## 1 Introduction  
[PERSON] et al. [1] and [PERSON] et al. [2] found that pulsed gas metal arc welding (P-GMAW) decreased the spatter and reduced the occurrence of incomplete fusion defects while providing a lower heat input and achieving high penetration. The P-GMAW process incorporated the benefits of axial spray transfer of one pulse one drop (OPOD) of GMAW, which improve the stability and quality of the welding process. A significant effort was made to analyze the effect of pulsing parameters on the metal transfer behaviors and optimize the pulsing parameters to achieve the desirable metal transfer mode of OPOD [3, 4]. The heat transfer and fluid flow of a molten pool can significantly influence properties such as weld pool geometry, temperature gradients, local cooling rates, and solidification structure. The driving forces for fluid flow in weld pools include the buoyancy force, electromagnetic force, surface tension gradient at the molten pool surface, and the impinging force of the arc plasma [5]. The observation of molten pool behavior presents an effective way to understand how to control the welding process and obtain valuable information for the formation of weld bead shape, which helps in the evaluation of welding quality. Extensive research has been conducted with the aim of monitoring the weld pool in arc welding [6, 7, 8]. [PERSON] et al. [9] correlated the weld penetration, weld bead width, and weld torch position using infrared images. [PERSON] and [PERSON] [10] used ultrasonic signals to estimate sidewall penetration. [PERSON] [11] reviewed the use of three-dimensional weld pool measurements. The 3D vision sensing system was proposed in order to measure the specular 3D weld pool in gas tungsten arc welding (GTAW), and [PERSON] et al. [12] measured 3D weld pool surface with weld joint penetration. [PERSON] et al. (2019) developed a neuro-fuzzy inference system based on three characteristic parameters for observation of the molten pool. Gravity influences the performance of welding, which affects molten pool behavior; hence, it is necessary to understand the gravity effect on weld pool convection, weld pool dimensions, microstructure evolution, and mechanical properties of the weld structure. [PERSON] and [PERSON] (2014) accomplished laser welding on 316 stainless steel in a simulated space environment. They determined that wide bead width and deep penetration were obtained in an environment with reduced gravity force. They also noticed an increase in porosity and suspended weld particles in such reduced gravity force conditions that may have led to a weaker weld joint. [PERSON] et al. (2015) studied the fusion zone shapes and profiles of weld beads in 304 stainless steel under simulated high gravity force. They found that the aspect ratio of the weld fusion zone at 5 G is smaller than that at 1 G. [PERSON] et al. (2016) performed welds related to different gravity directions such as vertical up position, vertical down position, and flat position. They found that the vertical up position caused 21% deeper penetration than the vertical down position. [PERSON] et al. (2017) stated that the direction of gravity significantly influences the weld pool shape associated with convection flows, which affect solidification morphology and primary dendrite spacing. The knowledge of how welding process parameters affect weld bead geometry is important because it can be applied to welding processes where optimal selection of input parameters is required for high productivity. [PERSON] et al. (2018) studied the weld bead geometry of beads on plate welds deposited using the flux-cored arc welding technique and showed that the percentage of dilution slightly increased with an increased welding current. [PERSON] et al. (2019) found that for a bead on plate welds of A36 steel produced using a single-wire process, the welding current had a strong influence on the size of the heat affected zone, with the size decreasing with the increasing welding current. [PERSON] et al.