METHOD OF MANUFACTURING A PERMANENT MAGNET

Abstract

A method of manufacturing a permanent magnet, including providing a powder composition, of which a first fraction includes ferromagnetic metal particles and a second fraction includes thermoplastic polymer particles; using the powder composition in a powder-bed based additive manufacturing process to form a part including ferromagnetic metal particles embedded in a fused thermoplastic polymer body; and subsequently conferring magnetism on the built part by arranging the finished part in a magnetic field.

Claims

1. A method of manufacturing a permanent magnet, comprising: providing a powder composition, of which a first fraction comprises ferromagnetic metal particles and a second fraction comprises thermoplastic polymer particles; using the powder composition in a powder-bed based additive manufacturing process to form a part comprising ferromagnetic metal particles embedded in a fused thermoplastic polymer body, the part being formed by applying the powder composition layer on layer corresponding to cross sections of the part and selectively solidifying the powder composition by application of laser radiation to fuse the powder at positions in each layer which correspond to the cross-section of the part in the layer; and conferring magnetism on the part by arranging the finished part in a magnetic field, wherein the first fraction is about 91.5% weight magnetic particles and the second fraction is about 8.5% weight polymer resin, wherein the polymer resin is a physical blend of about 6.8% weight polyamide 12 and about 1.7% weight of a low viscosity polyamide 12, and the magnetic particles comprise fine ground alloy powder including Neodymium-Iron-Boron powder, the fractions being mechanically mixed for the composite.

2. The method of claim 1, wherein the ground alloy is Nd—Pr—Fe—B alloy with a d50=65 microns.

3. The method of claim 1, wherein the ground allow is spherical particles of Nd—Pr—Fe—Co—Ti—B alloy with a d50=43 microns.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 illustrates a powder composition according to an embodiment of the invention;

[0024] FIG. 2 is a simplified diagram of an SLS apparatus during a build;

[0025] FIG. 3 shows a final stage in the inventive method;

[0026] FIG. 4 shows a cross-section through a permanent magnet manufactured using the inventive method.

[0027] In the drawings, like numbers refer to like elements throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] While this invention is susceptible of embodiments in many different forms, there is shown in the drawings, and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to embodiments illustrated. As used herein, the term “the invention” is not intended to limit the scope of the claimed invention and is instead a term used to discuss exemplary embodiments of the invention for explanatory purposes only.

[0029] FIG. 1 illustrates a powder composition 1 according to an embodiment of the invention. The diagram shows a mixture or dry blend of ferromagnetic particles 11 and thermoplastic polymer particles 12. The powder mixture can also be obtained by melt-compounding ferromagnetic metal particles with thermoplastic polymer particles to make composite pellets, which are then ground into a size suitable for use in an SLS apparatus. In the powder mixture, the ferromagnetic particles may be assumed not to have any magnetic properties, i.e. the particles would not be attracted to a magnet in the vicinity. The ferromagnetic particles 11 may comprise one or more of the alloys or compounds mentioned above. Similarly, the thermoplastic polymer particles 12 may comprise one or more of the materials mentioned above. In this embodiment, the ferromagnetic particles 11 may be assumed to make up at least 90 wt % of the powder composition 1. The remaining 10 wt % is given by the thermoplastic polymer particles 12 and—as appropriate—small quantities of additives such as a nucleation agent, a flow additive, an antioxidant, etc. The mean diameter of the ferromagnetic particles 11 can be up to 90 μm for such an exemplary powder mixture. The ferromagnetic particles 11 and the polymer particles 12 can have any regular or irregular shape.

[0030] FIG. 2 is a simplified diagram of an SLS apparatus 3 during a build. The diagram shows a partially completed part 2B supported on a build platform 30. This can be lowered by small increments so that the upper level of the partially completed part 2B remains at essentially the same level throughout the build. The part 2B is constructed in a layer-wise manner. For each layer, the powder composition 1 (comprising a blend of ferromagnetic particles 11, thermoplastic polymer particles 12 and optional additives as described above) is spread evenly over the base 30, as will be known to the skilled person, and a laser beam 31 is then guided to fuse the thermoplastic polymer only in a set of specific points in that powder layer 1L. The heat generated by the laser beam is sufficient to melt (i.e. fuse or sinter) the polymer, but does not affect the ferromagnetic material. The part will therefore comprise metal particles 11 embedded in fused polymer 120, as shown in the enlarged portion of the diagram. When the build is complete, the finished part is allowed to cool.

[0031] Referring to FIG. 3, in this exemplary embodiment, the finished part 2 has been formed so that it is stackable, and has been given a protective coating 22 to prevent oxidation of the ferromagnetic particles at the surface of the part 2. In this diagram, the finished part 2 is being magnetized. To this end, a sufficiently strong magnetic field 4 is generated, and the finished part 2 is placed in the field 4 for a suitable duration until the ferromagnetic particles are sufficiently saturated. This will result in magnetic properties being conferred on the finished part, i.e. the finished part will exhibit a certain remanence and will function as a permanent magnet 2PM as shown in FIG. 4.

[0032] FIG. 4 shows a cross-section through a permanent magnet 2PM manufactured using the inventive method. The diagram illustrates the persistent magnetic field 2F generated by the permanent magnet 2PM. The magnetic field 2F is the result of the magnetization process acting on the ferromagnetic metal particles 11 embedded in the fused thermoplastic polymer body 120.

[0033] The magnetic and structural properties of the finished part 2PM will depend to a large extent on the choice of powder composition and additive manufacturing process. A composite powder according to an aspect of the present invention can have a composition with up to 50% (dry weight) polymer powder and at least 50% (dry weight) ferromagnetic powder. As indicated above, the polymer powder can be chosen from one or more thermoplastic semi-crystalline polymers typically used in powder bed fusion processes such as copolyester, PA6, PA11, PA12, PP, PPS, and TPUs. Any one of these polymers, or a blend of two or more of these polymers, may be used in the composite powder to act as binder during the powder-bed fusion process.

[0034] The powder composition can comprise ferromagnetic particles in a fine powder, for example particles of a Neodymium-Iron-Boron (NdFeB) alloy, a Samarium-Cobalt (SmCo) alloy, ferrites of either Barium or Strontium, etc.

[0035] Various additives may also be included in the powder composition, for example a flow additive, an antioxidant, a nucleation agent, etc. The various fractions of the powder composition are preferably mixed to achieve a homogeneous dispersion of the ferromagnetic particles throughout the powder composition. Thorough mixing can be achieved by mechanical blending, melt compounding and subsequent grinding, chemical methods for mixing or coating the particles, etc., as will be known to the skilled person.

[0036] In one exemplary embodiment, a powder composition comprises 8.5 wt % polymer resin particles and 91.5 wt % ferromagnetic particles. To achieve a favourable melt viscosity for the magnetic composite, the polymer resin particles comprise 6.8 wt % of a high molecular weight Polyamide 12 and 1.7 wt % of a low-viscosity, high melt flow Polyamide 12. The ferromagnetic particles comprise Neodymium-Iron-Boron (NdFeB) alloy powder. The powder components were mechanically mixed for 30 minutes. The powder composition thus provided is then suitable for use in a commercial SLS machine.

[0037] In another exemplary embodiment, the powder composition may comprise ground neodymium alloy particles, for example a product such as MQP-AA4-15-7, i.e. Nd—Pr—Fe—B alloy particles with a mean diameter of 65 microns. Alternatively or in addition, the powder composition may comprise a product such asMQP-S-11 9, i.e. spherical particles of a Nd—Pr—Fe—Co—Ti—B alloy with a mean diameter of 43 microns.

[0038] A favourable formula for the inventive powder composition may comprise 91.5 wt % (or a volume fraction of 60%) neodymium alloy, 6.8 wt % PA12 and 1.7 wt % low viscosity, high melt flow PA12. These components are then dry-blended to obtain the powder composition for use in a laser sintering process, for example a powder bed fusion process. In a powder bed fusion process, as described above, layers of powder material are successively laid down in a build area, with a laser or some other type of electromagnetic or solidification energy being applied to each layer in a controlled manner according to the layer cross section of the object being built.

[0039] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0040] For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.