## 3 Results and Discussion  
### High-Speed Imaging  
In this section the results of the high-speed imaging are presented. Different camera settings were used to reveal different features of the arc phenomena and metal transfer. The results presented in this section were produced using a wire feeding speed of 150 ipm; travel speed of 7.5 ipm; peak current \(I_{p}=400\) A; background current \(I_{p}=50\) A; pulse frequency of 90 Hz; and voltage of 20 V.  
Figure 1 shows the images acquired when the arc radiation was filtered with a 515 \(\pm\) 10 nm narrow band pass filter for different camera sensor exposure times from 3.91 to 80 us. Each column in this figure shows the images from the start of peak phase to the onset of background current. Iron (Fe) has a strong emission peak (Fe I) at a wavelength of 516.74 nm [28; 29], and hence using a narrow band pass filter of 515 \(\pm\) 10 nm wavelength will selectively allow exposure to radiation mostly from Fe I. Hence, the bright regions on these images are composed mostly of iron vapor. Although iron has a peak emission line at 515 \(\pm\) 10 nm wavelength, some other radiation is also able to reach the camera sensor. Consequently, increasing exposure time will allow more radiation to reach the camera sensor, over-exposing the image, leading to image flare. This can be seen on the fifth row of Figure 1 for the exposure times of 20, 40, and 80 us. Despite the image flare due to the excessive sensor exposure, the exposure times of 3.91 and 6.25 us provide clear images of the iron vapor core formation and evolution during the peak current pulse. In Figure 1 at the times of 123.0 and 79.8 ms for the exposure time of 3.91 and 6.25 us, respectively, the light captured by the camera sensor is being emitted from the wire tip, weld pool, and the small droplet being transferred. Those regions are the main sources of iron vapor, owing to their high surface temperatures [11; 13]. As the current increases to the peak phase, the amount of vapor emitted from the wire tip increases due to the increase in wire tip temperature. This evolution of metal vapor region has been repeatedly predicted by numerical simulations [8; 9; 13] and reported in studies utilizing high-speed imaging [25]. Additionally, in Figure 1 for the exposure time of 3.91 and 6.25 us, one can see that the metal vapor around the secondary droplet is pushed downward away from the molten pool as the pulse current increases to a peak value, owing to the increase in plasma flow velocity as a consequence of the current increase [10]. This metal vapor pushed to lower temperature regions of the arc will allow the nucleation and growth of nanoparticles by condensation, leading to fume formation [13; 31] (see Supplemental Video S1 for details). Given that when the \(515\pm 10\,\mathrm{nm}\) wavelength filter is used, the image brightness is proportional to the amount of high temperature iron vapor, one could use this information to correlate the image pixel intensity to the amount of iron vapor present in a specific region. This is shown on Figure 2, where images were taken during the background current phase with an exposure time of \(80\,\mathrm{\SIUnitSymbolMicro s}\). The evolution  
Figure 1: Arc appearance for aperture of \(\mathrm{f}/22\) using at narrow band pass filter of \(515\pm 10\,\mathrm{nm}\) wavelength for different camera sensor exposure times. of pixel intensities along line A in Figure 2a are plotted in Figure 2b. It can be seen that the pixel intensity (gray value) increases with time, indicating an increase in the amount of iron metal vapor in the wire tip. This is in agreement with numerical simulation of [PERSON] et al. (2013), where it was shown that during background current phase, iron vapor accumulates at the wire tip and is not dragged down by the plasma flow due to the low flow velocities associated with small current values.