Process modeling of targeted control forming of sheet materials

Abstract

The primary objectives of the research project are to investigate the strain path evolution in the material and to develop a methodology that can be used to control the material flow in the forming process. The scope of this research work is to study and control the strain path of the material based on theoretical modeling, numerical simulations and experimental investigations. Numerical simulation is the main tool used to investigate the forming process in this project. The applications of various yield criteria, which have a critical impact on the accuracy of simulation results, were studied. The comparison between the simulation result and the experimental result show that there is no universal yield criterion suitable to all material types. Beside the simulation result, theoretical prediction of Forming Limit Curve (FLC), which is the main criterion of the necking phenomenon, is also greatly influenced by the choice of yield criterion. To certain material, the theoretical forming limit model can predict FLC more accurately if the model employs a particular yield criterion. In order to verify the phenomenon, theoretical prediction and experimental tests were both carried out to delineate the FLCs of two materials in this thesis. Targeted control forming process was proposed in the thesis which aims to build an innovative control strategy to move the material strain path from the necking risk area to the safe area, which is under the FLC. In order to achieve the target, the controllable working parameters including some geometrical factors were used in the control strategy. Blank Holding Force (BHF), which is used to restrain the material flow in the forming process, was adopted in the control strategy. The effect of both time variant and space variant BHF profiles on the strain path evolution were investigated. Two materials, SPCC (low carbon steel) and Al 6112 (aluminium alloy), were chosen as samples in this thesis. The strain path evolution under different BHF types were analyzed and summarized. The elements, enduring various strain states, had different performances when the magnitude of BHF varied. The changes of strain ratio of the elements were compared to pursue an effective BHF type, which can lead the strain path to the desired location. Besides BHF, the influences of geometrical features, the fillet radius of the die, the clearance between the die and the punch, and the blank size, on the strain evolution were also investigated. Comparison indicated that varying of fillet radius and blank size can lead to more significant deviation of strain path. Friction condition, as an indispensable factor in the forming process, was also studied. Taguchi method was used to determine the most significant friction pair among three ones (friction between blank holder and blank, between punch and blank, between die and blank) to simplify the process. Once the geometrical feature is settled, well designed process parameters including the BHF profile are helpful to control the material flow (strain path) to acquire the targeted product. The relationship between the strain path of the fracture risk elements and some process parameters, especially the BHF in different areas, was modeled based on the simulation results using Response Surface Methodology (RSM). The three dimensional response surfaces of the principal strain can easily guide the setup of the working parameters. The strain rate effect was additionally taken into account for the rate sensitive material to improve the accuracy of the control strategy. Neural network, one useful control method, was also adopted to build a control strategy for the targeted control forming process. It is more powerful to control a nonlinear process and can achieve a more accurate result based on large quantity sample data. In conclusion, a targeted control forming process can be achieved using various control strategies. The influential factors were investigated and their relationship with the strain path was modeled. The setup of BHF profile can control the strain path evolution of the elements, which endure various strain states, in the safe area of the principal strain diagram. The application of such a process can reduce the design time and increase the chance of obtaining a defect free product, even for less experienced engineers.