TEXTURE AND MICROSTRUCTURE EVOLUTION DURING ASYMMETRIC ROLLING OF MG ALLOYS

Asymmetric rolling (ASR) conditions can be achieved either by changing rotational velocities of each roll or diameters of the rolls. Under asymmetric deformation conditions, materials were subjected to enhanced shear deformation, and high-angle boundaries developed with increasing strain, and finer grains were formed by following recrystallization processes. Conventional rolling process has a symmetric plane in the center of the work piece and the speeds of the top and bottom rolls are the same. ASR process has differential speeds for the rolls and it is necessary to consider the whole thickness of the sheet. To consider plastic deformation at the elevated temperatures, constitutive equation for AZ31 magnesium was implemented to finite element method. Effective deformation rates, shear strains, velocity and locations of neutral points were investigated for the various reduction ratios and roll speed ratios. As the ratio between top and bottom roll speeds increases, shear strain increases. The peak shear strains show a diagonal distribution located from the inlet contact position of the higher speed roll to the outlet contact position of the lower speed roll. Neutral positions moved in the opposite directions during asymmetric rolling. In the higher speed roll side, the neutral position proceeded to the outlet, while it retreated to the inlet in the slower speed roll side. Main texture components of magnesium alloys during conventional rolling process were basal poles and some shear components due to by friction. Shear deformation increased by asymmetric rolling and the basal poles were splited. Higher friction deepened the shear layer into the sheets.