Abstract
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.
Highlights
Friction stir welding (FSW) was introduced by the welding institute (TWI) [1,2] as a solid-state joining process whereby a rapidly rotating tool is physically moved along the interface of two plates [3,4].As the weld-line bond is formed, the material from the advancing side (A.S) and retreating side (R.S)mix together to form the bonding layer [5], in a solid phase mixing process
Bobbin friction stir welding avariant variantwhere wherea asymmetrical symmetricaltool tool(consisting creates a fully penetrated butt joint by stirring and mixing of the plasticised connected by the tool pin) creates a fully penetrated butt joint by stirring and mixing of the plasticised material butt-joint position is shown in Figure material [9,15,16]
Besides demonstrating the recrystallized microstructure of the weld region in the S.Z, we focus on a thermomechanical comparison of grain size, morphology, and flow distribution in B.M, Heat Affected Zone (HAZ) and Thermo-mechanical Affected Zone (TMAZ) and S.Z different regions of the weld
Summary
A material that would be attractive to weld with the FSW method is aluminium A6082-T6 This is a useful marine grade aluminium alloy with excellent mechanical properties, good thermal weldability, good machinability, and excellent corrosion resistance [6]. One of the obstacles to better understanding the poor weldability is the difficulty of visualizing the microstructure of the weld region [10,11,12,13] This is problematic as the material responds poorly to known etchants, it is difficult to visualize the grain boundaries and the internal flow features [7,8,10]. The solid-phase joining of A6082-T6 plates by bobbin friction stir welding (BFSW) is problematic.
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