Production of Nd-Fe-B based powders for Laser-Powder-Bed-Fusion – the 3DREMAG Project
Stefan Riegg1, Joni Reijonen2, Olivier Tosoni3, Juergen Gassmann4, Lukas Schaefer1 and Oliver Gutfleisch1
1) Technical University of Darmstadt, Functional Materials, DE-64287 Darmstadt, Germany 
2) Technical Research Centre of Finland Ltd. (VTT), FI-02044 Espoo, Finland
3) Commissariat à l'énergie atomique et aux énergies alternatives – CEA-Liten, FR-38000 Grenoble, France
4) Fraunhofer IWKS, Magnetic Materials, DE-63457 Hanau, Germany
Keywords: Nd-Fe-B, Permanent Magnets, Additive Manufacturing, Powder Production
During the past years additive manufacturing (AM) became a part of the repertoire of standard industrial process routes. The method integrates effectively the increasing digitalization of production to the materials development. This is well established for the standard-metal-materials available for construction parts starting to create ways of mechanical functionalization by adapting the process parameters locally or by novel design ideas. [1] We want to address a different kind of functionality by defining additional functionality of the printed parts as an additional physical property such as electrical conductivity or a specific magnetic stray field. AM allows near-net-shape manufacturing of complex magnet shapes, avoiding significant machining waste during conventional manufacture of the final shape from sintered magnet blocks. However, the development of the AM process requires a qualified starting material for powder bed fusion, binder jetting or material extrusion. Since the supply of metal powders for AM is limited in general, and there is no Nd-Fe-B based powder designed for use in AM, the development of a Nd-Fe-B-based starting material for commercial production is the goal of the EIT Raw Materials funded project 3DREMAG [2]. 
Commercial powders designed for injection molding are already used for different AM techniques in combination with different polymers and binders [3 and references therein]. As a different example, self-produced Mn-Al powders were used in electron beam melting to produce hard magnetic and rare-earth element free fully dense bodies [4]. The printing process was followed by a heat treatment to achieve 90% of the hard magnetic tau-phase in the samples.
It is very challenging to obtain (a) fully dense Nd-Fe-B magnets by AM and (b) the specific microstructure of uniform micrometer sized Nd2Fe14B grains separated by a rare-earth rich grain boundary phase that has to be established to achieve high remanence and coercivity. To obtain sufficient amounts of powder materials that is qualified to be processed by powder bed fusion,the production route is outlined starting from ground strip cast flakes followed by a plasma spheroidization step. Extremely spherical particle morphology and proper particle size distribution to assure good flowability and packing density for the powder are desired. Special considerations are given to the process challenges such as oxidation, particle shape and size distribution, density of printed parts as well as magnetic anisotropy. Newest results will be discussed to encourage powder production for AM magnets with specific shapes for sensor or actuator applications.
[1] Matthias Gebauer, Markus Oettel, Bernhard Müller, Fraunhofer IWU: http://publica.fraunhofer.de/eprints/urn_nbn_de_0011-n-5592221.pdf
[2] https://3DREMAG.eu
[3] Christian Huber, Gerald Mitteralmskogler, Michael Goertler, Iulian Teliban, Martin Groenefeld, Dieter Suess, Additive manufactured isotropic NdFeB magnets by stereolithography, fused filament fabrication, and selective laser sintering, Materials 13 (2020) 1916
[4] I. Radulov, V. Popov Jr, A. Koptyug, F. Maccari, A. Kovalevsky, S. Essel, J. Gassmann, K. Skokov, M. Bamberger, Production of net-shape Mn-Al permanent magnets by electron beam melting, Additive Manufacturing 30 (2019) 100787


Production of Nd-Fe-B based powders for Laser-Powder-Bed-Fusion - the 3DREMAG Project

High-temperature shape memory alloys (HT-SMAs) are required in specific automotive, aerospace or energy applications [1], [2]. Common binary Ni-Ti SMAs are characterized by a limited transformation temperature and, thus, cannot be used for applications operating at 100°C or above. Ternary elements (Pt, Au, Hf, Zr) can be added to increase the transformation temperature [3]. The ternary Ni-Ti-Hf alloy is currently the most promising candidate material, however, pronounced brittleness and high costs due to the significant amount of Hf hinders its technological breakthrough. Alternative HT-SMAs are Co-Ni-Ga Heusler alloys. These materials undergo a first-order magnetostructural transformation (FOMST) from high-temperature B2-ordered austenite to tetragonal L10 low-temperature martensite [4]. A fully reversible superelastic response up to 500°C as well as excellent cyclic stability up to temperatures of 100°C have been reported for single-crystals [5]. However, polycrystalline Co-Ni-Ga suffers from intergranular cracking and a premature failure after several transformation cycles due to the anisotropic volume change of randomly crystallographic orientated grains. Many efforts in the fields of grain boundary and microstructure engineering have been done to synthesize HT-SMAs with favorable grain boundary configuration in order to fully prevent intergranular cracking and premature failure. It was reported that a columnar-grained microstructure with strong &lt;001&gt; texture and straight low-angle grain boundaries can overcome the structural and functional limitations in Cu-based SMAs [6]. Additive manufacturing (AM) is a very promising technique to synthesize HT-SMA since it allows a direct microstructure design. Selective laser melting (SLM) and directed energy deposition (DED) are two common AM techniques for metallic alloys. During the SLM process, pre-alloyed powder is molten layer-by-layer using a laser system operating under inert gas atmosphere to prevent oxidation. Microstructure can be directly tailored by different processing parameters like laser power, scanning velocity and scanning path. In contrast to the powder-bed based SLM, DED is a powder-stream based process using nozzles to directly transfer the powder material into a focused laser beam. Similar to the SLM process the microstructure can be directly tailored by varying the processing parameters such as laser power and scanning speed. In our present work, we compare the microstructure and magnetic properties of Co49Ni21Ga30 alloys processed by SLM and DED with single crystals as well as polycrystalline material prepared by conventional casting. The preferred columnar-grained microstructure could be obtained by proper choice of processing parameters for the different AM techniques [7], [8]. Fig. 1 shows the microstructure of Co-Ni-Ga fabricated by SLM. The electron backscatter diffraction (EBSD) analysis of the cross-section shows a polycrystalline columnar-grained microstructure along the building direction (BD) [7]. A similar columnar-grained microstructure is also observed in the DED processed material [8]. In addition, the DED sample is characterized by a strong &lt;001&gt; texture along BD. Due to these highly anisotropic microstructures, AM processed Co-Ni-Ga obtained by both, SLM and DED, shows excellent HT-SMA properties without any post-processing [7], [8]. To study the FOMST in more detail, temperature and field-dependent magnetization measurements were performed. Fig. 2 shows the temperature-dependent magnetization of additively processed samples, polycrystalline material in the as-cast condition and in a single crystalline state for different orientations. The SLM sample reveals the largest transition width and thermal hysteresis while the DED sample provides for a transformation behavior being very similar to the cast and single crystalline counterparts. Since the transformation behavior depends on parameters such as grain size, residual stresses, defects and precipitates [9] the influence of the processing parameters on these parameters is essential to understand the FOMST and functional properties in Co-Ni-Ga in more detail. This work was supported by the ERC Advanced Grant "CoolInnov" (No 743116), the CRC/TRR 270 “HoMMage” (DFG). TN acknowledges funding by DFG (No 398899207).

**References:**

[1] J. Ma, I. Karaman, R. D. Noebe, International Materials Reviews, vol. 55, pp. 257-315, (2010) [2] K. Otsuka, Ed., Shape memory materials, Cambridge Univ. Press, (1999) [3] M. Elahinia et al., Scripta Materialia, vol. 145, pp. 90-94, (2018) [4] A. Reul et al., Shap. Mem. Superelasticity, vol. 4, pp. 61-69, (2018) [5] P. Krooß et al., Shap. Mem. Superelasticity, vol. 1, pp. 6-17, (2015) [6] J.-L. Liu, H.-Y. Huang, J.-X. Xie, Materials & Design, vol. 64, pp. 427-433, (2014) [7] C. Lauhoff et al., Metall and Mat Trans A, vol. 51, pp. 1056-1061, (2020) [8] C. Lauhoff et al., Materials Research Letters, vol. 8, pp. 314-320, (2020) [9] F. Scheibel et al., Energy Technol., vol. 6, pp. 1397-1428, (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/237965b772da9e5c7d95872cd06b33fe.jpg)
Fig. 1: Microstructure of Co-Ni-Ga processed by SLM: (a) optical micrograph, the building direction is perpendicular to the surface, (b) EBSD inverse pole figure map of the cross-section revealing a polycrystalline columnar-grained microstructure along the building direction (BD)

![](https://assets.underline.io/uploads/markdown_image/1/image/ca9914c1650bb2f6243d137454ae4bf8.jpg)
Fig. 2: Temperature dependent magnetization of (a) Co-Ni-Ga single crystals, (b) cast polycrystalline Co-Ni-Ga and Co-Ni-Ga alloy processed by SLM and DED.

 Influence of additive manufacturing processes on the microstructure and magnetic properties of Co-Ni-Ga shape memory Heusler alloy

Recently, additive manufacturing (AM) of functional materials started integrating an additional physical property available besides the mechanical stability in printed parts. This additional property can be an increased electrical conductivity or as addressed here a specific magnetic stray field. AM allows near-net-shape manufacturing of complex magnet shapes, avoiding significant machining waste during conventional manufacture of the final shape from sintered magnet blocks. However, the development of the AM process in general requires a qualified starting material for each process such as powder bed fusion [1], binder jetting or material extrusion. Since the supply of metal powders for AM is limited in general, and there is no Nd-Fe-B based powder designed for use in AM, the development of a Nd-Fe-B-based starting material for commercial production is the goal of the EIT Raw Materials funded project 3DREMAG [2]. Commercial powders designed for injection molding are already used for different AM techniques in combination with different polymers and binders [3 and references therein]. E.g. self-produced Mn-Al powders were used in electron beam melting to produce polymer (and rare-earth element) free hard magnetic fully dense bodies [4]. The printing process was followed by a heat treatment to achieve 90% of the hard ferromagnetic tau-phase in the samples. It is very challenging to obtain fully dense Nd-Fe-B magnets by AM due to the specific microstructure of uniform micrometer sized Nd2Fe14B grains separated by a rare-earth rich grain boundary phase that has to be established to achieve high remanence and coercivity. To obtain sufficient amounts of powders that can be processed by powder bed fusion, Nd-Fe-B strip cast flakes are milled and further plasma spheroidized. First results of lab scale production of the powders and first AM tests will be presented and discussed.References: [1] Mateusz Skalon, Michael Görtler, Benjamin Meier, Siegfried Arneitz, Nikolaus Urban, Stefan Mitsche, Christian Huber, Joerg Franke and Christof Sommitsch, Influence of Melt-Pool Stability in 3D Printing of NdFeB Magnets on Density and Magnetic Properties, Materials 13 (2020), 139. [2] http://3dremag.eu [3] Christian Huber, Gerald Mitteralmskogler, Michael Goertler, Iulian Teliban, Martin Groenefeld, Dieter Suess, Additive manufactured isotropic NdFeB magnets by stereolithography, fused filament fabrication, and selective laser sintering, arXiv:1911.02881 [4] I. Radulov, V. Popov Jr, A. Koptyug, F. Maccari, A. Kovalevsky, S. Essel, J. Gassmann, K. Skokov, M. Bamberger, Production of net-shape Mn-Al permanent magnets by electron beam melting, Additive Manufacturing 30 (2019) 100787.

Nd-Fe-B based permanent magnetic materials design for additive manufacturing: 3DREMAG

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Presentations

Production of Nd-Fe-B based powders for Laser-Powder-Bed-Fusion - the 3DREMAG Project

Influence of additive manufacturing processes on the microstructure and magnetic properties of Co-Ni-Ga shape memory Heusler alloy

Nd-Fe-B based permanent magnetic materials design for additive manufacturing: 3DREMAG

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