Formability Predictions in Stamping and Process Parameter Optimization Based on the Inverse Approach Code Fast_Stamp
A simplified (one step) method called “inverse approach” (IA) for the numerical analysis of the stamping process has been continuously developed by the authors since the end of the eighties. In the present paper we recall the main finite element formulation aspects, for an efficient estimation of the large elastoplastic strains encountered in deep drawing operations. The capabilities of the code Fast_Stamp include: fixed (imposed) initial blank, bending effects due to tool curvatures, blankholder restraining forces (including drawbeads). Considerations of bending and unbending during forming stages can improve the IA regarding stresses distribution. In this case satisfactory estimation of springback can result. Since 1996 we also developed “math based” optimization algorithms and strategies. The cost functions and constraints are mainly expressed to reduce or control the thickness changes, the localized necking, the wrinkling tendency, the springback after forming. The design variables are describing the ...
COST 526 – Project F 3 Final Report Optimization of Process Parameters in Sheet Metal Forming
Figure 1 shows a simplified CAD representation of the tools and initial blank in the stamping process. During stamping, the metal sheet is submitted to large displacements and large elasto-plastic strains; rupture and wrinkles must be avoided, (Figures 2 and 3). The resulting part must also satisfy quality requirements regarding its geometry: after tools removing, the springback effect (Figure 4) can produce a non admissible final shape. The definition of the tools (punch, die, blankholders), choice of material (anisotropy, hardening) and process parameters (number of forming stages, forces to apply) must be performed for every new part to manufacture (there are several hundred parts obtained by stamping in a compact car). Improvement of the design and tryout procedures using numerical simulations may have a significant impact on the cost of the tools and on the reduction of the total time from design to manufacture, with also the possibility to provide better solutions than those determined from purely experimental tryout procedures.
Drawbead Forces in Sheet Metal Forming
During sheet metal forming on a double-action press, drawbeads on the blankholder supply a restraining force which controls the flow of metal into the die. As metal is drawn through the drawbead, the restraining force has two components, bending deformation and friction. In this study, these components were separated and analyzed. Drawing load, clamping load and strain were measured for sheet steels and aluminums which were drawn through a short section of simulated drawbead. The bending deformation component was isolated by replacing the shoulders and bead of the drawbead by rollers which reduced the friction to a negligible value. The experimental data is analyzed in terms of the materials effects which must be included in a mathematical model of drawbead forces. It was found that an isotropic hardening assumption would not adequately model the strain hardening but that a kinematic hardening assumption that takes cyclic hardening into account is necessary. It is also shown that strain rate hardening must be modeled for steel but not for aluminum. Friction is adequately represented by a coefficient of friction, µ. Experimental values of µ were obtained. Drawbead forces were calculated employing these experimental coefficients of friction and good agreement was found between the measured and calculated frictional components.