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

Key words: 
• laser additive manufacturing (LAM) /
• Nd-Fe-B permanent magnets /
• numerical simulation /
• microstructure /
• magnetic properties

Abstract: 

Nd-Fe-B permanent magnets play a crucial role in energy conversion and electronic devices. The essential magnetic properties of Nd-Fe-B magnets, particularly coercivity and remanent magnetization, are significantly influenced by the phase characteristics and microstructure. In this work, Nd-Fe-B magnets were manufactured 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 then clarified in detail. The results indicated that the solidification velocity (V) and cooling rate (R) are key factors in the phase selection. In terms of the VIM-casting Nd-Fe-B magnet, a large volume fraction of the α-Fe soft magnetic phase (39.7 vol.%) and Nd₂Fe₁₇B_x metastable phase (34.7 vol.%) are formed due to the low R (2.3 × 10^-1 °C s^-1), whereas only a minor fraction of the Nd₂Fe₁₄B hard magnetic phase (5.15 vol.%) is presented. For the LDED-processed Nd-Fe-B deposit, although the Nd₂Fe₁₄B hard magnetic phase also had a low value (3.4 vol.%) as the values of V (<10^-2 m s^-1) and R (5.06 × 10³ °C s^-1) increased, part of the α-Fe soft magnetic phase (31.7 vol.%) is suppressed, and a higher volume of Nd₂Fe₁₇B_x metastable phases (47.5 vol.%) are formed. As a result, both the VIM-casting and LDED-processed Nd-Fe-B deposits exhibited poor magnetic properties. In contrast, employing the high values of V (>10^-2 m s^-1) and R (1.45 × 10⁶ °C s^-1) in the LPBF process resulted in the substantial formation of the Nd₂Fe₁₄B hard magnetic phase (55.8 vol.%) directly from the liquid, while the α-Fe soft magnetic phase and Nd₂Fe₁₇B_x metastable phase precipitation are suppressed in the LPBF-processed Nd-Fe-B magnet. Additionally, crystallographic texture analysis reveals that the LPBF-processed Nd-Fe-B magnets exhibit isotropic magnetic characteristics. Consequently, the LPBF-processed Nd-Fe-B deposit, exhibiting a coercivity of 656 kA m^-1, remanence of 0.79 T and maximum energy product of 71.5 kJ m^-3, achieved an acceptable magnetic performance, comparable to other additive manufacturing processed Nd-Fe-B magnets from MQP (Nd-lean) Nd-Fe-B powder.