EBSD Characterization of Bobbin Friction Stir Welding of AA6082-T6 Aluminium Alloy

Abstract

Electron Backscatter Diffraction (EBSD) was used to determine microstructural evolution in AA6082-T6 welds processed by the Bobbin Friction Stir Welding (BFSW). This revealed details of grain-boundaries in different regions of the weld microstructure. Different polycrystalline transformations were observed through the weld texture. The Stirring Zone (SZ) underwent severe grain fragmentation and a uniform Dynamic Recrystallisation (DRX). The transition region experienced stored strain which changed the grain size and morphology via sub-grain-boundary transformations. Other observations were of micro-cracks, the presence of oxidization, and the presence of strain hardening associated with precipitates. Flow-arms in welds are caused by DRX processes including shear, and low and high angle grain boundaries. Welding variables affect internal flow which affects microstructural integrity. The shear deformation induced by the pin causes a non-uniform thermal and strain gradient across the weld region, leading to formation of mixed state transformation of grain morphologies through the polycrystalline structure. The grain boundary mapping represents the differences in DRX mechanism I different regions of the weld, elucidates by the consequences of the thermomechanical nature of the weld. The EBSD micrographs indicated that the localised stored strain at the boundary regions of the weld (e.g. flow-arms) has a more distinct effect in emergence of thermomechanical nonuniformities within the DRX microstructure.

Internal Flow Behaviour and Microstructural Evolution of the Bobbin-FSW Welds: Thermomechanical Comparison between 1xxx and 3xxx Aluminium Grades

“…The EBSD results confirmed that for the Al alloys with the FCC structure [37] the precipitation occurred at two positions: the HAGBs between the main grains and also within the grain at the LAGBs. The increased density of the participate particles deposited at the LAGBs contributes to strain hardening as the blocking barriers of dislocation movement.…”

“…The comparison between the microstructures shows that the grain morphology has a similar trend in the coarseness of the morphological features with the increase of the welding speeds. Although the dynamic recrystallized structure was formed for all the samples, the differences in apparent coarseness of the grain size indicate more severe plastic deformation has been increased by elevating the welding speeds [27,37]. Figure 9c and Figure 10c demonstrate a morphological comparison of the AA1100 and AA3003 samples in the maximum speeds (600 rpm and 400 mm/min), respectively.…”

“…The EBSD analysis can also denote the crystallographic misorientation information for the deformed and recrystallized grains texture. For the polycrystalline texture the main grainboundaries are recognised as the High-angle grain boundaries (HAGBs) threshold with a misorientation angle of >15 degrees [37]. Furthermore, the sub-grain scale alteration of the crystallographic orientation within the grains can be indicated by the sub-grain boundaries as the Low-angle grain boundaries (LAGBs) threshold.…”

3D-Printed Tool Shoulder Design for the Analogue Modelling of Bobbin Friction Stir Weld Joint Quality

“…Consequently, by the simultaneous pin action localized at the mid-SZ, the dissimilar joining bond layer forms at the interface of the Aluminium and Copper. It should be noted that the interruption in the refilling process disintegrates the deposition of the plastic layers transferred to the backing edge of the tool [31]. Therefore, the failure of the intermixing causes the flow discontinuity in the form of the void defect.…”

Dissimilar Friction Stir Welding of AA2024 and AISI 1018: Microstructure and Mechanical Properties

“…These IMCs deteriorate the mechanical properties [5] and lead to the rapid fracture of the joints [7]. Recently, new studies on friction stir welding (FSW) have been widely increasing with the focus on the joining of dissimilar metals [3,8], whereas pair metals in automotive and manufacturing industries are highly demanded. The FSW offers numerous advantages such as enhanced fatigue resistance, tensile properties, process quality, and reduced IMC formation, in addition to lower operating costs, energy efficiency, and being environmental and health friendly [9][10][11][12][13].…”