Wire arc additive manufacturing (WAAM) is categorized as a directed energy deposition (DED) technique that is specifically developed for fabricating large-scale metallic components with complex features . Compared with other AM techniques, WAAM is advantageous because of its high efficiency, high material usage, and low cost. Moreover, owing to its applicability to a variety of materials, this technique has been extensively employed in the aerospace, automotive, and rapid tooling industries [2, 3]. In the WAAM process, metallic components are produced when an electrical arc melts a wire-based feedstock and deposits it layer-upon-layer. Typically, gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and plasma arc welding (PAW) are applied in this process . WAAM enables high-density and superior mechanical properties of the components [5, 6]. However, the quality of the WAAM-fabricated components is inadequate to satisfy the general industrial applications because of disadvantages such as residual stress and distortion from excessive heat input [7-9]. Specifically, although the components have simple geometries, the geometric accuracy is not comparable to that of other AM techniques . Owing to the deposition process with a large layer, stair steps are inevitably produced on the side face of the components. This can significantly affect not only the surface quality of the components, but also the removal volume of materials in the finish machining process. Therefore, improving the surface quality is a key challenge in WAAM.
The surface quality of the WAAM-fabricated structures depends on several key parameters. Previous studies have reported certain factors that can affect the surface quality of the fabricated structures. Xiong et al.  investigated the influence of the interlayer temperature, traveling speed, wire feed speed, and constancy of the ratio of the wire feed speed to the traveling speed on the surface roughness of thin-wall parts fabricated using H08Mn2Si steel and reported that the surface quality of the thin-walled parts improved with the decrease in the interlayer temperature. Further, when the ratio of the wire feed speed to the traveling speed was constant, the surface roughness increased with an increase in the wire feed speed. In addition, the surface roughness decreased with a reduction in the wire feed speed and traveling speed. Prado-Cerqueira et al.  applied the cold metal transfer (CMT) advanced technique in the WAAM process, which was developed to prevent excessive heat input by controlling the wire feed speed and polarity. It was concluded that the simple CMT without polarity change improved the homogeneous hardness distribution, mechanical strength, and surface quality of the thin-wall parts fabricated using mild steel. Further, a shielding gas also plays an important role in the WAAM process in controlling the layer geometry, deposition rate, and spatter generation. Jurić et al.  prepared four types of shielding gases (97.5% Ar + 2.5% CO2, 95.5% Ar + 3% He + 1.5% H2, 95% Ar + 5% H2, 99.999% Ar) and compared the root-mean square height (Sq), arithmetic mean height (Sa), and mechanical properties of the specimens of Inconel 625 fabricated under each shielding gas. This revealed that 97.5% Ar + 2.5% CO2 shielding gas resulted in higher hardness and tensile strength of the specimens, but poor surface quality. Silwal et al.  focused on the WAAM process based on the surface tension transfer (STT) technique, and investigated the influence of the Ar and CO2 shielding gas mixture on the dimensional accuracy, mechanical properties, and microstructure of the wall structure fabricated using mild steel revealing that the higher CO2 content led to higher melt pool temperatures, whereas the lower CO2 content resulted in a dimensionally accurate geometry.
The first classification of metal transfer modes in GMAW was established by the International Institute of Welding (IIW) in 1976 . Subsequently, the effects of the welding process parameters on the metal transfer mode have been researched. Scotti et al. [16, 17] investigated the metal transfer behavior of carbon steel under different combinations of welding current, voltage, and gas composition and concluded that there were three classes of metal transfer modes: natural metal transfer, controlled metal transfer, and interchangeable metal transfer. For the natural metal transfer, two types of metal transfer modes, contact droplet transfer and free-flight droplet transfer to the weld pool, were observed. Further, the controlled metal transfer consisted of improved natural modes to obtain better process characteristics, such as spatter minimization, weld geometry control, and heat input stabilization. The interchangeable metal transfer was a mixed mode that consisted of two or more natural transfer modes. Teixeira et al.  observed the metal transfer behavior of mild steel under five types of shielding gases (Ar, Ar + 2% O2, Ar + 10% CO2, Ar + 25% CO2, and CO2) in GMAW. Metal transfer mode maps were plotted to identify the regions of each type of metal transfer revealing that the interchangeable globular/spray metal transfer was only obtained for Ar + 10% CO2 and Ar gases. In contrast, the Ar + 25% CO2 gas, owing to the high percentage of CO2, the transition current increased, resulting in a more difficult change to spray transfer. In addition, the presence of oxygen in the Ar + 2% O2 shielding gas resulted in smaller droplets than in pure argon. Furthermore, several studies have explored the impact of shielding gas on metal transfer behavior in the WAAM process. Wu et al.  investigated the influence of heat accumulation on bead formation, arc stability, and metal transfer behavior during the building of Ti6Al4V in GTAW-based WAAM. It was concluded that the heat accumulation led to variations in the bead geometry, which resulted in variations in the arc shape and droplet transfer mode. Ríos et al.  focused on the metal transfer behavior in PAW-based WAAM. This revealed that the metal transfer could be classified into three modes, and stable transfer was obtained when the surface of the molten pool was convex. Further, Liang et al.  investigated metal transfer behavior in a GMAW-based WAAM and determined that the interaction between the arc voltage and arc current was the most important factor influencing droplet transfer. The droplet radius decreases exponentially with an increase in the frequency of droplet transfer, whereas the deposition rate increased with an increase in the frequency of droplet transfer.
These results show that it is important to control the heat input and the associated droplet transfer mode during the WAAM process. The mechanism of surface defect formation in the GMAW-based WAAM process and its relation to the geometric accuracy of the fabricated structure are being elucidated. However, the variation in the metal transfer behavior with the increase in the number of layers under different gases on the geometry and surface roughness of the multilayer structures has not been fully elucidated. Although CO2 gas is a preferred shielding gas for the GMAW process of mild steels because it provides advantages such as higher welding speeds, greater penetration, and lower cost, its use is restricted because of problems associated with spattering and element losses due to oxidation. Meanwhile, it is known that Ar gas has a disadvantage in that the heat flux from an arc to the workpiece is relatively low, which limits the depth of the molten pool during welding . Further, in the WAAM process, stair steps are inevitably produced on the side face of the multilayer structure, thus, the effect of the gas characteristics on the penetration and surface roughness is an important consideration.
This study focused on the GMAW-based WAAM process and addressed the following deficiencies. First, the metal transfer behavior during the building was observed under various heat inputs and shielding gases, such as Ar and CO2. Second, the influence of the metal transfer behavior on the geometry and surface roughness of the single and multilayer structures was investigated. Finally, the chemical composition of the fabricated surface was analyzed using energy-dispersive X-ray spectroscopy (EDS). In the WAAM process, the cost of shielding gas cannot be ignored owing to the long processing time required to fabricate large-scale metallic components. In view of constantly increasing prices, the use of an expensive gas mixture can be avoided using a cheaper gas . Furthermore, the process parameter conditions that enable the elimination of interlayer cooling during the building are desired to improve the processing efficiency. Therefore, this study attempted to provide a simplified shielding gas selection process and verified the limitations of fabricating a thin-wall structure without interlayer cooling.