EP1082733A1

EP1082733A1 - Method for preparing a magnetic material by forging and magnetic material in powder form

Info

Publication number: EP1082733A1
Application number: EP99922227A
Authority: EP
Inventors: Daniel Fruchart; René Perrier De la Bathie; Sophie Rivoirard; Patricia De Rango
Original assignee: Rhodia Chimie SAS
Current assignee: Santoku Corp
Priority date: 1998-05-28
Filing date: 1999-05-26
Publication date: 2001-03-14
Anticipated expiration: 2019-05-26
Also published as: DE69906513T2; WO1999062080A1; DE69906513D1; FR2779267A1; EP1082733B1; CN1142562C; ATE236450T1; US6592682B1; TW558469B; FR2779267B1; AU3935399A; JP3668134B2; CN1310849A; JP2002516925A

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

PROCESS FOR PREPARING MAGNETIC MATERIAL BY FORGING AND MAGNETIC MATERIAL IN POWDER FORM

RHODIA CHEMISTRY

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 well known Their importance in the electrical or electronic industry is increasing

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

Another process consists in melting an alloy then in making it undergo a quenching on a wheel, in annealing it and in hot pressing or in coating the powder thus obtained with a resin or a polymer This process makes it possible to obtain bonded magnets 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 processes, with low yield or with insufficient results.

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

For this purpose, the method of the invention for the preparation of a magnetic material is characterized in that it comprises the following steps

- 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 ^"' '

According to a second variant, the method of the invention is characterized in that it comprises the following stages - 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 8 s ^"1 ,

- the product from the 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 persistence of at least 9kG

Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting 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 transition metal and at least one other element chosen from boron and carbon The process of the invention therefore starts from this case of alloys having the composition required to obtain the desired material This composition can be variable both by the nature of its constituents and by the respective proportions thereof

These are alloys comprising at least one rare earth and at least one transition metal 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 of the description, the elements of the group constituted by the yttπum and the elements of the periodic classification with atomic number included inclusively between 57 and 71 The periodic classification of the elements to which reference is made for the whole description is that published in the Supplement to the Bulletin of the French Chemical Society n ° 1 (January 1966) The rare earth of the alloy can be neodymium or even praseodymium Alloys based on of several rare earths Mention may more particularly be made of alloys based on neodymium and praseodymium In the case of an alloy of several rare earths, the neodymium and / or praseodymium may be major (s)

By transition elements is meant the elements of columns II a to VI a, VIII, Ib and Mb These transition elements can be more particularly here those chosen from 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 predominant Besides 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, of transition metal and of the other aforementioned element can vary within wide proportions. Thus, the content of rare earth can be at least 1% (the percentages given HERE are atomic percentages) and it can vary between 1% and 30% approximately, more particularly between 1% and 20% approximately The content of third element, in particular boron, can be at least 0.5% and it can vary between 0.5 and 30 % approximately, more particularly between 2 and 10% approximately For the additives, their content can be at least 0.05% and it can vary from 0.05 to 5% approximately

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 with the formula TR2Fe- | 4B, TR designating at least one rare earth, especially 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 transition metal and nitrogen. The process used in this case starts from alloys with the required composition of rare earth and transition metal to obtain the desired material All that has been said above concerning the rare earth, the transition element as well as any additives also applies HERE We can mention however more particularly the alloys based on samanum and iron from which magnetic materials based on samanum, iron and nitrogen will be obtained

It will be noted that the alloys used as starting materials have no or very few magnet properties. They have in particular no or very little coercivity and anisotropy. The alloys which are generally used consist of mainly monocrystalline grains, of large size, at least about 10 μm Here and for the whole of the description the sizes are measured by SEM The alloys can be in a massive form or in the form of a powder The alloys are generally heterogeneous from the point of view the size of the grains, the nature of the phases and the size of the particles in the case of a powder. The alloy can undergo, prior to the treatment of the invention, annealing at a temperature of at least 500 ° C. under inert atmosphere

The alloy as described above is placed in a sheath A cylindrical sheath is advantageously used The height of this sheath is preferably at least equal to the height of the alloy to be treated Its wall thickness is chosen 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 is carried out A metal sheath is generally used Preferably, the sheath is in steel The introduction of the alloy into the sheath can be done by pouring the molten alloy into 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 significant melting 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 indicated temperature under an inert atmosphere, for example under argon

It is however possible to proceed in a sealed envelope. It is meant by this 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 in 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 it can be brought to the required temperature without it being necessary to work in an atmosphere inert The next step in the process of the invention consists in forging the alloy in the sheath. Forging consists of a percussion, one in fact gives one or more hammer blows to 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 disposed in a sealed chamber surrounding the anvil of the forge. This chamber is connected to a source of inert gas and it comprises an opening through which the hammer of the forge can pass through a gasket

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

The mechanical power of the hammer blow must be such that the constituent grains of the alloy are broken. It can also be such that part of this power is used to heat the material, allowing several successive forges, without external heating of the material. '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 rate of at least minus 8s " ¹ , in particular at least 10 s ^-1 , more particularly at least 50s' ¹ and even more particularly 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 moment of 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 ^" ^, in particular at least 0.5 m. S"^'', more particularly at least Im.s ^"' ', and more particularly at least 4m s ^- '. Forging can be carried out with a reduction rate of at least 2. The reduction rate is defined by the initial height ratio (before forging) / final height (after forging) of the alloy / sheath assembly. This rate may more particularly be at least 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 Nd2Fe-j4B phase, this axis of easy growth is the axis a or b 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 at the end of the forging is in a cylindrical planar form, or possibly in the form of a capsule when a sealed envelope has been used as described above, the internal part of which comprises the metal alloy of flow and the peripheral or external part the flow duct. The alloy now consists of monocrystalline grains whose average size is at most 30 μm, more particularly at most 10 μm. The alloy has coercivity and is anisotropic. The magnetization axes are aligned parallel to the direction of forging.

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

The method of the invention may 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 at least one rare earth, at least one transition metal and nitrogen, preparation comprising a nitriding step, the complementary treatments are preferably carried out before this nitriding step.

The various complementary treatments which will now be described can be implemented in any order. As a complementary treatment, it is thus possible to subject the product resulting from the forging at least one annealing treatment to improve its magnetic properties. Different types of annealing treatment can be considered. A first type is made 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 the treatment can be between a few minutes and a few hours. Another type of annealing treatment can be carried out at a temperature of between 400 ° C. and 700 ° C., preferably also under an inert atmosphere of the argon type. The duration of the treatment can be between a few minutes and a few hours.

It is of course quite possible to carry out one or more annealing treatments of the same type or of a different type; for example, a treatment according to the aforementioned first type can be implemented, followed by a second treatment according to the second type.

As another complementary treatment, it is also possible to provide a process of .. decrepititation with hydrogen in order to obtain a powder with magnetic properties close to those of the solid product. Thus, one can subject the material obtained after forging and, optionally after at least one annealing treatment, to a hydriding so as to obtain a hydride of the alloy then to a dehydriding.

Hydriding and dehydrating treatments are known. The hydriding of the material can be carried out under a hydrogen atmosphere (for example at least equal to 0.1 MPa) at ambient temperature or else by thermally activating the material in an atmosphere containing hydrogen. For example, the material can be thermally activated 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 dehydriding treatment may optionally be followed by annealing of the first and / or of the second type mentioned above.

At the end of this treatment, a material is obtained in the form of a powder having advantageous magnetic properties. Thus, this material has a coercivity of at least 9kOe, more particularly of at least 9.5kOe and even more particularly of at least 10kOe in combination with a remanence of at least 9kG, more particularly of at least 9, 5kG and even more particularly at least 10kG. The material may have each of the coercivity values given above in combination with each of the remanence values also given above, for 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 which constitute the powder themselves are made up not of a single monocrystalline grain but of several monocrystalline grains with an average size of at least minus 0.1 μm. Thus, by way of example, the particles may have a size of a few tens of microns, in particular between approximately 10 and approximately 200 μm, more particularly between approximately 10 μm and approximately 100 μm, and be made up of ten grains of a few microns each.

As regards its composition, the material consists of the constituent elements which have been given above for the alloy and what has been described on this subject also applies here, the material being in particular based on at least one rare earth, 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 for examples 1 and 2, to the formula 5 for Example 3 and the formula

^N dl5,3 ^Fe 76.9 ^B 4.9 ^Cu 1 5 ^No 0.5 0.9 ^AI F ^or example 4.

The tests are carried out in a cylindrical steel sheath. In certain 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.

Table 1

Table 2

T- | : temperature during the first forging

T2: temperature during the second forging

E: speed of deformation during the first forging

Tr- _| : reduction rate after the first forging

Tr ₂ : total reduction rate after the second forging

Table 3

V- _| : hammer speed during the first forging 2 '• hammer speed during the second forging P- _j : mechanical power of the first hammer blow P: mechanical power of the second hammer blow

Table 4

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

Claims

1- Method for preparing a magnetic material, characterized in that it comprises the following steps:

- 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 "- '.

2- Process for the preparation of a magnetic material based on at least one rare earth, at least uα transition metal and nitrogen, characterized in that it comprises the following stages:

- 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 from the forging is subjected to a nitriding treatment.

3- A method according to claim 1 or 2, characterized in that forging is carried out with a material deformation speed of at least 10s ^" ", more particularly at least 50s ^"1 and even more particularly of at least 100s ^"' '.

4- A method according to claim 1, 2 or 3, characterized in that forging is carried out with a reduction rate of at least 2.

5- Method according to one of the preceding claims, characterized in that forging is carried out in a direction perpendicular to an axis of easy growth of the crystallites of the alloy.

6- Method 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- Method according to one of the preceding claims, characterized in that the alloy is based on at least one transition metal which is iron. 8- Method according to one of claims 1 or 3 to 7, characterized in that the alloy is based on at least one rare earth, at least one transition metal and boron.

9- Method according to one of claims 1 or 3 to 8, characterized in that the alloy further comprises copper.

10- Method according to one of the preceding claims, characterized in that the sheath is made of metal.

11- A method according to claim 9, characterized in that the sheath is made of steel.

12- Method according to one of the preceding claims, characterized in that the material obtained after the forging and, if necessary, before the nitriding treatment, is subjected to at least one annealing treatment.

13- Method according to one of the preceding claims, characterized in that the material obtained after the forging and, optionally after at least one annealing treatment, is subjected to a hydriding then to a dehydrating, the dehydriding possibly being followed by '' at least one annealing treatment and, if necessary, a nitriding treatment.

14- Magnetic material in powder form, characterized in that it has a coercivity of at least 9kOe and a persistence of at least 9kG.

15- Material according to claim 14, characterized in that it is based on at least one rare earth, at least one transition metal and at least one other element chosen from boron, carbon and l 'nitrogen.

16. Material according to one of claims 14 to 15, characterized in that it is in the form of a powder consisting of particles from 10 to 200 μm.

17- Material according to one of claims 14 to 16, characterized in that it is in the form of a powder whose particles consist of monocrystalline grains with an average size of at least 0.1 microns.

18- Material according to one of claims 14 to 17, characterized in that it is magnetically anisotropic.