## 1 Introduction  
Friction stir welding (FSW) is a solid-state joining method whereby instead of melting the material, a rotating tool ploughs through the interface of the two plates to be joined. The bonded structure is obtained by mixing the material, softened by heat and stirring, generated from friction between the rotating tool and the workpiece [1]. There is a class of FSW tools called bobbin tools [2; 3; 4], consisting of two shoulders, one on each side of the workpiece surfaces, connected by the tool pin (Figure 1). Recent welding trials using this symmetric tool configuration (comprises two shoulders connected by an axial pin) have highlighted some of the benefits for a fast and less complicated process, compared to the conventional FSW [5]. One of the critical problems in bobbin friction stir welding (BFSW) is to provide an applicable instrument for optimization of the joining parameters and their side-effects on final quality of the weld [6]. This situation is crucial for bobbin-tool technique as the process settings are still in the early stages of study. The general area of interest for the BFSW process is welding of light-alloys sheets, where there are many applications for this in marine, automotive, and aerospace fabrication processes [7; 8; 9]. Since, existence of defects can directly affect weld properties, making a reliable weld by the FSW process, in different situations and with different parameters, is difficult. Existence of a void as a defect is one of the challenges of work during bobbin-tool FSW process [10; 11]. The objective of this research is to elucidate the origins of the formation mechanism of a typical tunnel void in the weld region. Identification of the influential phenomena in the creation of a void is required in developing a multi-physics model, based on the principles of fluid mechanics. Bobbin-tool FSW appears to be a thermo-mechanical process as the frictional heat combined with the high plastic deformation is used for the joining process during the rotation and movement of the bobbin tool [12; 13; 14]. Also, changing the parameters of the process (e.g., feeding rate and rotational speed), or features of the bobbin tool (size and geometry of pin and shoulder), or the thickness and material properties of the weld plates may lead to unknown origin defects in the weld region [10; 11]. In other words, there is no solid theory about the parameters of the process and method of control to achieve the optimum machine settings. Developing a comprehensive theory describing the process will extend the range of applicable process parameters and overall productivity. Thus, a transition from trial and error to a science-based approach is required to link the welding mechanism and operational variables. For the first step in the investigation of weld region voids, it is intended to study the internal material flow regimes and how the failure of the flows causes weld defects [15]. However, the nature of the process makes it difficult to freeze the actual flow to validate a model [16; 17; 18]. In order to present an accurate multi-physics approach, near to real circumstances of the BFSW process, the basic interactions of the process parameters should be known in advance. Using an analogue model of the process and elucidation of relationships between effective parameters in creation and elimination of a void (tool features, process parameters and material capabilities), we can find the prerequisites of a numerical solution to effectively model the multi-physics of the situation. Our starting hypothesis for BFSW joining is based on a thermo-mechanical context of the process, as the solid-phase joining is simultaneously a result of plastic deformation and material displacement. Instead of aluminum plates, analogue modelling of FSW was performed using colored layers and grids of plasticine. This provides easier flow visualization compared to the metallographic measurements of the aluminum welds.