Influence of interlayer thermal cycling on microstructural evolution in WAAM processed carbon steel

Abstract 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 the quantification of grain size at each level through a computer vision approach based on digital image processing and segmentation techniques. The results, validated by transient thermal simulation, revealed a gradual microstructural transition along the build height: the lower layers contained fine acicular ferrite formed under rapid cooling, which evolved into equiaxed ferrite in the intermediate region due to recrystallization from repeated reheating, while the uppermost layers stabilized with coarser allotriomorphic ferrite as a result of slower cooling rates. This progressive change correlates with the increase in Δt₈/₅ time toward the top, explaining the corresponding reduction in hardness from 205–216 HV near the substrate to 132–163 HV in the upper region. These findings confirm that thermal cycling during WAAM leads to microstructural homogenization, which is essential for achieving consistent mechanical behavior across the build height.