Microstructural investigation of nanocrystalline Nd-Fe-B magnets fabricated by laser powder bed fusion

Abstract

Nd-Fe-B magnets were manufactured by laser powder bed fusion (LPBF) using commercially available spherical powder (MQP-S-11-9). The magnets were manufactured at four different volume energy densities but similar relative densities to investigate the relations between the process, structure, and characteristics. Unlike the nanocomposite microstructure of the initial powder, the microstructure of the LPBF samples had (Nd, Pr)2Fe14B grains separated by grain boundary phases through the remelting and solidification of the initial powder. The cooling rate of the melt pool increased as the volume energy density decreased, suppressing the crystallization of α-Fe in the grain boundary phase. When the volume energy density was less than 86 J/mm3, the crystallization of α-Fe was fully suppressed, and the grain boundary phases became amorphous, resulting in higher coercivity than that of the initial powder. The magnet with the strongest magnetic properties of Hci = 10.3 kOe, Br = 6.2 kG, and (BH)max = 7.5 MGOe was produced when LPBF was performed under the conditions of P90, V500, H70, and L30 (volume energy density = 85.7 J/mm3). The solidification behavior of Nd-Fe-B magnets during LPBF was analyzed in detail using thermodynamic simulations. In this study, the microstructure of the fusion zone (FZ) and the heat affected zone (HAZ) according to process parameters was analyzed, and the effect of the solidification microstructure on the magnetic properties of Nd-Fe-B magnets was reported.