## 3 Experimental Results and Discussion
### Material Flow Behavior on Weld Pool Surface
As explained above, the direction of Marangoni force is from the bottom surface toward the top surface of the weld pool for pure Ar and from the top surface toward the bottom surface for Ar + 0.5% O\({}_{2}\). Due to the change in keyhole shape, the shear force caused by plasma flow along the surface of keyhole wall varied. With pure Ar, because the keyhole diameter was smaller, most of the plasma flow pushed out at the bottom surface of the weld pool through the keyhole and only a small amount flowed in the upward and backward directions along the surface of the weld pool. This led to a weak upward and backward shear force (F\({}_{11}\)) on the top surface of the weld pool and a strong downward shear force (F\({}_{12}\)) along the keyhole wall. In this case, the combination of F\({}_{11}\) in the upward direction (weak force) and Marangoni force in the upward direction (weak force) controlled an upward material flow with low speed (0.24 m s\({}^{-1}\)). The keyhole diameter was larger, especially near the top surface of the weld pool. Figure 14: Schematic illustration of Marangoni force on the weld pool surface redrawn from [27]. (**a**) Pure Ar, (**b**) Ar + 0.5%O\({}_{2}\). -weld pool for Ar + 0.5% O\({}_{2}\). In this case, a large amount plasma flow pushed out in the upward and backward directions along the weld pool surface to produce a larger upward and backward shear force (F\({}_{21}\)); a small amount of plasma flow pushed down through the inside keyhole wall to cause a weak downward force (F\({}_{22}\)). In this case, the combination of F\({}_{21}\) in the upward direction (strong force) and Marangoni force (weak force) in the downward direction strongly accelerated the material flow in the upward direction toward the rear part of the weld pool (0.42 m s\({}^{-1}\)). As the backward heat transfer produced by the convection in the weld pool increased through the expansion of the keyhole diameter especially near the top surface, the weld pool length increased, but the width of the weld bead decreased when a small amount of oxygen was added to the argon shielding gas. As explained above, the shear force in KPAW was caused by the strong plasma gas and shielding gas flows along the keyhole and weld pool surfaces in the upward and backward directions on the upper side of the weld pool. With Ar + 0.5% O\({}_{2}\), due to the oxidation of the weld pool surface, the surface tension decreased. Consequently, the plasma gas flow and shielding gas flow easily pushed the fluid flow backward compared to the welding direction. As a result, the inclination angle of keyhole wall considerably increased, especially around the top surface, as demonstrated in Figure 8:0 This is different to the behavior of molten flow in the TIG welding process as shown in many previous papers, where the molten flow occurs in the downward direction due to (1) a weak shear force on the weld pool surface and (2) a strong Marangoni force. Summary, the shear force is the dominant driving force that depends on the shape of the keyhole. The slag appearance on the bottom surface and the change in weld bead width can cause changes in the mechanical properties of the welding joints such as tensile strength or residual strength [32]. However, as shown in another work, with a small amount of oxygen in the weld pool (less than 2.0%), the penetration can be changed due to the change in Marangoni convection, but mechanical properties such as tensile strength did not decrease [33]. [PERSON] et al. recently estimated the behavior of fluid flow inside the weld pool considering the influence of shielding gas compositions [34]. The results indicated that the velocity of molten flow with pure Ar is much lower than with Ar + 0.5% O\({}_{2}\). The plots showed that a downward convection flow occurred in the lower half of the weld pool with Ar + 0.5% O\({}_{2}\). The convection flow near the keyhole in the upper half of the weld pool did not seem to be observed. However, we observed a strong upward convection in the upper half of the weld pool.