Study On The Loading Paths Of The Tube Hydroforming Process

In automobile manufacturing industry, tube hydroforming (THF) will offer several advantages as compared to conventional manufacturing via stamping and welding. These advantages include: weight reduction, improved structural strength and stiffness, part consolidation, lower tooling cost due to fewer parts, fewer secondary operations. These advantages satisfy the requirements of the development of lightweight car. The THF technology has now been used in industry with the development of the computer and the super-high hydraulic pressure controlling technology. It can be forecast that the manufacturing technology of the automobile industry, aeronautics and astronautics industry, and shipbuilding industry will be improved by this technology.Because the effects of the material, the die shape and the loading condition, the forming workpieces may be limited by the failure modes of buckling, wrinkling and bursting during the THF process. To avoid the forming failures and improve pats'formability is a key technology for improving the competence of the enterprise. For a given part shape, die condition and tube material, the key technology is to reasonably control the internal pressure, the axial force and its relationship, that is loading path. Lots of literature research into the loading paths of the THF process, and achieved some usable results. But few efforts have been put into the rules of the loading paths, the mechanism of improving the formability by controlling the loading paths, the design principle and the design method of the reasonable loading paths.This dissertation studies on the loading paths of the THF through FEM and experiments methods. Firstly, a computer controlling THF equipment has been research and developed, its maximal internal pressure is 200MPa. The internal pressure, the axial force and the axial displacement can be controlled accurately. Then, the variation laws of the critical loading for avoiding different forming failures and the effect of different conditions on the critical loading forces for the free bulge process are studied. Thirdly, the mechanism of improving the formability by controlling the loading paths are analyzed from the view that stress station has effect on the formability, and the loading path has effect on the stress station. After that, the design principle for the reasonable loading paths is proposed. Based on this analysis, a design method integrated the adaptive simulation and the fuzzy logical control is proposed. At last, the method is used for some typical experiment parts.The main content of this thesis can be divided into five parts: 1. Research and develop a computer controlling THF equipmentA computer controlling THF equipment has been research and developed, its maximal internal pressure is 200MPa. The dissertation takes an exploratory study on the key technique of the integral structure, the hydraulic controlling system, the computer controlling system, the THF die system and the sealing system and so on. Then equipment is verified to be precise through quantities of experiments. It is the experiment tool for research.2. Simulation and experiment study on THFThis dissertation studies the three-dimensional FEM modeling method to improve the THF simulation precision, and put forward some guidelines for parameters selection of simulation process. The effect of the virtual time on the prediction precision in the THF process is analyzed when the dynamical explicit FEM is used. Through the comparison between experimental and simulation results, the FEM tool for the research is achieved. Then, two effects of the loading paths on the forming results are analyzed based on some experiment results. On the basis of the study above, this dissertation studied on the effect of several typical loading paths on the tube hydroformability, using numerical simulation and experiments.3. Study on the variation laws of the critical loading in THFThe critical loading window diagram (CLWD) is a diagram that is enclosed by the loading path of the critical internal pressure and the critical axial force, which are the critical loading when failures such as buckling, wrinkling or bursting have happened. The loading paths for forming a part should lies within the CLDW. Based on the plasticity theory and THF technology, this dissertation constructs the whole theoretical model for the CLWD, which is validated through experiments. Then, the effect laws of tube size, material and die shape on the CLWD are analyzed, and then put forward the selection range for different material, different tube size and die parameters, which limit the bound of the loading paths and avoid the forming failures of TFH. The CLWD can answer the question of how to avoid the forming failures from the theory.4. Study on the loading paths that improves the tube hydroformabilityIn CLWD, there are many loading paths for selection, but there is great differences formability for the hydroformed tubes between different loading paths are selected. To select an appropriate loading path, this dissertation studies the tube plastic deformation process in THF with theoretical methods, analyzes the stress and strain characteristics in hydroformed parts and finds the effect of the stress station on the formability. Then, obtains the ideal stress states for improving the formability in THF process. This dissertation proposes the basic concept to obtain the ideal loading path that leads to the ideal stress states in hydroformed parts, and then the theoretic model of the ideal loading path for the free bulge process are studied. According to the simulation and experiment results at different loading paths, this dissertation also put forward the design principle of reasonable loading paths, which answer the question of how to improve the formability of the tube from the theory. 5. Study on loading paths design method based on the combination of adapted simulation and fuzzy logical controlHow to obtain the reasonable loading path is still difficult for people, although its design principle is known. Based on the characteristic of THF technology and the design principle of loading paths, this dissertation put forward a design method for reasonable loading path, this method integrated the adaptive simulation and fuzzy logical control. The adaptive simulation avoids the try-and-error process of conventional loading path design method, it constructs a target function that assesses the defects tendency in hydroformed process, and then returns the simulation results to the next simulation step. In this way, the program can predict and rectify the forming process defects, which means fewer simulations and higher efficiency. In the identification of forming results and the modification of the process parameters, the fuzzy logical control method is used. In this way, more reasonable prediction of forming defects and adjustment of process parameters can be achieved. To validate the efficiency of the loading path design method, several experiments are used in the end. From the results, it's found that the design principal and the design method for loading path proposed in this dissertation are right and advanced. This method can answer the question of how to design the loading path in industry.This dissertation studies on the loading paths of THF mainly through theoretical deduction, numerical simulation and large amounts of experiments. The THF equipment, the mechanics of hydroforming, the design principle and the design method of loading path are researched into in this dissertation. It transforms the loading path design process that depended on experiments and experiences to a refined theoretical deduction process. Which provides theories guidance and a new method for the loading path design in THF.