Crack Intensity Reduction in Fe–6.5Si Alloy by Adding Cr and Controlling the Thermal Gradient

Abstract

Fe-6.5wt.%Si alloy is a promising soft magnetic material for electric motor applications owing to its high electrical resistivity and low core loss. However, the intrinsic brittleness of this alloy precludes fabrication of thin laminates using conventional rolling processes. Laser Powder Bed Fusion (LPBF) has therefore been considered as an alternative manufacturing route, offering geometric flexibility. Nevertheless, successful LPBF processing of Fe6.5wt.%Si has remained challenging due to its high silicon content. In this study, the Fe-6.5 wt.%Si alloy was modified by introducing 1wt.%Cr, and LPBF process variables were optimized to yield sound parts. The effect of Cr addition on suppressing the disorder–order phase transformation during solidification was investigated through Thermo-Calc® thermodynamic simulations and quantified via X-ray diffraction phase analysis. Crack morphology analysis from optical micrographs revealed a marked reduction in solidification and liquation cracks, attributed to the role of Cr in mitigating silicon segregation and consequently lowering the fraction of ordered phases. Preheating the build plate to 200°C was found to effectively eliminate vertical cracks by reducing thermal stresses; however, a limited number of horizontal cracks initiated at the sample edges and propagated inward, likely due to elevated thermal gradients at the perimeter. To address this issue, sacrificial walls were introduced at distances of 1.0 and 0.2 mm from the cube edges, locally reducing the cooling rates and effectively increasing the primary dendrite arm spacing (PDAS).The reduced cooling rate also led to lower lattice misorientation confirmed by EBSD, and a significant decrease in the crack length from ~2 to ~0.7 mm.