A. Duong, I. Smith, K. Snyder, O. Bishop, E. Carpenter, R. Barua
Virginia Commonwealth University,
United States
Keywords: Phase-separated Permanent Magnet Composites, Additive Manufacturing, Direct Energy Deposition
Summary:
As the United States strives to achieve net-zero carbon emissions by 2050, demand for permanent magnets - critical components of wide-ranging applications in the power and energy sector, including wind turbines, hydroelectric power generators, and electric vehicles - is expected to grow domestically and globally [1]. Neodymium-iron-boron and Samarium-Cobalt are currently the strongest commercially available magnets; however, their most significant drawback is that they contain rare-earth elements associated with market volatility, geopolitical sensitivity, and hazardous mining practices [1]. Among the possible alternatives for rare-earth magnets, Alnico materials (alloys containing Al, Ni, Co, and Fe) are promising, particularly for applications above 200 °C due to their low cost and thermal stability [1] These alloys consist of periodically distributed and crystallographically oriented ferromagnetic (Fe-Co)-rich nanostructures embedded in an (Al-Ni)-rich matrix formed by spinodal decomposition [2]. The materials processing route strongly influences the Alnico alloys' microstructure, which, in turn, plays a crucial role in tailoring the magnetic properties of this phase-separated system [2]. In this work, we examine process-structure-property correlations in Alnico alloys fabricated using magnetic-field-assisted direct energy deposition (DED) – a metal 3D printing approach characterized by solidification conditions that inherently produce directionally aligned grains and metastable precipitates [3]. Gas atomized pre-alloyed Alnico powders were built into cube-shaped samples (~5 mm in length) using a laser-based deposition system equipped with sensors for real-time monitoring of the printing process. The laser operating parameters (power, speed, beam radius) and the powder mass flow rate during printing were varied systematically in three experimental builds, resulting in a library of ~90 samples. Select specimens were heat treated in an inert atmosphere at about 850 °C for 10 minutes to solutionize the alloy fully and then slow-cooled for prolonged annealing in temperatures ranging from 550 °C to 800 °C for four hours. Results indicate that scanning strategies and heat treatment conditions strongly influence the magnetic properties of the 3D-printed magnets. In particular, the coercivity (Hc) of the as-printed magnets increases monotonically as a function of the energy density (E) during deposition (Note: E=P⁄(m ̇vD^2)). Following heat treatment, samples deposited with E >1.1 J·s/kg·mm3 demonstrate an increase in Hc, while those deposited with E >1.1 J·s/kg·mm3 show a distinct decrease. Structural characterization using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and magneto-optic Kerr effect (MOKE) microscopy indicate that the magnetic properties of the DED-processed Alnico magnets are closely correlated to the morphology and spatial dimensions of the spinodal decomposed phases. These findings demonstrate the feasibility of fabricating alnico permanent magnets with tailored textures using laser-based additive manufacturing routes. References: [1] Coey, J. M. D. "Perspective and prospects for rare earth permanent magnets." Engineering 6.2 (2020): 119-131. [2] Cui, Jun, et al. "Manufacturing processes for permanent magnets: Part I—sintering and casting." JOM 74.4 (2022): 1279-1295. [3] Svetlizky, David, et al. "Directed energy deposition (DED) additive manufacturing: Physical characteristics, defects, challenges and applications." Materials Today 49 (2021): 271-295.