Influence of Interlayer Thermal Cycling on Microstructural Evolution in WAAM Processed Carbon Steel

Wire Arc Additive Manufacturing (WAAM) has emerged as a cost effective and scalable process for fabricating metallic components. In carbon steel, the repeated thermal cycles during deposition significantly influence grain morphology and mechanical properties. This study investigates the evolution of grain size across ten sequentially deposited layers using GMAW based WAAM. An analysis was conducted through metallographic preparation and linear reconstruction of the deposited volume, allowing quantification of grain size at each level. The results show that initial layers exhibit columnar grains with acicular ferrite, and with continued deposition, thermal cycling promotes grain coarsening and recrystallization. Grain size increased from ~ 2 µm in the first layer to ~ 10 µm by the subsequent layers, indicating the onset of recrystallization-induced equiaxiality. The upper layers showed a higher presence of allotriomorphic ferrite, while the lower layers developed equiaxed ferrite due to repeated reheating. These findings confirm that thermal cycling during WAAM leads to microstructural homogenization, which is essential for achieving consistent mechanical behavior across the build height.