## 3 Results and Discussion
### Mechanism on Nanowire Flux Activated TIG Welding  
Generally, in a TIG welding process, there are several kinds of forces driving the melting metal flow and forging a shape of the welding pool [45; 46]. Those forces include shear force, Marangoni force, Lorentz force, and Buoyant force. However, during these forces, Lorentz force and Buoyant force are much weaker than the other dominant forces (shear force and Marangoni force) and thus are not considered to be dominant in TIG welding. Shear force is strong, but it always occurs in the downward direction. Thus, the Marangoni force, which can change direction and the shape of the welding pool as a dominant driving force, will be mainly discussed in this study. Figure 11: Tensile test fracture surface of (**a**) base metal, (**b**) no-flux TIG welding, (**c**) A-TIG weldments with microscale Fe\({}_{2}\)O\({}_{3}\) flux, and (**d**) A-TIG weldments with nanoscale Fe\({}_{2}\)O\({}_{3}\) flux. The difference in weld bead geometry of without-flux and with-flux weldments can be related with the heat input, and the reversal in Marangoni convection shown in Figure 12. The Marangoni flow is determined by the non-dimensional Marangoni number (Ma) defined in the following equation [47]:  
\[Ma=\frac{d\gamma}{dT}\frac{dT}{dx}\frac{L^{2}}{\eta a} \tag{5}\]  
where \((d\gamma/dT)\) is the surface tension gradient, \((dT/dx)\) is the temperature gradient, \(\eta\) is the viscosity, \(a\) is the thermal diffusivity and \(L\) is the characteristic length. The surface tension gradient \((d\gamma/dT)\) determines the direction of the flow, and it can produce Marangoni forces on the surface of a welding pool. Previous study showed that when the temperature is around 2000 K or less, the value of \((d\gamma/dT)\) will be negative and decrease with increasing temperature. In this condition, the surface tension shows the maximum value at the edge of the welding pool (since the temperature there is the lowest) and will create a convective flow from the center toward the edge as Figure 12a shows. Under this upward and backward flow around the top surface, an outward surface flow resulting in a wide, shallow welding molten pool will be induced. However, when adding flux (usually oxide compounds) the O (or S) content in-creases near the welding pool, which leads to a positive \((d\gamma/dT)\). Thus, the surface tension is greatest in the high-temperature region (at the center of the welding pool) and induces a radial inward flow (downward around the bottom surface of the welding pool). This means that the welding molten metal produces a downward flow starting from the welding pool center, yielding a deep and narrow pool (Figure 12b). Based on the temperature gradient part \((dT/dx)\), which determined the strength of the flow, it is obvious that for a similar \(T\), as \(dx\) is lower, the value of \((dT/dx)\) part will increase and thus increase the value of the flow force, which results in a deep welding penetration. Compared with no-flux TIG welding, both microscale and nanowire flux have a lower \(dx\), and contribute to improved penetration. Based on the study, for nanowire flux, the \(dT/dx\) value is higher than that of microscale flux. This could be attributed to the size effect of melting point [36; 49]. Due to the surface and quantum confinement effects of nanomaterials [50], there are fewer \"neighbors\" around the atoms at the surface than a microparticle, result in a lower binding energy per atom. The reduction of binding energy per atom results in a lower Gibbs free energy of oxides. Thus, the nanowire flux exhibits a lower thermal stability than microscale flux, indicating a higher dissolving rate under the same temperature and sufficient oxygen content for a more pronounced reversed Marangoni convection. It was also evident that the nanowire flux has a high heat input than microscale flux under the same current (Figure 7b), which results in a higher melting pool temperature, a high \(dT/dx\) and thereby a stronger Marangoni flow for a deeper penetration.