WO2017089488A1

WO2017089488A1 - Sintered permanent magnet

Info

Publication number: WO2017089488A1
Application number: PCT/EP2016/078719
Authority: WO
Inventors: Florence Servant; Cyril RADO; Olivier TOSONI; Fernand Vial
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives; Vial Consulting
Priority date: 2015-11-25
Filing date: 2016-11-24
Publication date: 2017-06-01
Also published as: FR3044161B1; FR3044161A1

Abstract

The invention relates to a method for producing a magnet, comprising the following steps: a) producing a molten base alloy having a R2M14B magnetic phase T1 and a rare earth-rich phase R selected from the group consisting of Nd, Pr, Dy, Tb, Ho, Gd, and the mixtures thereof, M being a metal element selected from the group consisting of Fe, Co, Ni, and the mixtures thereof, b) producing a molten additive alloy having an additive phase selected from the group consisting of the R'-Co, R'-Co-M', and R'-Co-M'-B phases, and the mixtures thereof, R' denoting a heavy rare earth, M' Cu and/or Al and/or Ga and/or Ni and/or Cr and/or Fe, c) hydriding/dehydriding said molten base and additive alloys, d) grinding so as to obtain base (a) and additive (b) particles, e) preparing a particle mixture (c) comprising base (a) and additive (b) particles, all of the base (a) and additive (b) particles from the particle mixture having a median size D₅₀ of between 1 µm and 4 µm, a ratio D₉₀/D₁₀ of less than 10, preferably less than 5, D₁₀, D₅₀, and D₉₀ being defined by volume, and an additive particle content by mass of between 0.1% and 2%, f) shaping and crystallographic orientation, (g) totally dehydriding and sintering at a sintering temperature of greater than the melting point of said rare earth-rich phase and the melting point of the additive (b) particles, h) preferably annealing the sintered part, and i) magnetizing said sintered part so as to obtain a magnet.

Description

Sintered permanent magnet

Technical area

The present invention relates to a sintered permanent magnet and a method of manufacturing such a magnet. State of the art

Among the permanent magnets currently known, sintered magnets consisting of magnetic grains Nd2Fei4B, called "NdFeB", are those that combine the highest remanence Br, or energy product (BH) _ma x, and the best coercitivity Hc (resistance to a demagnetizing field). They are conventionally used at room temperature or below 80 ° C for engine type applications, for hard disks or for nuclear resonance imaging. In recent years, NdFeB magnets have also been used for traction motors of electric hybrid vehicles, or for generators for wind turbines. In these applications, the magnets are brought to temperatures of up to 200 ° C. The magnetic properties of the NdFeB magnets (Br and Hc) decrease significantly with temperature. In particular, the magnets must maintain a good coercivity to prevent the demagnetizing fields from the entire magnetic system irreversibly demagnetize them. For this purpose, it is common to substitute heavy rare earths (atomic number between 64 and 71) to the Nd, up to 10% by total mass of the magnet, to increase the coercivity and make possible the use of these magnets above 80 ° C.

However, the manufacturing cost of the magnet increases considerably because heavy rare earths are less abundant than light rare earths. In addition, the energy product of the magnet decreases with the increase of the heavy rare earth content.

Recently, several teams have demonstrated that the location of heavy rare earths exclusively in a peripheral region of the NdFeB grains of a magnet makes it possible to improve the temperature resistance by considerably limiting the drop in the energy product.

For this purpose, a paste or powder containing a heavy rare earth is applied to the surface of the sintered magnet. Then a heat treatment between 850 and 960 ° C allows to infiltrate this heavy rare earth in the magnet by the grain boundaries, liquid in this range of temperatures. The diffusion in the solid grains being slower, it is limited to a fine peripheral region of the grains.

This technique was introduced by Shin Etsu [EP-1830371A1] and is now used industrially by a few NdFeB magnet producers.

This technique, however, is not suitable for thick magnets since the diffusion of heavy rare earths is limited to a few millimeters from the surface of the magnet. Thus, a thick magnet has heterogeneous properties, which degrade from the surface to the center of the magnet.

Other teams have tested the "powder-to-powder mix" technique. According to this technique, especially described in US Pat. No. 6,491,765 B2, US Pat. No. 6,527,874 B2 or US Pat. No. 6,537,385 B2, heavy rare earths are introduced into the core of the green part, before sintering, for example, as described in JP A-62-206802, using an additive powder of (Dy, Tb) -M with M selected from V, Nb, Al and Ti.

However, the sintering temperature necessary to densify the magnet, which is too high, causes the diffusion of heavy rare earths inside the magnetic grains. The heavy rare earths are therefore not localized in a thin layer around the magnetic grains, but distributed almost homogeneously within the grains. In addition, it is necessary to introduce a large amount of heavy rare earths. The price of the magnet is increased and the energy product greatly decreased.

There is therefore a need for a permanent magnet of great thickness having an energy product as close as possible to that of magnets NdFeB, suitable for applications at temperatures between 80 and 200 ° C and incorporating a small amount of rare earths heavy localized in a very small thickness under the outer surface of the magnetic phase grains.

An object of the present invention is to respond, at least partially, to this need. Summary of the invention

The present invention provides a method of manufacturing a magnet comprising the following steps:

a) manufacture of a molten base alloy comprising a magnetic phase T1 of type R2M14B and a phase comprising more than 80% by mass of a rare earth R selected from the group consisting of Nd, Pr, Dy, Tb, Ho, Gd and mixtures thereof, called "rare earth rich phase R", said rare earth-rich phase having not more than 1% of iron (in metallic form ), said fabrication comprising a melting of a base filler, then a cooling so as to obtain a base mass of said molten base alloy, and optionally a heat treatment for dissolving the iron of said rare earth-rich phase, said basic load comprising, for a total of 100%, the percentages being in mass on the basis of the basic load:

between 28% and 33% of said rare earth R, with Nd> 20%,

- 100% complement: metal element M selected from the group consisting of Fe, Co, Ni and their mixtures, with Ni + Co <5%, the iron optionally being able to be replaced, at least partially, by a replacement element chosen from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn and their mixtures, the content of replacement element being between 0 and 3%,

between 0.93% and 1.2% by weight of Bore B,

less than 1% of constituents other than the rare earth R, the metallic element M, the replacement element and boron B, or "pollutants", with O <1000 ppm,

b) independently of step a), that is to say before, after or simultaneously with step a), manufacturing a molten additive alloy comprising an additive phase selected from the group consisting of the phases R'-Co, R'-Co-M ', R'-Co-M'-B and mixtures thereof, R' denoting a heavy rare earth, Co designating Cobalt, M 'denoting copper Cu and / or Aluminum Al and / or Gallium Ga and / or Nickel Ni and / or Chromium Cr and / or Fe Iron, and B denoting the boron, and optionally crushing the molten additive alloy,

c) hydriding, at least partially, then, preferably, partial deshydruration of said molten base alloy, and

hydriding, at least partially, then, preferably, at least partial dehydration of said molten additive alloy,

molten base and additive alloys which can be hydrided and optionally dehydrided separately or in admixture, d) grinding the molten base alloys and additives from step c), separately or in mixture, so as to obtain base particles (a) and additive (b), respectively, all the base particles (a) having a median size D ₅ o of between 1 μηι and 4 μηι, a ratio D ₉₀ / Di _{0 of} less than 10, preferably less than 5, D ₁₀ , D ₅ o and D ₉₀ being defined by volume the additive particles (b) having a size of less than 30 μηι,

e) preparing a particulate mixture (c) comprising base particles (a) and additive (b), with a mass content of additive particles of between 0.1% and 2% (in percentage by mass on the base of the mass of all the base particles (a) and additive (b),

f) shaping the particulate mixture, and crystallographic orientation by applying a magnetic field of the particles of said particulate mixture, so as to obtain a green part;

g) completely dehydriding the green part and sintering the dehydrided green part at a sintering temperature higher than the melting temperature of said rare earth-rich phase and at the melting temperature of the additive particles (b), h) preferably annealing of the sintered part;

i) magnetizing said sintered part so as to obtain a magnet.

Surprisingly, the inventors have found that the distribution of the heavy rare earth R ', for example Dy and / or Tb, in the magnet is particularly homogeneous in the peripheral region of the magnetic grains, whatever the location of the these grains in the volume of the magnet, even on magnets of great thickness.

Without being limited by this theory, the inventors explain this result from the fact that, in the particulate mixture (c), the additive and base particles are intimately mixed, which allows the heavy rare earth R 'to be better distributed between the center and the surface of the magnet. In addition, they explain this result because in the dehydrided green part, the additive alloy is liquid at the temperatures used during sintering, which allows the heavy rare earth R 'to be well distributed to the periphery of the magnetic grains and lower the sintering temperature. Finally, the small particle size of the particulate mixture (c), and in particular base particles, allows sintering at a low temperature, and thus limit the diffusion of heavy rare earths from the periphery to the heart of the magnetic grains. Moreover, the manufacturing process is simplified since it is no longer necessary to carry out heavy rare earth diffusion after sintering.

A method of manufacturing a magnet according to the invention may also comprise one, and preferably several, of the following optional features:

in step a), in the base charge, said rare earth content R is preferably between 30% and 33%;

in the base charge, the total content of light rare earth (accounted for in the rare earth content R) is preferably between 20% and 33% by weight; in the base charge, R is preferably NdPr, with Nd between 80 and 75% of the mass of NdPr;

in the base charge, the total content of heavy rare earth (accounted for in the rare earth content R) is preferably between 0% and 10%;

in the basic filler, preferably in mass percentages, Ni + Co <4%, preferably Ni + Co <3%, preferably Ni + Co <2%, and preferably Ni + Co> 1%, more preferably Co> 1%;

in the basic load, preferably in percentages by weight,

0.1% <Al <2%, preferentially 0.15% <Al <0.5%, and / or

0.04% <Cu <0.2%, preferentially 0.08% <Cu <0.12%, and / or

Ga <0.5%, preferentially Ga <0.25%, and / or

- Nb <2%, preferentially Nb <0.05%, and / or

Zr <2%, preferentially Zr <0.05%, and / or

Ti <2%, preferentially Ti <0.05%, and / or

Mo <2%, preferentially Mo <0.05%, and / or

V <2%, preferentially V <0.05%, and / or

Hf <2%, preferentially Hf <0.05%, and / or

Ta <2%, preferentially Ta <0.05%, and / or

- W <2%, preferentially W <0.05%, and / or

Sn <1%, preferably Sn <0.2%;

in the base charge, preferably, Pr <10% and / or Dy <10% and / or Tb <10%; - in the basic charge, preferably,

Cr <2000 ppm, preferentially Cr <500 ppm, and / or O <800 ppm, preferentially O <500 ppm, preferentially O <300 ppm, and / or

- F <4000 ppm, preferentially F <2500 ppm, and / or

C <400 ppm, preferentially C <300 ppm, and / or

- If <3000 ppm, preferably Si <1500 ppm, and / or

- Ca <500 ppm, preferentially Ca <100 ppm, and / or

Li <1000 ppm, preferentially Li <300 ppm, and / or

Zn <1%, preferably Zn <0.05%, and / or

Mn <5000 ppm, preferentially Mn <1000 ppm, and / or

N <200 ppm, preferentially N <100 ppm;

at the end of step a), the molten base alloy has a lamellar structure consisting of R2M14B T1 phase lamellae, representing between 90% and 96% of the mass of the molten base alloy, and interlamellar phase rich in said rare earth R;

at the end of step c), the hydrided and optionally dehydrated molten base alloy comprises, in percentage by volume, more than 90% of magnetic phase T1 of type at the end of step c), hydrided and optionally dehydrided molten base alloy has, in percentage by volume, more than 3.5% and / or less than 6% of phase at the end of step c), the molten base alloy hydride and optionally dehydrided comprises, in percentage by volume, less than 1% boride phase type RM ₄ B ₄ ; at the end of step c), the hydrided and optionally dehydrided molten base alloy comprises, in percentage by volume, less than 3%, preferably less than 2%, preferably less than 1.5% of other phases. that the phases T1, RH ₂ and boride, and in particular nitride phases and / or oxides and / or carbides;

the rare earth rich phase R has a melting point of less than 900 ° C., preferably less than 800 ° C., preferably less than 700 ° C., preferably less than 650 ° C .;

in step b), the total content of heavy rare earth R 'is preferably between 70% and 95%, preferably between 76% and 89%, preferably between 78% and 86% of the total mass of the alloy; molten additive; in step b), the boron content is between 0 and 1%, preferably between 0 and 0.5% of the total mass of the molten additive alloy;

in step b), the content of M 'is between 0 and 50% of the total mass Co + M'; preferably, the hydrided and optionally dehydrided molten additive alloy has a composition adapted to form a eutectic from the two R 'and Co phases and / or the two R' and Co-M 'phases and / or the two phases R 'and Co-M'-B;

preferably, the hydrided and optionally dehydrided molten additive alloy has a melting temperature of less than or equal to 800 ° C., preferably less than or equal to 750 ° C .;

- Preferably, the median volume size of the base particles (a) of the particulate mixture is between 1 and 4 μηι, preferably less than 3 μηι and / or greater than 1 μηι;

- Preferably, the median volume size of the additive particles (b) in the particulate mixture is less than or equal to 30 μηι, 20 μηι, 10 μηι, 5 μηι, preferably less than 4 μηι, preferably less than 3 μηι, preferably less than 2.5 μηι, preferably less than or equal to the median volume size of the base particles (a);

in step d), the quantity of additive particles is preferably between 0.2% and 2%, as a percentage by weight based on the particulate mixture (c);

in step e), the sintering temperature is preferably between 850 ° C. and 1080 ° C., preferably between 900 ° C. and 970 ° C .;

the sintered part has a thickness greater than 4 mm, greater than 10 mm, greater than 20 mm, greater than 50 mm.

In one embodiment, step b) comprises a melting of an additive charge adapted so that the molten additive alloy obtained has a composition, in percentages by weight based on the molten additive alloy. , as

R '> 78%, and / or

Co> 10%, and / or

B <0.5%, and / or

- constituents other than R ', Co, B and M' <0,2%,

M ': 100% complement. The invention also relates to a sintered magnet, in particular manufactured according to a process according to the invention, said magnet comprising magnetic grains, preferably whose magnetic phase is of the R2M14B type, more than 80% by number of said magnetic grains presenting each a center and a peripheral region, the peripheral region containing a heavy rare earth, preferably Dysprosium and / or Terbium, in an amount greater than that of said center of the magnetic grain.

The magnetic grains are preferably bound by a binder phase whose lithium content, as a percentage by weight based on the binder phase, is less than 300 ppm.

Preferably, a magnet according to the invention also comprises one, and preferably several, of the following optional features:

- said peripheral region extends around the grain considered;

preferably, the magnetic grains represent more than 90% or even more than 95% of the mass of the magnet;

the mass content of heavy rare earth (originating from the molten additive alloy (R ') and possibly from the rare earth R of the molten base alloy) is preferably greater than 0.1% and less than 10%;

preferably, R and / or R 'is Dysprosium and / or Terbium;

- The total mass content of heavy rare earth in the peripheral region of any magnetic grain is preferably greater than the mass content of heavy rare earth in the center of said magnetic grain, the difference between the mass content of rare earth heavy in the region. peripheral and the mass content of heavy rare earth in the center of said magnetic grain being preferably greater than 0, 1%, 1%, 3%, 5%, and / or preferably less than 10%, preferably less than 7%;

- along any line passing right through any magnetic grain, and in particular along a line passing through the center of the magnetic grain, the total molar content of the rare earths is substantially constant, the relative difference maximum (difference between the maximum value and the minimum value, divided by the minimum value) preferably being less than 1%;

- between any two points of any magnetic grain, the relative deviation of the total molar content of rare earths (difference between the maximum value and the minimum value, divided by the minimum value) is less than 1%; in the peripheral region, the boron mass content is preferably greater than 0.93%;

the mass composition of the magnet is such that:

Cr <2000 ppm, preferably Cr <500 ppm, and / or

O <4000 ppm, preferably O <2500 ppm, and / or

F <300 ppm, preferably F <200 ppm, and / or

- C <2000 ppm, preferably C <800 ppm, and / or

- If <3000 ppm, preferably Si <1500 ppm, and / or

- Ca <500 ppm, preferably Ca <100 ppm, and / or

Li <1000 ppm, preferably Li <300 ppm, and / or

Zn <1% by weight, preferably Zn <0.05%;

preferably, the thickness of the peripheral region, that is to say the depth of diffusion of the heavy rare earth R ', is greater than 2 nm, at 5 nm, at 0.03 μηι, at 0 , 05 μηι, and / or less than 2 μηι, preferably 0.6 μηι, preferably 0.5 μηι;

the magnet has a thickness greater than 4 mm, greater than 10 mm, greater than 20 mm, greater than 50 mm;

the heavy rare earth R 'is distributed homogeneously in the magnet, in particular between the center and the periphery of the magnet;

the local content of heavy rare earth (R '+ heavy rare earth from R) in any region of the magnet is substantially equal to the average rare earth content of the magnet, said region having a volume greater than 0.02 cm ³ , or greater than 0.1 cm ³ and less than 1 cm ³ , for example 0.025 cm ³ ;

the ratio (TR _L -TR _L *) / TR _L * is between + 5% and -5%,

TR _L denoting the local content of heavy rare earth in any region of 0.025 cm ³ of the magnet,

- TR _L * designating the average content of heavy rare earth of the magnet.

The invention finally relates to the use of a magnet according to the invention or manufactured according to a method according to the invention in an environment at a temperature above 80 ° C. In particular, the invention relates to an apparatus selected from a traction motor or propulsion for a motor vehicle and a wind turbine generator comprising a magnet according to the invention and / or manufactured according to a method according to the invention. Brief description of the figures

Other characteristics and advantages of the invention will become apparent on reading the detailed description which follows and on examining the drawing, in which

FIG. 1 represents the microstructure of an NdFeB ribbon, after melting and rapid cooling by the quenching technique, the light bands corresponding to an interlamellar phase, rich in rare earths and the gray bands corresponding to the magnetic phase T1. ;

FIG. 2 very schematically illustrates an example of evolution of the local contents T in light rare earth (TRi) and heavy (TR _L ) at the periphery of a magnetic grain of a magnet according to the invention, along a passing axis. in the center of the grain, at the transition between the heart of the magnetic grain C and its peripheral region P.

Definitions

- A "powder" is a dry set of particles unrelated to each other. The percentages relating to a powder are therefore implicitly on the basis of the dry matter.

The particle size of a set of particles is classically evaluated by a particle size distribution characterization measured on the ground powder and carried out with a laser granulometer. The laser granulometer may be, for example, a Partica LA-950 from the company HORIBA.

- The percentiles or "percentiles" 10 (Ai ₀ ), 50 (A ₅ o), 90 (A ₉₀ ) and 99.5 (Α ₉₉ , ₅ ), and more generally "n" A _n of a property A d a population, for example of a population of particles are the values of this property corresponding to the percentages of 10%, 50%, 90%, 99.5% and n%, respectively, on the cumulative distribution curve relating to this Property, the values for this property are ranked in ascending order. In particular, the percentiles D _n relate to particle sizes of a powder. Percentages are by volume.

For example, 10%, by volume, of the particles have a size less than and 90% of the particles by volume have a size greater than or equal to D10. Percentiles for particle size can be determined using a particle size distribution using a laser granulometer.

The percentile 50 is classically called the "median" percentile. The percentile D ₅ o is conventionally called "median size". - "ppm" refers to "parts per million" by mass.

The term "sintering" is used to denote the consolidation by heat treatment of a preform, or "part with green", possibly with a partial or total melting of some of its constituents (but not of all its constituents, so that the preform is not transformed into a liquid mass).

- After sintering, the term "magnetic grains" grains from the magnetic phase T1. A magnetic grain is substantially constituted by a phase having the stoichiometry R2M14B, infiltrated at the periphery of the grain by the heavy rare earth R '.

- The "peripheral region" of a magnetic grain is the region in which the heavy rare earth R 'diffused during the manufacture of the magnet.

- A product is classically described as "melted" when it results from a process involving a melting of raw materials in the form of a liquid mass or "bath of molten material", then a solidification of this molten material, for example after casting in a mold or on a rotating wheel.

A phase is said to be "rich" in an element when it comprises more than 80% by weight of said element.

The melting temperature of an alloy is the minimum temperature from which all the particles of said alloy begin to melt at a pressure of 1 bar.

- A "secondary vacuum" corresponds to a pressure of less than 10 ^-4 mbar, preferably less than 5.10 ^{~ 5} mbar.

- A "primary vacuum" corresponds to a pressure of less than 10 ^{~ 2} mbar, preferably less than 10 ^{~ 3} mbar.

Unless otherwise indicated, "including", "comprising", "having", "including" or variations thereof are a non-exclusive inclusion.

Unless otherwise indicated, all contents are by mass.

detailed description

A magnet according to the invention can be manufactured according to steps a) to i).

In step a), a molten base alloy is made. Conventional melting processes, comprising a melting of a base charge, and then a cooling, can be implemented. The base load is suitable for the manufacture of the molten base alloy. It is preferably made of pure metals or alloys.

The metals are preferably introduced in metallic form.

Preferably, the base charge comprises a super-oichiometric amount of rare earth R, relative to the desired magnetic phase T1. The rare earth superstoichiometry R advantageously makes it possible to obtain a molten base alloy comprising, in addition to the magnetic phase T1, a phase rich in rare earth R.

The presence of Ni and Co, in particular Co, improves the remanence of the magnet and its resistance to corrosion. Their contents must however be limited, especially to improve the machinability of the magnet.

Preferably, no oxide is voluntarily introduced into the feedstock to ensure an oxygen content of less than 1000 ppm in the feedstock.

The base charge is heated, preferably under partial pressure of neutral gas, at a temperature preferably between 1350 ° C and 1550 ° C, so as to obtain a bath of molten material.

The bath of molten material is then preferably cast on a cooled rotating wheel. The molten material is thus solidified by quenching. The cooling rate is preferably between 100 K / s and 1000 K / s. The melted ribbons obtained preferably have a thickness of 0.12 to 0.5 mm, preferably 0.15 to 0.35 mm.

It is preferable to carry out a rapid cooling by quenching on a wheel in order to obtain a fine microstructure (FIG. 1) and to avoid an annealing step in order to eliminate the iron resulting from a slow crystallization.

As shown in FIG. 1, the typical microstructure has T1-phase lamellae and an interlamellar I phase rich in rare earth R. The rare earth-rich phase is easily identifiable by those skilled in the art.

The spacing of the lamellae is preferably between 1, 5 and 4 μηι, preferably between 1, 5 and 3 μηι, on average on all the lamellae.

Step b) can be performed independently of step a), that is to say before, after or during step a). A charge is prepared, called "additive charge", the composition of which is suitable for the manufacture of the molten additive alloy.

The sources of raw materials are dosed to obtain a molten additive alloy having the desired composition. This dosage does not pose a problem. Preferably, substantially pure sources are used.

The additive charge is preferably constituted, for more than 95%, more than 98%, more than 99%, by weight of heavy rare earth R ', cobalt, and optionally of transition metal M', namely Al and / or Cu and / or Ga and / or Ni and / or Cr and / or Fe, and / or Bore. The heavy rare earth R 'is preferably selected from the group consisting of Dy, Tb, Ho, Gd and mixtures thereof.

The composition of the additive filler is adapted so that the molten additive alloy from step b) has, in percentage on the basis of the molten additive alloy,

a mass content of heavy rare earths R 'preferably greater than or equal to 70%, preferably greater than or equal to 75%, preferably greater than or equal to 78%, and / or less than or equal to 95%, preferably less than or equal to or equal to 89%, preferably less than or equal to 86%, and / or

a cobalt mass content preferably greater than or equal to 5%, preferably greater than or equal to 10%, and / or in step b), the content of M 'is between 0 and 50%, preferably between 0 and 50%; and 30% of the total mass Co + M '

a boron B mass content preferably less than or equal to 1.5%, preferably less than or equal to 1%,

In one embodiment, the boron mass content is between 0 and 1%

In one embodiment, the cobalt-M 'mass content is between 14 and 22%. In one embodiment, the cobalt mass content is between 9 and 15%.

The transition metal M 'and the impurities are the complement to 100%. The mass content of transition metal M 'may be less than or equal to 50%, or less than or equal to 30%, based on the sum of the mass contents of transition metal M' and Cobalt. The content of impurities, that is to say constituting other than R ', Co, B and M', is preferably less than 5%, preferably less than 1%.

The composition of the additive filler is preferably adapted so that the additive powder resulting from step b) has a eutectic composition from the two phases R 'and Co and / or the two phases R' and Co- M 'and / or two phases R' and Co-M'-B.

The transition metal M 'is preferably Al and / or Cu and / or Ni and / or Cr and / or Fe.

The metals are preferably introduced in metallic form.

The additive charge is heated, preferably under partial pressure of neutral gas, so as to obtain a bath of molten material.

The bath of molten material is then cast, preferably at a temperature between 600 ° C and 900 ° C, preferably in a steel or copper mold water-cooled.

The cooling rate can be any.

An alloy of molten additive is thus obtained.

In step c), the molten base and additive alloys derived from steps a) and b) undergo a hydriding treatment, and preferably dehydriding.

Such a hydriding-dehydriding treatment is known to those skilled in the art, and has already been proposed for producing high magnetic property powders, for example in the documents EP 0 173 588 and EP 0 538 320. In a manner Generally, the hydriding-dehydriding treatment is conventionally carried out in a sealed reactor.

If necessary, crushing may be performed beforehand, for example to obtain particles whose largest dimension is less than 5 mm, preferably less than 1 mm. All conventional crushing techniques can be implemented.

In step a), the rapid cooling in ribbon form advantageously allows a hydriding treatment without such a preliminary reduction.

In step b), such crushing may be particularly useful if the casting of the melt leads to a mass of additive in the form of blocks or particles larger than 20 mm. Preferably, the additive alloy is then crushed roughly to obtain particles whose largest dimension is less than 1 mm, dry, preferably under a controlled atmosphere, that is to say under an inert gas.

In step c), the hydriding / dehydriding operations of the molten base alloys and additive may be carried out (optionally after crushing, in particular of the molten additive alloy) simultaneously after a mixing of these alloys or separately on the molten base alloy obtained at the end of step a) and on the molten additive alloy obtained at the end of step b).

The hydriding, at least partial, is preferably total. Partial dehydriding is optional, but preferred.

Stage c) consists first of all in putting the alloy (s) to be treated in contact with hydrogen (hydriding), typically by injecting hydrogen so as to obtain a hydrogen pressure in the reactor. between 1 and 3 bars absolute at room temperature.

The hydriding leads to fragmentation in the form of a coarse powder, which facilitates subsequent grinding.

Without wishing to be bound by the theory, the R ₂ Mi ₄ B phase of the molten base alloy, in contact with hydrogen, will undergo swelling leading to embrittlement of the material. Can be represented hydriding step of this phase by the following schematic reaction: 2 R2M14B + xH ₂ -> 2 R2M14BHX with x between 0 and 4. If surstcechiométrie rare earth R, the residual rare earth R the rare earth rich phase R is hydrided according to the following reaction R + 1, 5.H ₂ -> RH ₃ .

The hydriding of the molten additive alloy advantageously limits the sensitivity to oxidation and facilitates the grinding into fine particles.

In a second step, the hydrogen of the reactor is removed and can be advantageously recycled. A vacuum heat treatment, for example between 450 and 550 ° C, allows to dehydride one or more phase (s) previously hydrolyzed (s). The hydrogen released during this last step can be captured and recycled.

The lamellar phase, resulting from the magnetic phase hydride T1 of the molten base alloy, is dehydrided, preferably substantially completely, by the following schematic reaction, which leads again to the magnetic phase T1: 2 R2M14BHX -> 2 R2M14B + xH ₂ . Dehydriding of the phase rich in said rare earth R hydride is partial, following the following schematic reaction: RH ₃ -> RH ₂ + 0.5.H ₂ .

Although it is preferred, the dehydriding of the hydrided molten additive alloy is optional.

In step d), the base alloy and the additive alloy from step c) are ground separately and then mixed or ground together. In the latter case, if step c) of hydriding / desyhuring was carried out separately for both types of powders, then a mixing operation must be performed before grinding.

Preferably, the mixing carried out before the grinding operation requires the addition of a lubricant, preferably temporary, that is to say which is removed during sintering. The nature and amount of lubricant may be identical to those described in step e).

Preferably, the grinding of the hydrided and optionally dehydrided molten base alloy (resulting from step c)), leads to a set of particles whose median volume size is less than 4 μηι, preferably less than 3 μηι. , preferably less than 2.5 μηι and / or greater than 1 μηι.

Preferably, the grinding of the molten additive alloy hydrided and optionally dehydrided (from step c)), leads to a set of additive particles whose median volume size is less than 30 μηι, preferably less than 20 μηι, preferably less than 10 μηι, preferably less than 5 μηι, preferably less than 4 μηι, preferably less than 3 μηι, preferably less than 2,5 μηι, preferably less than the median size of all the basic particles.

In step e), a particulate mixture is prepared, excluding lubricant, of base particles and additive particles and having a median size D ₅ o in volume between 1 μηι and 4 μηι, preferably less than 3 μηι, preferably less than 2 μηι, and / or preferably greater than 1 μηι. The homogeneity of the particulate mixture (c) is improved and, during sintering, the diffusion of the heavy rare earth R 'resulting from the additive powder within the magnetic grains is limited.

Mixing of molten base alloy particles and molten additive alloy particles can thus be carried out on particles resulting from steps a) and b), or after step c), especially when the hydriding / dehydriding treatment is carried out separately on the molten base and additive alloys obtained in steps a) and b), or at the end of step d), on the base particles (a) and the additive particles (b) .

Preferably, the mixing carried out before the grinding operation includes the addition of a lubricant, preferably temporary, that is to say which is removed during sintering.

The preparation of the green part may also require the addition of lubricant after grinding.

The person skilled in the art knows how to determine the nature and the contents of the lubricant. The lubricant content is preferably less than 1%, 0.5%, 0.3%, 0.2%, by mass percentage based on the mass of the particles to be mixed.

Preferably, the total content of lubricant used before grinding only or before and after grinding is between 200 and 2500 ppm, preferably between 500 and 1200 ppm.

A solid lubricant is preferably used, for example zinc stearate in a content preferably of between 0.035% and 0.075%, preferably between 0.035% and 0.05% by weight, based on the mass of the particles to be mixed, and / or a liquid lubricant, for example based on decanoate, in a content preferably between 0.05% and 0.15%; preferably between 0.03% and 0.08%, in percentage by weight based on the mass of the particles to be mixed.

The particulate mixture (c) consists of the base particles (a), additive particles (b) and any lubricant particles.

The set consisting of the particulate mixture and the lubricant in any liquid form is called "starting charge".

Preferably, the base particles (a) and the additive particles (b) together represent more than 98%, preferably more than 99%, preferably more than 99.5%, preferably more than 99.8% preferably more than 99.9% of the mass of the feedstock. Preferably, the base particles (a) and the additive particles (b) together represent 100% of the non-temporary constituents of the feedstock.

At the end of step e), a substantially homogeneous starting charge is obtained.

In step f), the starting charge is preferably conventionally poured into a mold, for example in a die of compression equipment mounted on a press uniaxial, subjected to a pressure and a static axial or transverse magnetic field greater than 1, 5 Tesia, preferably greater than 2 Tesia or for example in a pulsed field greater than 3T, preferably greater than 5T, preferably greater than 7 Tesia to orient and compact the particles and to obtain a green part with a density preferably between 50% and 70% of the theoretical density of the material of the mixture of the additive and base particles (completely dense).

In step g), the green part is dehydrided and then sintered.

Those skilled in the art are familiar with the conditions for sintering.

A dehydriding stage of the green part is conventionally carried out, preferably at a temperature of between 600 and 800 ° C., preferably under a secondary vacuum, so as to avoid the demixing of the NdFeB phase in the presence of H ₂ .

Preferably, the sintering is carried out in an environment containing substantially no oxygen, water or hydrogen, preferably under secondary vacuum, preferably at a temperature greater than or equal to 850 ° C. and / or less than or equal to at 960 ° C, and preferably for 3 to 15 hours.

Under the effect of sintering, the rare earth rich phase of the base particles (a) and the additive particles (b) turn into a binder liquid phase which includes the magnetic grains and allows a diffusion of the heavy rare earth R 'in the magnetic grains. Then, the sintered parts are subjected to rapid cooling, preferably greater than 20 ° C / min, preferably about 30 ° C / min, between the sintering temperature and 50 ° C.

The density of the sintered part is preferably greater than 95% of the theoretical density of the material which constitutes them.

In step h), the sintered part is preferably annealed, or "tempered".

Annealing advantageously increases the resistance to demagnetization of the magnet. Those skilled in the art are familiar with the conditions for annealing.

The annealing is preferably carried out according to the following cycle:

heating of 50 ° C. at a temperature of between 800 and 900 ° C. at 5 ° C./min;

- Bearing between 800 and 900 ° C for 2 hours;

cooling from the bearing temperature to 50 ° C at 20 ° C / min; heating from 50 ° C to (460-650 ° C) at 5 ° C / min;

- Bearing at the temperature (460-650 ° C) for 3 hours;

- cooling of the temperature (460-650 ° C) at room temperature to 30 ° C / min.

The annealed sintered part can then be machined and / or surface treated, for example polishing or applying a coating to prevent oxidation and corrosion.

In step i), the annealed sintered piece is subjected to a complementary magnetization, preferably by applying a magnetic field tap greater than 2.6 Tesla, preferably greater than 4 Tesla, preferably greater than 5 Tesla.

At the end of step h), a magnet according to the invention is obtained.

Remarkably, the distribution of heavy rare earths is particularly homogeneous in the magnet, regardless of the thickness of the magnet.

The magnetic grains may all have the same composition, or have different compositions. In a preferred embodiment, more than 90%, more than 95%, more than 98%, more than 99% or even substantially 100% by mass of the magnetic grains have the same composition, and preferably consist of Nd2Fei4B.

The average size of the magnetic grains is preferably greater than 1.0 m and / or less than 7.0 μηι.

The binder phase results from the melting of the additive particles and the rare earth rich phase of the base particles.

The binder phase binds the magnetic grains together leaving a porosity. Preferably, the porosity of the magnet is less than 3%, preferably less than 1%.

Preferably, the magnetic grains and the binder phase together represent more than 95%, more than 98%, more than 99%, more than 99.5%, preferably substantially 100% of the mass of the magnet.

The melting of the additive particles and the rare earth rich phase of the base particles leads to a diffusion of the heavy rare earth R 'into the magnetic grains. The heavy rare earth concentration R 'in the peripheral region of a magnetic grain is advantageously homogeneous, regardless of the magnetic grain considered. Finally, scattering of the heavy rare earth R 'in the magnetic grain is limited to a shallow depth from the outer surface of the magnetic grain.

The inventors have found that the total molar content of the rare earths is substantially constant regardless of the magnetic grain considered. In particular, along any line crossing a magnetic grain, the maximum relative difference (difference between the maximum value for this content and the minimum value for this content, divided by the said minimum value) is typically less than 1%.

Examples

The following examples are provided for illustrative purposes and do not limit the invention. A molten base alloy having the following composition, in percentages by weight, is prepared according to step a):

Nd (80% by weight) - Pr (20% by weight): 31.5%

Dy <0, 1%

B: 0.96%

Co: 0.5%

Fe: 100% complement

Ti: 0.05%

Al: 0.2%

Cu: 0.12%

Ga: 0.1%

Nb + Zr + Mo + V + Hf + Ta + W <0.05%

Sn <0.2%

Cr + O + N + C + Si + F + Li + Ca + Zn <1%, with

Cr <500 ppm,

O <300 ppm,

F <200 ppm,

C <300 ppm,

If <1500 ppm,

Ca <100 ppm,

Li <300 ppm, and

Zn <0.05%. More specifically, in step a), the melting is carried out under partial pressure of argon (400 mbar) in an alumina crucible at a maximum temperature of 1450 ° C. The bath of molten material is cast on a water-cooled copper-based wheel having a rotational speed which makes it possible to obtain crystallized ribbons with a thickness in the range of 0.15 to 0.40 mm. ("Strip Casting").

The ribbons are then cooled to room temperature on a turntable cooled by conduction and convection. The spacing between two lamellae Nd2Fi ₄ B measured on the section of a ribbon is 3 μηι on average.

In parallel, a molten additive alloy of TbCoAICuB having the following composition, in percentages by weight, is prepared according to step b):

Tb: 80%

B: 0.5%

Co: 13.5%

Al: 4%

Cu: 2%

More precisely, the melting is carried out in an induction furnace in an alumina crucible. The bath of molten material is cast at a temperature close to 800 ° C under an argon partial pressure of 400 mbar and cooled in a steel mold having a thickness of 15 mm.

The mass of additive obtained, in the form of a block, is then crushed mechanically so as to obtain particles whose largest dimension is less than 1 mm.

The molten additive alloy constituting these particles is then mixed with the ribbons resulting from step a) in a weight ratio of 0.5 / 99.5 for 30 min in a mixer, in a chamber under a controlled atmosphere.

The mixture thus obtained is then subjected to an environment of pure hydrogen at an absolute pressure of 1.2 bar in a sealed chamber of a furnace at room temperature in rotation, for a time sufficient to hydride the entire material.

Then, the chamber is placed under a primary vacuum to evacuate hydrogen, then filled with argon at 0.7 bar, and finally heated at a temperature of 550 ° C for 2 hours for partial deshydruration. The coarse powder resulting from this treatment is then cooled to room temperature under argon. The coarse powder thus obtained is introduced into a mixer in which 0.05% zinc stearate, as a percentage by weight based on the total mass of the coarse powder, has been introduced beforehand. The mixing is carried out for 1 h 30.

The mixture obtained is then introduced into a fluidized bed gas jet mill to obtain a feedstock. The gas used is helium. The grinding pressure, the nozzle diameter and the selector speed are adjusted to obtain a median particle size of 3 μηι, measured by a laser granulometer.

The feedstock is then introduced into various rubber molds and is subjected to a magnetic field tap of 7 Tesla to orient the particles, followed by cold isostatic compaction at 1500 bar to obtain ten cylinders 15 mm in diameter and 20 mm in diameter. mm long.

In addition, three blocks of 46 * 78 * 35 mm ³ are made from the same feedstock by transverse compaction (uniaxial pressing and magnetic field applied perpendicular to the direction of pressing). The application of a continuous magnetic field of 2.0 Tesla allows an orientation of the particles according to the thickness of the block (35 mm) during the compaction.

For each of the samples (cylinders resulting from isostatic compaction and blocks), secondary vacuum sintering is performed according to the following thermal profile:

- heating at 2 ° C / min to 350 ° C and then plateau for 1 hour;

- Heating at 2 ° C / min up to 550 ° C and then plateau for 1 hour;

- Heating at 2 ° C / min up to 750 ° C and then bearing for 2 hours;

- Heating at 1 ° C / min to 970 ° C and then plateau for 6 hours.

Cooling is then carried out under secondary vacuum at 5 ° C / min. When the temperature reaches 700 ° C, argon, at an absolute pressure of 3 bar, is introduced, until the temperature reaches 50 ° C, to obtain a cooling rate greater than 30 ° C / min.

The samples are then returned under secondary vacuum according to the following thermal profile:

heating at 5 ° C / min from 50 ° C to 820 ° C;

- Bearing at 820 ° C for 2 hours;

- cooling at 20 ° C / min from 820 ° C to 50 ° C;

- heating at 5 ° C / min from 50 ° C to 510 ° C - Bearing at the temperature 510 ° C for 3 hours;

- cooling at 30 ° C / min to room temperature.

The cylinders are machined using a centerless grinding machine and a planar grinding machine equipped with a diamond grinding wheel to remove the oxide layer and obtain parallel surfaces to press them to the poles of the hysteresis.

The cylinders are then magnetized in a Bitter coil under a magnetic field of 5 Tesla.

At the heart of each block, four cylinders 15 mm +/- 0.02 mm in diameter and 20 mm +/- 0.02 mm long were extracted by electroerosion and a parallelism of +/- 0.02 mm between the two. bases of the cylinders. These cylinders are called "extracted cylinders" to distinguish cylinders already preformed in cylindrical form before sintering, called "original cylinders".

These cylinders extracted from the blocks are then magnetized in a Bitter coil under a magnetic field of 5 Tesla.

A magnetometer delivering a 2.7 Tesla magnetic field is used to measure the intrinsic properties (remanence and coercive field) in the second quadrant of the hysteresis cycle (characteristics of the "hardness" of magnetic materials).

Magnetic characteristics of the magnets are measured at 20 ° C and summarized in the tables below:

Table 1: original cylinders (average values on the 10 original cylinders)

Table 2: Extracted cylinders (average values on the 12 cylinders extracted)

Remarkably, the properties are substantially the same despite the fact that the thickness of the blocks was greater than the diameter of the original rolls. Thick magnets manufactured according to a method according to the invention from the blocks therefore have the same magnetic properties as the fine magnets, made from the original cylinders.

In comparison, the magnetic properties are almost identical to those obtained with 2 mm thick magnets having the same composition but made by the diffusion technique after sintering Dysprosium at the grain boundaries.

In addition, a conventional process without heavy rare earth diffusion at grain boundaries, called "two-alloy technique", employing a mixture of a heavy rare earth-free T1-type magnetic base particle powder and a powder of type R 2M14B, R 'being a heavy rare earth, leads to a lower coercitivity, less than 16 kOe for magnets having the same dimensions as said blocks. The results obtained with the invention are therefore remarkable, this conventional method being currently considered to provide the best results of coercitivity without resorting to a heavy rare earth diffusion process.

In addition, the original cylinders and extracts are observed by means of a scanning electron microscope (SEM-FEG) equipped with an EDX probe to analyze the different phases.

The observation shows the presence of a very fine peripheral region enriched with Dysprosium on the contour of the magnetic grains. The thickness of this peripheral region is 0.2 μηι on average. Unlike magnets manufactured according to the prior art, Dysprosium is substantially homogeneously distributed around the magnetic grains, but is substantially absent in the core of the magnetic grains if the base powder does not contain heavy rare earth. The effect of the heavy rare earth is thus preserved.

The size of the magnetic grains is less than 5 μηι on average and the rich Nd phase surrounds the grains continuously.

As now clearly apparent, the invention provides a method for producing a high-thickness magnet that has a high energy product, with coercivity greater than that obtained by homogeneous addition of heavy rare earth in magnetic grains. In particular, according to the invention, the increase in coercivity may be about 7 kOe for an average addition of 0.5% by mass of dysprosium while this increase in coercivity is only about 1.2 kOe for the same addition of dysprosium in the case where this element is distributed homogeneously in the grains of the magnetic phase.

Of course, the invention is not limited to the embodiments described, provided by way of illustration and not limitation.

Claims

A method of manufacturing a magnet comprising the steps of:

a) manufacturing a molten base alloy comprising a magnetic phase T1 of type R2M14B and a phase comprising more than 80% by weight of a rare earth R selected from the group consisting of Nd, Pr, Dy, Tb, Ho, Gd and their mixtures, called "rare earth rich phase R", said rare earth-rich phase having not more than 1% of iron, said manufacture comprising a melting of a base charge, then a cooling so as to obtain a base mass of said molten base alloy,

said basic charge comprising, for a total of 100%, the percentages being in mass on the basis of the basic charge:

between 28% and 33% of said rare earth R, with Nd> 20%,

100% complement: metal element M selected from the group consisting of Fe, Co, Ni and their mixtures, with Ni + Co <5%, the iron optionally being able to be replaced, at least partially, by a replacement element chosen from group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn and mixtures thereof, the content of replacement element being between 0 and 3%,

between 0.93% and 1.2% by weight of Bore B,

b) independently of step a), manufacturing a molten additive alloy comprising an additive phase selected from the group consisting of the R'-Co, R'-Co-M 'and R'-Co-M phases; -B and mixtures thereof, R 'denoting a heavy rare earth, Co denoting cobalt, M' denoting copper Cu and / or aluminum Al and / or gallium Ga and / or nickel nickel and / or chromium Cr and / or Fe iron, and B denoting the boron, and optionally crushing the molten additive alloy,

hydriding, at least partially, then, preferably, at least partial dehydration of said molten additive alloy, molten base and additive alloys which can be hydrided and optionally dehydrided separately or in admixture,

d) grinding the molten base alloys and additives from step c), separately or in mixture, so as to obtain base particles (a) and additive (b), respectively, all the base particles (a) having a median size D ₅ o of between 1 μηι and 4 μηι, a ratio D ₉₀ / Di _{0 of} less than 10, preferably less than 5, D ₁₀ , D ₅ o and D ₉₀ being defined by volume the additive particles (b) having a size of less than 30 μηι,

e) preparing a particulate mixture (c) comprising base particles (a) and additive (b), and with a mass content of additive particles of between 0.1% and 2%,

i) magnetizing said sintered part so as to obtain a magnet.

Method according to the preceding claim, wherein, in the particulate mixture, the median volume size of the additive particles (b) is less than or equal to the median volume size of the base particles (a).

Process according to the preceding claim, in which, in the particulate mixture, the median size of the base particles (a) is less than 3 μηι.

A process according to any one of the preceding claims, wherein step b) comprises melting an additive feedstock adapted for the obtained molten additive alloy to have a composition, in percentages by weight based on the molten additive alloy, such as

- R '> 70%, and / or

- Co> 5%, and / or

- B≤ 1, 0%, and / or - constituents other than R ', Co, B and M'<1, 0%,

- M ': 100% complement.

5. Method according to the immediately preceding claim, wherein step b) comprises a melting of an additive load adapted so that the molten additive alloy obtained has a composition, in percentages by weight on the basis of the molten additive alloy, such as

- R '> 78%, and / or

- Co> 10%, and / or

- B <0.5%, and / or

- constituents other than R ', Co, B and M' <0,2%,

- M ': 100% complement.

6. The method as claimed in claim 1, in which step b) comprises a melting of an additive charge adapted so that the additive alloy has a composition adapted to form a eutectic from the two. R 'and Co phases and / or two R' and Co-M 'phases and / or two R' and Co-M'-B phases.

7. A method according to any one of the preceding claims, wherein in step b) the heavy rare earth R 'is selected from the group consisting of Dy, Tb, Ho, Gd and mixtures thereof.

The process according to any of the preceding claims, wherein the additive alloy has a melting temperature of less than or equal to 800 ° C.

9. The process as claimed in any one of the preceding claims, wherein, in the basic feed, in percentages by weight:

the rare earth content R is between 30% and 33%; and or

- 1% <Ni + Co <4%; and or

- Pr <10% and / or Dy <10% and / or Tb <10%, and / or

the heavy rare earth content is less than or equal to 10%.

10. A method according to any one of the preceding claims, wherein the sintered part has a thickness greater than 4 mm.

1. A sintered magnet made according to a process according to any one of the preceding claims, having a thickness greater than 4 mm and comprising

magnetic grains, preferably whose magnetic phase is of the R2M14B type, M being chosen from the group consisting of Fe, Co, Ni and their mixtures, more than 80% by number of said magnetic grains, each having a peripheral region containing a ground rare heavy, preferably Dysprosium and / or Terbium, in an amount greater than that at the center of the corresponding magnetic grain, and

a binder phase binding said magnetic grains, the lithium content in the binder phase, as a percentage by weight based on the binder phase, being less than 300 ppm.

12. Magnet according to the immediately preceding claim, wherein the ratio (TR _L - TR _L *) / TR _L * is between + 5% and -5%,

TRL denoting the local content of heavy rare earth in any region of 0.025 cm ³ of the magnet,

- TR _L * designating the average content of heavy rare earth of the magnet.

13. Magnet according to any one of the two immediately preceding claims, wherein the thickness of the peripheral region is less than 0.5 μηι.

14. Magnet according to any one of the three immediately preceding claims, having a thickness greater than 50 mm.

15. Magnet according to the immediately preceding claim, the difference between the mass content of said heavy rare earth in the peripheral region and the center of said magnetic grain being greater than 1%.

16. Use of a magnet according to any one of the five immediately preceding claims in an environment at a temperature above 80 ° C.

An apparatus selected from the group consisting of a traction or propulsion motor for a motor vehicle and a wind turbine generator, said apparatus comprising a magnet according to any one of claims 1 to 15.