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.