## 3 Results and Discussion
### Arc Voltage and Arc Length  
Figure 6 shows the electric signals, featuring the variation of arc voltage as the current pulse progress. As can be seen in this figure, current values are nearly constant during the peak and background current phases. From Figure 4, one can see that as the pulse progresses during peak phase, the distance between electrode tip and weld pool decreases; however, the voltage is still increasing (see Figure 6). Despite that, the measured voltage increases during the peak phase, while the measured arc voltage decreases during background current (Figure 6). Additionally, notice a secondary peak in voltage signal during background current, indicated by the arrows. These occur following the moment of droplet detachment (see details in [27]). Arc length was measured (corresponding to the distance between wire tip and weld pool [2; 3]) for current values of 50 and 380 A, and the result is shown in Figure 7. For low values of welding  
Figure 4: Iron (Fe) metal vapour and argon (Ar) dominated regions during pulse peak phase (current of 400 A) captured using different bandpass filters and exposure times: (**a**) 515 \(\pm\) 10 nm filter and exposure time of 3.91 \(\upmu\)s, (**b**) 900 \(\pm\) 10 nm filter and exposure time of 20 \(\upmu\)s.  
Figure 5: Bandpass filters and exposure times that can be used for (**a**) droplet temperature measurement and (**b**) weld pool behavior (current 50 A) captured using different bandpass filters and exposure times. Figure 6: Variation of current and voltage for peak current of (**a**) 400 A and (**b**) 530 A.  
current (i.e., during the background phase), one can see in Figure 7a that voltage increases with the measured arc length. On the other hand, for higher values of welding current (as shown in Figure 7b), the measured arc voltage increases, even though the measured arc length decreases. A voltage fall in the GMAW arc column around 0.69-1.0 V/mm is recognized in the literature [32; 33; 34], and the results in Figure 7b contradict this widely-accepted proportionality between arc voltage and arc length. In Figure 4 it is possible to observe that the tapering position of the electrode does not shift upwards by a large amount. Even if one measures arc length as the distance between the beginning of the electrode tapered region and the weld pool [1], it would verify that arc voltage increases at a much higher rate than 0.69-1.0 V/mm [32; 33]. In Figure 4, although the current value is the same (i.e., 400 A), one can note that as the pulse progresses during peak phase, the shape of the iron vapor-dominated arc core is changing, which is due to the increasing amount of vapor generated. The height and diameter of this region was measured according to Figure 8a for different current values. The variation of arc voltage with the height of this region is shown in Figure 8b, where one can see that there is no clear relationship between these quantities, despite the fact that for a current of 400 A the voltage seems to decrease with increasing core height. In Figure 8c, the instantaneous voltage is plotted against the bottom diameter of this region, and one can note that voltage is proportional to the measured diameter. If arc voltage is plotted against the diameter/height ratios, the values have a linear relationship, as it is shown in Figure 8d. The linear relationship between measured voltage and the ratio between core diameter over height shown in Figure 8d is correlated to the decrease in column height as diameter increases. The physical basis for the increase of voltage with arc core diameter here is attributed to the fact that the bottom diameter increases with increasing amount of metal vapor formed, which will cause the cooling of the arc plasma and a consequent decrease in the arc plasma conductivity [10; 11]. [PERSON] et al. [8; 9] observed a shift in the arc attachment at the wire electrode tapering region, rather than the wire tip, and attributed this phenomenon to the decrease of electrical conductivity of the plasma at the tip of the wire electrode.