Microstructure evolution, composition distribution, and crack formation mechanisms of SS 316L/IN718 graded materials fabricated by laser directed energy deposition

The preparation of functionally graded materials (FGMs) via laser directed energy deposition (LDED) involves the coupling of multiple material parameters, the mechanisms of microstructural evolution and composition distribution under non-equilibrium solidification conditions, and the formation mechanisms of defects during the deposition process, which remain critical scientific challenges to be addressed. Taking the SS 316 L/IN718 material system as an example, this study establishes a multiphase, multicomponent fluid model and a thermomechanical coupling simulation framework. Combined with microstructural, crystallographic characterization, and microhardness analysis, it systematically investigates the microstructural transitions, composition distribution, and crack formation mechanisms under a 25 wt% compositional gradient condition. The study reveals that as the deposition layers transition from 100 % SS 316 L to 100 % IN718, the microstructure undergoes a discontinuous change from fine columnar and cellular crystals to coarse columnar and short dendritic crystals. This evolution is jointly influenced by variations in local temperature gradients and uneven solute distribution. Temperature accumulation and differences in the materials' thermal properties enhance molten pool stirring and remelting dilution effects, leading to significant segregation of solute elements (e.g., Nb, Mo). This further reduces the local solidification rate and destabilizes the solid-liquid interface. Cracks are primarily concentrated in the 50 % SS 316 L/50 % IN718 and 25 % SS 316 L/75 % IN718 gradient layers, attributed to liquation cracking caused by local compositional segregation, thermal stress concentration, and the presence of brittle carbides. Thermo-mechanical coupled simulations further confirm that the residual tensile stress is highest in the 25 % SS 316 L/75 % IN718 and 100 % IN718 gradient layers, making them the primary regions for crack initiation. This study proposes a comprehensive analytical method suitable for multi-material additive manufacturing (AM), providing theoretical guidance for compositional distribution regulation, microstructural design, and crack suppression in FGMs.