| Abstract:Aluminum alloys with high strength and toughness have great significance for civil economy and national security. Asymmetric rolling, a severe plastic deformation technique, is one of the main methods to improve performance of these Al alloys. Although there are many studies on differential speed rolling, researches on snake rolling-a relatively advanced asymmetric rolling technique, are very limited. In particular, systematic investigations on shape control and deformation homogeneity through the thickness direction in such rolling processes are still lacking.In this study, the deformation behavior in snake rolling of an AA7050alloy is investigated by finite element simulations, considering two asymmetric factors, i.e., speed mismatch and horizontal displacement between the rolls. A preliminary validation of the numerical results is conducted for selected conditions in rolling with horizontal displacements The results are expected to be useful in aiding the design of rolling processes. Major results and conclusions of this study are as follows:(1) During differential speed rolling, increasing of friction coefficient greatly reduces the rolling reduction corresponding to the neutral point. Under some specific reductions, there exists a critical speed ratio where the bending direction changes. These specific rolling reductions are larger than those corresponding to the curvature neutral point. Compared with symmetric rolling, only the differential speed rolling with a relatively high speed ratio can improve the magnitude of plastic strain and its homogeneity through the sheet thickness.(2) During horizontal-displacement rolling, the bending behavior of sheet is affected by the rolling reduction, friction coefficient and initial sheet thickness. The directions of shear stresses on the sheet surfaces in the cross shear regime (CSR) changes with the rolling reduction. Compared with symmetric rolling, the horizontal displacement of rolls can effectively improve the effective plastic strain near the sheet center but reduce those at the top and bottom surfaces, leading to a large through-thickness gradient of plastic deformation. Meanwhile, the shear strains in the transversal plane can be improved as a whole.(3) During snake rolling, the control of sheet shape is largely dependent on the selection of two asymmetrical factors, i.e., horizontal displacement and speed mismatch. Bending can be avoided by balancing the opposite effects of these two factors on the bending direction. Compared with horizontal-displacement or differential speed rolling: snake rolling can obtain reasonably straight sheets under more rolling reduction conditions; even under the same rolling reduction, there are various combinations of the two factors to keep the sheet straight; this technology is more effective to enhance shear deformation and its homogeneity across the sheet thickness, but is not advisable for improving the deformation homogeneity through the thickness.(4) Under various asymmetric rolling conditions, there is no general relationship between the CSR size and sheet curvature. The change of sheet curvature can be effectively explained by the through-thickness variation of the normal strain along the rolling direction at the exit in deformation zone. The sheet bends downward when the mean normal strain in the upper part of the sheet is larger than that at the lower part, and vice versa. The reduction of sheet curvature in snake rolling is not acquired by eliminating the CSR. It is found that the size of CSR will be non-linearly accumulated when the directions of shear stresses on the surfaces resulted from horizontal-displacement and speed mismatch are the same.(5) Comparison of experimental results obtained for horizontal displacement rolling using an AA7050alloy and the finite element simulations demonstrates that the simulations can reproduce reasonably well the sheet curvatures, and that the friction coefficient used in the simulations is appropriate. |
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