Additive manufacturing of Fe-6.5Si, Si and Fe-6.5Si/Si composites

Abstract

The increasing demand for high-efficiency electric motors requires soft magnetic cores with minimal eddy current losses. Although the addition of 6.5 wt.% Si to electrical steels significantly reduces these losses, the severe embrittlement of the material heavily limits conventional manufacturing routes involving rolling. This thesis explored Laser Powder Bed Fusion (L-PBF) as an innovative route to fabricate multi-material Soft Magnetic Composites (SMCs) with high electrical resistivity layers, aiming to mitigate magnetic losses. Initially, the processing of each individual constituent was investigated. For the L-PBF deposition of gas-atomized Fe-6.5Si, a Design of Experiments (DoE)/Box-Behnken statistical method successfully defined an optimized processing window. Aided by substrate preheating, the L-PBF process yielded highly dense and nearly crack-free samples. Microstructural analysis identified stable atomic ordering phases (B2 and D03), despite the high cooling rates inherent to the method. Subsequently, silicon (selected as the ideal low-electrical-conductivity material) was evaluated. Unlike Fe-6.5Si, the DoE/Box-Behnken approach failed to provide consistent, predictable trends for silicon. However, empirical optimization demonstrated that strict control of energy input and substrate preheating are essential to mitigate severe cracking susceptibility and structural porosity, achieving a relative density exceeding 99%. Finally, multi-material integration was evaluated through sequential experiments. Initial attempts encountered complex constraints due to thermomechanical issues, such as wettability mismatches and high melt-pool surface tension, which continuously jammed the recoater blade. By incorporating a continuous outer border to stabilize the geometry, it was possible to process silicon layers within a narrow energy window (VED = 42-47 J/mm3). Extrinsic hardware limitations, such as powder cross-contamination, combined with strong fluid flow within the melt pool, promoted intense liquid-state mixing. This prevented the formation of discrete segmented layers as originally designed. Instead of creating a segmented structural composite, this multi-material approach resulted in the successful in-situ synthesis of a homogeneous, silicon-rich soft magnetic matrix stabilized at 11-12 wt.% Si.