Abstract:
A method for producing permanent magnet alloy particles suitable for use in producing bonded permanent magnets. A melt or molten mass of a permanent magnet alloy having at least one rare earth element, at least one transition element, preferably iron, and boron is produced. The melt is inert gas atomized to form spherical particles within the size range of 1 to 1000 microns. The particles are heat treated in a nonoxidizing atmosphere for a time at temperature to significantly increase the intrinsic coercivity of the particles without sintering the particles to substantially full density. Thereafter, the particles are separated to produce a discrete particle mass. The particles during heat treatment may be maintained in motion to prevent sintering thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for producing permanent magnet alloy particles of a rare earth element containing permanent magnet alloy, which particles are suitable for use in producing bonded permanent magnets. 
     2. Description of the Prior Art 
     In various electrical applications, such as in electric motors, it is known to use bonded permanent magnets. Bonded permanent magnets are constructed of a dispersion of permanent magnet alloy particles in a bonding non-magnetic matrix of for example plastic. The permanent magnet particles are dispersed in the bonding matrix and the matrix is permitted to cure and harden either with or without magnetically orienting the dispersed particles therein. 
     Magnet alloys of at least one rare earth element, iron and boron are known to exhibit excellent energy product per unit volume and thus it is desirable to use these alloys in bonded magnets where low cost, high plasticity and good magnetic properties are required. It is likewise known with respect to these permanent magnet alloys that comminuting of these alloys to produce the fine particles required in the production of bonded magnets results in a significant decrease in the intrinsic coercivity of the alloy to a level wherein the particles are not suitable for use in producing bonded magnets. Hence, it is not possible to produce particles of these alloys for use in the production of bonded permanent magnets by comminuting castings of the alloy. 
     It is known to produce permanent magnet alloys of these compositions in particle form by inert gas atomization of a prealloyed melt of the alloy. The as-atomized particles, however, do not have sufficient intrinsic coercivity for use in producing bonded permanent magnets. 
     SUMMARY OF THE INVENTION 
     It is accordingly a primary object of the present invention to provide a method for producing permanent magnet alloy particles suitable for use in producing bonded permanent magnets wherein the required fine particle size in combination with the required coercivity is achieved. 
     Another object of the invention is to provide a method for producing permanent magnet alloy particles suitable for use in producing bonded permanent magents wherein the combination of particle size and coercivity is achieved without requiring comminution of a dense article, such as a casting, of the alloy to achieve the particles. 
     In accordance with the invention, and specifically the method thereof, permanent magnet alloy particles suitable for use in producing bonded permanent magnets are provided by producing a melt of a permanent magnet alloy comprising at least one rare earth element, at least one transition element and boron. The melt is inert gas atomized to form spherical particles within a particle size range of 1 to 1,000 microns. Thereafter, the particles are heat treated in a non-oxidizing atmosphere for a time at a temperature to significantly increase the intrinsic coercivity of the particles without sintering the particles to substantially full density. Thereafter, the particles are separated to produce a discrete particle mass. 
     Alternately, in acccordance with a second embodiment of the invention, heat treating may be conducted in a moving inert gas atmosphere while maintaining the particles in motion to significantly increase the intrinsic coercivity of the particles without substantially sintering the particles. 
     During heat treating, the intrinsic coercivity of the particles may be increased to at least 10,000 Oe. The heat treating temperature in accordance with the first embodiment of the invention may be less than 750° C. and less than 700° C. with respect to the second embodiment. 
     In the second embodiment of the invention the particles may be maintained in motion during heat treating by tumbling the particles in a rotating furnace. Alternately, a fluidized bed, a vibrating table or other conventional devices suitable for this purpose may be substituted for the rotating furnace. 
     After heat treating the particles may have a hard magnetic phase of Nd 2  Fe 14  B. 
     The rare earth element of the permanent magnet alloy may include neodymium or neodymium in combination with dysprosium. 
     The permanent magnet alloy may comprise, in weight percent, 29.5 to 40 total of at least one of the rare earth elements neodymium, praseodymium and dysprosium up to 4.5, 50 to 70 iron and the balance boron. Preferably, if dysprosium is present in combination with neodymium and/or praseodymium, the total content of all these elements is 29.5 to 40% with dysprosium being within the range of 0.7 to 4.5%. Alternatively, the permanent magnet alloy may comprise, in weight percent, 29.5 to 40% of at least one rare earth element neodymium, praseodymium, dysprosium, holmium, erbium, thulium, galium, indium or mischmetal, with at least 29.5% of this total rare earth element content being neodymium, up to 70% of at least one transition metal which may be iron, nickel and cobalt, with at least 50% iron, and 0.5 to 1.5% boron. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to presently preferred embodiments of the invention, which are described in the following examples. In the examples and throughout the specification and claims, all parts and percentages are by weight percent unless otherwise specified. 
     EXAMPLE 1 
     Difficulty in the Generation of Coercivity in Comminuted Cast Alloys (As-cast Alloys Comminuted to Various Particle Sizes) 
     Three alloys of the compositions in weight percent designated in Table I were melted, cast and then processed to powder particles of varying size. The particles were mixed with molten paraffin wax and then aligned in a 25 kOe field. The composite was kept in a weak magnetic field until the wax hardened. The composite was pulse magnetized in a 35 kOe field. The intrinsic coercivities of the powder-wax composites were measured using a hysteresigraph. The results are listed in Table II. 
     
                       TABLE I______________________________________Compositions of Cast Alloys (weight percent)Alloy Code   Nd     Dy         Fe   B______________________________________1            35.2   1.6        bal. 1.262            37.4   1.4        bal. 1.223            39.3   1.7        bal. 1.21______________________________________ 
    
     
                       TABLE II______________________________________Intrinsic Coercivity As a Functionof Particle Size - Crushed Cast AlloysAlloy Code   Particle Size (mesh)                      H.sub.ci (Oe)______________________________________1            -35 + 200     300        -60 + 200     450        5.4 microns*  11002            -35 + 200     350        -60 + 200     450        2.41 microns* 23003            -30 + 200     300        -60 + 200     600        5.6 microns*  900______________________________________ *Particle size listed in microns rather than by mesh size. 
    
     The composites had poor intrinsic coercivities rendering them unsuitable for use in a permanent magnet. Various heat treatments were conducted in an attempt to generate reasonable intrinsic coercivity in these ingot cast and crushed alloy composites. These attempts were unsuccessful. For example, after heat-treating samples of the crushed cast alloys of Table I for 3 hours at 500° C. the intrinsic coercivity H ci  (Oe) values decreased. Samples of each alloy that showed the highest H ci  values in the crushed and jet milled condition were loaded into a Vycor tube in an argon atmosphere and the tube was then evacuated. The powder in the Vycor tube was heat-treated at 500° C. for 3 hours. Test results on these powders were as follows: 
     
                       TABLE II-A______________________________________Intrinsic Coercivity of CrushedCast Alloys after Heat-Treatment*Alloy Code   Particle Size (mesh)                      H.sub.ci (Oe)______________________________________1            5.4 microns    5002            2.41 microns  13003            5.6 microns*  1100______________________________________ *Heat-Treatment  500° C. for 3 hours. 
    
     EXAMPLE 2 
     Lack of Adequate Coercivity in As-Atomized Powder 
     An alloy of the composition in weight percent 31.3 Nd, 2.6 Dy, 64.4 Fe, and 1.13 B was vacuum induction melted and inert gas atomized. The alloy particles were screened to various particle sizes. Wax samples were prepared as described in Example 1. The as-atomized powder did not exhibit any significant level of coercivity, Table III. 
     
                       TABLE III______________________________________Intrinsic Coercivity as a Functionof Particle Size: As-Atomized PowderParticle Size (mesh)            H.sub.ci (Oe)______________________________________ -60 + 100       2600-100 + 200       2600-200 + 325       3100-325             3800______________________________________ 
    
     EXAMPLE 3 
     Generation of Coercivity in Atomized Powders and Effect of Comminution on Heat Treated Atomized Powders 
     Inert gas atomized powder in the as-atomized condition of the composition in weight percent 31.3 Nd, 2.6 Dy, 64.4 Fe and 1.13 B was screened to a particle size of -325 mesh (44 microns). The powder was heat treated in vacuum at various temperatures for 3 hours. Heat treatment at relatively low temperatures (500°-625° C.) resulted in varying degrees of densification (sintering), Table IV. A sample from this partially sintered material was ground square then pulse magnetized in a 35 KOe field. The intrinsic coercivity of the partially sintered material was measured using a hysteresigraph. The remaining portion of the partially sintered material was crushed to a -325 mesh (44 microns) powder. Wax samples were prepared using the procedure described in Example 1. The intrinsic coercivity of each sample was measured. The results are listed in Table V. 
     It may be observed from the data listed in Table V that the heat treatment resulted in high levels of coercivity in the atomized powder. This heat treatment resulted in various degrees of partial sintering as listed in Table IV. When the high coercivity partially sintered mass was crushed to yield powder, the intrinsic coercivity was degraded somewhat but the degree of coercivity loss was considerably less than that for the powder obtained by crushing solid, fully densified, magnets. This experiment indicates that atomized powder can be heat treated to yield a loosely (partially) densified powder which can be readily comminuted to yield a powder with a reasonably high H ci . 
     
                       TABLE IV______________________________________Density Values for Partially Sintered*Heat Treated Atomized Powders______________________________________(Time of Heat Treatment - 10 Hours)        Temperature                   DensityAlloy        (°C.)                   (g/cm.sup.3)______________________________________A            500        4.56        525        4.14        550        4.33        575        4.14        600        4.19        625        4.19B            475        4.39        500        4.45        525        4.37        550        4.40        600        3.41        625        4.40C            475        4.26        500        4.30        525        4.45        550        4.33        575        4.07        600        4.60        625        4.37______________________________________Composition (wt. %)Alloy Code  Nd     Dy          Fe   B______________________________________A           29.5   4.5         bal. 1.00B           31.3   2.6         bal. 1.13C           33.5   0.7         bal. 1.00______________________________________ *Density of Fully Dense Solid NdDy-Fe-B Magnets is 7.55 g/cm.sup.3. 
    
     
                                           TABLE V__________________________________________________________________________Intrinsic Coercivity (KOe) as a Function ofHeat Treatment Temprature: Various RE-Fe-B Alloys__________________________________________________________________________(Time at Temperature - 10 Hours)       Temperature (°C.)AlloyCondition       475   500                525                   550   575                            600 625__________________________________________________________________________A    Part. sintered       N.M.  3.6*                14.6                   N.M.  15.7                            15.8                                15.4Powder 11.7  12.7                12.2                   12.7  12.8                            13.8                                13.8B    Part. sintered       3.6*  8.3*                9.6                   10.8  12.5                            13.2                                13.2Powder 9.6   10.3                8.8                   9.7    9.9                            10.6                                 9.3C    Part. sintered       5.1*  7.0*                7.7                   8.2    8.0                             9.3                                 9.0Powder 6.5   5.2                6.9                   7.5    7.2                             7.9                                 7.9__________________________________________________________________________Composition (wt. %)Alloy Code   Nd Dy         Fe B__________________________________________________________________________A            29.5           4.5        bal.                         1.00B            31.3           2.6        bal.                         1.13C            33.5           0.7        bal.                         1.00__________________________________________________________________________ N.M. = Not measured * = Sample was very soft and thus difficult to measure accurately. 
    
     EXAMPLE 4 
     Effect of Heat Treatment on Intrinsic Coercivity and Densification of Atomized Powders While in a Dynamic Heat Treatment Atmosphere 
     Inert gas atomized alloy spherical powder of the composition in weight percent 31.3 Nd, 2.6 Dy, 64.4 Fe and 1.13 B was heat treated in a flowing inert gas atmosphere rotating furnace apparatus to enable the generation of coercivity (generation of appropriate metallurgical structure by heat treatment required for desired H ci ) while minimizing the degree of sintering. When heat treated using similar time and temperature parameters as described in Example 3, the use of the rotating furnace apparatus minimized the amount of sintering and enabled a powder having adequate intrinsic coercivity for bonded magnets to be obtained, Table VI. 
     The intrinsic coercivity test results show that a significant improvement in intrinsic coercivity occurs when the as-atomized powder (H ci  =5800 Oe) is heat-treated at different temperatures up to 750° C. For the -325 mesh powder that did not partially sinter during the heat treatment in an inert gas atmosphere, the optimum temperature of heat treatment was below 700° C. Above this temperature, a drop in coercivity occurs. For the partially sintered spherical gas atomized powder that had been heated in the same temperature range in an inert gas atmosphere, prior to comminuting to -325 mesh, the optimum temperatures of heat treatment were below 750° C. 
     
                       TABLE VI______________________________________Intrinsic Coercivity of Heat-Treated,Gas Atomized -325 Mesh Powder AfterVarious TreatmentsWt. %(Alloy B - 31.3 Nd, 2.6 Dy, 1.1 B, Bal. Fe)      Heat      Heat-Treated      Treated   Partially Sintered PowderHeat Treatment,      Powder    Crushed to -325 Mesh Powder°C. H.sub.ci, Oe                H.sub.ci Oe______________________________________As-Atomized,      --        --H.sub.ci = 5800 Oe500, 10 hrs.      10,700    --550, 10 hrs.      12,000    11,500600, 10 hrs.      11,200    11,500600, 22 hrs.      10,600    12,000650, 10 hrs.      10,400    11,500700, 10 hrs.       6,300    12,000750, 10 hrs.       6,200     9,900______________________________________ 
    
     EXAMPLE 5 
     Gas atomized Alloy A (29.5% Nd, 4.5% Dy, 1.0% B, Bal. Fe) powder was heat treated in a flowing inert gas atmosphere rotating furnace at various times and temperatures and screened to different size fractions, Table VII. The furnace was constructed to provide an inert atmosphere and continuous movement and thus yield without sintering a heat treated powder with adequate H ci . 
     The intrinsic coercivity test results on samples of different size material show that very good coercivities are obtained regardless of the size of the spherical atomized powder. Higher values were obtained, however, on the size fractions above -325 mesh. 
     
                                           TABLE VII__________________________________________________________________________Intrinsic Coercivity of Heat-Treated Gas-Atomized Powder of Various Size FractionsWt. %(Alloy A - 29.5 Nd, 4.5 Dy, 1.0 B, Bal. Fe)Powder Size   500 C.-22 Hrs.           600 C.-10 Hrs.                   600 C.-22 Hrs.                           650C-22 Hrs.Mesh    Oe      Oe      Oe      Oe__________________________________________________________________________-325    10,800  11,100  11,100  10,300+325    14,600  15,500  15,700  15,000-30 to 60   15,400  13,800  ND      14,600 -60 to 100   15,700  14,600  ND      15,300-100 to 200   15,000  15,100  ND      13,900-200 to 325   12,600  13,700  ND      11,600__________________________________________________________________________ ND  Not Determined