Thermal modeling and microstructural evaluation of SS316L-IN718 single track deposits produced by directed energy deposition

Functionally Graded Materials (FGMs) enable gradual transitions of properties between dissimilar materials, making them attractive for advanced engineering applications. Directed Energy Deposition (DED) provides a pathway to fabricate such multi-material structures through controlled compositional gradients. In this study, SS316L-IN718 single track deposits were produced via DED to evaluate the effects of energy density and composition on melt pool geometry, microstructure and defect formation. A constant scanning speed of 11 mm/s was used with laser powers of 1500, 1800 and 2100 W, corresponding to energy densities of 54.54, 65.45 and 76.36 J/mm². Thermal simulations in ABAQUS showed strong agreement with experimental measurements, with errors below 2% for melt pool width and between 1.3% and 5.5% for depth. Higher IN718 fractions, however, exhibited errors up to 7%, attributed to unmodeled flow effects such as Marangoni convection and recoil pressure. Microstructural analyses revealed a transition from equiaxed grains in SS316L rich samples to columnar dendritic morphologies with increasing IN718 content, along with Laves phase formation and crack susceptibility, particularly at 25% IN718. EBSD confirmed greater misorientation, local deformation and grain refinement at higher IN718 contents, while the 50–50 blend showed the most balanced microstructure with homogeneous grains and low dislocation density. Suitable energy density ranges for multilayer FGM fabrication were identified as 45–50 J/mm² for 100% IN718, 45–55 J/mm² for 75% IN718, 50–55 J/mm² for 50% IN718, 50–60 J/mm² for 25% IN718 and 55–60 J/mm² for 100% SS316L. These findings define processing windows between insufficient fusion and excessive melting, providing a validated framework for reliable multilayer FGM production.