WO2001026424A1

WO2001026424A1 - Method for melting and solidifying without contact an electric conductor sample

Info

Publication number: WO2001026424A1
Application number: PCT/FR2000/002728
Authority: WO
Inventors: Pascale Gillon
Original assignee: Centre National De La Recherche Scientifique
Priority date: 1999-10-04
Filing date: 2000-10-02
Publication date: 2001-04-12
Also published as: EP1222841A1; FR2799335B1; AU7670300A; ATE284124T1; FR2799335A1; JP2003511239A; KR20020043611A; EP1222841B1; DE60016444D1

Abstract

The invention concerns a method for melting and solidifying an electric conductor sample, which consists in: melting the sample by induction using a high frequency alternating magnetic field; superposing on the alternating magnetic field a continuous magnetic field gradient with high intensity, so as to levitate the melted sample without contact with solid surfaces; and in reducing the intensity of the alternating magnetic field by increasing the intensity of the continuous magnetic field, so as to maintain the sample in levitation without contact with the solid surfaces; and reducing the temperature of the sample to obtain solidification without contact of said sample. The invention is applicable to samples comprising at least a metastable phase.

Description

PROCESS OF MELTING AND SOLIDIFICATION WITHOUT

CONTACT OF A CONDUCTIVE SAMPLE

ELECTRICITY.

The present invention relates to a process for melting and solidifying a conductive sample of electπcite. as well as an application of this proceeds to the manufacture of samples comprising at least one metastable phase

The levitation of molten metallic materials is a phenomenon of great interest in the engineering of processes for the preparation of the absence of walls in contact with liquid material. Thus, such a technique makes it possible to develop or transport very reactive metals, at a very high melting point, or requiring a high purity, to measure in the liquid state the physical properties of o ceπains alloys or to produce significant rates of supercooling in fluids. can lead to metastable phases by rapid solidification

A known technique for levitating a metallic material is electromagnetic lev itation, which consists in applying a high frequency alternating magnetic field to this material. The application of the magnetic field i D produces two effects the generation of induced currents flowing in 1 sample. which orodoute a heating by Joule effect and thus make melt the sample, and the creation of an electromagnetic force of repulsion, which raised and maintains 1 sample in levitation In addition, the sample has 1 melted state undergoes an intense electromagnetic stirring: o This technique has the disadvantage of being able to levitate only a small sample. In addition, the heating power and the levitation force are coupled because they are all

proportional to the square of 1 ntensite of the alternating kinetic field applied in such a way that it is not possible to dissociate its effects Finally, the sample undergoes instabilities, due in particular to internal mixing, which risk in particular causing significant lateral movements and even taking the sample out of the field area

It has been proposed a levitation technique to resolve these drawbacks, which consists in coupling a high alternating magnetic field

: o frequency with a strong magnetic field with a strong spatial variation This method is described for example in the article

Stabilized levitation melting of metallic mateπals by Pascale Gillon, second

International Conference E P M. Pans. May 27-29, 1997 It produces a stable levitation of massive samples that can weigh up to a hundred

^ grams, diamagnetic or paramagnetic This stabilization is explained by braking of the electromagnetic mixing by the intense continuous magnetic field.

The advantages provided by the levitation of molten metallic materials are however often limited, or even compromised, during the solidification of these materials. Indeed, after fusion, the molten sample is collected in a container where it solidifies, so that it is in contact with walls. Solidification seeds therefore appear at the contacts of the sample with the walls of the container and, moreover, the purity of the sample cannot be maintained. This is detrimental to supercooling and leads to premature solidification, preventing all the expected benefits from being obtained.

Several techniques exist for solidifying samples without contact. All are based on the elimination of the effect of gravity: in space shuttle, in free fall or in parabolic flights. However, such methods are subject to considerable constraints: available times. quantities that can be involved and / or costs involved, to which are added for on-board systems problems linked to the presence of flammable materials or materials with disturbing magnetic properties.

The present invention relates to a process for melting and solidifying an electrically conductive sample, allowing contactless solidification of the sample while overcoming the constraints present in known techniques of contactless solidification. The method of the invention thus makes it possible to process quantities of materials identical to those subjected to fusion, for the desired duration and economically, without being restricted by security measures linked to on-board systems. The invention also relates to the application of such a method to the manufacture of samples comprising at least one metastable phase.

To this end, the invention relates to a process for melting and solidifying an electrically conductive sample, in which:

- in a first step, the sample is melted by induction by means of an alternating magnetic field,

in a second step, a gradient of a continuous magnetic field is superimposed on the alternating magnetic field, so as to cause the molten sample to levitate without contact with solid surfaces, and

- In a third step, a solidification of the sample is produced. According to the invention, in the third step, the intensity of the alternating magnetic field is reduced by varying the intensity of the gradient of the continuous magnetic field, so as to keep the sample in levitation without contact with solid surfaces and to reduce the temperature of the sample to obtain a contactless solidification of this sample.

Thus, surprisingly, one succeeds in solidifying the sample without contact, by means of the same device as that used for the fusion without contact. Contrary to expectations, it is not necessary to stop the levitation to reduce the temperature and thereby solidify the sample, and can be to ^refrain from implementing complex technical weightless for this solidification without contact.

In this way, the liquid phase can be overheated without contact to remove all solidification germs, then cooled without contact. Solidification then does not take place at the thermodynamic solidification temperature, but the sample remains liquid below this temperature: this is the phenomenon of supercooling. When solidification occurs, the out-of-equilibrium conditions make it possible to manufacture metal phases which cannot form at equilibrium. These conditions favor the production of metastable phases. In addition, solidification can be very rapid and generate very fine microstructures based on small grains (for example nanograins), or even make it possible to obtain glasses.

Generally, the change of 1 ^"intensity of the magnetic field gradient is increased. However, for some materials, changes in magnetic susceptibility with temperature generates a magnetic force intensity increase, which requires maintenance or a decrease of the field continuous magnetic.

Preferentially. the alternating magnetic field is at high frequency, that is to say at a frequency greater than 1 kHz. and advantageously greater than

50 kHz. In addition, the continuous magnetic field preferably has a high maximum intensity, of induction greater than 0.3 T and advantageously greater than 3 T.

By "continuous magnetic field" is meant a time invariant field.

The DC magnetic field preferably produces a strong gradient (spatial variation), the product of ^the magnetic induction continuous magnetic field by the magnetic induction gradient having an intensity greater than 1 T ² / m and advantageously greater than 50 T ² / m.

The method according to the invention is applicable not only to diamagnetic materials, but also paramagnetic or ferromagnetic. The ^application of DC magnetic field in the second step produces the following two effects:

- a levitation which vertically raises ^the sample: thus detaching it from its support, it reduces heat loss by contact and can thus obtain a high overheating; this effect is particularly advantageous in the case ^of a cold crucible, for thermal contact peπes then prevent overheating;

- And a reduction, or even a stopping of the electromagnetic stirring, which stabilizes the shape of the liquid and fixes its position, making it possible to levitate the liquid sample in a stable manner. By "overheating" is meant a rise in temperature to a value greater than the melting temperature.

The combination of alternating and continuous magnetic fields keeps the sample levitating while controlling its position enough to avoid contact with solid surfaces. Thanks to this combination, it is possible to avoid the instabilities of positioning of the sample, while reducing its temperature.

In addition, the presence of the alternating field ensures a paπielle self-regulation of the system: if the sample is raised beyond the alternating field, it undergoes a lesser force of electromagnetic levitation. There is therefore a stable electromagnetic position along a vertical axis, which constitutes a soπe of potential wells. What is more, the alternating field also makes it possible to compensate for radial instabilities, in particular when the continuous magnetic field generates a levitation force which is not perfectly vertical.

Preferably, in the third step, the reduction in intensity of the alternating magnetic field is compensated for by an adapted variation in intensity of the gradient of the continuous magnetic field, so as to exert on the sample an approximately constant levitation force. The sample thus remains in substantially the same position during its solidification.

The variation in intensity of the gradient of the continuous magnetic field is advantageously produced by the variation in intensity of the continuous field himself. Such an operation is in fact simple to implement. According to another technique, this variation of the gradient is obtained by modifying the relative positioning of ^the sample in the DC field or by moving the field or of the sample. Finally, a third gradient of the variation in shape combines the first two techniques (modification of ^the field strength and the relative positioning of ^the sample).

According to a first form of implementation of the combination of alternating and continuous magnetic fields during the third step, the variation of the levitation force produced by the alternating magnetic field (induction) is evaluated over time, preferably taking into account the variations with the temperature of the magnetic properties of the sample. Indeed, these generally decrease with temperature, according to a function depending on the material considered. Once the variation in levitation force due to the decrease in induction and temperature has been evaluated, the gradient of the continuous magnetic field is modified so as to compensate for the decrease in the levitation force and to keep it approximately constant. Preferably, the gradient of the continuous magnetic field is controlled by the variation of the levitation force evaluated.

It is advantageous that the evaluation of the variations in levitation force due to the decreases in induction and temperature is calculated numerically in real time, by means of tables giving data relating to the magnetic properties of the material treated.

In a second form of implementation of the combination of alternating and continuous magnetic fields during the third step, the gradient of the continuous magnetic field is controlled by the movements of the sample. We thus directly compensate for the effects of variations in the force of levitation, which manifest spatially. Advantageously, the intensity of the gradient of the continuous magnetic field is automatically controlled. In an implementation variant, the adjustments are made manually. Preferably, in the second step, the intensity of the alternating magnetic field is adjusted so as to obtain an overheating of the sample. This overheating is preferably sufficient to produce a supercooling of ^the sample in the third step.

The intensity of the alternating magnetic field must then be high enough to overheat the liquid phase so as to dissolve all the germs solidification in the second step. The cooling of the sample in the third step thus only causes solidification in a supercooled state. Obtaining such supercooling is obtained by the combination of two characteristics of the process: contactless melting, which makes it possible to rise very high in temperature in a very homogeneous manner in the sample, and contactless solidification, which avoids the appearance of solidification germs compromising supercooling.

Advantageously, in the first step, no continuous magnetic field gradient is applied. This technique has the merit of its simplicity and is particularly suitable when using a cold crucible to melt the sample.

According to another mode of application of the continuous magnetic field, a gradient of continuous magnetic field is applied from the first step, that is to say before and during the fusion of the sample. It is thus possible to melt the sample in levitation. Such a method generally requires a gradual reduction of the DC field, as the AC field is increased to increase the heating of the sample. Indeed, it should preserve the spatial stability of the latter, and even preferentially to exercise ^on a sample approximately constant levitation force. We therefore generally proceed in four stages for the application of the continuous field: increase from zero to obtain levitation, reduction to compensate for the increase in the alternating field (first and second stages), increase to compensate for the decrease in the alternating field ( third step) and preferably reduction to zero to recover the solidified sample.

This technique is more complex to implement than the previous one (without any continuous field during the first step). However, it is particularly advantageous when it is desired to avoid contact between the liquid sample and the container containing it, in order to preserve the purity of this sample. In particular, it is advantageously used when using a refractory crucible as a container. In this way, it is in fact avoided to load the sample material with refractory impurities, which would be produced by reactions at the walls of the crucible during melting.

During the second stage, an advantageous mode of implementation consists in slightly reducing the intensity of induction, which can make it possible to considerably reduce the temperature without significantly disturbing the position of the sample.

Preferably, in a fourth step, the alternating and continuous magnetic fields are gradually reduced to zero, so as to recover the solidified sample.

The sample is preferably placed in a gradient zone of the continuous magnetic field, this gradient having an intensity decreasing upwards.

This decrease ^s' interpreted as covering in particular the area where the sample is brought in levitation using the AC and DC magnetic fields. In this way, is completed ^the partial obtained by means of the alternating field self-regulation, which results in a stable electromagnetic position ^along a vertical axis. Indeed, the component of the force of levitation due to the continuous field is less when ^the sample rises. ^The effect of gravity is thus combined with the effects of the alternating field and the gradient of the steady field, so as to prevent instabilities and to be able to maintain ^the sample in position.

Preferably, the continuous magnetic field is applied by means of a superconductive magnet. According to a preferred mode of implementation, the sample is melted in a cold crucible. This crucible is, for example, cooled copper. The use of a cold crucible makes it possible to rise very high in temperature and to avoid chemical reactions at the walls. In addition, the levitation of the sample makes it possible to avoid heat exchanges at the walls and thus to raise the temperature substantially above the melting point of the material considered.

In another embodiment, the sample is melted in a crucible made of refractory bricks. This mode of implementation is however not applicable to materials capable of chemically attacking the walls of the crucible, and it is then necessary to melt the sample in levitation by applying from the first step a continuous magnetic field, as explained previously.

In an advantageous embodiment using a superconductive magnet for the application of the continuous magnetic field and a cold crucible for melting the sample, this cold crucible is inductive and is positioned in a magnetic field gradient area of the superconducting magnet, capable of producing an upward directed vertical force on the sample.

The ^invention also concerns the application of the melting process and solidifying the manufacture of samples comprising at least a metastable phase. This process indeed allows obtaining metastable phases which cannot be obtained other than by supercooling.

Advantageously, this metastable phase is based on a titanium alloy, preferably TiAl.

The invention will be better understood and illustrated by means of an exemplary implementation which is in no way limitative, with reference to the appended figure, representing a contactless melting device used to implement the method of the invention.

A contactless electromagnetic and magnetic levitation non-contact melting device comprises (figure) an inductor 1 placed in a superconductive coil 2 and surrounding a cold crucible 3.

The cold crucible 3 is, for example, a hemispherical crucible made of sectored copper having an internal diameter of 16 mm, inserted in the inductor 1. The bottom of the crucible 3 is equipped with a retractable cooled finger 4, connected to a horizontal support 5 of vertical translation. The inductor 1 is, for example, an inductor with four turns supplied with high frequency alternating current. The inductive system comprising the inductor 1 and the cold crucible 3 is placed in a sealed enclosure 10, connected to a primary vacuum pump. This enclosure 10, resting on the support 5. is provided with an upper window 7 allowing monitoring by video camera 6 of the phenomena occurring in the enclosure 10.

The superconductive coil 2 is provided with a field hole of 120 mm in diameter and is capable of delivering a vertical magnetic field up to 8 T in the center. The enclosure 10 is inserted in the center of this coil 2.

In operation, a solid sample is first placed in the cold crucible 3 and a vacuum is produced on this sample. The following operations are carried out for treating the sample under a partial argon atmosphere.

At zero continuous field, the sample is heated and then melted by induction by increasing the intensity of the alternating magnetic field. Once the sample is liquid, the intensity of the continuous field is gradually increased until the levitation is obtained. As soon as the sample leaves the cold crucible 3. it this results in significant overheating of the liquid. The force responsible for levitation consists of a component from the alternating magnetic field (repulsion between inductor and metallic charge) and a component from the continuous magnetic field gradient related to the magnetic susceptibility of the material.

To solidify without contact, the temperature of the sample is lowered while keeping the total levitation force constant. The decrease in temperature is obtained by gradual decrease in the intensity of the alternating magnetic field. This operation produces two effects: a decrease in the electromagnetic component of the levitation force and a variation in the magnetic susceptibility (which is a function of temperature) which acts on the value of the magnetic component of the levitation force.

Solidification without contact is therefore obtained by compensating in real time during cooling for the variation in the levitation force by a variation in the intensity of the corresponding gradient of the continuous magnetic field.

Thanks to monitoring by video camera 6, levitation is detected as soon as free movements of the sample appear and the intensity of the continuous magnetic field can be adjusted as a function of the position of the sample. In addition, an infrared pyrometer preferably makes it possible to monitor the temperature of the sample during treatment. It is thus possible, if necessary, to determine the variations in the magnetic properties of the sample and to combine them with the variations in induction to determine the variation to be applied to the continuous magnetic field, therefore to the induced gradient.

Several samples of pure titanium and a TiAl alloy (50%) were made. They are in the form of an egg, without any trace of contact with a solid surface. Monitoring the surface temperature by infrared pyrometry revealed that supercooling was obtained.

Claims

CLAIMS 1. Process for melting and solidifying an electrically conductive sample, in which:

- In a third step, a solidification of the sample is produced, characterized in that in the third step, the intensity of the alternating magnetic field is reduced by varying the intensity of the gradient of the continuous magnetic field, so as to maintain the sample in levitation without contact with solid surfaces and to decrease the temperature of the sample to obtain a contactless solidification of said sample.

2. A method of fusion and solidification according to claim 1, characterized in that in the third step, the reduction in intensity of the alternating magnetic field is compensated for by an adapted variation in intensity of the gradient of the continuous magnetic field, so as to exert an approximately constant levitation force on the sample.

3. A method of fusion and solidification according to one of claims 1 or 2. characterized in that in the second step, the intensity of the alternating magnetic field is adjusted so as to obtain an overheating of the sample.

4. A method of melting and solidifying according to claim 3, characterized in that said overheating is sufficient to produce a supercooling of ^the sample in the third step.

5. A method of fusion and solidification according to any one of claims 1 to 4, characterized in that in the first step, no gradient of continuous magnetic field is applied.

6. A method of fusion and solidification according to any one of claims 1 to 5, characterized in that the sample is placed in a gradient zone of the continuous magnetic field, said gradient having an intensity decreasing upwards.

7. A method of fusion and solidification according to any one of the preceding claims, characterized in that the sample is melted in a cold crucible (3).

8. A method of fusion and solidification according to claim 7, characterized in that the cold crucible (3) is inductive and is positioned in a continuous magnetic field gradient zone of a superconductive magnet (2), capable of producing on the sample a vertical force directed upwards.

9. Application of the method according to any one of the preceding claims to the manufacture of samples comprising at least one metastable phase.

10. Application according to claim 9, characterized in that the metastable phase is based on a titanium alloy, preferably TiAl.