Insights into microstructural evolution in functionally graded additive manufacturing of IN625–CuCrZr alloys: A CALPHAD-based thermodynamic analysis and experimental study on the role of metastable miscibility gap

Functionally Graded Additive Manufacturing (FGAM) technique enables gradual and in-situ variation of composition within a component to achieve locally tailored microstructure and physical properties. This approach has recently been applied to fabricate multi-material components made of copper alloys for heat dissipation and Ni-based superalloys for superior mechanical properties, especially in aerospace applications. In this study, the successful FGAM of IN625 superalloy and CuCrZr alloy was demonstrated for the first time. Thin-wall samples without any major defects, such as cracks or lack of fusion, were fabricated by gradually varying the composition via a dual powder feeder laser directed energy deposition (L-DED) system. Further, to comprehend microstructural evolution during the process, CALPHAD simulations and characterization techniques (SEM, EDS, EBSD) were employed. A metastable miscibility gap was detected between the two alloys, which led to Cu-rich and Cu-lean regions in compositions with 50–75 wt.% CuCrZr. This resulted in mixed spheroidal and dendritic morphologies. X-CT analysis revealed Cu-lean spheroids clustering in specific regions, while EBSD showed coarse columnar grains in single-phase areas and fine equiaxed grains in compositions with a miscibility gap, where Cu-lean spheroids acted as nucleation sites. Scheil solidification simulations predicted M 2 C-type carbide formation within spheroids, confirmed by XRD, which contributed to a wide hardness variation of 180 to ∼800 HV across the component.