Arc stability, elemental transition, and interfacial cracking in bimetallic WAAM: interplay of deposition order and thermal history

Wire Arc Additive Manufacturing (WAAM) enables the efficient fabrication of large bimetallic structures; however, dissimilar ferrous/non-ferrous material combinations remain difficult to build because arc stability, elemental transition, and interfacial cracking do not evolve independently. This study investigates Stainless Steel 309 L (SS309L)–Inconel 625 bimetallic WAAM structures to reveal how deposition order and interfacial thermal history jointly govern these three critical phenomena. Bimetallic walls were fabricated by varying the deposition order, while current–voltage cyclograms, infrared thermography, X-ray computed tomography (XCT), and SEM–EDS were used to analyze arc behavior, cooling conditions, elemental transition, and crack formation. When the ferrous SS309L was deposited on the non-ferrous Inconel 625, pronounced arc instability was observed during the first interfacial layers, whereas the reverse sequence produced comparatively stable arc behavior. In addition, in the Inconel 625-first bimetallic structure, increasing total heat accumulation prolonged the interfacial solidification time, thereby delaying the attainment of the nominal composition of the subsequently deposited SS309L and extending the elemental transition across a greater number of layers. However, when solidification time remained short, segregation-assisted interfacial cracking occurred, indicating that insufficient thermal exposure promoted crack formation. Longer thermal exposure progressively suppressed cracking and enabled crack-free interfacial deposition, but only at the expense of a broader elemental transition zone. The results therefore reveal a fundamental process trade-off in Stainless Steel–Inconel bimetallic WAAM structures: conditions required for crack-free interfacial deposition also promote longer elemental transition, while deposition order primarily controls arc stability during ferrous/non-ferrous material transition.