Effect of low cast-rolling speeds on the microstructure and mechanical properties of twin-roll casting high Mg AA5059 alloy sheets

This study was undertaken to investigate the effect of low cast-rolling speeds (<15 mm/s) on the microstructure and the corresponding mechanical properties of twin-roll casting (TRC) high Mg AA5059 Al alloy sheets by metallographic microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), differential scanning calorimetry (DSC), and tensile testing. When low cast-rolling speeds were applied, the central segregation defects and micro-segregation defects disappeared significantly and the number of non-equilibrium eutectic phases abated remarkably. Nevertheless, the secondary dendrite arm spacing (SDAS) slightly increased and the degree of grain deformation in the core region of the cast-rolled sheet increased. The melt in the casting-rolling zone is in contact with the rolls for a longer period of time and the melt is also subjected to increased rolling force that together cause changes in the shape and depth of the sump in the twin-roll casting (TRC). Under the action of the change in the shape and depth of the sump, the temperature gradient along the thickness direction and the concentration gradient of the solute at the tip of the dendrite were significantly reduced and the solid solubility of the alloy was greatly improved. Eventually, the comprehensive mechanical properties of the high Mg AA5059 alloy sheet were enhanced.