Improving Productivity in the Laser Powder Bed Fusion of Inconel 718 by Increasing Layer Thickness: Effects on Mechanical Behavior

Build time is a critical contributor to overall part cost in the Laser Powder Bed Fusion (L-PBF) process, which in turn plays an important role in whether a part is made with this process or not. While there are many ways to improve productivity in the L-PBF process, this work focuses on one specific approach in the context of L-PBF of Inconel 718: increasing layer thickness, which reduces the number of slices for a given part, but is accompanied with the potential for a degradation in mechanical properties. The study is separated into two parts: the first establishes an optimal process at each of three different layer thicknesses (30, 60 and 80 µm). Having developed an optimum process, the study then obtains density, surface roughness and mechanical property data on specimens fabricated with the selected processes, along with SEM micrographs. Half the specimens are characterized and tested in the as-built condition, the other half go through a heat treatment process that includes Hot Isostatic Pressing (HIP). This study of a total of 600 specimens across six builds confirms that an increase in layer thickness has no significant effect on elastic modulus, but does show reductions in density and strength, along with slight increases in surface roughness. Microstructural studies show no significant differences in grain size and orientation, with a slight increase in carbides and delta precipitates with increasing thickness. Post-heat treatment mechanical strength for all thicknesses is comparable to forged values for Inconel 718, and limited data suggest improvements in excess of forged values can be obtained if specimens are machined prior to testing, with a narrowing of the differences attributable to layer thickness. This study also showed correlations between strength and density and revealed build location dependence for UTS.