Bonded magnets made with atomized permanent magnetic powders

Details Bonded magnets made with atomized permanent magnetic powders Bonded magnets made with atomized permanent magnetic powders

2001-02-28

2003-04-29

Sheehan, John (Department: 1742)

Compositions

Magnetic

With wax, bitumen, resin, or gum

C148S302000

Reexamination Certificate

active

06555018

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to magnets and methods for producing magnets. More specifically, the invention relates to magnets, particularly bonded magnets, made from atomized permanent magnetic powders, and methods for producing such powders and magnets.
BACKGROUND OF THE INVENTION
Bonded magnets are made from magnetic powders bonded together by binder, usually an organic or metallic resin. Although bonded magnets usually have lower magnetic energy compared to their fully-densified counterparts, such as sintered magnets, bonded magnets have wide industrial applicability because of their excellent formability—the ability of forming magnets in complex forms with high mechanical tolerance. In fact, the bonded magnet market has experienced the fastest growth of any permanent magnet market. Examples of bonded magnet applications include appliances, consumer electronics, automotives, factory automation, medical devices, computers, and office automation.
Bonded magnets are usually made from magnetic powders of Ferrite, Nd—Fe—B, Sm—Co, or Sm—TM (a combination of Co, Fe, Cu, Zr, and Hf), although recently other types of bonded magnets are also reported, such as the Sm—Fe—N type magnet disclosed in U.S. Pat. Nos. 5,750,044 and 5,186,766. One major growth area of bonded magnet is that of the Nd—Fe—B type. Many modifications have been made since the first bonded Nd—Fe—B magnet was disclosed in U.S. Pat. No. 4,902,361. For example, the use of special binders has been reported in U.S. Pat. Nos. 5,393,445; 5,149,477 and 5,376,291. A hybrid type of bonded magnet is reported in U.S. Pat. No. 5,647,886. The use of coating to improve corrosion resistance in bonded magnet is disclosed in U.S. Pat. No. 5,279,785. Anisotropic bonded magnets have been reported in U.S. Pat. Nos. 5,587,024 and 6,007,757.
Traditionally, the highest strength bonded magnets are made from rapidly solidified Nd—Fe—B powders produced by melt-spinning. In fact, melt-spinning still forms the basis for almost the entire bonded Nd—Fe—B magnet industry. In a melt-spinning process, a molten alloy mixture is flowed onto the surface of rapidly spinning wheel. Upon contacting the wheel surface, the molten alloy mixture forms ribbons, which solidify into flake or platelet particles. The flakes obtained through melt-spinning are relatively brittle and have a very fine crystalline microstructure. The flakes can also be further crushed or comminuted before being used to produce magnets. The cooling rate can be controlled by both the mass flow rate and the wheel spinning speed.
Although the melt-spinning process is the only commercially available process to achieve the necessary cooling rates to form good quality magnetic powders from Nd
2
Fe
14
B type alloy melts, it suffers from a number of drawbacks such as: microstructural non-homogeneity due to non-uniform quenching; a large number of voids existing between the powder particles that lead to low density and powder oxidation; and difficulties in magnet forming operations due to the large particle size and irregular shapes of the flakes.
Another potential method of producing rapidly solidified powders for making bonded magnets is atomization, although it has never been used widely on a commercial scale. Atomization is the breakup of a liquid into small droplets. Different types of atomization processes, such as gas atomization, water atomization, vacuum atomization, and centrifugal atomization, have been used for years to produce certain alloy powders. Although atomization has the potential of producing magnetic powders at a much higher mass flow rate than the melt-spinning process, it has not been commercially used to produce powders for making bonded magnets. One major drawback of atomization processes is that the cooling rate is generally lower than that of the melt-spinning processes, which usually results in inadequate quenching and poor magnetic properties of the magnetic powders. Attempts have been made in recent years to improve the applicability of atomization process in producing powders for making magnets. Atomized Nd—Fe—B powders have been used to make bonded magnets as given in U.S. Pat. No. 5,905,424. In U.S. Pat. No. 5,242,508, a spherical powder is given a protective coating to make a fully dense magnet. In U.S. Pat. No. 5,474,623, the method of making magnetically anisotropic spherical powder is reported. U.S. Pat. No. 6,022,424 discloses a method of using atomization to produce magnetic powders comprising a R
2.1
Q
13.9
B
1
structure.
The powders and bonded magnets disclosed in these references, however, all suffer from one or more of the following drawbacks: loss of intrinsic coercivity; corrosion instability in the magnet making process; internal magnetic shearing loss due to the characteristics of the magnetic powders; low volumetric loading due to the shape and other characteristics of the magnetic powders; difficulties in the loading and packing processes due to low flowability of the powders; high flux and remanence loss due to exposure to high temperatures; difficulties in processing due to high viscosity; and difficulties in producing small-dimension magnets with high magnetic strength and high part integrity due to the characteristics of the powders and methods used for producing bonded magnets from the powders.
When making a bonded magnet, magnetic powders, whether produced by melt-spinning or by atomization, are usually interspersed with a binder, which can be a polymer such as any thermoset or thermoplastic, or a metal such as zinc. Bonded magnets can then be formed from the powder-binder mixture by various processes—compression molding (compaction), injection molding, extrusion, calendering, thin layer, thin foil and thick sheets by using screen printing, spin casting, or slurry coating. Injection molding is usually used to produce bonded magnets of complex shapes with integrated components. However, injection molding requires good flowability of the magnetic powder-binder mixture, which usually is achieved by limiting the volume fraction of the magnetic powders, resulting in lower magnetic energy for the magnets. Compression molding usually produces magnets with relatively high energy because it can produce magnets from powder-binder mixtures with high volume fraction of magnetic powder due to its tolerance to lower flowability. However, compression molding suffers from its inability to produce very small magnets or magnets with complex shapes. Extrusion molding is a good process for continuous production of magnets, with a low cost of production. Even though extrusion molding does not have the strict requirement for flowability as does injection molding, the complexity of the magnets produced is limited due to the nature of the process.
In addition to the above mentioned drawbacks in the present production of bonded magnets using different processes, there are other general problems associated with the production of magnetic powders and bonded magnets such as material waste due to low process yield, cracking and/or distortion of the magnets due to limits on the loading of materials, and rejection due to dimensional variations. These problems all increase the cost and affect the magnetic properties of the final products.
SUMMARY OF THE INVENTION
The present invention provides magnets, particularly bonded magnets, that overcome or alleviate some or all of the drawbacks associated with the currently available bonded magnets. The invention also provides methods for producing the bonded magnets. Specifically, the present invention overcomes the above mentioned drawbacks by using magnetic powder made by atomization and composition control. More specifically, the present invention provides a bonded magnet made from magnetic powders that are obtained by an atomization process and comprise, by weight, about 15% to 25% of RE; about 0.8% to 2.0% of B; about 1% to 10% of T; and balanced with Fe, Co, or mixtures thereof, wherein RE is one or more rare earth elements selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Er, Gd, Tb, D

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