Effects of laser-energy density and build orientation on the structure-property relationships in as-built Inconel 718 manufactured by laser powder bed fusion

This study investigates the effects of build orientation and laser-energy density on the pore structure, microstructure, and tensile properties of Inconel 718 manufactured by laser powder bed fusion. Three different build conditions were selected for comparison based on previous research (namely, the conditions that resulted in the worst and best fatigue lifetimes): 0 degrees build orientation and 38 J/mm(3) laser-energy density, 0 degrees build orientation and 62 J/mm(3) laser-energy density, and 60 degrees build orientation and 62 J/mm(3) laser-energy density. Differences in porosity were measured between each build condition. In terms of microstructure, all three conditions exhibited a predominantly <001> texture in the build direction, grains elongated in the build direction, and a sub-grain structure oriented with the build direction that consisted of dislocation networks decorated with nano-scale precipitates. Build orientation (0 degrees versus 60 degrees with respect to the build plate) produced a difference in yield strength due to anisotropic grain morphology and effective grain size. The low laser-energy density specimens showed a significant decrease in all mechanical properties compared to the high laser-energy density specimens because the amount (6.91% by volume) and size of the lack-of-fusion porosity (from insufficient melting) surpassed a level at which microstructure (the grain and sub-grain structure) no longer governs quasi-static mechanical properties. This work provides insight that could enable the tunability of structure-property relationships in as-built Inconel 718 by optimizing laser-energy density and build orientation.