EP0269667B1 - Method for the preparation of permanent magnets by division of crystals - Google Patents

Abstract

Method for the preparation of permanent magnets at room temperature from an alloy containing at least a mixture of iron (Fe), boron (B) and rare earths (TR) including Yttrium, and for which there is a temperature range wherein said alloy is in two phases; one solid and brittle and the other one liquid. The method comprises heating said alloy under controlled atmosphere at a temperature sufficient to reach said temperature range, treating said alloy, and finally, optionally, allowing the treated alloy to cool. The method being characterized on the one hand in that said Fe/B-TR alloy is in a massive form, and on the other hand, in that the treatment of said massive alloy is carried out by bonding of the magnetic solid phase Fe-B-TR.

Description

The invention relates to a new process for producing high-performance permanent magnets by dividing the crystals of a magnetic phase in an alloy.

In the manufacture of permanent magnets, it was well known to use metal alloys Iron (Fe) - Boron (B) also comprising Rare Earths (TR). There are currently essentially two types of process for the manufacture of such magnets.

In the first process using powder metallurgy, described in European patent applications EP-A-0 101 552, 0 106 948 and 0 126 802, an alloy Fer-Bore-Rare Earth is produced which is ground in powder form, then oriented in a magnetic field that is cold pressed, sintered and finally subjected to heat treatment. If the magnets obtained in this way have excellent properties, however, this process has significant drawbacks. Indeed, the slightest pollution strongly alters the final properties. However, the pollution of the powder by the atmosphere is extremely rapid; this therefore requires working in a controlled atmosphere at room temperature, which increases manufacturing costs. In addition, it is necessary to use a grinding phase. However, the powders used have a high reactivity, in particular with respect to air, which unfortunately involves significant risks of explosion and fire.

The second process uses the micro-crystallization technique. This technique, described in European patents EP-A-0 125 752 or EP-A-0 133 758, essentially consists in melting an alloy of the type in question, then in subjecting it to a rapid hardening treatment on a roller, to crush and hot pressing, or coating the material obtained in a resin. This very fine jet technique of high temperature liquid soaked on a cold roller unfortunately leads to isotropic magnets, except to subject them to a creep and recrystallization operation which is always difficult to implement within a continuous process. In addition, as a high temperature melting process is used with very fine liquid ejection, an appropriate apparatus must be used and work in a controlled atmosphere in large enclosures with all the drawbacks that this entails.

Finally, in these two techniques, one necessarily goes through a phase during which the alloy is strongly divided.

The invention overcomes these drawbacks. It aims at a process of the type in question which is easy to implement, calls for more economical processing of raw materials, and leads to materials having improved properties.

• · on chauffe ledit alliage sous atmosphère contrôlée à une température suffisante pour atteindre ledit domaine de température;
• · puis, on traite cet alliage par corroyage à chaud;
• · et enfin, on laisse refroidir l'alliage traité.

This process for the preparation of permanent magnets at room temperature from a solid alloy containing at least a mixture of Iron, Boron and Rare Earths including Yttrium, for which there is a temperature range inside which said alloy is in two phases: one solid and fragile, and the other liquid, a process in which:

• · Said alloy is heated under a controlled atmosphere to a temperature sufficient to reach said temperature range;
• · Then, this alloy is treated by hot working;
• · And finally, the treated alloy is allowed to cool.

The treatment also consists in that the wrought is carried out with a wrought rate of at least ten, in order to refine the grains of the alloy into particles of a few microns and in that the wrought alloy is then subjected to a treatment. annealing and / or tempering at a temperature between 600 and 1000 ° C.

In other words, the invention consists first of all in no longer using an alloy in powder form but in a solid alloy comprising two phases, then in heating this solid alloy, and finally in subjecting it to strong mechanical stresses to induce a wrought with a high wrought rate and at a temperature enabling the magnetic crystals to break into particles of a few microns and finally, to subject this alloy to a heat treatment before cooling it.

• "atmosphère contrôlée", on désigne une atmosphère dont on contrôle la composition; il s'agit en pratique d'une atmosphère de gaz rares ou de vide, et ce afin d'éviter des réactions avec les Terres Rares;
• "corroyage", on désigne un traitement mécanique appliqué à un alliage métallique, destiné à provoquer l'affinement des grains de cet alliage; on peut citer des traitements de forgeage, de martelage, de laminage, de filage, de vibro-tassage (tassement par vibrations).

In the description and the claims, by:

• "controlled atmosphere" means an atmosphere whose composition is controlled; in practice it is an atmosphere of rare gases or vacuum, in order to avoid reactions with Rare Earths;
• "wrought" means a mechanical treatment applied to a metal alloy, intended to cause the refining of the grains of this alloy; mention may be made of forging, hammering, rolling, spinning, vibro-compaction (compaction by vibration) treatments.

• l'alliage massif est un alliage ternaire à base de Fer, de Bore et de Terres Rares, y compris l'Yttrium;
• en pratique, notamment pour des raisons substantielles d'économie et de propriétés mécaniques, la Terre Rare est choisie dans le groupe constitué par le Neodyme et le Praseodyme, qui dans ce cas est en plus grande proportion;
• les proportions respectives des différents constituants de cet alliage, pouvant contenir également d'autres agents de formation d'eutectiques, tels que l'Aluminium ou le Gallium, correspondent aux proportions habituelles, notamment celles décrites dans les demandes de brevets européennes citées dans le préambule;
• l'alliage se présente sous forme de lingots massifs, éventuellement sous forme de morceaux massifs; ainsi, en d'autres termes, lors de l'application des contraintes mécaniques du corroyage, on casse les cristaux magnétiques à chaud dans le liquide qui les entoure en phase finale;
• le chauffage de l'alliage massif est effectué par tout moyen connu, tel que effet Joule, induction, l'alliage pouvant être soit sous enveloppe étanche, soit sous vide, soit sous gaz rare;
• l'alliage massif ainsi chauffé est corroyé soit sous vide, soit sous gaz rare, ou dans un liquide non réactif, voire dans une enveloppe étanche pouvant subir les traitements mécaniques et thermiques, tels que par exemple une enveloppe en Fer doux ou en alliage à base de Fer;
• le chauffage est effectué à une température comprise entre 400 et 1050°C, de préférence au voisinage de 700°C, en tout cas à une température suffisante pour atteindre la plasticité de la phase eutectique non magnétique; on a constaté que si la température est inférieure à 400°C, on réduit l'alliage en poudre, ce qui ramène à la première technique exposée dans le préambule alors que si cette température excède 1050°C, on n'obtient plus le phénomène de corroyage, car les grains magnétiques deviennent trop malléables et grossissent au fur et à mesure du traitement;
• on développe les contraintes mécaniques de corroyage comme déjà dit par forgeage, martelage, filage, laminage ou tout autre traitement thermo-mécanique; on a constaté que la taille des cristaux magnétiques obtenus résulte du taux de corroyage appliqué dans les produits; ainsi, on a observé que l'on obtient de bons résultats avec un taux de corroyage supérieur à dix, avantageusement de l'ordre de vingt-cinq;
• après refroidissement éventuel, l'alliage traité subit un traitement de recuit et/ou de revenu à des températures comprises entre 600 et 1000°C voire plus, préférentiellement entre 700 et 900°C, ce qui améliore et stabilise les propriétés magnétiques, notamment la coercitivité.

Advantageously, in practice:

• the solid alloy is a ternary alloy based on Iron, Boron and Rare Earths, including Yttrium;
• in practice, in particular for substantial reasons of economy and mechanical properties, Rare Earth is chosen from the group consisting of Neodyme and Praseodyme, which in this case is in greater proportion;
• the respective proportions of the various constituents of this alloy, which may also contain other eutectic forming agents, such as Aluminum or Gallium, correspond to the usual proportions, in particular those described in the European patent applications cited in the preamble ;
• the alloy is in the form of solid ingots, possibly in the form of solid pieces; thus, in other words, during the application of the mechanical stresses of the working, the magnetic crystals are broken hot in the liquid which surrounds them in the final phase;
• the heating of the solid alloy is carried out by any known means, such as the Joule effect, induction, the alloy possibly being either in a sealed envelope, or under vacuum, or under rare gas;
• the solid alloy thus heated is wrought either under vacuum or under rare gas, or in a non-reactive liquid, or even in a sealed envelope which can undergo mechanical and thermal treatments, such as for example an envelope made of soft iron or of alloy with iron base;
• the heating is carried out at a temperature between 400 and 1050 ° C, preferably in the vicinity of 700 ° C, in any case at a temperature sufficient to reach the plasticity of the non-magnetic eutectic phase; we found that if the temperature is lower at 400 ° C, the alloy is reduced to powder, which brings back to the first technique exposed in the preamble whereas if this temperature exceeds 1050 ° C, we no longer obtain the phenomenon of wrought, because the magnetic grains become too malleable and increase in size as the treatment progresses;
• mechanical stresses are developed as already mentioned by forging, hammering, spinning, rolling or any other thermo-mechanical treatment; it has been found that the size of the magnetic crystals obtained results from the rate of wrinkling applied in the products; thus, it has been observed that good results are obtained with a degree of currying greater than ten, advantageously of the order of twenty-five;
• after possible cooling, the treated alloy undergoes an annealing and / or tempering treatment at temperatures between 600 and 1000 ° C. or even more, preferably between 700 and 900 ° C., which improves and stabilizes the magnetic properties, in particular the coercivity.

In other words, the fundamental characteristic of the invention consists in not using an alloy in powder form but in a solid alloy which is much more economical and less dangerous, then in treating this solid alloy by working this which no longer requires the use of complex and expensive equipment.

The manner in which the invention can be implemented and the advantages which ensue therefrom will emerge more clearly from the following exemplary embodiments, given by way of non-limiting illustration, in support of the single appended figure, which schematically represents an installation for placing implementing the method according to the invention.

This installation basically includes an anvil (1), on which rests a retaining ring (2), surrounded by a glass enclosure (3), defining a sealed chamber (4), connected by the inlet (5) to a Argon source not shown. The top of the sealed chamber has an opening (6) through which the hammer (7) of the exterior impact assembly (8) can pass through a seal (9). The sample (10) rests on the anvil (1) around the ring (2) in which the hammer (7) slides. The glass enclosure (3) is surrounded by induction coils (11).

Example 1 :

• 78 atomes de Fer;
• 6 atomes de Bore;
• 15,5 atomes de Néodyme;
• 0,5 atome d'Aluminium.

In a known manner, a massive sample is prepared (washer, molded cylinder, ingot, shot, ...) in an Iron / Boron / Rare Earth alloy, essentially comprising per hundred atoms:

• 78 iron atoms;
• 6 Boron atoms;
• 15.5 Neodymium atoms;
• 0.5 aluminum atom.

Is placed on the anvil (1), inside the ring (2), pieces of alloys of any shape. Argon is injected at (5) and by induction (11), the plate (10) is heated to 650 ° C for five minutes. When we reach this temperature, we hammer the wafer (10) for two minutes with the assembly (7, 8) developing a power of six Joules per stroke at a rate of one thousand eight hundred strokes per minute. We obtain a massive plate of twenty millimeters in diameter and five millimeters thick.

It should be noted that at this temperature, the fusible phase is a poorly identified mixture of metallic phases and even possibly of salts (rare earth fluorides and chlorides) and oxides. The main magnetic phase Nd₂Fe₁₄B remains present until at least 1050 ° C and during all mechanical or annealing treatments.

Then allowed to cool for three minutes until about 70 ° C.

The wafer thus obtained has an intrinsic coercive field of 300 kiloAmpers per meter (300kA / m), a density equal to 7.6, and a residual induction of 0.55 Tesla.

The material obtained has a quadratic crystal structure Nd₂Fe₁₄B.

This material is then subjected to an additional annealing operation for about thirty minutes at 800 ° C. carried out in the enclosure (4) for working.

A magnet is thus obtained having an intrinsic coercive field of 1000 kA / m, a residual induction of 0.85 Tesla, an internal energy of 1000 kiloJoules per cubic meter and a density of 7.6.

Example 2 :

Example 1 is repeated by applying a unidirectional and constant pressure during the annealing treatment to the sample (10). Strongly anisotropic magnets are thus obtained.

In these two examples 1 and 2, the hammering operation is undertaken only when the additional phases are sufficiently plastic to induce only the refinement of the crystals responsible for the magnetic properties.

Example 3 :

Three kilos of a solid NdFeB alloy, of atomic composition: Nd₁₅, ₅Fe₇₈B₆Al _0.5 , are produced. This massive alloy is poured into a soft iron container of diameter: sixty millimeters, of length: two hundred millimeters and six millimeters thick.

After cooling, the container is sealed tightly.

After heating the solid alloy in its container to 750 ° C., the assembly is spun into a die of suitable shape, for example in the form of a flat. A flat area of twenty-five by seven millimeters and several meters in length is then obtained with a wrought rate of 25 and an applied pressure of 13 kBar.

The magnet obtained is then cut to the desired length.

ces mesures étant effectuées suivant des directions perpendiculaires à la direction de filage.This magnet has the following characteristics:

these measurements being carried out in directions perpendicular to the spinning direction.

An annealing operation is then carried out under a controlled atmosphere of rare gas.

― H_Ci:

1000 kA/m

― H_CB:

480 kA/m

― Br:

0,85 Tesla,

― BH:

120 KJ/m³.

We then obtain the following characteristics:

- H _Ci :

1000 kA / m

- H _CB :

480 kA / m

- Br:

0.85 Tesla,

- BH:

120 KJ / m³.

In short, it has been found that the refinement of the crystals of the alloy significantly increases the coercivity of the assembly. In addition, as the industry most often requires anisotropic permanent magnets, anisotropy is obtained as already said by the application of strong unidirectional pressure on the treated material, the eutectic phase being in plastic phase.

It has been observed that the stress applied to the solid material increases the magnetic anisotropy in the direction of application. However, the amplitude of this phenomenon depends closely on the crystallographic orientation of the magnetic crystals before treatment: forging, spinning, etc.

In the case of any orientation of the magnetic crystals before treatment, slightly anisotropic magnets are obtained, with a difficult magnetization direction parallel to the spinning axis, and isotropic in the other two directions.

Furthermore, the direction of growth of the crystals is perpendicular to the direction of easy magnetization.

It is therefore necessary to control the growth direction of the magnetic crystals during the solidification phase of the solid alloy. Indeed, a unidirectional crystal growth makes it possible to distribute the directions of easy magnetization in a plane perpendicular to the direction of growth, but not in a defined direction.

Thus, by judiciously choosing the direction of the stress during the working in relation to the orientation of the crystals, it is then possible to obtain completely isotropic magnets.

The process according to the invention has numerous advantages over the process referred to in the preamble.

• la possibilité d'obtenir des aimants permanents à partir de la transformation de matières premières meilleur marché;
• une facilité et une rapidité de mise en oeuvre qui ne fait pas appel à du matériel sophistiqué;
• l'absence ou la quasi-absence de dangers pour l'environnement, notamment de risques d'explosion ou d'incendie, puisque l'on ne fait pas appel à des poudres.

We can cite:

• the possibility of obtaining permanent magnets from the processing of cheaper raw materials;
• ease and speed of implementation which does not require sophisticated equipment;
• the absence or near-absence of environmental hazards, in particular the risk of explosion or fire, since no powders are used.

In summary, this process is characterized by a consequent reduction in costs and the elimination of the dangers of manufacturing magnets of the Iron / Boron / Rare Earth type, which are more and more sought after.

In this way, this process can find numerous applications in the manufacture of permanent magnets, more particularly for the manufacture of electric motors, consumer motors, electronic equipment, loudspeakers.

Claims (6)

1. Process for preparing permanent magnets from a massive alloy containing at least one mixture of Iron (Fe), Boron (B) and Rare Earths (RE), including Yttrium, and for which there is a temperature range inside which said alloy is in two phases: one solid and fragile, and the other liquid, process in which:

· said massive alloy is heated in a controlled atmosphere at a sufficient temperature to attain the said temperature range;

· then this alloy is treated by hot welding with a rate of welding of at least ten, in order to refine the grains of the alloy into particles of some microns;

· then, the welded alloy undergoes a treatment of annealing and/or of tempering at a temperature of between 600 and 1000°C;

· and finally, the treated alloy is left to cool.

2. Process according to claim 1, characterized in that the massive alloy is a ternary Fe/B/RE alloy of which the quadratic magnetic phase RE₂Fe₁₄B is present during the whole of the treatments.

3. Process according to claim 2, characterized in that heating of the massive alloy to be welded is effected at a temperature of between 400 and 1050°C.

4. Process according to one of claims 1 to 3, characterized in that welding is effected by hammering, rolling, forging, extrusion, in a tight envelope made of an iron-based alloy.

5. Process according to claim 1, characterized in that the treatment of annealing and/or of tempering is effected after cooling.

6. Process according to one of claims 1 to 5, characterized in that during the annealing and/or tempering treatment, the alloy is subjected to an undirectional pressure.

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