Characterization of microstructure and magnetic properties of 3D printed bonded magnets made by fused deposition modeling

Bonded magnets are composite materials consisting of polymer matrix and magnetic powders, prepared by rapid solidification processes from Nd-Fe-B alloys. They are synthesised by the compounding process on a twin-screw extruder, whereby, the finished products of complex shapes can be made from bonded magnets using injection moulding or 3D printing by fused deposition modelling method (FDM). The main advantages of 3D printing are the possibility to produce parts with complex geometries that are not possible with traditional manufacturing techniques and low-cost production of small batches. The aim of the research work was to identify the optimum processing parameters, which would give 3D printed bonded magnets characteristics similar to those produced by injection moulding. The characterization of the microstructure of bonded magnets was made on cryo-fractured, conventionally mechanically prepared and ion beam polished samples. The microstructures of bonded magnets were analysed by stereo, optical and scanning electron microscopy. Additionally, the influence of the 3D printing parameters on the magnetic properties has been examined. The results of the research work have shown that desired magnetic properties of 3D printed bonded magnets can be obtained by optimizing the thickness of the printed layer, printing speed and flowrate. In addition, it was revealed that selection of the materialographic preparation method plays a crucial step for correct microstructural characterization. Namely, the impropriate sample preparation results in artifacts that are mostly misinterpreted as microstructural defects (pores, cracks, non-adherent layers, etc.) accidently caused during 3d printing.