Microstructure-controlled mechanical behavior of autogenous laser-welded IN625-AISI 304L dissimilar joints

An autogenous laser-welded dissimilar joint between Inconel 625 (IN625) and AISI 304 L austenitic stainless steel was produced to evaluate its microstructural changes and high-temperature structural performance. The laser weld exhibited full penetration with defect-free fusion and narrow weld fusion characteristics, explaining the high energy density and rapid solidification inherent to the process. Microstructural characterization highlights the heterogeneous solidification characteristics in the weld metals (WM), which solidify as cellular and columnar microstructures governed by localized thermal gradients and elemental compositions. Pronounced microsegregation of Nb and Mo occurred within interdendritic regions, promoting the formation of Laves phases, NbC, and Cr-rich carbides, as confirmed by SEM/EDS and elemental mapping. The weld fusion region near the AISI 304 L exhibits the occurrence of an unmixed zone (UZ) and partially mixed zone (PMZ), whereas the interfacial region near the IN625 exhibits the presence of PMZ along with uniformly distributed secondary phases. Mechanical assessment demonstrates a Vickers microhardness (HV) gradient across the weld joint, with enhanced WM hardness of 210 HV due to solid solution strengthening and interdendritic phases. Further, the destructive room temperature (RT) tensile testing indicates WM failure with an acceptable average tensile strength of 664.7±4.5 MPa and percentage elongation of 52±5.5%, comparable to the weaker base metal (AISI 304 L). The dissimilar weld illustrates the superior high-temperature (HT) tensile strength at 550 ºC (397 MPa), 600 ºC (359 MPa), and 650 ºC (304 MPa). Moreover, the reduced average impact toughness of the laser weld, i.e., 62± 3 J, is attributed to the dendritic heterogeneity and brittle secondary phases. Overall, the results demonstrate that laser welding enables sound IN625–304 L dissimilar joints with acceptable strength, though interdendritic segregation and precipitate formation remain critical factors influencing toughness. • The defect-free dissimilar laser-welded joint between Inconel 625 and AISI 304L was fabricated with almost no metallurgical defects. • The dissimilar laser weld demonstrates the segregation of elements forming laves phases and carbide phases across the weld matrix, grain boundaries, and interdendritic spaces. • High temperature tensile behaviour was investigated to highlight the mechanical stability under AUSC-relevant service conditions.