## 3 Results and Discussion
### Shallow Penetration with an Inclined Torch  
When the welding current entered into the small current phase with an inclined welding torch (case 3), the weld penetration depth decreased from 1.785 mm to 1.259 mm, whilst the width increased slightly from 2.849 mm to 2.871 mm compared with case 2, as shown by the dimensions of the FLP in Figure 3a for case 2 and Figure 4a for case 3. This significant decrease in penetration could be ascribed to the change in flow pattern in the molten pool, as schematically illustrated by Figure 4b. The figure shows that the trajectory of flow driven by droplet impingement was split into two branches at the SIP when the penetration depth decreased: (i) a branch flowing downward to the bottom, as depicted by the red curved arrows which represented the signs of the trajectories created by repeated droplet impingement (Figure 4c), and (ii) another branch flowing upward along the wall of the shallow level, as indicated by the gradually enlarged grain size with increased distance from the SIP (Figure 4d), which could also be interpreted as the outline of many droplet impingements. This change in flow pattern may have resulted from the constantly varying contradiction between the space provided for liquid flow and the space required by the momentum of flow propelled by droplet impingement. When the droplet impinged into the weld pool, it impacted and peeled the walls and then crashed against the bottom, as in case 2. However, because of the reduced penetration depth, the room supplied was far less than required, instantly intensifying the contradiction dramatically. As a result, the shrunken FLP region could no longer hold the violent flow, which caused molten metal to overflow the FLP region [6; 24], as schematically illustrated by the dashed circle arrow in Figure 4b. The overflowing effect could be proven by the width-to-depth aspect ratio [7]. In this study, the FLP aspect ratio increased from 1.596 to 2.281, an increase of 42.9 percent, due to the width being enlarged slightly, whereas the penetration was dramatically reduced by 41.7 percent compared to case 2, as shown in Figure 4e. The overflowing fluid then further exerted its influence on the trajectory of the impinging droplets by pushing them aside to the upward direction (Figure 4b). In the shallow level, the fluid flowed upward along the wall driven by electromagnetic forces (EMF) [25]; this flow may have acted as an inducing flow for droplet impingement. Hence, once pushed aside, the droplets were more involved in, and ultimately converged with the upward flow driven by EMF, as indicated by upward black dashed arrows. The upward flow shaped the grain morphology along the wall into three stages (Figure 4d): large size before the impact point (IP), reduced size at the IP and in a near neighboring region, and large size again far after the IP. The variation in the grain size with increased distance from the SIP, as illustrated by Figure 4f, may have resulted from the corresponding change in flow magnitude [3]. Different regions suffered different order-of-magnitude impacts and heat inputs from the droplets. The area before the IP suffered slightly and then the grain grew up, whereas the size of the grains at the IP decreased sharply to approximately half of that in the area before the IP, as shown by the local magnified image in Figure 4a and the optical micrograph in (Figure 4f,) which may have been because this area was impacted directly and heavily by droplet impingement, which carried abundant heat and momentum. From another perspective, the shortened columnar grains in the IP region along the upward wall (Figure 4d) provided evidence that droplet impingement had the ability to refine  
Figure 4: Welding torch inclined from the center line in the transverse section. (**a**) Molten pool profile; (**b**) schematic diagram of the molten pool behavior; (**c**) signs of downward flow; (**d**) signs of upward flow; (**e**) width, depth and width/depth ratio of the FLP; (**f**) columnar grain size along the upward wall.