DE3626406C2 - Process for the production of permanent magnets based on rare earth metals - Google Patents

Description

The invention relates to a process for the preparation of R-Fe-B Permanent magnet (R is a rare earth metal).

In practice, currently of the three following procedures used for the production of magnets of the R-Fe-B type:

• (1) The sintering process based on the technique of powder metallurgy is based (reference is made to document [1]: M. Sagawa et al., J. Applied Physics, Vol. 55 (6), March 15, 1984, Page 2083, as well as to the documents EP 0 101 552 A2, EP 0 106 948 A2, EP 0 134 304 A1 and EP 0 134 305 A1);
• (2) a method in which by means of a centrifugal casting Apparatus for producing an amorphous alloy ver is used, rapidly cooled tape pieces and from these means thermoforming a magnet is made (directed be on paper [2]: R.W. Lee; Applied Physics Letters, Vol. 46 (8), April 15, 1985, page 790, as well as to the Documents EP 0 108 474 A2 and EP 0 144 112 A1);
• (3) a method in which a mechanical alignment Treatment on the prepared according to the above method (2) Band pieces is carried out, by means of a two-stage Hot pressing technique (see the already mentioned publication [2] and in particular EP 0 133 758 A2).

In the sintering method of (1), an alloy block of the preferred composition Nd₁₅B₈Fe₇₇ by melting and Poured and the alloy block then turned into one fine powder with a particle diameter of about 3 microns processed. The powder is kneaded with the binder, which serves as a molding additive, and for obtaining a molded article in a magnetic field under pressure. The shaped body becomes a Hour sintered at about 1100 ° C in an argon atmosphere and then quenched to room temperature. After sintering will be the body is heat treated at about 600 ° C and thereby the Eigencentric force increased.

In the method (2) quenched pieces of tape of an R-Fe-B alloy of the general formula R _100-x (TM ₁ -y B _y ) _x are produced by means of the centrifugal casting apparatus which flings at an optimum substrate speed (50 × 90, 5y10). In this general formula, TM denotes a transition metal, especially Fe or a mixture of Fe and Co. The pieces of tape thus produced have a thickness of 30 μm and are composed of a collection of grains whose diameter is 0.1 μm or less , These pieces of tape are fragile and magnetically isotropic because the grains are isotropically distributed. The tape pieces are crushed into particles of suitable size and molded under pressure. At a pressure of about 700 MPa, the material is compressed to 85% by volume.

In the method (3), the quenched band pieces in a graphite mold or other suitable high temperature mold given in vacuum or an inert gas atmosphere preheated to about 700 ° C. When the temperature of the Band pieces has risen to a predetermined value, be they are exposed to unidirectional pressure and become one pressed together contiguous body. Temperature and time are not limited, although a temperature of 725 ± 250 ° C and a pressure of about 140 MPa to achieve a sufficient  Plasticity are favorable. The grains of the magnet are aligned in the printing direction.

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

This process is called the "two-stage hot pressing process". With it, anisotropic R-Fe-B type magnets become higher Dense created.

It is favorable, the particle diameter of the grains of Pieces of tape originally made by centrifugal casting were to make something smaller than the grain diameter at the maximum inherent coercive force occurs. In a certain way In fact, the grain size during the hot pressing will become great treatment coarser, so that if they are smaller beforehand as the optimum diameter, they after hot pressing treatment are optimal.

With the mentioned known techniques, magnets of the R-Fe B-type, but these techniques have some Disadvantage.

In the sintering process (1), the alloy must be fine powder be ground or ground. Alloys of the R-Fe-B type However, they are extremely active towards oxygen, so that their powder oxidized particularly easily. That is why the Oxygen concentration in the sintered body inevitably high.

When the powder is formed into a body, it needs one Additive such as zinc stearate. Although such a Additive is removed before the sintering process remains one  certain amount of the additive in the form of carbon in the Magnet back. The carbon affects the magnetic Properties of an R-Fe-B magnet strong.

The after the compression molding with the addition of the molding additive he holding blank is easily fragile and difficult to handle. It is therefore very tedious to keep these blanks in good condition bring the sintering furnace, which is a significant disadvantage provides. For the production of sintered magnets of the R-Fe-B type Therefore, expensive equipment is required. In addition, the Productivity extremely low, resulting in high production costs these magnets leads. The sintering method (1) is therefore not Satisfactory because of the inherently low raw material costs for this type of magnet due to the high production costs be compensated.

In the methods (2) and (3) becomes a vacuum spinner used, which is currently not very productive and is expensive.

Magnets produced by the method (2) have a bad temperature coefficient and are unfavorable for practical use.

In the process (3), the hot working (Hot pressing) in two stages. This However, the process is very inefficient. EP 0 133 758 A2 describes that Hotpreßversuche also on massive Alloy blocks were made. It is indeed a significant directional dependence of remanence, but not the expected improvement in coercive force Service. This is attributed to the fact that the grain size in massive alloy block is too big and hot pressing for such therefore unsuitable.  

From the book of Suren Cedigian "The Magnetic Materials, Fundamentals, Properties, Applications ", VDI-Verlag GmbH, 1973, pp 28-35, it is known that various Magnetic materials such as Vicalloy, Remendur, Cobalt-Gold Alloys, cobalt-iron-beryllium alloys, chromium-nickel Alloys, Cunife and Cunico alloys and Comalloy be processed differently. So it is stated that the as Remendur known iron-cobalt alloy melted in Ingots poured, hot forged and rolled, then opened Heated to 925 ° C and quenched in cold salt water. Through this treatment, the material is soft and then could reduced to small dimensions by cold rolling or drawing become. After mechanical stress, the material becomes tempered for about two hours at 600 ° C. Other Materials such as copper-nickel-iron alloys are used to improve the magnetic properties cold formed.

From DE 31 49 924 A1 is for a Sm, Cu, Fe and at least an element from the group Zr, Ti, Hf, Nb and V containing Magnetic alloy a method known by casting the Melt a stem macrostructure to achieve.

The object of the invention is to provide a method with the permanent magnets of the type mentioned at low cost ago can be put, the good features have.

This object is achieved by a method with the Characteristics in the claim solved.  

In the alloys according to claim the ingot is fine-grained by hot working and magnetically anisotropic. The subsequent heat treatment becomes high Values of coercive force achieved.

Because of the fine grain, moreover, the coercive force of the heat treated and heat treated Gußblocks relatively high.  

The invention proceeds from the reduction of the grain Hot working use. It is relatively light, the grain of the R₂Fe₁₄B compound in the cast state about the same size as to make that achievable by sintering. Now a cast block, containing the R₂Fe₁₄B phase of this grain size, Warm worked, then the grain is smaller and aligned.

example

First, alloys whose composition is shown in Table 1 is melted in an induction furnace and to Formation of stem zones poured into a mold. To Performing the hot working with a degree of deformation of about 50% or more (pressing in this example) was the ingot at a temperature of 1000 ° C for 24 Heat treated for a few hours to magnetically harden it. After Heat treatment was the average grain diameter of the sample included about 15 μm.

The properties of the obtained magnets are shown in Table 2 listed.

No. composition 1 Pr₈Fe₈₈B₄ 2 Pr₁₄Fe₈₂B₄ 3 Pr₂₀Fe₇₆B₄ 4 Pr₂₅Fe₇₁B₄ 5 Pr₁₄Fe₈₄B₂ 6 Pr₁₄Fe₈₀B₆ 7 Pr₁₄Fe₇₈B₈ 8th Pr₁₄Fe₇₂Co₁₀B₄ 9 Pr₁₄Fe₅₇Co₂₅B₄ 10 Pr₁₄Fe₄₂Co₄₀B₄ 11 Pr₁₄Dy₂Fe₈₁B₄ 12 Pr₁₄Fe₈₀B₄Si₂ 13 Pr₁₄Fe₇₈Al₄B₄ 14 Pr₁₄Fe₇₄Al₈B₄ 15 Pr₁₄Fe₇₀Al₁₂B₄ 16 Pr₁₄Fe₆₇Al₁₅B₄ 17 Pr₁₄Fe₇₈Mo₄B₄ 18 Nd₁₄Fe₈₂B₄ 19 Ce₃Nd₃Pr₈Fe₈₂B₄ 20 Nd₁₄Fe₇₆Al₄B₄

Table 2

As far as the casting magnet is concerned, (BH) max and iHc pass through the hot working greatly increased. This is because the grains are aligned by the hot working and the square shape of the BH curve is significantly improved.

Claims (1)

Process for the production of permanent magnets based on rare earth metals with the steps

• (a) preparing a melt of an alloy of one of the following compositions: Pr₈Fe₈₈B₄
  Pr₁₄Fe₈₂B₄
  Pr₂₀Fe₇₆B₄
  Pr₂₅Fe₇₁B₄
  Pr₁₄Fe₈₄B₂
  Pr₁₄Fe₈₀B₆
  Pr₁₄Fe₇₈B₈
  Pr₁₄Fe₇₂Co₁₀B₄
  Pr₁₄Fe₅₇Co₂₅B₄
  Pr₁₄Fe₄₂Co₄₀B₄
  Pr₁₄Dy₂Fe₈₁B₄
  Pr₁₄Fe₈₀B₄Si₂
  Pr₁₄Fe₇₈Al₄B₄
  Pr₁₄Fe₇₄Al₈B₄
  Pr₁₄Fe₇₀Al₁₂B₄
  Pr₁₄Fe₆₇Al₁₅B₄
  Pr₁₄Fe₇₈Mo₄B₄
  Nd₁₄Fe₈₂B₄
  Ce₃Nd₃Pr₈Fe₈₂B₄
  Nd₁₄Fe₇₆Al₄B₄
• (b) pouring the melt to form stem zones into a mold to obtain a ingot, excluding a ingot whose smallest dimension is less than 11 mm,
• (c) hot working the ingot having a degree of deformation of about 50% or more to obtain a fine crystal grain of the ingot and aligning the grain axes in a particular direction, and
• (d) heat treating the ingot at a temperature of 1000 ° C for 24 hours, after which the ingot has an average grain diameter of about 15 μm.