**2. Welding and High-Speed Camera System**

In this work, the electrode wire was connected to the positive pole of the welding power, while the workpiece was connected to the negative pole. For the convenience of the camera to shoot the arc and droplets, the welding torch was placed stationary over the workpiece, which can be moved by the motion platform at a constant speed.

Additionally, because the temperatures and rates of metal transfer are extremely high, the non-contact high-speed camera method has been widely used in droplet transfer. Figure 1 displays the schematic diagram of the welding and high-speed camera system, which can acquire synchronous voltage, current and image signal. The arc voltage was measured between contact tube and workpiece. The high-speed camera system was used at 10,000 fps and the sampling rate of the data acquisition card was 30 kHz. To lower the interference of bright arc, a laser light, whose wavelength is 850 nm, was used as a backlight. Other wavelengths of arc were weakened by a narrow-band filter centered at 850 nm between the arc and the camera. The laser light, arc, narrow-band filter and camera were all placed on the same horizontal line. As a result, the brightness of the arc was filtered to an appropriate range and the droplets can be clearly seen on the computer screen.

**Figure 1.** Schematic diagram of the welding and high-speed camera system.
