Molten pool crystallization behavior promoting amorphous and nanocrystalline grains in low coercivity additively manufactured soft magnetic material

Electrical steel with Si content (>6.5 wt% Si) can be used for up to 100 kHz frequency magnetic applications even without critical elements such as cobalt. Their brittleness makes it difficult to process conventionally into intricate shapes, limiting their scope for low core loss applications. Additive manufacturing may allow the processing of such challenging compositions provided the origin of nano crystallization within a molten pool, and high relative density of the manufactured components to result in the target magnetic properties are well understood. We report the microstructure-magnetic property correlation with the applied laser parameters of the additively manufactured Fe 73.5 Si 13.5 B 9 Cu 1 Nb 3 (at.%) soft magnetic alloy (8 wt% Si), commonly known as Finemet. The ability to tune the phase and the microstructure evolution by adjusting the volumetric energy density is demonstrated. This results in a matrix phase, which is either an amorphous-nanocrystalline mixture or completely crystalline. Microstructurally, the grain growth and the effects of undercooling in the solidified molten pool regions are described with an emphasis on the identified thermal gradient variations within a melt pool. A solute partitioning to thermal diffusion switch mechanism governs the crystallization, resulting in various morphologies (fine dendrites, micro/nanoscale equiaxed grains), and amorphous fractions. The samples were characterized as a dominant bcc α-Fe (Si) phase and a minor presence of the reordered D03 phase. Up to 97 % of the theoretical material density was achieved in the fabricated samples. Coercivity values ranging from 1.1 to 14.6 Oe (84–1166 A/m) and saturation magnetization from 124 to 136 emu/g (1.09–1.19 T) were measured using a vibrating sample magnetometer (VSM). We explain the recorded differences based on the melt-pool morphology, grain size variation, phases present, and residual stress as observed from the calorimetry study data. The investigation highlights the potential of additive manufacturing for Finemet alloys with desired magnetic properties, such as low coercivity and high magnetization values in the as-built condition. It also indicates an exciting prospect of controlling the magnetic properties by process parameters for target applications in high Si electric steel compositions.