Coercivity enhancement of waste Nd-Fe-B magnets by Pr70Cu30 grain boundary diffusion process

Abstract

Considerable quantities of Nd-Fe-B magnet wastes are produced every year worldwide. Some Nd-Fe-B magnet wastes in the bulk form, produced during manufacturing, have low coercivity and cannot meet the requirements for applications. Finding an effective way to reuse those wastes by improving the coercivity, without powdering or reproducing process, becomes very important for saving energy and raw materials in manufacture. In this work, the grain boundary diffusion process is carried out on waste Nd-Fe-B sintered magnets by using Pr70Cu30 as a diffusion medium. The effects of diffusion temperature, diffusion time, and annealing time on the magnetic properties of the magnets are investigated. It is found that the coercivity increases when the diffusion temperature increases from 500 to 800℃, the diffusion time increases from 1 to 3 h, or the annealing time increases from 1 to 3 h. By comparing the diffused sample with the simply heat treated sample, we find that the coercivity enhancement by grain boundary diffusion process indeed results from the infiltration of Pr and Cu elements. The coercivity of the magnet increases by 51.9%, from 7.88 kOe (1 Oe=79.5775 A/m) to 11.97 kOe, after 4-hour diffusion at 800℃ followed by 3-hour annealing, with a negligible reduction of remanence Br, achieving a 99.8% recovery of coercivity compared with the commercial N35 magnet. It is noted that 500℃ annealing for 3 h after 800℃ diffusion only slightly increases the coercivity by 4.6%, from 11.44 kOe to 11.97 kOe, which indicates that the annealing process after Pr-Cu grain boundary diffusion may be not indispensable. Based on the microstructure analysis, the diffusion of Pr and Cu is confirmed. However, the distributions of Pr and Cu are inhomogeneous within a range of tens of microns near the surface even though the diffusion has spread throughout the magnet. The structure of main phase grains separated by the continuous grain boundary phase is formed after the grain boundary diffusion process while the core-shell structure is not observed, which suggests that the modification of the grain boundary structure is the main reason for the coercivity improvement. Cu element plays an important role in forming continuous grain boundary phase. In addition, the electrochemical corrosion test shows that higher corrosion current is obtained in the diffused magnet than in the original magnet, though the corrosion potential is improved. The reduced corrosion resistance may be related to the increased RE-rich phase content and the formation of continuous grain boundary phase. The present work is of great importance for increasing the production yield of Nd-Fe-B magnets.