Research On Snake Rolling Of Thick Plate Of High Strength And Toughness Aluminum Alloy

The thick plate of high streghth and toughness aluminum alloy is very important structural material, widely used in industry of aerospace, aviation and vechicle. Hot rolling is the most key procedure to manufacture the thick plate of aluminum alloy, which can make the plate experience high degree of deformation, eliminate casting defect as well as refine the grain size, then the performance of the plate will be largely improved. However, limited by roll gap of rolling mill and ingot thickness, symmetrical rolling couldn't make the thick plate deformed sufficiently. As a result, the deformation, microstructure and performance of the plate between surface and center will be nonuniform, furthermore very high residual stress in the product will be issued. Theses kinds of problems already became the key difficulty in manufacturing thick plate of aluminum alloy, which should be sloved as soon as possible. Snake rolling can increase deformation of plate without increasing reduction, this is helpful to increase the deformation in plate center, meanwhile improve the homogeneity of deformation, microstructure and performance of the plate. So, the study on characteristic of snake rolling and application in manufacturing of the thick plate of aluminum alloy is very helpful for improving the development of aluminum industry of China.The present dissertation was funded by the subject "development of advanced manufacturing technology of aluminum alloy" in national "eleventh five year" plan of science and technology (2007BAE38B05) and the subject "Numerical simulation and computer aided design in processing of andvaced light material" in National Key Fundamental Research and Development Program of China (2010CB735811). This study will focus on the characteristic of snake rolling and application in manufacturing of thick plate of aluminum alloy. In the study, the criterion for plate entering the roll gap in snake rolling was worked out. By slab method, the influence of rolling parameters on rolling force, rolling torque and plate curvature was investigated. Also, the finite element model for snake rolling of 7150 aluminum alloy was constructed, which including models of heat transfer, material, friction and microstructural evolution. By using FEM software MSC.Marc and the subroutine program, the flow velocity of mass point in the plate, the horizontal and vertical force acting on rolls during single pass snake rolling was investigated, also the effect of rolling parameters on plate curvature in snake rolling was studied. Meanwhile, the influence of reduction and speed ratio on plate temperature, rolling force, equivalent plastic strain and recrystallization fraction during muti-passes snake rolling was studied, and optimized snake rolling route was carried out. Rolling experiments were performed to verify the FE model. The main conclusions of the dissertation are as follow:(1) Based on the horizontal force equilibrium, the criterion for plate entering roll gap in snake rolling was worked out. Compared with plate in symmetrical rolling, plate in snake rolling is easier to entering the roll gap. Beside of friction coefficient, reduction and roll diameter, offset distance also influences the plate to enter into the roll gap.(2) By using slab method, the expressions for calculating rolling force, rolling torque and plate curvature in snake rolling were worked out. Then, these expressions were used to investigate the influence of rolling parameters. The results show that, with increasing of speed ratio, rolling torque on upper roll increases, while rolling force and rolling torque on lower roll decrease; lager offset distance between rolls, reduction and lower friction coefficient always produce higher rolling force; smaller offset distance between rolls, friction coefficient and larger reduction are more likely to cause larger rolling torque both upon upper and lower rolls. Difference in shear strain and axial strain upon upper and lower surface of plate have opposite effect on plate bending, with the increasing of offset distance, the difference in shear strain increases but axial strain decreases, their combined effect enable the plate curvature decreases at first and then increases. The increasing of speed ratio only causes the increasing of the difference in shear strain, which causes the increasing of total plate curvature.(3) Single pass isothermal compression experiments with different temperatures and strain rates were performed on Gleeble1500D simulator, then the flow stress numerical model of 7150 aluminum alloy was constructed in terms of hyperbolic sine function. Temperature compensating Avrami equation was used to describe the dynamic transformation of recrystallization during snake rolling of 7150 aluminum alloy. By the double pass interval isothermal compression experiments, the fractional softening was calculated, then the numerical model of recrystallization fraction during snake rolling of 7150 aluminum alloy was constructed.(4) In plate deformation zone, the flow velocity of mass points in lower portion of the plate is faster than in upper portion. There is no clear difference in vertical force acting on rolls between snake rolling and symmetrical rolling, but the horizontal force acting on rolls in snake rolling is 52.1% higher than in symmetrical rolling. With the increasing of offset distance, the plate curvature increases at first and then decreases; with the increasing of temperature difference between top and bottom surface of the plate, speed ratio and reduction, the plate curvature increases; with the increasing of initial plate thickness and roll radius, the plate curvature decreases. (5) For the ingot of 400mm thickness after 80% reduction, the equivalent plastic strain in plate center and average equivalent plastic strain along thickness direction in the plate rolled by the optimized snake rolling is 36.6% and18.6% higher than the plate rolled by symmetrical rolling; standard variance of equivalent plastic strain and recystallization fraction is 38.9% and 41.1% lower respectively.(6) The biggest difference between the plate curvature measured in experiments and calculated by FEM is less than 12% of the experimental results in single pass snake rolling. In muti-pass snake rolling, the average and biggest difference of equivalent strain between experiment and FEM is 0.21 and 0.27 respectively, both are less than 8.3% of the biggest experimental result. So, the FE model of snake rolling of aluminum has enough precision.