Metal Additive Manufacturing Conference 2019

The current understanding of laser powder bed fusion process-microstructure-property relationships is inadequate to optimise the process. Microstructure-property relations in 316L stainless steel are explored in this study using heat treatment as an investigative tool. As-built material was heat treated to gradually remove microstructural features for evaluation of their influence on yield and failure behaviour. Samples were prepared from these materials for metallographic analysis and tensile testing. Image analysis of polished sections found porosity was primarily caused by lack of fusion with a preference for alignment along layer boundaries. Tensile testing revealed as-built material to be anisotropic with regard to yield strength and ductility. A theory explaining anisotropic ductility was proposed where pore orientation and loading direction are related to scanning strategy, stress concentrations and failure. The yield strength of as-built material was attributed to four main contributors; inherent lattice strength, grain boundaries, chemical segregation and dislocation density. Heat treatments were categorised into three regimes: recovery, homogenisation and annealing. Recovery heat treatment improved ductility while modestly decreasing strength and reduced the anisotropy of both. This treatment was recommended for structural parts. Post-test examinations of tensile samples found pore coalescence was prominent during failure of all samples. As-built samples exhibited significant brittle behaviour along layer-layer melt pool boundaries when stressed in the build direction, but more ductile behaviour when stressed in the build plane. Heat treatment increased ductility in both orientations. Keywords: Additive manufacturing, Laser powder bed fusion, 316L, Heat treatment, Characterisation, Porosity, Microstructure, Mechanical properties