Role of annealing twinning in microstructural evolution of high purity silver during friction stir welding

Abstract

Friction stir welding (FSW) with a relatively large heat input was applied to high purity Ag with a low stacking fault energy, which leads to profuse twinning. The microstructural evolution was examined along the material flow path during rapid cooling FSW. The frequent formation of annealing twinning induced by the heating caused the microstructural refinement, following the transformation from twin boundaries to normal high angle boundaries. The bulging of the high angle boundary, which is one of the typical mechanisms of discontinuous dynamic recrystallisation did not occur. Irrespective of the initial grain size, the grain size obtained after the FSW was approximately 8 µm due to the dynamic balance between the grain boundary migration and the annealing twin formation.