## 3 Results and Discussion  
### Input/Output Data for Prediction of Penetration Condition  
Figure 4 depicts a schematic of the changes in the state of the back-bead with respect to variations in the WFR and root gap. At a 1.0-mm gap, PJP (class 0) occurred with WFRs ranging from 7.0 to 1.0 m/min, while a CJP (class 1) was achieved at a WFR of 11.0 m/min. At a WFR of 12.0 m/min, burn-through (class 2) occurred without back-bead formation. At  
Figure 3: Back-bead measurement method and output data classification. a 2.0 mm gap, PJP (class 0) occurred at a WFR of 7.0 m/min. In addition, for WFR values ranging from 8.0 to 9.0 m/min, a mixture of PJP (class 0) and CJP (class 1) was observed. For WFR values ranging from 10.0 to 11.0 m/min, CJP (class 1) was achieved. However, for a WFR of 12.0 m/min, burn-through (class 2) occurred (Figure 4a). Figure 4b shows the cross-sectional shape of the weld joint when PJP, CJP, and burn-through occurred. Figure 5 illustrates the current and voltage waveforms during welding performed using a DC pulse. The current and voltage waveforms were analyzed to assess the validity of structuring the input data before their composition. The average welding current/voltage and pulse cycle were verified. The average peak current (\(I_{P}\)), average base current (\(I_{B}\)), duration of the peak current (\(T_{IP}\)), and duration of the base current (\(T_{IB}\)) were determined from the current waveform. The average peak voltage (\(V_{P}\)), average base voltage (\(V_{B}\)), duration of the peak voltage (\(T_{VP}\)), and duration of the base voltage (\(T_{VP}\)) were determined from the voltage waveform. Table 2 summarizes the analysis of the welding current/voltage waveform with respect to variations in the WFR. The analysis was performed solely in instances where CJP occurred to examine the variations in the welding current/voltage with respect to the WFR. The root gap was maintained at 3.0 mm to induce CJP even at a lower WFR, and a ceramic backing tape was attached to the back of the specimens to prevent burn-through. An increase in the WFR resulted in an increase in the average current/voltage and a decrease in the pulse cycle. From the analysis of the current waveform, as the WFR increased from 7.0 to 12.0 m/min, the peak current \(I_{P}\) and its duration \(T_{IP}\) remained almost constant at 465 A and 1.77 ms, respectively. In contrast, the base current \(I_{B}\) increased from 36 to 65 A, whereas its duration \(T_{IB}\) decreased from 2.14 to 0.12 ms. The voltage waveform, which is dependent on the current waveform, exhibits a similar trend. The peak voltage \(V_{P}\) and its duration \(T_{VP}\) were similar, measuring 33.9 V and 1.60 ms, respectively. \(V_{b}\) exhibits an increasing trend, whereas \(T_{VB}\) shows a decreasing trend. The peak term remained relatively consistent as the WFR changed. However, variations in the pulse cycle and base term indicate that  
Figure 4: Gap and WFR changes: (**a**) schematic of penetration-type classification; and (**b**) cross-section image according to classification. there are differences in the calculated dynamic resistance, highlighting their potential as input data through STFT transformation. During the welding experiments in this study, at a gap of 0.2 mm and WFR values of 8.0 and 9.0 m/min, a region was observed (Figure 4a) where PJP and CJP coexisted. Under the same welding conditions, PJP and CJP mixtures were formed, prompting the need to examine variations in the welding current/voltage owing to these phenomena. Figure 6 illustrates the welding process conducted at a root gap of 3.0 mm with a WFR of 9.0 m/min. The welding phenomena were captured using a high-speed camera while simultaneously measuring the welding waveform. The average current/voltage for PJP (class 1) and CJP (class 2) were 265 A/28.7 V and 269 A/29.6 V, respectively.