EP3717147B1 - Low velocity and low frequency aluminium casting method - Google Patents

Description

TECHNICAL AREA

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

PRIOR ART

During 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 cause of macroscopic segregations when their lifetime is of the same order of magnitude as the characteristic solidification times. These phenomena lead, in the solidified ingot, to local depletion or local enrichment of 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 negative central macrosegregation, resulting from a depletion of eutectic alloying elements, along a central vertical axis of the ingot. These macrosegregations have been described in the work of John Wiley et al "Direct-Chill Casting of light alloys", Wiley Editor, 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 underlying 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;
• The transport of grains in the supercooled zone under the effect of gravity, Archimedes' force and natural or forced convection;
• Flow in the mushy zone induced by volumetric shrinkage at solidification, which may be assisted by metallostatic pressure;
• The flow of the liquid in the pasty zone caused by mechanical deformations;
• Forced flows which may result from pouring, injection or release of gas, mixing, vibration, etc. which 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 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 described less often 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 alloying 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 differentiated manner from the atoms of the metal making up the ingot. The directions L, TC and TL are defined with respect to the directions of the parallelepiped-shaped ingot. The direction L corresponds to the direction of casting, the direction TC corresponds to the direction parallel to the smallest dimension of the ingot, also called thickness and the direction TL being the third direction, also called transverse direction. Other means make it possible to visualize this phenomenon, for example echography 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, the intermittent macrosegregation is most marked in the vicinity of the T/2.5 region, and typically between T/2.3 and T/3.3, 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, denotes 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 soon 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 the cases of casting of aluminum alloys filled typically according to formats with a thickness greater than 300 mm, this thickness threshold itself depending on the casting speed.

The documents EP 2 682 201 A1 and US 2015/283606 A1 relate to the continuous and semi-continuous casting of metal, in particular aluminium. These devices comprise a crystallizer which is open at both ends in the casting direction, means for introducing a molten mass into the crystallizer, as well as electromagnetic inductors.

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

The reduction or elimination of continuous macrosegregations, for example central macrosegregation, has already been described. In particular, it has been shown that the application of a magnetic field, for mixing or flow braking purposes, 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 through which a direct current passes.

The inventors have 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 segregations, or even to eliminate them, so as to better control the mechanical properties of the products resulting from casting.

DISCLOSURE OF THE INVENTION

A first object of the invention is a process for forming an aluminum alloy ingot in a mold as described in appended claim 1.

In other words, if T/2 denotes half the thickness of the ingot, the technical plating effect is obtained between T/2 - T/4 and T/2 + T/4.

Mean Lorentz force during a period means a Lorentz force determined according to a time interval corresponding to the inverse of the frequency.

Preferably, a single sliding magnetic field whose amplitude is varied periodically according to a frequency is applied. That is to say that during several successive periods, a single sliding magnetic is applied, the amplitude of which is varied periodically according to a single frequency.

The method makes it possible to obtain a significant reduction in intermittent macro-segregations in the middle zone.

In the middle zone, the angle of inclination of the forehead is between 0° and 90°. The lower it is, the more the front is oriented parallel to the vertical axis. It is the same for the angle of the Lorentz force. An angle value of 0° corresponds to a vertical orientation.

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

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

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

Preferably, the magnetic field is modulated, at the frequency, between a minimum value and a maximum value. Preferably, the minimum and maximum value are constant for several successive periods. Thus, the average Lorentz force is constant during several successive periods, for example during at least 10 successive periods.

The aluminum alloy can in particular be chosen from alloys of the 2XXX, 6XXX or 7XXX type. Advantageously, the thickness of the mold is greater than 300 mm. Advantageously, the thickness of the ingot is greater than 300 mm.

The method comprises, prior to casting, a modeling of the Lorentz force applied to at least one point of the front, so as to define, taking into account the thickness of the mold, a frequency value and/or a value of casting speed allowing obtaining an average Lorentz force, whose angle of inclination with respect to the vertical, is less than the angle, at the said point, formed by the face with respect to the vertical . Preferably, this modeling is carried out at different points, along the forehead, along the transverse axis. The modeling can make it possible to define a value of frequency and/or a value of casting speed making it possible to obtain an average Lorentz force whose angle of inclination, with respect to the vertical, is less than 4° to the angle of inclination formed by the forehead with respect to the vertical.

A second object, not claimed, of the invention is an ingot, in particular an aluminum alloy ingot, obtained by a process according to the first object of the invention.

• 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 plusieurs 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 différents éléments considérés.

The ingot obtained by a method according to the first object of the invention is characterized by a spectral intensity criterion (ζ) of 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 several elements of the alloy whose individual content is greater at 0.5%, the profile being established in said direction TC, said maximum amplitude being determined in a range of spatial periods comprised 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 various elements considered.

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

FIGURES

• The figures 1A to 1D illustrate an example of a device allowing implementation of a method according to the invention. The figures 1A and 1B present the main components of the device. The figures 1C and 1D respectively represent a spatial and temporal distribution of the amplitude of a sliding magnetic field, according to an exemplary embodiment.
• The figure 2 illustrates an average Lorentz force, applied to part of the marsh of a flow, implementing the invention.
• The figures 3A, 3B, 3C and 3D show results of simulations making it possible to obtain the orientation of an average Lorentz force in the marsh, at the level of the front, respectively at different frequencies, for a casting speed of 55 mm per minute.
• The figures 3E, 3F, 3G and 3H show results of simulations making it possible to obtain the orientation of an average Lorentz force in the marsh, at the level of the front, respectively at different frequencies, for a casting speed of 35 mm per minute.
• The figures 4A and 4B are curves established by considering different frequencies respectively, and representing an evolution of an angle, called differential angle, along the front, the differential angle representing a difference between the angles, with respect to the vertical, respectively formed by the force of Lorentz mean and the front. On the figure 4A , a casting rate of 55 mm per minute was considered. On the figure 4B , a casting rate of 35 mm per minute was considered.
• The Fig. 4C shows an abacus making it possible to define an operating range as a function of the casting speed (axis of abscissas) and the thickness of the casting (axis of ordinates), to obtain an orientation of the Lorentz force according to the invention, in a middle zone extending between T/2±T/4.
• The figure 5A represents a horizontal profile, parallel to an axis defining the thickness of an ingot, representative of a concentration of an alloying element in an ingot, as well as smoothed profiles, obtained by different smoothings of the horizontal profile.
• The figure 5B shows a profile representative of the intermittent macro-segregation affecting an ingot obtained by casting.
• The Figures 5C and 5D are representative profiles of intermittent macro-segregation, represented in a Fourier space. The abscissa axis corresponds to the spatial period, expressed in mm. The profiles were obtained from ingots respectively formed with and without implementation of the invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The figures 1A and 1B illustrate a mold allowing an implementation of the invention. In this example, an aluminum alloy 1 flows into a mold 2, through an opening 2i. Casting takes place along a vertical Z axis, through the mold. The ingot mold is delimited by a peripheral enclosure whose section, in a horizontal plane XY, is parallelepipedic. The mold defines a frame, parallel to a longitudinal axis Y, along a width W, and, parallel to a transverse axis X, defining a thickness T. The width W is greater than the thickness T. The thickness T corresponds to a distance between two vertical walls 2p delimiting the mold 2. The casting forms a parallelepipedic ingot. A plane, called median plane M, extends at mid-thickness (T/2), parallel to the vertical axis Z and to the longitudinal axis Y of the ingot. The thickness T is preferably between 300 mm and 750 mm. It has been found that intermittent macrosegregations appear markedly when the thickness T exceeds 300 mm. In the examples or simulations mentioned in this description, we considered a thickness T equal to 525 mm. The width W is equal to 1650 mm. The length, along the vertical axis, can reach several meters, for example between 3 and 10 meters.

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

Under the effect of cooling, a zone of solid alloy 1s is formed, close to the cooled enclosure, around a zone of liquid alloy 1ℓ, designated by the term “swamp”. The interface between the marsh 1ℓ and the solid zone 1s forms a front 10. After cooling, the ingot, also designated by the term “product”, is formed. The front 10 has a slope, with respect to the vertical, which varies according to the thickness. The angle of the front is called an angle α between the tangent to the front, at a point, and the vertical, i.e. the Z axis. The lower the angle of the front α, the greater the tangent to the forehead is oriented vertically. The angle of the front is α represented on the figure 2 . The angle of the forehead varies along the transverse axis X.

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

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

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

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

The magnetic field B applied to the liquid alloy 1ℓ, is an alternating field, of amplitude B ₀ and of frequency f . The effect of this magnetic field is to apply swamp mixing, under the effect of Lorentz forces acting on the liquid alloy 1ℓ. Indeed, the application of a magnetic field B generates, in the alloy, the formation of an electric current J resulting, within the liquid alloy 1ℓ, 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 proportionality relation. The 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 into the swamp, so as to obtain effective mixing of the liquid. The frequency f is all the lower as the thickness of the product is high. 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 et orienté selon l'axe vertical Z. Par amplitude, on entend la valeur maximale que prend une grandeur périodique. L'application d'un champ magnétique glissant se traduit, en un point du marais, par une variation périodique de son amplitude. Ainsi, l'amplitude du champ magnétique en un point du marais varie en fonction du temps, entre une amplitude minimale $B_0^{\mathit{min}}$

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

.The generator 5 is able to generate a sliding magnetic field. The term sliding magnetic field designates an alternating magnetic field, whose amplitude B ₀ is not constant, and varies between a minimum value and a maximum amplitude $B_{}^{}$${}_0^{\mathit{max}}$

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

propagating along a propagation axis Δ, preferably rectilinear and oriented along the vertical axis Z. By amplitude is meant the maximum value taken by a periodic quantity. The application of a sliding magnetic field results, at a point in the marsh, in a periodic variation of its amplitude. Thus, the amplitude of the magnetic field at a point in the marsh varies as a function of time, between a minimum amplitude $B_{}^{}$${}_0^{\mathit{min}}$

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

.

The sliding magnetic field generator 5 may consist of several electromagnetic inductors arranged around the peripheral enclosure. On the figure 1B , three pairs 5 ₁ , 5 ₂ and 5 ₃ of electromagnetic inductors have been shown. The upper part 5s of the inductors is positioned at the level of the free surface 1 _sup of the liquid alloy. Each inductor has a phase shift of 90° between the upper part 5s and the lower part 5i. In the examples described below, a device was used as described in the application WO2014/155357 , and more precisely according to the configuration described in connection with FIGS. 19 and 20A, in which three inductors, oriented along the vertical axis Z, are arranged opposite each large face of the ingot. By large face of an ingot is meant a face extending along the longitudinal axis Y and the vertical axis Z. Each inductor comprises one or more coils. In this example, each coil is placed at a distance of 185 mm of the mold. In a way Generally, the distance between a coil of an inductor and the mold can be between 130 mm and 200 mm.

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 relative to the latter. For example, it is possible to generate a sliding magnetic field by rotating a permanent magnet.

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

.The distance λ separating two amplitude maxima of the magnetic field is the wavelength of the sliding magnetic field. The figure 1C represents an example of the distribution of the amplitude B ₀ of a magnetic field sliding along a propagation axis Δ at a time t (solid line), and at a time t+Δt (dotted line). On the axis of propagation, a coordinate r corresponding to the position of a point of the marsh has been represented. The figure 1D illustrates a temporal evolution of a sliding alternating magnetic field at this point. This evolution is periodic, and takes place according to a period P. The application of a sliding magnetic field results, at a point in the marsh, in a periodic variation of its amplitude. Thus, the amplitude of the magnetic field at a point in the marsh varies as a function of time, between a minimum amplitude $B_{}^{}$${}_0^{\mathit{min}}$

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

.

où σ désigne la conductivité électrique.The Lorentz force, at a point of coordinates r in 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: $$F_0\left(r\right)=\frac{1}{2}{\mathit{<figure-callout\; id="0"\; label="\sigma f\lambda B"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">\sigma f\lambda B</figure-callout>}}_0^2\left(r\right)$$

where σ denotes the electrical conductivity.

The amplitude of the Lorentz force at a point r in the marsh depends on the square of the amplitude of the magnetic field applied at this point.

The inventors have observed that the appearance of intermittent macro-segregations 11 can be limited by adjusting the electromagnetic stirring when the mean Lorentz force applied to the liquid alloy 1ℓ flowing at the front 10 has a certain orientation, and this in a median zone of the marsh, extending symmetrically on either side of the median plane M, between T/2 - T/4 and T/2 + T/4. The thickness of the middle zone M corresponds to half the thickness of the ingot. By average Lorentz force is meant an average of the Lorentz force during a period P of the magnetic field. The period P of the magnetic field corresponds to the time interval separating two successive maxima or minima of the magnetic field, as represented on the figure 1D . The period P corresponds inverse to the frequency f . The inventors have observed that in the middle zone, at the interface of the marsh 1ℓ, and the solid alloy 1s, at the level of the front 10, the angle β formed by the average Lorentz force F , with respect to the vertical, must advantageously be less than the angle α of the front, previously mentioned, corresponding to the angle between the tangent to the front and the vertical, the angles α and β being oriented in the same direction. That is, it is advantageous that the direction of the average Lorentz force F is more vertical than the direction of the tangent to the front. Thus, in the middle zone, at the interface between the swamp and the front, the average Lorentz force F is oriented towards the solid 1s alloy, and not towards the liquid 1ℓ alloy. This condition is illustrated in the picture 2 . This figure shows a section of a casting along an XZ plane. The position of the median plane M corresponds to the thickness T/2.

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

Under these particular conditions, the Lorentz force F tends to press the liquid alloy 1ℓ of the marsh against the front 10, which limits the formation of intermittent macro-segregations. The Lorentz force is said to be plating. It allows the formation of a convective laminar flow along all or part of the front 10, limiting the appearance of intermittent macro-segregations.

Preferably, a single sliding magnetic field whose amplitude is varied at a frequency f is applied to obtain the effect of pressing the liquid alloy 11 of the marsh against the front 10.

As described below, the phenomenon of plating of the liquid alloy by the Lorentz force against the front 10 is all the more marked as the casting speed V and the frequency f are low.

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

The figures 3A, 3B, 3C and 3D show the orientation of the mean Lorentz force, obtained by simulation, at different points of a front 10. In these figures, a part of a front 10 has been shown, along a plane XZ, extending between the plane median M (abscissa x=0) and a wall of the ingot mold (abscissa x=0.26). The abscissa axis represents a position along the transverse axis X and the ordinate axis represents a position along the vertical axis Z. The frequencies considered are respectively equal to 5Hz ( Figure 3A ), 1Hz, 0.5Hz and 0.2Hz ( 3d figure ), the casting speed being 55 mm/min. It is observed that by considering the same position on the front 10, the lower the frequency f , the greater the angle β of the average Lorentz force F is weak. Thus, in the same position on the front, the average Lorentz force F tends to tilt vertically as the frequency decreases.

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

The figures 3E, 3F, 3G and 3H show respectively the average orientation of the Lorentz force, obtained by simulation, at different points of a front 10, the frequencies being respectively equal to 5 Hz, 1 Hz, 0.5 Hz and 0.2 Hz.

On the figures 3A to 3H , so-called normalized forces have been represented, each force being normalized by its intensity, so as to better show the evolution of the orientation of the mean Lorentz force on the front 10, as a function of the position along the front .

The comparison of figures 3A to 3H shows that the lower the frequency, the greater the range of thickness Δx according to which the Lorentz force becomes tight. The range of thickness Δx extends from the median plane M towards the wall of the mold, along the transverse axis X. It widens as the frequency decreases. Moreover, at the same frequency, the lower the casting speed, the greater the range of thickness in which the Lorentz force is binding. It is therefore advantageous to favor both a low frequency f , preferably less than 2 Hz, or even 1 Hz, and a low casting speed, preferably less than 45 mm/min, or even 40 mm/min.

Based on simulations as shown in the figures 3A to 3H , the inventors have determined an evolution, along the transverse axis X, of an angle θ, called differential angle, representing a difference between, at the same point, the angle of the front α and the angle of the Lorentz force β, i.e. θ = α - β. It is recalled that the angles α and β are oriented in the same direction. When θ > 0, α > β: the Lorentz force is more inclined than the tangent to the front. So she's cheeky.

The figures 4A and 4B show the evolution of the differential angle θ as a function of a position x on the front 10, along the transverse axis X. The abscissa axis represents the position x, expressed in meters, on the front along of the transverse axis. Just as on the figures 3A to 3H , the coordinate x=0 corresponds to the position T/2, the coordinate x=0.26 corresponding to the wall 2p of the mold. The figures 4A and 4B were obtained by considering respectively a casting speed of 55 mm/min and 35 mm/min. On each figure, the simulations of the orientation of the mean Lorentz force F were carried out by successively considering several frequencies f , between 5 Hz and 0.2 Hz. In each figure, straight lines corresponding to the values θ = 0° and θ = 4° when the frequency is equal to 1 Hz. The Lorentz force is flattening when θ > 0, but the inventors consider that it is advantageous for θ ≥ 4°. It is thus possible to define, on each configuration, a range of thickness, in which the Lorentz force is binding, from the median plane M (thickness T/2).

We have represented, on the figures 4A and 4B , the thickness ranges Δx (θ = 0°) and Δx (θ = 4°) for f = 1 Hz. figures 3A to 3H , it is observed that the thickness ranges are all the more important as the frequency is low and the casting speed is low. Optimal results are obtained for f ≤ 2 Hz, or even f ≤ 1 Hz, and when the casting speed is 35 mm. The lower the frequency, the greater the intensity of the Lorentz force applied to the liquid alloy bordering the front, in the thickness range T/2 - T/4. This reinforces the intensity of the Lorentz force and increases the technical effect sought. In other words, to obtain a significant reduction in intermittent macro-segregations, an orientation of the Lorentz force as previously described is necessary. However, its intensity must be sufficient to obtain a plating of the liquid alloy 1ℓ, against the front 10. This is why it is preferable to modulate the magnetic field according to a relatively low frequency f .

Using simulations taking into account different casting thicknesses, the inventors established an abacus, represented on the Fig. 4C , making it possible to define an operating range for which the Lorentz force is considered to be sufficiently tight, that is to say when the differential angle θ is greater than or equal to 4°. This abacus is the subject of the Fig. 4C . The abscissa and ordinate axes of the chart correspond respectively to the casting speed V and to the thickness T of the ingot. The thickness being determined, the chart makes it possible to define the casting speed and the maximum frequency making it possible to place oneself in the conditions of implementation of the invention. On this chart, crosses show the experimental test conditions described below, in connection with Table 1.

This chart depends on the number and characteristics of the inductors, their positioning relative to the mold, the dimensions of the latter and the operational parameters of the installation, in particular relating to the applied magnetic field. The person skilled in the art, knowing the characteristics of the installation, can carry out simulations aimed at obtaining the orientation of the mean Lorentz force F at different points along the front 10, along the transverse axis X. He can then determine a frequency range and a casting speed range for which θ≥0°, or advantageously θ≥4°, is obtained, so as to implement the invention and obtain the desired technical effect, that is to say a limitation of intermittent macro-segregation between T/2 and T/4, and more particularly between T/2.3 and T/ 3.3.

Experimental tests were carried out, using alloys of the 7010 and 7035 type, the thickness T of the ingot being equal to 525 mm. Each pour was made using a fixed frequency. Between the different castings, the frequency f was varied between 0.250 Hz and 0.850 Hz. Each casting was carried out using a fixed casting speed. Between the different castings, the casting speed was varied between 35 mm/min and 55 mm/min.

Table 1 summarizes the experimental test conditions, five of which implement the invention. Tests 1 to 3 were carried out from a 7010 alloy, while tests 4 to 6 were carried out from a 7035 alloy. The parameters of each test are the frequency f and the casting speed V. During each test, a 1650 mm × 525 mm format ingot was produced.

The Ref 7010 and Ref 7035 tests are reference tests, carried out without electromagnetic stirring. <u>Table 1</u> Test Alloy V f ΔC _meso Fourier ΔC _meso Fourier (mm/min) (Hz) (W/4) (W/4) (W/2) (W/2) Ref 7010 7010 45 0.82 0.012 0.71 1 7010 40 0.475 0.44 0.005 2 7010 40 0.850 0.50 0.005 3 7010 45 0.250 0.56 0.006 Ref 7035 7035 35 0.59 0.012 0.75 0.014 4 7035 35 0.475 0.32 0.005 0.41 0.006 5 7035 35 0.270 0.46 0.007 6 7035 55 0.475 0.59 0.013 0.51 0.008

Each of these tests is represented by a cross on the abacus of the Fig. 4C . It can be seen that tests 1, 2, 3, 4 and 5 are considered to implement the invention if reference is made to the abacus represented on the Fig. 4C . Under these conditions, a Lorentz force is obtained between T/2 and T/4. Test 6 is outside the invention, because taking into account the thickness of the ingot and the casting speed, the chart indicates a maximum frequency of 0.1 Hz, whereas the frequency used during this test is amounted to 0.475 Hz.

The ingots formed were characterized by analyzing horizontal profiles (along the TC axis) of a radiograph taken at mid-width along a vertical plane L/TC, these profiles being calibrated for obtain the spatial distribution of heavy alloy elements such as Zn and/or Cu. For the 7010 alloy, the alloying elements considered are Zn and Cu. For the 7035 alloy, the alloying element considered is Zn. The terms L and TC, known to those skilled in the art, correspond to the dimension of the ingot along the vertical axis and along the so-called “short cross” axis. Intermittent macro-segregation can be characterized by a maximum difference in mass of an alloy element in the zone most marked by intermittent macro-segregations, i.e. between T/2.3 or T/ 3.3. As previously indicated, T/n denotes a distance relative to an edge of the ingot, along a horizontal axis, T/2 corresponding to the center of the ingot.

To quantify the intermittent macro-segregation, the concentration profiles were processed as illustrated in the figure 5A . The profile obtained with a resolution of 0.1 mm is the raw profile referenced profile A, and represented in dotted lines on the figure 5A . A first smoothing is carried out, according to a sliding average over 2 mm, the smoothed profile obtained being referenced profile B, represented by a solid line on the figure 5A . This smoothing makes it possible to attenuate the effect of micro-segregations, which correspond to local fluctuations in concentrations.

A second smoothing of the raw profile is carried out, according to a sliding average of 50 mm, to overcome intermittent macro-segregations, and to obtain a central continuous segregation profile, or base profile, referenced profile C. This profile is represented in dashes on the figure 5A . The basic profile C is subtracted from the smoothed profile B to obtain a so-called corrected profile D, representative of the intermittent macro-segregation. The latter is represented on the figure 5B . As can be seen on the figure 5B , the corrected profile D is mainly representative of intermittent macro-segregation, and is not or only slightly affected by central continuous macro-segregation and by micro-segregations. Intermittent macro-segregation is characterized by determining a concentration difference Δ C _meso = C _max - C _{min ·} C _max and C _min designate respectively the maximum and minimum concentrations measured on the corrected profile. It is considered that a value of Δ C _meso less than 75% of a reference value, obtained on a cast ingot without mixing, constitutes a satisfactory result, that is to say a significant reduction in intermittent macro-segregation .

The concentration difference Δ C _meso was measured at W/4 and/or at W/2, that is to say according to planes perpendicular to the median plane, parallel to the transverse axis X and whose coordinate, according to the longitudinal axis Y, is respectively equal to W/4 and W/2.

The corrected profile D has been normalized by the nominal concentration of the alloy element considered (Zn and Cu for the 7010 alloy, Zn for the 7035 alloy). The profile thus normalized was analyzed by Fourier transform, so as to identify the spatial period characterizing the intermittent macrosegregation. When intermittent macrosegregation is severe, one or more amplitude peaks in the range 8 mm - 25 mm are generally observed. An adimensional spectral intensity criterion ζ is determined which corresponds to the maximum amplitude of the Fourier components in a spatial period range between 8 and 25 mm. The products obtained by the process according to the invention preferably have a criterion ζ of less than 0.01, preferably less than 0.007 and preferably less than 0.005. The ζ criterion was measured at W/4 and/or at W/2. The figure 5C and 5D show respectively an example of a distribution of spatial periods between 0 and 30 mm for examples 4 and 6, on several profiles. In these figures, the ordinate axis represents the spectral intensity, while the abscissa axis represents the spatial period, expressed in mm.

On the ingots cast without stirring, corresponding to the reference measurements, the ζ criterion is greater than 0.01, with typical values of 0.012 at W/4 or 0.014 at W/2.

• pour les essais 1 et 2, ΔC_meso à W/2 est respectivement de 0,44 et 0,50, la valeur de référence étant 0,71 ;
• pour l'essai 3, ΔC_meso à W/4 est de 0,56 la valeur de référence étant 0,82.

Tests 1, 2, and 3 were obtained using a 7010 alloy. The casting speed was equal to 40 mm/min for tests 1 and 2 and 45 mm/min for test 3. The frequency f was respectively equal to 0.475 Hz, 0.850 Hz and 0.250 Hz. On each of these tests, we obtained a difference in concentration Δ C _meso significantly lower than the reference measurements:

• for tests 1 and 2, ΔC _meso at W/2 is 0.44 and 0.50 respectively, the reference value being 0.71;
• for test 3, ΔC _meso at W/4 is 0.56, the reference value being 0.82.

On each of these tests, the spectral intensity criterion ζ in the spatial period range between 8 and 25 mm is systematically less than 0.001.

• pour les essais 4 et 5, ΔC_meso à W/4 est respectivement de 0,41 et 0,46, la valeur de référence étant 0,75 ;
• pour l'essai 4, ΔC_meso à W/2 est de 0,32 la valeur de référence étant 0,59.

Tests 4, 5, and 6 were obtained using a 7035 alloy. The casting speed was equal to 35 mm/min for tests 4 and 5 and 55 mm/min for test 6. The frequency f was respectively equal to 0.475 Hz, 0.270 Hz and 0.475 Hz. On tests 4 and 5, we obtained a difference in concentration Δ C _meso significantly lower than the reference measurements:

• for tests 4 and 5, ΔC _meso at W/4 is 0.41 and 0.46 respectively, the reference value being 0.75;
• for test 4, ΔC _meso at W/2 is 0.32, the reference value being 0.59.

For tests 4 and 5, the spectral intensity criterion ζ in the spatial period range between 8 and 25 mm is less than 0.001.

Test 6, being outside the invention, does not make it possible to obtain a reduction in intermittent macrosegregations. The values obtained are comparable to the reference values.

The invention may be implemented for the production of ingots intended for components for which the requirements in terms of quality are high, for example components linked to applications in the aeronautical field.

Claims (7)

1. Method for forming an aluminium alloy ingot (1) in an ingot mould (2), the ingot mould defining a parallelepiped, such that the ingot formed extends parallel to a longitudinal axis (Y), along a width (W) and parallel to a transverse axis (X), along a thickness (T), the thickness being less than the width, the ingot defining a median plane (M) extending along the mid-thickness (T/2), parallel to the longitudinal axis (Y), the method comprising the following steps of:

   - preparing the liquid aluminium alloy;

   - casting the liquid aluminium alloy in the ingot mould along a vertical axis (Z), the alloy being cooled, during casting, by a surface flow of a cooling liquid (3) so as to form a solid alloy (1s) extending around a liquid alloy (1ℓ), referred to as a sump, said solid alloy forming a front (10) at the interface with the sump (1ℓ), the front being inclined at an angle of inclination (α) to the vertical axis, the angle of inclination of the front varying along the transverse axis (X);

   - displacing the solid alloy (1s), along the vertical axis (Z), at a casting velocity (V);

   - during the casting, applying a travelling magnetic field, the amplitude whereof is varied periodically according to a frequency (f), the magnetic field being generated by at least one magnetic field generator disposed at the periphery of the ingot mould, so as to apply a Lorentz force (F) at various points of the sump, the average Lorentz force (F), during a period (P) of the magnetic field, being inclined relative to the vertical axis (Z) at an angle of inclination (β), referred to as the angle of inclination of the Lorentz force, the latter varying along the transverse axis;

   - the travelling magnetic field propagating along a propagation axis parallel to the vertical axis;

   - the sump comprising a median zone, extending symmetrically on either side of the median plane (M), the thickness (T) whereof corresponds to a half-thickness of the ingot;

   the method being characterised in that:

   - the frequency (f) is less than 5 Hz;

   - and in that the casting velocity (V) and the frequency (f) are such that throughout the median zone of the sump, at the interface between the liquid alloy and the solid alloy, at the front (10), the angle of inclination (β) of the force is strictly less than the angle of inclination (α) of the front; the angles of inclination of the front and of the force being oriented in the same direction, such that in the median zone, the Lorentz force (F) acts on the liquid alloy (1ℓ), at the interface, by pressing it against the front (10), and where, prior to casting, the Lorentz force (F) applying at at least one point of the front (10) is modelled so as to define, taking into account the thickness (T) of the ingot mould, a frequency value (f) and/or a casting velocity value (V) enabling an average Lorentz force (F) to be obtained, the angle of inclination (β) whereof relative to the vertical is less than the angle, at said point, formed by the front (10) relative to the vertical.
2. Method according to claim 1, wherein the angle of inclination (β) of the average Lorentz force is at least 4° less than the angle of inclination (α) of the front, such that the average Lorentz force ( F ) is more inclined, towards the vertical, than the front (10) and wherein the modelling allows a frequency value and/or a casting velocity value to be defined, which allows an average Lorentz force ( F ) to be obtained, the angle of inclination (β) whereof, relative to the vertical, is 4° less than the angle of inclination (α) formed by the front (10) relative to the vertical.
3. Method according to any one of the preceding claims, wherein the frequency (f) is less than 2 Hz or less than 1 Hz.
4. Method according to any one of the preceding claims, wherein the casting velocity (V) is less than 45 mm/minute.
5. Method according to any one of the preceding claims, wherein the average Lorentz force ( F ) is constant over a plurality of successive periods.
6. Method according to any one of the preceding claims, wherein the aluminium alloy is selected from alloys of the 2XXX, 6XXX or 7XXX series.
7. Method according to any one of the preceding claims, wherein the thickness of the ingot formed is greater than 300 mm.

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