Enhancing mechanical performance in bimetallic WAAMed structures through strategic deposition order

This study investigates the influence of deposition order in bimetallic stainless steel 309L (SS309L) / Inconel 625 fabricated by Wire Arc Additive Manufacturing (WAAM) on its microstructure, elemental diffusion, and mechanical characteristics. Two deposition strategies were examined: SS309L followed by Inconel 625, and the reverse order. Microstructural analyses revealed that starting with SS309L led to a shorter elemental transition zone (∼1.5 layers), whereas the reverse sequence produced an extended zone up to 4–5 layers, implying delayed phase stabilisation in the SS309L region. Vickers hardness tests showed that the first case exhibited a stable distribution along the build direction, with interface hardness up to ∼270 HV. In contrast, the reverse order produced a broader high-hardness region exceeding 300 HV in the early SS layers due to delayed ferritic phase formation. Tensile tests revealed that the first case achieved 15% higher ultimate tensile strength (UTS) compared to the second. Furthermore, Digital Image Correlation (DIC) analysis indicated strain was more concentrated near the interface in the first strategy, suggesting stronger interfacial bonding. These findings highlight deposition order plays a crucial role in controlling interfacial integrity and anisotropic mechanical response in multi-material WAAM structures, providing insights for fabricating functionally graded components in industry.