BACKGROUND-The solid-phase joining of A6082-T6 plates by bobbin friction stir welding (BFSW) is problematic. Better methods are needed to evaluate the microstructural evolution of the weld. However, conventional Al reagents (e.g., Keller's and Kroll's) do not elucidate the microstructure satisfactorily, specifically regarding grain size and morphology within the weld region. APPROACH-We developed innovative etchants for metallographic observations for optical microscopy. RESULTS-The macrostructure and microstructure of A6082-T6 BFSW welds were clearly demonstrated by optical microscopy analysis. The microetching results demonstrated different microstructures of the Stir Zone (S.Z) distinct from the Base Metal (B.M) and Heat Affected Zone (HAZ) & Thermo-mechanical Affected Zone (TMAZ). The micrographs showed a significant decrease in grain size from 100 µm in B.M to ultrafine 4-10 µm grains for the S.Z. Also, the grain morphology changed from directional columnar in the B.M to equiaxed in the S.Z. Furthermore, thermomechanical recrystallization was observed by the morphological flow of the grain distortion in HAZ and TMAZ. The etchants also clearly show the polycrystalline structure, microflow patterns, and the incoherent interface around inclusion defects. ORIGINALITY-Chemical compositions are identified for a suite of etchant reagents for metallographic examination of the friction-stir welded A6082-T6 alloy. The reagents have made it possible to reveal microstructures not previously evident with optical microscopy.

Bobbin friction stir welding (BFSW), with its fully penetrated pin and double-sided shoulder, can provide high rates of heat generation. This produces solid-state thermo-mechanical grain refinement. In this paper, the microstructure evolution of the welded joints of AA6082-T6 obtained using BFSW process was investigated with a focus on grain refinement. Two sheets of the AA6082-T6 alloy were butt-welded with a fixed-gap bobbin tool. The microstructure at a mid-weld transverse cross-section was evaluated using optical microscopy and electron backscatter diffraction (EBSD). Significant grain refinement was observed, with a decrease in grain size from 100 µm in directional columnar grain morphology of the base metal, to an ultrafine size-less than 10 µm-for the equiaxed grains in the stirring zone. The EBSD results showed that with BFSW processing, secondary phase precipitation patterns were produced that are distinct from the primary artificial age-hardening precipitates created by the T6 tempering cycle. The severe plastic deformation and heat generation appear to accelerate dynamic recrystallization and precipitation during the BFSW process. The microstructural studies confirmed that the BFSW process can provide a highly efficient thermodynamically activated grain refinement in the solid-state without requiring additional processes such as heat treatment or external means of grain refinement.

Bobbin friction stir welding (BFSW) is an innovative variant for the solid state welding process whereby a rotating symmetrical tool causes a fully penetrated bond. Despite the process development, there are still unknown variables in the characterization of the process parameters which can cause uncontrolled weld defects. The entry zone and the exit zone consist of two discontinuitydefects and removing them is one of the current challenges for improving the weld quality. In the present research, the characteristic features of the entry and exit defects in the weld structure and formation mechanism of them during the BFSW processing was investigated. Using stacked layers of multi-colour plasticine the material flow, analogous to metal flow, can be visualised. By using different colours as the path markers of the analogue model, the streamline flow can be easily delineated in the discontinuity defects compared with the metal welds. AA6082-T6 aluminium plates and multi-layered plasticine slabs were employed to replicate the entry-exit defects in the metal weld and analogue samples. The fixed-bobbin tool utilized for this research was optimized by adding a thread feature and tri-flat geometry to the pin and closed-end spiral scrolls on both shoulder surfaces. Samples were processed at different rotating and longitudinal speeds to show the degree of dependency on the welding parameters for the defects. The analogue models showed that the entry zone and the exit zone of the BFSW are affected by the inhomogeneity of the material flow regime which causes the ejection or disruption of the plastic flow in the gap between the bobbin shoulders. The trial aluminium welds showed that the elimination of entry-exit defects in the weld body is not completely possible but the size of the defects can be minimized by modification of the welding parameters. For the entry zone, the flow pattern evolution suggested formation mechanisms for a sprayed tail, island zone and discontinuity-channel. For the exit zone a keyhole-shaped discontinuity is discussed as a structural defect.

One of the difficulties with bobbin friction stir welding (BFSW) has been the visualisation of microstructure, particularly grain boundaries, and this is especially problematic for materials with fine grain structure, such as AA6082-T6 aluminium as here. Welds of this material were examined using optical microscopy (OM) and electron backscatter diffraction (EBSD). Results show that the grain structures that form depend on a complex set of factors. The motion of the pin and shoulder features transports material around the weld, which induces shear. The shear deformation around the pin is non-uniform with a thermal and strain gradient across the weld, and hence the dynamic recrystallisation (DRX) processes are also variable, giving a range of observed polycrystalline and grain boundary structures. Partial DRX was observed at both hourglass boundaries, and full DRX at mid-stirring zone. The grain boundary mapping showed the formation of low-angle grain boundaries (LAGBs) at regions of high shear as a consequence of thermomechanical nature of the process.

Bobbin friction stir welding with a double-sided tool configuration produces a symmetrical solid-state joint. However, control of the process parameters to achieve defect-free welds is difficult. The internal flow features of the AA6082-T6 butt-joints in bobbin friction stir welding were evaluated using a set of developed reagents and optical microscopy. The key findings are that the dark curved patterns (conventionally called 'flow-arms'), are actually oxidation layers at the advancing side, and at the retreating side are elongated grains with a high-density of accumulation of sub-grain boundaries due to dynamic recrystallization. A model of discontinuous flow within the weld is proposed, based on the microscopic observations. It is inferred that the internal flow is characterized by packets of material ('flow patches') being transported around the pin. At the retreating side they experience high localized shearing at their mutual boundaries, as evidenced in high density of sub-grain boundaries. Flow patches at the advancing side are stacked on each other and exposed to oxidization.

Bobbin Friction Stir Welding (BFSW) is a thermomechanical process containing severe plastic deformation by mechanical stirring and Dynamic Recrystallization (DRX) during recooling. Here we report the three-dimensional characteristics of the micro-flow patterns within the aluminium weld structure. The Surface topography observations by Atomic Force Microscopy (AFM) show the stirred-induced microstructural evolution where the rearrangement of dislocations at the sub-grain scale, and the subsequent High- and Low-Angle Grain Boundaries (HAGBs, LAGBs) exhibit specific alterations in grain size and morphology of the weld texture. The dislocations interaction in different regions of the weld structure also was observed in correlation to the thermomechanical behaviour of the BFSW process. These micro-flow observations within the weld breadth give a new insight into the thermomechanical characteristics of the FSW process during the stirring action where the plastic flow has a key role in the formation of the weld region distinct from the base metal.

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The potential position for tunnel defect within the structure of bobbin-tool friction stir welds was studied by analogue modelling. The welding process was simulated on layered plasticine slabs instead, compared to the aluminum plates. Observations in the modelled structure showed a high possibility for a continuous channelled discontinuity, like a tunnel-shaped void defect, in the entry zone of the weld, which mirrors the metal welding. The anatomy of tunnel defect in the entry zone was explained according to the mechanics of material during the plastic deformation process.