Microstructure characteristics and crystallographic orientation in dissimilar friction stir welding

This study examines the microstructural properties such as grain structure, grain boundary misorientation, and crystallographic texture of friction stir welding (FSW) joints under two cooling media (air and water cooling). The microstructural evolution of FSW joints was characterized using light and electron microscopy techniques. Specifically in electron microscopy, inverse pole figure maps, pole figures, and orientation distribution functions were evaluated for both conditions of FSW joints using Electron Back Scattered Diffraction (EBSD). The water-cooling FSW (WCFSW) joint exhibited the smallest grain sizes at the stir zone (SZ) location compared to the corresponding condition in the air-cooling FSW (ACFSW) joint. EBSD analysis revealed that grain size in the SZ was about 7.6 µm in the ACFSW joint and further refined to 3.2 µm in the WCFSW joint. The misorientation angle distribution of water cooling conditions generated high-angle grain boundaries. The main texture components observed in the FSW samples were Brass [{110} < 112>] and Goss [{110} < 001>]. Additionally, Orientation Distribution Function (ODF) analysis found a higher texture intensity in water-cooled joints than in air-cooled joints. The WCFSW joint also had higher ultimate tensile strength values and a faster strain hardening rate. This is because the grains were smaller, more dislocations were created, and suitable texture components and intensity were favorable.