Characterizing the influence of thermal history on phase balance and performance in CMT-WAAM 316 L wall

As an advanced welding process with precise droplet control, Cold Metal Transfer (CMT) is ideal for wire arc additive manufacturing (WAAM). However, optimizing its use for stainless steel requires a detailed understanding of the resulting microstructural and mechanical properties. This study evaluates these characteristics to ensure performance in 3D-printed applications. Three (bottom, middle, and top) specimens were took from deposited CMT-WAAM wall and were characterized by microstructural analyses (OM, SEM and XRD) and microhardness and tensile tests to evaluate the variation of their microscopic, macroscopic and mechanical properties, respectively. The microstructure results indicate highly sensitive to the varying thermal gradients and cooling rates across the build direction. The bottom layers exhibit a coarser/skeletal δ-ferrite morphology, with content of 8.21%, due to efficient substrate heat dissipation. While heat accumulation in the middle layers results in a coarser/started-equiaxed ferritic structure with inter-dendritic SDAS of 23.79 μm. Conversely, as rapid cooling from ambient air limits the δ-ferrite/γ-austenite transformation, the top layers exhibited an increased residual δ-ferrite with content of 11.28% and inter-dendritic SDAS of 4.12 μm. With this considerable increase in ferrite content, the hardness at the top region ranged highly from 181 to 200 Vickers hardness (HV). In addition, tensile tests show mechanical anisotropy, with peak strengths at the bottom and top of UTS ≈ 520 MPa and high ductility exceeding 80% elongation. Ultimately, these insights provide the scientific basis required to tailor the microstructural integrity and industrial scalability of large-scale 316 L SS components.