Laser powder bed fusion of Invar 36 soft magnets for dimensionally stable magnetic shielding applications

This work investigates the laser powder bed fusion (LPBF) of Invar 36 alloy to develop a high-performance magnetic shielding prototype with superior dimensionally stable characteristics. A design of experiment approach was employed to optimise laser processing parameters for maximum build integrity and superior soft magnetic performance. The as-fabricated (AF) samples show a single phase γ-FCC structure, characterised by different levels of porosity and crystallographic anisotropy. The AF optimised condition exhibits a high-density solid volume of 99.89%, which corresponds to the lowest coercive field (H c ) of 274 A/m, the highest saturation magnetisation ( M s )(13×10 4 A/m) and a Curie temperature ( T c ) of 235 ° C. Post-processing was applied to the AF optimum condition, which consists of a hot isostatic pressing (HIP) process, followed by a heat treatment process in a hydrogen atmosphere (HIP+HT). Following the HIP+HT process, the microstructure density increases to 99.99% through pores collapse during HIP, accompanied by stress relief and a significant reduction in dislocation density. The magnetic properties exhibit a significant improvement following post-processing: the M s and T c increase to 15×10 4 A/m and 245 ° C, respectively, owing to enhanced chemical homogeneity, oxide reduction and the improved Ni-Fe ferromagnetic ordering. Meanwhile, the H c decreases to 181 A/m due to the decrease in porosity and dislocation density. The magnetic shielding performance was assessed on a tubular prototype in DC and AC modes in the transverse and axial directions. Under the DC mode, the AF condition shows shielding factors ( SF ) of 3.25 and 4.75 in the axial and transverse directions, respectively, which increase to 18 and 75 after post-processing. In AC mode at 1000 Hz, the SF improved from 3 (axial) and 5 (transverse) in the AF condition to 60 and 245, respectively, with post processing. The AF optimised condition shows a coefficient of thermal expansion of 1.7×10 -6 / ° C that is improved to 1.5×10 -6 / ° C following post-processing due to the improvement in chemical homogeneity.