Characterizing the Influence of Thermal History on Phase Balance and Performance in CMT-WAAM 316L Wall

Cold metal transfer (CMT) is an advanced welding process developed by Fronius International, primarily designed for wire arc additive manufacturing (WAAM), a type of 3D metal printing. Combining the advantages of gas metal arc welding (GMAW) with precise droplet control, CMT is particularly well suited for additive manufacturing applications. Therefore, the use of stainless steel specifically for these applications requires knowledge of its microstructural and mechanical properties. In our recent study, a vertical wall was 3D-printed using 316L stainless steel via the CMT process. Three samples of bottom, middle, and top were taken of this as-deposited wall and were characterized by microstructural analyses by XRD, SEM, and OM, and extensive hardness testing via Vickers microhardness to evaluate the variation of their properties at the microscopic and macroscopic levels, respectively. The microstructure results indicate highly sensitive to the varying thermal gradients and cooling rates across the build direction. The bottom layers exhibit a fine, skeletal ferrite morphology due to efficient substrate heat dissipation, while heat accumulation in the middle layers results in a coarser, more equiaxed structure. Conversely, the top layers undergo a secondary refinement and exhibit increased residual ferrite content, as rapid cooling from ambient air limits the completion of the peritectic reaction. The hardness ranged from 181 to 200 Vickers hardness (HV), with a considerable increase at the top layers due to concentration of δ ferrite. These results provide a valuable contribution to the design of steel structures using 3D-printed stainless steel.