EP0408453B1 - Apparatus for an electromagnetic nozzle for controlling a jet of liquid metal - Google Patents

Description

The present invention relates to an electromagnetic nozzle device which can be used in particular at the outlet of a crucible for the stabilized casting of a liquid metal with variable flow rate in the form of ultra-clean material intended in particular for the atomization of metal powders such as for manufacture of superalloy parts for aeronautical applications.

The known and currently used methods for the preparation of superalloys, in particular based on nickel such as those which are particularly targeted by the present invention, involve melting operations in a crucible of refractory material of ceramic type carried out in a vacuum furnace . During this operation, a metal / ceramic reaction takes place, which inevitably results in the presence of ceramic inclusions in the material obtained. A refining of the superalloy is therefore made necessary each time the application conditions impose the obtaining of a so-called super-clean superalloy. This is particularly the case for nickel-based superalloys intended for aeronautical applications, such as parts of aeronautical turbine engines or other propulsion assemblies. Various known techniques are used to obtain such an inclusive refining, for example by remelting in a cooled crucible, the fusion being obtained either by electric arc, or by electron beam or by plasma beam.

• des diamètres de busette assez importants sont imposés afin d'éviter le risque de bouchage ;
• instabilité du jet de métal liquide ;
• difficultés importantes pour faire varier le diamètre du jet liquide en cours d'opération.

Whatever technique is used, however, when casting molten metals for use, whether filling a mold or atomizing the liquid metal to obtain powders, it becomes necessary either 'operate a tilting of the oven, in a first case, or to use at the outlet of the liquid metal a nozzle of refractory material, in a second case. In the first case, the control of the flow rate and the mass of the molten metal can hardly be obtained and in the second case where this problem is solved, other drawbacks appear:

• fairly large nozzle diameters are imposed in order to avoid the risk of clogging;
• instability of the liquid metal jet;
• significant difficulties in varying the diameter of the liquid jet during operation.

• d'une part, une pollution chimique dûe à la réaction du métal liquide à haute température avec les oxydes contenus dans les matériaux réfractaires constituant les parois ;
• d'autre part, une pollution physique dûe à l'abrasion des parois de la busette par le passage du métal en fusion.

Furthermore, the contact between liquid metal and solid walls of the nozzle leads to double pollution of the metal:

• on the one hand, chemical pollution due to the reaction of the liquid metal at high temperature with the oxides contained in the refractory materials constituting the walls;
• on the other hand, physical pollution due to abrasion of the walls of the nozzle by the passage of molten metal.

In particular applications, in particular of known techniques of preparation by atomization by gas of liquid metals, these pollutions cause the presence of numerous inclusions in the metallic powders and it is in particular recognized that the presence of such inclusions in rotating parts of aeronautical engines, for example based on nickel, can be the cause of defects in service performance of these parts subjected to oligocyclic fatigue stresses and in particular causes premature ruptures of parts subjected to high stresses at high temperature. These problems have led to a reduction in the grain size of the powders, which results in very poor particle size yields in the preparation of these powders. Attempts at a solution have been proposed, on the basis of the use of an electromagnetic nozzle which allows the confinement of the jet of liquid metal without contact with the walls. FR-A-2 316 026, FR-A-2 396 612 and FR-A-2 397 251 have thus described electromagnetic devices operating at high frequency in which a copper screen is necessary to obtain the desired confinement.
The industrial implementation of such devices, such as on an installation for atomizing powders of nickel-based superalloys, however presents serious difficulties. FR-A-2 457 730 can eliminate the copper screen, but the device operating at low frequency requires in many applications to use significant powers, not compatible with industrialization, as soon as significant reductions in the jet of liquid metal are becoming necessary, especially in powder atomization techniques.

An electromagnetic nozzle device making it possible to avoid the drawbacks of known prior devices is characterized in that the electromagnetic induction with turns is associated with a magnetic field concentrator device disposed between said coil inductor and the outlet walls of the crucible which it surrounds externally, said magnetic field concentrator device consisting of four to eight three-dimensional sectors separated by radial slots, each sector comprising a diametrically external semi-cylindrical wall and a semi-wall -cylindrical diametrically internal, coaxial and of smaller height, the respective four edges of these walls being joined by planar portions and the cavity thus formed being cooled by water, said internal and external walls comprising turns forming an electromagnetic inductor.

• la figure 1a représente, selon une demi-vue schématique en coupe par un plan vertical passant par l'axe de symétrie, un dispositif de busette électromagnétique conforme à l'invention,
• la figure 1b représente, selon une demi-vue schématique en coupe par un plan horizontal, le dispositif concentreur de champ magnétique de la busette électromagnétique représentée à la figure 1a ;
• la figure 2 représente, selon une vue schématique en coupe par un plan vertical, un creuset d'un type connu dit creuset refroidi de lévitation équipé du dispositif de busette électromagnétique représenté sur les figures 1a et 1b ;
• La figure 3 montre un détail de la figure 2 lors de l'évacuation d'un jet de métal liquide hors du creuset;
• La figure 4 montre un détail analogue à celui de la figure 3 dans le cas de l'application du dispositif de busette électromagnétique conforme à l'invention à un creuset réfractaire classique.

The remarkable results obtained are also conditioned by the choice of specific dimensioning parameters as well as by determined parameters for defining the applied magnetic field, in particular the frequency and intensity of the field.
Other characteristics and advantages of the invention will be better understood on reading the description which follows of an embodiment of the invention, with reference to the appended drawings in which:

• Figure 1 a is a diagrammatic half-view in section through a vertical plane passing through the axis of symmetry, an electromagnetic nozzle device according to the invention,
• Figure 1 b shows, in a schematic half sectional view in a horizontal plane, the magnetic field concentrator device of electromagnetic nozzle shown in Figure 1a;
• 2 shows, in a schematic view in section through a vertical plane, a crucible of a known type said cooled crucible levitation equipped with the electromagnetic nozzle device shown in Figures 1 a and 1 b;
• Figure 3 shows a detail of Figure 2 during the evacuation of a jet of liquid metal out of the crucible;
• Figure 4 shows a detail similar to that of Figure 3 in the case of the application of the electromagnetic nozzle device according to the invention to a conventional refractory crucible.

FIGS. 1 a and 1 b show detailed views of an electromagnetic nozzle device produced in accordance with the invention usable for controlling the jet of liquid metal, in particular at the outlet of the crucible of a installation for casting molten metal as partially shown in FIG. 2. The nozzle comprises an electromagnetic inductor 1, of a type known per se, comprising several turns 1 a and the implementation of which (power supplies, etc.) is also known per se and does not will not be the subject of a more detailed description. The inductor 1 is disposed at the outlet of a crucible 2 and externally surrounds the walls of said crucible. Between said inductor 1 and said walls of the crucible 2 is placed a device 3 magnetic field concentrator. The field concentrator 3 is sectorized and in fact, the field concentration effect appears as soon as a slot is present. To avoid deformation or deflection of the jet due to a higher magnetic field intensity in front of a slot, the field concentrator 3 is produced in an even number of equal sectors distributed symmetrically. For ease of production and in the applications targeted by the invention for the casting of metals or the atomization of superalloys, in particular based on nickel, the number of sectors provided is eight but it can be reduced to four. According to the embodiment of a particular geometry of the sectors 4 of the field concentrator 3 according to the invention and shown in Figures 1 a , 1 b , and 2, each sector 4 is made of copper plates and has a radially outer wall 4 has a semi-cylindrical wall arranged vertically relative to the crucible 2 and a wall radially internal 4b semi-cylindrical, coaxial with the previous one but of lower height. The respective four edges of these wall elements 4a and 4b are joined by four planar wall portions, upper 4c, lower 4d and lateral 4e and 4f. the internal cavity 5 thus formed inside each sector 4 is filled with cooling water. Each semi-cylindrical wall 4a and 4b has turns 6a and 7a so as to form an electromagnetic inductor. The sectors 4 of the magnetic field concentrator 3 are separated by radial slots 3a. The crucible 2 of a type known per se has walls 8 whose particular geometry makes it possible to keep most of the liquid metal 9 in levitation. Said walls 8 include cooling tubes 10 supplied by a water box 11. The liquid metal is discharged at the outlet of the crucible 2 through an orifice 12 masked by a cooled finger 13 capable of being retracted.

The detail of the lower part of the crucible 2, opened after retraction of the finger 13, shown in FIG. 3 shows the evacuation of a jet of liquid metal from the crucible. Originally, at the top of the crucible outlet, the jet of liquid metal has a diameter coincident with that of the material nozzle 14 located at the bottom of the crucible 2. As soon as the vein of liquid metal reaches the height of the 3 magnetic field concentrator of the electromagnetic nozzle, the metal jet has a reduction in section 15.

If, instead of a cold levitation crucible, as shown in FIGS. 2 and 3, a conventional refractory crucible, intended for example for the atomization of powders, is used, the lower part of this crucible 20, schematically represented in the FIG. 4 comprises an orifice 31 at the level of which the concentrator 3 of magnetic fields is positioned which causes a reduction in section 15 which separates the metal from contact with the wall 32a of the material nozzle 32.

dans laquelle :

• µ est la perméabilité magnétique dans le vide
• σ est la conductivité électrique du métal liquide,
• R est le rayon du jet de métal liquide,
• ω est la pulsation du champ magnétique, reliée à la fréquence f par ω = 2 π f,

La fréquence minimale f₁ obtenue est donc : $$\text{f₁ = 1 /π ω σ R²}$$

This result is obtained by the creation of an intense magnetic field over a very localized area which results from the use of the electromagnetic nozzle with magnetic field concentrator 3 according to the invention. A conventional coil inductor having to produce the same result would have a very large bulk, incompatible with the constraints imposed by the control of the jet of liquid metal. In fact, by the choice adapted to the application of dimensioning parameters and of adequate position of the electromagnetic nozzle and in particular of the magnetic field concentrator 3, axisymmetric forces directed towards the axis of the jet of liquid metal are generated. If the jet approaches wall 14a, said electromagnetic nozzle creates a restoring force which recenters the jet in the axis of the nozzle. This restoring force requires an intense magnetic field, the minimum frequency of which must be such that the penetration depth of the magnetic field and of its induced currents in the jet is less than the radius R of the liquid metal jet, which is expressed by the relation next : $$\text{µ σ ω R²> 2}$$

in which :

• µ is the magnetic permeability in a vacuum
• σ is the electrical conductivity of the liquid metal,
• R is the radius of the liquid metal jet,
• ω is the pulsation of the magnetic field, connected to the frequency f by ω = 2 π f,

The minimum frequency f₁ obtained is therefore: $$\text{f₁ = 1 / π ω σ R²}$$

• limitation des puissances mises en oeuvre ;
• risques d'amorçages électriques entre les différents secteurs 4 du concentreur 3 de champ magnétique ou entre ceux-ci et le jet métallique ;
• augmentation avec la fréquence des pertes dans l'inducteur 1 et le concentreur de champ 3 ;
• efficacité du dispositif mesurée par le coefficient de contraction X, exprimé en pourcents et défini par :
  X = ( de - ds) / de

avec de, diamètre de la veine liquide en entrée de la busette et ds, diamètre de la veine liquide en sortie de la busette.The restoring force is obtained when the magnetic field generates an increasing force in the radial direction from the surface of the jet, which causes, in conservative flux, a similar variation in the axial direction. Taking into account the exploitation of an essentially surface pressure effect, the efficiency of the device increases with frequency. Increasing the frequency also has the advantage of reducing the mixing effects of the liquid metal. However, practical limits which can be determined experimentally for each application are imposed on the frequencies. A maximum frequency f₂ is thus determined from the following criteria:

• limitation of the powers used;
• risks of electric strikes between the different sectors 4 of the magnetic field concentrator 3 or between them and the metal jet;
• increase with frequency of losses in inductor 1 and field concentrator 3;
• device efficiency measured by the contraction coefficient X, expressed in percent and defined by:
  X = (de - ds) / de

with de, diameter of the liquid stream at the inlet of the nozzle and ds, diameter of the liquid stream at the outlet of the nozzle.

A frequency domain f such as:
100 Hz <f, <10⁶ Hz in which the jet of liquid metal is not only channeled but also contracted is thus obtained.

The intensity B of the applied magnetic field is determined as a function of the magnetic pressure P _m exerted on the periphery of the jet of liquid metal to counterbalance the effects of surface tension and the forces of inertia and which is sought in the application concerned , depending on the relationship:
Pm = B² / 2 µ

The application of these conditions to a sample of nickel-based superalloy melted in crucible 2, shown in FIG. 2, in which the diameter of the material nozzle 14 is 15 mm made it possible to obtain a diameter 2R of metal liquid at the outlet of the 6mm electromagnetic nozzle, i.e. a contraction coefficient X, as defined above, of 60%.

The following results are obtained, expressed in values of the coefficient X of contraction as a function of the range of frequencies applied:
For 10² Hz <f <10⁶ Hz, X> 10%
for f <10² Hz or f> 10⁶ Hz, X <10%
and for 5.10³ Hz <f <5.10⁵ Hz, X> 50%.

The electromagnetic nozzle device with field concentrator device according to the invention and which has just been described thus makes it possible to ensure, by means of a choice of implementation parameters adapted to each application according to the criteria which have been indicated. the desired results and in particular a separation of the liquid metal from the walls of the reflow crucible, in particular at the level of the material nozzle leaving the crucible, thus avoiding any contact between walls and liquid metal and thereby any risk of pollution.

The device also has the advantage of ensuring stability of the jet of liquid metal contracted over a significant distance and thus a laminar flow is obtained over a distance which can be greater than ten times the outlet diameter of the electromagnetic nozzle. , the compactness of the device according to the invention facilitates the installation at the outlet of the crucible of an installation of the "super clean" type of reflow by electron beam, by plasma beam or, as in the example described, by reflow in a cold crucible, a casting installation (in a mold for example) or finally a powder atomization installation.

Claims (6)

1. Electromagnetic nozzle device disposed at the outlet of a metal-melting crucible (2), including an electromagnetic inductor (1) having turns (1a), characterised in that it furthermore includes a magnetic-field concentrating device (3) disposed between the said inductor (1) having turns and the outlet walls of the said crucible (2) which it surrounds on the outside and consisting of at least four three-dimensional sectors (4) separated by radial slots (3a), which are uniformly disposed around the said outlet of the crucible, including a water-cooled internal cavity (5) carrying, respectively in the diametrally external wall (4a) and in the diametrally internal wall (4b), turns (6a, 7a) forming an electromagnetic inductor.
2. Electromagnetic nozzle device according to Claim 1, in which the said sectors (4) are eight in number and have a diametrally external wall (4a) shape in the form of a segment portion of a vertical-axis cylinder and a diametrally internal wall (4b) shape in the form of a segment portion of a coaxial cylinder of smaller height, the four edges of one external segment and of the associated internal segment being joined together by plane portions (4c, 4d, 4e, 4f).
3. Electromagnetic nozzle device according to Claim 2, in which the walls of the sectors (4) of the field concentrator are made from copper.
4. Electromagnetic nozzle device according to any one of the preceding claims, in which the magnetic field applied to the jet of molten metal has a frequency located within an optimum range which is defined for each application between a minimum frequency f₁ given by the following formula: $$\text{f1 = 1/πµσR²}$$

   

   in which µ is the magnetic permeability in vacuum,
   σ is the electrical conductivity of the liquid metal in question and
   R is the radius of the jet of liquid metal, and a maximum frequency f₂ determined experimentally by taking into account the following factors:

   - available power,

   - risk of arcing,

   - limitation of the losses in the inductor and in the field concentrator, and

   - effectiveness measured by the contraction coefficient X, such that: $$\text{X = (de - ds)/ de}$$ where de is the diameter of the liquid stream at the inlet of the nozzle and ds is the diameter of the liquid stream at the outlet of the nozzle, whereas the intensity B of the applied magnetic field is connected to the desired magnetic pressure Pm being exerted at the periphery of the jet by the relationship: $$\text{Pm = B² / 2 µ}$$

   
5. Electromagnetic nozzle device according to Claim 4, in which the said optimum range of the frequencies f of the magnetic field for a contraction coefficient X greater than 50% is given by: $$\text{5.10³ Hz < F < 5.10⁵ Hz}$$

   
6. Electromagnetic nozzle device according to any one of Claims 1 to 5, characterised in that it is disposed at the outlet of a crucible (2) used for atomising a liquid metal in order to obtain powders of ultraclean material, especially superalloy powders.

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