Ni80Fe15Mo5 permalloy has been proven to be the most effective candidate for magnetic shielding materials because it can be rapidly magnetized in weak magnetic fields, which means high magnetic permeability and magnetic induction intensity as well as low coercive force. Meanwhile, Permalloy has excellent mechanical properties, strength and toughness. Permalloy has great application potential in aerospace, telecommunications, instrumentation and other fields due to its sensitive response to weak magnetic signals and excellent mechanical properties.
Additive manufacturing has a negative impact on the magnetic properties of permalloy. SLM samples usually exhibit smaller grain sizes and a larger number of grain boundaries, which hinders the rotation or movement of magnetic domains and deteriorates magnetic properties. Dislocations can all affect magnetism by generating 90° and 180° magnetic domain walls. Therefore, as a key post-treatment technology, heat treatment is crucial for improving the microstructure and thermal properties of superalloys prepared using SLM. Heat treatment can eliminate the related phases of harmful impurities, reduce internal stress, and obtain a uniform recrystallized structure to achieve disorder. Annealed components can reduce the obstruction of grain boundaries to magnetic domains, thereby promoting the volatilization of impurity elements such as carbon, sulfur and phosphorus to purify the alloy. This process reduces coercive force and enhances the saturation magnetization intensity.
Liu et al. carried out isothermal annealing of SLM Fe-50%Ni components for 1-2 hours within the temperature range of 800 to 1300°C. Due to the relaxation of residual stress and the homogenization of microstructure after annealing, the increase in magnetic permeability and the decrease in coercive force occur simultaneously. Li et al. reported that recrystallization annealing does not significantly change the preferred orientation of grain texture. The microstructure evolution, including the elimination of residual stress, the change of grain boundary density, the elimination of substructural walls of the unit cell and the re-homogenization of composition, enhances the comprehensive magnetic property of Ni-15Fe-5Mo permalloy.
Furthermore, a previous study indicated that after heat treatment of the SLM Fe-50%Ni sample, the maximum relative permeability before heat treatment increased from 1000 to 5000, while the coercive force decreased from 2.5 Oe to 1.25 Oe. The Ni-15Fe-Mo permalloy powder used in this study was produced by Shanghai Truer Technology Co., Ltd. The particle size of this powder is D10=22.9μm, D50=34.3μm, D90=51.8μm, and the average particle size is 38.2μm. The relative density of Ni-15Fe-Mo permalloy prepared using SLM pre-alloying powder is 99.75%. When VED=81.63 J/mm3, it shows a uniform microstructure without elemental segregation. The microstructure is mainly composed of epitaxial columnar and honeycomb-like grains, with honeycomb-like substructural walls. This study aims to clarify the microstructure evolution of Ni-15Fe-Mo permalloy formed by SLM and the mechanism influencing its mechanical and soft magnetic properties. The magnetic properties were improved through the optimization of process parameters and post-annealing heat treatment. The results of this study can provide valuable insights for controlling the microstructure and performance of the samples.
