Microstructure evolution of Cu–30Zn during friction stir welding

The microstructure evolution of Cu–30Zn during the deformation and cooling stages of friction stir welding was separately investigated by employing the in-process rapid cooling, tool “stop action”, and subsequent short-time annealing. A pure copper foil was inserted into the butting surfaces of the two workpieces to show the microstructure evolution path. The microstructure along the material flow path was investigated by the EBSD technique. At the weld’s upper part, continuous material flow occurs nearly over the whole range of the shoulder. The initial coarse grains are refined by the discontinuous dynamic recrystallization (DDRX) accompanied by the annealing twinning. During the material flow, the grain structure evolution is dominated by the annealing twinning during the thermally activated grain boundary migration and the subsequent twin destruction due to further deformation, resulting in a nearly constant grain size. Finally, normal grain growth occurs at the cooling period. At the lower part, the material transfers in a very thin layer near the probe surface and rapidly forms stable band structures. Due to the lower heat generation and higher strain rate, the mechanical twinning occurs in front of the probe. These deformation twins can provide additional nucleation sites for the DDRX via the twin destruction caused by further deformation. The higher strain rate in the weld’s lower part contributes to the finer grains than that of the upper part grains. However, due to the shoulder’s coverage, the lower part undergoes a longer cooling period than the upper part, and thus more significant grain growth occurs.