## 1 Introduction  
The, 6 XXX series of aluminum alloys are widely used in different fields because of their high strength-to-weight ratio [1; 2]. These components are often fabricated into industrial structures by fusion welding processes [2]. Gas metal arc welding (GMAW) is an important fusion welding method of joining aluminum alloys [3]. In GMAW, the weld profile and microstructure are strongly affected by the molten pool behavior [4], which in turn is strongly affected by various driving forces, such as the arc pressure, plasma shear force, Marangoni force and electromagnetic force (EMF) [4; 5]. However, in GMAW, the molten pool behavior is more complex because it involves the impingement of molten droplets into the weld pool, relative to gas tungsten arc welding which has no droplet impingement [4; 6]. [PERSON] and [PERSON] [6] found that a detached droplet is accelerated by the plasma arc during its flight toward the workpiece. When a droplet was transferred into the molten pool, the flow pattern was mainly determined by the droplet impact [4; 7]. [PERSON] et al. [8] simulated two different metal droplet velocities and found that the weld penetration was primarily determined by the metal droplet impact force. Moreover, by developing a unified model considering both the heat and mass transfer in the electrode, arc plasma and molten pool, [PERSON] and [PERSON] [9] found that the penetration of the -weld pool was mainly determined by the impingement of droplets. After droplet impingement, a \"crater\" is created in the weld pool [10], and the finger-like penetration (FLP) geometry of the weld bead is formed [8; 11]. As shown in the lower right corner of Figure 1, the weld profile with the FLP in the transverse section was characterized by a shallow and wide penetration in the upper region, labeled zone A in Figure 1, and a deep and narrow penetration in the lower weld [12], labeled zone B in Figure 1. Zone B is known as FLP.  
Although the geometric characteristics of the FLP are well recognized, the grain morphology of this type of weld bead has not been fully investigated. Previous research has mainly focused on various methods for the refinement of grains, such as ultrasonic stirring [13], pulsed ultrasonic stirring [14], electromagnetic stirring [15], and arc oscillation [16], and grains in the central part of a weld bead which were often set as the focus and evaluated indicator for the validity of a method. This may be because grains along a weld wall have been widely recognized as having a columnar morphology [17; 18]. However, a weld bead with FLP characteristics displays a quite different result in this study. So far, the difference in grain morphology between the walls of the FLP and the walls of the shallow region has not been reported, nor has the mechanism causing the variation.  
As mentioned above, droplet impingement plays a significant role in determining the flow pattern, temperature distribution and resultant weld profile and grain morphology. In this study, we focus on the weld profile and grain morphology of aluminum alloys based on droplet impingement with the welding torch held perpendicular to the base metal. Subsequently, to investigate the effect of the impingement direction, the welding torch is held at a 10\({}^{\circ}\) inclination in the transverse section. Finally, to further understand how droplet impingement is affected by penetration depth, the double pulsed-gas metal arc welding (DP-GMAW) method, which can produce both deep and shallow penetration in a single travel, was employed.