*2.1. Experimental System*

The experimental system consisted of welding system, high-speed photography system, and welding electrical signal synchronous acquisition system, as shown in Figure 2.

**Figure 2.** Schematic of high-speed photography system for weld pool observation.

EWM Phoenix 521 (EWM Hightec Welding GmbH, Mundersbach, Germany), EWM Cold Arc (EWM Hightec Welding GmbH, Mundersbach, Germany), and Fronius TPS5000 (FRONIUS, Pettenbach, Austria) were selected as power sources of welding system to provide needed waveforms. The welding position was PA(Flat position, as per ISO 6947). The movement of the workbench was controlled by servo motor. Current sensor, voltage sensor and signal acquisition card constituted the welding electrical signal synchronous acquisition system. Acquisition frequency of signal acquisition card is 1.5 <sup>×</sup> 106 Hz. The high-speed photography system was used to recorded the behavior of the weld pool.

In order to capture the side-view of welding pool during the welding process, the high speed photography camera was at position one of Figure 2, which is on the same horizontal plane as the welding test plate. Dynamic information about the weld pool oscillation from the high speed photography pictures was obtained by tracing the pool surface as a function of time. The shooting angle is perpendicular to the welding seam in the same plane. A light-emitting-diode (LED) was used as its excitation light source at position one, whose wave length was 850 nm, and continual output was 3 W. A laser source 850 nm near infrared filter was used to filter out strong arc during welding process. Acquisition frequency of position one is 10,000 Hz. The shooting picture is shown in Figure 3a. Positions of the camera and backlight source need to be changed to capture the contour and the flow behavior of the weld pool surface, as shown in position two of Figure 2. Different narrow-band filters were selected to obtain different information of the weld pool surface which have different spectral characteristics. The high speed camera was equipped with 850 nm near infrared filter to obtain the contour information of the weld pool with the laser shined by the same type laser source mentioned above at position two, as shown in Figure 3b. 650 nm near infrared filter was installed to obtain metal flow information of the weld pool with no laser shined, as shown in Figure 3c. Acquisition frequency of position two of Figure 2 was 2000 Hz. During video capturing, the camera was placed at an angle deviation of about 0–5◦. In the analysis, the effect of these angles was not taken into account. However, based on the geometry employed, it is estimated that average errors are of the order of 1.5%.

**Figure 3.** Pictures captured for different condition: (**a**) side-view shooting picture; (**b**) overhead shooting effect (850 nm near infrared filter was added); and (**c**) overhead shooting effect (650 nm near infrared filter was added).
