Microstructure of NiTi superelastic alloy manufactured by selective laser melting

Abstract : 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 〈0 0 1〉 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).