Magnetic shielding promotion via the control of magnetic anisotropy and thermal Post processing in laser powder bed fusion processed NiFeMo-based soft magnet

The aim of this study is to promote the magnetic shielding characteristics of laser powder bed fusion (LPBF) processed NiFeMo alloy. This was achieved via controlling the crystallographic texture of the builds to increase the grain population along the easy axis of magnetisation, as well as the use of post-process hydrogen heat treatment (HT) and hot isostatic pressing (HIP) processes. The as-fabricated microstructure typically demonstrates weak magnetic properties due to the alignment of the crystallographic orientation/spin order along the [100] hard axis of magnetisation, which is parallel to the build direction since it is also the preferred growth direction during solidification in cubic materials. Tilting the build orientation to align the easy magnetisation axes [110] and [111] along the build principal directions results in an improvement in the magnetic shielding characteristics normal and transverse to the build principal directions. Furthermore, the HT/HIP processes further promoted the soft ferromagnetic characteristics, with the best magnetic shielding properties being registered for the [111] tilted sample following both HIP and HT, demonstrating 60–100 folds improvement compared with the as-fabricated condition. The improved ferromagnetism following HIP + HT was due to several combined effects, including stress relief, consolidation of gas pores, recrystallisation, and grain growth. The post-processing sequence (HT + HIP vs. HIP + HT) appeared to affect the resulting magnetic characteristics. Finally, the tensile properties for the builds were characterised to ensure that both functional and mechanical behaviours would achieve the required performance.