Process gas influence on microstructure and mechanical behavior of Inconel 718 fabricated via selective laser melting

The influence of process gas on porosity, microstructure, and mechanical properties of nickel-based superalloy Inconel 718 (IN718) produced via selective laser melting (SLM) was investigated. Density and surface roughness of as-built parts were similar for samples built under nitrogen and argon environments. After hot isostatic pressing (HIP) and precipitation-hardening heat treatments, argon-processed specimen had finer, more uniform grain structures, and increased room-temperature tensile failure strains (> 24%) and yield strengths (> 4%) compared with nitrogen-processed specimen. Elemental analysis showed no variation in composition or nitride precipitate concentration with environment. Larger melt pool structure and heat-treated grain size were produced under argon when laser focus shift was varied away from focal plane, while under nitrogen grain refinement was observed when focus shift is moved in the positive direction. These changes in microstructure are reflected in mechanical properties; the highest strength and ductility is achieved in argon-processed samples at the laser focal plane. To clarify the contribution of thermal conductivity, density, and refractive index of process gas to these observations, samples were built under helium and surface finish and microstructure were evaluated. It was determined that laser interaction with process byproducts is most responsible for differences in as-printed and heat-treated microstructures under each protective environment.