Resistance sintering of NdFeBCo permanent magnets and analysis of their properties

Abstract

Permanent NdFeBCo magnets are produced by resistance sintering, which is a recent approach in the field of powder metallurgy. Classified as an electrical current assisted sintering (ECAS) process, its ultrafast characteristics enable quick densification, in the order of hundreds of milliseconds, which is considerably shorter than in conventional sintering, where up to several hours are required. Furthermore, no admixed powder lubricant is required, thus reducing contaminations and increasing effective density. By optimization of the electrical current density, sintering time and compaction pressure, magnetic discs with up to 97% relative density are presented. High-energy impact ball milling of the particular powder is shown to be important for subsequent magnetization. Sieve analysis shows the influence of the ball milling on the starting powder before sintering. Final properties are evaluated by hardness indentation, SEM and XRD analyses. The magnetic properties are evaluated and compared to both sintered and bonded commercial Nd magnets. The obtained magnetic properties are furthermore in accordance with the technical datasheet of the original powder, meaning that optimal properties are achieved by resistance sintering and magnetization. Finally, for the magnets to be suitable for commercial use, nickel plating of the magnets for enhancing corrosion resistance is shown to be possible by electroplating.