EP1082733B1 - Method for preparing a magnetic material by forging and magnetic material in powder form - Google Patents

Abstract

The invention concerns a method for preparing a magnetic material by forging, characterised in that, in a first embodiment, it comprises the following steps; placing in a sheath an alloy based on at least one rare earth, at least one transition metal and at least one other element selected among boron and carbon; bringing the whole alloy to a temperature not less than 500° C.; forging the whole at a deformation speed of the material not less than 8 s-1. After forging, it is possible to subject the resulting product to at least one annealing and hydridation then dehydridation, in another embodiment, it consists in starting with an alloy based on at least one rare earth and one transition metal and proceeding as in the first embodiment. After forging and, optionally, annealing, hydridation and dehydridation treatments, the resulting material is subjected to nitriding. The invention also concerns a magnetic material in power form, characterised in that has a coercivity not less than 9 kOe and retentivity not less than 9 kG.

Description

The present invention relates to the preparation of a magnetic material by forging as well as a magnetic material in powder form.

Permanent magnets based on iron, boron and rare earths are good known. Their importance in the electrical or electronic industry is growing.

There are two main types of process for preparing these magnets. The first uses powder metallurgy for the preparation of dense magnets or sintered.

Another method consists in melting an alloy and then subjecting it to wheel, anneal it and hot press or coat the powder thus obtained with a resin or a polymer. This process allows bonded magnets to be obtained. The powder and the magnet obtained by the implementation of this process are most often isotropic. To obtain an anisotropic powder or magnet, it is currently necessary to use expensive, low-yielding processes or whose results are insufficient.

There is therefore a need for a process for the production of anisotropic products which is simpler to implement, possibly more economical or improved yield, and which leads to products with satisfactory properties or even improved.

The object of the invention is the development of such a method.

• on place dans une gaine un alliage à base d'au moins une terre rare, d'au moins un métal de transition et d'au moins un autre élément choisi parmi le bore et le carbone;
• on porte l'ensemble à une température d'au moins 500°C;
• on soumet l'ensemble à un forgeage avec une vitesse de déformation du matériau d'au moins 8s^-1.

For this purpose, the process of the invention for the preparation of a magnetic material is characterized in that it comprises the following stages:

• an alloy based on at least one rare earth, at least one transition metal and at least one other element chosen from boron and carbon is placed in a sheath;
• the assembly is brought to a temperature of at least 500 ° C;
• the assembly is subjected to forging with a material deformation speed of at least 8s ^-1 .

• on place dans une gaine un alliage à base d'au moins une terre rare et d'au moins un métal de transition;
• on porte l'ensemble à une température d'au moins 500°C;
• on soumet l'ensemble à un forgeage avec une vitesse de déformation du matériau d'au moins 8s^-1;
• on soumet à un traitement de nitruration le produit issu du forgeage.

According to a second variant, the method of the invention is characterized in that it comprises the following steps:

• an alloy based on at least one rare earth and at least one transition metal is placed in a sheath;
• the assembly is brought to a temperature of at least 500 ° C;
• the assembly is subjected to forging with a material deformation speed of at least 8s ^-1 ;
• the product resulting from forging is subjected to a nitriding treatment.

The invention also relates to a magnetic material in powder form, characterized in that it has a coercivity of at least 9kOe and a remanence of at least minus 9kG.

Other characteristics, details and advantages of the invention will also appear more fully on reading the description which follows, as well as the various concrete but non-limiting examples intended to illustrate it.

The present invention applies, according to its first variant, to the preparation of magnetic materials based on at least one rare earth, at least one metal of transition and at least one other element chosen from boron and carbon. The process of the invention therefore starts in this case from alloys having the composition required for get the desired material. This composition can be variable both by the nature of its constituents only by the respective proportions thereof.

These are alloys comprising at least one rare earth and at least one metal of transition and which additionally contain at least one other element chosen from boron and carbon. Such alloys are well known.

By rare earth is meant, for the whole description, the elements of the group consisting of yttrium and the elements of the periodic table number atomic inclusive between 57 and 71. The periodic classification of elements referred to for the entire description is that published in the Supplement to the Bulletin de la Société Chimique de France n ° 1 (January 1966).

The rare earth of the alloy can be neodymium or even praseodymium. We can use alloys based on several rare earths. We can cite more particularly alloys based on neodymium and praseodymium. In the case of an alloy of several rare earths, neodymium and / or praseodymium may be in the majority.

By transition elements is meant the elements of columns IIIa to VIIa, VIII, Ib and llb. These transition elements can be more particularly here those chosen in the group comprising iron, cobalt, copper, niobium, vanadium, molybdenum and nickel, these elements can be taken alone or in combination. according to a preferred variant, the transition element is iron or even iron in combination with at least one element from the aforementioned group, iron being the majority.

In addition to the aforementioned elements, the alloy may include additives such as gallium, aluminum, silicon, tin, bismuth, germanium, zirconium or titanium taken alone or in combination.

The respective proportions of rare earth, transition metal and the other above can vary widely. So the rare earth content can be at least 1% (the percentages given here are percentages atomic) and it can vary between 1% and 30% approximately, more particularly between 1% and about 20%. The content of third element, in particular boron, can be at least minus 0.5% and it can vary between 0.5 and 30% approximately, more particularly between 2 and About 10%. For additives, their content can be at least 0.05% and it can vary from about 0.05 to 5%.

By way of example of alloys, mention may very particularly be made of neodymium / iron / boron alloys, in particular those comprising also copper. Mention may also be made, as alloys which can be used more particularly in the context of the present invention, of those which have a phase corresponding to the formula TR ₂ Fe ₁₄ B, TR designating at least one rare earth, very particularly neodymium.

The invention also applies, according to its second variant, to the preparation of magnetic materials based on at least one rare earth, at least one metal of transition and nitrogen. The process used in this case starts from alloys having the required composition of rare earth and transition metal to obtain the material wish. All that has been said above concerning the rare earth, the element of transition as well as any additives also applies here. However, we can cite more particularly the alloys based on samarium and iron from which will obtain magnetic materials based on samarium, iron and nitrogen.

It should be noted that the alloys used as starting materials do not have or very few magnet properties. In particular, they have little or no coercivity and anisotropy. The alloys that are generally used consist of mainly monocrystalline grains, of large size, of at least 10 μm approximately. Here and for the whole description, the sizes are measured by SEM. Alloys can be in a massive form or in the form of a powder. The alloys are generally heterogeneous in terms of grain size, nature of the phases and of the particle size in the case of a powder.

The alloy may undergo, prior to the treatment of the invention, annealing at a temperature of at least 500 ° C under an inert atmosphere.

The alloy as described above is placed in a sheath. We use advantageously a cylindrical sheath. The height of this sheath is preferably at less equal to the height of the alloy to be treated. Its wall thickness is chosen in such a way so that it does not burst during forging but this thickness must remain relatively small. The material of the sheath must be as plastic as possible at the temperature at which forging takes place. A sheath is generally used metal. Preferably, the sheath is made of steel.

The introduction of the alloy into the sheath can be done by casting the molten alloy in it or by any mechanical means starting from an ingot or powder.

The alloy-sheath assembly is then brought to a temperature of at least 500 ° C. The maximum temperature not to be exceeded is that beyond which there is a risk of produce a significant fusion of the grains of the alloy. This temperature is more precisely between 600 ° C and 1100 ° C, more particularly between 800 ° C and 1000 ° C. The alloy is brought to the temperature indicated under an inert atmosphere, by example under argon.

It is however possible to proceed in a sealed envelope. We mean by that that once the alloy is placed in the sheath, the lower part and the upper part of the assembly formed by the sheath and the alloy are sealed by a cover made of a material which can be of the same nature as that of the sheath and the cover is welded to the sheath. The alloy is thus isolated from the outside and can be brought to the required temperature without that it is necessary to work under an inert atmosphere.

The next step of the process of the invention consists in subjecting a forging to the alloy in the sheath. Forging consists of a percussion, one gives indeed one or several hammer blows on the alloy / sheath assembly. Forging takes place on the alloy / sheath assembly at the temperature indicated above. When the sheath is not sealed, the alloy / sheath assembly is placed in a sealed chamber surrounding the anvil of the forge. This chamber is connected to a source of inert gas and it includes an opening through which the forge hammer can pass through a seal.

Generally, the number of hammer blows is between 1 and 10.

The mechanical power of the hammer blow must be such that the grains constituting the alloy are broken. It can also be such that part of this power is used for heating the material, allowing several successive forges, without external heating of the alloy. Thus, this power can be for example at least about 1 kilowatt per gram of material (kW / g), more particularly at least 5kW / g. Such a power corresponds to a material deformation speed of at least 8 s ^-1 , in particular of at least 10 s ^-1 , more particularly of at least 50s ^-1 and even more particularly of at least 100s ^-1 . The rate of deformation of the material is defined by the expression (dh / h) / dt, dh / h designating the ratio (initial height-final height) / initial height, the height being that of the alloy / sheath assembly, dt designating the duration of the crushing which is equal to dh / (v / 2), v being the speed of the hammer at the time of the impact and v / 2 being considered, as a first approximation, as the average speed during the crushing, this average speed can indeed be defined as the ratio (initial speed-final speed) / 2 i.e. (v-0) / 2.

Such a power corresponds to devices in which the speed of the hammer is at least 0.3 ms ^-1 , in particular at least 0.5 m.s ^-1 , more particularly at least 1m.s ^-1 , and even more particularly at least 4m.s ^-1 .

Forging can be carried out with a reduction rate of at least 2. The rate of reduction is defined by the ratio initial height (before forging) / final height (after forging) of the alloy / sheath assembly. This rate can be more particularly at least minus 5.

According to a preferred embodiment of the invention, forging is carried out in a direction perpendicular to an axis of easy growth of the crystallites of the alloy. In the case of the Nd ₂ Fe ₁₄ B phase, this easy growth axis is the a or b axis of the quadratic mesh. Forging in this case allows the axes c to pass from an equatorial distribution to a substantially unidirectional distribution.

The product obtained after forging is in a flat form cylindrical, or possibly in the form of a capsule when a sealed envelope as described above, the internal part of which comprises the alloy starting metal and the peripheral or external part the starting sheath. The alloy is now made up of monocrystalline grains whose average size is at most 30µm, more particularly at most 10µm. The alloy has coercivity and it is anisotropic. The magnetization axes are aligned parallel to the direction of the forging.

According to the second variant of the invention and with a view to obtaining a material magnetic based on at least one rare earth, at least one transition metal and of nitrogen, the product from the forging is subjected to a nitriding treatment. The nitriding treatment is done in a known manner. The nitrogen content of the material obtained can be of the same order as that given above for boron, more in particular, it can be between 2 and 15%.

The method of the invention can also comprise, after the forging step, other complementary steps implementing treatments which will be described below. In the case of the preparation of a magnetic material based on minus a rare earth, at least one transition metal and nitrogen, preparation comprising a nitriding step, the additional treatments are implemented preferably work before this nitriding step.

The different complementary treatments which will now be described can be implemented in any order.

As an additional treatment, it is thus possible to subject the product from at least one annealing treatment to improve its properties magnetic.

Different types of annealing treatment can be considered. A first type takes place at a temperature which can be between 700 ° C. and 1100 ° C. The treatment is preferably carried out under an inert atmosphere, for example under argon. The duration of treatment can be between a few minutes and a few hours.

Another type of annealing treatment can be conducted at a temperature between 400 ° C and 700 ° C, preferably also under an inert atmosphere of the type argon. The duration of treatment can be between a few minutes and a few hours.

It is of course quite possible to carry out one or more treatments of annealing of the same or different type; for example we can implement a treatment according to the first type mentioned above and then a second treatment according to the second type.

As another complementary treatment, it is also possible to provide a hydrogen decrepitation process to obtain a powder with properties magnetic close to those of the massive product. So we can submit the material obtained after forging and, optionally after at least one annealing treatment, at hydriding so as to obtain a hydride of the alloy and then dehydriding.

Hydriding and dehydrating treatments are known. Hydriding the material can be done under a hydrogen atmosphere (for example at least equal to 0.1MPa) at room temperature or by thermally activating the material in an atmosphere containing hydrogen. For example, we can thermally activate the material up to a temperature below 500 ° C, preferably below 300 ° C. Dehydriding can be achieved by heating the hydrated material to a temperature of at least 500 ° C under vacuum. The temperature and the heating time are chosen so as to obtain complete dehydriding. The treatment of dehydriding can be optionally followed by annealing of the first and / or second aforementioned type.

At the end of this treatment, a material is obtained in the form of a powder. having interesting magnetic properties. So this material has a coercivity of at least 9kOe, more particularly at least 9.5kOe and even more particularly at least 10kOe in combination with a remanence of at least 9kG, more particularly at least 9.5kG and even more particularly at least 10kg. The material can have each of the coercivity values given above in combination with each of the remanence values also given above, by example a coercivity of 9kOe in combination with a remanence of 9.5kG. The material has a crystalline texture which makes it magnetically anisotropic. The particles that make up the powder themselves are made up of not just one monocrystalline grain but of several monocrystalline grains with an average size of at least minus 0.1 µm. So, for example, the particles can have a size of a few tens of microns, in particular between approximately 10 and approximately 200 μm, more particularly between around 10µm and around 100µm, and be made up of ten grains of a few microns each.

Regarding its composition, the material consists of the elements components which have been given above for the alloy and what has been described thereon also applies here, the material being in particular based on at least one rare earth, at least at least one transition metal and at least one other element chosen from boron, carbon and nitrogen.

Examples will now be given.

The alloy used corresponds to the formula Nd _15.3 Fe _76.8 B _4.9 Cu _1.5 Al _1.5 for examples 1 and 2, to the formula Nd _15.5 Fe ₇₈ B ₅ Cu _1.5 for example 3 and with the formula Nd _15.3 Fe _76.9 B _4.9 Cu _1.5 Nb _0.5 Al _0.9 for example 4.

The tests are carried out in a cylindrical steel sheath. In some cases the alloy undergoes two hammer blows (first forging and second forging).

The characteristics of the starting material are given in table 1, the forging conditions in tables 2 and 3 and the magnetic properties of the solid materials obtained in table 4. Examples mass (g) sheath and alloy diameter (mm) height (mm) sheath thickness (mm) 1 20,18 12 25 2 2 15.76 12 20 1 3 20.31 12 25 1 4 19.98 12 24.5 1 Examples T ₁ (° C) T ₂ (° C) E (s ^-1 ) Tr ₁ Tr ₂ 1 980 890 95.0 4.39 6.25 2 1060 - 112.5 5.90 - 3 995 - 95.6 6.00 - 4 1000 - 92 6 - T ₁ : temperature during the first forging T ₂ : temperature during the second forging E: speed of deformation during the first forging Tr ₁ : reduction rate after the first forging Tr ₂ : total reduction rate after the second forging Examples V ₁ (ms ^-1 ) V ₂ (ms ^-1 ) P ₁ (kW / g) P ₂ (kW / g) 1 4.75 4 10.3 70 2 4.54 - 13.9 - 3 4.78 - 9.8 - 4 4.48 - 8.3 - V ₁ : hammer speed during the first forging V ₂ : hammer speed during the second forging P ₁ : mechanical power of the first hammer blow P ₂ : mechanical power of the second hammer blow Example Hc coercivity Remanence Br Energy product kOe kA / m kG T MGOe kJ / m ³ 1 9.5 756 10 1 17.5 139 2 10.0 796 10 1 16 127 3 9.5 756 10 1 17.5 139 4 10.1 804 9.9 0.99 21 167

The remanence values given in Table 4 show that the products are anisotropic.

Claims (18)

1. Process for the preparation of a magnetic material, characterized in that it comprises the following steps:

   an alloy based on at least one rare earth, on at least one transition metal and on at least one other element chosen from boron and carbon is placed in a sheath;

   the assembly is heated to a temperature of at least 500°C;

   the assembly is subjected to a forging operation with a strain rate of the material of at least 8 s^-1.
2. Process for the preparation of a magnetic material based on at least one rare earth, on at least one transition metal and on nitrogen, characterized in that it comprises the following steps:

   an alloy based on at least one rare earth and on at least one transition metal is placed in a sheath;

   the assembly is heated to a temperature of at least 500°C;

   the assembly is subjected to a forging operation with a strain rate of the material of at least 8 s^-1;

   the product after forging is subjected to a nitriding treatment.
3. Process according to claim 1 or 2, characterized in that forging is carried out with a strain rate of the material of at least 10 s^-1, more particularly of at least 50 s^-1 and even more particularly of at least 100 s^-1.
4. Process according to claim 1, 2 or 3, characterized in that forging is carried out with a reduction ratio of at least 2.
5. Process according to one of the preceding claims, characterized in that forging is carried out in a direction perpendicular to an easy growth axis of the crystallites of the alloy.
6. Process according to one of the preceding claims, characterized in that the alloy is based on at least one rare earth, which is neodymium or samarium.
7. Process according to one of the preceding claims, characterized in that the alloy is based on at least one transition metal, which is iron.
8. Process according to one of claims 1 or 3 to 7, characterized in that the alloy is based on at least one rare earth, on at least one transition metal and on boron.
9. Process according to one of claims 1 or 3 to 8, characterized in that the alloy also comprises copper.
10. Process according to one of the preceding claims, characterized in that the sheath is made of metal.
11. Process according to claim 9, characterized in that the sheath is made of steel.
12. Process according to one of the preceding claims, characterized in that the material obtained after forging, and, where appropriate, before the nitriding treatment, is subjected to at least one annealing treatment.
13. Process according to one of the preceding claims, characterized in that the material obtained after forging, and, optionally, after at least one annealing treatment, is subjected to a hydriding treatment and then to a dehydriding treatment, it being possible for the dehydriding treatment to be optionally followed by at least one annealing treatment and, where appropriate, a nitriding treatment.
14. Magnetic material in powder form, based on at least one rare earth, on at least one transition metal and on at least one other element chosen from boron and carbon, obtained by the process according to one of claims 1, 3 to 13, characterized in that it has a coercivity of at least 9 kOe and a remanence of at least 9 kG.
15. Metallic material in powder form, based on at least one rare earth, on at least one transition metal and on nitrogen, obtained by the process according to one of claims 2 to 13, characterized in that it has a coercivity of at least 9 kOe and a remanence of at least 9 kG.
16. Material according to either of claims 14 and 15, characterized in that it is in the form of a powder consisting of 10 to 200 µm particles.
17. Material according to one of claims 14 to 16, characterized in that it is in the form of a powder, the particles of which consist of single-crystal grains having an average size of at least 0.1 µm.
18. Material according to one of claims 14 to 17, characterized in that it is magnetically anisotropic