The effects and utility of homogenization and thermodynamic modeling on microstructure and mechanical properties of SS316/IN718 functionally graded materials fabricated by laser-based directed energy deposition

As driven by the need of structural integration and functionalization in modern society, functionally graded material (FGM) with multiple materials is highly demanded for rapidly growing industrial needs. FGMs with smooth gradients in composition and mechanical properties can prevent failure caused by mismatched interfaces and stress concentration. This work first investigates the effects of homogenization and aging heat treatment on the microstructure evolution and mechanical properties of 316 stainless steel (SS316)/ Inconel 718 (IN718) FGMs first. The FGMs fabricated with a decent gradient compositional control by laser-based directed energy deposition. The compositional evolution, the aging precipitation behaviors with different homogenization times, and the microhardness of FGMs were studied via microstructure characterization and high-throughput simulation, to elucidate their effects on gradient smoothing and mechanical properties. Homogenization not only eliminates the heterogeneity inherited from the additive manufacturing (AM) process but also provides a practical way to smooth the gradient of composition and microstructure for improved gradient properties. Furthermore, it directly affects the subsequent precipitation behaviors in the FGMs, which highly depends on the diffusion degree of the elements in the matrix and grain boundaries. This work also conducted a comprehensive study on the heat treatment of SS316/IN718 FGMs, which will provide a theoretical and practical basis for the further development and application of FGMs. Besides, the uncertainty of thermodynamic modeling is evaluated and quantified the prediction accuracy for further improvement in the future.