Elemental diffusion influenced by process parameters in LPBF of IN625 on IN738 substrate: A focused investigation of Ti behavior and its role in defect formation (cracks and pores)

This work investigates the influence of process parameters on elemental diffusion and defect formation during laser powder bed fusion (LPBF) of IN625 deposited onto an IN738 nickel-based superalloy substrate. Special attention is given to the behavior of titanium (Ti) due to its tendency to segregate and contribute to cracking. By systematically varying laser power and scan speed, the study shows that higher scan speeds combined with lower laser powers promote Ti enrichment at the interface, increasing the likelihood of solidification cracking. Maximum elemental enrichment occurs at a medium power level (150 W) and a linear energy density between 0.67 and 1 J/mm. The investigation also identifies the heating conditions under which ductility dip cracking can occur in the substrate. The results explain the simultaneous occurrence of both high-energy and low-energy cracks in the build-up layer and substrate during LPBF. Higher laser powers promote a more uniform elemental distribution, reducing segregation and mitigating the formation of cracks and pores. Two main types of pores—lack-of-fusion (LOF) and keyhole pores—were observed, with their formation closely linked to melt pool geometry and energy input. At lower energies, below 0.333 J/mm (linear energy density), samples processed at 100, 150, and 200 W exhibit irregular LOF pores. A safe processing window was identified at a laser power of 200 W and scan speeds at or below 150 mm/s, enabling defect-free deposition. These findings offer valuable guidance for optimizing LPBF processes involving dissimilar superalloys, especially in applications where structural integrity and metallurgical compatibility are critical.