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In this work, the effect of friction stir processing (FSP) on microstructure and mechanical properties of an Al-5Mg-0.2Sc-0.1Zr alloy was investigated in detail. The results showed that the as-cast alloy exhibited equiaxed grain structure with a large number of primary Al3(Sc,Zr) particles inside. FSP imposed a significant grain-refining effect on the alloy, changing the average grain size of 50.5 μm to 2.4 μm, which is attributed to the occurrence of dynamic recrystallization and generation of nanoscaled Al3(Sc,Zr) precipitates. These Al3(Sc,Zr) dispersoids intense impeded the movement of dislocation and grain boundaries and hindered the growth of recrystallized grains, resulting in a remarkable grain-refining strengthening, substructure strengthening and precipitation strengthening effect. On the other hand, the residual primary Al3(Sc,Zr) particles gave rise to a loss in ductility.

Friction-stir processing (FSP) imposed a remarkable grain refining effect on Al-5Mg-0.2Sc alloy, changing typical dendritic microstructure into fine and equiaxed recrystallized grains. Such effect was found to be related to intense plastic deformation and high-temperature exposure, resulting in occurrence of dynamic recrystallization. During this process, Al3Sc dispersoids due to the addition of 0.2 % Sc in a high density were precipitated and effectively hindered the migration of grain boundaries and subsequent grain growth, giving rise to a significant grain-refining strengthening effect. On the other hand, these Al3Sc precipitates caused an additional substructure strengthening and precipitation strengthening effect. An attractive comprehensive property of Al-5Mg-0.2Sc alloy subjected to FSP was obtained.

Primary particles with faceted cubic morphology were produced in as-cast Al-Mg alloy due to the addition of Sc. The cross-section of the particles revealed some eutectic structure composed of multilayer of ‘Al3Sc + α-Al + Al3Sc····’. At the cooling rate of 200 – 300 K/s, Al3Sc primary phase nucleated initially on oxides within the melt and developed to a cubic structure with a ‘cellular-dendritic’ mode of growth. The formation of α-Al structural shells was attributed as a reason for the segregation of Mg-rich lamellar dendrites at later stages. A growth mechanism for multilayer structure was proposed using the results presented.