Lizhong Liu

Recent Development on Theory and Application of Variable Gauge Rolling, a Review

Variable gauge rolling (VGR) is a new technology for producing the materials which have the advantage of lightweight due to optimized thickness according to load distribution. The new progresses in the theoretical research and application of VGR are reviewed in this paper. Two basic equations, VGR-f and VGR-s, were deduced. The former is a new differential equation of force equilibrium, and the latter is a new form of formula for the law of mass conservation. Both of them provide a new base for the development of VGR analysis. As the examples of VGR’s application, tailor rolled blank (TRB) and longitudinal profile (LP) plate are introduced. Now TRBs are only produced in Germany and China, and have been used in the automotive manufacturing to play an important role in lightweight design. LP plates have been used in shipbuilding and bridge construction, and promised a bright prospect in reducing construction weight. In addition, new technologies and applications of VGR emerge constantly. Tailor welded strips and tailor rolled strips with variable thickness across the width can be used for progressive die and roll forming. The 3D profiled blank can be obtained by two-step rolling process. Tailor tubes with the variable wall thickness are an efficient way to reduce the weight. The blank with tailored thickness and mechanical property is also under development. Above products based on the tailored ideas provide a new materials-warehouse for the designers to select so as to meet the needs of weight reducing and material saving.

Grain statistics effect on deformation behavior in asymmetric rolling of pure copper foil by crystal plasticity finite element model

The grain statistics effect was investigated through asymmetric rolling of pure copper foil by a realistic polycrystalline aggregates model and crystal plasticity element finite model. A polycrystalline aggregate model was generated and a crystal plasticity-based finite element model was developed for each grain and the specimen as a whole. The crystal plasticity model itself is rate dependent and accounts for local dissipative hardening effects and the original orientation of each grain was generated based on the orientation distribution function (ODF). The deformation behaviors, including inhomogeneous material flow, decrease of contact press and roll force with the increase of grain size for the constant size of specimens, were studied. It is revealed that when the specimens are composed of only a few grains across thickness, the grains with different sizes, shapes and orientations are unevenly distributed in the specimen and each grain plays a significant role in micro-scale plastic deformation and leads to inhomogeneous deformation and the scatter of experimental and simulation results. The slip system activity was examined and the predicted results are consistent with the surface layer model. The slip band is strictly influenced by the misorientation of neighbor grain with consideration of slip system activity. Furthermore, it is found that the decrease of roll force and the most active of slip system in surface grains are caused by the increase of free surface grain effect when the grain size is increased. The results of the physical experiment and simulation provide a basic understanding of micro-scaled plastic deformation behavior in asymmetric foil rolling.

Size Effect on Mechanical Properties and Texture of Pure Copper Foil by Cold Rolling

To study the size effect on the properties of copper, tensile tests were performed with pure copper foil (thickness range from 25 μm to 300 μm) by cold rolling. A pronounced size effect was observed at a thickness of 76 μm. The results showed that ultimate strength increased as sample thickness decreased from 300 μm to 100 μm, however, this was decreased as the thickness changed from 76 μm to 25 μm with ultrahigh strain, with the same trend of dislocation density and micro stain. The rolling texture were consisted of copper {112}<111>, brass {011}<211>, and S {123}<634>. These features seemed to be linked to the increase of fraction of surface grain to volume, which led to lower districting on the dislocation slip.

Symmetric and Asymmetric Rolling Pure Copper Foil: Crystal Plasticity Finite Element Simulation and Experiments

A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientation development during foil rolling. Simulations were successfully carried out by using crystal plasticity finite element method (CPFEM), and a novel computational framework is presented for the representation of virtual polycrystalline grain structures. It has been found that asymmetric rolling (ASR) is more efficient in producing plastic deformation since it develops additional shear strain and activity of slip system compared with symmetric rolling (SR). For ASR, increase in the length of the shear zone, and decrease in the amount of the pressure and roll force would lead to further reduction. The shear strain path in SR and ASR is strictly influenced by the misorientation of neighbor grains, and corresponding {1 1 1} pole figures offer direct evidence of the spread of crystallographic orientation around the normal direction. The activity of slip systems was examined in detail and found that the predicted results are consistent with the surface layer model. The accuracy of the developed CPFEM model is verified by the fact that the simulated results of roll force coincide well with the experimental results.

Springback behavior of tailor rolled blank in U-shape forming

The springback of tailor rolled blanks with quenching and partitioning steels was investigated. In order to find out the springback behavior and related influence factors for the novel sheets, both experimental and simulation methods have been used to compare and analyze the springback characteristics of equal thickness blanks and tailor rolled blanks in U-channel forming. From the results, the overall springback angles of tailor rolled blanks at thin and thick sides are respectively 106.79° and 99.705°, which are both lower than those of the corresponding equal thickness blanks. Due to the existence of the thickness transition zone, the stress distribution in thin and thick sides of blanks is changed. The location of dangerous region in thin side of tailor rolled blanks is closer to the end of side, and the thick side moved to the middle of straight wall, which are different with the equal thickness blanks. Afterwards, the released quantities of tangential stress and strain per unit section of blank are adopted to calculate relative springback angles and give novel evaluation criteria for qualitatively analyzing the amount of springback angles. By comparing the results, it shows that the tangential strain method is more suitable for the actual situation.

Theoretical analysis of minimum metal foil thickness achievable by asymmetric rolling with fixed identical roll diameters

Abstract A novel approach is proposed for computing the minimum thickness of a metal foil that can be achieved by asymmetric rolling using rolls with identical diameter. This approach is based on simultaneously solving Tselikov equation for the rolling pressure and the modified Hitchcock equation for the roller flattening. To minimize the effect of the elastic deformation on the equal flow per second during the ultrathin foil rolling process, the law of conservation of mass was employed to compute the proportions of the forward slip, backward slip, and the cross shear zones in the contact arc, and then a formula was derived for computing the minimum thickness for asymmetric rolling. Experiment was conducted to find the foil minimum thickness for 304 steel by asymmetric rolling under the asymmetry ratios of 1.05, 1.15 and 1.30. The experimental results are in good agreement with the calculated ones. It was validated that the proposed formula can be used to calculate the foil minimum thickness under the asymmetric rolling condition.