Fundamental Research On Three Opposite Points Incremental Forming Of Sheet Metal

Sheet metal forming is widely used in manufacturing industry. Conventional process which depends on the product-specific dies and is characterized by long cycle, low efficiency and high cost, is difficult to adapt to small batch production, multi-variety production and trial manufacture of new products. Single Point Incremental Forming(SPIF) which has received an universal attention by researchers around the world is well suitable for small batch and customized production due to its increased flexible, reduced cost and the removal of the need for traditional dies. However, there exist some problems in practical application for SPIF plagued due to inherent geometric inaccuracy and lack of deformation control strategies.To improve the accuracy of incremental forming, a process called Three Opposite Points Incremental Forming(TOPIF) is studied from the perspective of reducing the deforming area and controlling the local deformation in this paper. The deformation process and its mechanism, process parameter selection and forming experiments in TOPIF were studied mainly. The main research works are as follows:(1) The local plastic deformation and the elastic bending deformation were studied and analyzed in TOPIF. The distributions of local deformation and the influence of local deformation on the integral geometry were analyzed based on conical yield and deformation constraint condition. And then, the concept of the equivalent layered model was put forward.On the basis of this, the equivalent simply-supported circular plate model was established,and the expression of elastic deflection for the equivalent forming layer was solved, and the elastic bending deformation analysis was implemented, and the conclusion that the elastic deformation is much smaller than the plastic deformation in TOPIF was obtained. Through the analysis on the local plastic deformation and the elastic bending deformation of the equivalent forming layer in both SPIF and TOPIF, the reason that TOPIF results inconsiderably enhanced geometric accuracy and increased strategy to control the local deformation as compared to SPIF, is explained theoretically.(2) The mechanical characteristics of the plastic deformation of the equivalent forming layer were studied and discussed in TOPIF. By using of membrane analysis and plane strain condition, the mechanics equations to describe the deformation regularity were established,and the equivalent virtual stress and strain were resolved, and the deformation state and deformation mode were analyzed. These provide some theoretical basis for understanding the nature of deformation mechanism and guiding the planning of tool path and the design of process parameters during forming in TOPIF.(3) A basic deformation algorithm was studied based on the normal yield for TOPIF. By analyzing the trajectory projected from initial blank to the target shape, the layered plastic displacement during deformation was solved, and the deformation and historical displacement for any node were calculated. It provides a fundamental basis for predicting geometry and sheet thinning.(4) The deformation mechanism and characteristics were discussed by using of the numerical analysis method which was established based on LS-DYNA in TOPIF. The pyramid and truncated cones were simulated by using of incremental theory and plastic potential correlation algorithm. The simulation results show that the presented model and algorithm are practical for the simulated parts in TOPIF. It was confirmed that TOPIF is characterized by high geometric accuracy, small area of local deformation and effective deformation control. It was found that the primary modes of deformation in TOPIF are bending, extrusion and shear. The basic reason for this is the introduction of the opposite auxiliary and supporting tools.(5) Experimental studies of effects of process parameters on forming quality were performed. Some fundamental conclusions and rules for design and selection of the process parameters were obtained. The effects of the contour trajectory which was designed based on the linear interpolation in discrete domain on geometry were analyzed by experiments performed on a custom-designed and fabricated machine by Zhejiang university of technology.The impacts of step size and tool parameters on the geometry and surface precision were discussed through experimental campaigns. A new expression for predicting the sheetthickness after forming was derived based on the mathematical description of the final surface and a normal projection of the initial blank onto the final shape. Take the frustum parts as examples, numerical simulation and experiment results show that the presented method and theoretical analysis on geometric accuracy, principal strain, sheet thickness and surface topography are practical for the frustum parts in TOPIF. Finally, some experimental parts with complex features, such as ellipsoid part, concave-convex part and so on, were formed by the custom-designed and fabricated TOPIF machine.