Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magn
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Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magnetic properties Lennart Leich1,*
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, Arne Ro¨ttger1, Rene Kuchenbecker1, and Werner Theisen1
Institut für Werkstoffe, Lehrstuhl Werkstofftechnik, Ruhr Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
Received: 15 June 2020
ABSTRACT
Accepted: 28 September 2020
This study investigates the compaction of nanocrystalline NdFeB magnet powder by electro-discharge sintering (EDS). On this account, process parameters, microstructure, and the associated magnetic properties of the EDS-densified nanocrystalline NdFeB specimens were investigated by varying the discharge energy EEDS and compression load pEDS. Although optimized process parameters could be evaluated, three different microstructures (fully densified zone, insufficiently densified zone, and melted zone) are present in the EDScompacted specimens. Thereby, volume fractions of these formed three different microstructures determine the resulting mechanical and magnetic properties of the specimens. For all specimens, the intrinsic coercivity Hc,J deteriorates with increasing discharge energy, as the generated Joule heat leads to microstructural changes (grain growth, dissolution of magnetic phases), which reduces the magnetic properties. The compression load has less influence on the coercivity Hc,J, as it only affects the initial resistance of the pre-compacted powder loose. The residual induction Br deteriorates with increasing the discharge energy due to microstructural changes. An increase in the compression load pEDS results in an increase in the specimens’ density and thus promotes the residual induction Br.
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The Author(s) 2020
1 Introduction As an enabler and driver of technology, permanent magnets have gained increasing importance over the last half-century and they are a critical part of many high-tech products, such as electric vehicles, electric generators of wind turbines, and consumer
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https://doi.org/10.1007/s10854-020-04562-6
electronics [1]. High-performance rare-earth-based permanent magnets such as NdFeB are indispensable in the miniaturization of electrical devices, the development of highly efficient electric motors, and energy conversion in general [2]. The energy product (BH)max of NdFeB, which is the most important quantity for permanent magnets as it describes the
J Mater Sci: Mater Electron
energy stored in the magnet, is the highest among all permanent magnets in the case of NdFeB magnets. It is theoretically above 500 kJ/m3 and thus exceeds other permanent magnets at room temperature, which can be attributed to the hard magnetic phase Nd2Fe14B [3]. Due to the increasing electrification (electromobility, energy storage) it can be assumed that the need for NdFeB magnets will increase in the future. On this account, the research and development of new alloying concepts or more efficient manufacturing processes in the field of NdFeB magnets ar
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