Effect of heat treatment on functionally graded 304L stainless steel to Inconel 625 fabricated by directed energy deposition

Defining appropriate heat treatments for compositionally functionally graded materials (FGMs) is challenging due to varying processing conditions in terminal alloys and gradient regions. Therefore, it is critical to investigate the impact of potential heat treatments on phase transformations and the resulting mechanical properties along the entire compositional path. In the present study, we applied heat treatments at 700 °C, 900 °C, and 1150 °C on a stainless steel 304L (SS304L) to Inconel 625 (IN625) FGM fabricated using directed energy deposition (DED) additive manufacturing (AM). The microstructure and hardness, as a function of layer-wise composition and applied heat treatment, were characterized. The applicability of computational methods previously developed by the team to predict experimentally observed phases by the hybrid Scheil-equilibrium approach was evaluated. This approach improves the accuracy of predicting phases formed after heat treatment compared to equilibrium thermodynamic calculations using the overall layer compositions and provides a simple pathway to assist in designing heat treatment for FGMs.