## 1 Introduction  
The application of aluminum in automotive parts is increasing to reduce vehicle weight. Various joining processes are applied to join aluminum parts [1; 2]. Among the various joining processes, gas metal arc welding (GMAW) is an efficient welding process that is particularly suitable for welding metal plates. Whereas a general GMAW process uses a single arc, the tandem GMAW process used in this study comprises two independent welding machines, each with its own power source, welding torch, wire drive, and welding wire. Owing to its system characteristics, the tandem GMAW process has a higher deposition rate and welding speed than the conventional single-wire GMAW process, implying that the tandem GMAW process can improve welding productivity [3; 4; 5; 6; 7; 8]. However, the welding system and arc behavior of tandem GMAW are more complex than those of conventional single-wire GMAW such that it is necessary to establish the effects of process parameters on the weld bead shape, which is directly related to weld quality, and welding quality in tandem with the GMAW process. [PERSON] et al. [8] developed a model to analyze the influence of the wire feed rate (WFR), torch angle, welding speed, and nozzle-to-plate distance on the weld bead height, width, and penetration depth in the GMAW process through the design of experiments (DOEs). The fit of the model was confirmed through an analysis of variance (ANOVA). [PERSON] et al. [9] optimized the strength according to the welding parameters (current, voltage, gas flow rate, torch angle, welding speed,wire diameter, and electrode feed rate) in aluminum GMAW using the Taguchi method. [PERSON] et al. [10] optimized the tensile strength and mass deposition rate according to the welding current, WFR, and welding speed among the GMAW parameters using the DOE. The results confirmed that the WFR and welding current are important variables. [PERSON] et al. [11] derived the optimal ultimate tensile strength through an L9 orthogonal array by optimizing the welding current, voltage, and gas flow rate among the welding parameters in the GMAW of aluminum alloy. [PERSON] et al. [12] optimized the welding parameters (welding current, voltage, and bevel angle) to obtain better impact strength using the Taguchi L9 orthogonal array in GMAW and proved that the current and voltage are the main factors through ANOVA. [PERSON] et al. [13] performed GMAW on metals with narrow gaps and optimized welding parameters, such as the rotation angular velocity, rotation angular amplitude, WFR, welding speed, and sidewall stay time, to optimize the welding quality. [PERSON] et al. [14] performed GMAW on the shape of a T-fillet joint and developed a mathematical model to predict the bead shape according to the welding parameters (welding current, welding voltage, welding speed, and wire extension). Consequently, the deviation between the predicted and actual values was less than 1.0 mm. [PERSON] et al. [15] investigated the effects of welding current and torch position parameters, including the torch-aiming position, travel angle, and work angle, on the bead geometry in a single-lap joint GMAW. [PERSON] et al. [16] analyzed the effect of welding parameters on the formation of gas metal arc welding--gas tungsten arc welding (GMAW-GTAW) double-arc welding and provided optimal conditions for obtaining high-quality and good forming. [PERSON] et al. [17] compared a 6.35 mm thick 6061 aluminum alloy with the GMAW process using the hybrid GTAW-GMAW and confirmed that the GTAW-GMAW promoted grain structure refinement and reduced porosity in the welded metal. In addition, the GTAW-GMAW process reduced the degradation in the heat-affected zone under the as-veld condition and increased the welding speed by 40%. [PERSON] et al. [18] developed a model that optimizes the parameters through a Gaussian process regression model for the bead shape in a tandem flux cored arc welding process. [PERSON] et al.