Spatial control of microstructure and material hardness in functionally graded stainless steels by DED-LB/M and in-situ alloying

Duplex stainless steels (DSS) are characterized by a two-phased microstructure (δ-ferrite and γ-austenite) with equal phase fractions, providing an exceptional combination of high strength, toughness, and corrosion resistance. This duplex microstructure is conventionally achieved by a precise thermo-mechanical process (e.g., hot rolling) followed by multiple post-processing steps (coating, joining, assembly) to meet the requirements in high-performance applications (e.g., advanced wear and corrosion resistance). Laser directed energy deposition of metals (DED-LB/M) enables simultaneous processing of multiple materials in a single component, allowing for the customization of the functionality while reducing the number of process steps required. In this study, a 1.4462 DSS was manufactured by DED-LB/M and compositionally modified (in-situ alloyed) with increasing proportions of elemental Cr and/or Mo powder to control both the phase formation and material hardness. Subsequent solution annealing (1050 °C; 2 h) and quenching homogenized the as-built microstructure within each grading increment. Microstructure analysis (phase fraction, morphology, and grain size using electron backscattered diffraction) was correlated with the local chemical composition by energy dispersive X-ray spectroscopy. Hardness profiles along the grading direction indicated a gradual increase in material hardness due to the stabilization of δ-ferrite (+ 69 HV10) or σ-phase (+ 683 HV10) with the addition of Cr and/or Mo. This approach demonstrates that in-situ alloying in DED-LB/M facilitates the spatial control of phase structures and the customization of functional properties. Components can now be manufactured in a single process with smooth compositional transitions and locally enhanced material properties, e.g. ductile core with wear and corrosion resistant shell.