Reutilization of Dy strips in rotating diffusion to enhance coercivity of sintered Nd-Fe-B magnets

As a new evolutive grain boundary diffusion technology, rotating diffusion technology exhibits many advantages such as easy to achieve mass production and reutilization of heavy rare earth materials for diffusion. In this study, the magnetic properties and microstructure of sintered Nd-Fe-B magnets with enhanced coercivity by rotating diffusion using Dy strips were studied. The optimal rotating diffusion parameters was obtained at 800 °C for 8 h, and the coercivity of diffused magnet increased from 12.51 kOe to 17.38 kOe. A (Nd,Dy)2Fe14B core-shell microstructure was observed in the surface region of the Nd2Fe14B grains after the rotating diffusion. In order to evaluate the reutilization rate of heavy rare earth materials, multiple reutilization experiments of Dy strips by rotating diffusion were implemented. The coercivity enhancement declines slightly with the increase of rotating diffusion times. Due to the evaporation of Dy strips, the diffusion process results in uneven, rough surface of Dy strips with many small concaves, which reduces the saturated vapor pressure of Dy strips, leading to less Dy evaporation and diffusion, and in turn to lower coercivity. But it is worth to mention that the coercivity of the fifth rotating diffusion magnet still reaches to 16.67 kOe, which is 33% higher than that of the original magnet. It is therefore concluded that the reutilization of Dy strips in rotating diffusion for sintered Nd-Fe-B magnets with enhanced coercivity is feasible and rotating diffusion technology could realize the mass production.