Effect of Grain Size on the Mechanical Properties and Formability of AA8021 Aluminum Foil

AA8021 aluminum foils of 40 μm gauge with different grain sizes were prepared by four final annealing processes. The effect of grain size on the mechanical properties and formability was investigated by tensile and Erichsen cupping test. The microstructure before and after test was characterized by optical microscope, scanning electron microscope, electron backscatter diffraction, and surface profiler. The results showed that, as the grain size increased, both the ultimate strength, the yield strength, the elongation and the Erichsen value of aluminum foil dramatically decreased. With the increase of grain size, the ratio of free surface grain to volume increased and the number of grain boundaries decreased, which resulted in the decrease of strength, according to the surface layer model and grain-boundary strengthening effect. During the deformation process, a bigger grain size led to a higher surface roughness and a more non-uniform strain distribution in the matrix, which promoted the premature occurrence of fracture. As a result, a limited elongation and low formability were obtained. Fracture surfaces analysis found that both tensile and Erichsen cupping specimens showed a typical knife edge rupture with several dimples. The formation of dimples was mainly attributed to the presence of Al-Fe phases.