## 3 Experimental Results and Discussion
### Material Flow Behavior on Weld Pool Surface
More slag was created when using mixed gas compared with pure Ar due to two factors: the penetration of oxygen from the air and oxygen from the shielding gas. Generally, in the KPAW process with an open keyhole, a strong shear force always occurs in the downward direction along the keyhole wall from the top surface toward the bottom surface of the weld pool due to the strong plasma flow exiting a small nozzle diameter. However, in one of our recent experiments, we found that the shear force in KPAW with an open keyhole consists of two components: along the keyhole wall in the downward direction from the top surface toward to the bottom surface, and another in the upward direction from the middle part of the keyhole toward the top side in backward direction from the keyhole toward the rear part of weld pool [18]. Depending on the keyhole shape, the strength of components can be changed. This can cause a change in the convective flow of the weld pool. The Marangoni force is a surface force acting on the surface of a weld pool and a keyhole wall. As shown in a previous study, Marangoni forces can be produced by the reversion of the surface tension gradient (\(d\sigma/dT\)) on a weld pool surface, as indicated in Figure 14[27]. [PERSON] et al. proposed that at a low temperature (about 2000 K or less), the surface tension decreases with increasing temperature (\(d\sigma/dT\) always has a negative value). In this case, the surface tension at the weld pool edge is higher than at the center of weld pool, creating convective flow from the center toward the edge. Marangoni flow occurs from high temperature zones toward low temperature zones. However, adding a small amount of activated elements such as oxygen (0.5 wt%), the temperature coefficient of  
Figure 12: Weld bead width on the top surface of the base metal. (**a**) pure Ar, (**b**) Ar + 0.5% O\({}_{2}\). Figure 13: Welding slag appearance on the bottom surface of the base metal. (**a**) Pure Ar, (**b**) Ar + 0.5% O\({}_{2}\). surface tension can change from negative to positive (\(d\sigma/dT\) always has a positive value), changing the direction of Marangoni flow from outward to inward [28]. These results showed that the Marangoni force direction was upward and backward around the top surface of the weld pool under pure Ar and downward around the bottom surface of the weld pool for Ar + 0.5% O\({}_{2}\). Lorentz force (an electromagnetic force) is caused by electric current flowing through the base metal. As a result, this magnetic field induces convective flow. Generally, the Lorentz convection is inward and downward [29]. The simulation results of an individual force indicated that the velocity caused by this force was much lower than that caused by Marangoni force [26]. Therefore, this force is not considered to be a main force affecting the weld pool in KPAW. Buoyant force is an upward force acting in the weld pool. This force always occurred from the bottom surface toward the top surface of the weld pool. However, this force was much weaker than other dominant forces (shear force and Marangoni force) [18]. As a result, buoyant force is not considered to be a dominant driving force in KPAW. In a surface fluid flow experiment of the TIG welding process, [PERSON] et al. estimated that the maximal speed is about 0.1 m s\({}^{-1}\) at the center of the arc [30]. This value is much lower than the velocity value in this study. The main driving forces in the TIG welding process are Marangoni force and shear force [31]. Based on this difference, the KPAW shear force is much stronger than in TIG welding. Based on the explanation above, our results are discussed through schematic illustration of the influence of shear and Marangoni forces in Figure 15. In this case, the convective patterns of the weld pool are controlled principally by the interaction of shear force and Marangoni force.