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

• It is the first work to study the magnetic hardening of Nd-Fe-B alloy by deformation processes from the point of view of mechanical grain fraction. • For the first time, Finite-Discrete-Element-Method modelling was applied to study the Nd-Fe-B alloys. • With the help of FDEM simulations, it is possible to understand the influence of different process parameters on the microstructural evolution. • Based on the knowledge gained from this work, the processing route was methodically optimized. A new processing route with better process controllability and improved product properties was developed. 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 Nd 2 Fe 14 B 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 Nd 2 Fe 14 B 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.