Fracture criterion for predicting edge-cracking in Hot rolling of twin-roll casted AZ31 Mg alloy

Abstract

Edge-cracking as one of main defects for rolled Mg alloy sheets is inevitable due to the combined characteristics of the material and process. This study aims to quantitatively analyze the damage distributions induced by rolling deformation in transverse direction (TD) for twin-roll casted (TRCed) AZ31 Mg alloy, and optimize the Freudenthal fracture criterion to predict the crack initiation and propagation during the hot rolling. FEM simulations and laboratory experiments of edge-cracking behaviors during the rolling process were carried out at temperatures of 250∼400 °C and reduction rates of 30%∼45%. Overall cracking indexes, including temperature- and reduction-dependent transverse depth of edge-cracking, distributions of damage based on the Freudenthal criterion, and critical damage values to fracture were discussed. Results show that the fracture mode of edge cracks is dominated by 45° crossed shear cracking in the longitudinal (RD–ND) section of rolled sheets, and the cracking depth along the TD increases as the rolling temperature decreases and reduction rate increases. The transverse damage decreases linearly from the edge to the interior during the rolling when using the Freudenthal criterion, which can be attributed to the combined effects of temperature difference and effective stress difference along the TD. The critical damage value of Freudenthal criterion was successfully obtained for establishing the criterion for edge-cracking prediction in AZ31 Mg alloy rolling, which is within the range of 0.59∼0.88 and has a clear linear relationship with Ln(Zener-Hollomon parameter, Z).