## 3 Results and Discussion
### Welding Torch Inclined in the Transverse Section  
The profile of the weld pool was significantly different when the welding torch was set at an angle of 10\({}^{\circ}\) from the center line in the transverse section (case 2), with the disappearance of the SIP on the non-action side (Figure 3a). This phenomenon possibly arose because when the welding torch is inclined, heat input becomes asymmetric about the longitudinal central plane, outlined in References [24; 25], as shown in Figure 3b. Because of the asymmetric heat input, more base metal melted on the left side of the axial line of the welding wire than on the right side, causing the bottom to be uneven. Thus, when the droplet impacted the uneven bottom of the weld pool, the more molten region (in this case, the left side) had a considerably lower resistance to the impingement because of its liquid state than the solid-state region on the right side [11; 26]. Thus, instead of impacting both sides at the same order of magnitude, as in case 1, the droplet impacted harder on the left side due to its weak resistance. The original uneven solid bottom became more uneven because of the additional hard impact and heat input from droplet impingement. Next, an asymmetric crater formed because the left side bottom was considerably lower than the right side bottom; this crater could serve as a confined channel to redirect the droplet impingement [27]. Then, when another droplet impinged, it tended to crash heavily against the solid walls on the impact action side (IAS), thus melting and paring the solid-state wall, whilst flowing downward until finally smashing against the bottom. The dimensions of the FLP were measured with 2.849 mm in width and 1.785 mm in depth, and the width-to-depth aspect ratio was 1.596. Next, the boundaries were expanded downward in the same mode, as depicted by the black dashed lines in Figure 3b. However, there was no crashing against the other side wall during the entire downward flow process because its liquid state facilitated the flowability for droplet impingement. This lack of crashing may have accounted for the remnant SIP on the IAS, and the disappearance of SIP on the non-impact action side (NIAS). Figure 3: Welding torch inclined from the center line in the transverse section. (**a**) Molten pool profile with the sharp inflection point (SIP) not present on the non-impact action side (NIAS); (**b**) schematic diagram of the molten pool behavior; (**c**) columnar grain on the NIAS in the FLP level; (**d**) shortened columnar grain on the impact action side (IAS) at the shallow level. Figure 3c,d demonstrates the grain morphology distribution characteristics at different positions of the weld pool walls. The grains along the wall of the IAS in the FLP, where droplet impact occurred, were refined with no discernible difference from the same area in case 1. However, the columnar grains emerged along the walls of the NIAS in the FLP (Figure 3c), whereas refined grains were observed at the same position in case 1. This phenomenon may have occurred because both the momentum and heat obtained from the superheated droplets were consumed most at, and shortly after crashing against the wall of the IAS until they reached the bottom. This interpretation is supported by Figure 3d, which clearly demonstrates the variation trend of columnar grains in the bottom region. The figure shows that the columnar grains with small sizes emerged in the bottom region (zone A in Figure 3d) because of the weak existence of droplet impingement. Afterward, when the fluid flowed to the NIAS region, the droplet-impingement-driven-flow had nearly exhausted of all its momentum and heat, losing its ability to run violently throughout the FLP region, thus providing the grains in this region with the opportunity to grow without interruption into a columnar morphology with greater size in the NIAS region (zone B in Figure 3d).