Foundamental Study On Tube Hydroforming Process

Tube hydroforming (THF) is an advanced plastic forming technology widely used in making complicated-shaped hollow parts with thin walls. Tube hydroforming technology offers several advantages: high accuracy, flexible forming, low energy consumption, high matierial utilization ratio, low cost, etc. This dissertation focuses on the basic research of the key problems of thin wall tube hydroforming, using the combination of analytical and experimental approach. The study mainly concentrates in six aspects:(1) A new material property test device is designed and manufactured to obtain exact tube material property. The device is used in obtaining the basic data: bulging height versus internal pressure curve, namely PH curve, needed in evaluating tube material forming properties. Compared to the existing device, it bulges the tube freely by fixing the two endings of the tube,.so it .can not only work with high efficiency, but also accurately estimate the tube material property under biaxial tension.(2) Based on the self-made tube material forming property test device, a new analytical model suitable for THF is created. This analytical model is based on plastic theory, planar stress assumption, static equilibrium equations,.and introduices the assumption that tube bulging contour function is square. By testing and analytical calculating on aluminum alloy LF21M and stainless steel 304L, compared with the result of finite element simulation, it is showed that the model is simple and convenience, and it is accurate for normal materials used in THF like aluminum alloy, low-carbon steel and stainless steel.(3) A new solution of tube material properties is proposed to obtain tube material property efficiently and accurately---adaptive and reserved FEM simulation solution. A series of criterions are set up by analyzing the influence of material properties on PH curve in THB test. On this basis, using analytical results as original value, the process of FEM simulation is real-time monitored by a computer program. Then the material parameters are being adjusted according the modification principle until the good result is obtained. This method is validated through experiments and finite simulations,and it is showed that it is more advantaged compared with pure analytical method or pure reserved FEM simulation. (4) A new forming pressure model of tube hydro-bulging is established to optimaize the THB test. The influences of tube geometry and material properties on bulging results is analyzed. It is showed that the forming pressure not only is influenced by the material property, but also depends on the thickness to diameter ratio and bulging width. For relative bulging height, it is only depends on work hardening index and the ratio of bulging width and tube diameter but not on tube thickness, diameter, work hardening coefficient and thick anisotropy index.(5) In order to obtain accurate friction coefficient between blank and die, a new friction coefficient testing device is established and some common lubricants used in tube hydroforming are tested. The method is based on Duncan theroy. The fricition coefficient is obtained by setting up the contact force model between blank and friction column. The results show that the device is simple and convenient, and can obtain the accurate fricition coefficient quickly..(6) Aiming at the drawback of traditional T-shaped tube forming method: expensive facitlity and complicated technics, a new method--- feeding driven THF is proposed. The new method is dependent on decreasing of interior forming volume caused by axial feeding to create the forming pressure Compared to traditional method, this method is more in need of a high-pressure relief valve control system than expensive high-pressure hydraulic pump system. Based on the method, T-shaped tube quality is analyzed theoretically on the basis of this method. It is showed that, for T-shaped part, as a safe forming region of relief pressure is existed, proper parts can only be formed within the region. Safe region is mainly dependent on material properties. Materials with good plastic behavior have large safe region. Besides, thickness distribution and useful height mainly depend on axial feed but not internal pressure.