Abstract:
A method for producing a fully dense permanent magnet article by placing a particle charge of the desired permanent magnet alloy in a container, sealing the container, heating the container and charge and extruding to achieve a magnet having mechanical anisotropic crystal alignment and full density.

Description:
This application is a continuation of application Ser. No. 889,760, filed July 28, 1986 abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     For various permanent magnet applications, it is known to produce a fully dense rod or bar of a permanent magnet alloy, which is then divided and otherwise fabricated into the desired magnet configuration. It is also known to produce a product of this character by the use magnet particles, which may be prealloyed particles of the desired permanent magnet composition. The particles are produced for example by either casting and comminution of a solid article or gas atomization of a molten alloy. Gas atomized particles are typically comminuted to achieve very fine particle sizes. Ideally the particle sizes should be such that each particle constitutes a single crystal domain. The comminuted particles are consolidated into the essentially fully dense article by die pressing or isostatic pressing followed by high-temperature sintering. To achieve the desired magnetic anisotrophy, the crystal particles are subjected to alignment in a magnetic field prior to the consolidation step. 
     In permanent magnet alloys, the crystals generally have a direction of optimum magnetization and thus optimum magnetic force. Consequently, during alignment the crystals are oriented in the direction that provides optimum magnetic force in a direction desired for the intended use of the magnet. To provide a magnet having optimum magnetic properties, therefore, magnetic anisotrophy is achieved with the crystals oriented with their direction of optimum magnetization in the desired and selected direction. 
     This conventional practice is used to produce rare-earth element containing magnet alloys and specifically alloys of neodymium-iron-boron. The conventional practices used for this purpose suffer from various disadvantages. Specifically, during the comminution of the atomized particles large amounts of cold work are introduced that produce crystal defects and oxidation results which lowers the effective rare-earth element content of the alloy. Consequently, rare-earth additions must be used in the melt from which the cast or atomized particles are to be produced or in the powder mixture prior to sintering in an amount in excess of that desired in the final product to compensate for oxidation. Also, the practice is expensive due to the complex and multiple operations prior to and including consolidation, which operations include comminuting, aligning and sintering. The equipment required for this purpose is expensive both from the standpoint of construction and operation. 
     Permanent magnets made by these practices are known for use with various types of electric motors, holding devices and transducers, including loudspeakers and microphones. For many of these applications, the permanent magnets have a circular cross section constituting a plurality of arc segments comprising a circular permanent magnet assembly. Other cross-sectional shapes, including square, pentagonal and the like may also be used. With magnet assemblies of this type, and particularly those having a circular cross section, the magnet is typically characterized by anisotropic crystal alignment. 
     During mechanical working the crystals will tend to orient in the direction of easiest crystal flow. This results in mechanical, crystal anisotrophy. The preferred orientation from the standpoint of optimum directional magnetic properties is desirably established in the optimum crystal magnetization direction by this mechanical crystal anisotrophy. 
     OBJECTS OF THE INVENTION 
     It is accordingly a primary object of the present invention to provide a method for producing fully dense, permanent magnet alloy articles having mechanical anisotropic crystal alignment by an efficient, low-cost practice. 
     An additional object of the invention is to provide a method for producing permanent magnet articles of this type wherein cold work resulting from comminution and oxidation of the magnet particles with attendant excessive loss in effective alloying elements, such as rare-earth elements, including neodymium, may be avoided. 
     A further object of the invention is to provide a method for producing permanent magnet alloy articles of this type wherein the steps of comminution of the atomized particles and alignment in a magnetic field may be eliminated from the production practice to correspondingly decrease production costs. 
     Another object of the invention is to produce a permanent magnet characterized by anisotropic radial crystal alignment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic showing of an anisotropic, transverse aligned and anisotropic, transverse magnetized magnet article in accordance with prior art practice; 
     FIG. 2 is a schematic showing of one embodiment of an anisotropic, radial aligned and anisotropic, radial magnetized magnet article in accordance with the invention; and 
     FIG. 3 is a schematic showing of an additional embodiment of an anisotropic, radial aligned and anisotropic, radial magnetized arc-section articles constituting a magnet assembly in accordance with the invention. 
    
    
     SUMMARY OF THE INVENTION 
     Broadly, the method of the invention provides for the production of a fully dense permanent magnet alloy article by producing a particle charge of a permanent magnet alloy composition from which the article is to be made. The charge is placed in a container and the container is evacuated, sealed and heated to elevated temperature. It is then extruded to achieve mechanical anisotropic crystal alignment and to compact the charge to full density to produce the desired fully dense article. The particle charge may comprise prealloyed, as gas atomized particles. Extrusion may be conducted at a temperature within the range of 1400 to 2000 F. 
     The permanent magnet article of the invention may be characterized by mechanical anisotropic crystal alignment which may be radial. The magnet article preferably has an arcuate peripheral surface and an arcuate inner surface and is characterized by mechanical anisotropic radial crystal alignment and corresponding anisotropic radial magnetic alignment. The magnet article may have a circular peripheral surface and an axial opening defining a circular inner surface. Also the magnet article may include an arc segment having an arcuate peripheral surface and a generally coaxial arcuate inner surface. The alloy of the magnet may comprise neodynium-iron-boron. 
     In accordance with the invention, mechanical radial alignment of the extruded magnet results in the crystals being aligned for optimum magnetic properties in the radial direction rather than axially. In a cylindrical magnet, during magnetization if the center or axis is open, one pole is on the inner surface and the other is on the outer surface in a radial pattern of magnetization. With the magnet of the invention the crystal alignment and magnetic poles may extend radially. Therefore, the magnetic field is uniform around the entire perimeter of the magnet. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By the use of as atomized powder and specifically as gas atomized power, comminution is avoided to accordingly avoid additional or excessive oxidation and loss of alloying elements, such as neodymium, and to eliminate cold working or deformation that introduces crystal defects. With the extrusion practice in accordance with the invention the desired mechanical radial anisotropic crystal alignment is achieved by the extrusion practice without requiring particle sizes finer than achieved in the as atomized state and without the use of a magnetizing field from a high cost magnetizing source. Consequently with the extrusion practice in accordance with the invention both consolidation to achieve the desired full density and anisotropic crystal alignment is achieved by one operation, thereby eliminating the conventional practice of aligning in a magnetic field prior to consolidation. The crystal alignment may be radial as well as anisotropic for magnet articles having arcuate or circular structure. 
     With reference to the drawings, FIG. 1 shows a prior art circular magnet, designated as 10, that is axially aligned and magnetized with the arrows indicating the alignment and magnetized direction, and N and S indicating the north and south poles, respectively. Because of the axial alignment, the magnetic field produced by this magnet would not be uniform about the periphery thereof. FIG. 2 shows a magnet, designated as 12, having a center opening 14. By having the magnet radially aligned and radially magnetized in accordance with the invention, as indicated by the arrows, the magnetic field produced by this magnet will be uniform about the periphery of the magnet. FIG. 3 shows a magnet assembly, designated as 16, having two identical arc segments 18 and 20. As may be seen from the direction of the arrows, the magnet segments 18 and 20 are radially aligned and magnetized in a like manner to the magnet shown in FIG. 2. This magnet would also produce a magnetic field that is uniform about the periphery of the magnet assembly. 
     As will be demonstrated hereinafter, the extrusion temperature is significant. If the temperature is too high such will cause undue crystal growth to impair the magnetic properties of the magnet alloy article, specifically energy product. If, on the other hand, the extrusion temperature is too low effective extrusion both from the standpoint of consolidation to achieve full density and mechanical anisotropic crystal alignment will not be achieved. 
     SPECIFIC EXAMPLES 
     Particle charges of the following permanent alloy compositions were prepared for use in producing magnet samples for testing. All of the samples were of the permanent magnet alloy 33Ne, 66Fe, 1B, in weight percent, which was gas atomized by the use of argon to produce the particle charges. The alloy is designated as 45H. Particle charges were placed in steel cylindrical containers and extruded to full density to produce magnets. 
     
                                           TABLE I__________________________________________________________________________Magnetic Properties of Extruded magnets.Material: Alloy 45H -10 mesh powderDie Extrusion      MeasuringSize    Temperature      Direction             Br  Hc  Hci BHmax                             HkInch    °F.      (as extruded)             Gauss                 Oe  Oe  MGOe                             Oe__________________________________________________________________________0.75    1600   axial  4100                 3200                     8400                         3.2 1550      radial          1  7800                 5900                     9300                         12.4                             3400      radial          2  7800                 6900                     9350                         12.8                             35000.75    1700   axial  3920                 3000                     8730                         3.0 1400      radial          1  7600                 5380                     8800                         11.1                             2650      radial          2  7600                 5380                     8620                         11.6                             28000.75    1800   axial  3700                 2800                     8150                         2.7 1400      radial          1  7580                 5100                     8000                         11.2                             2450      radial          2  7100*                 4850*                     8000                         9.4*                             24000.75    1900   axial  3500                 2400                     5650                         2.3 1000      radial          1  6800                 4420                     6400                         8.8 2200      radial          2  6700                 4350                     6350                         8.6 19000.625    1900   axial  3800                 2800                     7000                         2.6 1150      radial          1  7150                 4450                     6700                         9.2 2050      radial          2  7200                 4450                     7670                         9.4 21000.75    2000   axial  3900                 2800                     6700                         2.9 1100      radial          1  6800                 4880                     5900                         7.6 1500      radial          2  7000                 4000                     6100                         8.0 1700**0.75    1900   axial  4350                 2150                     10650                         3.4 1300      radial          1  6000                 4100                     10600                         6.3 1650      radial          2  6200                 4200                     10250                         6.8 1600**0.75    2000   axial  1500                 800 1900                         0.3 200      radial          1  5500                 3000                     7400                         4.0 700      radial          2  5000                 2800                     7300                         3.4 700__________________________________________________________________________ *Sample chipped **As-cast 30B alloy extruded at 2000 F. 
    
     The samples were extruded over the temperature range of 1600-2000 F. 
     As may be seen from the data presented in Table I, remanence (Br) and energy product (BH max ) are affected by the extrusion temperature. Specifically, the lower extrusion temperatures produced improved remanence and energy product values. At each temperature a drastic improvement in these properties was achieved with radial alignment, as opposed to axial alignment. This is believed to result from the fact that recrystalization is minimized during extrusion at these lower temperatures. Consequently, during subsequent annealing crystal size may be completely controlled to achieve optimum magnetic properties. 
     
         ______________________________________Compac-  Measur-tion   ingTemp.  Direc-   Br      Hc   Hci  BHmax Hk   density(°F.)  tion     Gauss   Oe   Oe   MGOe  Oe   gm/cc______________________________________1550   axial    5800    2820 4300 4.8    950 7.52  radial   5380    2800 4400 4.2   860  radial   5250    2700 4350 3.9   7501500   axial    6050    3350 5350 5.9   1050 7.52  radial   5600    3200 5450 5.2   1050  radial   5500    3150 5400 5.0   1100______________________________________ 
    
     Table II reports magnetic properties for magnets of the same composition as tested and reported in Table I, except that the magnets were not extruded but were produced by hot pressing. The magnetic properties were inferior to the properties reported in Table I for extruded magnets. 
     
                                           TABLE III__________________________________________________________________________Magnetic Properties of Extruded Magnets Measuredalong Radial Directions.           Temper-Powder  Die           atures                Br  Hc  Hci BHmax                                HkMagnetmesh    inch           °F.                gauss                    Oe  Oe  MGOe                                Oe__________________________________________________________________________EX-34A-10     0.875           1550 7900                    5400                        7800                            12.4                                2950                7700                    5400                        7780                            12.0                                3000EX-34B-10     0.875           1550 7500                    5200                        7520                            11.0                                2800                7600                    5300                        7600                            11.6                                3000EX-33A-10     1.00           1550 7220                    5000                        7400                            10.4                                2650                7200                    4900                        7300                            10.0                                2700EX-33B-10     1.00           1550 6900                    4700                        7200                            9.0 2350        &#34;  &#34;    6900                    4700                        7300                            9.2 2400                8200                    5100                        7350                            12.0                                2350EX-10-10     0.75           1600 7700                    5750                        8800                            12.3                                3400                7620                    5700                        8750                            12.0                                3400EX-36A-10 +60 0.875           1600 7600                    5100                        7680                            10.9                                2800                7480                    5050                        7650                            10.4                                2400EX-36B-10 +60 0.875           1600 7500                    5080                        7700                            10.8                                2550                7500                    5100                        7800                            10.7                                2650EX-37A-10 +60 0.875           1600 7550                    4800                        7000                            10.6                                2450                7500                    4860                        7030                            10.4                                2450EX-38A-60 +120        0.875           1600 7680                    5040                        7200                            11.0                                2550                7600                    5000                        7100                            11.2                                2650EX-38B-60 +120        0.875           1600 7700                    5200                        7500                            11.7                                2720                7800                    5220                        7500                            12.0                                2650EX-39B-60 +120        0.875           1600 7500                    5150                        7900                            10.6                                2600                7700                    5280                        7800                            11.6                                2750EX-40-120    +325        0.875           1600 7350                    4700                        6630                            10.1                                2210                --  --  --  --  --EX-42B-325    0.875           1600 7900                    5880                        8500                            12.9                                3600                7900                    5800                        8300                            13.0                                3600EX-30-10     1.00           1600 7300                    5200                        7900                            10.7                                3100__________________________________________________________________________ 
    
     It may be seen from the data reported in Table III that the magnetic properties of the extruded samples are not affected by particle size over the size range tested and reported in Table III. 
     
                       TABLE IV______________________________________Magnetic Properties of Extruded Magnets Measured inRadial Directions after Various Heat Treatments.Alloy 45H, -10 +60 meshExtrusion Temperature: 1600° F.Die Opening(inch)/Angle(degree): 0.875/50  Heat Treatment              Br      Hc   Hci   BHmax HkSamples  °C.-hours              gauss   Oe   Oe    MGOe  Oe______________________________________EX-36A as-extruded 7600    5100 7680  10.9  2800              7480    5050 7650  10.4  2400&#34;       550-1      7500    5250 8150  10.8  2750              7700    5280 8000  11.6  2730&#34;       550-3      7600    5200 7920  11.2  2650              7500    5200 7820  10.8  2750&#34;       550-6      7600    5200 7850  11.2  2550              7550    5200 7800  11.2  2650&#34;      1060-3      7800    5750 8500  12.6  3600              7800    5700 8400  12.6  3600&#34;      1000-3      7800    5500 8000  12.4  3200              7620    5400 7900  11.6  3250&#34;      1010-3      7800    5450 7900  12.2  3300              7750    5400 7850  12.0  3200&#34;      1035-12     7680    5500 7650  12.0  3200              7650    5400 7650  12.0  3300EX-36B as-extruded 7500    5080 7700  10.8  250              7500    5100 7800  10.7  2650&#34;       800-2      7680    5700 9000  12.0  3300              7640    5650 8900  12.0  3350&#34;       900-3      7700    5850 9120  12.4  3650              7400    5600 9000  11.0  3450&#34;      1060-3      7600    5600 8300  12.0  3400              7700    5600 8320  12.0  3350______________________________________ 
    
     Table IV shows the effect of heat treatment after extrusion on the magnetic properties. It appears from this data that at a heat-treating temperature of 800 C. or above both remanence and energy product are improved. 
     
                       TABLE V______________________________________Magnetic properties of Extruded Magnets in theAs-Extruded and Die-upsetted conditionSample: EX-10, Alloy 45H, -10 meshExtrusion Temperature: 1600 ° F.Die Opening(inch)/ Angle(degree): 0.75/50             Br      Hc    Hci   BHmax HkConditions    Direction             gauss   Oe    Oe    MGOe  Oe______________________________________as-extruded    axial    4100    3200  8400  3.2   1550    radial   7800    5900  9300  12.4  3400    radial   7800    6900  9350  12.8  3500Die-Upsetted    axial    6800    5700  8600  8.2   1750    radial   4900    3450  8340  4.4   1350    radial   5300    3650  7300  4.9   1450______________________________________ 
    
     An extruded sample magnet (sample EX-10) was tested to determine magnetic properties in the as extruded condition. The sample was then die upset forged and again tested to determine magnetic properties. The data presented in Table V indicates the significance of the &#34;radial properties&#34; achieved as a result of the extrusion operation in accordance with the practice of the invention.