One of the main challenges in additive manufacturing (AM) of metals is to manufacture high quality materials and ensure the performance of AM materials in service duties. This challenge can only be solved when the relationships between build process parameters, microstructure and deformation behaviour are understood. This present study is part of holistic efforts at Imperial College to reveal such relationships. In this study, we present our study of porosity condition, grain morphology, texture and metastable phases in AM stainless steel 316. To provide samples for mechanical and microstructural study, cylindrical samples of stainless steel 316 were printed by powder-bed laser melting with a bi-directional hatch pattern. Scanning electron microscopy and electron backscattered diffraction were used to investigate fine microstructures (such as grain morphology, texture and crystal phases) after 3D printing and deformation. Subsequently, a detailed 3D structure of columnar grains in as-printed 316 steel is constructed thanks to microscopic observation. Most of grains in as-built samples have a spherical bowl morphology, and being stacked on others to form the columnar structure. Examinations on microstructures show that the small sub-grains in as-printed samples is likely responsible for high yield strength at room temperature (significantly higher than that of conventional steel). In addition, residual stresses after rapid cooling probably promote the deformation-induced twinning that assists the plasticity during deformation, leading to a good ductility of the AM steel (almost as same as that of conventional 316 steel). Currently, a more detailed study is being undertaken to confirm this hypothesis.
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