Process optimisation for Fe-based nanocrystalline soft-magnetic alloys produced with laser additive manufacturing

Fe-based amorphous and nano-crystalline alloys have emerged as promising
substitutes for silicon steel in power electronics and electrical machines due to their
lower coercivity (Hc) and significantly reduced core loss. Nevertheless, these
amorphous/nano-crystalline materials still exhibit a lower saturation flux density (Bs)
compared to silicon steel. Enhancing the power density and efficiency of advanced
electronic devices relies on the development of Fe-based nanocrystalline alloys with
high Bs and low core loss. However, the production of application-sized parts from
Fe-based nanocrystalline alloys with high Bs faces challenges due to their low glass-
forming ability. This limits the manufacturability of such alloys, making
nanocrystallization a difficult process. Utilising high cooling rates during laser
powder bed fusion (LPBF) process presents a favourable opportunity for fabricating
Fe-based amorphous/nanocrystalline alloys with intricate geometries. This work
demonstrated an extensive process optimization for Fe-based nanocrystalline alloys
to produce parts with high bulk density and soft-magnetic behaviour.
The optimization of process parameters in the LPBF technique has been examined by
considering the volumetric energy input (E), which encompasses key build
parameters such as laser power (P), scan speed (v), layer thickness (t), and hatch
spacing (h). This investigation focused on understanding the impact of these major
process parameters on the physical and magnetic properties of Fe-based
amorphous/nanocrystalline composites ((Fe87.38Si6.85B2.54Cr2.46C0.77, mass %)) fabricated
through LPBF. Various combinations of process parameters involving P and v were
applied while considering t and h.
The magnetic properties exhibit notable variations attributed to the amorphous phase
content and the presence of nanocrystalline phases within the microstructure, and
their size is greatly influenced by the process parameters. The microstructure
undergoes transformation during the laser scanning process, resulting in the
formation of molten pools (MP) and heat-affected zones (HAZ) due to the significant
thermal gradient between laser tracks. The molten pools primarily consisted of α-
Fe(Si) nanograins, while the heat-affected zones typically contained clusters of
nanocrystalline Fe2B and Fe3Si. The size and quantity of these nanocrystallites played
a crucial role in determining the magnetic properties. For this reason, the detailed
microstructural analysis was performed to comprehend the change in the soft-
magnetic behaviour, depending on the process parameters.