Heterogeneous liquid–solid melt pools in EB-PBF of porous inconel 625/BNi-2: A general strategy for strength–permeability synergy in mixed-melting powder systems

Abstract

To enhance tensile strength and functionality while maintaining porosity for transpiration cooling, this study introduces NiCrSiB (BNi-2) powder into the electron beam powder bed fusion (EB-PBF) process. By integrating beam defocusing, porous Inconel 625 alloys with uniform, interconnected micron-scale pores were fabricated. Comprehensive analyses of morphology, porosity, microstructure, mechanics, and permeability were conducted, with comparisons to BNi-2-free and powder metallurgy (PM) counterparts. BNi-2 improved pore uniformity, approaching PM quality. Lower line energy increased pore density and spatial uniformity. A quantitative multi-scale link between porosity, pore shape, and tensile strength was established, revealing that "low beam current + high scan speed" defines a highly sensitive and efficient process window, where small parameter changes can trigger functional shifts. Melt pool calculations revealed a stable heterogeneous state with liquid–solid coexistence, forming four zones: unmelted and partially melted powder zone (UMP/PMP), melted BNi-2 zone (MBZ), melted Inconel 625 zone (MIZ), and mixed MBZ&MIZ zone (MIX), driven by heterogeneous melting and Cr, Mo, Si, B segregation. BNi-2 raised tensile strength to 166 MPa at 27 % porosity (277 % higher than EB-PBF baseline), and 80 MPa at 33 % porosity—142 % and 33 % higher than BNi-2-free and PM samples. Isotropic permeability reached 4.32–4.55 d (124 % of PM). This study achieved bidirectional optimization of structure and performance, and established a novel mechanism identification pathway and parameter design criterion applicable to the EB-PBF fabrication of porous metals. It provides a valuable and generalizable reference for structure–function synergistic regulation across different alloy systems.