Microstructure and mechanical properties of Al–Mg–Er sheets jointed by friction stir welding

1.7 mm thick cold-rolled Al–Mg–Er alloy sheets under as-rolled and annealed conditions were subjected to friction stir welding (FSW). The as-rolled sheet showed fibrous tissues with a high density of dislocations. After annealing, the fibrous tissues were replaced by coarse, nearly-equiaxed grains ∼18 μm in size and the density of dislocations decreased greatly, resulting in significantly reduced hardness of the annealed sheet. After FSW, the nugget zone (NZ) of both the FSW joints of as-rolled and as-annealed sheets consisted of fine equiaxed recrystallised grains with sizes of 4.5 μm and 4.3 μm, respectively. For the joint of the as-rolled sheet, the dislocations in the NZ and thermomechanically affected zone (TMAZ) were significantly reduced and the NZ showed lower hardness than the base material (BM). However, for the joint of the as-annealed sheet, the NZ exhibited higher hardness than the BM due to significant grain refinement in the NZ. The joint of the as-rolled sheet showed a joint efficiency of 68% and fractured in the NZ but the joint of the as-annealed sheet had a joint efficiency of nearly 100% and fractured in the BM.