Influence of metal transfer behavior under Ar and CO₂ shielding gases on geometry and surface roughness of single and multilayer structures in GMAW-based wire arc additive manufacturing of mild steel

Wire arc additive manufacturing (WAAM) is advantageous for fabricating large-scale metallic components; however, a high geometric accuracy as that of other AM techniques cannot be achieved because of the deposition process with a large layer. This study focuses on the WAAM process based on gas metal arc welding (GMAW). To clarify the influence of shielding gas used to protect a molten metal during fabrication on the geometric accuracy of the built part obtained via the GMAW-based WAAM process, the influence of the metal transfer behavior on the geometry and surface roughness of the fabricated structures was investigated via visualization using a high-speed camera when single and multilayer depositions were performed under different heat inputs and gases. It is known that Ar gas is not suitable for welding steel because it cannot provide the desired arc stability and weld bead characteristics. The results reveal that the arc is stable in the multilayer deposition of the WAAM process, and the short-circuit of the metal transfer at the heat input of 1.17 kJ/cm enables smoothing of the fabricated surface with large irregularities. Better arc stability under Ar gas is achieved when the oxygen content of the fabricated surface is 22 wt%. Furthermore, the short-circuit between the metal droplet and the fabricated surface, where the molten pool is insufficiently formed, resulted in a hump formation. The results indicate that proper use of Ar gas can improve the surface quality when depositing on rough surfaces.