## 3 Experimental Results and Discussion
### Material Flow Behavior on Weld Pool Surface  
To verify the reliability of the research results, the experimental observations were repeated more than 10 times for each shielding gas. The experimental results showed the same tendency of zirconia particles in most experiments. Typical consecutive tracer particle movement images for Ar + 0.5% O\({}_{2}\) are presented in Figure 9 from 13 until 28 ms.  
Four typical results for each case of the shielding gas are shown in Figure 10. The tracer particles in a1, a2, a3, and a4 showed nearly the same tendency (Figure 10a). In this case, tracer particles' motion was observed for 130 ms. Firstly, the zirconia particle sank to the bottom surface before entering the weld pool. Then, it accelerated in an upward direction behind the keyhole. After accelerating in the upward direction, it reversed from straight motion to rotational motion behind the keyhole before entering the welding slag layer on left and right edges of the weld pool surface. Figure 8: Temperature distribution on the surface along keyhole wall in the case of Ar. (**a**) Observed images, (**b**) Temperature distribution. Figure 9: Typical continuous tracer particle movement images. (**a**) t = 13 ms, (**b**) t = 18 ms, (**c**) t = 23 ms, (**d**) t = 28 ms. Figure 10b shows that after upward and backward acceleration from inside the keyhole, the tracer particles in b1, b2, b3, and b4 displayed translational movement toward to the ending part of the weld pool. In this case, the zirconia motion was observed for 30 ms. Accordingly, the moving time of the zirconia particle in pure Ar case was longer than that with Ar + 0.5% O\({}_{2}\). Due to oxidation on the weld pool surface, we observed that the slag on the weld pool surface was significantly increased when using Ar + 0.5% O\({}_{2}\). The average speed of material flow in the two cases was computed; the average speed was about 0.24 and 0.42 m s\({}^{-1}\) in pure Ar and Ar + 0.5%O\({}_{2}\), respectively. Weld pool surface shapes are depicted in Figure 11, which compares the weld pool length in both pure Ar and the mixture of Ar and 0.5% oxygen. The weld pool on the top surface in pure Ar case was longer, but its width decreased in comparison to with Ar + 0.5% O\({}_{2}\). The keyhole diameter was larger, especially around the top surface of the weld pool in Ar + 0.5% O\({}_{2}\). The weld pool length was about 11 mm for pure Ar, but about 14 mm for Ar + 0.5% O\({}_{2}\). Slag was observed at the ending part of the weld pool, especially for the mixed shielding gas. This issue can be explained based on the difference in the weld pool length and the oxidation of the weld pool surface by the oxygen in the shielding gas. Since the weld pool for the mixed shielding gas was much longer than that of pure shielding gas, the shielding effect was much reduced. The covered efficiency was low, producing the slag described above. The widths of weld bead after welding are exhibited in Figure 12. Due to the higher flow velocity of the weld pool convection and the zirconia particle being directly accelerated backward to the ending  
Figure 11: Weld pool surface appearance. (**a**) pure Ar (**b**) Ar + 0.5% O\({}_{2}\). Figure 10: Four typical series of images of observed convection on the weld pool surface for both pure Ar and Ar + 0.5% O\({}_{2}\). (**a**) pure Ar, (**b**) Ar + 0.5% O\({}_{2}\). part of the weld pool, the width of the weld bead was narrower when the shielding gas was admixed with 0.5% O\({}_{2}\). The weld bead width was about 7.0 mm for pure Ar and about 5.0 mm for Ar + 0.5% O\({}_{2}\). Figure 13 displays the slag on the bottom surface of the base metal after welding. At a high temperature during the welding process, oxygen from the air participated in oxidation reactions to form oxides, especially on the bottom surface due to an insufficient shielding effect. The welding slag on the bottom surface of the base metal consequently increased for Ar + 0.5% O\({}_{2}\).