Julia Löfstrand: Design and additive manufacturing of soft-magnetic materials and metallic glasses

Abstract

Fe-based metallic glasses as soft-magnetic cores in energy applications increase the efficiency and significantly lower the energy losses during use, compared to conventional electrical steels. Metallic glass production calls for rapid cooling from high temperatures, solidifying the liquid before it crystallises. The additive manufacturing technique laser-based powder bed fusion is an increasingly popular production method for these materials. The material is melted stepwise, in small melt pool volumes at a time, allowing the heat to dissipate fast. The extreme thermal environment in laser-based powder bed fusion additive manufacturing enables the production of metallic glasses without shape restrictions. However, thermal cycling can also lead to residual stress and it is challenging to find a stable process window where defects like cracks, pores, or lack-of-fusion are not deteriorating the material properties. A great challenge is also to control the devitrification, nucleation and growth of crystalline phases commonly occurs, especially in the heat affected zone around the melt pool.

This work explores production and devitrification of metallic glasses in extreme thermal environments, with the intention to evaluate composition and production methods for soft-magnetic materials and metallic glasses. The influence of boron concentration and heating rate on Fe-Co-Nb-B metallic glasses is captured with millisecond precision using high brilliance synchrotron diffraction techniques. We are able to show the shift in the initial phase formation from α-Fe to the metastable Fe23B6 for higher boron concentrations (20 at.%) and high heating rates. The devitrification at slow heating rates was also investigated, using both calorimetry and magnetometry. The experimental description of the devitrification was additionally complemented with modelling. Using the same in-situ setup, we also evaluated the impact of oxygen contamination in a Zr-based metallic glass. Two different devitrifi-cation modes, homogeneous and heterogeneous, are measured and successfully mod-elled, for a selection of oxygen doping levels. Additionally, two studies are presented where Fe-based metallic glass and metallic glass composites are manufactured using laser-based powder bed fusion. The process development was successfully supported by Kerr microscopy imaging of melt pools of single tracks and the magnetic and structural properties of the produced parts were evaluated.