Toward understanding the microstructure characteristics, phase selection and magnetic properties of laser additive manufactured Nd-Fe-B permanent magnets

Abstract Nd-Fe-B permanent magnets are critical components for energy conversion and electronic devices. The key magnetic properties of Nd-Fe-B magnets, especially the coercivity and remanent magnetization, are strongly dependent on the phase characteristics and microstructure. In this work, Nd-Fe-B magnets were prepared using vacuum induction melting (VIM), laser directed energy deposition (LDED) and laser powder bed fusion (LPBF) technologies. The microstructure evolution and phase selection of Nd-Fe-B magnets were clarified in detail. The results indicated that the solidification velocity (V) and cooling rate (R) are key factors in determining the phase selection. In terms of the VIM-casting Nd-Fe-B magnet, a large volume fraction of the soft magnetic α-Fe phase (39.7 vol.%) and Nd2Fe17Bx metastable phase (34.7 vol.%) are formed due to the low R (2.3×10-1 ℃/s), while the hard magnetic Nd2Fe14B phase is only 5.15 vol.%. With increasing V (<10-2 m/s) and R (5.06×103 ℃/s), part of the soft magnetic α-Fe phase (31.7 vol.%) was suppressed, more Nd2Fe17Bx metastable phases (47.5 vol.%) were formed in the LDED-processed Nd-Fe-B magnet, and the hard magnetic Nd2Fe14B phase also had a low value (3.4 vol.%). As a result, the casting- and LDED-processed Nd-Fe-B magnets exhibit poor magnetic properties. In contrast, the high V (>10-2 m/s) and R (1.45×106 ℃/s) led to the formation of the hard magnetic Nd2Fe14B phase (55.8 vol.%) from liquid, and the α-Fe phase and Nd2Fe17Bx phase precipitation were suppressed in the LPBF-processed Nd-Fe-B magnet. Furthermore, the strong crystallographic texture on the {001} crystal plane is another reason for the remanence enhancement in the LPBF-processed Nd-Fe-B magnets. Consequently, a coercivity of 656 kA/m, a remanence of 0.79 T and maximum energy product of 71.5 kJ/m3 was achieved in the LPBF-processed Nd-Fe-B magnet, which indicated excellent magnetic performance, comparable to other additive manufacturing processed Nd-Fe-B magnets from MQP (Nd-lean) Nd-Fe-B powder.