Experimental measurement of strain distribution in the nugget zone during friction stir welding using an identical-material tracer

Quantifying local strain distribution within the nugget zone during Friction Stir Welding (FSW) remains experimentally challenging because of severe plastic deformation and complex three-dimensional material flow. This study presents an identical-material tracer insert technique in which an anodized AA5083 foil was embedded within an AA5083 workpiece, enabling direct experimental strain measurement while eliminating the thermo-mechanical property mismatch associated with dissimilar tracers. FSW was performed at a rotational speed of 1025 rpm and a traverse speed of 67 mm/min. Based on tracer deformation morphology and logarithmic thickness-reduction analysis, the nugget zone was divided into three distinct sub-regions: the shoulder-affected zone (SAZ), compressive zone, and pin-affected zone (PAZ). The maximum strain values in these regions were approximately 2.0, 0.4 (compressive), and 1.1, respectively. Electron backscatter diffraction analysis revealed progressive grain refinement from the SAZ (13.0 ± 4.2 μm) to the PAZ (5.0 ± 1.8 μm), while the high-angle grain boundary fraction increased from 68.5% to 89.3%. Despite experiencing the highest strain, the SAZ exhibited comparatively coarser grains because of greater thermal exposure from the rotating shoulder. The observed microstructural evolution was governed by the combined effects of strain and thermal exposure, with continuous dynamic recrystallization playing a dominant role in grain refinement. The results establish a clear quantitative relationship between local strain heterogeneity and recrystallization behaviour, demonstrating that the identical-material tracer technique provides a reliable and physically representative approach for strain quantification during FSW.