Additive Manufacturing of Soft Magnetic Fe50Ni

Soft magnetic alloys are widely utilized in electric motors, transformers, and sensors because of their high saturation magnetization, low coercivity, and high permeability. In this research, the magnetic and microstructural properties of Fe50Ni samples manufactured with two additive manufacturing processes, electron beam freeform fabrication (EBF3) and selective laser melting (SLM), were characterized and correlated with processing parameters.

Using wire and powder feedstock, samples were printed with varying scan speed, beam power, and energy density. Samples were characterized by optical and scanning electron microscopy, electron backscatter diffraction, x-ray dispersive spectroscopy, and vibrating sample magnetometry to correlate the resulting magnetic response with material properties.

While both the EBF3 and SLM processes were successful in printing Fe50Ni, the EBF3 depositions exhibited a high degree of printability with no visible porosity or cracking. In comparison, the SLM printed samples resulted in a wide printability map, yielding both excellent and poor quality samples. Significant porosity was observed in many of the SLM samples, with relative densities ranging from 63.7 to 99.0%. High quality SLM samples were deposited at a volumetric energy density of 76.39 J/mm3, in strong agreement with literature values (60.93 to 83.33 J/mm3) and the EBF3 values (87.48 J/mm3). In addition, the two processes resulted in significantly different melt pool sizes (3.429 mm2 for EBF3, 0.0036 mm2 for SLM) due to absorption and penetration differences in the two heat sources. The differing melt pools impacted the deposition, solidification, and cooling rates. The predicted maximum cooling rate for high quality samples was 5.2x10-3 K/s for EBF3 and 1.8x10-7 K/s for SLM, which resulted in a significant variation in grain size, ranging from 566 to 1064 µm for EBF3 and 16 to 81 µm for SLM.

The structure insensitive magnetic property, saturation magnetization (Ms), exhibited minimal variance, remaining relatively unchanged through the varied processing parameters for both EBF3 and SLM. All printed samples were shown to be disordered FCC, with saturation magnetization values ranging from 141 to 148 emu/g, near the literature value of 154 emu/g for Fe50Ni. No significant change in saturation magnetization was observed between the feedstock materials and the final samples, which is consistent with unchanging phase and composition through the printing process.

With respect to the structure sensitive magnetic properties, several microstructural aspects of the material contribute. In general, large grains, minimal porosity, and preferred texture orientation are desired to enhance the structure sensitive magnetic properties. The samples had measured coercivities (Hc) ranging from 1.15 to 6 Oe, and permeabilities (µ) ranging from 37 to 160. The EBF3 samples exhibited large FCC columnar grains (~280 µm), with strong Goss texture. The SLM samples generally had less Goss texture, which weakened as porosity increased.

Overall, samples printed by EBF3 exhibited lower coercivity values in comparison to SLM printed samples. The lower coercivity associated with EBF3 suggests that soft magnetic components printed by EBF3 may have merit. EBF3 samples exhibit large grains and a strong Goss texture, both of which are favorable for the structure sensitive properties of soft magnetic Fe50Ni. While the research is still in its infancy, the production of additively manufactured soft magnetic materials shows promise for more customizable, novel soft magnetic components.