Optimization of a thixoforming process for Nd-Fe-B permanent magnets by the finite-discrete-element-method

Microstructural engineering, especially grain size modification, is an important aspect of improving the magnetic properties of Nd-Fe-B alloys. Previous work of the authors proved that thixoforming in terms of rotary swaging can be an effective and economically feasible method for grain refinement. Brittle Nd2Fe14B grains are fragmented by the rotary swaging process due to mechanical loadings. However, improved hard magnetic properties are limited by the heterogeneous grain size distribution after the forming processes. In order to gain a better understanding of the influences of process parameters on the grain fracture processes, a numerical study based on a Finite-Discrete-Element model with Cohesive Interface Elements is carried out in this work. This approach allows the simulation of crack patterns in brittle Nd2Fe14B grains and thus to observe the crack formation during processing. Diameter reduction ratio, impact velocity, impact angle and grain size differences are investigated by parameter variation. Based on the knowledge gained, an optimized process route was designed. Qualitative experimental validation confirms the identified conditions that allow for more homogeneous particle size distributions achievable with the new process route.