## 4 Discussion  
In GTAW of the thick-sheet aluminum alloys structure shown in Figure 1, the welding position changes from the climbing position to the flat position. The fundamental difference between these two positions is the orientation of weld pool relative to gravity, as shown in Figure 13. In climbing welding, the gravity action on the weld pool has two components, one is perpendicular to the sheet surface (in sheet thickness direction) and the other is parallel to the sheet surface (lateral direction). In contrast, when flat welding, the gravity acts on the weld pool only in the sheet thickness direction, while the component parallel to the sheet surface is zero. Such a difference in the gravity action direction has a notable influence on the flow of molten metal in the weld pool and weld formation quality as revealed in this study, which is similar to those reported for the welding of steel and titanium alloys by laser and GMAW welding processes.  
For flat welding, the metal flows dominantly downwards, and the velocity magnitudes are largest in the middle region of the weld pool. Such a flow pattern indicates a high penetrating capability, which is very critical in welding thick sheets. Excessive downward movement of molten metal at the bottom, however, may lead to weld sagging and should therefore be avoided, as found for not only the aluminum alloy in this study, but also steels and titanium alloys reported in other research [6; 8]. For climbing welding, fluid flow is enhanced in lateral direction (afterwards), which results in a longer weld pool and makes the molten metal apt to flow out of the weld pool and form uneven weld surface. Meanwhile, the velocity is decreased in thickness direction, which leads to a weakened heat transfer between the top and the bottom portions of the weld pool and a decreased penetrating capability. This is manifested by the higher temperatures on the top portion and a smaller high-temperature  
Figure 12: Weld formation for two welding positions: (**a**) climbing welding; and (**b**) flat welding.  
Figure 13: Orientations of weld pools relative to gravity for the two welding positions: (**a**) climbing welding (start end); and (**b**) flat welding (finish end).  
region (with temperature higher than 909 K) at the bottom part of the climbing weld pool. Both the accelerated lateral metal outflow and the decreased penetrating capability could make the weld pool unstable, and finally lead to inconsistent weld bead when climbing welding.  
To obtain good weld formation in climbing welding, it is therefore necessary to maintain enough penetrating capability and avoid excessive lateral flow of molten metal. One possible measure that may be taken is to employ an electrode with a sharper tip (smaller tip angle), which may generate a more concentrated arc and higher arc pressure, and then lead to improved weld pool stability and better weld formation. For welding with constantly changing welding positions, it may also be necessary to adjust manually or automatically the welding parameters (e.g., welding current and voltage) based on the on-line monitoring of the penetration state during welding process. Moreover, high-energy laser beams can be employed in combination with the tungsten arc to form a more penetrating and robust hybrid welding process in welding thick aluminum alloy sheets with changing welding positions.