Manufacturing process of a highly coercive magnet

Abstract

Method for manufacturing a highly coercive magnet The invention relates to a method for manufacturing a highly coercive magnet 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) subjecting the 2nd powder to a hydrogenation-disproportionation treatment, c) mixing the 1st powder with the 2nd powder obtained at the end of step b), d) subjecting the mixture obtained at the end of step c) to a compacting step so as to obtain a compacted part, e) subjecting the compacted part obtained at the end of step d) to a pre-sintering step, f) subjecting the pre-sintered part obtained at the end of step e) to a sintering step.

Description

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

[0002] In the context of the present invention, the term “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 chosen from rare earths (in particular rare earths: La, Ce, Pr, Nd, Dy, Gd, Tb, Ho).

[0004] TR is predominantly neodymium (Nd). Therefore, the most common example of these permanent magnets is the NdFeB type magnet in which the 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), permanent magnets of the NdFeB type are commonly used in various applications including magnets in the motors of electric or hybrid vehicles, electrical appliances (e.g. household appliances or air conditioning), electronic devices (e.g. hard drives) and wind turbine generators.

[0006] However, given the current problems of environmental protection and the depletion of natural resources, particularly rare earths, in order to satisfy a constantly growing demand which may at least 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, devices or motors, once these devices are no longer in use, in order to limit the production of these magnets using only virgin materials extracted from deposits.

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

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

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

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

[0011] A 3rd known recycling route is the so-called "powder" route during which the recovered magnets (for example from out-of-use devices) are reduced to the form of powders. 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 being able to readjust the compositions of the new magnets manufactured from recycled magnets by mixing different powders and thus having a certain freedom as to the final form of these new magnets. Furthermore, in the case of sintered magnets, this recycling route is based on processes for producing magnets by powder metallurgy which are already implemented and perfectly mastered.

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

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

[0015] It is known from the manufacturing processes of permanent magnets comprising heavy rare earths that the substitution of neodymium (and to a lesser extent praseodymium) by heavy rare earths in the entire magnetic phase TR2Fei4B is accompanied by a reduction in remanence which can be limited by using a mixture of a powder low in heavy rare earths and a powder rich in heavy rare earths or by diffusing the heavy rare earths from the surface of the sintered magnets and along the grain boundaries. In this way, the heavy rare earths are preferentially located at the periphery of the magnetic grains, that is to say in the regions most critical for demagnetization, so as to obtain a structure known as "core-shell". The peripheral region of a magnetic grain is therefore the region into which the heavy rare earths have diffused during the manufacture of the magnet.

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

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

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

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

[0020] The method for manufacturing a highly coercive magnet, preferably from recycled magnets, is based on the so-called "powder" recycling route and makes it possible to obtain 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 said heavy rare earths.

[0021] The subject of the invention is a method for manufacturing a highly coercive magnet (preferably from recycled magnets), which comprises at least the following steps: a) we have: - a first powder comprising grains of a magnetic phase TR2Fei4B, metal-based compounds and a grain boundary phase rich in rare earth, and op- tionally grains of a non-magnetic phase TRFe4B4, said first powder being free of heavy rare earth or the mass content of heavy rare earth expressed in relation to the mass of the first powder being less than 1%, - 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 as “hydrogenation disproportionation”) so as to decompose the magnetic phase TR2Fei4B 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 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 1st powder is mixed with the 2nd powder obtained at the end of step b) in a mass ratio which depends on the heavy rare earth content of the 2nd powder so as to obtain a mixture,

[0023] 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 pre-sintering step which comprises the following sub-steps: - heating from room temperature to a first temperature between 200°C and 400°C at a heating rate of 1°C to 5°C / minute, - hold at the said first temperature between 200°C and 400°C for a period between 1 hour and 3 hours,

[0024] - heating from said 1st temperature between 200°C and 400°C to a 2nd temperature between 400°C and 500°C at a heating rate of 1°C to 5°C / minute, - holding at said 2nd temperature between 400°C and 500°C for a period between 1 hour and 3 hours, - heating from said 2nd temperature between 400°C and 500°C to a 3rd temperature between 550°C and 650°C at a heating rate of 1°C to 5°C / minute, - holding at said 3rd temperature between 550°C and 650°C for a period between 1 hour and 3 hours, - heating from said 3rd temperature between 550°C and 650°C to a 4th temperature between 650°C and 750°C at a heating rate of 5°C to 10°C / minute, - holding at said 4th temperature between 650°C and 750°C for a period of between 10 minutes and 30 minutes,

[0025] 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 from said 4th temperature between 650°C and 750°C to a 5th temperature between 850°C and 1050°C at a heating rate of 5°C to 25°C / minute, - holding at said 5th temperature between 850°C and 1050°C for a period of between 2 hours and 24 hours.

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

[0028] During step f) of sintering the mixture of the 1st powder and the 2nd powder, a liquid phase is formed (by eutectic reaction) between the different metal-based compounds, the TR-rich phase, the magnetic phase TR2Fei4B and the optional non-magnetic phase TRFe4B4 of the 1st powder. This liquid phase has a high mass content of rare earths, of the order of 90% by mass at 700°C, 80% at 900°C and 70% at 1000°C. During the temperature increase under vacuum, the rare earth hydrides formed in step b), in particular the heavy rare earth hydrides, are transformed into metallic heavy rare earths which dissolve in said liquid phase so as to enrich it.These heavy rare earths now in the form of metallic heavy rare earths are then available to create a so-called "core-shell" structure as mentioned above around the grains of the 1st powder during the densification of the 1st powder and the 2nd powder which occurs during sintering.

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

[0030] Step b) of the manufacturing method according to the invention thus makes it possible to dispense with a step of diffusion of these heavy rare earths within the grains of the magnetic phase TR2Fei4B of the 2nd powder; which is a slow process and which would create a concentration gradient within the grains rich in heavy rare earths of the 2nd powder. This would make it possible to ultimately only recover a small fraction of the quantity of said heavy rare earths. In addition, in this case of diffusion, the content of heavy rare earths at the periphery of the grains of the 1st powder would be lower than the content of heavy rare earths in the grains of the 2nd powder.

[0031] On the contrary, with the manufacturing method according to the invention, the extraction of the 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 at the periphery of the grains of the magnetic phase TR2Fei4B of the 1st powder at higher contents than the heavy rare earth content in the grains of the 2nd powder (preferably which has been 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.

[0032] During pre-sintering step e), the compacted part obtained at the end of step d) is dehydrided. This operation consists of eliminating almost all of the hydrogen contained in the compacted part. The hydrogen is essentially present in the form of hydrides TRHx with x close to 2. This involves reducing the overall hydrogen content in the compacted part, which is for example of the order of 2000 ppm (0.2% by mass) to a value for example less than 100 ppm (0.01% by mass), preferably less than 50 ppm (0.005% by mass). This operation makes it possible to obtain better magnetic properties after sintering.

[0033] More precisely, in order to properly sinter an NdFeB magnet, it is first necessary to completely desorb the hydrogen present inside it in order to then be able to densify it optimally. This desorption of hydrogen is carried out during the pre-sintering step e). However, it is known that the recombination of the Nd2Fe[4B] magnetic phase can take place at the same time as the total desorption of hydrogen. This is problematic because the heavy rare earths would again be trapped in said magnetic phase and would then be less available to diffuse to the periphery of the grains of the 1st powder. The advantage provided by the hydrogenation-disproportionation step b) would then be lost before even reaching the plateau of sintering.

[0034] Therefore, to overcome this problem, it is essential to decorrelate as well as possible the desorption of hydrogen and the recombination of the grains of the magnetic phase Nd2Fei4B.

[0035] Against all expectations, the inventors discovered that the selection of the temperature and pressure intervals in step b) as detailed above makes it possible to obtain a beta phase whose mass content, expressed relative to the total mass of the 2nd powder, can advantageously be between 5% and 50%. This beta phase contains almost all of the boron. However, the presence of almost all of the boron in the beta phase increases the diffusion distances of the latter during the sintering step; which contributes to decorrelating the desorption of hydrogen and the recombination of the grains of the magnetic phase Nd2Fei4B.

[0036] Furthermore, the selection of the temperature and pressure intervals in step b) makes it possible to obtain, at the end of step b), grains of micron size, advantageously between 1 μm and 20 μm. This micron size makes it possible to increase the diffusion distances of the boron, iron and rare earths during the sintering step, and thus to avoid excessively rapid recombination of the grains and to allow the heavy rare earths to diffuse correctly at the periphery of said grains.

[0037] Finally, the inventors have surprisingly discovered that carrying out a pre-sintering step e) and a sintering step f) as described above also contributes to desorbing the hydrogen without recombining the magnetic phase.

[0038] More precisely, the implementation of these steps e) of pre-sintering and f) of sintering according to the parameters as described above makes it possible, as soon as the desorption is finished, to maximize the quantity of heavy rare earths in the liquid phase and, as an additional advantage, to reach 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 at the periphery of the grains of the 1st powder.

[0039] Thus, the originality of the present invention lies in the decorrelation of the desorption of hydrogen and the recombination of the grains of the magnetic phase Nd2Fel4B thanks to the selection of the parameters as described above for the implementation of the steps b) of hydrogenation-disproportionation, as well as the steps of e) pre-sintering and f) of sintering.

[0040] In the context of the present invention, an additional advantage is to co-sinter the 1st powder and the 2nd powder at a lower temperature, because the heavy rare earths have already been extracted from the grains of the magnetic phase TR2Fei4B of the 2nd powder. This makes it possible to locate the heavy rare earths at the extreme periphery of the grains of the 1st powder and therefore 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 approximately 1% by mass.

[0041] This gain in terms of dilution of the manufacturing process according to the invention is particularly interesting, because it makes it possible to envisage the recycling, in a short route, of quantities of recycled magnets rich in heavy rare earths, which will remain limited and to make it possible to manufacture significant quantities of magnetically efficient magnets with a minimum input of critical raw materials.

[0042] The various technical characteristics of the manufacturing method according to the invention are described below in more detail.

[0043] In the context of the present invention, the term "TR-rich grain boundary phase" means a metallic phase containing more than 70% by mass of rare earth combined with metals, for example metals chosen from iron, copper and aluminum.

[0044] The metal-based compounds of the 1st powder may comprise metals selected from iron, copper, aluminum, gallium, titanium and zirconium.

[0045] The 1st powder may comprise in mass percentages expressed relative to the mass of said 1st powder: - between 90% and 99%, preferably between 95% and 97%, of grains of the magnetic phase of type TR2Fei4B, - between 0.5% and 3%, preferably between 0.5% and 2%, of the metal-based compounds,

[0046] - between 1% and 10%, preferably between 2% and 5%, of the rich grain boundary phase in rare earth, - optionally between 0.1% and 5%, preferably between 0.1% and 1%, of grains of the non-magnetic phase of type TRFe4B4.

[0047] The 1st powder may comprise, in mass percentages expressed relative to the mass of said 1st 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%.

[0048] As explained above, the mass content of heavy rare earths is less than 1% in the first powder.

[0049] The first powder preferably has a particle size of between 3 μm and 7 μm.

[0050] The first powder may have been obtained from: - virgin raw materials (e.g. pure metals and / or alloys) which are completely free of heavy rare earths or whose mass content is less than 1%, or - recycled magnets which are completely free of heavy rare earths or whose mass content is less than 1%, or - a mixture of virgin raw materials and recycled magnets which are completely free of heavy rare earths or whose mass content is less than 1%.

[0051] When the first powder has been obtained totally or partly from virgin raw materials, the latter are chosen from pure metals and / or alloys. Preferably, these are pure metals.

[0052] When the first powder has been obtained totally 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 under the English name: Jet Mill type grinding).

[0053] First of all, 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.

[0054] The molten material bath 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 molten ribbons obtained can have a thickness of between 0.1 and 0.5 mm, preferably between 0.15 and 0.35 mm.

[0055] The decrepitation step under hydrogen makes it possible to obtain a first powder whose particle size is between 50 μm and a few millimeters.

[0056] Decrepitation under hydrogen can be carried out at a temperature between 10°C and 500°C, preferably between 20°C and 150°C, and at a hydrogen pressure between 0.01 MPa and 5 MPa, preferably between 0.08 MPa and 0.25 MPa.

[0057] The duration of the decrepitation step under hydrogen can be between 1 hour and 5 hours.

[0058] The gas jet grinding step makes it possible to obtain a first powder whose median size is between 2 pm and 10 pm, preferably between 3 pm and 6 pm, with a particle size fineness whose ratio of 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 grinders marketed by the company Hosokawa-Alpine under The trade names AFG100, AFG200 and AFG400 can be used for this gas jet grinding stage. They comprise a sealed chamber into which an inert gas under a pressure of between 2 and 8 bars is introduced through three converging nozzles, and the powder to be ground through a hopper to control the feed rate. The gas flow carries the powder in its wake and releases it by passing through a vortex generated by a system called a "cyclone". In order to improve the particle size fineness, this equipment can be equipped with an inertial selector which prevents the largest particles from leaving the grinding chamber.

[0059] When the first powder has been obtained totally or partly from recycled magnets, the latter may have been subjected to a step of decrepitation under hydrogen and / or gas jet grinding.

[0060] The technical characteristics of the hydrogen decrepitation and gas jet grinding step may be those which have been described for obtaining the first powder from virgin raw materials.

[0061] The 2nd powder preferably has a particle size comparable to that of the first powder so as to facilitate the mixing of the 1st powder with the 2nd powder in step c) of the manufacturing process according to the invention. The 2nd powder preferably has a particle size between 3 μm and 7 μm.

[0062] The 2nd powder may comprise, in mass percentages expressed relative to the mass of said 2nd 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%.

[0063] This means that the 2nd powder comprises, in mass percentages expressed relative to the mass of said 2nd powder, between 1% and 10%, preferably between 2% and 5%, of heavy rare earth.

[0064] The 2nd powder may have been obtained from: - virgin raw materials (e.g. pure metals and / or alloys) with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%, or - recycled magnets with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%, or - a mixture of virgin raw materials and recycled magnets with a mass content of heavy rare earths between 1% and 10%, preferably between 2% and 5%.

[0065] As explained above, the method for manufacturing a magnet according to the invention preferably uses recycled magnets.

[0066] This is why, in a preferred embodiment of the invention, the 2nd powder was obtained solely from recycled magnets. These recycled magnets comprise heavy rare earths. The mass percentage of these heavy rare earths, expressed relative to the mass of said 2nd powder, is between 1% and 10%, preferably between 2% and 5%.

[0067] 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.

[0068] The decrepitation step under hydrogen makes it possible to obtain a 2nd coarse powder whose particle size is between 50 μm and a few millimeters.

[0069] The technical characteristics of the hydrogen decrepitation and gas jet grinding step for obtaining the 2nd powder can be those which have been described for obtaining the 1st powder from virgin raw materials or recycled magnets.

[0070] When the 2nd powder has been obtained totally or partly from recycled magnets, the latter may have been subjected to a step of decrepitation under hydrogen and / or gas jet grinding. The technical characteristics of the step of decrepitation under hydrogen and gas jet grinding may be those which have been described for obtaining the 1st powder from virgin raw materials.

[0071] When the 2nd powder has been obtained totally 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.

[0072] The technical characteristics of the wheel casting, hydrogen decrepitation and gas jet grinding steps may be those described above for obtaining the first powder from virgin raw materials.

[0073] The hydrogenation-disproportionation treatment may be carried out under vacuum, namely by heating the 2nd powder under vacuum to a temperature as described above before the introduction of hydrogen. In another embodiment of the invention, the 2nd powder is heated to a temperature as described above after the introduction of hydrogen.

[0074] The 2nd powder can be heated until the desired temperature is obtained as described above for step b) with a heating rate of between 1°C / minute and 30°C / minute.

[0075] In step b), the temperatures as described above are appropriate so as not to cause agglomeration of particles between them.

[0076] In step b), the pressures as described above are appropriate so as not to cause a loss of texture of the magnet.

[0077] For example, at a pressure of 0.8 bar, the treatment temperature of step b) may be between 925°C and 1025°C.

[0078] The duration of the hydrogenation-disproportionation treatment can be between 10 minutes and 3 hours. This duration depends on the hydrogenation-disproportionation treatment temperature: it is shorter at high temperature. Indeed, too long a treatment duration at high temperature would cause macroscopic heterogeneities within the microstructure of the magnet obtained with the manufacturing process.

[0079] At the end of 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 therefore preserve the mixture of rare earth hydrides of chemical formula TRHX, iron, iron boride (Fe2B) and the beta phase, up to room temperature (i.e. approximately 20°C).

[0080] Optionally, before carrying out step c) of the manufacturing method according to the invention, the 2nd powder is fractured in order to improve the contact between the rare earth hydrides and the liquid phase during step f) so that the heavy metallic rare earths dissolve as well as possible in the liquid phase. This step may be necessary if the particles of the 2nd powder have reagglomerated during the hydrogenation-disproportionation treatment, but also if their microstructure is not the most suitable, namely in particular if the hydrides are located within the grains rather than at the periphery. The 2nd powder may have been fractured by at least one grinding technique chosen from gas jet grinding, planetary grinding, attrition grinding and cryogenic grinding.

[0081] 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.

[0082] 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 carry out compacting step d).

[0083] Compacting step d) can be carried out using transverse, axial, cold isostatic compaction or isostatic pressing with rubber (also known as the name "RIP", namely the English acronym for "Rubber Isostatic Pressing"), so as to obtain a compacted part called a "green part". For example, the mixture is compacted by applying a uniaxial pressure of between 50 MPa and 300 MPa.

[0084] 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.

[0085] In an advantageous embodiment of the invention, at the end of step c) and before carrying out the compacting step d), the mixture obtained at the end of step c) is subjected to a magnetic field to orient the grains of the mixture and ultimately obtain an anisotropic magnet which has a high remanence. Preferably, the magnetic field is greater than 1 Tesla, more preferably greater than 2 Tesla. Preferably, the magnetic field does not exceed 8 Tesla. This application of the magnetic field can be carried out when the mixture obtained at the end of step c) has been poured into the mold.

[0086] Preferably, pre-sintering step e) is carried out under secondary vacuum so as to avoid demixing of the TRFeB phase, and in the presence of hydrogen. A secondary vacuum corresponds to a pressure of less than 104mbar, preferably less than 5.105mbar.

[0087] Then, step f) of sintering is carried out so as to obtain a magnet. This involves the consolidation by heat treatment of the compacted part, with possibly a partial or total melting of some of its constituents (but not all of its constituents, so that the compacted part is not transformed into a liquid mass).

[0088] The sintering step f) is advantageously carried out in an environment containing substantially no oxygen, water or hydrogen, preferably under secondary vacuum.

[0089] 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.

[0090] In an advantageous embodiment of the invention, at the end of step f), the magnet thus obtained is subjected to cooling. Preferably, this is rapid cooling, namely greater than 20°C / min, more preferably approximately 30°C / min, from the sintering temperature to room temperature or, where appropriate, to the temperature at the start of the optional annealing step described below.

[0091] The magnet obtained at the end of step f), where appropriate at the end of cooling if this is carried out, can then be subjected to an annealing step.

[0092] Indeed, annealing makes it possible to increase the resistance to demagnetization of the magnet. A person skilled in the art is familiar with the conditions for carrying out the step of annealing.

[0093] For example, if the magnet has been subjected to rapid cooling to a temperature of 50°C, the annealing step may comprise the following thermal profile: - heating from 50°C to 820°C at 5°C / min; - a stage at 820°C for 2 hours; - cooling from 820°C to 50°C at 20°C / min; - heating from 50°C to a temperature between 460°C and 650°C at 5°C / min; - a stage at a temperature between 460°C and 650°C for 2 hours; - cooling from a temperature between 460°C and 650°C to 50°C at 30°C / min.

[0094] In an advantageous embodiment of the invention, at the end of the sintering step f), where appropriate at the end of the cooling if this is implemented or of the annealing step if this is implemented, the magnet can be machined and / or undergo a surface treatment, for example polishing or the application of a coating to prevent oxidation and corrosion.

[0095] At the end of the manufacturing process described above, a magnet does not have its own magnetization. The magnet can thus be subjected to a complementary magnetization: for example, the magnet can be subjected to a magnetization field parallel to the direction of alignment of the magnetic field used to orient the grains of the mixture and obtain an anisotropic magnet as described above. The magnetic field can have an intensity greater than 4 Tesla, preferably greater than 5 Tesla. These high values ​​are generally obtained in pulsed mode.

[0096] In other words, at the end of sintering step f), the magnet thus obtained can be subjected to at least one of the following steps (namely one of these steps or any combination thereof) chosen from: - cooling, for example a cooling step as described above; - an annealing step, for example an annealing step as described above; - a machining and / or surface treatment step, for example a machining and / or surface treatment step as described above; - a complementary magnetization step, for example a complementary magnetization step as described above.

[0097] These optional steps carried out after the sintering step f) are perfectly within the reach of those skilled in the art.

[0098] 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, a form of implementation of the method for manufacturing a magnet according to the invention.

[0099] EXPERIMENTAL PART:

[0100] Preparation of the I ^powder

[0101] A 1st powder comprising in mass percentages expressed relative to the mass of said 1st 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 as follows.

[0102] Initially, virgin raw materials were available in massive form of the various metals as detailed above and in the quantities also indicated above (in other words the “basic charge”).

[0103] This base charge was heated. The melting was carried out under partial pressure of argon (400 mbar) in an alumina crucible at a maximum temperature of 1450°C so as to obtain a molten bath.

[0104] The bath of molten material was poured onto a water-cooled copper-based wheel having a rotation speed allowing the production of crystallized ribbons with a thickness of between 150 pm and 400 pm, with an average thickness of 250 pm.

[0105] The ribbons thus obtained were collected in a tank cooled by circulating water so as to cool them to room temperature.

[0106] The ribbons were then placed in a sealed enclosure of a furnace for a decrepitation step.

[0107] The decrepitation step was carried out in the following manner. The enclosure was placed under primary vacuum (i.e. a pressure of less than 1 mbar, preferably less than 102 mbar), then filled with hydrogen to reach a pressure of 2 bars. Next, the enclosure was placed under primary vacuum to evacuate the hydrogen, then it was heated to a temperature of 550°C for 2 hours to obtain partial dehydration, then cooled to room temperature (i.e. approximately 20°C) under argon.

[0108] The coarse powder thus obtained was then homogenized in a mixer into which 0.05% by mass of zinc stearate had been introduced, the mass percentage of zinc stearate being expressed relative to the mass of said coarse powder. Zinc stearate is a lubricant which facilitates the establishment of a fluidized bed during the gas jet milling step. This homogenization lasted 1 hour and a half.

[0109] 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 granulometer was 5 μm.

[0110] Preparation of the 2™e-powder

[0111] A 2nd powder comprising in mass percentages expressed relative to the mass of said 2nd powder: - Nd: 21.7%, - Pr: 6.6%, - Dy: 4.0%, - B: 1.07%, - Co: 1.01%, - AI: 0.69%, -Cu:0.15%, - Fe: 100% complement, was prepared in the following manner.

[0112] Initially, recycled magnets with dimensions of 8 x 28 x 5 mm were available, which included the various metals as detailed above and in the quantities also indicated above.

[0113] The recycled magnets were placed in a sealed enclosure of a furnace for the decrepitation step. The enclosure was placed under primary vacuum, then filled with hydrogen to reach a pressure of 0.8 bars. This treatment made it possible to hydride the entire material but also to peel off the metal coatings.

[0114] The coarse powder thus obtained was heated under hydrogen in the same enclosure at 950°C for 3 hours under 0.8 bar of hydrogen after a heating ramp of 5°C / min. The whole was then cooled naturally under hydrogen to room temperature.

[0115] The powder obtained was then introduced under a controlled atmosphere into a grinding bowl, along with 8 mm diameter stainless steel balls for a ball / powder ratio of 1:2. This bowl was immersed in a liquid nitrogen bath until thermalization. The whole was then ground in a vibrating mill, then the contents of the bowl were transferred into a glove box. The size of the 2nd powder was between 1 μm and 20 μm.

[0116] Then, according to step c) of the manufacturing process, the 1st powder was mixed with the 2nd powder obtained at the end of step b) in a mass ratio which depends on the heavy TR content of the 2nd powder for 30 minutes in a mixer, in an enclosure under a controlled atmosphere.

[0117] 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.

[0118] The mixture obtained at the end of step c) was then introduced into different cylindrical rubber molds 22 mm high and 14 mm in internal diameter which were subjected to a magnetic field of 7 Tesla to orient the particles.

[0119] Then, step d) of compacting the manufacturing process according to the invention was carried out by subjecting the mixture contained in these different molds to cold isostatic compaction at 1500 bar so as to obtain compacted parts.

[0120] Then, step e) of pre-sintering of the manufacturing process according to the invention was carried out on these parts compacted under secondary vacuum according to the following thermal profile: - heating at 5°C / min from room temperature to the first temperature of 300°C, then holding for 2 hours at the said first temperature of 300°C, - heating at 5°C / min from the first temperature 300°C to a second temperature of 500°C, then holding for 2 hours at said second temperature of 500°C, - heating at 5°C / min from the said 2nd temperature of 500°C to a 3rd temperature of 600°C, then holding for 2 hours at 600°C, - heating at 5°C / min from said 3rd temperature of 600°C to a 4th temperature of 700°C, then holding for 30 minutes at said 4th temperature of 700°C.

[0121] Then, 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 from said 4th temperature of 700°C to a 5th temperature of 975°C at a heating rate of 10°C / minute, - hold at the said 5th temperature of 975°C for a period of 12 hours.

[0122] At the end of this last stage at said 5th temperature of 975°C, argon was introduced until an absolute pressure of 2 bar was reached in order to obtain magnets.

[0123] Then, the magnets were subjected to cooling with a cooling rate of 15°C / min from 975°C to 30°C.

[0124] Then, the magnets were then subjected to a secondary vacuum annealing step according to the following thermal profile: - heating at 5°C / min from 50°C to 820°C, - stage at 820°C for 2 hours, - cooling at 20°C / min from 820°C to 50°C, - heating at 5°C / min from 50°C to 500°C, - temperature hold at 500°C for 2 hours, - cooling at 15°C / min from 500°C to room temperature.

[0125] The magnets thus obtained in cylindrical shape were machined by means of a rec- and a diamond wheel to remove the oxide layer and obtain parallel surfaces.

[0126] Furthermore, so-called “comparative” magnets were manufactured from 100% of the first powder, in the same way as these magnets according to the invention.

[0127] The magnetic properties of the magnets according to the invention and of the comparative magnets are detailed in Table 1 below in which: - Br is the remanence (expressed in T), - Hcj is the coercivity (expressed in kA / m), - (BH)max is the maximum energy product (expressed in kJ / m3), - Dy is the mass content of dysprosium (expressed in %), - coercivity gain (expressed in kA / m)

[0128] [Tableauxl] Br (T) Hq (kA / m) (kl / ni3) Dy (% by mass) Coercivity gain (kA / m) Comparative magnets 1.29 1305 322 Ô - Magnets according to the invention 1.18 1496 266 1.00 191

[0129] To understand the interest 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 in coercivity of the magnets according to the invention is 191 kA / m. Related to the quantity 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 in 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 method of the invention. These experiments demonstrate that the present invention makes it possible to obtain magnets with an optimized gain in coercivity.

Claims

1. Claims Method for manufacturing a highly coercive magnet, characterized in that it comprises at least the following steps: a) we have: - a 1st powder comprising grains of a magnetic phase TR2 Fei4B (“TR” designating an element or a combination of two or more elements chosen from rare earths), metal-based compounds and a grain boundary phase rich in rare earth, and optionally grains of a non-magnetic phase TRFe4B4, said 1st powder being free of heavy rare earth or the mass content of heavy rare earth expressed in relation to the mass of the 1st powder being less than 1%, - a 2nd powder 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, 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 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, - 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 1st powder is mixed with the 2nd powder obtained at the end of step b) in a mass ratio which depends on the heavy rare earth content of the 2nd 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 pre-sintering step which comprises the following sub-steps: - heating from ambient temperature to a first temperature between 200°C and 400°C at a heating rate of 1°C to 5°C / minute, - holding at said first temperature between 200°C and 400°C for a period between 1 hour and 3 hours, - heating from said first temperature between 200°C and 400°C to a second temperature between 400°C and 500°C at a heating rate of 1°C to 5°C / minute, - holding at said 2nd temperature between 400°C and 500°C for a period of between 1 hour and 3 hours, - heating from said 2nd temperature between 400°C and 500°C to a 3rd temperature between 550°C and 650°C at a heating rate of 1°C to 5°C / minute, - holding at said 3rd temperature between 550°C and 650°C for a period between 1 hour and 3 hours, - heating from said 3rd temperature between 550°C and 650°C to a 4th temperature between 650°C and 750°C at a heating rate of 5°C to 10°C / minute, - holding at said 4th temperature between 650°C and 750°C for a period of 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 from said 4th temperature between 650°C and 750°C to a 5th temperature between 850°C and 1050°C at a heating rate of 5°C to 25°C / minute, - holding at said 5th 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 1st powder comprises, in mass percentages expressed relative to the mass of said 1st powder: - between 90% and 99%, preferably between 95% and 97%, of grains of the magnetic phase of type TR2Fei4B, - 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 1st powder comprises, in mass percentages expressed relative to the mass of said 1st 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 chosen from the group consisting of Fe, Co, Ni, taken alone or in 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 in 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 2nd powder comprises, in mass percentages expressed relative to the mass of said 2nd powder: - between 27% and 35% of rare earth, including between 1% and 10%, preferably between 2% and 5%, of heavy rare earth, - between 0.9% and 1.2% of 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 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 in 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 1st powder has been 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 step, followed by - a step of decrepitation 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 2nd powder has been obtained from: - virgin raw materials whose mass content of heavy rare earths is between 1% and 10%, preferably between 2% and 5%, or - recycled magnets whose mass content of heavy rare earths is between 1% and 10%, preferably between 2% and 5%, or - a mixture of virgin raw materials and recycled magnets whose mass content of heavy rare earths is between 1% and 10%, preferably between 2% and 5%.

9. Manufacturing method according to claim 8, characterized in that said 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. Manufacturing method according to any one of claims 1 to 9, characterized in that the 2nd powder has been fractured by at least one grinding technique chosen from gas jet grinding, planetary grinding, attrition grinding and cryogenic grinding.

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