Flat-top laser powder bed fusion of non-weldable Ni-based superalloy: crack mitigation through microstructural engineering for enhanced mechanical properties

Laser powder bed fusion (L-PBF) of high-γ′ Ni-based superalloys is limited by cracking and microstructural heterogeneity. Here, a flat top (FT) laser beam was used to fabricate Inconel 939, and its cracking behaviour, microstructure, and tensile properties were compared with those produced by a Gaussian beam. At 850 W and 350 mm/s, FT-LPBF produced specimens with 99.95% relative density and no cracks detected in the examined sections. Compared with the Gaussian conditions, FT processing generated flatter melt pools, continuous columnar grains, and a lower fraction of high-angle grain boundaries. The as-built FT specimen achieved an ultimate tensile strength (UTS) of 1211 MPa and elongation of 18.8% at room temperature. From 800–1000°C, its UTS decreased from 983 to 284 MPa, while elongation increased from 18.4% to 29.4%. Thermal-fluid simulations suggest that a more uniform temperature field and reduced lateral thermal gradients contribute to stable epitaxial growth. These findings demonstrate that a more uniformly distributed laser beam profile can improve the printability and tensile performance of IN939 without post-processing.