Laser Powder Bed Fusion Processing of Soft Magnetic Fe-Ni-Si Alloys: Effect of Hot Isostatic Pressing Treatment

Laser powder bed fusion (L-PBF)-processed high-silicon steel has great advantages in freely designed electric engines, and various studies have been conducted in this field. However, the analysis of both the mechanical and magnetic properties, focusing on the multiscale microstructure under as-fabricated and heat-treated conditions, which is indispensable for industrial applications, has not been performed. In this study, an Fe–Ni–Si sample was fabricated using the L-PBF process. Subsequently, the following hot isotropic pressing (HIPing) process was employed as a post heat treatment step for the Fe–Ni–Si alloys. The effects of HIPing on the microstructure were investigated, focusing on the metastable stable phase transformation in the Fe–Ni–Si system. X-ray diffraction results showed single-phase fcc γ (Fe, Ni) in the L-PBF-processed samples before and after HIPing. Moreover, the acicular Ni/Si-rich structure (formed in the as-fabricated L-PBF sample because of its high cooling rates) transformed to the equilibrium austenite, Ni3Si, and FeNi3 phases during HIPing. After HIP, the compressive modulus and strength increased from 11 GPa and 650 MPa to approximately 18 GPa and 900 MPa, respectively. The magnetic properties were evaluated via a hysteresis loop, and the coercivity increased from 1.8 kA/m and to 2.9 kA/m after the HIPing process.