Method for producing a highly textured magnet

Abstract

The invention relates to a method for producing a highly textured magnet comprising the following steps: a) providing: -a 1st powder comprising grains of a magnetic phase TR2Fe14B, - a 2nd powder comprising heavy rare earth elements and grains of a magnetic phase TR2Fe14B, b) subjecting the 2nd powder to a hydrogenation-disproportionation treatment, c) mixing the 1st powder with the 2nd powder obtained at the end of step b), d) subjecting the mixture obtained at the end of step c) to a magnetic field, e) subjecting the mixture obtained at the end of step d) to a compacting step so as to obtain a compacted part, f) subjecting the compacted part obtained at the end of step e) to a sintering step so as to obtain a magnet.

Description

DESCRIPTION TITLE: Manufacturing process of a highly textured magnet

[0001] The present invention relates to a method for manufacturing a highly textured magnet, preferably from recycled magnets. More specifically, the invention relates to permanent magnets of the TRFeB type.

[0002] In the context of the present invention, the term "highly textured magnet" means a magnet whose remanence measured parallel to the axis of easy magnetization (hereinafter abbreviated "B / ") is very largely greater than that measured perpendicular to the axis of easy magnetization (hereinafter abbreviated "Br 1 "), namely that the ratio (B / / - B^J / B / / is greater than 0.9.

[0003] In the context of the present invention, “TR” designates an element or a combination of two or more elements chosen from rare earths (in particular rare earths: La, Ce, Pr, Nd, Dy, Gd, Tb, Ho).

[0004] TR is predominantly neodymium (Nd). Therefore, the most common example of these permanent magnets is the NdFeB type magnet in which the crystalline phase NdzFe^B is the main phase, namely an alloy of neodymium, iron and boron to form a tetragonal crystal system.

[0005] Due to their excellent magnetic properties of good coercivity (i.e. resistance to demagnetization) and high remanence (i.e. high magnetic force), NdFeB permanent magnets are commonly used in various applications, including magnets in electric or hybrid vehicle motors, electrical appliances (e.g. household appliances or air conditioning), electronic devices (e.g. hard drives) and wind turbine generators.

[0006] However, given the current problems of environmental protection and the depletion of natural resources, particularly rare earths, in order to satisfy a perpetually growing demand and at least one that could exceed the supply projections for these permanent magnets for these different cutting-edge technologies, it is necessary to be able to efficiently recycle the magnets contained in these different devices, devices or motors, as soon as these devices are no longer in use, in order to limit the production of these magnets from only virgin materials extracted from deposits.

[0007] In the context of the present invention, the term “recycled magnets” means: - magnets which have been recovered for further use (for example during waste sorting operations) from devices, appliances, motors or other products containing magnets and which were no longer in use, and also - magnets corresponding to production waste, for example waste from the production of magnets (especially because they are defective) and which are thus revalued.

[0008] There are different ways of recycling NdFeB permanent magnets.

[0009] A l èreOne of these routes is so-called "direct" recycling, during which recovered magnets (for example from disused devices) are reused in block form, possibly after one or more surface treatments and machining. In other words, according to this route, magnets are produced directly from recycled magnets. There is no mandatory treatment leading to a modification of the physicochemical properties of the recycled magnets, but simply a light external treatment or cutting for the desired shaping. However, it is possible to carry out a rare earth diffusion treatment on these magnets from their surface, which modifies their physicochemical properties and therefore their magnetic properties, mainly the coercivity.

[0010] A 2 èmeThe recycling process is a so-called "indirect" or "long" process during which the chemical elements constituting the recovered magnets (for example from disused devices) are separated in the form of oxides, using pyrometallurgical or hydrometallurgical treatments. These oxides thus obtained are reintroduced upstream of the manufacture of new magnets as raw materials in the synthesis of metals, then of precursor alloys.

[0011] A 3 ème A well-known recycling route is the so-called "powder" route, in which recovered magnets (e.g. from disused devices) are reduced to powder form. These powders are then diluted in polymers to make magnets. bonded or they are densified by heat treatments to obtain sintered magnets.

[0012] The so-called "powder" method has the advantage of being able to readjust the compositions of new magnets made from recycled magnets by mixing different powders and thus having a certain freedom as to the final form of these new magnets. In addition, in the case of sintered magnets, this recycling method is based on powder metallurgy magnet production processes that are already implemented and perfectly mastered.

[0013] However, this so-called "powder" recycling route (just like the so-called "direct" route) does not allow optimal use of heavy rare earths (Dy or Tb) which are present in the magnetic phase of certain permanent magnets, mainly as a substitute for neodymium (and to a lesser extent for praseodymium), in order to improve their coercivity and their temperature resistance. For example, the Dy content can be up to 10% by mass on average for operating temperatures of 150 to 180°C. Indeed, heavy rare earths make it possible to increase the magnetocrystalline anisotropy of the magnetic phase and therefore the resistance to demagnetization.

[0014] In this regard, it should be remembered that there are two categories of rare earths: - heavy rare earths including: europium (Eu), gadolimium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and yttrium (Y), - light rare earths including: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd) and samarium (Sm).

[0015] It is known from the manufacturing processes of permanent magnets comprising heavy rare earths that the substitution of neodymium (and to a lesser extent praseodymium) by heavy rare earths in the entire TF FewB magnetic phase is accompanied by a reduction in remanence which can be limited by using a mixture of a powder poor in heavy rare earths and a powder rich in heavy rare earths or by diffusing the heavy rare earths from the surface of the sintered magnets and along the grain boundaries. In this way, the heavy rare earths are preferentially located at the periphery of the magnetic grains, i.e. in the most critical regions for demagnetization, so as to obtain a structure known as "core-shell". The peripheral region of a magnetic grain is therefore the region into which the heavy rare earths have diffused during the manufacture of the magnet.

[0016] In other words, knowledge on the manufacturing processes of magnets comprising heavy rare earths underlines the importance of locating the heavy rare earths in a peripheral region of the magnetic grains, in order to improve the gains in magnetic performance of the magnets thus obtained while also having the advantage of limiting the quantity of heavy rare earths necessary for their manufacture.

[0017] However, during the so-called "powder" recycling route, it is possible to co-sinter several powders, at least one of which may have been obtained from recycled magnets that contained heavy rare earths. But in this case, a significant portion of these heavy rare earths will not be able to diffuse outside the grains and will therefore remain in the magnetic phase of said powder rather than feeding a core-shell type effect at the periphery of the grains initially devoid of heavy rare earths. This is why, in order to obtain magnetic performances comparable to those obtained with manufacturing processes for permanent magnets comprising heavy rare earths from virgin raw materials, during this so-called "powder" recycling route, it is known to add heavy rare earths from primary sources (in other words, heavy rare earth deposits).Or, if one wishes to recycle magnets rich in heavy rare earths (typically a content greater than 2.5% by mass), it is also known to mix a l. ère powder from these recycled magnets rich in heavy rare earths with a 2 ème powder free of heavy rare earths or at least with a low content of heavy rare earths (typically a content of less than 1% by mass). The co-sintering of these two powders only allows a small part of the heavy rare earths contained in the powder to be diffused. ère powder. With such a mixture of powders, the increase in coercivity is of the order of 150 kA / m per percentage of heavy rare earth added.

[0018] The inventors of the present invention sought to optimize the use of heavy rare earths present in recycled permanent magnets during the so-called “powder” recycling route.

[0019] The inventors thus sought to improve the so-called "powder" recycling route by proposing a new process for manufacturing a highly textured magnet, preferably from recycled permanent magnets, of the TRFeB type and containing heavy rare earths, which is based on this recycling route but which also presents other original technical characteristics which are detailed below, allowing: - obtaining a magnet with high magnetic performance (i.e. good coercivity while maintaining high remanence), without necessarily adding heavy rare earths from virgin raw materials (e.g. from deposits); which minimizes environmental impacts and is more energy efficient; - optimal recovery of permanent magnets containing high levels of heavy rare earths (for example between 2.5 and 10% by mass) from end-of-life devices or appliances (in other words, magnets to be recycled); - the possibility of using recycled magnet powders at a rate of 10% to 50% by mass compared to the total mass of powders used for the manufacture of said magnet; - the use of current production lines for the manufacture of magnets.

[0020] The manufacturing process of a highly textured magnet, preferably from recycled magnets, is based on the so-called "powder" recycling route and makes it possible to obtain a highly textured magnet in which the heavy rare earths are located at the periphery of the grains of the magnetic phase, in order to obtain good coercivity while maintaining high remanence, and even optimizing its magnetic performance compared to that of recycled magnets containing said heavy rare earths.

[0021] The subject of the invention is a method for manufacturing a highly textured magnet (preferably from recycled magnets), which comprises at least the following steps: a) we have: - of a l ère powder comprising grains of a magnetic phase TRzFe^B, metal-based compounds and a rare earth-rich grain boundary phase, and optionally grains of a non-magnetic phase TRFe4B4, said l èrepowder being free of heavy rare earth or the mass content of heavy rare earth expressed relative to the mass of the l ère powder being less than 1%, - of a 2 ème powder (preferably obtained from recycled magnets), comprising heavy rare earths, the mass content of heavy rare earths, expressed relative to the mass of said 2 ème powder, being between 1% and 10%, preferably between 2% and 5%, said 2 ème powder containing grains of a magnetic phase TRzFe^B, b) we subject the 2 ème powder to a hydrogenation-disproportionation treatment (also known as “hydrogenation disproportionation”) so as to decompose the magnetic phase TRzFe^B into a mixture containing rare earth hydrides of chemical formula TRH Xin which x is the atomic ratio of H / TR and is for example between 2 and 3, iron, iron boride (FezB) and a so-called beta phase which comprises TR, iron and boron, said hydrogenation-disproportionation treatment being carried out: - either at a temperature between 850°C and 950°C and at a hydrogen pressure greater than or equal to 0.3 bar and less than 0.5 bar, - either at a temperature between 925°C and 1025°C and at a hydrogen pressure greater than or equal to 0.5 bar and less than 1 bar, - either at a temperature between 950°C and 1100°C and at a hydrogen pressure greater than or equal to 1 bar and less than or equal to 10 bar, c) the l is mixed ère powder with the 2 ème powder obtained at the end of step b) in a mass ratio which depends on the heavy rare earth content of the 2 èmepowder so as to obtain a mixture, d) the mixture obtained at the end of step c) is subjected to a magnetic field, e) the mixture obtained at the end of step d) is subjected to a compacting step so as to obtain a compacted part, f) the compacted part obtained at the end of step e) is subjected to a sintering step so as to obtain a magnet.

[0022] Because the 2 ème powder contains heavy rare earths, during step b) of the manufacturing process according to the invention, heavy rare earth hydrides are formed.

[0023] It could be expected that the hydrogen treatment of step b) would induce a loss of texture of the magnet manufactured with the manufacturing process according to the invention. Indeed, it is known that hydrogenation-disproportionation can change the texture of the material subjected to such treatment. In addition, the microstructure of a powder decomposed by hydrogenation-disproportionation also influences the texture of the magnetic phase grains in the magnet after recombination of the latter during sintering. For example, a fine lamellar microstructure recombines in a textured manner, whereas a coarse spherical microstructure recombines in a poorly textured manner in the form of finer grains. In other words, a poor level of texture of a powder decomposed by hydrogenation-disproportionation leads to the formation of recombined powder grains that are isotropic; resulting in a magnet with a significant loss in remanence.

[0024] However, against all expectations, the inventors discovered that the selection of the temperature and pressure intervals in step b) as detailed above makes it possible to obtain a beta phase whose mass content, expressed relative to the total mass of the 2 ème powder, can advantageously be between 5% and 50%. The presence of this beta phase, as well as its magnetically anisotropic structure within said microstructured mixture of step b) makes it possible to counter this loss of texture potentially expected due to the implementation of this treatment under hydrogen and thus to obtain with the manufacturing method according to the invention a highly textured magnet.

[0025] Furthermore, the beta phase contains a proportion of more heavy rare earths than the hydride phases, with a partition coefficient that can be greater than 5. The heavy rare earths present in this beta phase are linked to a complex crystallographic structure and will therefore be a priori less available to diffuse to the periphery of the magnetic grains of the l ère powder. However, the selection of temperature and pressure ranges in step b) as detailed above makes it possible to obtain a beta phase whose mass content is a compromise allowing a high degree of texture of the magnet without trapping too many heavy rare earths.

[0026] Finally, the selection of the temperature and pressure intervals in step b) as detailed above makes it possible to obtain at the end of this step b) grains of micron size, advantageously between 1 μm and 20 μm. This micron size has the advantage, during the sintering step, of avoiding too rapid recombination of these grains and thus allow the heavy rare earths to diffuse correctly around the periphery of said grains.

[0027] The mass ratio of step c) can be determined as follows: for example if the 2 ème powder contains X% of heavy rare earths and that we wish to achieve in the final magnet an overall content of Y%, the mass ratio of the mass m2 of the 2 ème powder on the mass ml of the l ère powder is m2 / ml = Y / (XY).

[0028] During step f) of sintering the mixture of the l ère powder and 2 èmepowder, a liquid phase is formed (by eutectic reaction) between the different metal-based compounds, the TR-rich phase, the magnetic phase TRzFe^B and the optional non-magnetic phase TRFe4B4 of the l ère powder. This liquid phase has a high mass content of rare earths, of the order of 90% by mass at 700°C, 80% at 900°C and 70% at 1000°C. During the temperature increase under vacuum, the rare earth hydrides formed in step b), in particular the heavy rare earth hydrides, are transformed into metallic heavy rare earths which dissolve in said liquid phase so as to enrich it. These heavy rare earths, now in the form of metallic heavy rare earths, are then available to create a so-called "core-shell" structure as mentioned above around the grains of the liquid phase. ère powder during densification of the l ère powder and 2 ème powder that is produced during sintering.

[0029] Thus, thanks to step b) of the manufacturing process according to the invention, the heavy rare earths present in the 2 ème powder (preferably a 2 ème powder that was obtained from recycled magnets) are extracted from the magnetic phase by being transformed into hydrides. This increases the chemical activity of the heavy rare earths. Indeed, during the rise in temperature in vacuum during sintering, the transformation of these hydrides into metallic heavy rare earths makes them available in the liquid phase that also forms during sintering.

[0030] Step b) of the manufacturing process according to the invention thus makes it possible to avoid a step of diffusion of these heavy rare earths within the grains of the magnetic phase TRzFe^B of the 2 ème powder; which is a slow process and would create a concentration gradient within the grains rich in heavy rare earths of the 2 èmepowder. This would ultimately allow only a small fraction of the quantity of said heavy rare earths to be recovered. In addition, in this case of diffusion, the rare earth content heavy on the periphery of the grains of the l ère powder would be lower than the heavy rare earth content in the grains of the 2 ème powder.

[0031] On the contrary, with the manufacturing process according to the invention, the extraction of heavy rare earths from the magnetic phase of the 2 ème powder increases their chemical potential. At the end of step f), the heavy rare earths are located on the periphery of the grains of the magnetic phase TRzFe^B of the l ère powder at higher contents than the heavy rare earth content in the grains of the 2 èmepowder (preferably obtained from recycled magnets), so that the coercivity of the magnet thus obtained is increased very significantly, even though the said magnet contains small quantities of heavy rare earths.

[0032] In the context of the present invention, an additional advantage is to co-sinter the l ère powder and the 2 ème powder at lower temperature, because the heavy rare earths have already been extracted from the grains of the magnetic phase TRzFe^B of the 2 ème powder. It thus makes it possible to locate the heavy rare earths at the extreme periphery of the grains of the l ère powder and therefore to obtain a very marked "core-shell" structure which makes it possible to approach the gains in coercivity of those obtained by diffusion at the grain boundaries. But the invention makes it possible to overcome the limits in terms of magnet thickness compared to the diffusion process by the surface of the magnet.

[0033] This gain in terms of dilution of the manufacturing process according to the invention is particularly interesting, because it makes it possible to envisage the recycling, in a short route, of quantities of recycled magnets rich in heavy rare earths, which will remain limited and to make it possible to manufacture significant quantities of magnetically efficient magnets with a minimum input of critical raw materials.

[0034] The various technical characteristics of the manufacturing process according to the invention are described below in more detail.

[0035] In the context of the present invention, the term "TR-rich grain boundary phase" means a metallic phase containing more than 70% by mass of rare earth combined with metals, for example metals chosen from iron, copper and aluminum.

[0036] Metal-based compounds of the l èrepowder may include metals selected from iron, copper, aluminum, gallium, titanium and zirconium.

[0037] The l ère powder may comprise in mass percentages expressed relative to the mass of said l ère powder: - between 90% and 99%, preferably between 95% and 97%, of grains of the magnetic phase of type TRzFe^B, - between 0.5% and 3%, preferably between 0.5% and 2%, of metal-based compounds, - between 1% and 10%, preferably between 2% and 5%, of the rare earth-rich grain boundary phase, - optionally between 0.1% and 5%, preferably between 0.1% and 1%, of grains of the non-magnetic phase of type TRFe4B4.

[0038] The l ère powder may comprise, in mass percentages expressed relative to the mass of said l ère powder: - between 27% and 35% rare earth, - between 0.9% and 1.2% boron, - 100% complement of at least one metallic element M chosen from the group consisting of Fe, Co, Ni, taken alone or as a mixture thereof, the sum of the mass percentages of Ni and Co being less than or equal to 5%, and optionally Fe being partially replaced by at least one replacement element chosen from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn, taken alone or as a mixture thereof, the content of the replacement element(s) being less than or equal to 3%.

[0039] The l ère powder preferably has a particle size between 3 pm and 7 pm.

[0040] The l ère powder may have been obtained from: - virgin raw materials (e.g. pure metals and / or alloys) which are completely free of heavy rare earths or whose mass content is less than 1%, or - recycled magnets which are completely free of heavy rare earths or whose mass content is less than 1%, or - a mixture of virgin raw materials and recycled magnets which are completely free of heavy rare earths or whose mass content is less than 1%.

[0041] When the l ère powder was obtained wholly or partly from virgin raw materials, the latter being chosen from pure metals and / or alloys. Preferably, these are pure metals.

[0042] When the l ère powder was obtained wholly or partly from virgin materials, the latter may have been subjected to the following stages: - a wheel casting stage, followed - a decrepitation step under hydrogen and / or gas jet grinding (also known under the English name: Jet Mill type grinding).

[0043] First, the mixture of virgin raw materials (in other words the "base charge") is heated, preferably under partial pressure of neutral gas or under vacuum, to a temperature advantageously between 1350°C and 1550°C, so as to obtain a bath of molten material.

[0044] The molten material bath is then poured onto a cooled rotating wheel. The molten material is then solidified by quenching. The cooling rate can be between 500 K / s and 5000 K / s. The resulting molten ribbons can have a thickness of between 0.1 and 0.5 mm, preferably between 0.15 and 0.35 mm.

[0045] The decrepitation step under hydrogen makes it possible to obtain a l ère powder with a particle size between 50 pm and a few millimeters.

[0046] Decrepitation under hydrogen can be carried out at a temperature between 10°C and 500°C, preferably between 20°C and 150°C, and at a hydrogen pressure between 0.01 MPa and 5 MPa, preferably between 0.08 MPa and 0.25 MPa.

[0047] The duration of the hydrogen decrepitation step can be between 1 hour and 5 hours.

[0048] The gas jet grinding step allows to obtain a l ère powder whose median size is between 2 pm and 10 pm, preferably between 3 pm and 6 pm, with a particle size fineness whose ratio of 9 ème decile at 1 erdecile, or in other words "D90 / D10", is less than 10, preferably less than 5. Commercial equipment such as mills marketed by the company Hosokawa-Alpine under the trade names AFG100, AFG200 and AFG400 can be used for this gas jet milling step. They comprise a sealed chamber in which An inert gas under a pressure of between 2 and 8 bars is introduced through three converging nozzles, and the powder to be ground through a hopper to control the feed rate. The gas flow carries the powder in its wake and releases it by passing through a vortex generated by a system called a "cyclone". In order to improve the particle size fineness, this equipment can be equipped with an inertial selector which prevents the largest particles from leaving the grinding chamber.

[0049] When the l èrepowder was obtained totally or partly from recycled magnets, the latter may have been subjected to a step of decrepitation under hydrogen and / or gas jet grinding.

[0050] The technical characteristics of the hydrogen decrepitation and gas jet grinding stage can be those which have been described for obtaining the l ère powder from virgin raw materials.

[0051] The 2 ème powder preferably has a particle size comparable to that of the first powder so as to facilitate mixing of the l ère powder with the 2 ème powder in step c) of the manufacturing process according to the invention. The 2 ème powder preferably has a particle size between 3 pm and 7 pm.

[0052] The 2 ème powder may comprise, in mass percentages expressed relative to the mass of said 2 ème powder: - between 27% and 35% rare earth, including between 1% and 10%, preferably between 2% and 5%, heavy rare earth, - between 0.9% and 1.2% boron, - 100% complement of at least one metallic element M chosen from the group consisting of Fe, Co, Ni, taken alone or as a mixture thereof, the sum of the mass percentages of Ni and Co being less than or equal to 5% and, optionally Fe being partially replaced by at least one replacement element chosen from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn, taken alone or as a mixture thereof, the content of the replacement element(s) being less than or equal to 3%.

[0053] This means that the 2 ème powder comprises, in mass percentages expressed relative to the mass of said 2 ème powder, between 1% and 10%, preferably between 2% and 5%, of heavy rare earth.

[0054] The 2 ème powder may have been obtained from: - virgin raw materials (e.g. pure metals and / or alloys) with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%, or - recycled magnets with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%, or - a mixture of virgin raw materials and recycled magnets with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%.

[0055] As explained above, the method for manufacturing a magnet according to the invention preferably uses recycled magnets.

[0056] Therefore, in a preferred embodiment of the invention, the 2 èmepowder was obtained only from recycled magnets. These recycled magnets include heavy rare earths. The mass percentage of these heavy rare earths, expressed relative to the mass of said 2 ème powder, is between 1% and 10%, preferably between 2% and 5%.

[0057] Advantageously, the 2 ème powder was obtained from recycled magnets which were subjected to the following treatment: - a decrepitation step under hydrogen, optionally followed - gas jet grinding.

[0058] The decrepitation step under hydrogen makes it possible to obtain a 2 ème coarse powder with a particle size between 50 pm and a few millimeters.

[0059] The technical characteristics of the hydrogen decrepitation and gas jet grinding stage to obtain the 2 èmepowder may be those which have been described for obtaining the l ère powder from virgin raw materials or recycled magnets.

[0060] When the 2 ème powder was obtained totally or partly from recycled magnets, the latter may have been subjected to a decrepitation step under hydrogen and / or gas jet grinding. The technical characteristics of the step of hydrogen decrepitation and gas jet grinding may be those described for obtaining the l ère powder from virgin raw materials.

[0061] When the 2 ème powder was obtained wholly or partly from virgin materials, the latter may have been subjected to the following stages: - a wheel casting stage, followed - a decrepitation step under hydrogen and / or gas jet grinding.

[0062] The technical characteristics of the wheel casting, hydrogen decrepitation and gas jet grinding stages can be those described above for obtaining the l ère powder from virgin raw materials.

[0063] The hydrogenation-disproportionation treatment can be carried out under vacuum, namely by heating under vacuum the 2 ème powder at a temperature as described above before the introduction of hydrogen. In another embodiment of the invention, the 2 ème powder is heated to a temperature as described above after the introduction of hydrogen.

[0064] The 2 ème powder can be heated to the desired temperature as described above with a heating rate between 1°C / minute and 30°C / minute.

[0065] In step b), the temperatures as described above are appropriate so as not to cause agglomeration of particles between them.

[0066] In step b), the pressures as described above are appropriate so as not to cause a loss of texture of the magnet.

[0067] For example, at a pressure of 0.8 bar, the treatment temperature of step b) may be between 925°C and 1025°C.

[0068] The duration of the hydrogenation-disproportionation treatment can be between 10 minutes and 3 hours. This duration depends on the hydrogenation-disproportionation treatment temperature: it is shorter at high temperatures. Indeed, too long a treatment duration at high temperatures would cause macroscopic heterogeneities within the microstructure of the magnet obtained with the manufacturing process.

[0069] At the end of the hydrogenation-disproportionation treatment, the 2 ème powder can be subjected to cooling under hydrogen, in order to prevent the recombination of the elements to reform the magnetic phase TRzFe^B, and therefore preserve the mixture of rare earth hydrides of chemical formula TRH X , iron, iron boride (FezB) and beta phase, up to room temperature (i.e. approximately 20°C).

[0070] Optionally, before carrying out step c) of the manufacturing method according to the invention, the 2 ème powder is fractured to improve the contact between the rare earth hydrides and the liquid phase during step f) so that the heavy metallic rare earths dissolve better in the liquid phase. This step may be necessary if the particles of the 2 èmepowder have reagglomerated during the hydrogenation-disproportionation treatment, but also if their microstructure is not the most suitable, namely in particular if the hydrides are located within the grains rather than on the periphery. The 2 ème powder may have been fractured by at least one grinding technique selected from gas jet grinding, planetary grinding, attrition grinding and cryogenic grinding.

[0071] Step c) of mixing the l ère powder and 2 ème powder obtained at the end of step b) is advantageously carried out for at least 30 minutes, preferably more than one hour, so as to obtain a homogeneous mixture.

[0072] The mixture obtained at the end of step c) can be poured into a mold (having the negative shape of the magnet to be manufactured) to then carry out step e) of compacting.

[0073] During step d), the mixture obtained at the end of step c) is subjected to a magnetic field to orient the grains of the mixture and ultimately obtain an anisotropic magnet which has a high remanence. Preferably, the magnetic field is greater than 1 Tesla, more preferably greater than 2 Tesla. Preferably, the magnetic field does not exceed 8 Tesla. This application of the magnetic field can be carried out when the mixture obtained at the end of step c) has been poured into the mold.

[0074] The compaction step e) can be carried out using transverse, axial, cold isostatic compaction or isostatic pressing with rubber (also known as “RIP”, the English acronym for “Rubber Isostatic Pressing"), in order to obtain a compacted piece called a "green piece". For example, the mixture is compacted by applying a uniaxial pressure of between 50 MPa and 300 MPa.

[0075] The density of the compacted part obtained at the end of step e) is advantageously between 50% and 70% of the theoretical density of said mixture obtained at the end of step c).

[0076] In an advantageous embodiment of the invention, at the end of step e) and before carrying out step f) of sintering, the compacted part obtained at the end of step e) is dehydridated. This operation consists of eliminating almost all of the hydrogen contained in the compacted part. The hydrogen is essentially present in the form of hydrides TRHx with x close to 2. This involves reducing the overall hydrogen content in the compacted part, which is for example of the order of 2000 ppm (0.2% by mass) to a value for example less than 100 ppm (0.01% by mass), preferably less than 50 ppm (0.005% by mass). This operation makes it possible to obtain better magnetic properties after sintering.

[0077] This optional step of dehydration of the compacted part can be carried out by subjecting said compacted part to a temperature between 600°C and 800°C, preferably under secondary vacuum so as to avoid demixing of the TRFeB phase, and this in the presence of hydrogen. A secondary vacuum corresponds to a pressure lower than 10 -4 mbar, preferably less than 5.10 -5 mbar.

[0078] Then, step f) of sintering is carried out so as to obtain a magnet. This involves the consolidation by heat treatment of the compacted part, with possibly a partial or total melting of some of its constituents (but not all of its constituents, so that the compacted part is not transformed into a liquid mass).

[0079] The sintering step f) is advantageously carried out in an environment containing substantially no oxygen, water or hydrogen, preferably under secondary vacuum and at a temperature of between 850°C and 1050°C and for a duration of between 3 hours and 24 hours so as to obtain said magnet.

[0080] At the end of step f) of the manufacturing process according to the invention, a magnet is obtained whose density is advantageously greater than 7.4 g. cm -3 .

[0081] In an advantageous embodiment of the invention, at the end of step f), the magnet thus obtained is subjected to cooling. Preferably, this is rapid cooling, namely greater than 20°C / min, more preferably approximately 30°C / min, from the sintering temperature to room temperature or, where appropriate, to the temperature at the start of the optional annealing step described below.

[0082] The magnet obtained at the end of step f), where appropriate at the end of cooling if this is carried out, can then be subjected to an annealing step.

[0083] Indeed, annealing increases the magnet's resistance to demagnetization. Those skilled in the art are familiar with the conditions for carrying out the annealing step.

[0084] For example, if the magnet has been subjected to rapid cooling to a temperature of 50°C, the annealing step may include the following thermal profile: - heating from 50°C to 820°C at 5°C / min; - a stage at 820°C for 2 hours; - cooling from 820°C to 50°C at 20°C / min; - heating from 50°C to a temperature between 460°C and 650°C at 5°C / min; - a stage at a temperature between 460°C and 650°C for 2 hours; - cooling from a temperature between 460°C and 650°C to 50°C at 30°C / min.

[0085] In an advantageous embodiment of the invention, at the end of the sintering step f), where appropriate at the end of the cooling if this is implemented or of the annealing step if this is implemented, the magnet can be machined and / or undergo a surface treatment, for example polishing or the application of a coating to prevent oxidation and corrosion.

[0086] At the end of the manufacturing process described above, a magnet does not have its own magnetization. The magnet can thus be subjected to a complementary magnetization: for example, the magnet can be subjected to a magnetization field parallel to the direction of alignment of the magnetic field used to orient the grains of the mixture and obtain an anisotropic magnet as described above. The field magnetic field can have an intensity greater than 4 Tesla, preferably greater than 5 Tesla. These high values ​​are generally obtained in pulsed mode.

[0087] In other words, at the end of sintering step f), the magnet thus obtained can be subjected to at least one of the following steps (namely one of these steps or any combination thereof) chosen from: - cooling, for example a cooling step as described above; - an annealing step, for example an annealing step as described above; - a machining and / or surface treatment step, for example a machining and / or surface treatment step as described above; - a complementary magnetization step, for example a complementary magnetization step as described above.

[0088] These optional steps carried out after sintering step f) are perfectly within the reach of those skilled in the art.

[0089] The present invention will be better understood with the aid of the detailed description of the experimental part below which describes, by way of non-limiting example, an embodiment of the method of manufacturing a magnet according to the invention.

[0090] Description of the figures:

[0091] [Fig. 1] Figure 1 is a photograph taken using a scanning electron microscope (backscattered electron mode) of the mixture obtained at the end of step b) of hydrogenation-disproportionation which was carried out at a hydrogen pressure of 0.8 bar and at a temperature of 900°C for a plateau time of 3 hours.

[0092] [Fig. 2] Figure 2 is a photograph taken using a scanning electron microscope (backscattered electron mode) of the mixture obtained at the end of step b) of hydrogenation-disproportionation which was carried out at a hydrogen pressure of 0.8 bar and at a temperature of 950°C for a plateau time of 3 hours.

[0093] [Fig. 3] Figure 3 is a photograph taken using a scanning electron microscope (backscattered electron mode) of the mixture obtained at the end of step b) of hydrogenation-disproportionation which was carried out at a hydrogen pressure of 0.8 bar and at a temperature of 1050°C for a plateau duration of 3 hours.

[0094] EXPERIMENTAL PART:

[0095] Preparation of the l ère powder

[0096] A l ère powder comprising in mass percentages expressed relative to the mass of said l ère powder: - 33.5% of a mixture of the two rare earths Nd and Pr (according to the following mass percentages: 75% of Nd and 25% of Pr, these mass percentages being expressed in relation to the total mass of said two rare earths); - B: 0.99%; Co: 0.5%; Al: 0.2%; Cu: 0.12%; Ga: 0.10%; - impurities: O: 160 ppm, N: 13 ppm, H: 14 ppm, C: 180 ppm, S: 28 ppm, - Fe: 100% complement, was prepared as follows.

[0097] Initially, virgin raw materials were available in massive form of the various metals as detailed above and in the quantities also indicated above (in other words the “basic charge”).

[0098] This base charge was heated. Melting was carried out under partial pressure of argon (400 mbar) in an alumina crucible at a maximum temperature of 1450°C so as to obtain a molten bath.

[0099] The molten material bath was poured onto a water-cooled copper-based wheel with a rotation speed allowing the production of crystallized ribbons with a thickness between 150 pm and 400 pm, with an average thickness of 250 pm.

[0100] The ribbons thus obtained were collected in a tank cooled by circulating water so as to cool them to room temperature.

[0101] The ribbons were then placed in a sealed enclosure of an oven for a decrepitation step.

[0102] The decrepitation step was carried out as follows. The enclosure was placed under primary vacuum (i.e. a pressure lower than 1 mbar, preferably lower than 10' 2 mbar), then filled with hydrogen to reach a pressure of 2 bars. Then, the enclosure was put under primary vacuum to evacuate the hydrogen, then it was heated at a temperature of 550°C for 2 hours to obtain partial dehydration, then cooled to room temperature (i.e. approximately 20°C) under argon.

[0103] The coarse powder thus obtained was then homogenized in a mixer into which 0.05% by mass of zinc stearate had been introduced, the mass percentage of zinc stearate being expressed relative to the mass of said coarse powder. Zinc stearate is a lubricant which facilitates the establishment of a fluidized bed during the gas jet milling step. This homogenization lasted 1 hour and a half.

[0104] The resulting homogenized powder was then introduced into a fluidized bed gas jet mill. The gas used was nitrogen. The grinding pressure, nozzle diameter, and selector speed were adjusted to obtain a èrepowder whose median particle size measured online by a laser granulometer was 5 pm.

[0105] Preparation of the 2 ème powder

[0106] A 2 ème powder comprising in mass percentages expressed relative to the mass of said 2 ème powder: - Nd: 22.2%, - Pr: 6.81%, - Dy: 4.5%, - B: 1.07%, - Co: 0.5%, - Al: 0.5%, - Cu: 0.09%, - Fe: 100% complement, was prepared as follows.

[0107] Initially, recycled magnets measuring 8 x 28 x 5 mm were available, which included the various metals as detailed above and in the quantities also indicated above.

[0108] The recycled magnets were placed in a sealed furnace chamber for the decrepitation step. The chamber was placed under primary vacuum, then filled with hydrogen to reach a pressure of 0.8 bars. This treatment made it possible to hydride the entire material but also to peel off the metal coatings.

[0109] The coarse powder thus obtained was heated under hydrogen in the same enclosure at 950°C for 3 hours under 0.8 bar of hydrogen after a heating ramp of 5°C / min. The whole was then cooled naturally under hydrogen to room temperature.

[0110] The resulting powder was then introduced under controlled atmosphere into a grinding bowl, along with 8 mm diameter stainless steel balls for a ball / powder ratio of 1:2. This bowl was immersed in a liquid nitrogen bath until thermalization. The whole was then ground in a vibrating mill, then the contents of the bowl were transferred into a glove box. The size of the 2 ème powder was between 1 pm and 20 pm.

[0111] Furthermore, two other magnet powders called "comparative 1" and "comparative 2" were prepared in exactly the same way as this 2 ème powder, but with the only difference that the hydrogenation-disproportionation temperature (in other words the heating temperature under hydrogen) was 900°C for “comparative powder 1” and 1050°C for “comparative powder 2”, in order to compare their structures with that of 2 ème powder.

[0112] Figure 1 is a scanning electron microscope photograph of the comparative powder 1. In this photograph of Figure 1, the phases of rare earth hydrides with the chemical formula TRH X , as well as iron and iron boride are visible. The absence of the beta phase is noted.

[0113] Figure 2 is a scanning electron microscope photograph of the 2 èmepowder. In this photograph in Figure 2, the phases of rare earth hydrides with the chemical formula TRH X , beta phase, as well as iron and iron boride are visible. In addition, the size of the rare earth hydride phase is micron; which will allow appropriate recombination of the grains during the sintering step.

[0114] Figure 3 is a scanning electron microscope photograph of the comparative powder 2. In this photograph of Figure 3, the phases of rare earth hydrides with the chemical formula TRH X , beta phase, as well as iron and iron boride are visible. However, the size of the rare earth hydride phase is nanometric. This nanometric size is not satisfactory because it could cause the grains to recombinate too quickly during the sintering stage.

[0115] Then, according to step c) of the manufacturing process, the l ère powder was mixed with the 2 ème powder obtained at the end of step b) in a mass ratio which depends on the heavy TR content of the 2 ème powder for 30 minutes in a mixer, in a controlled atmosphere enclosure.

[0116] The mass ratio of the mass m2 of the 2 ème powder on the mass ml of the l ère powder was equal to m2 / ml = 25 / 75.

[0117] The mixture obtained at the end of step c) was then introduced into different cylindrical rubber molds of 22 mm height and 14 mm internal diameter which were subjected to a magnetic field of 7 Tesla to orient the particles in accordance with step d) of the manufacturing process according to the invention.

[0118] Then, step e) of compacting the manufacturing process according to the invention was carried out by subjecting the mixture contained in these different molds to cold isostatic compaction at 1500 bar so as to obtain compacted parts.

[0119] Then, step f) of sintering of the manufacturing process according to the invention was carried out on these parts compacted under secondary vacuum according to the following thermal profile: - heating at 5°C / min from room temperature to 300°C, then holding for 2 hours at 300°C, - heating at 5°C / min from 300°C to 500°C, then holding for 2 hours at 500°C, - heating at 5°C / min from 500°C to 750°C, then holding for 2 hours at 750°C, - heating at 2.5°C / min from 750°C to 975°C, then holding for 12 hours at 975°C.

[0120] At the end of the last stage (sintering), argon was introduced until an absolute pressure of 2 bar was reached in order to obtain magnets.

[0121] Then the magnets were subjected to cooling with a cooling rate of 15°C / min from 975°C to 30°C.

[0122] Then, the magnets were then subjected to a secondary vacuum annealing step according to the following thermal profile: - heating at 5°C / min from 50°C to 820°C, - stage 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 500°C, - temperature hold at 500°C for 2 hours, - cooling at 15°C / min from 500°C to room temperature.

[0123] The resulting cylindrical magnets were machined using a grinding machine and a diamond wheel to remove the oxide layer and obtain parallel surfaces.

[0124] Furthermore, so-called "comparative" magnets have been manufactured from 100% of the l ère powder, and this in the same way as these magnets according to the invention.

[0125] The magnetic properties of the magnets according to the invention and of the comparative magnets are detailed in Table 1 below in which: - Br is the remanence (expressed in T), - Hcj is the coercivity (expressed in kA / m), - (BH)max is the maximum energy product (expressed in kJ / m 3 ), - Dy is the mass content of dysprosium (expressed in %), - density is the density of the magnet (expressed in g / cm 3 ).

[0126] Table 1

[0127] As explained above, the magnets according to the invention were manufactured with 25% by mass of recycled magnet powders. The loss of their remanence is 0.07 T, i.e. 5.43% in relative terms less than the comparative magnets (i.e. manufactured with 0% recycled magnet powders). But considering that the lower mass density and the presence of Dy contribute to reducing the remanence of the magnets according to the invention, this allows us to affirm that the loss of texture, if it exists, remains very limited. This very slight loss of remanence must be compared to that which would have been obtained by using an essentially isotropic decomposed powder. In this case, mixing 25% of isotropic powder with an anisotropic powder should cause a loss of approximately 12.5% ​​in remanence, which is not at all what is observed with the magnets according to the invention. This allows us to conclude that the manufacturing process according to the invention makes it possible to improve the texture of the magnets manufactured.

Claims

CLAIMS 1. Method for manufacturing a highly textured magnet, characterized in that it comprises at least the following steps: a) there is provided: - of a l ère powder comprising grains of a magnetic phase TRzFe^B (“TR” designating an element or a combination of two or more elements chosen from rare earths), metal-based compounds and a rare earth-rich grain boundary phase, and optionally grains of a non-magnetic phase TRFe4B4, said l ère powder being free of heavy rare earth or the mass content of heavy rare earth expressed in relation to the mass of the l ère powder being less than 1%, - of a 2 ème powder comprising heavy rare earths, the mass content of heavy rare earths, expressed relative to the mass of said 2 ème powder, being between 1% and 10%, preferably between 2% and 5%, said 2 èmepowder containing grains of a magnetic phase TRzFe^B, b) we subject the 2 ème powder to a hydrogenation-disproportionation treatment so as to decompose the magnetic phase TRzFe^B into a mixture containing rare earth hydrides of chemical formula TRH X in which x is the atomic ratio of H / TR, iron, iron boride (FezB) and a so-called beta phase which comprises TR, iron and boron, said hydrogenation-disproportionation treatment being carried out: - either at a temperature between 850°C and 950°C and at a hydrogen pressure greater than or equal to 0.3 bar and less than 0.5 bar, - either at a temperature between 925°C and 1025°C and at a hydrogen pressure greater than or equal to 0.5 bar and less than 1 bar, - either at a temperature between 950°C and 1100°C and at a hydrogen pressure greater than or equal to 1 bar and less than or equal to 10 bar, c) the l is mixed èrepowder with the 2 ème powder obtained at the end of step b) in a mass ratio which depends on the heavy rare earth content of the 2 ème powder so as to obtain a mixture, d) the mixture obtained at the end of step c) is subjected to a magnetic field, e) the mixture obtained at the end of step d) is subjected to a compacting step so as to obtain a compacted part, f) the compacted part obtained at the end of step e) is subjected to a sintering step so as to obtain a magnet.

2. Manufacturing method according to claim 1, characterized in that the l ère powder comprises, in mass percentages expressed relative to the mass of said l ère powder: - between 90% and 99%, preferably between 95% and 97%, of grains of the magnetic phase of type TRzFe^B, - between 0.5% and 3%, preferably between 0.5% and 2%, of metal-based compounds, - between 1% and 10%, preferably between 2% and 5%, of the rare earth-rich grain boundary phase, - optionally between 0.1% and 5%, preferably between 0.1% and 1%, of grains of the non-magnetic phase of type TRFe4B4.

3. Manufacturing method according to claim 1 or 2, characterized in that the l ère powder comprises, in mass percentages expressed relative to the mass of said l ère powder: - between 27% and 35% rare earth, - between 0.9% and 1.2% boron, - 100% complement of at least one metallic element M chosen from the group consisting of Fe, Co, Ni, taken alone or as a mixture thereof, the sum of the mass percentages of Ni and Co being less than or equal to 5%, and optionally Fe being partially replaced by at least one replacement element chosen from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn, taken alone or as a mixture thereof, the content of the replacement element(s) being less than or equal to 3%.

4. Manufacturing method according to any one of claims 1 to 3, characterized in that the 2 ème powder comprises, in mass percentages expressed relative to the mass of said 2 ème powder: - between 27% and 35% rare earth, including between 1% and 10%, preferably between 2% and 5%, heavy rare earth, - between 0.9% and 1.2% boron, - 100% complement of at least one metallic element M chosen from the group consisting of Fe, Co, Ni, taken alone or in a mixture of these, the sum of the percentages mass of Ni and Co being less than or equal to 5% and, optionally Fe being partially replaced by at least one replacement element chosen from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn, taken alone or as a mixture thereof, the content of the replacement element(s) being less than or equal to 3%.

5. Manufacturing method according to any one of claims 1 to 4, characterized in that the l ère powder was obtained from: - virgin raw materials which are completely free of heavy rare earths or whose mass content is less than 1%, or - recycled magnets which are completely free of heavy rare earths or whose mass content is less than 1%, or - a mixture of virgin raw materials and recycled magnets which are completely free of heavy rare earths or whose mass content is less than 1%.

6. Manufacturing method according to claim 5, characterized in that the virgin raw materials have been subjected to the following steps: - a wheel casting stage, followed - a decrepitation step under hydrogen and / or gas jet grinding.

7. Manufacturing method according to claim 5, characterized in that the recycled magnets have been subjected to a step of decrepitation under hydrogen and / or gas jet grinding.

8. Manufacturing method according to any one of claims 1 to 7, characterized in that the 2 ème powder was obtained from: - virgin raw materials with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%, or - recycled magnets with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%, or - a mixture of virgin raw materials and recycled magnets with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%.

9. Manufacturing method according to claim 8, characterized in that the 2 ème powder was obtained from recycled magnets which were subjected to the following treatment: - a decrepitation step under hydrogen, optionally followed - gas jet grinding.

10. Manufacturing method according to any one of claims 1 to 9, characterized in that the 2 ème powder has been fractured by at least one grinding technique selected from gas jet grinding, planetary grinding, attrition grinding and cryogenic grinding.

11. Manufacturing method according to any one of claims 1 to 10, characterized in that in step d), the mixture obtained at the end of step c) is subjected to a magnetic field greater than 1 Tesla, preferably greater than 2 Tesla.

12. Manufacturing method according to any one of claims 1 to 11, characterized in that at the end of step e) and before carrying out step f) of sintering, the compacted part obtained at the end of step e) is dehydrided.

13. Manufacturing method according to any one of claims 1 to 12, characterized in that the magnet obtained at the end of sintering step f) is subjected to at least one of the steps chosen from cooling, an annealing step, a machining and / or surface treatment step and a complementary magnetization step.

14. Manufacturing process according to any one of claims 1 to 13, characterized in that the duration of the hydrogenation-disproportionation treatment of step b) is between 10 minutes and 3 hours.

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