or more force parameters takes place in a time interval less than or equal to the characteristic durations of the recirculations generated by natural convection. These times vary depending on the thickness of the ingot and the casting speed. Considering thicknesses e between 300 mm and 700 mm, and casting speeds of between 30 mm / min and 80 mm / min, the characteristic times of 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). Thus, the force parameters vary in a time interval Δ t determined as a function of these characteristic times. The term “variation” is understood to mean a significant variation of at least 10% of the force parameter considered, and preferably of at least 20% or even 30% of the force parameter.

The variation of a force parameter can be periodic, the time period of variation being of the order of a characteristic recirculation duration, that is to say between 20 seconds and 10 minutes depending on the conditions of dimensions and speed of the casting. Preferably, in the marsh, during the time period of variation, the maximum density Lorentz force varies by at least 30 Nm ^-3 , and advantageously at least 40 Nm ^-3 , and preferably at least 50 Nm ^-3 , and even more preferably at least 60 Nm ^-3 .

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

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

Advantageously, during the casting, the Lorentz force of maximum intensity is greater than 80 N / m ³ , preferably greater than 100 N / m ³ , preferably greater than 120 N / m ³ , even more preferably. greater than 140 N / m ³ . The inventors have in fact observed that the suppression of intermittent macrosegregations was not optimum when the force was too low, as shown in Example 5 ( Fig 20 a to d). The minimum value from 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.

du champ magnétique sont modifiées respectivement en faisant varier la fréquence et l'amplitude du courant d'induction circulant dans des inducteurs. Pour cela, le procédé peut comprendre une étape préalable de définition d'un domaine de fonctionnement, c'est-à-dire une plage de variation de la fréquence et/ou de l'intensité du courant d'induction. Cette étape préalable comprend la détermination d'une ou de plusieurs valeurs de couples fréquence/intensité, dites valeurs critiques, générant, à la surface libre 1_sup du marais, une résonance, la résonance se traduisant par l'apparition d'oscillations significatives de ladite surface libre 1_sup, cette dernière étant représentée sur la figure 1A. Ces oscillations significatives sont généralement observées à l'œil nu. Par oscillation significative, on entend par exemple une oscillation dont l'amplitude est supérieure ou égale à 5 mm selon l'axe vertical Z. Par exemple, la fréquence du courant est fixée et on augmente l'intensité du courant d'induction jusqu'à ce qu'une oscillation significative soit observée.According to one embodiment, the frequency f and / or the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

of the magnetic field are modified respectively by varying the frequency and the amplitude of the induction current flowing in the inductors. For this, the method can comprise a preliminary step of defining an operating domain, that is to say a range of variation of the frequency and / or of the intensity of the induction current. This preliminary step includes the determination of one or more values of frequency / intensity pairs, called critical values, generating, at the free surface 1 _sup of the marsh, a resonance, the resonance resulting in the appearance of significant oscillations of said free surface 1 _sup , the latter being shown on the figure 1A . These significant oscillations are usually seen with the naked eye. By significant oscillation is meant, for example, an oscillation whose amplitude is greater than or equal to 5 mm along the vertical axis Z. For example, the frequency of the current is fixed and the intensity of the induction current is increased up to that a significant oscillation is observed.

du champ magnétique glissant, permet de s'approcher temporairement de la courbe de résonance R, par exemple de façon périodique, tout en restant dans la zone de stabilité. Cela permet de maximiser l'intensité de la force de Lorentz, et donc le brassage du marais, tout en restant dans des configurations de sécurité acceptables. En effet, au voisinage de la courbe de résonance, l'effet de brassage est particulièrement important.By considering different critical values of frequency (or intensity), it is possible to experimentally determine a resonance curve R, in a frequency / intensity plane corresponding to the different pairs (frequency / intensity) at which a resonance is observed at the free surface of the marsh. From this curve R, a range of variation of the intensity and / or of the frequency is determined, so as to avoid or limit the appearance of a resonance of the free surface of the marsh. In fact, the resonance curve delimits a zone of stability and a zone of instability, in which the casting can become dangerous. However, modulating the frequency or intensity of the induction current, and therefore the frequency f or the maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

of the sliding magnetic field, makes it possible to temporarily approach the resonance curve R, for example periodically, while remaining in the zone of stability. This maximizes the intensity of the Lorentz force, and therefore swamp mixing, while remaining within 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, that is to say the dimensions of the mold, the casting speed, the configuration of the applied magnetic field, the latter depending on the magnetic field generator, c ' that is to say inductors or permanent magnet (s) used. An R resonance curve is shown on the figure 2 , this curve having been obtained by casting an ingot with a thickness of 525 mm x 1650 mm, at a casting speed of 45 mm / min, a magnetic stirring being carried out by the application of a magnetic field by three inductors arranged in front of each large face of the ingot and out of phase by 90 ° to form a horizontal electromagnetic pump element, as previously mentioned. In this figure, there is also shown graphs representing a percentage of the intensity of a Lorentz force, called nominal, 100% corresponding to the intensity of the maximum induction current usable in the installation when the frequency is equal at 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 are 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 of the frequency of the induction current flowing through an inductor. The inventors have found that it is advantageous to vary the frequency because the variation in the penetration of the field which results therefrom makes it possible to vary the force gradient in the thickness and the depth of the liquid well more effectively. In addition, the power electronics make the frequency variation faster than the variation intensity; which gives an additional degree of freedom towards the weaker periods of unsteady forcing. It is in fact advantageous to decouple the hydrodynamic characteristic times from the characteristic solidification times in order to avoid intermittent macrosegregations.

Another example of a curve is shown on the figure 8 and will be commented on later in connection with the examples. On the figures 2 and 8 , we have shown the resonance curve R, determined experimentally, as well as the curve representing a Lorentz force whose intensity is equal to 10% of the nominal Lorentz force defined beforehand.

The variation of one or more force parameters can in particular make it possible to alternate periods during which the dimensionless number of Hartmann Ha is respectively low, typically less than 1, and high, typically greater than 3, or even 5. The dimensionless number of Hartmann Ha is a number commonly used in the field of magnetohydrodynamics. It represents a ratio between the magnetic viscosity and the viscosity of a charged liquid flowing in a magnetic field. The greater this number, the greater the contribution of Lorentz forces. Preferably, the dimensionless Hartmann Ha number alternates with a ratio between weak and strong values of at least 3 or of at least 5. Such a configuration is preferred, since 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 natural convection predominates.

As described in connection with the examples presented below, the products obtained by a method according to the invention exhibit intermittent macrosegregation which is limited compared to methods of the prior art, or even not perceptible. In the following examples, the characterization of the products was carried out by analyzing horizontal profiles (along the TC axis) of an X-ray taken at mid-width along a vertical L / TC plane, these profiles being calibrated to obtain the distribution spatial elements of heavy alloys of Zn or Cu type. The areas enriched in such heavy elements, more absorbent, appear in the form of dark spots on the negative of the radiographs taken and therefore light spots on the radiographs presented. An example of obtaining the Zn concentration profile from an X-ray of an Al-Zn alloy is shown on the figure 4 .

The terms L, TL and TC, known to those skilled in the art, correspond respectively to the dimension of the ingot along the vertical axis, the so-called “long transverse” axis and along the so-called “short transverse” axis.

Complementarily or alternatively, chemical analyzes can be carried out along horizontal profiles, so as to quantify the spatial distribution of said chemical elements along the TC axis. Intermittent macrosegregation can be characterized by a maximum deviation in mass of an alloying element, in this case Zn, in the zone most marked by intermittent macrosegregation, that is to say in the vicinity of T / 2.5 .

To quantify intermittent macrosegregation, the concentration profiles, obtained by radiography or by any other method, with a spatial resolution of 0.1 mm were processed as shown in Figure figure 5A . The profile obtained with the resolution of 0.1 mm is the raw profile referenced profile A. A sliding average over 2 mm makes it possible to avoid microsegregation, the smoothed profile obtained is referenced profile B. Another sliding average of the raw profile over 50 mm makes it possible to get rid of intermittent macro-segregations, and to obtain the continuous macro-segregation profile, the profile obtained being a so-called basic profile, referenced profile C. Profile C is subtracted from profile B to obtain a so-called corrected profile , corresponding to intermittent macrosegregation, the corrected profile being referenced profile D. Such a profile is shown on the Figure 5B . As can be seen from this figure 5B , the corrected profile is mainly representative of intermittent macrosegregation, and is not or only slightly affected by central continuous macrosegregation and by microsegregation. Such a corrected profile makes it possible to characterize the intermittent macrosegregation.

où max (C_ZA) et min (C_ZA) désignent respectivement les concentrations maximale et minimale de l'élément considéré mesurées entre T/2.3 et T/3.3.We can then calculate a maximum difference in concentration in an analysis zone Z _A located between T / 2.3 and T / 3.3, this maximum difference being able to be expressed according to the following equation: $$\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZA}}=\max \left({\mathrm{VS}}_{\mathrm{ZA}}\right)-\min \left({\mathrm{VS}}_{\mathrm{ZA}}\right)$$

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

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

où max (C_ZR) et min (C_ZR) désignent respectivement les concentrations maximale et minimale de l'élément considéré mesurées entre T/6 et T/12.The maximum difference Δ C _ZA can be normalized by the nominal concentration C ₀ of the element considered. The products according to the invention preferably have a value of such a standardized ratio of less than 10% and preferably less than 8% or even less than 6%. However, the absolute value of 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, in order to characterize the products obtained by the method according to the invention, to calculate, as a reference, a maximum deviation in a reference zone Z _R not very sensitive to intermittent macro-segregations, located between T / 6 and T / 12, this maximum deviation being able to be expressed according to the following equation: $$\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZR}}=\max \left({\mathrm{VS}}_{\mathrm{ZR}}\right)-\min \left({\mathrm{VS}}_{\mathrm{ZR}}\right)$$

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

We thus obtain a dispersion criterion ε making it possible to evaluate the intermittent macrosegregation for the element considered: $$\mathrm{\epsilon }=\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZA}}/\mathrm{\Delta }{\mathrm{VS}}_{\mathit{ZR}}$$

To avoid local variations in composition, it is advantageous, in order to determine Δ C _ZA and ΔC _ZR , to calculate an average over at least 5 concentration profiles at least 10 mm apart.

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

According to a nomenclature known to those skilled in the art, T / n denotes a distance from 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 by the nominal composition of the element. Such an analysis makes it possible to identify spatial periods characterizing intermittent macrosegregation. Intermittent macrosegregation has periods between 8 and 25 mm. When intermittent macro-segregation is important, we therefore observe a peak in the amplitude of the Fourier components for spatial periods between 8 and 25 mm. One determines an adimensional criterion of spectral intensity ζ which corresponds to the maximum amplitude of Fourier components in a spatial period range between 8 and 25 mm, normalized by the nominal concentration C ₀ of the element considered. The products obtained by the process according to the invention preferably have a criterion critère less than 0.01, preferably less than 0.007 and preferably less than 0.005.

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

In the case where the sum of two elements is considered, the values for normalizing the maximum deviation Δ C _ZA , and / or the Fourier transform correspond to the sum of the nominal concentrations of the elements considered.

The rectangular cross-section ingots obtained by the process according to the invention can be used as they are cast or after wringing, optionally after dissolution and quenching and aging for the alloys with age hardening. Advantageously, the ingots of rectangular section obtained by the process according to the invention are rolled and / or forged.

Example 1.

An AA7035 alloy was cast without electromagnetic stirring. The composition of the cast alloy comprising 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 grain refining was carried out with an AITiB 5: 1 refining concentration of 1Kg / t. The casting speed was 35 mm / min. The figure 3 shows a mid-width x-ray of the ingot on an L / TC plane, in which the central negative macrosegregation and intermittent macrosegregation are clearly identifiable. On the figure 6A , different horizontal raw profiles of the Zn content have been shown, along a TC axis, as well as smoothed profiles B are obtained by a sliding average over 2 mm deduced from the figure 3 . Radiography makes it possible to quantify only the elements causing a contrast with respect to aluminum, namely in this case Zn. This remark applies to Example 2 below. The central negative macrosegregation is clearly observed, maximum at T / 2, intermittent macrosegregation being observed between T / 2.3 and T / 3.3. On the figure 6B , the various profiles corrected for the Zn content (profiles D), along a TC axis, obtained after subtraction of each smoothed profile (profile B) by a base profile (profile C) representative of the continuous macrosegregation, have been shown.

The value of the maximum deviations of the Zn content was 0.75% by weight for Δ C _ZA and 0.19% by weight for ΔC _ZR , the value of the maximum deviations normalized in the analysis zone and in the reference zone thus being 13.3% and 3.5% respectively. The value of the dispersion criterion ε as defined by equation (6) was 3.9. The Fourier transform of each profile has been calculated, and is represented on the figure 7 , after normalization by the nominal composition of Zn: 5.6% by weight. The x-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 mm and 25 mm. The spectral intensity criterion ζ, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm, normalized by the nominal concentration C ₀ of Zn, was for all the profiles at least 0.01.

Example 2.

In a second example, a casting of an AA7035 alloy was carried out with electromagnetic stirring. The composition of the cast alloy included a nominal Zinc concentration of 5.6% by weight and a nominal Magnesium concentration of 1.3% by weight. The ingot size was 1650mm x 525mm. The grain refining was carried out with an AITiB 5: 1 refining concentration of 1Kg / t. The casting speed was 35 mm / min. The electromagnetic stirring was obtained by placing, opposite each face L / TL of the ingot, (corresponding to a YZ plane in the reference mark indicated on the figure 1A ), three inductors oriented along the vertical axis Z, traversed by an alternating current, frequency 0.25 Hz, phase-shifted with respect to each other by 60 ° and spaced from each other by 0.6 m, 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 the opposite direction. The inductors generated a magnetic field sliding along a horizontal plane, the sliding axis being parallel to the TL direction, the wavelength λ was 3.6 m. The maximum density of the Lorentz force induced in the liquid marsh was varied between about 180 N / m ³ and 240 N / m ³ with a variation speed of 2 Nm ^-3 .s ^-1 by modifying the nominal value of the current in the inductors. The resonance curve, corresponding to these conditions of casting, is shown on the figure 8 . The variation in the intensity of the induction current is represented in this figure by a double arrow.

The figure 9 shows an X-ray of the ingot according to an L / TC plane, on which the central negative macrosegregation at T / 2 is identifiable. On the figure 10A different gross horizontal profiles of the Zn content (profile A) and smoothed (profiles B) have been shown, along a TC axis. One distinguishes the negative central macrosegregation, maximal at T / 2. On the figure 10B , the different horizontal profiles of the Zn content have been shown, along a TC axis, of corrected profile type (D profiles) obtained after subtraction of the profile corresponding to the continuous macrosegregation.

The value of the maximum deviations of the Zn content was 0.24% by weight for Δ C _ZA and 0.28% by weight for Δ C _ZR , the value of the maximum deviations normalized in the analysis zone and in the zone of reference being respectively 4.3% and 5%. The value of the dispersion criterion ε as defined by equation (6) was 0.9: intermittent macrosegregation in the analysis zone between T / 2.3 and T / 3.3 has been eliminated. The Fourier transform of each profile has been calculated, and is represented on the figure 11 , after normalization by the nominal composition of Zn: 5.6% by weight. The x-axis represents the spatial period, between 0 and 30 mm. There are no longer any predominant peaks observed. The spectral intensity criterion ζ, which corresponds to the maximum amplitude of the Fourier components between 8 and 25 mm normalized by the nominal concentration C ₀ of Zn, was for all the profiles less than 0.005.

Example 3

In this example, an AA 7050 alloy was cast without electromagnetic stirring. The composition of the alloy was 6.3% by weight Zn, 2.2% by weight Mg and 2.1% by weight Cu. The format of the ingot was 1650x525mm. The grain refining is carried out using an AITiC3: 0.15 refining wire with an addition rate of 1 kg / tonne. The casting speed was 45mm / min. It constitutes the reference of Example 4.

The figure 12 shows an X-ray of the ingot according to an L / TC plane, on which the central negative macrosegregation at T / 2 is identifiable. On the figure 13a , we have shown the smooth horizontal profile of the sum of two elements Zn and Cu (profiles B) along an axis TC, deduced from the radiography of the figure 12 . Indeed, the radiography only makes it possible to quantify the elements causing a contrast with respect to aluminum, namely in this case Zn and Cu. This remark applies to Examples 4 and 5 which follow. On the figure 13b , we have represented the different horizontal profiles of the Zn + Cu concentration, along a TC axis, of corrected profile type (D profiles) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum deviations 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 as defined by equation (6) was 4.4. The figure 14 represents the Fourier transform of each profile, after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The x-axis 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 normalized by the sum of the nominal compositions in 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, an alloy was cast in AA 7050. 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 1650x525mm. The grain refining is carried out using an AITiC3: 0.15 refining wire with an addition rate of 1 kg / tonne. The casting speed was 45mm / min. The electromagnetic stirring was obtained by placing, opposite each face L / TL of the ingot, (corresponding to a YZ plane in the reference mark indicated on the figure 1A ) three coils oriented along the z axis and traversed by an alternating current which was out of phase, in the central coil, by 90 ° with respect to the current in the extreme coils. The wavelength of the sliding field was 2.4 m. The electromagnetic pump elements thus obtained were arranged in a mirror with respect to each face L / TL of the ingot, the sliding direction being parallel to the cross-long direction, the generated sliding diverging from the mid-width of the ingot. The unsteady forcing was obtained by imposing a cyclic variation of the frequency of the alternating electric current which traversed the coils, as illustrated by the double arrow in the frequency vs intensity diagram of the figure 15 . The maximum density of the Lorentz force thus generated by the variation of the frequency between 0.450 and 0.600Hz was varied between approximately 110 N / m ³ and 150 N / m ³ over a period of 3 min which corresponds to a speed of variation of about 0.22 N / m ³ / s.

The figure 16 shows an X-ray of the ingot according to an L / TC plane, on which the central negative macrosegregation at T / 2 is identifiable. Intermittent macrosegregations are very strongly attenuated compared to the reference ( Fig 12 ), as shown in figures 17a and 17b .

On the figure 17a , the smooth horizontal profile of the sum of the Zn + Cu elements (profiles B) along an axis TC, deduced from the radiography of the figure 16 . On the figure 17b , the different horizontal profiles of the sum of the two elements Zn and Cu have been shown, along an axis TC, of corrected profile type (Profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum deviations of the Zn + Cu content was 0.30% by weight for ΔC _ZA and 0.14% for ΔC _ZR . The value of the dispersion criterion ε as defined by equation (6) was 2.2. Intermittent macro-segregation in the analysis area has therefore been reduced and is shown on the left. figure 18 , after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The x-axis 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 normalized by the sum of the nominal compositions in Zn and Cu, was for all the profiles less than 0.005.

Example 5

In this example, a casting of AA7050 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 contents of the other elements were all less than 0.5% by weight. The cross section of the ingot was 1650x525mm. The grain refining is carried out using an AlTiC3: 0.15 refining wire with an addition rate of 1 kg / tonne. The casting speed was 45mm / min. The electromagnetic stirring was obtained by placing, opposite each face L / TL of the ingot, (corresponding to a YZ plane in the reference mark indicated on the figure 1A ) three coils oriented along the z axis and traversed by an alternating current which was out of phase, in the central coil, by 90 ° with respect to the current in the extreme coils. The wavelength of the sliding field was 2.4 m. The electromagnetic pump elements thus obtained were arranged in a mirror with respect to each face L / TL of the ingot, the sliding direction being parallel to the cross-long direction, the generated sliding diverging from the mid-width of the ingot.

The unsteady forcing was obtained by imposing a variation from zero in the intensity of the alternating electric current flowing through the coils, as illustrated by the arrows in the frequency vs intensity diagram of the figure 19 . The intensity of the maximum Lorentz volume force thus generated by the variation in intensity typically varied from 0 N / m ³ to 140 N / m ³ in 4 min, which corresponds to a rate of variation of 0.58 N / m3 / s. Subsequently, the intensity of the maximum Lorentz volume force was varied between 140 N / m ³ and 360 N / m ³ in 5 min, which corresponds to a rate of change of 0.73 N / m 3 / s.

The results obtained are illustrated by the two radiographed vertical slices represented on the figure 20a (variation in intensity between 0 N / m ³ to 140 N / m ³ in 4min) and figure 21a (variation of the force between 140 N / m ³ to 360 N / m ³ in 5min) which are in continuity with one another.

On the figure 20b , the smooth horizontal profile of the sum of the major elements Zn + Cu (profiles B) along a TC axis, deduced from the radiography of the figure 20a . On the figure 20c , the different horizontal profiles of the sum of the Zn + Cu elements have been shown, along a TC axis, of corrected profile type (D profiles) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum deviations of the Zn + Cu content was 0.70% by weight for ΔC _ZA and 0.22% for ΔC _ZR . The value of the dispersion criterion ε as defined by equation (6) was 3.2. The figure 20d represents the Fourier transform of each profile, after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The x-axis 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 normalized by the sum of the nominal compositions in Zn and Cu, was for all the profiles less than 0.01. It is noted however that the criterion of spectral intensity ζ shows values higher than 0.005.

On the figure 21b , the smooth horizontal profile of the sum of the major elements Zn + Cu (profiles B) along a TC axis, deduced from the radiography of the figure 21a . On the figure 21c , the different horizontal profiles of the sum of the major elements Zn + Cu have been shown, along an axis TC, of corrected profile type (Profiles D) obtained after subtraction of the profile corresponding to the continuous macrosegregation. The value of the maximum deviations of the Zn + Cu content was 0.37% by weight for ΔC _ZA and 0.15% for ΔC _ZR . The value of the dispersion criterion ε as defined by equation (6) was 2.4. The figure 21d represents the Fourier transform of each profile, after normalization by the sum of the nominal compositions of Zn and Cu: 8.3% by weight. The x-axis 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 normalized by the sum of the nominal compositions in Zn and Cu, was for all the profiles less than 0.005.

It is thus observed that the suppression of intermittent macrosegregations is improved if the force is greater than 140 N / m ³ . Indeed, when the force is too low, it is observed that the values of the dispersion criterion ε of spectral intensity ζ are greater than the preferred values of the invention. The inventors thus assume that an unsteady forcing which would consist of periodically interrupting the sliding field would not make it possible to advantageously suppress the intermittent macrosegregations.

Claims (14)

1. Method for casting an aluminium alloy ingot in a substantially rectangular ingot mould comprising the following steps:

   - preparing the aluminium alloy;

   - casting the aluminium alloy in the ingot mould, 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, application of a magnetic field of which the amplitude (B₀) is periodically varied according to a frequency (f), said magnetic field being generated by at least one magnetic field generator arranged at the periphery of the ingot mould, 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 $\left(B_0^{\mathit{max}}\right)$

   

   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 (F_max );

   the method being characterised in that a magnetic parameter referred to as a force parameter, governing the Lorentz force of maximum intensity (F_max ), is variable in a predetermined time interval (Δt), said parameter being:

   ▪ said maximum amplitude $\left(B_0^{\mathit{max}}\right)$

   

   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 (F_max ) propagating along the axis of propagation.
2. Method according to claim 1, wherein the section of the ingot mould, 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 than 300 mm and preferably at least 400 mm.
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 (F_max ), propagating along the axis of propagation, varies by at least 30 N.m^-3 in a time interval (Δt) 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 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 (Δt).
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 direction of casting.
7. Method according to any of the preceding claims, wherein during the casting, the variation in the force parameter is periodical, the period being between 20s 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 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 mould.
9. 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 (1_sup) of the aluminium alloy flowing in the ingot mould, 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. 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 (R), 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. Method according to any of claims 1 to 7, wherein at least one generator is a permanent magnet, the method comprising:

    - a variation in a distance between the permanent magnet and the ingot mould;

    - 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. Method according to any of the preceding claims, wherein the aluminium 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. Aluminium alloy ingot, obtained by the method object of any of claims 1 to 12 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: $$\mathrm{\epsilon }=\mathrm{\Delta }{C}_{\mathit{ZA}}/\mathrm{\Delta }{C}_{\mathit{ZR}}$$

    

    $$\mathrm{\Delta }{C}_{\mathit{ZA}}=\max \left({\mathrm{C}}_{\mathrm{ZA}}\right)-\min \left({\mathrm{C}}_{\mathrm{ZA}}\right)$$

    

    $$\mathrm{\Delta }{C}_{\mathit{ZR}}=\max \left({\mathrm{C}}_{\mathrm{ZR}}\right)-\min \left({\mathrm{C}}_{\mathrm{ZR}}\right)$$

    

    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 (ZA), 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 measured in a reference zone (ZR), 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 of the element considered according to the direction TC.
14. Aluminium alloy ingot, according to claim 13 of which a spectral intensity criterion (ζ) is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said spectral intensity criterion being calculated by:

    - Determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent 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,

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

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