## 2 Material and Experimental Methods  
### Measurement of Material Flow on Weld Pool Surface  
The experimental setup is portrayed in Figure 1. This experimental system consisted of a welding torch, a welding power source (NW-300 ASR, Nippon Steel Welding & Engineering Co., Ltd., Tokyo, Japan), base metal, shielding gas, plasma gas, an HSVC (Memrecam Q1v-V-209-M8, Nac Co., Ltd., Osaka, Japan), an actuator (THK E56-06-0300H-TS, THK Co., Ltd., Tokyo, Japan), a band pass filter, and a diode laser power (CNDC24B7, Japan Servo Co., Ltd., Osaka Japan). The base metal was SUS 304 stainless steel in \(300\times 100\times 4\) mm (length \(\times\) width \(\times\) height) sheets. Its chemical composition is presented in Table 1. Two kinds of the shielding gas were applied: pure Ar and Ar + 0.5% O\({}_{2}\). Pure Ar was employed for plasma gas in all experiments. The HSVC, connected to the computer, captured videos at 1000 frame per second (fps). The welding speed of 3 mm s\({}^{-1}\) was controlled by an actuator. A filter with wavelength of 940 nm was attached in front of the HSVC. The distance from the filter to the center point of the keyhole was 300 mm. A high-power diode laser with power of 30 W and wavelength of 940 nm was employed. The purpose of the diode laser was to overcome the brightness of the arc plasma. Using both the band-pass filter and the laser power, the brightness of the main arc was considerably decreased. Hence, the weld pool surface and the motion of zirconia particles were clearly observed during welding. The magnification of the weld pool images was controlled by a lens (Micro-Nikon 105 mm f/2.8, Nikon Co., Ltd., Osaka, Japan). A tungsten electrode (2% thorium) with a 3.2 mm diameter was used. The full tip angle of the electrode was 60\({}^{\circ}\), the nozzle diameter for plasma gas was 2.4 mm, and the nozzle diameter for the shielding gas was 12 mm. Other welding conditions are listed in Table 2.  
To detect the convection flow on weld pool surfaces, we used 0.3 mm diameter zirconia particles. To put zirconia particles in holes on the base metal, in each of the experiments, we drilled six 2-mm-deep and 1-mm-diameter holes in two lines (three holes per line; Figure 2). Afterward, the holes were cleaned using ethanol and acetone and were then dried in the air. After drying, one zirconia particle was placed in each hole. Finally, the base metal was fixed and welded under the stated welding conditions. Afterward, a computer running Dipp-motion software (Detech Co., Ltd., Osaka, Japan) was used to calculate the velocity of the molten flow on the weld pool surface. To determine the velocity of fluid flow, images with resolution of \(640\times 480\) pixels were set up for the HSVC to capture real images of \(24\times 23\) mm. After, based on the HSVC resolution and the dimensions of the real images, the transferred distance of particles was converted from pixels of the HSVC to the real dimensions of the weld pool for each time step. Then, the velocity was determined.  
Figure 1: Experimental setup for observing the weld pool surface.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Element** & **C** & **Si** & **Mn** & **Ni** & **Cr** \\ \hline Wt \% & 0.06 & 0.44 & 0.96 & 8.19 & 18.22 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of base metal (SUS 304).  
Figure 2: Schematic illustration of holes on the base metal surface.