Micro-cracking mechanism of RENÉ 108 thin-wall components built by laser powder bed fusion additive manufacturing

Abstract

This study investigates the effect of wall thickness on the micro-cracking mechanism in thin-wall components produced by laser powder bed fusion (LPBF) using a hard-to-weld nickel-based superalloy, RENÉ 108. Microstructure analysis shows higher fraction of micro-cracking for thicker parts and a discontinuity at the 1 mm wall thickness. All micro-cracks exhibit interdendritic behavior, suggesting micro-crack formation in the final stages of solidification. Two finite element modeling (FEM) approaches are employed to evaluate the stress states at the beam and layer scales during laser processing. The beam-scale model shows positive stress triaxiality within the melt pool above the solidus temperature, supporting the view that solidification cracking is the dominant micro-cracking mechanism. The layer-scale model predicts higher stress triaxiality with increasing part thickness, favoring the finding for longer and larger number of micro-cracks in thicker parts. Hence, the anomalous micro-cracking behavior observed in the 1 mm part cannot be explained using the current models. Detailed microstructure analysis reveals larger variation in the primary dendrite arm spacing (PDAS) and cooling rate for the 1 mm part during LPBF. A higher thermal gradient is expected under these conditions, thereby explaining the anomalous effect observed between 0.25 mm and 5 mm wall thicknesses.