Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2001-06-08
2003-07-15
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S120000, C148S301000, C148S302000, C148S303000
Reexamination Certificate
active
06592682
ABSTRACT:
The present invention relates to the preparation of a magnetic material by forging and to a magnetic material in powder form.
Permanent magnets based on iron, boron and rare earths are well known. Their importance in the electrical or electronics industry is growing.
There are two main types of process for preparing these magnets. The first makes use of powder metallurgy in order to prepare dense or sintered magnets.
Another process consists in melting an alloy and then quenching it on a wheel, in annealing it and in hot pressing or encapsulating the powder thus obtained in a resin or a polymer. This process makes it possible to obtain bonded magnets. The powder and the magnet obtained by implementing this process are usually isotropic. In order to obtain an isotropic powder or magnet, it is currently necessary to use expensive processes which are inefficient or which give inadequate results.
There is therefore a need for a process for producing anisotropic products which is simpler to implement, is possibly more economical or has an improved efficiency and which leads to products with satisfactory or even improved properties.
The subject of the invention is the development of such a process.
To this end, the process 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, 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
.
According to a second version, the process of the invention is 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 it 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.
The invention also relates to a magnetic material in powder form which is characterized in that it has a coercivity of at least 9 kOe and a remanence of at least 9 kG.
Further characteristics, details and advantages of the invention will become even more apparent on reading the description which follows, together with the concrete but non-limiting examples intended to illustrate it.
The present invention applies, according to its first version, to the preparation of magnetic materials 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. The process of the invention therefore starts in this case with alloys having the composition required for obtaining the desired material. This composition may vary both in regard to the nature of its constituents and the respective proportions of them.
The invention involves alloys which comprise at least one rare earth and at least one transition metal and which also contain at least one other element chosen from boron and carbon. Such alloys are well known.
Throughout the description, the term “rare earth” should be understood to mean one of the elements of the group formed by yttrium and the elements of the Periodic Table having an atomic number of between 57 and 71 inclusive. The Periodic Table of the Elements to which reference is made throughout the description is the one published in the Supplément au Bulletin de la Société Chimique de France [
Supplement to the Bulletin of the Chemical Society of France
] No. 1 (January 1966).
The rare earth of the alloy may be neodymium or else praseodymium. Alloys based on several rare earths may be used. Mention may more particularly be made of alloys based on neodymium and praseodymium. In the case of an alloy of several rare earths, neodymium and/or praseodymium may be the major component(s).
The term “transition elements” should be understood to mean the elements of Columns IIIa to VIIa, VIII, Ib and IIb. In the present case, these transition elements may be more particularly those chosen from the group comprising iron, cobalt, copper, niobium, vanadium, molybdenum and nickel, it being possible for these elements to be taken alone or in combination. According to a preferred version, the transition element is iron or else iron in combination with at least one element of the aforementioned group, iron being the major component.
Apart from the aforementioned elements, the alloy may comprise additives such as gallium, aluminium, 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 may vary widely. Thus, the rare-earth content may be at least 1% (the percentages given here are atomic percentages) and it may vary between approximately 1% and 30%, more particularly between approximately 1% and 20%. The content of the third element, especially boron, may be at least 0.5% and it may vary between approximately 0.5 and 30%, more particularly between approximately 2 and 10%. In the case of the additives, their content may be at least 0.05% and it may vary from approximately 0.05 to 5%.
By way of examples of alloys, mention may most particularly be made of neodymium/iron/boron alloys, especially those which also comprise 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 satisfying the formula RE
2
Fe
14
B, RE denoting at least one rare earth, most particularly neodymium.
The invention also applies, according to its second version, to the preparation of magnetic materials based on at least one rare earth, on at least one transition metal and on nitrogen. The process used in this case starts with alloys having the composition, in terms of rare earth and of transition metal, required to obtain the desired material. Everything that was stated above in regard to the rare earth, the transition element and the optional additives also applies in this ease. However, mention may more particularly be made of alloys based on samarium and iron, from which alloys magnetic materials based on samarium, iron and nitrogen will be obtained.
It will be noted that the alloys used as starting products do not have the properties of magnets, or do so very slightly. In particular, they have a very small or zero coercivity and exhibit very little or no anisotropy. The alloys that are used generally consist of mostly large, single-crystal grains with a size of at least approximately 10 &mgr;m. Here, and for the entire description, sizes are measured by SEM. The alloys may be in bulk form or in powder form. The alloys are generally heterogenous with regard to the grain size, to the nature of the phases and, in the case of a powder, to the particle size.
Prior to the treatment of the invention, the alloy may be annealed at a temperature of at least 500° C. in an inert atmosphere.
The alloy as described above is placed in a sheath. Advantageously, a cylindrical sheath is 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 in such a way that it does not burst during forging, but this thickness must remain relatively small. The material of which the sheath is composed must be as plastic as possible at the temperature at which forging takes place. Generally, a metal sheath is used. Preferably, the sheath is made of steel.
The alloy may be introduced into the sheath by the molten alloy being cast into it, or by any mechanical means starting with an ingot or with powder.
The alloy/sheath assembly is then heated to a temperature of at least 500° C. The maximum temperature not to be exceeded is that above which there is a risk of significant melting of the grains or particles of the alloy occurring. This temperature is more specifically betwe
Fruchart Daniel
Perrier De La Bâthie Rene
Rango Patricia
Rivoirard Sophie
Santoku Corporation
Sheehan John
Stevens Davis Miller & Mosher LLP
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