Microstructure and mechanical properties of an asymmetric twin-roll cast AZ31 magnesium alloy strip

The microstructure along the thickness of a strip produced by asymmetric twin-roll casting was found to be significantly inhomogeneous. There were many banded structures with flow form near the upper surface of the strip, whereas a fine dendritic structure dominated near the lower surface of the strip. Precipitates of Mg17Al12 and Al8Mn5 were dispersively distributed throughout the as-cast strip. Recrystallization of the strip during homogenization first occurred in the banded structures. After rolling and annealing, the strip consisted of fine grains measuring approximately 5 μm in size. It was concluded that the shear strain caused by the difference in linear velocity between the surfaces of the upper and lower rolls resulted in the banded structures of the strip.