Effects of Scan Speed on Crack Elimination, Microstructural Evolution, and Mechanical Properties of IN738LC Alloy Processed by Laser Powder Bed Fusion

Cracking represents a critical issue in γ’-strengthened Ni-based superalloys processed via laser powder bed fusion. This study systematically investigated the influence of scan speed (800–1200 mm/s) on the crack elimination mechanism, microstructural evolution, and mechanical properties of LPBF-processed IN738LC alloy. Near-defect-free IN738LC parts were successfully produced with a relative density of 99.6% and a crack density of only 0.025%. The results indicate that as the scan speed increased from 800 mm/s to 1100 mm/s, a flatter melt pool (S4) was obtained, which reduced the proportion of high-angle grain boundaries. The cooling rate also increased from 13.68 K/μs to 15.96 K/μs, promoting grain refinement and the dispersion precipitation of MC carbides. The refined grains effectively suppressed stress concentration and inhibited crack propagation along grain boundaries. The optimized process (1100 mm/s) achieved optimal comprehensive mechanical properties. Compared to a scan speed of 800 mm/s, the ultimate tensile strength, yield strength, and elongation at room-temperature increased from 1075 MPa, 820 MPa, and 13.2% to 1179 MPa, 871 MPa, and 21.1%, respectively, while hardness increased from 365 HV1.0 to 387 HV1.0. This study demonstrated that the microstructure and mechanical properties of LPBF-processed IN738LC alloy can be tailored via controlling the thermal history of the melt pool, providing a foundation for processing high-crack-sensitivity alloys utilizing laser powder bed fusion.