Numerical Simulation And Analysis For Single-Point Incremental Forming Process

Single-point incremental forming (SPIF) is a flexible forming technology for three-dimensional sheet metal parts. Through controlling the vertical feeding and horizontal trajectory of the tooling,continuous local plastic deformation is achieved.And three-dimensional shape contour of the parts is formed layer atop layer.Compared with traditional stamping process,SPIF has many advantages.First of all,it does not need solid die,economizing material and costs on die designing and manufacturing.Secondly,it can enhance the mechanical properties such as the strength of the formed products.Meanwhile,it is able to machine high-intensity materials which are difficult to deform. Lastly, it is easy to achieve CAD/CAM integration and automation.Research to SPIF is still at the stage of exploration. While the theoretical analysis is lacked and immature, the theory on the forming and controlling of SPIF is still to be developed and improved, which is also a hot topic of current research. SPIF technology combines the rapid manufacturing technology with the plastic forming technology. It is a comprehensive interdisciplinary technical problem. The development of this technology will promote the development of the rapid manufacturing technology and the plastic forming technology. So it's necessary to research into SPIF technology.In this paper, SPIF process is studied by numerical simulation method. On one hand, it is easy to observe the effect of technological parameters and forming track on the forming result, by changing the technological parameters or the forming trajectory. On the other hand, simulation results can also be compared with the follow-up experiment results, providing the constitution of experimental program with reference. It can also reduce the number of experiments and improve the feasibility of the experimental program.The main research contents and conclusions are as follows:1) The analysis of the forming mechanism of SPIF process and the establishment of the FE model of SPIFThe sheet forming process of both Dieless SPIF and Die SPIF are analyzed. The plastic deformation process is divided into five stages: elastic pre-tension, bending along the surface of the tool hemisphere, reversed bending to the anterior bending, unloading after the process finished, and rebounding. Analysis of the stress and strain state of SPIF process is done, and a method of calculating the shell thickness and thickness reduction rate in SPIF process is brought out.To carry out numerical simulation correctly, reasonable FE model is very important. In this paper, FE models of both Dieless SPIF and Die SPIF are established according to the mechanical models, following the modeling steps in ANSYS software. Modeling process involves the selection of the element types and the material model and the setting of boundary conditions. This will be helpful to establish the FE model of SPIF correctly. There are three important principles that should be followed during the trajectory designing, they are uniform deformation zone, moderate vertical feeding and horizontal offset.2) Analysis of simulation results of SPIFIn this paper, the SPIF processes of parts of different shapes are simulated, to investigate the forming principle and to analyse its formability from different aspects.There are a few differences between the Die SPIF and Dieless SPIF process at forming accuracy and thickness-reduced region. The accuracy at the border between the bottom and the wall in Die SPIF is poor, while it happens at the border between the edge and the wall in Dieless SPIF. In addition, the positions where the metal is stacked are also not the same between the two processes. When forming box shaped part with corners, the thinnest region is on the wall in Die SPIF, while it happens at the corners in Dieless SPIF.When forming spherical surface parts, more attention must be paid to the feeding value. A lot of defects may be caused because of inappropriate feeding. In this paper, the calculating formula of the feeding value is brought forth, in order to provide reference for follow-up study.Through analysis of the change of shell thickness from different region, the theoretical calculations of shell thickness and thickness reduction rate are verified. This will provide guidance for the actual production. A few defects, such as crack, wrinkling and impress, happen during the simulation, then the causes of the defects are discussed, and corresponding measures are brought out to inhibit the defects.3) Analysis of the factors impacting SPIF processIn this paper, simulation results of SPIF process with different technological parameters are analysed, and the impact on the SPIF process from the initial thickness, the tooling radius, the forming obliquity and the transition manner between layers are discussed. With the increasing forming depth, the thinner the shell is, the more likely crack will happen. When the shell thickness increases, the deformation resistance will increase, and so will the equivalent stress. The tooling radius relates to the machining accuracy at local area. If the tooling radius is small, the machining accuracy at the corners will be good. But because of small contact area, stress concentration phenomenon will be more serious and it may lead to crack. If the tooling radius increases, the distribution of stress will be more uniform, but there may be poor machining accuracy at the corners. If the forming obliquity is too small, defects such as crack and wrinkling will be easily caused, while impress will be caused if the obliquity is too large. Meanwhile, the larger the forming obliquity is, the more uniform the distribution of shell thickness will be. There may be impress at the feeding position using ordered feeding method, which can be improved by dispersing the feeding positions. The forming result using gyroidal feeding method is well, with uniform stress state and shell thickness distribution.4) The feasibility analysis of combining SPIF process with multi-point forming processThere are two ways to be discussed of the combination of SPIF process and multi-point forming, which are called mixed forming and step-by-step forming. The forming processes in the two ways are both simulated and anslysed. After that, the forming effect and function of both two methods are discussed. It is proved that the combination of SPIF process with multi-point forming process is feasible.Compared with Die SPIF, the accuracy of mixed forming is almost the same. Because the mould is built-up by the multi-point punches, which can be controlled, part with more shapes can be formed without special molds. But the efficiency of mixed forming is not increased obviously. The deformation of step-by-step forming takes on a more uniform deformation state and a reasonable stress distribution. However, there may be serious thinning at the border between the SPIF area and multi-point forming area, and if the SPIF area is too small, metal stacking may be happens at the center region. Compared with the simplex SPIF, step-by-step forming can obviously enhance the forming efficiency.