Mitigation of interfacial brittleness and property enhancement in WA-DED SS316L–NAB structures using an IN718 buffer layer

Wire arc directed energy deposition (WA-DED) enables the fabrication of multi-material structures for demanding marine applications; however, direct deposition of SS316L and nickel-aluminium bronze (NAB) creates a brittle, crack-prone interface due to thermodynamic incompatibility in the Fe–Cu binary system. This study investigates the influence of an intermediate IN718 buffer layer on mitigating interfacial Fe–Cu brittleness in SS316L–NAB structures via Fe–Ni and Ni–Cu sequential transitions. Bimetallic (BM) SS316L–NAB and trimetallic (TM) SS316L–IN718–NAB architectures were fabricated and compared through microstructural and mechanical analysis. The BM interface is reaction-dominated, exhibiting a submicron transition width (∼0.3 μm), a continuous AlNi intermetallic, elevated micro-strain (5.1–8.8 × 10 −3 ), and interfacial cracking. The TM architecture produces a crack-free interface with an extended compositional transition width (∼1 μm), stabilised FCC phase fraction (∼90%), refined crystallite size (14–21 nm), reduced microstrain (2.3–5.4 × 10 −3 ), and progressive interfacial hardness (up to 331 HV 0.5 ). Mechanically, TM achieves an average yield strength of ∼362 MPa, approximately 46% higher than BM (∼248 MPa), attributed to dislocation strengthening (∼110–130 MPa), solid-solution, mismatch-induced strengthening, and Orowan dispersoid strengthening. These results establish IN718 as a metallurgically active diffusion mediator that ensures crack-free graded compositional accommodation.