# An Image-Processing Method for Extracting Kinematic Characteristics of Droplets during Pulsed GMAW
## 2 Experimental System and Welding Conditions  
Figure 1 shows the GMAW system that can realize synchronous voltage, current, and image signal acquisition. Considering that the frequency of arc behavior and droplet transfer is pretty high in GMAW, a high-speed camera at 10,000 fps was used in this study to catch detailed images of the process. However, the welding arc cannot be photographed without filter lens protection. Otherwise, the extremely bright arc could cause huge interference in the images and even damage the photosensitive elements of camera. The brightness of the arc should be filtered to an appropriate range. Depending on the spectrogram of a welding arc, the energy of arc light in the infrared band is relatively weak [17]. Thus, a narrow-band filter centered at 850 nm was placed in the lens, and a laser light with a wavelength of 850 nm was used as a backlight to enhance the background brightness and magnify the contrast of images.  
In addition, an adjustable ND (neutral density) filter was added in front of the lens. The filter is composed of a pair of polarizers. According to [PERSON]'s law, light intensity can be adjusted by changing the relative angles \(\theta\) of the two polarizers [18]. Furthermore, because of different response characteristics of visible light and infrared light, this ND filter has a significant effect on visible light reduction but not the 850-nm infrared laser [19]. As a result, the intensity of arc light in the image can be strengthened or weakened, while the intensity of the laser background merely changes with the ND filter.  
Two groups of images under the same single typical pulse current but different polarization angle of the ND filter are shown in Figure 2. Arc behavior can clearly be seen in Figure 2b (translucent arc). With the increase in current, the arc becomes brighter; however, the droplet is covered by the arc at peak current. After that, the contour of droplet surrounded by the translucent arc becomes clear when  
Figure 1: Schematic diagram of gas metal arc welding (GMAW) synchronous signal acquisition system.  
the brightness of the arc decreases. In contrast, a clear contour of the droplet but no arc can be found in Figure 2c (transparent arc). Arc light is totally removed by the narrow-band filter and ND filter.  
Arc behavior is necessary for studying metal transfer, as the droplet is mainly pushed into the molten pool by the arc force [20]. For the image processing of droplets, it is not expected to have the interference of arc light in images, as seen in Figure 2b. Thus, Figure 2c, which filters all the arc light, would have an advantage in the image processing. However, the arc does play a significant role in droplet transfer. Although a clear droplet profile and less interference of arc light can be found in Figure 2c, the key information of the arc for analyzing the droplet transfer is lost.  
Therefore, for the research of droplet transfer, it is a better choice to keep the information of the arc and droplet at the same time in images. From this point of view, Figure 2b, which has both a translucent arc and clear droplet, is more appropriate for comprehensively studying droplet transfer. As a result, images with a translucent arc like Figure 2b were selected as the study object of the image processing in this paper.  
The ER50-6 wire of 1.2 mm diameter was used, and the CTWD (contact tube-to-work distance) was selected as 18 mm. Additionally, Table 1 shows the other welding conditions of the pulsed GMAW.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Average Current (A)** & **Average Voltage (V)** & **Welding Speed (mm/s)** & **Gas Type** & **Flow Rate of Gas (I/min)** \\ \hline
110 & 24 & 4 & 82\% Ar + 18\% CO\({}_{2}\) & 18 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Welding conditions of the pulsed gas metal arc welding (GMAW).  
Figure 2: _Cont._