Method for producing sheet ingots by vertical casting of an aluminium alloy

Abstract

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

1 . A method for casting an aluminum alloy ingot in a substantially rectangular ingot mold comprising the following steps:
 preparing the aluminum alloy;   casting the aluminum alloy in the ingot mold, along a vertical casting axis, the alloy being cooled, during the casting, by a runoff of a coolant in contact with the solidified metal;   during the casting, applying a magnetic field of which the amplitude (B 0 ) is periodically varied according to a frequency (ƒ), said magnetic field being generated by at least one magnetic field generator arranged at the periphery of the ingot mold, in such a way as to apply a Lorentz force (F) at different points of a liquid portion of the alloy in the process of solidification;   the magnetic field applied being a traveling magnetic field, propagating along an axis of propagation, in such a way that a maximum amplitude (B 0   max ) of the magnetic field propagates along said axis of propagation, defining a propagation wavelength (λ), said traveling magnetic field driving a propagation, along said axis of propagation, a Lorentz force of maximum intensity;   wherein the force parameter, governing the Lorentz force of the maximum intensity, is variable in a predetermined time interval, said parameter being:   said maximum amplitude of the magnetic field;   and/or said frequency of the magnetic field;   and/or the propagation wavelength of the magnetic field;   in such a way as to obtain a modulation, in said time interval, of said Lorentz force of the maximum intensity propagating along the axis of propagation.   
     
     
         2 . The method according to  claim 1 , wherein the section of the ingot mold, in a horizontal plane, defines a thickness and a length, the thickness being less than or equal to the length, the thickness being greater than 300 mm and preferably at least 400 mm. 
     
     
         3 . The method according to  claim 1 , wherein the frequency of the magnetic field is less than 5 Hz, or 2 Hz or 1 Hz. 
     
     
         4 . The method according to  claim 1 , wherein the Lorentz force of the maximum intensity, propagating along the axis of propagation, varies by at least 30 N.m −3  in the predetermined time interval between 20 seconds and 10 minutes. 
     
     
         5 . The method according to  claim 1 , wherein the magnetic field is such that the absolute value of the variation of the density of the maximum Lorentz force is greater than or equal to 0.05 N.m −3 .s −1  during said predetermined time interval. 
     
     
         6 . The method according to  claim 1 , wherein the axis of propagation of the maximum amplitude of the magnetic field belongs to a plane parallel to the direction of casting. 
     
     
         7 . The method according to  claim 1 , wherein during the casting, the variation in the force parameter is periodical, the period being between 20 s and 20 minutes, or between 1 minute and 15 minutes, or between 2 minutes and 10 minutes. 
     
     
         8 . The method according to  claim 1 , wherein the generators are electromagnetic inducers, each electromagnetic inducer having a current flowing through it referred to as induction current, the method comprising, during said time interval:
 a variation in the intensity of the induction current;   and/or a variation of a frequency of the induction current;   and/or a variation of a distance between an electromagnetic inducer and the ingot mold.   
     
     
         9 . The method according to  claim 8 , comprising a variation in the intensity or in the frequency of the induction current flowing through an inducer, the method comprising:
 a prior step of defining at least one critical value of the intensity and of the frequency of the induction current generating, on a free surface of the aluminum alloy flowing in the ingot mold, a resonant wave;   a determination of a range of variation in the intensity or in the frequency of the induction current according to said critical value defined beforehand.   
     
     
         10 . The method according to  claim 9  comprising, during said prior step, a definition of a plurality of critical values of the intensity and of the frequency of the induction current, in such a way as to define a resonance curve, representing the values of intensity and of frequency generating a resonance of said free surface, the method comprising a determination of a range of variation in the intensity or in the frequency of the induction current in a range delimited by said resonance curve. 
     
     
         11 . The method according to  claim 1 , wherein at least one generator is a permanent magnet, the method comprising:
 a variation in a distance between the permanent magnet and the ingot mold;   and/or a rotation of the permanent magnet, and a variation in the rotation speed of the magnet;   and/or a rotation of two permanent magnets.   
     
     
         12 . The method according to  claim 1 , wherein the aluminum alloy is chosen from alloys of types 2XXX, 5XXX, 6XXX or 7XXX and wherein the thickness is at least 400 mm or 450 mm. 
     
     
         13 . The method according to  claim 1 , wherein the dimensionless Hartmann number, at at least one point of the liquid portion of the alloy, varies at least by a factor of 3, even by a factor of 5, in said predetermined time interval. 
     
     
         14 . An aluminum alloy ingot obtained by the method according to  claim 1 . 
     
     
         15 . The aluminum alloy ingot according to  claim 14  having, for an element of the alloy, of which the content by weight is greater than 0.5%, or for the sum of two elements of the alloy of which the individual content by weight is greater than 0.5%, a dispersion criterion less than 3.3, preferably less than 3, more advantageously less than 2.5, even more advantageously less than 2 and preferably less than 1.5, the dispersion criterion being defined according to the following expressions:
 ε=ΔC ZA /ΔC ZR    
 ΔC ZA =max (C ZA )−min (C ZA ), ΔC ZR =max (C ZR )−min (C ZR ), where: 
 max (C ZA ) and min (C ZA ) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the two elements considered measured in a zone of analysis, having intermittent macrosegregations, for example between T/ 2 . 3  and T/ 3 . 3 ; 
 max (C ZR ) and min (C ZR ) respectively designate the maximum and minimum concentrations of the element considered or of the sum of the two elements considered measured in a reference zone, considered as little affected by the intermittent macrosegregations, for example between T/ 6  and T/ 12 ; 
 said concentrations being measured on at least one profile established at mid-width in a vertical plane L/TC and according to a direction TC, said profile being representative of said intermittent macrosegregations of the element considered according to the direction TC. 
 
     
     
         16 . The aluminum alloy ingot according to  claim 14 , wherein a spectral intensity criterion is less than 0.01, preferably less than 0.007 and preferably less than 0.005, said spectral intensity criterion being calculated by:
 determining a maximum amplitude of a Fourier transform of a profile representative of an intermittent macrosegregation of an element of which the content by weight is greater than 0.5% or the sum of two elements of the alloy of which the individual content is greater than 0.5%, the profile being established according to said direction TC, said maximum amplitude being determined in a range of spatial periods between 8 and 25 mm,   standardizing said maximum amplitude by a nominal concentration of said element or by the sum of the nominal concentrations of the two elements considered.