DE3626406A1 - METHOD FOR PRODUCING PERMANENT MAGNETS BASED ON RARE EARTH METALS - Google Patents

Description

The invention relates to a method for producing Permanent magnets based on rare earth metals and the magnets produced by the process.

In practice, the following three methods are currently in use for the production of magnets of the R-Fe-B type made:

(1) The sintering process based on the technique of powder metalluria is based on (Reference 1: M. Sagawa et al., Japan Applied Physics, Vol. 55 (6), March 15. 1984, page 2083);

(2) a method in which by means of a centrifugal casting device, to make an amorphous alloy is used, rapidly cooled pieces of tape and from these made a magnet using the binder technology are referred to (Document 2: R.W. Lee; Applied Physics Letters, Vol. 46 (8), April 15, 1985, page 790);

(3) a method in which mechanical alignment treatment on those prepared by the above method (2) Band pieces is executed, by means of a two-stage hot pressing technique (see the already mentioned Document 2).

In the sintering process according to (1), an alloy block is used made by melting and casting and the alloy block then to a fine powder with a particle diameter of about 3 µm processed. The magnetic powder is kneaded with the binder, which is used as a mold additive serves, and for obtaining a shaped body in a magnetic field molded under pressure. The molded body is one hour  sintered at about 1,100 ° C in an argon atmosphere and then quenched to room temperature. After sintering the body is heat treated at about 600 ° C and thereby the self-coercive force increases.

In method (2), using a centrifugal casting device, the one with an optimal substrate speed hurls, quenched pieces of an R-Fe-B Alloy. These pieces of tape have a thickness of 30 µm and consist of a collection of grains together whose diameter is 0.1 µm or less. These pieces of tape are fragile and magnetically isotropic, because the grains are distributed isotropically. The band pieces are crushed into particles of suitable size and the particles kneaded with resin and molded under pressure. With a print from about 700 MPa the material to 85 volume percent condensed.

In process (3), the quenched band pieces are into a graphite mold or other suitable high temperature mold given that in vacuum or an inert gas atmosphere was preheated to about 700 ° C. If the temperature of the band pieces has risen to a predetermined value they are subjected to unidirectional pressure. Temperature and time are not limited, though a temperature of 725 ± 250 ° C and a pressure of about 140 MPa to achieve sufficient plasticity are cheap. The grains of the magnet are in Printing direction slightly aligned, but overall isotropic.

Using a larger cross-sectional shape then another hot press treatment, generally at a temperature of 700 ° C and a pressure of 70 MPa for a few seconds. The thickness of the Material is then half of its original thickness reduced and a magnetic alignment parallel to  Pressing direction introduced, making the alloy anisotropic becomes.

This process is called the "two-stage hot pressing process". It is used to make anisotropic magnets of the R-Fe-B type high density created.

It is advantageous to determine the particle diameter of the grains of the band pieces, originally by centrifugal casting were produced, somewhat smaller than the grain diameter to make at which the maximum self-coercive force occurs. To some extent the grains are during the heat press treatment coarser, so that if it is before are smaller than the optimal diameter, according to hot press treatment are optimal.

With the known techniques mentioned magnets of R-Fe-B type manufactured, however, have these techniques some disadvantages.

In the sintering process (1), the alloy has to be fine powder be ground or ground. Alloys of R-Fe-B types, however, are exceptional to oxygen active, so that your powder oxidizes particularly easily. Therefore the oxygen concentration in the sintered body inevitably high.

When the powder is molded, an additive such as for example zinc stearate. Although such an additive before a certain amount remains after the sintering process of the additive in the form of carbon in the magnet. The carbon affects the magnetic properties of an R-Fe-B magnet strong.

The one obtained after the compression molding with the addition of the molding additive Blank is fragile and difficult to make  handle. It is therefore very troublesome to keep these blanks in good condition Bring condition in the sintering furnace, which is a clear one Represents disadvantage. For the production of sintered magnets of the R-Fe-B type therefore expensive systems are required. In addition, productivity is extremely low, what leads to high manufacturing costs of these magnets. The sintering process (1) is therefore unsatisfactory since the inherently low raw material costs for this type of magnet to be compensated for by the high manufacturing costs.

In processes (2) and (3), a vacuum centrifugal casting device is used used, which is currently not very productive and is expensive. The crystals of by the method (2) Manufactured magnets are isotropic, which is why the energy product is low and the hysteresis loop is clearly from deviates from the square shape. Produced by the process (2) Magnets have a poor temperature coefficient and are disadvantageous for practical use.

The method (3) is unique in that the hot working is carried out in two stages. This method however, is very inefficient.

The object of the invention is to provide a method with the permanent magnet of the type mentioned at low cost can be produced, the excellent properties own and not the above in detail Disadvantages explained.

This object is achieved by a method with solved the features of claim 1.

In this process, the ingot is hot worked at a temperature of preferably 500 ° C or more  subjected to a fine crystal grain of the block and align the grain axes in a certain direction, which makes the casting alloy magnetically anisotropic becomes.

To improve the magnetic properties and in particular to increase the self-coercive force of the resulting Magnet is the one marked in claim 2 Starting material used and through a heat treatment magnetically hardened at a temperature of 250 ° C or more.

For binder magnets (resin-bonded magnets) a Alloy this composition using the Ability to easily form a hydrogenated compound powdered. The fine powder comes with an organic Binder kneaded and then to obtain the binder magnet hardened.

In order to obtain a binder magnet by the usual pulverization, taking care that a fine grain can be easily obtained by hot working, care is taken to ensure that each powder particle contains a plurality of the magnetic R ₂ Fe ₁₄ B grains. The powder is then kneaded with organic binder and cured to achieve the binder magnet.

As previously described, each of the existing methods for the production of a permanent magnet on the basis of a rare earth has its own critical disadvantage, that the powder is difficult to handle or productivity is low. To avoid this Magnetic hardening in the solid state became a disadvantage examined. This resulted in the following: (1) In the The alloy composition range according to claim 1 the alloy becomes fine-grained and heat treated  anisotropic. (2) In the area of the alloy according to claim 2 sufficient self-coercive force is achieved by Heat treatment of the casting block reached. (3) By pulverization the casting block of an alloy according to claim 2 by means Hydrogen crackling and kneading the powder with an organic binder and curing the mixture you get a binder magnet. (4) The casting block consists of a large number of finer after heat treatment Grains together, which is why the one made from them Powder consists of a variety of fine grains and too a binder magnet can be processed.

In the method according to the invention, hot working, by which the casting block is made anisotropic, in one step be and does not need to be two-stage, as with that quenching method disclosed in the aforementioned document 2. Because of the fine grain is beyond the self-coercive force of the hot-worked body clearly increased. Since there is no need to use the casting block pulverize, there is no need to strictly control the atmosphere for sintering etc. This significantly reduces the system costs. Another Advantage of the invention is that the obtained Binder magnet is not initially isotropic, like the one after obtained magnet known quenching process, but that an anisotropic binder magnet can easily be obtained can. The advantages of an R-Fe-B magnet with excellent Properties and low cost are therefore fully achieved.

The magnetization of a solid alloy is known from "The Lecture Meeting of Japanese Institute of Metals", Fall 1985, Lecture No. 544. However, this document relates to small samples of the composition Nd _16.2 Fe _50.7 Co _22.6 V _1.3 B _9.2 , which is melted in air and sprayed with argon gas, from which the samples are then taken. It is therefore believed that fine grain formation occurs through quenching when small samples are taken.

Experiments have shown that with the composition mentioned in this document the grain of the main phase Nd ₂ Fe ₁₄ B becomes coarse when a conventional casting process is used. Although it is possible to make the alloy of the composition Nd _16.2 Fe _50.7 Co _22.6 V _1.3 B _9.2 anisotropic by hot working, it is very difficult to obtain a sufficient self-coercive force for the resulting body for a permanent magnet to reach.

It has also been shown that to achieve a magnet sufficient self-coercive force itself the usual casting process the composition of the starting material should contain little boron, as with the Alloy of claim 2 is the case.

As the document 1 shows, the prior art considers the typical optimal composition of an R-Fe-B magnet R ₁₅ Fe ₇₇ B ₈ . R and B are more strongly represented in this composition than in the composition R _11.7 Fe _82.4 B _5.9 , which is equivalent in terms of the atomic percentages of the main phase compound R ₂ Fe ₁₄ B. This is explained by the fact that in order to achieve a sufficient self-coercive force not only the main phase but also a non-magnetic phase rich in R and a rich in B are necessary.

In the composition according to claim 2, the self-coercive force maximum if the proportion of B is less than is in the usual composition. Generally show Compositions with a low boron content are strong Decrease in self-coercive force when the sintering process is applied, which is why this composition range  little importance was attached.

With the usual casting process, however, you get the high one Self-coercive force only in the composition range according to claim 2, while the composition rich in boron, which is the main composition for the sintering process is not sufficient self-coercive force leads.

The following is assumed as the reason for this. The intrinsic coercive force of the magnet itself depends primarily on the nucleation model, both in the sintering process and in the casting process according to the invention. This is confirmed by the fact that the magnetization recurve of the magnets showed a steep increase in both methods, such as that of SmCo ₅ . A magnet of this type has a self-coercive force according to the single domain model. If the grain of the R ₂ Fe ₁₄ B compound with large magnetic crystal anisotropy is too large, the Bloch walls run through the grain, so that movement of the Bloch walls slightly inverts the reversal of magnetization, thereby reducing the self-coercive force. If, on the other hand, the grain of the R ₂ Fe ₁₄ B compound is below a certain size, the Bloch walls disappear from the grain. In this case, since the remagnetization is effected solely by rotating the magnetization, the self-coercive force decreases.

In order to achieve a sufficient coercive force, the R ₂ Fe ₁₄ B phase must therefore have an appropriate grain diameter of approximately 10 µm. In the sintering process, the grain diameter can be suitably adjusted by adjusting the diameter of the powder particles before sintering. In the casting process, however, the grain diameter of the R ₂ Fe ₁₄ B compound is determined when the liquid material solidifies. Therefore, it is necessary to control the composition and the consolidation with great care.

The composition is particularly important. If more than 8 atomic percent of boron is contained, there is a likelihood that the grain of the R ₂ Fe ₁₄ B phase of the magnet will be larger than 100 µm after casting. In this case, it is difficult to obtain a sufficient self-coercive force in the cast state without using the quenching device of document 2. In the area of the composition with a low boron content according to claim 2, however, the grain diameter of the magnet can easily be reduced by adjusting the type of shape, the mold temperature, etc. In any case, however, the grain of the main phase R ₂ Fe ₁₄ B becomes finer by the heat treatment, so that after the heat treatment the intrinsic coercive force of the magnet has increased.

The range of the composition in which a sufficient self-coercive force is achieved in the cast state, that is to say the composition poor in boron, can conversely also be referred to as an iron-rich composition. During solidification, Fe appears first as the main phase and then the R ₂ Fe ₁₄ B phase due to the peritectic reaction. Since the cooling rate is much greater than the rate of the equilibrium reaction, the sample solidifies in such a way that the R ₂ Fe ₁₄ B phase surrounds the primary Fe phase. Since this composition range is low in boron, the boron-rich phase, as occurs in the magnet of R ₁₅ Fe ₇₇ B ₈ , which is the typical composition suitable for the sintering process, is necessarily of such a small amount that it can be almost neglected . The heat treatment specified in claim 2 serves to diffuse the primary Fe phase and to maintain the equilibrium state. The self-coercive force of the resulting magnet depends strongly on the diffusion of Fe. Next, the binder magnet claimed in claim 4 will be explained.

Binder magnets are currently being quenched produced from document 2. However, since that Powder made with the quenching process on the isotropic There is aggregation of polycrystals, their Is 0.1 µm or less, the powder is magnetically isotropic. Therefore, using the quenching process an anisotropic magnet cannot be obtained and the benefits the magnets of the R-Fe-B type, namely low cost and excellent properties are not achieved. If an R-Fe-B type magnet is to be manufactured, the magnet's own coercive force is sufficient held high that the pulverization by means of hydrogen crackling done, the little mechanical distortion caused. In this way, a binder magnet can be realized with high self-coercive force. The main advantage of this method is that in contrast to the method according to document 2, an anisotropic Magnet can be made.

Finally, the binder magnet claimed in claim 5 are explained.

There are two reasons why the binder magnet of the R-Fe-B type only using the special pulverization process of claim 4 can be produced:

First of all, it should be noted that the critical radius of the individual domain of the R ₂ Fe ₁₄ B compound is much smaller than that of SmCo ₅ etc. and is in the submicron range. It is extremely difficult to achieve such small powder particle diameters using conventional mechanical pulverization. In addition, the powder obtained is too active and is therefore easily oxidized and ignited. Due to the grain diameter, the self-coercive force of the magnet obtained is very small. The inventors examined the relationship between the grain diameter and the resulting self-coercive force. It was found that the self-coercive force was at most a few 800 A / cm and did not increase even when a surface treatment was carried out on the magnet.

Another problem is that of mechanical processing caused distortion. If, for example, a magnet with an inherent coercive force of 80 A / cm in the sintered state mechanically is pulverized, has the resulting powder with a particle diameter of 20 to 30 µm Coercive force less than 0.8 A / cm. With mechanical Pulverization of a SmCo magnet, from which adopted will have the same nucleation model such a decrease in the self-coercive force does not occur, rather, a powder of sufficient coercive force can be used easy to manufacture. The cause of this phenomenon is assumed that the influence of distortion, etc., at the pulverization and processing of a magnet of R-Fe-B- Type is caused to be of significant size. This Influence poses a critical problem when a small one Magnet, for example for a rotor magnet of a stepper motor a wristwatch from the sintered magnetic block is cut out.

The above two reasons, that is, the critical radius is small and the effect of mechanical distortion is large, mean that the binder magnet cannot be obtained with the usual pulverization. To obtain powder with sufficient self-coercive force, the powder must be prepared so that its powder particles contain a large number of R ₂ Fe ₁₄ B grains, as disclosed in document 2. However, the quenching method of document 2 is problematic in terms of its productivity. In addition, it is actually impossible to manufacture such powders by pulverizing the sintered body. Since the grains grow to a certain extent during sintering, it is necessary to make the grain or particle diameter smaller than the ultimately desired diameter before sintering. However, when the particle diameter of the powder is small, its oxygen concentration is extremely high and the property of the magnet is not satisfactory.

The permissible grain diameter of the R ₂ Fe ₁₄ B connection after sintering is currently approximately 10 µm. The self-coercive force is reduced to almost zero after pulverization.

The invention makes use of the reduction in size of the grain by hot working. It is relatively easy to make the grain of the R ₂ Fe ₁₄ B compound about the same size as that obtained by sintering in the cast state. If a casting block which contains the R ₂ Fe ₁₄ B phase of this grain size is then heat-treated, the grain becomes smaller and aligned, and the pulverization then takes place. In this way, since the particle diameter of the powder for binder magnets is between 20 and 30 μm, it is possible for the powder particles to contain a large number of R ₂ Fe ₁₄ B grains, so that a powder with sufficient intrinsic coercive force is obtained. In addition, this powder is not isotropic as in the quenching process according to document 2, but can be aligned in the magnetic field, that is, an anisotropic magnet is produced from this type of powder. When pulverized by hydrogen crackling, the self-coercive force becomes even better.

The following are the selection of the composition for a permanent magnet produced according to the invention  considerations are explained:

An element or a combination is used as a rare earth element of elements from the group Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Mo, Er, Tm, Yb and Lu selected. The best magnetic property is achieved with Pr. In practice therefore, Pr, a Pr-Nd alloy, a Ce-Pr-Nd alloy etc. used.

To increase the self-coercive force, it is sometimes beneficial a small amount of a heavy rare earth metal such as Dy, Tb etc., Al, Mo, Si etc.

The main phase of a magnet of the R-Fe-B type is R ₂ Fe ₁₄ B. If the proportion of R is less than 8 atomic percent, this connection does not occur, but a cubic body-centered structure like that of α-iron, which is why the magnetic properties are not particularly good. On the other hand, if the proportion of R is more than 30 atomic percent, the amount of the non-magnetic R-rich phase increases, and the magnetic properties become much worse. Therefore, the suitable range of the proportion of R is between 8 and 30 atomic percent. The preferred range for R for a cast magnet is between 8 and 25 atom percent.

Boron is an essential element for the appearance of the R ₂ Fe ₁₄ B phase. If the proportion of boron is less than 2 atomic percent, the rhombohedral R-Fe type occurs and a high self-coercive force is not obtained. If, on the other hand, as in the magnet produced by the known sintering process, the boron content is more than 28 atomic percent, the non-magnetic, boron-rich phase increases and the remanence significantly decreases. The favorable upper limit of the boron content for a shaped magnet is 8 atom percent. With more than 8 atomic percent boron, the fine R ₂ Fe ₁₄ B phase is not obtained unless special cooling is carried out and the self-coercive force is low.

Cobalt is the active element for improving the Curie point and basically has the effect of substituting Fe and producing R ₂ Co ₁₄ B. However, this R ₂ Co ₁₄ B compound has a small crystalline anisotropy field, and the more the proportion of this compound increases, the lower the magnet's own coercive force. In order to achieve a coercive force of more than 800 A / cm, which is considered sufficient for a permanent magnet, the proportion of cobalt should be 50 atomic percent or less.

As described in Shang Maocai et al. "Proceedings of the 8th International Workshop on Rare-Earth Magnets ", 1985, page 541, aluminum has the effect to increase the self-coercive force. The mentioned publication however only refers to sintered magnets, although the same effect occurs with cast magnets.

However, since aluminum is a non-magnetic element, With a high aluminum content, the remanence decreases, and with a share of more than 15 atomic percent, the Remanence reduced to the value of hard ferrite. A such a magnet does not achieve the possible high quality of a rare earth magnet. Therefore, the proportion of Aluminum is 15 atomic percent or less.

Preferred exemplary embodiments are described below with reference to the drawings of the invention explained. Show it:

Fig. 1 the inventive production method of a magnet of the R-Fe-B type,

Fig. 2 shows the alignment process of the magnet alloy by hot working by extrusion,

Fig. 3 shows the alignment process of the magnet alloy by hot working by rolling, and

Fig. 4 shows the alignment process of the magnet alloy by means of hot working by pressing.

example 1

Reference is made to FIG. 1, which shows the method according to the invention for producing a permanent magnet.

First, an alloy of the desired composition melted in an induction furnace and into a mold poured. So that the magnet becomes anisotropic, the samples then subjected to various types of hot working. This example was not from the general Molding process, but from a special molding process Made use of, namely the so-called Liquid Dynamic Compaction procedure, which is described in the publication 5: T.S. Chin et al., "J. Appl. Phys." 59 (4), 2/15/1986, page 1297 is and very effective in achieving one fine grain by quenching it.

In this exemplary embodiment, hot working was carried out according to one of the types shown in FIGS . 2 to 4, namely extrusion, rolling or compression molding, specifically at a temperature of 1,000 ° C.

As far as extrusion is concerned, a isostatic pressure on the sample additionally a device to apply pressure to the sample from the Form side provided. As for rolling and compression molding, became the speed of rolling or pressing set so that the forming speed  was minimal. No matter what type of hot processing is used will, the axis of easier magnetization of the grain is aligned parallel to the direction in which the alloy is forced.

In the process of FIG. 1, alloys of the composition shown in Table 1 were melted and processed into magnets. The type of hot processing used in each case is given in the table for the individual samples.

The tempering after hot machining was carried out at one Temperature of 1,000 ° C for 24 hours.

Table 1

The properties of the magnets obtained are shown in Table 2 listed. Remanence is used for comparison purposes specified a sample in which the hot working is not took place.

Table 2

From Table 2 it can be seen that each of the hot working variants the remanence is increased, the samples were made magnetically anisotropic.  

Example 2

This example is based on the general casting process.

First alloys, the composition of which are shown in Table 3 is melted in an induction furnace and poured into a mold to form stem zones. After performing the hot working with a hot working efficiency of about 50% or more (pressing in this example) the ingot was at a temperature annealed from 1,000 ° C for 24 hours to make it magnetic to harden. After tempering, the mean grain diameter was the sample at about 15 µm.

In the case of cast magnets, a flat anisotropic magnet is obtained by processing the sample into the desired shape without hot working, taking advantage of the anisotropy of the stem zone. For a binder magnet, hydrogen absorption in a hydrogen atmosphere of about 1 MPa and hydrogen desorption under a pressure of 10 ^-3 Pa were repeated in a container made of 18-8 stainless steel at room temperature, and the samples were then pulverized and kneaded with 4% by weight of epoxy resin . The mass was then molded under pressure, applying a magnetic field of 8 kA / cm perpendicular to the direction of pressure.

The properties of the magnets obtained are shown in Table 4 listed.  

Table 3

Table 4

As for the cast magnet, (BH) max and iHc are greatly enlarged by hot working. This is because the grains are aligned by hot working and the square shape of the BH curve is significantly improved. In contrast, in the quenching process of document 2, iHc tends to decrease due to hot working. Accordingly, one of the great advantages of the present invention is that the self-coercive force is significantly improved.

Example 3

In this example, the samples were after hot working pulverized and mixed with binder.

Samples Nos. 2 and 8 of Table 3 of Example 2 were processed separately to form a powder with a particle diameter of approximately 30 μm using a crusher or a plate mill (measured using a Fischer Subsieve Sizer). The grain diameter of the Pr ₂ Fe ₁₄ B or Pr ₂ (FeCo) ₁₄ B compound in the powder particles was 2 to 3 µm.

Of the two types of powder obtained, the powder was Composition of No. 2 kneaded with 2% by weight of epoxy resin, the mixture is then shaped in a magnetic field and the preserved compact body cured.

The powder of the composition of No. 8 was carried out after a silane coupling reagent treatment with nylon 12 kneaded at about 280 ° C, the proportion of the powder Was 40 volume percent, and then injection molded. The properties of the magnets obtained are shown in Table 5 listed.  

Table 5

Table 5 shows that the intrinsic coercive force iHc is approximately at the same level as in Example 2, where hydrogen crackling was used.

As can be seen from the description above, The method according to the invention creates a permanent magnet sufficient self-coercive force only through execution hot working without pulverizing the Cast blocks as in the conventional sintering process.

Hot processing is carried out in one stage and not in two stages like the quenching process, and it doesn't work only that the magnet becomes anisotropic, but also increases the self-coercive force.

The manufacturing process of the Permanent magnets therefore compared both with the known Sintering process as well as the known quenching process significantly simplified.

Through hydrogen crackling or pulverization the samples after hot processing can be according to the invention an anisotropic binder magnet can also be created.

The reference numbers used in FIGS . 2 to 4 have the following meaning:

• 1 hydraulic stamp,
  2 form,
  3 magnetic alloy,
  4 direction of easy magnetization,
  5 printing direction,
  6 roller,
  7 direction of movement,
  8 direction of roll rotation,
  9 stamps,
  10 base plate,
  11 direction of movement of the base plate,
  12 Vertical movement of the stamp.

Claims (5)

1. A method for producing permanent magnets based on rare earth metal with the steps
Melting an alloy of 8 to 30 atomic percent R (R stands for at least one of the rare earth metals including yttrium), 2 to 28 atomic percent boron, 50 or less atomic percent cobalt, 15 or less atomic percent aluminum and the rest iron and other process-related inevitable impurities,
Casting the molten alloy to obtain a ingot and
Heat treating the ingot at a temperature of 250 ° C or more to obtain a fine crystal grain of the ingot and to align the grain axes in a specific direction, thereby magnetically hardening the ingot. 2. The method according to claim 1, characterized in that that the starting material is a magnetic casting alloy  from 8 to 25 atomic percent R, 2 to 8 atomic percent Boron, 50 or less atomic percent cobalt, 15 or less Atomic percent aluminum and the rest iron and other process-related is inevitable impurities, and that the magnetic hardness by heat treatment at one temperature of 250 ° C or more is carried out. 3. The method according to claim 1, characterized in that the heat treatment at one temperature of 500 ° C or more is carried out. 4. The method according to claim 1, characterized in that taking advantage of the alloy their ability to form slightly hydrogenated compounds, is pulverized, the powder with an organic Binder is kneaded and the mixture to achieve a resin-bonded magnet is cured. 5. The method according to claim 4, characterized in that the powder is prepared in such a way that even after the pulverization of each powder particle contains a plurality of R ₂ Fe ₁₄ B grains.