Experimental study of breakage marks in wire arc additive manufacturing assisted by active cooling of thin-walls

Wire arc additive manufacturing has recently gained significant attention, and a complete understanding of the process remains a target. In this technology, the manufacturing of thin walls can produce considerable heat gradients and thermally induced distortions, potentially resulting in surface characteristics that may be incorrectly considered defects. Under water-assisted deposition conditions, unique and unexpected etching-like surface markings were consistently observed on carbon-steel prewalls that preserved their hot-rolling oxide scale. This work investigates whether the shape of these marks can serve as process-informative evidence for the rapid visual assessment of thermal-stress severity and cooling efficacy, while also preventing their misdiagnosis as faults. A systematic problem-solving methodology integrating brainstorming with hypothesis falsification was employed: the characteristics of the marks were specified, alternative hypotheses were proposed, and specific experiments were developed to refute them. Tensile loading, three-point bending, and shear-cutting tests were employed to replicate the mechanism under monitored deformation and temperature settings. The findings suggest that the marks mainly result from the cracking and fragmentation of the hot-rolling oxide scale, caused by thermally induced deformation of the pre-wall surface. In mechanical tests, elongations beyond roughly 1.3–1.4% resulted in significant oxide-scale cracking, but much smaller elongations caused minimal marks. The findings substantiate oxide-scale fragmentation as the principal source of the observed markings and illustrate that a falsification-driven methodology can effectively address unforeseen process anomalies.