## 1 Introduction  
Aluminum alloys have been widely used in manufacturing aeronautical and aerospace structures thanks to their high specific strengths and moderate cost. Gas tungsten arc welding (GTAW) is one of the main welding processes employed in welding such structures of aluminum alloys, within which complex three-dimensional (3D) weld paths may exist [1, 2]. In GTAW of such 3D welds, it is often impractical to realize and maintain the conventional flat welding position by manipulating the structures for fixed arc because of their large dimensions and/or weights. Instead, the posture of the welding torch is manipulated during welding according to the trajectory of a weld path while the structure is fixed and welded. Under such conditions, other welding positions, besides the flat welding position, are inevitable. However, it has been found that weld formation defects, such as poor bead surface appearance, inconsistent width and depth, undercuts, etc. are more apt to occur for welding positions (e.g., vertical welding) other than the flat position. This becomes even more serious when the aluminum alloy sheets are thick (with thickness greater than 5.0 mm) [3]. The welds with poor formation quality need be repaired, or even scraped when the defects are too serious, which then lead to increase in cost and waste of materials. Hence, it is imperative to investigate the causes and possible solutions to the poor weld formation in the GTAW of a thick-sheet aluminum alloy structure using varying welding positions. As we know, the weld formation is closely related to the fluid flow behavior of molten metal in weld pools [4; 5], which can be affected by the welding positions taken during welding. The horizontal welding position has been demonstrated to be more effective than flat welding position in reducing the weld formation defects of undercut and sagging when welding a thick high strength steel by laser beam [6]. This was attributed to the more desirable fluid flow state for the horizontal weld pool that resulted from the joint action of surface tension, recoiling pressure and gravity [7]. In contrast, in laser welding of Ti6 Al4V titanium alloy, the flat welding position led to better weld profiles than the horizontal welding position, in terms of weld undercut, excess weld metal, and excess penetration [8]. In addition, for vertical welding positions, the undercuts of vertical up welds were greater than that of vertical down welds for the same welding parameters. Burn-through holes and inconsistent weld bead widths were formed in vertical up welds, which were absent in vertical down welds. All these phenomena were believed resulted from the different actions of gravity on the weld pool metals for different welding positions [9]. For gas metal arc welding (GMAW) of thick steel plates, it has been revealed through both numerical modeling and experimental observation that the welding positions (flat, overhead, and vertical) could significantly affect the fluid flow behavior and free surface profile of weld pools, which in turn result in different weld formation quality, in terms of humping, overflow, etc., due to the action of gravity [10; 11; 12; 13]. Numerical modeling by [PERSON] et al. [14] indicated that completely different flow patterns might result from the different orientations of specimens in GTAW of the 6061 aluminum alloys. Studies by [PERSON] et al. [15; 16] showed that the gravitational orientation could considerably influence the solidification microstructures in terms of the primary dendrite spacing in GTAW of an Al-Cu alloy. Despite existing research carried out on the influence of welding position on the welding quality in welding of steel and titanium alloys via laser and GMAW welding processes, no research has been reported so far on the characteristics of fluid flow behavior for different welding positions and their relationship with the weld formation quality in GTAW of thick aluminum alloy sheets.