3.1.1. Oscillograms

Figure 2 shows the oscillograms for the standard GMAW condition. For the root pass, one can notice a periodic repetition with frequency of approximately 3 Hz (Figure 3a). By correlating the electrical signals to the high speed images, it was observed that this repetition is a consequence of arc attachment to the groove sidewalls. As a consequence of the arc attachment to the sidewall, the power source promotes an instantaneous increase in the arc current, as shown in detail in Figure 3b. It is possible that this was caused by the auto-regulation system of the power source, which interprets the attachment of the arc to sidewalls as an increase in wire feed speed, consequently increasing the current to keep the melting rate constant while the arc length is reduced due to the arc attachment to the groove walls.

On the other hand, one can notice that this repetitive behavior did not occur in the fill and cap passes (Figure 2c,d), likely due to a reduced degree of arc constriction, and the shortest path to the electron conduction being the bottom of the groove and not the sidewalls. On the other hand, the severity of short-circuits in the cap pass was higher when compared to the filler pass. This suggests that the distance between the droplet (as shown in the high speed frames, Figure 6) and the substrate is lower, favoring short-circuits.

Figure 3 shows the oscillograms for the CW-GMAW specimens. It is noted that the periodic pattern observed in the oscillograms of GMAW for root pass was not observed for the entire sampling period in CW-GMAW of 2000 ms. This suggests the sidewall erosion was mitigated during CW-GMAW, since sidewall penetration causes the periodicity in the observed signal. Moreover, the short-circuit severity in the fill/cap pass is less prominent than that observed in the GMAW specimen.

**Figure 2.** Typical oscillograms for the GMAW condition: (**a**) root pass; (**b**) detail of the root inside the period of repetition; (**c**) fill pass; and (**d**) cap pass.

**Figure 3.** Typical oscillograms for the CW-GMAW condition: (**a**) root pass; and (**b**) fill/cap pass.
