## 3 Results and Discussion  
### Welding Torch Perpendicular to the Base Metal  
When the welding torch was perpendicular to the top surface of the base metal in the transverse section (case 1), FLP was commonly observed in the fusion zone [8; 12], as shown in Figure 2a. The formation of the weld profile with FLP was mainly due to the combination of the outward flow of the molten metal at a shallow level, and downward flow at a deep level. The outward flow was responsible for the wide but shallow penetration, and the downward flow was responsible for the deep and narrow penetration. Under the combined driving forces, including a drag force induced by the difference in tangential velocity between the plasma and the molten pool, and a Marangoni force caused by surface tension and its spatial gradient [5; 19], the molten metal flowed outward. Next, more of the solid base metal at the liquid/solid interface was molten, thereby increasing the width of the weld pool. The heat input occurred over a large area, which could not melt deeper because of the lack of a powerful downward flow. Therefore, a wide but shallow concave region formed. The remarkable surface depression caused by arc pressure following a Gaussian distribution thinned the fluid layer between the arc and solid base metal [20], thus decreasing the buffering of the weld puddle from the droplet impact [4; 21]. The acceleration of the droplets due to the plasma drag force [6], combined with the weakened buffering effect of the weld puddle, made it easier for droplets to reach the bottom boundary of the weld pool, even with substantial redundant momentum. Moreover, because of the heat input before droplet impingement, the solid bottom was heat-softened [22]. Thus, when an accelerated and superheated droplet impinges on a thin fluid layer, it flows downward and crashes against the solid but already-softened bottom; creating a crater. Note that the heat and momentum carried by droplets weakened and the size of the droplets decreased as they flowed downward, and after crashing, the droplets scattered into pieces to flow to both sides [23], as represented by the green dashed arrows in Figure 2b. Then, the crater became increasingly deep under \begin{table}
\begin{tabular}{c c} \hline \hline
**Process Parameters** & **Value** \\ \hline Mean voltage (V) & 21.7 \\ Mean current (A) & 96 \\ Wire diameter (mm) & 1.2 \\ Welding speed (mm/s) & 59 \\ Wire feeding rate (mm/s) & 2.8 \\ First phase current (A) & 114 \\ Second phase current (A) & 78 \\ First phase time (ms) & 180 \\ Second phase time (ms) & 180 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Welding process parameters for DP-GMAW. Figure 1: Schematic diagram of the current waveform and longitudinal sections of the double pulsed-gas metal arc welding (DP-GMAW) weld pool and the inclination of the welding torch. the successive impingement of droplets, as shown by the black dashed line in Figure 2b. Finally, a deep penetration formed. Meanwhile, because of the significant difference in velocity between the droplet and the neighboring liquid in the molten pool and the reduced viscosity caused by increased temperature, only a small fraction of the fluid near the droplets was heated and accelerated, and this fluid could only melt a narrower area than at the shallow level. Therefore, a distinct separating landmark, namely a sharp inflection point (SIP), formed between the shallow and the FLP levels. The grain morphology along the different walls of the molten pool differed considerably, including both columnar grains (Figure 2c) at the shallow level, and refined equiaxed dendrites (Figure 2d) at the FLP level. The difference in grain morphology could be ascribed to the contradiction between the room required by the momentum obtained by the molten metal from droplet impingement and the space which could be provided by the FLP.