Mechanistic process–microstructure–cracking correlations in laser powder bed fusion of the Ni-based superalloy IN738

This study addresses the mechanistic interplay among processing parameters, microstructure, and cracking behavior in LPBF of difficult-to-print nickel-based superalloy IN738LC. To this end, key process parameters were systematically varied. Advanced microstructural characterization, combined with Scheil solidification simulations, was performed to elucidate microstructure and crack formation mechanisms under rapid solidification conditions. The results demonstrate that melt pool geometry plays a critical role in crack behavior, with an optimal melt pool width-to-depth aspect ratio showing the most favorable crack behavior. Reducing the hatch spacing resulted in lower crack density despite enhanced grain coarsening compared to increasing laser power, owing to improved densification and an optimized melt pool width-to-depth ratio. Microscopic analysis revealed a non-equilibrium microstructure of as-built condition consisted of a γ matrix with dendritic/cellular substructures, nanoscale carbides at cell boundaries, and sporadic Al-based oxides; no γ′ precipitates were detected. These features, consistent with Scheil's predictions, locally reduce microstructural coherency and promote crack initiation. Crack formation is strongly promoted by elemental segregation and dispersed oxide formation, which reduce ductility, together with the high thermal stresses inherent to the LPBF process.