Metal treatment – Stock – Magnetic
Reexamination Certificate
1998-11-06
2001-04-03
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C147S019001
Reexamination Certificate
active
06210495
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for obtaining magnetically anisotropic materials, in particular for permanent magnets, by solidifying a liquid alloy in an orienting field. This process applies to the production of alloys or magnets containing rare earth elements and transition metals, and more particularly to magnets of samarium-cobalt type.
DESCRIPTION OF RELATED ART
From patent FR 2660107 a process is known for obtaining a magnetic material having a directional metallurgical structure, consisting of using a liquid alloy, bringing said alloy to a temperature such that a composition containing crystallites is obtained, submitting this composition to a magnetic field producing sedimentation during its cooling, in such manner as to obtain a temperature gradient in the sedimentation zone.
It is learnt from this document therefore that solidification must be conducted both in a magnetic field while controlling solidification by slow cooling, and by applying a heat gradient in such manner as to orient grain growth in a preferential direction.
The product obtained has a microstructure which comprises grains whose growth was made in the direction of the heat gradient, these grains simultaneously having magnetocrystalline anisotropy that is usually oriented parallel to the direction of the magnetic field applied during solidification.
In general, such process cannot go further than producing magnetically anisotropic monocrystals having a grain size, in the case of samarium-cobalt based alloys for example, that is greater than 500 &mgr;m; this is a major disadvantage since magnets containing such grains have, in particular, insufficient coercivity.
Also, the implementation of said process comes up against considerable practical difficulties. It requires the simultaneous use of an orienting field and of a complete system allowing rigorous control over slow cooling (20°/h) during solidification in order to obtain the heat gradient in the desired direction.
The process is in fact little productive and difficult to industrialize. In particular, the fact that the initial composition requires the presence of crystallites and the use of slow, controlled cooling, means that it is unsuitable for directly obtaining magnets or cast or moulded products from a liquid metal.
With this type of casting or moulding process, the liquid is usually superheated to avoid undue setting (no crystallites are present therefore) and undergoes fast, uncontrolled cooling and solidification speeds.
Patent application EP 474566 similarly describes a process for preparing a polycrystalline magnetic body of R BaCuO type (R being a rare earth element), consisting of preparing a composition such that in its molten state it comprises crystallites of said body, of heating to a few degrees above melting point so that some crystallites remain, cooling slowly until solidification and applying an orienting magnetic field at least as from the time when the composition starts becoming liquid and up until its solidification.
Such processes therefore entail real operating difficulties requiring the presence of crystallites and the use of slow and/or controlled cooling, and can only achieve products which, even though they have good remanence (Br), have insufficient coercivity (Hcj), in particular for magnets of samarium-cobalt type.
In the face of these difficulties, a more industrial process has been researched for obtaining magnetically textured materials, that is productive, easy to implement and, at the same time, can improve the magnetic characteristics of the products obtained.
SUMMARY OF THE INVENTION
The invention is a process for obtaining a solid, magnetically anisotropic material from an alloy containing a rare earth element and a transition metal that is fully liquid at a temperature significantly higher than its melting point, optionally cast or moulded to obtain a shaped part, which is solidified by natural or forced, non-controlled cooling in the presence of a continuous, static magnetic field.
The process of the invention may be completed after cooling by a magnetic hardening heat treatment stage, comparable to that customarily used for the production of magnets by powder metallurgy (PM).
The magnetic material obtained usually has a remanent magnetization over saturation magnetization ratio Br/Bs of at least 80%.
Also, with the process of the invention it is generally possible to obtain a material having a coercivity (Hcj) at least equal to, and even greater than, that obtained with a material of the same composition produced by PM. For material of samarium-transition metal type, coercivity is at least 8 kOe and typically at least 25 kOe.
With this process it is possible, easily and economically by simple melting-solidification under usual conditions, to obtain a polycrystalline magnetic material or magnets of any shape by simple casting or moulding under economical, industrial conditions.
Under these conditions, the temperature of the initial liquid alloy may largely exceed the melting point of the alloy, for example by at least 10° C., even 150° C., since the presence of crystallite (or seed) subsisting in the liquid is not necessary, and cooling is conducted with no particular control over rate or heat gradient, at speeds that are in general of at least 100° C./sec.
The static magnetic field is generally greater than 2T.
It appears unusual that under such operating conditions, in particular the fast cooling rate, it is possible to obtain an improved magnetically anisotropic material, in particular in the case of samarium-cobalt; even higher speeds, for example of at least 500° C./s, may even be used to improve the metallurgical quality of the parts obtained without jeopardizing their magnetic quality.
Indead according to the prior art, the use of a slow cooling speed at the time of solidification is justified by the fact that the grains (crystallites) must be given the opportunity to appear and have the time to orient themselves under the action of the field. Also, the presence of crystallites is justified since, if they are not present during solidification, germination may occur simultaneously at all points of the liquid and too quickly for grain orientation to take place. The presence of a heat gradient in a given direction also contributes to promoting grain growth in preferential direction and to promoting texturing of the solid magnetic material.
For men of the art, therefore, slow, controlled cooling and the presence of pre-existing crystallites appear essential in order to obtain a textured material. In opposition to this conclusion, the present invention shows that this is not the case, in particular for Sm—Co based materials for which the action of the orienting field alone is sufficient to obtain an anisotropic (textured) material regardless of cooling conditions; it is possible that this unexpected texturing result may be due to another phenomenon.
The microstructure of the solid obtained is generally homogeneous, containing grains whose average size is less than 500 &mgr;m, which are magnetically anisotropic through crystallographic orientation produced by the applied orienting magnetic field.
The initial liquid alloy preferably has the composition of the solid material it is desired to obtain, in order to avoid having to collect and remove a supernatant liquid, from which one of the components of the alloy has been removed, during solidification. The superheated liquid alloy can be cast in a mould in which solidification can take place by natural or forced, non-controlled cooling in a magnetic field.
The process of the invention advantageously applies to the production of magnetic materials containing samarium and a transition metal (M) (such as cobalt, iron, copper, zirconium) preferably having the atomic composition Sm (Co,Cu,Fe,Zr)
x
, where x varies between 5 and 9, in particular alloys of Sm
2
M
17
or SmM
5
type.
The structure of these materials can be orientated by applying an orienting magnetic field of sufficient intensity regardless of superheating or
de la Bathie Rene Perrier
Legrand Beatrice
Tournier Robert
Centre National de la Recherche Scientifique
Dennison, Scheiner Schultz & Wakeman
Sheehan John
LandOfFree
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