Patent Description:
NdFeB sintered permanent magnets are widely used in high-tech fields such as electronic equipment, medical equipment, electric vehicles, household products, robots, etc. In the past few decades of development, NdFeB permanent magnets have been rapidly developed and the residual magnetic properties have basically reached the theoretical limit. However, the gap between the coercive force and the theoretical value is still very large, so improving the coercive force of the magnet is a major research hotspot.

Heavy rare earths terbium (Tb) or Dysprosium (Dy) are added for greatly improving the magnetic coercivity of the NdFeB magnets. According to one conventional manufacturing process, Tb or Dy are directly mixed into the magnet alloy powders, but consume large amounts of Tb or Dy thereby significantly increasing the material costs. According to an improved manufacturing process, the amount of Tb or Dy can be greatly reduced by applying the grain boundary diffusion technology, but still the material costs are very high for the heavy rare earths. Therefore, it is still important to continuously reduce the total content of heavy rare earths in the NdFeB magnet.

Although increasing the coercivity is most effective through diffusing heavy rare earths, the abundance of heavy rare earths is low and accordingly the price is expensive. Therefore, more and more researchers are preparing heavy rare earth alloys with low melting point to obtain with improved coercivity.

<CIT> discloses NdFeB magnets which are diffused with Tb, Dy or Ho, contain an M2 boride phase, an HR enrichment layer and a specific core-shell structure including an (R,HR)-Fe(Co)-M1 phase covering the main phase. In <CIT> the diffusion source is a hydride powder of an R1 - R2-M type alloy, whose melting point is <NUM>-<NUM>. <CIT> provides a magnet characterized by a grain-bounded epitaxial layer, namely a two-particle boundary phase RXHoyCuZX1, is proposed to greatly increase the performance of the magnet after diffusion.

Further examples may be found in <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>. <CIT> and <CIT> disclose that mixing the pulverized magnetic powder with a non-magnetic low melting point (LMP) alloy powder are consolidated or sintered into a bulk magnet. Firstly, the method is easy to result in inhomogeneous distribution of the NdFeB. Secondly, The RM alloy is well known skilled person. The ratio of RM alloy to the NdFeB is the difficult point and the method applied also is critical. The traditional method of mixing the pulverized magnetic powder with RM alloy powder can not get low melting point grain boundaries uniformly. And the diffusion source is diffused into the magnet unevenly and results in poor performance of NdFeB. Designing magnets with low melting point grain boundaries and coordinating with multiple diffusion sources is important.

In the above techniques, the magnets are to form a specific phase or use low-cost diffusion sources for reducing the production cost of the magnets. However, there is still a need to further reduce the content of heavy rare earths of NdFeB magnets.

There is provided a method of preparing a sintered NdFeB magnet as defined in claim <NUM>.

Further embodiments of the invention could be learned form the dependent claims and the following description.

<FIG> shows a SEM image using ZISS electron microscopy of the microstructure of an exemplary Nd-Fe-B permanent magnet after diffusion and aging.

Reference will now be made in detail to embodiments. The present disclosure, however, may be embodied in various different forms, and should not be construed as being limited to only the illustrated embodiments herein. Rather, these embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the aspects and features of the present disclosure to those skilled in the art.

The present invention provides a low-heavy rare earth magnet (i.e. a sintered NdFeB magnet including a low content of heavy rare earth elements) and a corresponding manufacturing method. A special diffusion source for the diffusion process is coated onto a sintered NdFeB magnet of a well-defined magnet composition. Diffusion and aging results to the formation of a high-performance magnet with a specific phase structure. Even in the presence of reduced heavy rare earth contents, the magnet shows a greatly increased coercivity. It is assumed that the combination of the specific grain boundary structure and the diffusion source can greatly improve the coercivity.

There is provided a method of preparing a sintered NdFeB magnet comprising the following steps:.

According to one embodiment, in step (S2) a weight content of Cu is <NUM>%≤Cu≤<NUM>%, a weight content of Al is <NUM>%≤Al≤<NUM>%, and a weight content of Ga is <NUM>%≤Ga≤<NUM>%, each with respect to the total weight of the flake alloy sheets and the low melting point powder.

According to another embodiment, in the NdFeB alloy of step (S1) R is at least one element of Nd and Pr, and M is at least one element of Co and Ti. Further, the NdFeB alloy sheets may be mechanically crushed into flake alloy sheets of <NUM> - <NUM>.

According to another embodiment, in the diffusion source of step (S4).

In step (S2), the dehydrogenation temperature is <NUM> - <NUM>.

According to another embodiment, in step (S2), an average particle size D50 of the low melting point powder is <NUM> - <NUM> measured by laser diffraction (LD). Further, an average particle size D50 of the NdFeB magnet powder may be <NUM> - <NUM> after jet milling measured by laser diffraction (LD). The measurement method may be performed according to ISO <NUM>-<NUM>. According to the IUPAC definition, the equivalent diameter of a non-spherical particle is equal to a diameter of a spherical particle that exhibits identical properties to that of the investigated non-spherical particle.

According to another embodiment, in step (S3), the sintering temperature of the NdFeB magnet is <NUM> - <NUM> and the sintering time is <NUM> - <NUM>.

According to another embodiment, in step (S5), the diffusion temperature of NdFeB magnets is <NUM> - <NUM> and the diffusion time is <NUM> - <NUM>.

According to another embodiment, in step (S5), an aging temperature is <NUM> - <NUM>, an aging time is <NUM> - <NUM>, an aging heating rate is <NUM> - <NUM>/min, and an aging cooling rate is <NUM> - <NUM>/min.

A sintered NdFeB magnet is obtained by the above-mentioned preparation method.

A phase structure of the sintered NdFeB magnet may comprise:.

A thickness of the obtained sintered NdFeB magnet may be <NUM> - <NUM>.

(S2) The flake alloy sheets, low melting point powders and lubricant for mechanical mixing and stirring are put into the hydrogen treatment furnace for hydrogen absorption and dehydrogenation treatment. The NdFeB magnet powders are prepared by jet milling.

(S3) The NdFeB magnet are prepared by magnetic field orientation molding, sintering treatment.

(S4) The NdFeB magnet is machined into the desired shape after sintering, and then a low-heavy rare earth diffusion source film are coated with the NdFeB magnet.

(S5) The Low-heavy rare earth magnets are prepared by diffusion and aging processing.

Preferably, wherein: in step (S1), the NdFeB alloy raw material compositions of weight percentage are, respectively, <NUM>% ≤ R≤<NUM>%, <NUM>% ≤ B≤ <NUM>%, <NUM>≤Gd≤<NUM>%,<NUM>≤Ho≤<NUM>%,<NUM>% ≤M≤<NUM>%, the R including at least two elements of Nd, Pr, Ce, La, Tb, Dy, the M including at least one element of Co, Mg, Ti, Zr, Nb, Mo, the rest is Fe. The mixed low melting point powders contain NdCu, NdAl and NdGa, whose weight percentage is <NUM>%≤NdCu≤<NUM>%, <NUM>%≤NdAl≤<NUM>%, <NUM>%≤NdGa≤<NUM>%.

Preferably, a low-heavy rare earth diffusion source is atomized milling, amorphous alloy sheets or ingot casting.

Preferably, wherein: in step (S2), the dehydrogenation temperature is <NUM> - <NUM>.

Preferably, wherein: in step (S2), the particle size of the low melting point powders is <NUM> - <NUM>. The particle size of NdFeB magnets alloy powders is <NUM> - <NUM> after jet milling.

Preferably, wherein: in step (S3), the sintering temperature of NdFeB magnets is <NUM> - <NUM>, the sintering time is <NUM> - <NUM>;
Preferably, wherein: in step (S5), the diffusion temperature of NdFeB magnets is <NUM> - <NUM>, the diffusion time is <NUM> - <NUM>, the aging temperature is <NUM> - <NUM>, and the aging time is <NUM> - <NUM>. The aging temperature of the NdFeB magnet is heated at a rate of <NUM> - <NUM>/min, and the cooling rate is <NUM> - <NUM>/min.

The beneficial effects of using the above further scheme are:
A grain boundary magnet with low melting point is designed and a special diffusion source with special phase structure are coated with the magnet. A low-heavy rare earth NdFeB magnet with specific grain boundary structure is obtained by diffusion and aging treatment; The coercivity is greatly improved through the synergy of magnet composition and diffusion source.

The diffusion magnet matrix contains NdCu, NdAl and NdGa of the low melting point phase, which is conducive to increasing the diffusion coefficient of the magnet grain boundary, thereby improving the diffusion efficiency of the diffusion source;
The crystal phase structure distribution of the diffusion source is the RM phase and RHM phase, which can improve the diffusion coefficient, therefore it is beneficial to enter the magnet for the element of the diffusion source. This way can well form a magnetic isolation effect in the low-heavy rare earth NdFeB magnet, and realize the role of improving the coercivity.

The low-heavy rare earth magnet has a characteristic phase, and the characteristic phase Fe mass content <<NUM>%, which has non-ferromagnetic properties and can have a good magnetic isolation effect;
The present invention can reduce the heavy rare earth content in the magnet very well, can greatly reduce the cost of the magnet, the process is simple, can achieve mass production.

The preparation process of exemplary sintered NdFeB magnets will now be described in detail.

NdFeB alloy raw materials are mixed with different ratios of NdCu, NdAl, and NdGa and a conventional lubricant is added. Magnet compositions No. <NUM> - <NUM> are summarized in Table <NUM> below.

The preparation method of the NdFeB alloy was as follows:
The NdFeB alloy raw materials are smelted in a strip casting process to obtain NdFeB alloy sheets, and the obtained alloy sheets are mechanically crushed into flake alloy sheets of <NUM> - <NUM> size.

NdCu, NdAl and NdGa as low melting point powders with a particle size range of <NUM> - <NUM> are mixed and added to the flake alloy sheets.

The mixed materials of the flake alloy sheets, low melting point powders and lubricant are put into the hydrogen treatment furnace for hydrogen absorption and dehydrogenation treatment, wherein the dehydrogenation temperature is <NUM> - <NUM>. The low melting point alloy powders are coating the flake alloy sheets. NdFeB powders are prepared by air milling and the NdFeB powder particle size is <NUM> - <NUM>.

The addition of a lubricant during the jet milling step is well-known. Any common type of lubricant und its dosage can be used. There is no specific restriction.

The NdFeB alloy powders after the air flow grinding is oriented molding and pressed into the blank by isostatic pressure.

The pressing blank of NdFeB is sintered in vacuum, and quickly cooled by argon, and then the blank is heat-treated including a primary tempering and secondary aging. The sintered magnet performance is tested, and the specific process conditions and magnet characteristic are shown in Table <NUM>.

The sintered NdFeB magnet is mechanically processed to obtain the desired shape and then a diffusion source film is coated on the sintered NdFeB magnet. The weight of Dy on the sintered NdFeB magnet is <NUM>. %, and the weight of Dy in Dy alloy on the sintered NdFeB magnet is <NUM>.

An increase in coercivity after diffusion of the Dy alloy reaches <NUM> - <NUM> kA/m, and the process allows to reduce the production cost of the magnet due to the low Dy content.

The diffusion sources based on Dy alloys and magnet characteristics of the sintered NdFeB magnets are shown in Table <NUM>.

Pure diffusion examples of Dy and magnet characteristics of the sintered NdFeB magnets are shown in Table <NUM>.

Based on the above data, the NdCu, NdAl, NdGa phase powders are added to the grain boundary of the NdFeB alloy flakes, whose grain boundary has a low melting point. The grain boundary channel of NdFeB permanent magnets are suitable for the diffusion, especially when the diffusion source is a heavy rare earth alloys. The coercivity increases significantly to △Hcj > <NUM> kA/m after diffusion, and the coercivity is significantly better than in case of diffusion of pure Dy.

Specifically, the various embodiments of Table <NUM> and the comparative examples of Table <NUM> are analyzed as follows:.

From the above, it can be seen that after diffusion and aging the coercivity of the examples of Table <NUM> is significantly better than the coercivity of the comparative examples of Table <NUM>.

Claim 1:
A method of preparing a sintered NdFeB magnet comprising the following steps:
(S1) Smelting of the raw materials of a NdFeB alloy to obtain strip casting NdFeB alloy sheets and mechanically crushing the NdFeB alloy sheets into flake alloy sheets, wherein the NdFeB alloy has the following composition in weight percentage:
<NUM>%≤R≤<NUM>%, <NUM>%≤B≤<NUM>%, <NUM>≤Gd≤<NUM>%, <NUM>≤Ho≤<NUM>%, and <NUM>≤M≤<NUM>%,
where R is at least one element of Nd, Pr, Ce, La, Tb, and Dy,
M is at least one element of Co, Mg, Ti, Zr, Nb, and Mo, and
the rest of the NdFeB alloy is Fe;
(S2) Mechanically mixing the flake alloy sheets, a low melting point powder and a lubricant, followed by hydrogen absorption and dehydrogenation treatment of the mixture and jet milling of the product to obtain a NdFeB magnet powder,
wherein the dehydrogenation temperature is <NUM> - <NUM>,
wherein the low melting point powder contains at least one component selected form NdCu, NdAl and NdGa and a weight percentage of the components is <NUM>%≤NdCu≤<NUM>%, <NUM>%≤NdAl≤<NUM>%, and <NUM>%≤NdGa≤<NUM>% with respect to the total weight of the flake alloy sheets and the low melting point powder;
(S3) Pressing and forming the NdFeB powder to a blank and sintering the blank to obtain a sintered NdFeB magnet;
(S4) Mechanically processing the sintered NdFeB magnet to a desired shape, and then forming a diffusion source film on the surface of the sintered NdFeB magnet, wherein diffusion source film includes a diffusion source of formula RxHyM<NUM>-x-y, wherein
R is at least one of Nd, Pr, Ce, La, Ho, and Gd,
H is at least one of Tb and Dy,
M is at least one of Al, Cu, Ga, Ti, Co, Mg, Zn, and Sn, and
where x and y are set to be <NUM>%<x≤<NUM>% and <NUM>%<y≤<NUM>% in weight percentage;
and (S5) Performing a diffusion process and aging to obtain the final sintered NdFeB magnet.