Localised control of phase formation in a high-carbon low alloy steel by laser powder bed fusion

The ability of additive manufacturing processes to control solidification at a microscopic scale is of particular interest in the development of components with topology-optimised microstructures. Leveraging laser powder bed fusion (L-PBF), we present a new methodology to develop site-specific “microstructure architectures” consisting of dissimilar phases. By controlling the laser focal offset in a bespoke L-PBF machine, we tailor the melt pool shape and control the volume of material that undergoes re-heating during each melting event. Using a high-carbon low alloy steel as a case study material, we demonstrate control over the formation of tempered and un-tempered phases at the resolution of the laser spot size (~102 µm) and in 3-D throughout the build. Although the tensile strength of the steel is limited by the presence of keyhole porosity and solidification cracking, our strategy showcases the opportunity to vary hardness site-specifically and by up to 2.4 GPa, opening the path to producing alloys with novel microstructure and property complexity.