Microstructural evolution and mechanical performance of Inconel 625 fabricated by CMT cycle-step wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) based on cold metal transfer (CMT) is a promising technique for fabricating large-scale nickel-based superalloy components; however, controlling heat input and microstructural evolution remains a significant challenge. In this study, thin-walled Inconel 625 components were fabricated using three CMT deposition strategies: continuous, high cycle-step (HCS), and low cycle-step (LCS) modes. The influence of deposition mode on bead geometry, microstructure, elemental segregation, phase formation, crystallographic characteristics, and mechanical properties was systematically investigated. Macroscopic examination confirmed stable and defect-free deposition for all processing conditions. Microstructural characterization revealed that continuous deposition promoted coarse columnar dendrites, pronounced Nb–Mo segregation, and the formation of brittle Laves phases in the interdendritic regions. In contrast, the CMT-HCS mode produced a refined mixed microstructure, whereas the CMT-LCS mode generated fine equiaxed and cellular grains with minimal elemental segregation and suppressed Laves phase formation. X-ray diffraction confirmed the predominance of the γ-Ni FCC phase in all samples, with the CMT-LCS mode exhibiting the smallest crystallite size (19.06 nm) and the highest dislocation density (2.75 × 10−3 nm−2). Mechanical testing demonstrated that the CMT-LCS mode achieved the highest average microhardness (276 ± 2.01 HV), average ultimate tensile strength (UTS) (764 ± 8.25 MPa), average yield strength (439 ± 5.25 MPa), and average elongation (50.6 ± 2.21%), representing an 11.04% improvement in average UTS and a 7.81% increase in average microhardness compared with the CMT continuous mode. These findings demonstrate that the CMT-LCS deposition strategy provides superior thermal control, microstructural refinement, and mechanical performance for WAAM-fabricated Inconel 625 components.