Microstructure of NiTi superelastic alloy manufactured by selective laser melting

• SLM processing is considered as “Metallic Self Micro-Mold Casting” (MSµMC). • SLM NiTi exhibits complex microstructures with patches, micro-lamellae, nano-cells. • Chemical micro-distribution is assumed between lamellae due to rapid solidification. • Grain parallel to laser direction is oriented along <001> direction. • Grain perpendicular to laser direction is oriented along <101> and <111> directions. Nitinol is used in dental tools for its superelasticity, shape memory and biocompatibility. Selective laser melting (SLM) on a powder bed is a potential process for manufacturing such as dental tools. Microstructural studies are conducted on NiTi-SLM using a parallel strip strategy and 90° laser beam rotation. The multi-scale (nano-, micro- and mesoscopic) microstructure is revealed. Similar “cast” type microstructure features are found. The SLM melt is casted in a “micro-mold”. Fine micro-grains with random crystallographic orientation are formed near the mold walls and served as seats for the rapid growth of columnar dendritic grains along the <001> direction with epitaxial crystallographic relationships. The growth of the columnar grains curves towards the center of the “micro-mold”, coinciding with the center of the laser beam. It is proposed that SLM processing resembles “Metallic Self Micro-Mold Casting” (MSµMC) and/or “Self Micro-Molding” (S2M).