Microstructural and mechanical characteristics of CMT-WAAM fabricated SS321 and Hastelloy C-276 functionally graded material

Wire arc additive manufacturing (WAAM) has emerged as a promising technique for fabricating large-scale and complex metal components due to its high deposition rate, cost efficiency, and design flexibility. In this study, cold metal transfer (CMT)-based WAAM was used to fabricate a functionally graded material (FGM) comprising SS321 and Hastelloy C-276. The resulting thin-wall structure was free from solidification cracks and fusion defects, exhibiting a sound bimetallic interface. Optical microscopy revealed equiaxed and columnar dendrites in SS321, while Hastelloy C-276 showed fine cellular and elongated dendrites. X-ray diffraction confirmed γ-austenite with minor δ-ferrite in SS321. Energy-dispersive X-ray spectroscopy indicated a smooth compositional gradient with minimal segregation, ensuring strong metallurgical bonding. Microhardness gradually declined with build height due to thermal accumulation, ranging from 220 to 197 HV in SS321 and 272 to 236 HV in Hastelloy C-276, with a localized peak at the interface due to alloy intermixing. Interface tensile specimens exhibited a UTS of 675 MPa, YS of 436.7 MPa, and an elongation of 48.22%. Fractures in vertical specimens occurred near the SS321 side, confirming it as the weaker zone. SEM fractography revealed ductile failure characterized by micro-voids coalescence and fine dimples. CMT-WAAM proves effective for fabricating FGMs with smooth transitions and dependable mechanical integrity.