Localized Crack-Free Welding for DS 247LC Superalloy Correlated with Single-Mode Fiber Laser and CET Theory

In this study, we systematically investigated the mechanisms enabling localized solidification crack-free welding of directionally solidified (DS) 247LC superalloy using a single-mode fiber laser, with a particular focus on the correlation with the columnar-to-equiaxed transition (CET) theory. Welding was performed on a single DS grain under twelve conditions, varying heat input (1, 1.5, 2 J/mm), welding speed (500, 750 mm/s), and energy density (7.1, 14.0 J/mm<sup>2</sup>). The weld bead geometry was found to depend on the energy density, resulting in either heat conduction mode (7.1 J/mm<sup>2</sup>) or keyhole mode (14.0 J/mm<sup>2</sup>). In the heat conduction mode, solidification cracking was completely suppressed across all conditions, while in the keyhole mode, cracking was prevented under some conditions but appeared along the bead centerline under specific high heat input conditions (C-3 and C-4). Crystallographic analysis revealed that reduced epitaxial growth rates and increased high-angle grain boundaries were associated with crack formation in these cases. CET maps were calculated using the Thermo-Calc Additive Manufacturing module to elucidate the relationship between weld morphology and crack susceptibility, reflecting rapid solidification behavior under laser welding conditions. The results showed that heat conduction mode welds remained within the columnar growth region, while keyhole mode welds under higher heat input shifted towards the CET boundary or equiaxed region, correlating strongly with crack formation. These findings demonstrate that the suppression of CET and the promotion of epitaxial growth are critical for achieving crack-free welding in DS 247LC superalloy using single-mode fiber lasers, providing practical guidelines for advanced manufacturing and repair of turbine components under rapid solidification conditions.