Hierarchical core–shell microstructure emergence in B₄C-modified 316 L stainless steel by LPBF process

Laser powder bed fusion (LPBF) of 316 L stainless steel has drawn increasing interest for structural applications, yet further improvements in mechanical performance remain desirable. In this study, a high-throughput experimental strategy was employed to optimize the LPBF parameters and B₄C content, with the aim of enhancing microstructural control and mechanical strength. The addition of B₄C ceramic particles led to the formation of a hierarchical core–shell microstructure, composed of a Ni-enriched interfacial layer, a nanocrystalline shell, and a Cr-rich transition zone. This interfacial architecture significantly refined the grain structure and altered the solute distribution, resulting in a remarkable increase in ultimate tensile strength from 786 MPa (316 L) to 1309 MPa (316 L + 1 wt.%B4C). We systematically analyzed the formation mechanism of the hierarchical core–shell microstructure and its contribution to strengthening through dislocation interactions, solute segregation, and strain field accommodation. These findings highlight the synergistic effects of process optimization and interface engineering in the design of high-performance metal matrix composites via additive manufacturing.