Mechanical properties and microstructure evolution of selective laser melting Inconel 718 along building direction and sectional dimension

In this study, Inconel 718 cylindrical components with a built height of 320 mm were fabricated by selective laser melting (SLM) technology to investigate mechanical properties variation along the building direction. Based on computed tomography (CT) scanning, the entire samples were free of pores above 60 mu m. The mechanical properties and microhardness were characterized along the longitudinal direction for samples with diameters of 5 mm, 10 mm, and 15 mm. The experimental results showed that the strength of the SLM Inconel 718 samples decreases gradually from bottom to top, while the corresponding ductility exhibits an inverse relationship along the building direction. The part with 15 mm diameter presents an overall higher strength and lower ductility compared to parts with 5 mm and 10 mm diameters. According to grain size statistics, the high cooling rate of Inconel 718 closer to the bottom leads to grain refinement, which corresponds well to the Hall-Petch relationship. According to TEM analysis results, the Laves phase and carbides can be identified on the sub-grain boundaries of the as-printed sample. A further APT reconstruction model indicated that the sub-grain boundary is composed of Nb-rich granular particles. In addition, detailed analyses on microstructure, dissolution of precipitates, and mechanical properties are discussed based on a quantitative characterization.