Effect of Ag nano-additivation on microstructure formation in Nd-Fe-B magnets built by laser powder bed fusion

Abstract: Laser powder bed fusion (PBF-LB/M) enables the near-net shape production of permanent magnets with complex geometry while reducing material waste. However, controlling the microstructure and optimizing magnetic properties remain challenging due to rapid solidification and intrinsic heat treatment effects occurring during both inter-layer and intra-layer processing. Surface additivation of the feedstock powder with Ag nanoparticles (NPs) is a concept that has been shown to increase the coercivity of PBF-LB/M-produced Nd-Fe-B magnets. Using atom probe tomography (APT) and transmission electron microscopy (TEM), we reveal that Ag nano-additivation promotes heterogeneous nucleation of the Nd2Fe14B phase, leading to refined, equiaxed grains and increased stability of the Ti-Zr-B-rich intergranular phase. The intrinsic heat treatment, influenced by layer-wise processing, further affects the distribution of Ag-rich regions, impacting grain growth and intergranular phase composition across different regions of the melt pool. Compared to the unadditivated sample, the Ag-additivated sample exhibits a significantly finer grain structure and a changed intergranular phase, which contribute to enhanced domain wall pinning and coercivity. These microstructural changes directly modify the magnetic domain structure, as evidenced by Lorentz transmission electron microscopy (TEM). Our results highlight that the interplay between nano-additivation and in-process heat treatment provides a novel pathway for tailoring the microstructure and enhancing the magnetic performance of permanent magnets.