Researchers at the Department of Energy’s Oak Ridge National Laboratory have demonstrated that permanent magnets produced by additive manufacturing can outperform bonded magnets made using traditional techniques while conserving critical materials.
Scientists fabricated isotropic, near-net-shape, neodymium-iron-boron (NdFeB) bonded magnets at DOE’s Manufacturing Demonstration Facility at ORNL using the Big Area Additive Manufacturing (BAAM) machine. The result, published in Scientific Reports, was a product with comparable or better magnetic, mechanical, and microstructural properties than bonded magnets made using traditional injection molding with the same composition.
The additive manufacturing process began with composite pellets consisting of 65 volume percent isotropic NdFeB powder and 35 percent polyamide (Nylon-12) manufactured by Magnet Applications, Inc. The pellets were melted, compounded, and extruded layer-by-layer by BAAM into desired forms.
NdFeB permanent magnets are frequently classified into sintered and bonded magnets. While sintered magnets retain full density and offer high energy product, bonded magnets have high degree of net-shape formability and intermediate energy product. Bonded permanent magnets are fabricated by blending magnetic powders with a polymer as binder, and then molded into desired shapes utilizing several commercial processing methods including injection molding, compression molding, extrusion, and calendering. Recently, bonded permanent magnets have experienced accelerated industrial applications due to their advantages such as intricate shapes, low weight and cost, superior mechanical properties and corrosion resistance, etc. Nd2Fe14B was first discovered as a strong permanent magnet in 1984. It adopts a tetragonal crystal structure (P42/mnm) with the easy magnetic axis along the c axis. It possesses high magnetic energy product as large as 512 kJ/m3 (64 MGOe), with a Curie temperature Tc = 585 K and a high magnetic anisotropy constant K1 of 4.5 MJ/m3 arising from the strong spin-orbit coupling in Nd. In fact, developing better NdFeB bonded magnets has been heavily researched. Magnet powder properties, processing temperature, loading factor, magnet density and degree of orientation are critical process variables for improving magnetic and mechanical properties of NdFeB bonded magnets
(a) Image of the nozzle depositing layers of magnetic materials on the print bed; (b) Schematic of the melt and extrude process
Magnetic properties of Big Area Additive Manufacturing (BAAM) and Injection Molding (IM) fabricated NdFeB bonded magnets
While conventional sintered magnet manufacturing may result in material waste of as much as 30 to 50 percent, additive manufacturing will simply capture and reuse those materials with nearly zero waste, said Parans Paranthaman, principal investigator and a group leader in ORNL’s Chemical Sciences Division. The project was funded by DOE’s Critical Materials Institute (CMI).
Using a process that conserves material is especially important in the manufacture of permanent magnets made with neodymium, dysprosium—rare earth elements that are mined and separated outside the United States. NdFeB magnets are the most powerful on earth, and used in everything from computer hard drives and head phones to clean energy technologies such as electric vehicles and wind turbines.
The printing process not only conserves materials but also produces complex shapes, requires no tooling and is faster than traditional injection methods, potentially resulting in a much more economic manufacturing process, Paranthaman said.
“Manufacturing is changing rapidly, and a customer may need 50 different designs for the magnets they want to use,” said ORNL researcher and co-author Ling Li. Traditional injection molding would require the expense of creating a new mold and tooling for each, but with additive manufacturing the forms can be crafted simply and quickly using computer-assisted design, she explained.
Future work will explore the printing of anisotropic, or directional, bonded magnets, which are stronger than isotropic magnets that have no preferred magnetization direction. Researchers will also examine the effect of binder type, the loading fraction of magnetic powder, and processing temperature on the magnetic and mechanical properties of printed magnets.
Alex King, Director of the Critical Materials Institute, thinks that this research has tremendous potential. “The ability to print high-strength magnets in complex shapes is a game changer for the design of efficient electric motors and generators,” he said. “It removes many of the restrictions imposed by today’s manufacturing methods.”
“This work has demonstrated the potential of additive manufacturing to be applied to the fabrication of a wide range of magnetic materials and assemblies,” said co-author John Ormerod. “Magnet Applications and many of our customers are excited to explore the commercial impact of this technology in the near future,” he stated.
Bonded magnets offer a better option for making low cost intricate shapes from isotropic powder when the high magnetic performance of sintered magnets is not required. Injection molding is a well-established method for fabricating complex shaped bonded magnets. This novel alternate approach – Big Area Additive Manufacturing (BAAM) – can fabricate near-net-shape isotropic NdFeB bonded magnets. Magnetic and mechanical characterizations demonstrate that the BAAM fabricated magnets can compete with or outperform the injection molded magnets. In addition, additive manufacturing offers significant advantages such as cost effectiveness (no tooling required), fast speed (simple procedure), and capability of producing parts of unlimited in sizes and shapes. Therefore, BAAM provides an effective method in realizing arbitrary shape with minimum cost and waste, and has the potential to revolutionize large-scale industry production of bonded magnets. In the future work, the effect of binder type, loading fraction of the magnetic powder, anisotropic particles, and processing temperature on the magnetic and mechanical properties of the printed bonded magnets will be investigated.
Abstract – Big Area Additive Manufacturing of High Performance Bonded NdFeB Magnets
Additive manufacturing allows for the production of complex parts with minimum material waste, offering an effective technique for fabricating permanent magnets which frequently involve critical rare earth elements. In this report, we demonstrate a novel method – Big Area Additive Manufacturing (BAAM) – to fabricate isotropic near-net-shape NdFeB bonded magnets with magnetic and mechanical properties comparable or better than those of traditional injection molded magnets. The starting polymer magnet composite pellets consist of 65 vol% isotropic NdFeB powder and 35 vol% polyamide (Nylon-12). The density of the final BAAM magnet product reached 4.8 g/cm3, and the room temperature magnetic properties are: intrinsic coercivity Hci = 688.4 kA/m, remanence Br = 0.51 T, and energy product (BH)max = 43.49 kJ/m3 (5.47 MGOe). In addition, tensile tests performed on four dog-bone shaped specimens yielded an average ultimate tensile strength of 6.60 MPa and an average failure strain of 4.18%. Scanning electron microscopy images of the fracture surfaces indicate that the failure is primarily related to the debonding of the magnetic particles from the polymer binder. The present method significantly simplifies manufacturing of near-net-shape bonded magnets, enables efficient use of rare earth elements thus contributing towards enriching the supply of critical materials.
SOURCES – Nature Scientific Reports, Oak Ridge National Labs
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