Use of weight-saving materials to produce lightweight components with enhanced dimensional control is important to the automotive industry. This has increased the need to understand the material behavior with respect to the forming process at the microstructural level. A test matrix was developed based on the orthogonal array of Taguchi design of experiment (DOE) approach. Experiments were conducted for the V-bending process using 6022-T4 AA to study the variation of springback due to both process and material parameters such as bend radius, sheet thickness, grain size, plastic anisotropy, heat treatment, punching speeds, and time. The design of experiments was used to evaluate the predominate parameters for a specific lot of sheet metal. It was observed that bend radius had greatest effect on springback. Next, finite element simulation of springback using ANSYS implicit code was conducted to explore the limits regarding process control by boundary values versus material parameters. 2-D finite element modeling was considered in the springback simulations. A multilinear isotropic material model was used where the true stress-strain material description was input in discrete form. Experimental results compare well with the simulated predictions. It was found that the microstructure of the material used in this study was processed for sheet metal forming process.

Sheet metal forming is one of the most common forming processes used in the industry, especially in the automotive industry. It becomes a common sense, that by increasing the accurate simulation of sheet metal forming, the industry can save dramatic cost in trial-and-error process when designing the sheet metal forming tools. In past decades, considerable studies have been done in the field of numerical analysis of metal forming processes, particularly in springback prediction. Significant progresses have been made, but the accuracy of simulation results still needs to improve. One reason is that, in the typical sheet metal forming analysis the tools are considered as absolute un-deformable rigid bodies. The deformation of the tools, which happens in the real production, is not taken into account. Another reason may be that the classic simulation considers the friction coefficient between the tools and blank as constant. However, the actual friction condition depends on a number of parameters. The objective of this thesis work is trying to investigate how it will affect the springback prediction results when either the tool deformation or more complex friction conditions are considered. The purpose is nothing related with the precise simulation about the true problem or how accuracy the simulation results show compared to the experiment results. The work is only to give an emphasis hint that how the FE-model and friction mode chosen affects the springback results when doing a numerical analysis. A simple model called flex rail is used for sheet metal forming simulations with three different friction models. A comparison between the results clearly shows a difference when advanced friction models are applied. A 3D elastic solid model is created to compare the result with rigid model. The results show the difference when deformation of the tools is taken into account. Finally, an actual case with tools from the industry is investigated. The tools are from SAAB Cars Body Components. This case is to investigate the possibility and necessity of applying the advanced friction model and elastic tools when a complex real industry problem is faced. Further study is needed to do with comparison experimental data to verify the accuracy when these models are used.

Sheet metal hydroforming(flexforming) is a process generally used in the manufacturing of aerospace parts in which a rubber diaphragm forms the sheet on a die with the help pressurized fluid and by this aspect it is different from the conventional stamping process. Some defects occur in the parts that are manufactured by this method and they are not different from the general sheet metal forming defects. Wrinkling, tearing and springback are among those defects. Variety of parts makes difficult to encounter these defects arising the detailed investigation of this process. In this work, the flexforming process was modeled by finite element method in order to investigate the operation windows of the problem. Various two and three-dimensional models were established with and without diaphragm, using explicit and implicit approach for time integration and using solid and shell elements for the blank. Using the material Aluminum 2024-T3 alclad sheet alloy, three basic experiments were conducted: Bending of a straight flange specimen, bending of a contoured flange specimen and bulging of a circular specimen. By these experiments the effects of blank thickness, die bend radius and forming pressure have been investigated. Experimental results were compared with finite element results to verify the computational models. Then, three selected aerospace sheet parts were analyzed and success of the model in the real life applications is proved.

Aluminum based alloys have wide area of usage in automotive and defense industry and bending processes are frequently applied during production. One of the most important design criteria of bending processes is springback, which can be basically defined as elastic recovery of the part during unloading. To overcome this problem, heat treatment is generally applied to the workpiece material to refine tensile properties. In this study, the effect of heat treatment on springback characteristics of aluminum studied both numerically by using finite element analysis and experimentally. For this purpose, two different materials are selected and various heat treatment procedures are considered. The aluminum sheets having thickness of 1.6 mm, 2 mm and 2.5 mm are bent to 60&amp / #730 / , 90&amp / #730 / and 120&amp / #730 / . The von Mises stress distributions, plastic strain values and punch load values and comparison of the numerical and experimental results are also given.

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<p>Advanced high strength steels (AHSS) exhibit significant higher springback and different fracture modes in forming processes and these problems cannot be accurately predicted using conventional simulation methods in many cases. In this thesis, new simulation technologies have been developed to improve the predictability for AHSS forming. The technologies integrated various aspects of simulation techniques, including development of material models and local formability criteria, calibration of the models with experimental data, and simulation method and parameter optimisations. Both laboratory and full scale parts were used to validate the simulation technologies developed. These technologies are originally applied to solve AHSS forming problems.</p> <p>The springback predictions have been significantly improved using the newly developed simulation technology. The technologies include the implementation of the smooth contact to reduce contact errors, modification of mass scaling to reduce dynamic effect, implementation of isotropic/kinematic hardening model and optimization of simulation parameters. Shear fracture (a stretch bending fracture on a small radius) have been successful predicted using Modified Mohr Coulomb (MMC) fracture criterion. Both laboratory experiments and full scale parts have been used to validate the predictions. Shearing and pre-forming effects on hole expansion and edge stretching have been investigated. A new approach was introduced to evaluate AHSS sheared edge deformation and quality by measuring material flow line angle change on a shearing edge. Shearing processes were simulated using MMC failure criterion and the sheared edge deformation has been integrated to hole expansion simulation to produce a more accurate prediction. The pre-forming effect on edge cracking has been investigated through both experiments and simulations. The limit strains have been measured by experiments. Simulation technology was also developed to predict surface strains of pre-form and subsequent stretching. Formulation of plane stress characteristics considering normal anisotropy have been developed and applied to analyze the flange deformations and optimum blanks for cup drawing. The method of plane strain characteristics has been used to predict earing throughout the entire cup drawing process.</p> / Doctor of Philosophy (PhD)

Bending process is an important forming process in most industrial fields. Springback and cross-section distortion are commonly faced problems in bending process. Springback behavior of closed and open section beams changes with different parameters such as cross-section type, cross-section dimensions, bend radius and bend angle. For closed sections like tube, the dominating problem is cross-section distortion. The thickness of the tube at intrados (inner surface of tube being in contact with die) increases, whereas the thickness of the tube at extrados (outer surface of tube) decreases. Furthermore, another cross-section distortion type for tubes is flattening at extrados which is undesirable in some manufacturing operations. The present research, using finite element method, focuses on investigating the springback behavior of commonly used aluminum beams which are T-Shaped, U-Shaped and tubular for different cases. A series of analyses is performed for a beam and the changing parameters in the analyses are bend radius and thickness. Furthermore, for tubes, the effects of axial force on springback behavior are investigated. It is seen that the axial force causes stretching and the springback angles are decreased. Moreover, in order to overcome cross-section distortion in flattening for tubes, different internal pressures are used and the effects of internal pressure are investigated. By applying appropriate internal pressure, the flattening distortion is mostly eliminated. Conclusions are drawn revealing springback behaviors and cross-section distortions with respect to bend radius, bend angle, thickness, axial pull and internal pressures. They are in good agreement with other published researches and experimental results. Therefore, the models can be used to evaluate tooling and process design in bending operations.

This study focuses on the effects of different constitutive models in sheet metal forming operations by considering the cylindrical and square cup drawing and V-bending simulations. Simulations are performed using eight different constitutive models / elastic plastic constitutive model with isotropic hardening, elastic plastic constitutive model with kinematic hardening, elastic plastic constitutive model with combined hardening, power law isotropic plasticity, piecewise linear isotropic plasticity, Barlatthree-parameter, cyclic elastoplastic and Hill&rsquo / 48 model.The numerical analyses are accomplished by using three different 1 mm thick sheet materials / St12 steel, Al-5182 aluminum and stainless steel 409 Ni. An explicit finite element code is used in the simulations. For square cup drawing, three different blank holder forces / 2 kN, 4 kN and 5 kN are considered for St12 steel, whereas only 5 kN blank holder force is applied for stainless steel 409 Ni and Al-5182 aluminum. A number of experiments are carried out and analytical calculations are utilized to evaluate the results of simulations. In cylindrical cup drawing, simulation results of different constitutive models show good agreement with analytical calculations for thickness strain and effective stress distributions. In square cup drawing, simulation results of all the models displayed good agreement with the experimental results for edge contour comparisons, although the distributions of effective stress vary for different models within the cup. The numerically and experimentally obtained springback amounts are also in good agreement. The simulation results obtained for piecewise linear isotropic plasticity and power law isotropic plasticity models show better agreement with the analytical solutions and experiments.