Influence of layer thickness on the printability of nickel alloy 718:A systematic process optimization framework

Abstract

Laser-powder bed fusion (L-PBF) is a metal additive manufacturing (AM) process involving layer-by-layer metal parts fabrication. It has been successfully used in the literature to manufacture nickel alloy 718 parts. However, the process parameter selection has generally been based on either intensive experimental methods or expensive computational simulations. As such, these methods test a limited region of the parameter space. Thus, hindering the evaluation of possible parameter combinations that can prove superior to those in the tested space. Moreover, most of the previous studies combine the process parameters into single metrics, e.g., linear energy density (LED) or volumetric energy density (VED). Both metrics have been shown in the literature to provide an incomplete understanding of the produced parts and inconsistent mechanical properties and microstructures. This paper uses a systematic framework developed by the authors that covers the entire scanning speed-laser power (v-P) parameter space, estimates the maximum hatch spacing at each v-P combination, and detects the good printability region. This framework defines the good printability region as the region that is free of the main possible porosity defects, namely, lack of fusion, keyholing, and balling. Furthermore, to cover the effect of the fourth process parameter (layer thickness), the framework is applied to 60 and 90μm layer thicknesses. This extension of the framework is performed to test the possibility of increasing the volumetric build rate (VBR) by increasing the layer thickness. Finally, the printability is validated using density and porosity measurements, while performance is evaluated using tensile tests. Using the framework, parts with 99.99% densification were fabricated successfully for both layer thicknesses. Moreover, tensile strength values of 1059.1 and 1063.5 MPa and volumetric build rates of 7.10 and 6.92 mm3/s were achieved for the as-printed parts for the 60 and 90μm layer thicknesses, respectively. However, these VBR values are constrained by the conservative hatch spacing criterion used in the framework. Further optimization of the hatch spacing can provide higher VBR values for the 90μm layer thickness.