Investigations on the microstructure and crack formation of IN738LC samples processed by selective laser melting using Gaussian and doughnut profiles

Abstract

This paper contributes to a better understanding of processing the nickel-based superalloy IN738LC using selective laser melting (SLM). Initially, the basic workability of IN738LC using SLM is demonstrated. Subsequently, a comparison between a Gaussian and a doughnut profile is carried out, and clear correlations between the choice of process parameters and the resulting imperfections depending on the chosen laser beam profile are shown. Electron backscatter diffraction (EBSD) measurements show a significant influence of scan strategy and build direction on the texture of the sample, independent of the used laser beam profile. Regardless of the laser beam profile, two contradicting trends complicate the defect-free processing of IN738LC, i.e. a reduction in the crack density can only be realized with increasing porosity. Through microstructural investigations, observed hot cracks are identified to be solidification cracks. Based on a broad experimental study in combination with a numerical solidification study, a theory about the crack initiation mechanism is presented. Finally, by using atom probe tomography (APT), the element zirconium is confirmed as a possible reason for the occurrence of solidification cracks.