**1. Introduction**

Bobbin friction stir welding (BFSW) is an innovative variant of the friction stir welding (FSW) process [1], whereby a double-sided rotating tool physically ploughs along the interface of two butted plates [2] without needing the backing anvil and the axial force during the process [3]. The dynamic interaction between the workpiece and the non-consuming tool creates a severe friction condition at the contact interface [4]. Consequently, significant heat is generated due friction which can locally soften the workpiece material sufficient for plastic yielding and stirring at the bonding track [5]. The stirring action arises from the rotation speed (ω) and advance speed (*V*). While the tool advances along the weld line (Figure 1), the mutual interaction of the speeds (ω*, V*) transports the softened mass from leading edge of the rotating tool to be deposited at the rear or trailing edge of the tool [6].

The side of the weld-seam where the direction of tool rotation is the same as the tool progress is called the advancing side (AS) and the opposite side of the weld-seam is the retreating side (RS) [7]. The region located between the AS and RS borders, named the stirring zone (SZ) [8], experiences a thermomechanical plasticizing and then deposition and consolidation in the weld locus [9]. The adjacent region outside the SZ is called the Thermo-Mechanical Affected Zone (TMAZ). The microstructure of this region is formed by the stress-strain fields and heating flux which are induced by the friction and heat generation effects of the stirring action [10]. The next region between the TMAZ and the Base Metal (BM) is the Heat Affected Zone (HAZ). This region is exposed to thermal fields of the stirring process which alter the microstructure [11].

**Figure 1.** Schematic of the BFSW process for a butt-joint position.

Comparing with the Conventional-FSW (CFSW), in the BFSW the fully-penetrated pin requires more control during the process, as an inconsistency between the process variables can cause more severe failures. The double opposing shoulders system provide a greater contact surface for the frictional heat generation from both sides of the workpiece [9]. The process also replaces the CFSW backing or anvil support plate with the BFSW rotating shoulder at the lower side of the workpiece [12,13]. The CFSW case requires a downward axial load [14] on the tooling, whereas the BFSW requires a compression ratio [15] (the variance between the inner edge-to-inner edge biting gap of the shoulders and the actual thickness of the plate). These differences cause differences in the flow regimes of the two processes [16,17]. In the FSW literature, the onion rings in the conventional-FSW weld structure have been attributed to discontinuous flow during tool-material interaction [18].

As the temperature in FSW processes is lower than in fusion welding, it is categorized as a metal forming process [19]. The internal flow regimes related to the plasticized mass play the main role in the welding mechanism rather than metallurgical transformations of melt-and-solidification. In general, the FSW technique is proposed for materials with a high capability of the dynamic plastic deformation. Aluminium that responds well to large plastic deformation is a good candidate material for FSW. In particular, marine grade AA6082-T6 aluminium alloy with good machinability properties would be attractive to be processed by the BFSW, but shows poor weldability in the conventional fusion welding. One of the obstacles to a better understanding of the actual flow regimes is the need to visualise the details of the flow features for the cross section of the weld [8,20]. This is challenging for AA6082-T6 alloy as the weld region responds poorly to conventional etchant reagents [21]. This problem arises because of the low contrast between grains and grain boundaries for the AA6082-T6 microstructure where the precipitate particles are uniformly dispersed within a supersaturated solid solution treated by artificially ageing per the T6 cycle. Also, severe plastic deformation and grain fragmentation during the BFSW process reduce grain size to ultrafine.

While there is an extensive literature on the microstructural characteristics of the FSW welds [22], the flow mechanism also needs to be identified within the weld structure [23–25]. In this regard, there have been attempts to elucidate the heat flow [26] and material flow [27] mechanisms in BFSW, as it is expected to be different to conventional-FSW [28].

The aim of this work is to identify the causality between flow regimes and physical defects. The approach is to visualise plastic deformation features of AA6082-T6 BFSW welds with new etchants [21], using optical metallography. These reagents show the details and complexity of the plastic flow patterns within the stirring zone, even when there is an element of grain refinement due to the thermomechanical plastic deformation. A benefit of this approach is achieving a detailed microstructural analysis with conventional etching methods and optical metallography, rather than the more costly processes of electropolishing, or electron metallography (e.g., SEM, EBSD, and TEM).

## **2. Materials and Methods**

For the weld trials, rolled plates of AA6082-T6 (Al–Si–Mg–Mn family) were used as the workpiece. The chemical composition of the AA6082-T6 aluminium alloy as the base metal is reported in Table 1.


**Table 1.** Element composition of the AA6082-T6 aluminium alloy (wt %). Data from [29].

BFSW tests were conducted using a geometrically full-featured (including threads, flats and scrolls) bobbin tool manufactured from H13 tool steel with hardness of 560 HV. The butt-joint weldments comprised two pieces of similar plates with dimensions of 250 mm (length) × 75 mm (width) × 6 mm (thickness). The welding trials involved two sets of operation speeds; spindle rotational speed (ω) and weld travel rate (*V*).

The aim of the research was to identify the flow features of the BFSW weld in a defect-free structure compared with the presence of defects. A sample of each condition was procured. The defective weld was produced with welding parameters of ω = 400 rpm, V = 350 mm/min. The good weld was produced at ω = 650 rpm and V = 400 mm/min. The same tool was used in both cases. These process parameters were identified from previous work [30]. That reference also describes the tool geometry in detail, see also Table 2.

**Table 2.** Parameters of the BFSW process for the AA6082-T6 weld trials.


The BFSW experiments were performed on a 3-axis CNC machining centre (2000 Richmond VMC Model, 600 Group brand, Sydney, Australia) with a Fanuc control unit and 14-horsepower spindle motor capacity. There were no preheating or post-weld processes before or after the welding process. The direction of tool rotation was clockwise relative to the advancing direction of the welding (Figure 1). Table 2 gives more details of the bobbin-tool and the welding operation. After welding the quality of joints were first checked by visual examination, and then cross sectioned by an electro-discharge

machine wire cut through the middle of the weld seam (perpendicular to the welding direction). The resulting surfaces were subjected to metallographic measurements. For metallographic analysis, firstly the specimens were prepared using standard mechanical polishing with different grades of SiC sand papers (600-grit, 800-grit and 1200-grit). To achieve a mirror surface, the micro-polishing step was conducted on a micro-cloth pad with a 3 μm diamond paste, and finally a 0.05 μm colloidal silica solution. The etching process was designed to remove the oxide film, and then delineate the flow lines. The polished specimens were first pre-etched for 3 min in a solution of (5 g NaOH + 1 g NaCl + 80 mL H2O) at 70 ◦C. The developer mixtures are shown in Table 3, which describes the composition and other conditions of the chemical solutions (time and temperature). After completion of the etching process, the samples were rinsed in ethanol, and then dried with warm air. The flow patterns of the BFSW weld region were studied using stereoscopic and light optical microscopes (Olympus Metallurgical Microscope, Tokyo, Japan). In some cases, where the microscopic features needed to be clarified in more detail, the cross-section sample was re-polished and re-etched with other reagents. For some metallurgical validating, the etched samples were also subjected to elemental mapping using the scanning electron microscope (SEM) (JEOL 6100, JEOL Inc., Peabody, MA, USA) equipped with energy-dispersive X-ray spectrometer (EDS) detector (Oxford Instruments plc, Abingdon, UK).


**Table 3.** Different reagent compositions with separate sequences of processing.
