Effect Of Forming Stages On Blank Formability In Incremental Sheet Forming Process

Incremental Sheet Forming(ISF) is an innovative forming technology without dedicated dies. In ISF, the final part shape is the result of cumulating all localized plastic deforma-tions which generated by continuously moving of a simple forming tool over sheet met-al. By this method, parts could be manufactured with low forming force, highly flexible, cost reduction and fully utilize the formability of material, so ISF process is very suita-ble for small batch and rapid prototype production, which meet the growing demands of modern market to highly customized products. Therefore, ISF process will have bright future.In order to find methods for reducing excessive sheet thinning when forming the steeply walled parts with incremental sheet forming process, three Finite Element Me-thod (FEM) models for simulating single-stage, double-stages and multi-stages strate-gies were developed with ABAQUS, and the simulation results, such as sheet thickness distribution and equivalent plastic strain history, were obtained. Then, on the basis of these data, the influence of forming stages on sheet metal formability was studied, and the results will give some reference for manufacturing complex parts.Firstly, based on elastic-plastic finite element theory, the approach of numerical simulation for modeling ISF process was planed with consideration of both precision and efficiency. A reasonable FEM model for forming frustum pyramid was established using S4R shell element, nonlinear isotropic elastic-plastic material model, coulomb friction model and considering strategies of dynamic load of spatial tool path. Experi-mental apparatus was designed and the simulation results, such as sheet thickness dis-tribution, profile shape along part radial and so on, were validated by experiment. The principal strain history of node A1 and A2 are in a good agreement with the deformation mechanics of ISF proposed by Jackson and etc.Secondly, ISF processes with different parameters were simulated. Effects of half-apex angle, vertical step size between consecutive contours and tool diameter on part thickness distribution, equivalent plastic strain, maximum equivalent plastic stress, forming force and so on were studied. The results show that half-apex angle is the do-minating process parameter to impact the formability of material, as the half-apex angle bigger, the smallest thickness of part increases and the deformation of sheet is also uni-form. Small step size and big tool diameter have a good influence on strain distribution and surface quality. Process parameters have no influence on maximum equivalent plas-tic stress. Increasing step size and/or increasing tool diameter, the forming force added.Thirdly, on the basis of geometry analysis, the FEM models for simulating sin-gle-stage and double-stages of ISF processes to form a cone with 30°half-apex angle were established respectively. The results of geometry shape, thickness distribution and equivalent plastic strain were compared and found that double-stages forming strategies can greatly improve the formability of sheet because of the increased deformation area and the uniformity thickness distribution. The maximum equivalent plastic strain at node A, B, C is reduced by 66.2%, 81.9% and 36.0% respectively. Node B has the larg-est reduction; this delays the appearance of crack in the part formed by single-stage strategy. The maximum deformation area is nearer to the parts'bottom surface. In addi-tion, these results are validated by experiments.At last, procedure of designing ISF process was introduced. And as an example, the structure of a complex part with vertical wall surface was analyzed and the 3D geo-metry of process CAD model was obtained. Considering the formability of this process CAD model under ISF, a multi-stages forming strategy was proposed. To validate the forming strategy whether reasonable or not, a FEM model for simulating multi-stages of ISF process was established and the data, such as stress, strain and so on, were trans-mitted successfully between stages. The simulation results show that the minimum thicknesses of each stage are in a good agreement with design.