Study For Multiple-step Incremental Air-bending Forming And Springback Of High-strength Sheet-metal

Air-bending operation is the most important process of sheet metal forming. This single-step air-bending processing is suitable for workpieces of simple profile. However, parts with complex surface can not be machined by the single-step air bending. To solve this problem, a multiple-step incremental air-bending forming is utilized. This forming process is a flexible sheet-metal-forming technology that uses principles of stepped manufacturing. It transforms the complicated geometry information into a series of parameters of single-step, and then the plastic deformation is carried out step by step through the computer numerical controlled movements of the punch and sheet feed for getting the desired part. Therefore, the process is cost-effective to form large complex parts in small to medium batches such as semiellipse-shaped workpieces which could be used as crane boom, telescopic arm of the concrete pump truck, boom of bridge, and petroleum piping, etc, which can reduce the production costs and improve production efficiency.The air-bending of sheet metal forming is one kind of elastic-plastic deformations, where the springback phenomenon is inevitable. Because of the existence of the big springback for high-strength sheet metal, the precision of products and subsequent assembly operations are severely affected, the workload of mold-try, mold-repair and correction after shaping is largely increased, and product development cycle is obviously prolonged, also restrict the more application and popularization of the incremental air-bending forming of sheet metal. Springback is a complex problem due to multi-coupled factors and highly nonlinear, which leads to springback prediction, springback modeling and springback correction more difficult to be studied intensively. At present time, little is reported on the incremental air-bending forming process and warpage deformation analysis for large-sized workpiece which are relative to springback factor. Therefore, studying incremental air-bending process, finding out high efficiency and reliable method of the springback prediction, springback modeling and springback correction, improving the forming precision of the part and eliminating warpage deformation, which are being a major problem to be solved in sheet metal incremental air-bending forming field.Based on the tests for air-bending of sheet metal, a prediction model of springback radius and punch radius is developed with GA(genetic algorithm) and BPNN(back propagation neural network). BPNN is prone to getting into local extrema and convergence is slow. To overcome these drawbacks, this study attempts to combine GA, avoiding local minima and achieving global convergence quickly and correctly by searching in several regions simultaneously, with BPNN to minimize the total mean squared error (MSE) between actual output of the network and the desired output through optimizing the weights of neural network. A mathematical model of springback is developed with dimensional analysis and orthogonal test. Dimensional analysis is used to confirm the relationship of the physical variables with the principle of dimensional homogeneity in physical laws and the Buckingham pi law. And the corresponding relationship between the sheet metal springback radius and the sheet thickness, yield strength,the punch radius,the Young's module could be solved quantitatively. A model of springback correction is established with off-line closed-loop control system. Introducing feedback control idea in automatic control theory to multiple-step incremental air-bending forming of sheet metal, an off-line closed-loop control iterative algorithm, combined by Fast Fourier and wavelet transform, is developed. With this algorithm, the mould surface of sheet metal incremental air-bending forming could be properly corrected, and the springback errors of the formed workpieces could be effectively reduced. Combining the above three models with numerical simulation, which can be taken as a new approach for sheet metal multiple-step incremental air-bending forming, tool design and warpage deformation analysis.Based on Hill's yielding criterion and exponential strain hardening law as well as plane strain condition, ABAQUS 3-D finite-element model (FEM) are proposed in this paper for investigating the incremental air-bending forming process. An elasto-plastic constitutive equation about the relationship of stress and strain in air-bending is deduced. And solves the key technologies such as sheet feed rate, die opening and the loading of punch displacement.The explicit algorithm of ABAQUS/explicit module fitted for dynamic and non-linear analysis could be used to simulate the sheet metal. forming process, and the implicit algorithm of ABAQUS/standard module adapted to static and steady analysis to simulate the springback process. And the above algorithms are used in a large-scale semiellipse-shaped workpiece's each step forming and each step springback simulation, which including numerical simulation of forming process and warpage defect. This method can obtain the optimal air-bending tool parameters and forming process parameters. Also reveals the non-uniform deformation of press-brake lathe bed is the main reason of causing distortion and dislocation as well as warpage deformation.In this paper, sheet metal springback prediction model and springback mathematical model as well as springback correction model are combined with the numerical simulation, and the multiple-step incremental air-bending machining of a semiellipse-shaped workpiece is performed on the basis of the combination of simulation and models. Manifested by the experiment for incremental air-bending forming of this workpiece, the numerical simulation method proposed yields satisfactory performance in optimizing air-bending tool parameters and forming process parameters, improving workpiece forming accuracy, and eliminating workpiece warpage defect.