Method and apparatus for producing magnetically anisotropic Nd-Fe-B magnet material

Abstract: A method of producing a magnetically anisotropic Nd-Fe-B magnet material comprises the steps of: charging a green body produced by pressing a flaky or powdery material consisting of an amorphous alloy and/or a finely crystallized alloy, into a cavity (80) defined by a die (3) having a hole extending therethrough, a lower plunger (4) inserted into said hole; and an upper plunger (1) having a larger diameter than that of said hole; pressing said green body by elevating said lower plunger while keeping a temperature thereof (pressing temperature) at 600-850°; elevating the resulting pressed body by elevating said lower plunger (4) while heating; and upsetting said pressed body while keeping a temperature thereof (upsetting temperature) at 600-850°C to provide it with magnetic anisotropy.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method and an apparatus for producing a magnetically anisotropic Nd-Fe-B magnet material having large coercive force and magnetic energy product by compressing a green body and treating it to have magnetic anisotropy, the green body being obtained by cold forming of flakes or powders obtained by pulverizing an Nd-Fe-B magnetic ribbon formed by a rapid quenching method. It is conventionally known as disclosed in JP-C-61-34242 to produce an Nd-Fe-B sintered magnet by preparing an ingot by melting a mixture of Nd, Fe, B and if necessary additional elements, pulverizing it to form powder, sintering and heat-treating it. Further, it is known that an Nd-Fe-B magnet can be provided with a high coercive force by making the crystal grains of the magnet finer, for instance, to an average grain size of about 0.01-0.5µm. This fine crystal-type Nd-Fe-B magnet is disclosed, for instance, in EP-A-0 133 758. It is also disclosed that die upsetting can provide the magnet with magnetic anisotropy, but it is not disclosed how a pressed powder body having fine crystal grains can be efficiently formed into a compressed body with magnetic anisotropy, and what deformation by upsetting can increase the magnetic properties of the compressed body. OBJECT AND SUMMARY OF THE INVENTION The present invention is aimed at providing a method of efficiently producing a magnetically anisotropic Nd-Fe-B magnet of a substantially fine crystal type. More specifically, it is aimed at providing a magnetically anisotropic Nd-Fe-B magnet material with high coercive force and magnetic energy product by carrying out the compression of a pressed powder body and giving magnetic anisotropy thereto more efficiently than conventional methods. Another object of the present invention is to provide an apparatus for producing a magnetically anisotropic Nd-Fe-B magnet by upsetting in such a manner as to form such a final shape as a field magnet for voice coils (mostly fan shape), a field magnet for generators (mostly arc segment shape), a magnet for speakers (mostly doughnut shape) or a field magnet for flat motors (mostly circular shape). To achieve the above objects, the present invention provides the method defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing the steps according to one embodiment of the present invention: Fig. 2 is a graph showing the influence of the compression temperature on coercive force: Fig. 3 is a graph showing the relations between a strain rate and magnetic properties: Fig. 4 is a graph showing relations similar to those of Fig. 3 for the magnet of Nd₁₄Fe₈₀B₆; Fig. 5 is a graph showing the demagnetization curves of the upset magnets (Nd₁₄Fe₈₀B₆) of the present invention: Fig. 6 is a graph showing the demagnetization curves of the magnet "a" (Nd₁₄Fe₇₇B₈Ga₁) of the present invention: Fig. 7 is a view showing a separate-type die for making it easier to withdraw an upset magnet according to another embodiment of the present invention; and Fig. 8 is a view showing a die apparatus having a separate-type upper plunger according to a further embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION With respect to the composition of the magnet material, Pr and Nd have substantially the same effects, so that part or all of Nd may be replaced by Pr. A substitution of part of Nd with Dy or Tb provides a fine crystal-type magnet with particularly high coercive force and excellent thermal stability. Specifically, x is 0-0.3 when A is Dy or Tb. When the total of Nd and the A element is less than 11 atomic %, sufficient intrinsic coercive force iHc cannot be obtained, and when it exceeds 18 atomic %, the Br decreases. Therefore, it is 11-18 atomic %. When B is less than 4 atomic %, an R₂Fe₁₄B phase, a main phase of the alloy of the present invention cannot be fully formed, giving low residual magnetic flux density Br and intrinsic coercive force iHc. On the other hand, when B exceeds 11 atomic %, a magnetically undesirable phase appears, resulting in the decrease in Br. Therefore, B is 4-11 atomic %. By substituting part of Fe with Co, the Curie temperature of the alloy can be enhanced, reducing the temperature variation of residual magnetic flux density thereof. When the amount of Co substituted for Fe (y) exceeds 0.3, the Curie temperature is elevated but the anisotropy constant of the main phase decreases, making it difficult to obtain high iHc. Therefore, y is 0.3 or less. M is one or more elements other than Nd, Dy, Pr, Fe, Tb, Co and B, and it is preferably Ga, Zr, Hf, Nb, Ta, Si, Zn, Aℓ or Ti. When the amount (z) of the additional element M exceeds 3 atomic %, a large decrease in Br takes place. Thus, z is 3 or less. Incidentally, the alloy of the present invention may contain Nb coming from ferroboron. In the present invention, when an average size of fine crystal grains in the alloy exceeds 0.5µm, the iHc of the alloy decreases and the irreversible loss of flux thereof at 160°C becomes 10% or more, extremely lowering thermal stability thereof. On the other hand, when the average size is less than 0.01µm, the iHc is also low, unable to provide the desired permanent magnet. Therefore, the average size of the fine crystal grains in the alloy is limited to 0.01-0.5µm. In the present invention, flaky or powdery starting material may be prepared by the following procedures. First, an alloy of a predetermined composition is prepared by high-frequency melting or arc melting, etc., and the alloy is rapidly quenched to form flakes. The rapid quenching can be carried out by either of a single roll method or a double roll method, and materials for the roll may be Fe, Cu, etc. When Cu is used for the roll, it is preferably plated with Cr. The rapid quenching is conducted in an inert gas atmosphere such as Ar, He, etc. to prevent the oxidation of the alloy. The resulting flakes are pulverized to 100-200µm or so. The coarse powder thus formed is pressed at room temperature to provide a green body. The green body is placed in a cavity of a die 3 for compression as shown in Fig. 1, and it is kept at temperatures between 600°C and 850°C by high-frequency heating. It is compressed by elevating a lower plunger 4, and elevated to an upsetting position by elevating the lower plunger 4 while heating with a high-frequency heater 2, and then upset at temperatures of 600-850°C by lowering an upper plunger 1 while keeping the lower plunger 4 stationary. If the compression of the green body is insufficient, magnetic anisotropy cannot be fully obtained by die upsetting. Thus, it is important to carry out compression at 600-850°C to produce a pressed body having a relatively small crystal grain size. When the compression temperature is lower than 600°C, the green body to be pressed has too much resistance to deformation, making the compression difficult. On the other hand, when it exceeds 850°C, crystal grains grow too much, resulting in extreme decrease in intrinsic coercive force iHc. Fig. 2 shows how the coercive force varies depending upon the compression temperature in the pressed powder body of Nd₁₄Fe₈₀B₆. For alloys to which the present invention is applied, the compression and the die upsetting are preferably conducted at 700-760°C. As is shown in Figs. 1 (c) and (d), the upsetting of the pressed body at 600-850°C, particularly 700-760°C, can provide an anisotropic flat plate. In this case, a lowering speed of the upper plunger, namely how fast strain is given to the pressed body greatly affects the magnetic properties of the resulting magnet alloy. A strain rate is defined herein as h/h₀, wherein h is a distance of the upper plunger lowered a second, and h₀ is a height of the pressed body. The magnetic properties depend upon the strain rate as shown in Fig. 3. When the strain rate is too small, a cycle of upsetting process becomes too lengthy. Thus, it is preferably about 1x10⁻⁵/sec or more. On the other hand, when it is too fast, extreme decrease in residual magnetic flux density Br(4πIr) ensues. Thus, it is preferably about 1x10⁻¹/sec or less. More preferably, with the strain rate between about 4x10⁻⁴/sec. and about 4x10⁻²/sec., magnetically anisotropic magnet material with large magnetic energy product can be obtained by the upsetting. If necessary, the resulting flat plate may be subjected to a heat treatment of rapid quenching after heating at 600-800°C to increase the iHc thereof. By selecting a proper shape of the die cavity for effecting the upsetting, the magnetically anisotropic upset magnet in the shape of disc, doughnut or fan can be produced. The flat plate may be pulverized to form powder for anisotropic resin-bonded magnets. The present invention will be further described in detail by the following Examples. Example 1 An Nd₁₄Fe₈₀B₆ alloy was prepared by arc melting, and formed into flakes by a single roll method in an Ar atmosphere. The flakes obtained at a roll peripheral speed of 30 m/sec. were in an irregular shape having a thickness of about 30µm. It was found by X-ray diffraction that they were composed of a mixture of an amorphous phase and a finely crystalline phase phase. These flakes were pulverized to 32 mesh or less, and formed into a green body by die pressing. The pressing pressure was 0.6 kbar , and no magnetic field was applied. The green body had a diameter of about 15mm and a height of about 30mm, and the density of the green body was 8.5g/cm³. This green body 51 was placed in a cavity 80 as shown in Fig. 1, and compressed at 700°C under pressure (about 0.2 kbar) by the lower plunger 4 to provide a pressed body 52 of about 15mm in diameter and about 20mm in height. This pressed body 52 had a density of 7.4g/cm³which was sufficiently high as compared to the theoretical density of 7.5g/cm³. While continuing high-frequency heating, it was raised by elevating the lower plunger 4, and after the upper surface 89 of the lower plunger reached the top surface of the hole 90, the upper plunger 1 was lowered to compress this pressed body 53 at 700°C to conduct the upsetting thereof to provide a disc-shaped, magnetically anisotropic magnet material 54 having a diameter of about 40mm. With the lowering speed of the upper plunger varied, the relations between the strain rate and the magnetic properties were investigated. The results are shown in Fig. 4, in which the term "H.T." denotes the magnetic properties of the alloy cooled with water after die upsetting at 690°C for 30 minutes. As a result of investigating production conditions, two types of magnet materials showing the magnetization curves as shown in Fig. 5 were obtained by proper working conditions. The compression ratio h₀/h was set at 3.5 or more. Example 2 With the alloy composition of Nd₁₄Fe₇₇B₈M₁, Example 1 was repeated to prepare an upset magnet with magnetic properties as shown in the following Table. The magnet containing Ga was further investigated with respect to composition, working conditions and heat treatment conditions. The Nd₁₄Fe 79.25 B₆Ga 0.75 magnet having the magnetization curve "a", which was prepared by hot pressing at 620°C, upsetting at 675°C and heat treating at 650°C for 1 hour and then water cooling is shown in Fig. 6. The curve "b" shows the magnet prepared by pulverizing the magnet "a" by a disc mill to 105-250µm and compressing it. Example 3 As is shown in Fig. 7, with a concentrically dividable die 3′, the upset magnet 54 can be easily withdrawn by elevating the lower plunger 4 and an inner die 1 simultaneously. And with a concentrically dividable upper plunger 1′ shown in Fig. 8, the formed magnet can be withdrawn laterally after elevating the upper plunger. As a further embodiment, the die may have a plurality of steps on the upper side to carry out multi-step upsetting. As described above in detail, the present invention can provide a magnetically anisotropic Nd-Fe-B magnet material with high coercive force and high magnetic energy product. And the following effects are attained: (1) As compared to a method in which upsetting is carried out after replacing a die after compression while heating, operation efficiency is greatly high, saving much time and energy. (2) In the case of conducting upsetting by exchanging a die after the compression step, the temperature of the pressed body is inevitably lowered, resulting in the decrease in magnetic properties, particularly intrinsic coercive force iHc of the resulting magnet for unknown reasons, but such deterioration of the magnetic properties does not substantially take place in the present invention. (3) With a separate-type die structure, the upset magnet material with good properties can be produced with higher efficiency.

Claims: A method of producing a magnetically anisotropic Nd-Fe-B magnet material having an average crystal grain size of 0.01∼0.5 µm, comprising the steps of producing a green body by pressing a flaky or powdery material consisting of an amorphous alloy and/or a finely crystallised alloy represented by the formula (Nd 1-x A x )₁₁₋₁₈(Fe 1-y Co) bal B₄₋₁₁M z wherein 0 ≦ x ≦ 1; y ≦ 0.3; z ≦ 3; M represents at least one element other than Nd, Dy, Pr, Fe, Tb, Co and B; and A represents at least one of the elements Dy, Pr and Tb, pressing the green body at a temperature of 600∼850 °C using a die (3) which has a hole extending therethrough, a lower plunger (4) inserted in the hole, and an upper plunger (1), and upsetting the pressed body at a temperature of 600∼ 850 °C, to provide it with magnetic anisotropy, characterised in that a die (3) is used in which the upper plunger (1) has a larger diameter than the hole in the die, that the pressing step is performed by elevating the lower plunger (4), that the pressed body is elevated after the pressing step by further elevating the lower plunger (4), and that the pressing and upsetting steps are performed while the temperature of the body is continuously kept at said temperature of 600∼850 °C. The method of claim 1, wherein M represents one or more elements selected from Ga, Zr, Hf, Nb, Ta, Si, Zn, Aℓ and Ti. The method of claim 1 or 2, wherein the pressing temperature and the upsetting temperature are respectively within the range of 700-760°C. The method of any of claims 1 to 3, wherein in the upsetting step, said magnet material is plastically deformed at a strain rate of about 1x10⁻⁵/s to about 1x10⁻¹/s, preferably about 4x10⁻⁴/s to about 4x10⁻²/s. The method of any of claims 1 to 4, wherein both of the pressing step and the upsetting step are carried out under reduced pressure of 13 Pa or less. The method of any of claims 1 to 5, wherein the upsetting step is carried out with an appropriate die of such a shape as to form a fan-shaped, arc segment-shaped or doughnut-shaped magnet material. The method of any of claims 1 to 6, wherein said pressed body has a density which is 95% or more of the theoretical density thereof. An apparatus for producing a magnetically anisotropic Nd-Fe-B magnet material, comprising a die (3) having a hole extending therethrough, an upper plunger (1) and a lower plunger (4) axially movable within the hole of said die (3), a cavity (80) defined by the die (3) and the upper and lower plungers (1, 4), means (2) for heating a pressed Nd-Fe-B body at a temperature of 600∼850 °C, and airtight means for keeping the pressed body under reduced pressure, characterised in that the upper plunger (1) has a larger diameter than the hole of said die (3) and is movable to a position where it closes said hole. The apparatus of claim 8, wherein the die and/or the upper plunger is/are made of separate coaxial cylindrical members (3, 3'; 1, 1'). The apparatus of claim 8 or 9, wherein the die has a plurality of concentric steps on its upper side, and a plurality of upper plungers having different diameters are provided.

Inventors: IWASAKI KATSUNORI, NOZAWA YASUTO, TANIGAWA SHIGEHO, IWASAKI, KATSUNORI, NOZAWA, YASUTO, TANIGAWA, SHIGEHO
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1988
Main classification: H01F41
IPC codes: H01F41, H01F1, B22F9, B22F3