Metal Injection Moulding of NdFeB based on Recycled Powders

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1 Metal Injection Moulding of NdFeB based on Recycled Powders Carlo Burkhardt 1, Philipp Imgrund 1, Oxana Weber 1, Thomas Schlauf 2, Karsten Pischang 3, Joamin Gonzalez-Gutierrez 4, Christian Kukla 5, Malik Degri 6, Ivor Rex Harris 6, Lydia Pickering 6, Allan Walton 6 1 OBE Ohnmacht & Baumgärtner GmbH & Co KG, Turnstr. 22, Ispringen, Germany 2 FOTEC Forschungs- und Technologietransfer GmbH, Viktor-Kaplan-Straße 2, A-2700 Wiener Neustadt, Austria 3 PT+A GmbH, Fritz-Meinhardt-Str. 32, Dresden, Germany 4 Montanuniversitaet Leoben, Department of Polymer Engineering and Science, Chair of Polymer Processing, Otto Gloeckel-Str. 2, 8700 Leoben, Austria 5 Montanuniversitaet Leoben, Industrial Liaison Department, Peter Tunner Str. 27, 8700 Leoben, Austria 6 School of Metallurgy & Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom Abstract As the raw materials used for permanent magnets production is critical to the low carbon future envisioned by the European Union, the Horizon 2020 project Resource Efficient Production of Magnets (REProMag aims to address the issue of sustainability of RE permanent magnets by developing an innovative automated manufacturing route called SDS process (Shaping, Debinding and ). This process will allow economically efficient production of net shape or near net shape magnetic parts with complex structures and geometries, whilst being absolutely waste-free through the use of fully recycled raw material and a 100% material efficiency in the consequent processing steps shaping, debinding and sintering. The REProMag SDS processing route is based on the use of powder obtained from end of life rare earth magnets by using the hydrogen decrepitation (HD) process. A proprietary binder system has been developed for producing a mouldable MIM feedstock, having a chemical composition optimised for the processing of the highly reactive magnetic powder. The first prototypes were processed in modified injection moulding equipment that were consequently debinded and sintered under tailored conditions. An overview of the project, including the processing steps and their challenges, the influence of debinding and sintering conditions on the microstructure and magnetic properties of isotropic sintered MIM parts are presented and discussed. Special attention is given to temperature control, gas pressure conditions and atmospheres during thermal debinding and sintering. Keywords Nd-Fe-B, permanent magnet, recycling, HD-process, metal injection moulding (MIM), REProMag 1. Introduction As the preferences of consumers shift towards hitech, miniaturised and green products, the material security regarding the supply of rare-earth elements becomes more and more of a concern. To reduce Europe s dependence on foreign supply, technologies must be developed to not only use the resource more efficiently through optimisation of design, but also to minimise production waste. Over the last ten years several groups have been carrying out research into metal injection moulding (MIM) of NdFeB powder to produce isotropic or anisotropic rare earth magnets with reduced raw material consumption and higher geometrical complexity than would be possible in the conventional press and sintering approach. [1,2,3] However, the MIM process poses several challenges, including the high reactivity of the powder to oxygen and carbon pickup, which often results in poor remanence and coercivity in the final magnets. [1,2,3] To address these issues and to realise a circular recycling economy, the feasibility to produce metal injection moulded magnets based on recycled NdFeB material is investigated in the REProMag Horizon 2020 project. The SDS processing route used in this work is based on the use of powder obtained from end of life sintered rare earth magnets which have been processed using the hydrogen decrepitation (HD) process [4,5]. A proprietary binder system has been developed for producing a mouldable MIM feedstock, having a chemical composition optimised for the processing of the highly reactive magnetic powder. First prototypes were processed in a modified mould without magnetic alignment in order to achieve isotropic green parts that were consequently debinded and sintered under tailored conditions.

2 2. Experimental The NdFeB powders were produced using the Hydrogen Decrepitation (HD) process from end of life sintered magnets of single composition, as given in table 1. Tab. 1. Starting composition of the used magnets Nd Dy Pr Fe Co B Cu Nb Al wt% To obtain a processable powder, the sintered NdFeB scrap magnets were placed on a porous metal mesh inside a custom built 300 litre hydrogen reactor vessel and then exposed to 1.3 bar of hydrogen at room temperature. To compensate for the H 2-absorption of the alloy, the pressure was kept on a constant level during the hydrogenation process by subsequently introducing more hydrogen until the pressure remained stable for 20 minutes, signaling the end of the reaction. To avoid oxygen intake the powder was transferred in a closed container into a glove box under protective atmosphere. It was then burr milled and sieved down to a particle size of <45 µm. The analysis of the chemical composition of the recycled powders was performed with WDXRF (Wavelength Dispersive X-Ray Fluorescence) on a PANalytical PW 1400 wave length dispersive detector, using Rh Kαradiation; the particle size was measured using a Retsch LA-950 Laser Scattering Particle analyser. The powder morphology was investigated with a Scanning Electron Microscope, Jeol JCM 6000 NeoScope. The powder was then mixed at elevated temperatures under protective atmosphere with a proprietary, multi-component binder system. To protect the NdFeB material against possible oxygen intake during the following injection moulding step, in one feedstock variant the powder particle surfaces were coated completely by binder ingredients in a dedicated multi-step mixing sequence. To produce isotropic magnets, the feedstock was injection moulded on an Arburg Allrounder 270U machine without an external magnetic field. The injected parts were solvent debinded in a LÖMI debinding unit EBA 200; afterwards thermal debinding was carried out in reductive atmosphere in four trials (in chapter 3 named sintering run 1-4) with different heating ramps between 0.1 and 2 K min -1 and dwell times at various temperatures between 300 and 550 C, followed by sintering at 1100 C for one hour in an argon atmosphere, using an Elnik 3045 MIM furnace. The carbon and oxygen contents of the sintered samples were measured by hot gas ex-traction on a LECO CS 230 gas analyser (C-content) and a LECO TS 400 CGHE analyser (O-content). The density was determined using the Archimedes method. To perform magnetic characterizations, the samples were pulse magnetized with a 3.5T field and analyzed on a Metis HyMPulse pulse field magnetometer at room temperature. 3. Results and discussion 3.1.Powder Fig. 1 (a) shows the microstructure of the starting material before hydrogen decrepitation using an SEM in backscattered mode. The dark phase is the Nd 2 Fe 14 B matrix phase, the light grey/white areas show a well distributed phase. As can be seen also in fig. 1 (b), which represents the same material in a higher magnification, there is some evidence of porosity. Pore Fig. 1. BSE-SEM images of the starting material before hydrogen decrepitation with (a) a magnification of 500x and (b) of 1000x After HD-treatment as described in chapter 2 (experimental), the particle size analysis of the recycled powders used for the sample production provided the following results: Tab. 1. Particle size distribution of the recycled NdFeBpowders D10 [µm] D50 [µm) D90 [µm]

3 The SEM images of the recycled powders (fig. 2a and 2b) indicate particle sizes of mainly around microns which is close to the starting grain size in the sintered magnets (10µm). However, particle size analysis showed a wide distribution of sizes, which indicates that some of the powder is not single crystal in nature. As these multi-crystal areas of the recycled material however should have a preferred magnetic orientation, a subsequent milling procedure was not carried out at this stage. The chemical composition of the used NdFeB-type material is given in table MIM processing Because of the high reactivity of the Nd-phase in the material, a standard commercial feedstock system e.g. based on POM cannot be employed for MIM processing of NdFeB-type powders [1]. Other binder systems based e.g. on methylcellulose dissolved in water or wax based systems have either not proven to be successful [2] or are only suitable for lab scale production, because of their limited processability, [3]. In order to provide a material for competitive large scale manufacturing, the development of a tailored feedstock system therefore was necessary. As the phase is known to be very reactive with respect to carbon (C) and oxygen (O 2 ) to form Ndcarbides and/or Nd-oxides special care has been taken to processing atmospheres and the mixing and kneading process, which was carried out either as a single-step or a multi-step procedure. Based on a powder/thermoplastic blend with additives; the chosen main binder is soluble in a commercially available, cheap and safely processable solvent and thus removable in a standard solvent debinding reaction vessel. The backbone binder is debindable in a thermal process in a reductive atmosphere at rather low temperatures of approximately 300 C, which is beneficial with respect to the reaction kinetics of the NdFeB-material in its hydrided form. For this work, feedstocks based on two different binder systems were prepared; employing variations of the backbone binder content and processing with and without protective atmosphere. Table 3: Feedstock types under investigation Feedstock # Binder Compounding Debinding 1 Proprietary Single-step, air Acetone 2 Proprietary, mod. backbone 3 Proprietary Single-step, air Three-step, inert atmosphere Acetone Heptane Fig. 2: BSE - SEM images of the recycled powders with (a) a magnification of 500x and (b) of 1700x Table 2: Chemical composition of the recycled NdFeBpowders B Co Dy Fe Nb Nd Pr wt.% Mn Ni Si Zr O N C S ppm < <10 All feedstock variants could be injection-moulded on non-modified MIM serial equipment using melt temperatures between 110 C (feedstock #1 and #2) and 150 C (feedstock #3). Changeover pressures ranged between bar (feedstock #1 and #3) and 1000 bar (feedstock #2) with tool temperatures at C for all feedstock types. 3.3 Discussion With solvent debinding, feedstock #1 and #2 reached debinding rates of more than 93 wt.% of the initial amount of base polymer. With the multi-step processed feedstock #3 however, where the powder particles were pre-coated with binder ingredients, debinding rates of only 78 wt.% could be achieved.

4 (a) (b) For feedstock 1 and 2 sintered in runs 2, 3 and 4, both remanence and coercivity remain unsatisfactory. This can be attributed to the excess oxygen contained in the samples, which will be concentrated in the grain boundary phase. As previously reported, if the grain boundary phase is oxidised then liquid phase sintering cannot be achieved due to the higher melting point of Nd 2 O 3 [4]. Feedstock 3, which features good carbon and oxygen levels after sintering, showed low B r and jh c values for sintering run 3; however reasonable magnetic properties with a remanence of 0.57 T and a coercivity value of 970 ka/m were recorded for nonaligned samples made out of this feedstock in sintering run 4. Here, the thermal debinding temperature and the switchover temperature from reducing to inert gas atmosphere were optimised with respect to sintering run 3. These changes lead to a microstructure that is very similar to the starting material before the HD-process (see fig.1). As can be seen in the SEM images in fig. 4, the light grey/white phase appeared to be widely distributed and finely dispersed in the dark grey Nd 2 Fe 14 B matrix phase. Fig. 3: Carbon (a) and oxygen (b) content of the sintered parts For the thermal debinding, slow heating ramps have proven to be beneficial as reported elsewhere [1,3]. It was found that with feedstock #3, heating rates of 0.5K/min are sufficient to avoid carbon or oxygen contamination of the sintered samples, as can be seen from fig. 3a and 3b. Magnetic characterisation was not carried out for sintering run 1, as the carbon content of the samples was so high that the magnetic properties would have been very poor. All other samples were characterised at room temperature, with the results summarised in table 4. Table 4: Magnetic properties of MIM samples from recycled powders run 2 run 3 run 4 Feedstock 1 Feedstock 2 Feedstock Pore Fig. 4. SEM images of the sintered MIM-magnets, feedstock 3, sintering run 4 with (a) a magnification of 500x and (b) of 1000x

5 As can be seen especially in fig. 4b, the porosity of the samples (black areas) is significantly larger than in the starting material, leading to the relatively low density of the samples of only approx. 7.0 g/cm³. The observed porosity is not surprising as the oxygen content of the starting sintered magnet and hence the HD powder ( ppm) is higher than would commonly be observed for a starting cast alloy for magnet production (typically ppm). When conventional sintering techniques have been applied to reprocess these HD powders then similar porosity has been observed [4]. To overcome this problem then extra rare earth can be added to the hydrogenated powders in the form of NdH 2.7. In the next set of MIM experiments blended powders of HD NdFeB powder + 2-3at% NdH 2.7 will be injection moulded. This should overcome the density problem and lead to higher remanence and coercivity values in these materials. Fig. 5 shows the demagnetisation curve of a nonaligned sample in the as-sintered state. The coercivity is similar to that observed in conventionally sintered NdFeB magnets made from these recycled materials [5]. The remanence at present is relatively low. This is attributed to the low density and due to the fact that the NdFeB material is not aligned in the die during injection moulding. In the next series of experiments an aligning field of 1 T will be applied to the material inside the die in order to produce an anisotropic magnet. Fig. 5: Demagnetisation curve for a non-aligned MIM- NdFeB magnet produced by the SDS method from recycled powder (feedstock 3, sintering run 4, compare to bold values in table 4) 4. Conclusion With the present work, the feasibility of a MIM process using 100% recycled NdFeB powder has been successfully demonstrated. It has been shown that control of oxygen and carbon content by suitable selection of the feedstock preparation route, solvent, thermal debinding and sintering conditions is of major importance to obtain reasonable hard magnetic properties for an isotropic magnet. Further research will be conducted to improve the magnetic properties and to develop a processing route for anisotropic NdFeB magnets based on these recycled powders by injecting the samples into a tooling with an applied external field. This will then be compared to MIM magnets made from primary NdFeB alloys. 5. Acknowledgement Research has been done as part of the project Resource Efficient Production for Magnets REProMag financed by the European Commission under the Horizon 2020 FoF-Framework under grant agreement [6]. Authors thank Dr. M. Krispin and Dr. G. Rieger at Siemens AG, München and Dr. Benjamin Podmiljsak at Institut Jožef Stefan, Ljubljana for magnetic measurements and Dr. Christian Gierl-Mayer, Technical University of Vienna for measurements of the chemical composition, oxygen and carbon measurements. 6. References [1] T. Hartwig, L. Lopes, P. Wendhausen and N. Ünal, Metal Injection Molding (MIM) of NdFeB Magnets, The European Physical Journal Conferences 01/2014; 75: [2] O. Yamashita, Magnetic Properties of Nd-Fe-B Magnets Prepared by Metal Injection Molding, Journal of the Japan Society of Powder and Powder Metallurgy 42(9): (1995) [3] C. Weck, T. Hartwig, Sintern von spritzgegossenen NdFeB-Magneten, Workshop Magnetwerkstoffe Vom Design zum Recycling Fraunhofer IFAM Bremen [4] M. Zakotnik, E. Devlin, I.R. Harris and A.J. Williams, Hydrogen Decrepitation and Recycling of NdFeB-type Sintered Magnets, Journal of Iron and Steel Research International 13: (2006) [5] A. Walton, H. Yi, N.A. Rowson, J.D. Speight, V. Mann, R.S. Sheridan, A. Bradshaw, I.R. Harris, A.J. Williams,The Use of Hydrogen to Separate and Recycle Neodymium-Iron-Boron-type Magnets from Electronic Waste, Journal of Cleaner Production 104 (2015) [6] C. Burkhardt, Resource Efficient Production of Magnets, REProMag Concept and Objectives, ( )