Upscaling the 2-Powder Method for the Manufacturing of Heavy Rare-Earth-Lean Sintered didymium-Based Magnets

The 2-powder method (2PM) is compared with the grain boundary diffusion process (GBDP) for the production of high coercivity didymium-based permanent magnets with low amount of heavy rare-earth (HRE). The advantage of using 2PM for large magnets is demonstrated. A coarse HRE-free main phase powder (D50 = 5.5 mu m) and a fine, HRE containing anisotropy powder (D50 = 2.1 mu m), are produced by strip casting, hydrogen decrepitation, and target milling, with all processing steps in the 20 kg range. The powders are subsequently blended in different concentrations to reach a HRE content of 0, 1, 2, and 3 wt% in the sintered magnets. While remanence is only slightly reduced by this process, compared with the plain didymium magnet, the coercivity of the magnets is increased by up to over 750-1687 kA m(-1). The microstructural and magnetic characterization both confirm the formation of a HRE-enriched shell region enclosing the Nd2Fe14B grains. In contrast to magnets produced by GBDP, the amount of Dy substitution and thickness of the shells is constant throughout the entire volume of the magnet.