Investigation of metallurgical mechanism governing the disorder/order transformation in high-silicon steels manufactured by L-PBF

Laser-Powder-Bed-Fusion is a promising technology for manufacturing ferromagnetic components in High- Silicon steels to overcome the problem of intrinsic brittleness. To date, the mechanisms governing the formation of the ordered-brittle B2 and D03 phases, from the primary disordered-ductile A2 phase, are still unclear for processes involving rapid solidification. In this framework, a deep understanding of steel microstructural evolution is needed to identify the most suitable additive techniques and to define the best manufacturing parameters. The purpose of this work is to understand the mechanisms involved in the L-PBF fabrication of Fe–6.5 wt.% Si steel components, enabling a comparison with melt-spinning and Electron-Beam-Powder-Bed-Fusion. The work is developed around two hypotheses: (i) the disorder/order transformation is driven by thermal cycling in the already solidified layers due to repeated laser passes, (ii) the formation of ordered phases occurs during the solidification stage. The study is based on mathematical modelling coupled to experimental approach. Differential-Scanning-Calorimetry is applied to derive the kinetics of formation of ordered phases. Finite- Element-Method is adopted to evaluate the temperature evolution during the L-PBF process, and directional dendrite growth and Scheil’s model to calculate, respectively, the solidification map and micro-segregation. The analysis shows that the intrinsic brittleness of the Fe–6.5 wt.%Si component is promoted by ordered phases presence due to Si and C micro-segregation under conditions of rapid solidification and high undercooling at the dendritic front. On the other hand, high cooling rates do not allow the component to remain within the critical temperature long enough to form ordered phases during printing process.