Recrystallization mechanisms in 5251 H14 and 5251 O aluminum friction stir welds

The combination of deformation and heat in the nugget of friction stir welding (FSW) tends to transform by dynamic recrystallization the large grains of the base metal into a microstructure where the grains are completely equiaxed and strongly disorientated. To follow the different stages of dynamic recrystallization in the nugget, the evolution of the microstructure of two aluminum samples, cold rolled or annealed, is observed from the parent metal to the nugget using electron back-scattered diffraction (EBSD). The dynamic recrystallization mechanisms in each region of the weld strongly depend on the initial microstructural state of the aluminum sheet, in particular on the deformation energy stored. So, a static recrystallization prior to a continuous dynamic recrystallization was evidenced for the initially cold rolled Al alloy, while a geometric dynamic recrystallization occurred for the initially annealed microstructure. Fractions of low angle boundaries, crystallographic texture and grain size were compared in both weld nuggets.