Abstract:
A method is provided for manufacturing a multi-layered magnetic rod in which a steel powder is reduced and extruded through a die. To improve the magnetic properties, a layer of rare earth magnetic power formed around the low alloy steel in the extruder chamber. A second layer of coating formed around the rare earth powder and the three layers are co-extruded through the die to produce a layered rod having improved magnetic properties. The resulting magnetic rod may be machined using conventional machining methods.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 09/567,110, filed May 8, 2000. This application also claims the benefit of U.S. Provisional Patent Application No. 60/175,502, filed Jan. 11, 2000, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to a multi-layer structure permanent magnet and a method for manufacturing the same using an extrusion process to form the multi-layer structure.  
         BACKGROUND OF THE INVENTION  
         [0003]    Permanent magnets are used in many applications. One such application is in the creation of brushless electric motors where they are used to replace the traditional alnico or ceramic magnets. Such magnets are useful, but are limited in terms of energy versus size. Higher energy in a smaller package is generally desirable in most industrial and commercial settings. Rare earth magnets, because of their extremely high energy, are finding increasing use in applications such as brushless DC motors.  
           [0004]    The manufacture of rare earth magnets is a laborious and expensive process. As is described in U.S. Pat. No. 4,902,357, the production of rare earth magnets begins with sintering rare earth material. Following the sintering process, the magnet is solution treated and aged at elevated temperatures to achieve the desired magnetic properties.  
           [0005]    Rare earth magnets are also very difficult to machine because they are mechanically hard and brittle. It is simply not practical to machine rare earth magnets or structures containing such magnets after they are magnetized during the manufacturing process steps, because the machined particles adhere strongly to the magnets. Because of the extremely high coercive forces of rare earth magnets it is also not practical to construct a magnetized rotor or stator of rare earth magnets and then remagnetize the structure as is commonly done with alnico magnets.  
           [0006]    Because commercial desire is strong to employ rare earth magnets in applications such as DC brushless motors, the industry continues to look for methods of manufacturing rare earth magnets that are low in cost and are produced to near net shapes, thus having virtually no machining or relatively reduced machining.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to an extrusion method for producing a multi-layered structure with high-energy ring magnets at low cost and which can be readily machined to its final size. More specifically, the invention is directed to a method for producing a multilayer rod having the desired magnetic properties and ease or limited need of machining.  
           [0008]    In this invention an extrusion process is employed. A first layer of low alloy steel powder is injected into a chamber of an extrusion machine. A rare earth metal powder is then injected into the extrusion chamber to form a circumferentially disposed second layer around the first layer. The material so disposed in the extrusion chamber is then extruded through a die to form a rod with the concentric layering intact and having magnetic properties not found in the base materials  
           [0009]    A third layer, if desired, may be extruded around the rare earth metal at the same time. The third layer if used forms an outer skin of the extruded rod and protects the rare earth metal layer from mechanical fragmentation and corrosion. If a third layer is not desired an antioxidant coating may be applied. Then a stainless steel or an aluminum retention cap is inserted to provide protection against magnet integrity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a plan view, partially in section, of the die portion of a rod extruder in accordance with the present invention.  
         [0011]    [0011]FIG. 2 is a plan view, partially in section, of the extruder shown in FIG. 1 with a portion of the material extruded into a rod.  
         [0012]    [0012]FIG. 3 is a perspective view of a magnet formed from the method of the present invention showing the lines of magnetic force.  
         [0013]    [0013]FIG. 4 is a cross section view of the multilayer magnet shown in FIG. 3.  
         [0014]    [0014]FIG. 5 is a plan view, partially in section, of the die portion of a tube extruder.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    An extrusion machine  10  in accordance with the present invention is shown in FIG. 1. The extruder  10  comprises a container body  12  having a chamber  16  for holding material, a die  18 , and an opening  20 . A hydraulically operated punch  14  is sized to fit tightly in the chamber  16 . During operation, the punch  14  is moved into the chamber  16  compressing and reducing the material  22 , 24 ,  26 . High pressure is generated.  
         [0016]    As seen in FIG. 2, as the punch  14  further compresses the materials  22 ,  24 ,  26 , so as to cause them to extrude through the die  18  and exit the machine through the opening  20 . The high pressure causes the materials  22 ,  24 ,  26  to solidify into the rod  28  of the die  18  even after the materials  22 ,  24 ,  26  are removed from the extruder  10 . Once the extrusion process is completed, the extruded rod  28  is stripped or removed from the extrusion machine. Typically, extrusion machines have a stripping plate (not shown) for removing the extruded rod  28  from the die  18 .  
         [0017]    In accordance with the present invention, it has been found that when steel bars or wires are extruded, the reduced product becomes magnetized. Improved magnetic properties are obtained when powdered rare earth magnet material is used. As shown in FIG. 2, a first powder  22 , preferably low alloy steel, is injected into the center of chamber  16  with a second powder  24 , preferably of Nd 2 Fe 14 B alloy, circumferentially layered around the first  22 . An optional third layer  26  of powdered material is preferably layered circumferentially around the Nd 2 Fe 14 B alloy layer  24 . When these layered materials  22 ,  24 ,  26  are reduced and extruded through the die  18 , the resulting extruded form contains multiple layers as seen in FIG. 3 and FIG. 4. The purpose of these layers will be made clearer herein. Once extruded, the resulting rod  28  has radial magnetic properties not displayed in the base materials alone. The radial lines of force  32  are shown in FIG. 3.  
         [0018]    As the extrusion process forces the material to flow through the die, the material is compressed. Magnetic domains in the rare earth material will be aligned only when the extrusion process is sufficient to allow the material to flow. The magnetic lines of force will be aligned perpendicular to the direction of the material flow. If the powdered material is merely depressed the resulting product will not exhibit usable magnetic properties. Heat may be applied to the chamber to aid compression. Generally, the materials will be compressed to at least about one-half their original volume. Preferably, the compression will be about one-third or about 30% of the original volume of the powders. Most preferably, the compression will be by a factor of about 16 to 1.  
         [0019]    Because the outer layer of the magnet is a powered/plastic material, any conventional machining processes can be used to form the rod thereafter, for example, if a higher degree of concentricity is required, the magnetic rod  28  can be turned or ground. Applications such as brushless DC motors require that a shaft be inserted through the center of the magnet  28 . In this case, a hole may be drilled through the inner low alloy steel layer  22  to allow insertion of the shaft. This machinability provides a lower cost and more flexibility over the traditional methods for producing magnets that require specialized sintering, grinding, bonding and cleaning operations.  
         [0020]    If machinability of the outer surface of the rod  28  is not required, it is contemplated that the outer layer  26  can be replaced by a coating. A typical coating such as epoxy, nickel, or aluminum chromate would provide the rod  28  with corrosion and oxidation protection.  
         [0021]    It should be appreciated that another advantage to the present invention is that the magnetic strength of the rod  28  may be easily altered. By changing the ratio of Nd 2 Fe 14 B alloy  24  to low alloy steel powder  22 , in the rod  28 , the magnetic properties can be changed to the desired levels. Because the process of extruding metals is well developed, the rod  28  can be manufactured with a high degree of reproducibility. This allows for a product with predictable and consistent magnetic properties.  
         [0022]    Referring to FIG. 5, there is shown an alternate embodiment for a tube extruder  34 . In this embodiment, the piston  36  has a mandrel  38  extending through the container body  12  and the die  18 . The mandrel  38  has the effect of blocking the flow of material  22 ,  24 , and  26  from the center of the die  18 . The result is a multilayer thin wall magnetic tube  40  having an inner wall formed from the low alloy steel powder, an intermediate layer  24  formed from a rare earth metal and an outer layer  26  formed from low alloy steel or other materials depending on applications.  
       EXAMPLES  
       [0023]    An extrusion chamber was injected with 300 grams of low alloy steel, 300 grams of Nd 2 Fe 14 B alloy circumferentially layered around the low alloy steel and 1200 grams low alloy steel circumferentially layered around the Nd 2 Fe 14 B alloy. The materials were extruded with the piston generating 110 to 130 ksi. This extrusion achieved a reduction of 16 to 1 by volume. The resulting extruded rod was 24 inches long and ⅞ inch in diameter and had a lower than targeted specific gravity of 7.64 gm/cc and lower than 30 MGOe of magnetic energy.  
         [0024]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.