Functionally Graded Additive Manufacturing of Inconel 625 and CuCrZr Alloys

Abstract

Significant advancements over the past decade have transformed metal additive manufacturing from a prototyping tool into a full-fledged production process. These developments have enabled the use of lighter, stronger, and more cost-effective additively manufactured components in aerospace, automotive, and energy industries. As qualification efforts progress, research is increasingly focused on advanced capabilities such as combining multiple alloys within a single build to create functionally graded structures, eliminating the need for additional joints. In that regard, Functionally Graded Additive Manufacturing (FGAM) is a layer-by-layer process that varies composition and/or microstructure within a component to achieve locally tailored properties. A new class of FGAMs combining highly heat-conductive CuCrZr alloy with Inconel 625 superalloy has gained considerable attention for aerospace applications, leveraging the former’s high heat dissipation and the latter’s excellent mechanical properties. This can be done through the Laser Directed Energy Deposition (L-DED) technique; however, the implementation remains a material-processing challenge due to the noticeable thermophysical mismatch between the two alloys. This dissertation provides a comprehensive investigation into the FGAM of IN625-CuCrZr alloys, encircling process parameter optimization, gradient path development, and microstructural and defect formation analysis through advanced characterization, CALPHAD-based thermodynamic simulations, and finite element modeling. In that regard, process parameters have been optimized from single-track to multilayer scales, and the effect of process parameters on the microstructure has been studied, more specifically on CuCrZr alloy as there was a big gap in the literature. Further, the FGAM of IN625-CuCrZr has been exercised for two geometries of thin wall and cuboid, incorporating both sharp and gradual compositional transitions. Sharp transitions led to delamination at the interface, while gradual transitions resulted in structurally sound builds. In the gradual transition zone, the presence of a metastable miscibility gap between the liquid of the two alloys led to the formation of distinct Cu-lean and Cu-rich phases in the microstructure, a phenomenon predicted through CALPHAD-based thermodynamic simulations. The formation of solidification cracking in the gradient region of the cuboid geometry was further investigated using Kou’s cracking susceptibility criterion. In support of these findings, a multi-step numerical investigation of heat transfer in both thin wall and cuboid geometries was conducted using finite element analysis. First, a hybrid statistical–numerical thermal model was developed and implemented in the scale of single tracks through user-defined subroutines (DFLUX, USDFLD, and FILM) in Abaqus software. This model enabled high-fidelity prediction of melt pool geometry and thermal history and was validated against experimental melt pool dimensions and in-situ thermocouple measurements. Subsequently, the validated heat source model was used to simulate the thermal behavior during FGAM processing of both geometries. The thermal simulations highlighted the critical role of geometry on cooling rates and temperature distributions, providing deeper understanding into cracking behavior and how geometry-dependent thermal history influence microstructure and defect formation during FGAM of IN625-CuCrZr alloys. Overall, this work establishes a robust experimental–computational framework for FGAM of dissimilar alloys using L-DED process. It introduces a scalable strategy for depositing functionally graded IN625–CuCrZr structures with controlled transitions and minimized defects. The modeling and characterization approaches developed here can be extended to other material systems, while the insights into miscibility gap, solidification behavior, and cracking mechanisms lay the groundwork for future microstructure design and process control in metal additive manufacturing.