Method for manufacturing a highly textured magnet

Abstract

Method for manufacturing a highly textured magnet The invention relates to a method for manufacturing a highly textured magnet comprising the following steps: a) we have: - a 1st powder comprising grains of a TR2Fe14B magnetic phase, - a 2nd powder comprising heavy rare earths and grains of a TR2Fe14B magnetic phase, b) we subject the 2nd powder to a hydrogenation-disproportionation treatment, c) we mix the 1st powder with the 2nd powder obtained at the end of step b), d) we subject the mixture obtained at the end of step c) to a magnetic field, e) we subject the mixture obtained at the end of step d) to a compaction step so as to obtain a compacted part, f) we subject the compacted part obtained at the end of step e) to a sintering step so as to obtain a magnet.

Description

Title of the invention: Method for manufacturing 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 TRFeB type permanent magnets.

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

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

[0004] TR is predominantly neodymium (Nd). This is why the most common example of these permanent magnets is the NdFeB type magnet in which the Nd2Fei4B crystalline phase is the main phase, namely an alloy of neodymium, iron and boron allowing the formation of a tetragonal crystal system.

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

[0006] However, given the current problems of environmental protection and depletion of natural resources, in particular rare earths, in order to satisfy a perpetually growing demand and at least one that may exceed the supply projections for these permanent magnets for these different advanced technologies, it is necessary to be able to efficiently recycle the magnets contained in these different devices, equipment or motors, as soon as these devices are out of 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, "recycled magnets" means: - magnets that have been recovered (for example during waste sorting operations) from appliances, devices, motors or for reuse other products including magnets which were no longer in use, and also - magnets corresponding to production waste, for example waste from the production of magnets (because they are defective in particular) and which are thus re-valued.

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

[0009] One of these approaches is so-called "direct" recycling, in which recovered magnets (for example, from end-of-life equipment) are reused in block form, possibly after one or more surface treatments and machining. In other words, according to this approach, 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 to achieve the desired shape. It is, however, possible to perform a rare-earth diffusion treatment on these magnets from their surface, which modifies their physicochemical properties and therefore their magnetic properties, primarily coercivity.

[0010] A second recycling route is an "indirect" or "long" route in which the chemical elements constituting recovered magnets (for example, from end-of-life devices) are separated into oxides through pyrometallurgical or hydrometallurgical processes. These oxides thus obtained are reintroduced upstream in the manufacture of new magnets as raw materials in the synthesis of metals, and subsequently of precursor alloys.

[0011] A third known recycling method is the so-called "powder" method, in which recovered magnets (for example, from end-of-life appliances) are reduced to powder form. These powders are then diluted in polymers to manufacture bonded magnets, or they are densified by heat treatments to obtain sintered magnets.

[0012] The so-called "powder" route has the advantage of allowing the composition of new magnets made from recycled magnets to be readjusted by mixing different powders, thus providing some freedom regarding the final shape of these new magnets. Furthermore, in the case of sintered magnets, this recycling route relies on powder metallurgy magnet production processes that are already implemented and fully mastered.

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

[0014] In this regard, it should be recalled that two categories of rare earths are distinguished: - 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 manufacturing processes for permanent magnets comprising heavy rare earths that the substitution of neodymium (and to a lesser extent praseodymium) by heavy rare earths throughout the magnetic phase TR2Fei4B results in a decrease in remanence. This decrease can be limited by using a mixture of a heavy rare earth-poor powder and a heavy rare earth-rich powder, 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 regions most critical for demagnetization, so as to obtain a structure known as the "core-shell" structure. The peripheral region of a magnetic grain is therefore the region into which the heavy rare earths diffused during the manufacture of the magnet.

[0016] In other words, knowledge of 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 required for their manufacture.

[0017] However, during the so-called "powder" recycling process, it is possible to co-sinter several powders, at least one of which may have been obtained from recycled magnets containing heavy rare earth elements. But in this case, a significant portion of these heavy rare earth elements will not be able to diffuse out of the grains and will therefore remain in the magnetic phase of said powder rather than contributing to a core-shell effect at the periphery of the grains initially devoid of heavy rare earth elements. Therefore, in order to obtain magnetic performance comparable to that obtained with permanent magnet manufacturing processes incorporating heavy rare earth elements from virgin raw materials, it is known to add heavy rare earth elements from primary sources (in other words, heavy rare earth deposits) during this so-called "powder" recycling process.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 first powder from these recycled magnets rich in heavy rare earths with a second powder free of heavy rare earths or at least with a low heavy rare earth content (typically less than 1% by mass). Co-sintering these two powders only allows a small portion of the heavy rare earths contained in the first powder to diffuse. With such a powder mixture, the increase in coercivity is on 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 process.

[0019] The inventors thus sought to improve the so-called "powder" recycling method by proposing a new manufacturing process for a highly textured magnet, preferably from recycled permanent magnets of the TRFeB type and containing heavy rare earths, which is based on this recycling method but also presents other original technical characteristics which are detailed below, enabling: - obtaining a magnet with high magnetic performance (i.e., good coercivity while maintaining high remanence), and this without necessarily adding heavy rare earths from virgin raw materials (for example 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 out-of-use devices or equipment (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 for a highly textured magnet, preferably from recycled magnets, is based on the so-called "powder" recycling route and allows the obtaining of 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 invention relates to a method for manufacturing a highly textured magnet (preferably from recycled magnets), which comprises at least the following steps: a) We have: - of a first powder comprising grains of a magnetic TR2Fe4B phase, metal-based compounds and a rare-earth-rich grain boundary phase, and optionally grains of a non-magnetic TRFe4B4 phase, said first powder being free of heavy rare earth or the mass content of heavy rare earth expressed relative to the mass of the first powder being less than 1%, - of a 2nd powder (preferably obtained from recycled magnets), comprising heavy rare earths, the mass content of heavy rare earths, expressed relative to the mass of said 2nd powder, being between 1% and 10%, preferably between 2% and 5%, said 2nd powder containing grains of a magnetic phase TR2Fei4B,

[0022] b) the 2nd powder is subjected to a hydrogenation-disproportionation treatment (also known by the English term "disproportionation hydrogenation") so as to decompose the magnetic phase TR2FeMB into a mixture containing rare earth hydrides of chemical formula TRHX in which x is the atomic ratio of H / TR and is for example between 2 and 3, iron, iron boride (Fe2 B) and a so-called beta phase which includes TR, iron and boron, said disproportionation hydrogenation 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 first powder is mixed with the second powder obtained at the end of step b) in a mass ratio that depends on the heavy rare earth content of the second powder so as to obtain a mixture,

[0023] d) the mixture obtained at the end of step c) is subjected to a magnetic field,

[0024] e) the mixture obtained at the end of step d) is subjected to a compaction step in order to obtain a compacted part,

[0025] f) the compacted part obtained at the end of step e) is subjected to a sintering step in order to obtain a magnet.

[0026] Because the 2nd powder contains heavy rare earths, during step b) of the manufacturing process according to the invention, heavy rare earth hydrides are formed.

[0027] One might expect that the hydrogen treatment in step b) would induce a loss of texture in the magnet manufactured using the manufacturing process according to the invention. Indeed, it is known that hydrogenation-disproportionation can alter the texture of the material subjected to such treatment. Moreover, the microstructure The texture of a powder decomposed by hydrogenation-disproportionation also influences the texture of the magnetic phase grains in the magnet after their recombination during sintering. For example, a fine lamellar microstructure recombines in a textured manner, while a coarse spherical microstructure recombines with little texture as finer grains. In other words, an incorrect texture level in 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 of remanence.

[0028] However, contrary to expectations, the inventors discovered that selecting the temperature and pressure ranges 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 second powder, can advantageously be between 5% and 50%. The presence of this beta phase, as well as its magnetically anisotropic structure within the microstructured mixture of step b), makes it possible to counteract the potentially expected loss of texture resulting from this treatment under hydrogen, and thus to obtain, with the manufacturing process according to the invention, a highly textured magnet.

[0029] Furthermore, the beta phase contains a higher proportion of heavy rare earth elements than the hydride phases, with a partition coefficient that can exceed 5. The heavy rare earth elements present in this beta phase are bound to a complex crystallographic structure and will therefore, a priori, be less available to diffuse to the periphery of the magnetic grains of the first 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 that ensures a high degree of magnet texture without trapping too many heavy rare earth elements.

[0030] Finally, the selection of temperature and pressure ranges 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 preventing excessively rapid recombination of these grains and thus allowing the heavy rare earths to diffuse properly around the periphery of said grains.

[0031] The mass ratio of step c) can be determined as follows: for example if the 2nd powder contains X% of heavy rare earths and we wish to achieve in the final magnet an overall content of Y%, the mass ratio of the mass m2 of the 2nd powder to the mass ml of the 1st powder is m2 / ml = Y / (XY).

[0032] During step f) of sintering the mixture of the 1st powder and the 2nd powder, a liquid phase forms (by eutectic reaction) between the different compounds at The first powder consists of a metal base, the TR-rich phase, the magnetic phase TR2Fe4B, and the optional non-magnetic phase TRFe4B4. This liquid phase has a high rare earth content, approximately 90% by mass at 700°C, 80% at 900°C, and 70% at 1000°C. During the vacuum heating process, the rare earth hydrides formed in step b), particularly the heavy rare earth hydrides, are transformed into metallic heavy rare earths, which dissolve in the liquid phase, enriching 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 first powder during the densification of both the first and second powders, which occurs during sintering.

[0033] Thus, thanks to step b) of the manufacturing process according to the invention, the heavy rare earths present in the second powder (preferably a second powder 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 temperature rise under 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.

[0034] Step b) of the manufacturing process according to the invention thus eliminates the need for a diffusion step of these heavy rare earths within the grains of the TR2Fei4B magnetic phase of the second powder; this is a slow process that would create a concentration gradient within the heavy rare earth-rich grains of the second powder. This would ultimately result in only a small fraction of the quantity of said heavy rare earths being utilized. Moreover, in this diffusion scenario, the heavy rare earth content at the periphery of the grains of the first powder would be lower than the heavy rare earth content in the grains of the second powder.

[0035] Conversely, with the manufacturing process according to the invention, the extraction of heavy rare earths from the magnetic phase of the 2nd powder increases their chemical potential. At the end of step f), the heavy rare earths are located on the periphery of the TR2FeMB magnetic phase grains of the 1st powder at higher concentrations than the heavy rare earth content in the grains of the 2nd powder (preferably obtained from recycled magnets), so that the coercivity of the magnet thus obtained is increased very significantly, even though said magnet contains small quantities of heavy rare earths.

[0036] In the context of the present invention, an additional advantage is to co-sinter the first and second powders at a lower temperature, since the heavy rare earths have already been extracted from the TR2Fei4B magnetic phase grains of the second powder. This allows the heavy rare earths to be located at the extreme periphery of the first powder grains. powder, and thus obtain a very pronounced "core-shell" structure which makes it possible to approach the coercivity gains obtained by grain boundary diffusion. But the invention makes it possible to overcome the limitations in terms of magnet thickness compared to the diffusion process through the surface of the magnet.

[0037] This gain in terms of dilution of the manufacturing process according to the invention is particularly interesting, because it makes it possible to consider the recycling, in a short time, of quantities of recycled magnets rich in heavy rare earths, which will remain limited and to make it possible to manufacture large quantities of magnetically efficient magnets with a minimum input of critical raw materials.

[0038] The various technical characteristics of the manufacturing process according to the invention are described in more detail below.

[0039] In the context of the present invention, "TR-rich grain boundary phase" means a metallic phase containing more than 70% by mass of rare earth combined with metals, for example metals selected from iron, copper and aluminum.

[0040] The metal-based compounds of the first powder may include metals selected from iron, copper, aluminum, gallium, titanium and zirconium.

[0041] The first powder may comprise, in mass percentages expressed relative to the mass of said first powder: - between 90% and 99%, preferably between 95% and 97%, of TR2Fei4B type magnetic phase grains, - between 0.5% and 3%, preferably between 0.5% and 2%, of metal-based compounds,

[0042] - between 1% and 10%, preferably between 2% and 5%, of the rich grain boundary phase rare earth elements - optionally between 0.1% and 5%, preferably between 0.1% and 1%, of non-magnetic phase grains of type TRFe4B4.

[0043] The first powder may comprise, in mass percentages expressed relative to the mass of said first powder: - between 27% and 35% rare earth elements, - between 0.9% and 1.2% boron, - complement to 100% 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 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 in a mixture of these, the content of the replacement element(s) being less than or equal to 3%.

[0044] The first powder preferably has a particle size between 3 µm and 7 µm.

[0045] The first powder may have been obtained from: - virgin raw materials (e.g., pure metals and / or alloys) that are totally free of heavy rare earth elements or whose mass content is less than 1%, or - recycled magnets that are completely free of heavy rare earth elements or whose mass content is less than 1%, or - a mixture of virgin raw materials and recycled magnets which are totally free of heavy rare earths or whose mass content is less than 1%.

[0046] When the first powder has been obtained wholly or partly from virgin raw materials, these are chosen from pure metals and / or alloys. Preferably, they are pure metals.

[0047] When the first powder has been obtained wholly or partly from virgin materials, the latter may have been subjected to the following steps: - a wheel casting stage, followed - a decrepitation step under hydrogen and / or gas jet grinding (also known by the English name: Jet Mill type grinding).

[0048] First, the mixture of virgin raw materials (in other words the "basic 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.

[0049] The molten material is then poured onto a cooled, rotating wheel. The molten material is thus 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.

[0050] The hydrogen decrepitation step makes it possible to obtain a first powder whose particle size is between 50 pm and a few millimeters.

[0051] Hydrogen decrepitation 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.

[0052] The duration of the hydrogen decrepitation step can be between 1 hour and 5 hours.

[0053] The gas jet grinding step makes it possible to obtain a first powder with a median particle size between 2 µm and 10 µm, preferably between 3 µm and 6 µm, with a particle size distribution whose ratio of the 9th decile to the 1st decile, or in other words "D90 / D10", is less than 10, preferably less than 5. Commercial equipment such as mills marketed by Hosokawa-Alpine under the trade names AFG100, AFG200 and AFG400 can be used for this gas jet grinding step. They comprise a sealed chamber into which an inert gas at a pressure between 2 and 8 bar is introduced by Three converging nozzles feed the powder to be ground via a hopper, allowing for controlled feed rate. The gas flow carries the powder along and releases it as it passes through a vortex generated by a system called a "cyclone." To improve particle size, this equipment can be fitted with an inertial selector that prevents larger particles from escaping the grinding chamber.

[0054] When the first 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.

[0055] The technical characteristics of the hydrogen decrepitation and gas jet milling stage can be those that have been described for obtaining the first powder from virgin raw materials.

[0056] The second powder preferably has a particle size comparable to that of the first powder so as to facilitate the mixing of the first powder with the second powder in step c) of the manufacturing process according to the invention. The second powder preferably has a particle size between 3 µm and 7 µm.

[0057] The second powder may comprise, in mass percentages expressed relative to the mass of said second powder: - between 27% and 35% rare earth elements, of which between 1% and 10%, preferably between 2% and 5%, are heavy rare earth elements, - between 0.9% and 1.2% boron, - complement to 100% 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 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 in a mixture of these, the content of the replacement element(s) being less than or equal to 3%.

[0058] This means that the 2nd powder comprises, in mass percentages expressed in relation to the mass of said 2nd powder, between 1% and 10%, preferably between 2% and 5%, of heavy rare earth.

[0059] The second powder may have been obtained from: - virgin raw materials (e.g., pure metals and / or alloys) with a mass content of heavy rare earth elements between 1% and 10%, preferably between 2% and 5%, or - recycled magnets with a heavy rare earth mass content between 1% and 10%, preferably between 2% and 5%, or - a mixture of virgin raw materials and recycled magnets whose content The mass percentage of heavy rare earths is between 1% and 10%, preferably between 2% and 5%.

[0060] As explained above, the manufacturing process for a magnet according to the invention preferably uses recycled magnets.

[0061] Therefore, in a preferred embodiment of the invention, the second powder was obtained solely from recycled magnets. These recycled magnets contain heavy rare earth elements. The mass percentage of these heavy rare earth elements, expressed as a percentage of the mass of said second powder, is between 1% and 10%, preferably between 2% and 5%.

[0062] Advantageously, the 2nd powder was obtained from recycled magnets which were subjected to the following treatment: - a decrepitation step under hydrogen, optionally followed by - gas jet grinding.

[0063] The hydrogen decrepitation step makes it possible to obtain a 2nd coarse powder whose particle size is between 50 pm and a few millimeters.

[0064] The technical characteristics of the hydrogen decrepitation and gas jet milling step for obtaining the 2nd powder can be those that have been described for obtaining the 1st powder from virgin raw materials or recycled magnets.

[0065] When the second powder was obtained wholly or partly from recycled magnets, the latter may have been subjected to a hydrogen decrepitation and / or gas jet grinding step. The technical characteristics of the hydrogen decrepitation and gas jet grinding steps may be those described for obtaining the first powder from virgin raw materials.

[0066] When the 2nd powder has been obtained wholly or partly from virgin materials, the latter may have been subjected to the following steps: - a wheel casting stage, followed - of a decrepitation step under hydrogen and / or gas jet grinding.

[0067] The technical characteristics of the wheel casting, hydrogen decrepitation and gas jet milling stages can be those described above for obtaining the first powder from virgin raw materials.

[0068] The hydrogenation-disproportionation treatment can be carried out under vacuum, namely by heating the second powder under vacuum to a temperature as described above before the introduction of hydrogen. In another embodiment of the invention, the second powder is heated to a temperature as described above after the introduction of hydrogen.

[0069] The second powder can be heated to the desired temperature as described above with a heating rate between 1°C / minute and 30°C / minute.

[0070] In step b), the temperatures as described above are appropriate so as not to cause an agglomeration of particles together.

[0071] In step b), the pressures as described above are appropriate so as not to cause a loss of texture of the magnet.

[0072] By way of example, at a pressure of 0.8 bar, the processing temperature of step b) can be between 925°C and 1025°C.

[0073] The duration of the hydrogenation-disproportionation treatment can range from 10 minutes to 3 hours. This duration depends on the hydrogenation-disproportionation treatment temperature: it is shorter at higher temperatures. Indeed, an excessively long treatment time at high temperatures would cause macroscopic heterogeneities within the microstructure of the magnet obtained by the manufacturing process.

[0074] Following the hydrogenation-disproportionation treatment, the 2nd powder can be subjected to cooling under hydrogen, in order to prevent the recombination of the elements to reform the magnetic phase TR2Fei4B, and thus preserve the mixture of rare earth hydrides of chemical formula TRHX, iron, iron boride (Fe2B) and the beta phase, until room temperature (namely about 20°C).

[0075] Optionally, before step c) of the manufacturing process according to the invention, the second powder is fractured to improve contact between the rare earth hydrides and the liquid phase during step f) so that the heavy metallic rare earths dissolve optimally in the liquid phase. This step may be necessary if the particles of the second powder have re-agglomerated during the hydrogenation-disproportionation treatment, but also if their microstructure is not optimal, in particular if the hydrides are located within the grains rather than at the periphery. The second powder may have been fractured by at least one grinding technique selected from gas jet milling, planetary milling, attrition milling, and cryogenic milling.

[0076] Step c) of mixing the 1st powder and the 2nd 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.

[0077] 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 compaction.

[0078] 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 with high remanence. Preferably, the magnetic field is greater than 1 Tesla, more preferably greater than 2 Tesla. Preferably, the 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.

[0079] Step e) of compaction can be carried out using transverse, axial, cold isostatic compaction, or rubber isostatic pressing (also known as "RIP," the English acronym for "Rubber Isostatic Pressing"), so as to obtain a compacted part referred to as a "green part." For example, the mixture is compacted by applying a uniaxial pressure of between 50 MPa and 300 MPa.

[0080] 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).

[0081] In an advantageous embodiment of the invention, after step e) and before step f) of sintering, the compacted part obtained after step e) is dehydrated. This operation consists of removing almost all of the hydrogen contained in the compacted part. The hydrogen is essentially present in the form of TRHx hydrides with x close to 2. The aim is to reduce the overall hydrogen content in the compacted part, which is, for example, on 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.

[0082] This optional step of dehydrating 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 to avoid demixing of the TRFeB phase, and in the presence of hydrogen. A secondary vacuum corresponds to a pressure below 10⁴ mbar, preferably below 5 x 10⁵ mbar.

[0083] Next, step f) of sintering is carried out in such a way 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).

[0084] 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 between 850°C and 1050°C and for a duration between 3 hours and 24 hours in order to obtain said magnet.

[0085] 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.cm3.

[0086] In an advantageous embodiment of the invention, at the end of step f), the magnet thus obtained is subjected to cooling. Preferably, this is a re rapid cooling, i.e. above 20°C / min, more preferably about 30°C / min, from the sintering temperature to room temperature or, where appropriate, to the temperature of the start of the optional re-annealing step described below.

[0087] The magnet obtained at the end of step f), where applicable after cooling if this is implemented, can then be subjected to an annealing step.

[0088] Indeed, annealing increases the magnet's resistance to demagnetization. A person skilled in the art is familiar with the conditions for carrying out the annealing step.

[0089] 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 plateau at 820 °C for 2 hours; - a 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 plateau at a temperature between 460°C and 650°C for 2 hours; - a cooling of the temperature between 460°C and 650°C to 50°C at 30°C / min.

[0090] In an advantageous embodiment of the invention, after step f) of sintering, where applicable after cooling if implemented or after the annealing step if implemented, the magnet can be machined and / or undergo surface treatment, for example polishing or the application of a coating to prevent oxidation and corrosion.

[0091] Following the manufacturing process described above, a magnet does not possess its own magnetization. The magnet can therefore be subjected to 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 magnetic field can have an intensity greater than 4 Tesla, preferably greater than 5 Tesla. These high values ​​are generally obtained in pulsed mode.

[0092] In other words, at the end of step f) of sintering, 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: - a cooling, for example a cooling stage as described above; - a re-annealing step, for example a re-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.

[0093] These optional steps carried out after step f) of sintering are perfectly within the reach of a person skilled in the art.

[0094] 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, one embodiment of the process of manufacturing a magnet according to the invention. Brief description of the drawings

[0095] [Fig-1] The [Fig. 1] is a photograph taken by scanning electron microscopy (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.

[0096] [Fig.2] [Fig.2] is a photograph taken with 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.

[0097] [Fig.3] The [Fig.3] is a photograph taken by scanning electron microscopy (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 time of 3 hours.

[0098] EXPERIMENTAL SECTION:

[0099] Preparation of the powder

[0100] A first powder comprising, in mass percentages expressed relative to the mass of said first 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%; AI: 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 in the following way.

[0101] Initially, virgin raw materials were available in massive form in the various metals as detailed above and in the quantities also indicated above (in other words, the "basic charge").

[0102] This basic charge was heated. The melting was carried out under partial pressure argon (400 mbar) in an alumina crucible at a maximum temperature of 1450°C in order to obtain a melting bath.

[0103] The molten material bath was poured onto a water-cooled copper-based wheel with a rotation speed enabling the production of crystallized ribbons with a thickness between 150 pm and 400 pm, with an average thickness of 250 pm.

[0104] The ribbons thus obtained were collected in a container cooled by circulating water so as to cool them down to ambient temperature.

[0105] The ribbons were then placed in a sealed chamber of an oven for a decrepitation step.

[0106] The decrepitation step was carried out as follows. The chamber was placed under primary vacuum (i.e., a pressure below 1 mbar, preferably below 102 mbar), then filled with hydrogen to reach a pressure of 2 bar. Next, the chamber was placed under primary vacuum to remove the hydrogen, then heated to a temperature of 550°C for 2 hours to achieve partial dehydration, and then cooled to ambient temperature (i.e., approximately 20°C) under argon.

[0107] 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 that facilitates the establishment of a fluidized bed during the jet milling step. This homogenization lasted one and a half hours.

[0108] The homogenized powder thus obtained 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 first powder whose median particle size, measured online by a laser particle size analyzer, was 5 µm.

[0109] Preparation of the 2™e-powder

[0110] A second powder comprising, in mass percentages expressed relative to the mass of said second powder: - Nd: 22.2%, -Pr: 6.81%, - Dy: 4.5%, - B: 1.07%, - Co: 0.5%, - AI: 0.5%, - Cu: 0.09%, - Fe: 100% complement, was prepared in the following manner.

[0111] Initially, recycled magnets of dimensions 8 x 28 x 5 mm were available, comprising the various metals as detailed above and in the quantities also indicated above.

[0112] The recycled magnets were placed in a sealed chamber of a furnace for the decrepitation step. The chamber was initially evacuated, then filled with hydrogen to reach a pressure of 0.8 bar. This treatment hydridified the entire material and also peeled off the metallic coatings.

[0113] The coarse powder thus obtained was heated under hydrogen in the same chamber at 950°C for 3 hours under 0.8 bar of hydrogen after a heating ramp of 5°C / min. The mixture was then cooled naturally under hydrogen to room temperature.

[0114] The resulting powder was then introduced under a controlled atmosphere into a grinding bowl, along with 8 mm diameter stainless steel balls at a ball-to-powder ratio of 1:2. This bowl was immersed in a liquid nitrogen bath until thermalization. The mixture was then ground in a vibratory mill, and the contents of the bowl were transferred to a glove box. The size of the second powder ranged from 1 µm to 20 µm.

[0115] Furthermore, two other magnet powders called "comparative 1" and "comparative 2" were prepared in exactly the same way as this 2nd powder, but with the sole 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 the 2nd powder.

[0116] [Fig. 1] is a scanning electron microscope photograph of comparative powder 1. In this photograph of [Fig. 1], the phases of rare-earth hydrides with chemical formula TRHX, as well as iron and iron boride, are visible. The absence of the beta phase is noted.

[0117] Figure 2 is a scanning electron microscope image of the second powder. This image of Figure 2 shows the rare-earth hydride phases with the chemical formula TRHX, the beta phase, as well as iron and iron boride. Furthermore, the size of the rare-earth hydride phase is micron-sized, which will allow for proper recombination of the grains during the sintering step.

[0118] Figure 3 is a scanning electron microscope image of comparative powder 2. In this image of Figure 3, the rare-earth hydride phases with the chemical formula TRHX, the 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 unsatisfactory because it could lead to excessively rapid recombination of the grains during the sintering step.

[0119] Next, according to step c) of the manufacturing process, the first powder was mixed with the 2nd powder obtained at the end of step b) in a mass ratio which depends on the content of heavy TR of the 2nd powder for 30 minutes in a mixer, in a chamber under controlled atmosphere.

[0120] The mass ratio of the mass m2 of the 2nd powder to the mass ml of the 1st powder was equal to m2 / ml = 25 / 75.

[0121] The mixture obtained at the end of step c) was then introduced into different cylindrical molds of 22 mm in height and 14 mm in internal diameter made of rubber 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.

[0122] Next, step e) of compaction of 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 in order to obtain compacted parts.

[0123] Then, step f) of sintering of the manufacturing process according to the invention was carried out on these compacted parts under secondary vacuum according to the following thermal profile: - heating at 5°C / min from ambient temperature up to 300°C, then holding at 300°C for 2 hours, - 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.

[0124] At the end of the last (sintering) stage, argon was introduced until an absolute pressure of 2 bar was reached in order to obtain magnets.

[0125] Next, the magnets were subjected to cooling with a cooling rate of 15°C / min from 975°C to 30°C.

[0126] Then, the magnets were subsequently subjected to a secondary vacuum annealing step according to the following thermal profile: - heating at 5°C / min from 50°C up to 820°C, - plateau at 820°C for 2 hours, - Cooling at 20°C / min from 820°C down to 50°C, - heating at 5°C / min from 50°C up to 500°C, - maintain a temperature of 500°C for 2 hours, - cooling at 15°C / min from 500°C down to ambient temperature.

[0127] The magnets thus obtained in cylindrical shape were machined using a grinder and a diamond grinding wheel to remove the oxide layer and obtain parallel surfaces.

[0128] Furthermore, so-called "comparative" magnets were manufactured from 100% of the first powder, in the same way as these magnets according to the invention.

[0129] The magnetic properties of the magnets according to the invention and of 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 / m3), - Dy is the mass content of dysprosium (expressed in %), - density is the density of the magnet (expressed in g / cm3).

[0130] [Tables 1] Br (T) Hcj (kA / m) (BH)max (kJ / m3) Dy (% mass) Density jg;Wj Comparative magnets 1.29 1305 322 0 7.50 Magnets according to the invention 1 22 1482 285 1.12 7.42

[0131] As explained above, the magnets according to the invention were manufactured with 25% by mass of recycled magnet powders. Their loss of remanence is 0.07 T, or 5.43% less relative than the comparative magnets (i.e., manufactured with 0% recycled magnet powders). However, considering that the lower mass density and the presence of Dy contribute to reducing the remanence of the magnets according to the invention, it can be stated that the loss of texture, if it exists, remains very limited. This very slight loss of remanence should be compared to that which would have been obtained using a predominantly isotropic decomposed powder. In that case, mixing 25% isotropic powder with an anisotropic powder should result in a loss of approximately 12.5% ​​in remanence, which is not at all what is observed with the magnets according to the invention. This leads to the conclusion that the manufacturing process according to the invention improves the texture of the manufactured magnets.

Claims

1. Demands A method for manufacturing a highly textured magnet, characterized in that it comprises at least the following steps: a) we have: - of a first powder comprising grains of a magnetic phase TR2 Fei4B (“TR” designating an element or a combination of two or more elements selected from the rare earths), metal-based compounds and a rare earth-rich grain boundary phase, and optionally grains of a non-magnetic phase TRFe4B4, said first powder being free of heavy rare earths or the mass content of heavy rare earths expressed relative to the mass of the first powder being less than 1%, - of a 2nd powder comprising heavy rare earths, the mass content of heavy rare earths, expressed in relation to the mass of said 2nd powder, being between 1% and 10%, preferably between 2% and 5%, said 2nd powder containing grains of a magnetic phase TR2Fei4B, b) the 2nd powder is subjected to a hydrogenation-disproportionation treatment so as to decompose the magnetic phase TR2FeuB into a mixture containing rare earth hydrides of chemical formula TRHX in which x is the atomic ratio of H / TR, iron, iron boride (Fe2B) and a so-called beta phase which includes 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, - or 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 first powder is mixed with the second powder obtained at the end of step b) in a mass ratio that depends on the heavy rare earth content of the second 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 compaction 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. A manufacturing process according to claim 1, characterized in that the first powder comprises, in mass percentages expressed relative to the mass of said first powder: - between 90% and 99%, preferably between 95% and 97%, of TR2Fe4B type magnetic phase grains, - 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 TRFe4B4 type non-magnetic phase grains.

3. A manufacturing process according to claim 1 or 2, characterized in that the first powder comprises, in mass percentages expressed in relation to the mass of said first powder: - between 27% and 35% of rare earth, - between 0.9% and 1.2% of boron, - complement to 100% of at least one metallic element M selected from the group consisting of Fe, Co, Ni, taken alone or in mixtures 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 selected from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn, taken alone or in mixtures thereof, the content of the replacement element(s) being less than or equal to 3%.

4. A manufacturing process according to any one of claims 1 to 3, characterized in that the second powder comprises, in mass percentages expressed relative to the mass of said second powder: - between 27% and 35% rare earth elements, of which between 1% and 10%, preferably between 2% and 5%, are heavy rare earth elements, - between 0.9% and 1.2% boron, - making up to 100% at least one metallic element M selected from the group consisting of Fe, Co, and Ni, taken alone or in mixtures 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... less one replacement element chosen from the group consisting of Al, Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn, taken alone or in a mixture of these, the content of the replacement element(s) being less than or equal to 3%.

5. A manufacturing process according to any one of claims 1 to 4, characterized in that the first powder was obtained from: - virgin raw materials which are totally free of heavy rare earths or whose mass content is less than 1%, or - recycled magnets which are totally 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 totally free of heavy rare earths or whose mass content is less than 1%.

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

7. A manufacturing process according to claim 5, characterized in that the recycled magnets have been subjected to a hydrogen decrepitation step and / or gas jet grinding.

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

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

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

11. A manufacturing process 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. A manufacturing process 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 dehydrated.

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

14. A 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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