Method for producing a highly coercive magnet

Abstract

The invention relates to a method for producing a highly coercive magnet, the method comprising the following steps: a) providing: - 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) the 2nd powder is subjected to a hydrogenation-disproportionation treatment; c) the 1st powder is mixed with the 2nd powder obtained at the end of step b); d) the mixture obtained at the end of step c) is subjected to a compacting step so as to obtain a compacted part; e) the compacted part obtained at the end of step d) is subjected to a presintering step; f) the presintered part obtained at the end of step e) is subjected to a sintering step.

Description

DESCRIPTION TITLE: Process for manufacturing a highly coercive magnet

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

[0002] In the context of the present invention, "highly coercive magnet" means a magnet whose coercivity is greater than 1400 kA / m at room temperature.

[0003] In the context of the present invention, "TR" designates 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 NdzFe^B crystalline phase is the main phase, namely an alloy of neodymium, iron and boron allowing to form 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, particularly 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 reuse (for example, during waste sorting operations) from appliances, devices, motors or other products containing magnets that were no longer in use, and also - magnets corresponding to production waste, for example waste from magnet production (because they are defective) and which are thus re-valued.

[0008] There are different ways to recycle NdFeB type permanent magnets.

[0009] One l èreOne of these methods is so-called "direct" recycling, in which recovered magnets (for example, from end-of-life devices) are reused as blocks, possibly after one or more surface treatments and machining. In other words, with this method, 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. However, it is 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 2 èmeThe recycling process is an "indirect" or "long" route in which the chemical components of recovered magnets (for example, from end-of-life appliances) are separated into oxides through pyrometallurgical or hydrometallurgical treatments. These oxides are then reintroduced upstream in the manufacture of new magnets as raw materials in the synthesis of metals and subsequently precursor alloys.

[0011] A 3 ème A known recycling method is the so-called "powder" method, in which recovered magnets (for example, from end-of-life appliances) are reduced to powder. 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" process has the advantage of allowing the composition of new magnets made from recycled magnets to be readjusted by mixing different powders allow for a certain degree of freedom regarding the final shape of these new magnets. Furthermore, in the case of sintered magnets, this recycling method relies on powder metallurgy magnet production processes that are already implemented and fully mastered.

[0013] However, this so-called "powder" recycling method (just like the "direct" method) does not allow for optimal use of the heavy rare earth elements (Dy or Tb) present in the magnetic phase of some 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, heavy rare earth elements increase the magnetocrystalline anisotropy of the magnetic phase and therefore its resistance to demagnetization.

[0014] In this regard, it is worth recalling that there are two categories of rare earth elements: - 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 containing heavy rare earth elements that the substitution of neodymium (and to a lesser extent praseodymium) with heavy rare earth elements throughout the TF FewB magnetic phase results in a decrease in remanence. This can be mitigated by using a mixture of a heavy rare earth-poor powder and a heavy rare earth-rich powder, or by diffusing the heavy rare earth elements from the surface of the sintered magnets and along the grain boundaries. In this way, the heavy rare earth elements are preferentially located at the periphery of the magnetic grains, i.e., in the regions most critical for demagnetization, thus obtaining a structure known as the "core-shell" structure. The peripheral region of a magnetic grain is therefore the region into which the heavy rare earth elements diffused during the magnet's fabrication.

[0016] In other words, knowledge of the manufacturing processes of magnets including 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 amount of heavy rare earths needed 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 the 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 achieved with permanent magnet manufacturing processes using 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 "powder" recycling process.Alternatively, 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 derived from these recycled magnets rich in heavy rare earths with a 2 ème Powder free of heavy rare earth elements, or at least with a low heavy rare earth element content (typically less than 1% by mass). Co-sintering these two powders only allows a small portion of the heavy rare earth elements contained in the powder to diffuse. ère powder. With such a mixture of powders, 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 coercive 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 features, detailed below, which allow: - obtaining a magnet with high magnetic performance (namely excellent 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 existing production lines for the manufacture of magnets.

[0020] The manufacturing process for a highly coercive magnet, preferably from recycled magnets, is based on the so-called "powder" recycling method and allows the production of a 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 the said heavy rare earths.

[0021] The invention relates to a method for manufacturing a highly coercive 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 TRzFe^B phase, metal-based compounds and a rare-earth-rich grain boundary phase, and optionally grains of a non-magnetic TRFe4B4 phase, said l èrepowder being free of heavy rare earth or the mass content of heavy rare earth expressed relative to the mass of the 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 as a percentage of 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 submit the 2 ème powder undergoes hydrogenation-disproportionation treatment (also known as "hydrogenation-disproportionation") to decompose the magnetic TRzFe^B phase 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 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, - 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 ère powder with the 2 ème powder obtained at the end of step b) in a mass ratio that 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 compaction step so as to obtain a compacted part, e) the compacted part obtained at the end of step d) is subjected to a pre-sintering step which includes the following sub-steps: - heating from ambient temperature to a l ère temperature between 200°C and 400°C at a heating rate of 1°C to 5°C / minute, - to address said l ère temperature between 200°C and 400°C for a duration of between 1 and 3 hours, - heating of said l ère temperature between 200°C and 400°C up to a 2 ème temperature between 400°C and 500°C at a heating rate of 1°C to 5°C / minute, - level at the said 2 ème temperature between 400°C and 500°C for a duration of between 1 and 3 hours, - heating of said 2 èmetemperature between 400°C and 500°C up to a 3 ème temperature between 550°C and 650°C at a heating rate of 1°C to 5°C / minute, - level at the said 3 ème temperature between 550°C and 650°C for a duration of between 1 and 3 hours, - heating of said 3 ème temperature between 550°C and 650°C up to a 4 ème temperature between 650°C and 750°C at a heating rate of 5°C to 10°C / minute, - level at the aforementioned 4 ème temperature between 650°C and 750°C for a duration between 10 minutes and 30 minutes, f) the pre-sintered part obtained at the end of step e) is subjected to a sintering step in order to obtain a magnet, said sintering step comprises the following sub-steps: - heating of said 4 ème temperature between 650°C and 750°C up to a 5 èmetemperature between 850°C and 1050°C at a heating rate of 5°C to 25°C / minute, - level at the aforementioned 5 ème temperature between 850°C and 1050°C for a period of between 2 hours and 24 hours.

[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] 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 we wish to achieve an overall content of Y% in the final magnet, 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).

[0024] During step f) of sintering the mixture of the l ère powder and 2 èmeIn the powder, a liquid phase forms (by eutectic reaction) between the different metal-based compounds: the TR-rich phase, the magnetic TRzFe^B phase, and the optional non-magnetic TRFe4B4 phase. ère powder. This liquid phase has a high rare earth content, on 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), particularly the heavy rare earth hydrides, are transformed into metallic heavy rare earths, which dissolve in the liquid phase, thus enriching it. These heavy rare earths are now in the form of heavy rare earths. Metallic materials are then available to create a so-called "core-shell" structure, as mentioned above, around the grains of the l ère powder during the densification of the l ère powder and 2 èmepowder that is produced during sintering.

[0025] 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 (obtained from recycled magnets) is 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.

[0026] Step b) of the manufacturing process according to the invention thus makes it possible to eliminate a step of diffusion of these heavy rare earths within the grains of the magnetic phase TRzFe^B of the 2 èmepowder; which is a slow process and would create a concentration gradient within the heavy rare earth-rich grains of the 2 ème powder. This would ultimately allow only a small fraction of the quantity of these heavy rare earths to be utilized. Moreover, in this diffusion scenario, the heavy rare earth content at the periphery of the grains of the ère powder would be lower than the heavy rare earth content in the grains of the 2 ème powder.

[0027] Conversely, 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 localized at the periphery of the grains of the magnetic phase TRzFe^B of the l ère powder with higher concentrations 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 said magnet contains small amounts of heavy rare earths.

[0028] During pre-sintering step e), the compacted part obtained at the end of step d) is dehydrated. This operation consists of removing almost all of the hydrogen contained in the compacted part. The hydrogen is mainly 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 allows for better magnetic properties after sintering.

[0029] More specifically, in order to properly sinter an NdFeB magnet, the hydrogen present inside must first be completely desorbed before it can be optimally densified. This hydrogen desorption is carried out during the pre-sintering step (e). However, it is known that recombination of the NdzFe^B magnetic phase can occur simultaneously with the complete desorption of hydrogen. This is problematic because the heavy rare earth elements would be trapped again in this magnetic phase and would then be less available to diffuse to the periphery of the grains. ère powder. The advantage provided by step b) of hydrogenation-disproportionation would then be lost before even reaching the sintering plateau.

[0030] Therefore, to overcome this problem, it is essential to uncouple hydrogen desorption and grain recombination as much as possible in the magnetic phase NdzFe^B.

[0031] Against all 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 2 ème The powder content can advantageously be between 5% and 50%. This beta phase contains almost all of the boron. The presence of almost all of the boron in the beta phase increases its diffusion distances during the sintering step; this helps to decouple hydrogen desorption and grain recombination from the magnetic NdzFe^B phase.

[0032] Furthermore, the selection of temperature and pressure ranges in step b) allows for the production of micron-sized grains, advantageously between 1 µm and 20 µm. This micron size increases the diffusion distances of boron, iron, and rare earth elements during the sintering step, thus preventing excessively rapid recombination of the grains and allowing the heavy rare earth elements to diffuse properly around the periphery of said grains.

[0033] Finally, the inventors surprisingly discovered that carrying out a pre-sintering step e) and a sintering step f) as described above also helps to desorb hydrogen without recombining the magnetic phase.

[0034] More specifically, implementing these steps (e) of pre-sintering and (f) of sintering according to the parameters described above allows, as soon as desorption is complete, for maximizing the quantity of heavy rare earths in the liquid phase and, as an additional advantage, reaching the sintering plateau as quickly as possible thanks to the heating ramp for reaching this plateau described above. Indeed, as explained above, the heavy rare earth hydrides are transformed into metallic heavy rare earths, and the heavy rare earths are then available to diffuse to the periphery of the grains of the liquid. ère powder.

[0035] Thus, the originality of the present invention lies in the decorrelation of hydrogen desorption and recombination of the grains of the magnetic phase Nd2Fel4B through the selection of parameters as described above for the implementation of steps b) hydrogenation-disproportionation, as well as steps e) pre-sintering and f) sintering.

[0036] Within the framework of the present invention, an additional advantage is to co-fry the l ère powder and the 2 ème powder at a lower temperature, because the heavy rare earths have already been extracted from the grains of the magnetic TRzFe^B phase of the 2 ème powder. This allows the heavy rare earth elements to be located in the extreme periphery of the grains of the l èrepowder and thus to obtain a very marked “core-shell” structure which makes it possible to exceed the gain in coercivity obtained for magnets whose heavy rare earths have been added in the mass, namely 150 kA / m for an addition of heavy rare earths of about 1% by mass.

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

[0041] The l ère powder may comprise in mass percentages expressed in relation to the mass of said l ère powder: - between 90% and 99%, preferably between 95% and 97%, of TRzFe^B 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 non-magnetic phase grains of type TRFe4B4.

[0042] The l ère powder may include, in mass percentages expressed relative to the mass of said l ère 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%.

[0043] As explained above, the mass content of heavy rare earths is less than 1% in the l ère powder.

[0044] The l ère powder preferably has a particle size between 3 pm and 7 pm.

[0045] The l ère 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 l èreThe 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.

[0047] When the l ère powder was obtained wholly or partly from virgin materials, which may have undergone 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 "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.

[0049] The molten material is then poured onto a cooled, rotating wheel. The molten material is thus solidified by quenching. The cooling rate can range from 500 K / s to 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 allows us to obtain a ère powder with a particle size between 50 µm 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 and 5 hours.

[0053] The gas jet grinding stage makes it possible to obtain a èrepowder with a median size between 2 µm and 10 µm, preferably between 3 µm and 6 µm, with a particle size distribution whose ratio of 9 ème decile at the 1st erThe 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 milling stage. They consist of a sealed chamber into which an inert gas, at a pressure between 2 and 8 bar, is introduced through three converging nozzles, and the powder to be ground is fed through a hopper that allows control of the feed rate. The gas flow carries the powder in its wake 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 the largest particles from exiting the milling chamber.

[0054] When the l èrepowder 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 grinding stage can be those described for obtaining the l ère powder made from virgin raw materials.

[0056] The 2 ème powder preferably has a particle size comparable to that of the first powder in order to facilitate mixing of the 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 µm and 7 µm.

[0057] 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 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 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.

[0059] 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 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 with a mass content of heavy rare earths 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] That is why, in a preferred embodiment of the invention, the 2 èmeThe 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 relative to the mass of said 2 ème powder, is between 1% and 10%, preferably between 2% and 5%.

[0062] Advantageously, the 2 ème The powder was obtained from recycled magnets that underwent the following treatment: - a decrepitation step under hydrogen, optionally followed - of a gas jet crusher.

[0063] The hydrogen decrepitation step allows us to obtain a 2 ème coarse powder with a particle size between 50 µm and a few millimeters.

[0064] The technical characteristics of the hydrogen decrepitation and gas jet milling stage for obtaining the 2 èmepowders may be those that have been described for obtaining the l ère powder made from virgin raw materials or recycled magnets.

[0065] When the 2 ème The powder was obtained wholly or partly from recycled magnets, which may have undergone 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 powder. ère powder made from virgin raw materials.

[0066] When the 2 ème powder was obtained wholly or partly from virgin materials, which may have undergone 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 grinding stages can be those described above for obtaining the l ère powder made from virgin raw materials.

[0068] The hydrogenation-disproportionation treatment can be carried out under vacuum, namely by heating the 2nd under vacuum 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.

[0069] The 2 ème powder can be heated to the desired temperature as described above for step b) 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 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] As an example, at a pressure of 0.8 bar, the processing temperature of step b) can be between 925°C and 1025°C.

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

[0074] Following the hydrogenation-disproportionation treatment, the 2 ème The powder can be subjected to hydrogen cooling to prevent the recombination of elements to reform the magnetic TRzFe^B phase, thus preserving the rare-earth hydride mixture with the chemical formula TRH X , iron, iron boride (FezB) and beta phase, up to room temperature (i.e. about 20°C).

[0075] Optionally, before carrying out step c) of the manufacturing process according to the invention, the 2 ème The 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 as efficiently as possible in the liquid phase. This step may be necessary if the particles of the 2 èmePowders may re-agglomerate during the hydrogenation-disproportionation treatment, but also if their microstructure is not optimal, specifically 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 milling, planetary milling, attrition milling and cryogenic milling.

[0076] 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 an hour, so as to obtain a homogeneous mixture.

[0077] The mixture obtained at the end of step c) can be poured into a mold (presenting the negative shape of the magnet to be manufactured) to carry out step d) of compaction.

[0078] Step d) of compaction can be carried out using transverse, axial, cold isostatic compaction, or rubber isostatic pressing (also known as RIP, the acronym for "Rubber Isostatic Pressing"), in order to obtain a compacted part referred to as a "green part". For example, the mixture is compacted by applying a uniaxial pressure between 50 MPa and 300 MPa.

[0079] The density of the compacted part obtained at the end of step d) is advantageously between 50% and 70% of the theoretical density of the final magnet.

[0080] In an advantageous embodiment of the invention, after step c) and before compaction step d), the mixture obtained after 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 magnetic field does not exceed 8 Tesla. This application of the magnetic field can be carried out after the mixture obtained after step c) has been poured into the mold.

[0081] Preferably, step e) of pre-sintering is carried out under secondary vacuum to avoid demixing of the TRFeB phase, in the presence of hydrogen. Secondary vacuum corresponds to a pressure below 10 -4 mbar, preferably less than 5.10 -5 mbar.

[0082] Next, step f) of sintering is carried out in order 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).

[0083] Step f) of sintering is advantageously carried out in an environment containing substantially no oxygen, water or hydrogen, preferably under secondary vacuum.

[0084] 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 -3 .

[0085] 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 about 30°C / min, from the sintering temperature to ambient temperature or, where applicable, to the temperature of the start of the optional annealing step described below.

[0086] The magnet obtained at the end of step f), if applicable after cooling if this is implemented, can then be subjected to an annealing step.

[0087] Indeed, annealing increases the magnet's resistance to demagnetization. Those skilled in the art know the conditions for carrying out the annealing process.

[0088] 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.

[0089] 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.

[0090] After 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.

[0091] 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: - 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.

[0092] These optional steps carried out after step f) of sintering are perfectly within the reach of a person skilled in the art.

[0093] 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 for manufacturing a magnet according to the invention.

[0094] EXPERIMENTAL SECTION:

[0095] Preparation of the l ère powder

[0096] One l ère powder comprising in mass percentages expressed in relation 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 in the following way.

[0097] 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 "base charge").

[0098] This basic 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 in order to obtain a molten pool.

[0099] The molten material was poured onto a water-cooled copper-based wheel with a rotation speed that allowed 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 container cooled by circulating water in order to cool them down to room temperature.

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

[0102] The decrepitation step was carried out as follows. The chamber was placed under primary vacuum (i.e., a pressure less than 1 mbar, preferably less than 10⁻¹⁰). 2 mbar), then filled with hydrogen to reach a pressure of 2 bars. 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, then cooled to ambient temperature (approximately 20°C) under argon.

[0103] The resulting coarse powder was then homogenized in a mixer containing 0.05% zinc stearate by mass, the percentage of zinc stearate being expressed relative to the mass of the coarse powder. Zinc stearate acts as a lubricant, facilitating the formation of a fluidized bed during the jet milling stage. This homogenization process lasted one and a half hours.

[0104] 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 ère powder whose median particle size measured online by a laser particle size analyzer was 5 pm.

[0105] Preparation of the 2 ème powder

[0106] A 2 èmepowder comprising in mass percentages expressed relative to the mass of said 2 ème powder: - Nd: 21.7%, - Pr: 6.6%, - Dy: 4.0%, - B: 1.07%, - Co: 1.01%, - Al: 0.69%, - Cu: 0.15%, - Fe: 100% complement, was prepared in the following way.

[0107] We initially had recycled magnets measuring 8 x 28 x 5 mm which included the different metals as detailed above and in the quantities also indicated above.

[0108] The recycled magnets were placed in a sealed chamber inside a furnace for the decrepitation stage. 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 removed the metallic coatings.

[0109] The resulting coarse powder was heated under hydrogen in the same chamber to 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.

[0110] The resulting powder was then introduced under a controlled atmosphere into a grinding bowl, along with 8 mm diameter stainless steel balls at a 1:2 ball-to-powder ratio. 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 2 ème powder was between 1 pm and 20 pm.

[0111] Next, 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 that depends on the heavy TR content of the 2 èmepowder for 30 minutes in a mixer, in a controlled atmosphere chamber.

[0112] 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.

[0113] The mixture obtained at the end of step c) was then introduced into different cylindrical molds of 22 mm height and 14 mm internal diameter made of rubber which were subjected to a magnetic field of 7 Tesla to orient the particles.

[0114] Next, step d) 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.

[0115] Then, step e) of pre-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 l ère temperature of 300°C, then hold for 2 hours at said temperature ère temperature of 300°C, - heating at 5°C / min of the l ère temperature 300°C up to a 2 ème temperature of 500°C, then plateau for 2 hours at said temperature. ème temperature of 500°C, - heating at 5°C / min of said 2 ème temperature from 500°C up to a 3 ème temperature of 600°C, then held at 600°C for 2 hours. - heating at 5°C / min of said 3 ème temperature of 600°C up to a 4 ème temperature of 700°C, then hold for 30 minutes at said 4 ème temperature of 700°C.

[0116] Next, step f) of sintering of the manufacturing process according to the invention was carried out on these pre-sintered parts under secondary vacuum according to the following thermal profile: - heating of said 4 èmetemperature of 700°C up to a 5 ème temperature of 975°C at a heating rate of 10°C / minute - level at the aforementioned 5 ème temperature of 975°C for a period of 12 hours.

[0117] At the end of this last level at the aforementioned 5 ème At a temperature of 975°C, argon was introduced until an absolute pressure of 2 bar was reached in order to obtain magnets.

[0118] Next, the magnets were subjected to cooling with a cooling rate of 15°C / min from 975°C to 30°C.

[0119] Then, the magnets were subjected to a secondary vacuum annealing stage 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.

[0120] The resulting cylindrical magnets were machined using a grinder and a diamond grinding wheel to remove the oxide layer and obtain parallel surfaces.

[0121] Furthermore, so-called "comparative" magnets were manufactured from 100% of the l ère powder, and in the same way as these magnets according to the invention.

[0122] 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 / m 3 ), - Dy is the mass content of dysprosium (expressed in %), - gain in coercivity (expressed in kA / m)

[0123] Table 1

[0124] To understand the value of the invention, it is necessary to estimate the gain in coercivity per percentage of Dy in the final magnets. Compared to the comparative magnets, the gain The coercivity of the magnets according to the invention is 191 kA / m. Relative to the amount of Dy introduced, this represents a gain of 191 kA / m / %Dy. As explained above, it is known that when Dy from virgin materials is introduced into the manufacture of a magnet, this gain in coercivity is only about 150 kA / m / %Dy. This value is lower than the gain in coercivity obtained with magnets manufactured according to the manufacturing process of the invention. These experiments demonstrate that the present invention makes it possible to obtain magnets with an optimized gain in coercivity.

Claims

CLAIMS 1. Method for manufacturing a highly coercive 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 compacting step so as to obtain a compacted part, e) the compacted part obtained at the end of step d) is subjected to a compacting step pre-sintering which includes the following sub-steps: - heating from room temperature to a l ère temperature between 200°C and 400°C at a heating rate of 1°C to 5°C / minute, - level with the said l ère temperature between 200°C and 400°C for a duration between 1 hour and 3 hours, - heating of said l ère temperature between 200°C and 400°C up to a 2 ème temperature between 400°C and 500°C at a heating rate of 1°C to 5°C / minute, - landing at said 2 ème temperature between 400°C and 500°C for a duration between 1 hour and 3 hours, - heating of said 2 ème temperature between 400°C and 500°C up to a 3 ème temperature between 550°C and 650°C at a heating rate of 1°C to 5°C / minute, - level at said 3 ème temperature between 550°C and 650°C for a duration between 1 hour and 3 hours, - heating of said 3 ème temperature between 550°C and 650°C up to a 4 ème temperature between 650°C and 750°C at a heating rate of 5°C to 10°C / minute, - level at said 4 èmetemperature between 650°C and 750°C for a period between 10 minutes and 30 minutes, f) the pre-sintered part obtained at the end of step e) is subjected to a sintering step so as to obtain a magnet, said sintering step comprises the following sub-steps: - heating of said 4 ème temperature between 650°C and 750°C up to 5 ème temperature between 850°C and 1050°C at a heating rate of 5°C to 25°C / minute, - level at the said 5 ème temperature between 850°C and 1050°C for a period of between 2 hours and 24 hours.

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 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%.

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 said 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 selected grinding technique among 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 at the end of step c) and before carrying out step d) of compacting, the mixture obtained at the end of step c) is subjected to a magnetic field, preferably a magnetic field greater than 1 Tesla.

12. Manufacturing method according to any one of claims 1 to 11, 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.

13. Manufacturing process according to any one of claims 1 to 12, characterized in that the duration of the hydrogenation-disproportionation treatment of step b) is between 10 minutes and 3 hours.

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