*4.3. Stability Analysis of the Electrical Signal*

In this work, a one self-designed multiple-sensor-signal fusion system was employed to online collect the welding voltage and current signals. Using collected signals, the stability analysis of the welding current, which was closely relative to the stability of welding process and quality of weld bead, was conducted. After sampling all of nine welding current signals, three current signals corresponding to different stabilities, which were L3, L5 and L7 according to above analyses, were selected to make detailed stability analyses, corresponding results are shown in Figures 8–10. Figure 8 shows the transient welding current waveform and corresponding possibility density function (PDF) results of welding current of L3. The PDF of the welding current, together with the transient welding current, were commonly employed methods to evaluate the stability of welding process [28,29]. As shown in Figure 8a, the larger values of welding current appeared in 37.7–37.8 s and 38–38.5 s; the values were above 350 A; this was because the welding currents were unstable, which can induce the metal transfer non-uniform, and short-circuits occurred so that the transient welding currents sharply increased. While in Figure 8b, two convex peaks appeared at 17 A and 36 A; this was because the base current in TBP and TPP were respectively 17 A and 36 A. another convex peak appeared at 316 A, which was the peak value of the pulse current. However, there are some current possibility densities distributed beyond 316 A; this was because the welding current was unstable, and big volume metal transfer may

occur, which can also induce short-circuits and make the welding current sharply increase; this was why the welding current can be distributed from 316–400 A. Also, the welding current can be distributed from 0 to 17 A, the 0 A of the welding current denoted the open-circuit occurring. The appearance of short-circuit and open-circuit showed that the welding current was unstable.

**Figure 8.** Welding current in L3 experiment, (**a**) Transient welding current waveform, (**b**) Possibility density function (PDF) of the welding current.

Corresponding figures of L5 are shown in Figure 9. The signal in Figure 9a was so regular, which denoted that the process had no short-circuit or open circuit. While in Figure 9b, the possibility density distributions were concentrative on three convex peaks, which corresponded to 21, 32 and 316 A. These three current values were the base currents and peak currents in TPP and TBP. The variation of possibility density distribution curve had large slopes, which meant the current sharply changed. In addition, there was no current distributed in 0 A and beyond 316 A, which also demonstrated that no short-circuit or open circuit occured, and the currents were smoothly switched between bases and peak values.

**Figure 9.** Welding current in L5 experiment, (**a**) Transient welding current waveform, (**b**) PDF of the welding current.

Figure 10 shows the corresponding analyses results of L7. The signals in Figure 10a were regular, which denoted that no short-circuit and open circuit occurring. While in Figure 10b, the current possibility density distribution concentrated on three convex peaks, which were 19, 35 and 316 A corresponding to the base currents and peak current in TPP and TBP. However, the slopes of variation

curve were smaller than that in Figure 9b. In addition, there were small flat slopes at 19 A and 35 A, which showed that the current cannot be stable in these setting values. Moreover, the current possibility density increased from 175 to 316 A, the amplitude was about 0.001%, which meant the current had a small variation over this range.

**Figure 10.** Welding current in L7 experiment, (**a**) transient welding current waveform, (**b**) PDF of the welding current.

According to detailed analyses and comparisons of Figures 8–10, it can be noticed that the welding current in Figure 8 was the most unstable, whose stability can be ranked as "Poor". The welding current in Figure 9 had the highest stability and can be ranked as "Excellent". The stability of welding current in Figure 10 was a little worse than that of Figure 9, and can be ranked as "Good". The stability of the welding currents in between that of Figure 8 and that of Figure 10 can be ranked as "Medium". In addition, in the above three experiments, it can be noticed that the peak values of the current pulses were the same, both in different experiments and in two current pulse sets, but the base values were different in different experiments and different current pulse sets. It was because the base values can be determined by the twin pulse frequency TPF and twin pulse current change, which were different in the above experiments.
