Microstructural and mechanical insights into 1.2709 maraging steel produced by direct energy deposition

Abstract

The present work focuses on the characterization of the ultra-high-strength 1.2709 maraging steel produced by the Direct Energy Deposition (DED) technique, either in its as-built or as-built + heat-treated state. Scanning electron microscope micrographs and X-ray diffraction patterns showed that the heat treatment (namely, solution annealing and aging) had minimal impact on the microstructure changes of the maraging steel. The material is characterized by fine cellular or dendritic microstructure containing several percent of the ductile γ-austenite phase in both as-built and as-built + heat-treated states. A small amount of the Ni₃Mo_0.5Ti_0.5 intermetallic phase was observed even in the as-built state of the material. The heat treatment caused a substantial improvement of the mechanical properties through the homogeneous precipitation of nano-sized needle-shaped Ni₃Mo_0.5Ti_0.5 intermetallic phase. Tensile yield strength increased from 753 to 1957 MPa, ultimate tensile strength—from 991 to 2024 MPa, and microhardness—from 350 to 700 HV0.1. The present results are also compared with those obtained for the same material produced by the more commonly used Laser Powder Bed Fusion (L-PBF) technique. Despite having a coarser microstructure with a presence of γ-phase than the LPBF-printed material, the DED-printed maraging steel exhibited greater precipitation hardening while maintaining 5% ductility after heat treatment.