Correlation between microstructure and mechanical properties in additively manufactured Inconel 718 superalloys with low and high electron beam currents

Abstract

This study explored the correlation between microstructure and mechanical properties in additively manufactured Inconel 718 superalloys with low and high electron beam (EB) currents. The relative densities of the as-built Inconel 718 superalloys with low EB currents ranged from 99.45 to 99.79 %, while the alloys with high EB currents demonstrated relatively high relative densities ranging from 99.82 to 99.96 %. The microstructures of the alloys with low EB currents had columnar grain microstructures with a strong <0 0 1> texture parallel to the built direction (BD). The high-angle grain boundary density of the alloys with low EB currents was lower than with high EB currents. In addition, the alloys contained fine γ″ precipitates with an average size of 32 nm. Although the fracture-initiating sites for the alloys consisted of defects, such as micro-cracks and gas-entrapped pores, the alloys with low EB currents demonstrated excellent tensile strength and good uniform elongation due to the single-crystal-like microstructures and fine γ″ precipitates. The γ″ precipitates easily grew in deep and narrow melt pools due to sufficient thermal transfer with increasing penetration depth. However, the growth of γ″ precipitates was limited in the shallow and wide melt pools, and the shallow and wide melt pools resulted in single-crystal-like microstructures and strong <0 0 1> textures parallel to the BD. Therefore, the morphology of the melt pool, determined by the EB current, played an important role in the single-crystalline microstructure and growth of the γ” precipitates.