For numerical modeling and predictive analysis of warm hydroforming, better understanding of material properties (i.e. Flow curves) is required at elevated temperatures and high strains. Hydraulic bulge testing is a suitable method to obtain this information. However, analysis of the test data is not standardized as there are numerous approaches developed and adopted throughout the years. In this study, first, different approaches for hydraulic bulge analysis were compared with stepwise experiments to determine the best combination of approaches in obtaining accurate flow curves at different temperatures and strain rates. Then, three different grades of stainless steels (AISI 201, 301 and 304) were tested at various hydroforming conditions to determine the effect of pressure, temperature and strain rate on formability (i.e. cavity filling and thinning). These experimental findings were then used to be compared with predicted values from FEA. Results showed that material model works accurately in predicting the formability of materials in warm hydroforming.

O propósito deste trabalho é a modelagem experimental e estudo do controle do processo de hidroconformação de tubos. Assim, o trabalho visa o projeto e a construção de um dispositivo servo-controlado de hidroconformação de tubos com um sistema de controle digital. O trabalho compreende o projeto e a construção de um dispositivo a ser acoplado em uma prensa hidráulica, a instrumentação dos equipamentos usados e a implantação do sistema de controle automático do processo através de um computador PC e de placas de interface A/D e D/A. Os aplicativos de controle foram desenvolvidos em linguagem de alto nível no sistema operacional Windows. No projeto do aplicativo, inicialmente foram realizadas rotinas para testes do sistema em malha aberta. As demais rotinas são aquelas associadas às funções matemáticas do modelo fenomenológico do sistema de hidroconformação, aquelas destinadas ao controlador de malha fechada. O tipo de estratégia de controle a ser utilizada foi definida no decorrer do projeto e foi baseada em um modelo de processo não linear, linearizado em torno de cada ponto de operação. Para fins de obtenção do modelo nominal para o controlador, os atuadores e sensores tiveram suas dinâmicas desprezadas face a dinâmica do processo e suas curvas de processo foram levantadas experimentalmente. / This works aims is the experimental modeling of a tube hydroforming (THF) “T" branch, and de THF process automatic control study. Thus, the design and the construction of a servo-controlled hydraulic device for THF, with a automatic digital control system, is embedded in our objectives. Design and construction of device to append on a hydraulic press, implantation of the measurement equipment and implementation of the control system algorithms through a PC with I/O interface boards is necessary. Control algorithms were developed in Hi-level language for windows operating system. The application design was based on experimental initial tests performed with no feed-backing controlling mode. Routes related to phenomenological mathematical model of the THF process were validate against the literature database, and were devoted to the feed-backing controller mode. Control strategy to employ in final application was defined during the process calibration, based on the non-linear characteristics of the “T" branch THF. To obtain the final load path model sensors and cylinders had their dynamics neglected because the THF dynamic is very much higher, and had their behavior curves experimentally raised.

Tube hydroforming is a forming method which has several advantages. By using pressure in combination with material feeding it is possible to manufacture products with high structural integration and tight dimensional tolerances. The forming method is especially suited for aluminium alloys which have a relatively low ductility. Finite Element simulations are used extensively in the sheet metal stamping industry, where the methodology has contributed to a better understanding of the process and the new prediction capability has significantly reduced costly die tryouts. Similarly, the tube hydroforming industry can benefit from Finite Element simulations, and this simulation methodology is the topic of this dissertation. Deep drawing and tube hydroforming have a basic difference, namely that the latter process essentially is a force controlled process. This fact, in combination with the anisotropic behaviour of aluminium tubes, enforces a need for accurate constitutive descriptions. Furthermore, the material testing needs to account for the specifics of tube hydroforming. The importance of proper material modelling is in this work shown for hydrobulging and hydroforming in a die with extensive feeding. The process parameters in hydroforming are the inner pressure and the material feeding, where a correct combination of these parameters is crucial for the success of the process. It is here shown, that Finite Element simulations together with an optimisation routine are powerful tools for estimating the process parameters in an automated procedure. Finally, the reliability and quality of the simulation results depend on how failure is evaluated, which in the case of hydroforming mainly oncerns wrinkling and strain localisation. Since tube hydroforming often is preceded by bending operations this fact also demands the criteria to be strain path independent. In this work, it is shown that the prediction of strain localisation depends on the ability to predict diffuse necking, which in turn is strongly related to the chosen constitutive model.

Die surface shape may improve the flow of materials, reduce stress concentration of the products, and decrease the processing load to extend the life of die. The objective of this paper is to show that how to design the die surface shape of T-shape protrusion hydroforming with different diameters. A finite element code DEFORM 3D is used to simulate the process of THF, including adaptive simulation to predict the internal pressurization in the tube, and utilize flow net distribution to predict the axial feeding stroke and counter punch (CP) movement. After the amendment to the loading path, the flowability and appearance of the product quality will achieve the best results. Experiments of T-shape warm hydroforming of magnesium alloy AZ61 tubes are. The forming temperature is set as 250¢J. The simulated loading paths are used. From the comparisons of product shape, thickness distribution between analytical and experimental values, the validity of this analytical model is verified.. Keywords: Tube hydroforming, Finite element simulation, Die surface design.

The tube hydroforming (THF) technology has been widely used in manufacturing the lightweight and high strength components. The success of THF is largely dependent on the selection of the loading paths: internal pressure vs. time and axial feeding vs. time. The Finite element method is used to simulate the forming result of different loading paths and reduce the cost of die-testing. T-shape tube hydroforming is investigated adaptive simulation by combining FEM code LS-DYNA with fuzzy logic controller subroutine is proposed. During the simulation process, subroutines can adjust the loading paths according to the values of the minimum tube thickness and its variance. Then, the purpose of better thickness distribution of the formed tube at the side branch is achieved. Comparing with other linear loading paths, this adaptive control method got better results. In experiments, the validity of LS-DYNA applied in THF process is verified and the experimental results by adaptive simulation are better than those by the linear loading paths.

The microstructures and mechanical properties of the AZ31 Mg tubes fabricated by one-pass forward piercing tube extrusion operated at 250-400oC and 10-2-100 s-1 are examined. The grain size is refined from the initial ~75

Tube hydroforming process is a relatively new technology compared to conventional manufacturing via stamping and welding. However there is not much knowledge available for the product or process designers. The objective of this study will determine the flow stress and forming limit diagram of tubular materials to discuss the formability of tubes. Firstly, a mathematical model is proposed to examine the plastic deformation behavior of a thin-walled tube at different process parameters during the bulge hydroforming process without axial feeding. In the formulation of this mathematical model, an ellipsoidal surface and non-uniform thinning in the free bulged region and sticking friction between the tube and die are assumed. In the sticking friction mode, the elements after contact with the die do not move or slide. The effects of various forming parameters, such as the die entry radius, the bulge length, anisotropy, the initial thickness of the tube, etc., upon the forming pressures are discussed systematically. Secondly, an analytical model combined with hydraulic bulge tests is proposed to evaluate the properties of tubular materials considering anisotropy effect. Annealed AA6011 aluminum tubes and SUS409 stainless steel tubes are used for the bulge test. The tube thickness and radius at the pole and the internal forming pressure are measured simultaneously during the bulge test. The anisotropic values are obtained from tensile tests. From above experimental data, the effective stress - effective strain relations can be derived by this analytical model. The finite element method is used to conduct the simulations of hydraulic bulge forming with the flow stresses obtained by the above-mentioned model. The analytical forming pressures versus bulge heights are compared with the experimental results to validate the approach proposed in this study. Finally, this study also establishes the Forming Limit Diagram (FLD) of aluminum tubular material. An experimental system of tube hydroforming in which axial feed is applied to carry out the hydraulic bulge-forming test of the annealed aluminum alloy tubes. Furthermore, Hill¡¦s new yield criterion is also used to predict the Forming Limit Curves of sheets. The predicted forming limit diagrams are compared with the experimental data. The results of this study can provide useful knowledge for process design. In addition, the process parameters of flow stress and forming limit diagram obtained can improve the accuracy of the simulation results in industrial and academic fields.

In this study, the non-binary coding, elitist strategy, increasing mutation rate, extinction, and immigration strategy are used to improve the simple genetic algorithms. The improved search technique can reduce the possibility of falling into the local optimum due to the premature convergence in a large searching space, and increase the chance of finding out the near-optimal parameters. The hydraulic forming machine used in this thesis, includes a power source of a hydraulic motor and a actuator of two hydraulic cylinders. Both the internal pressure and axial force are controlled to hydroform the tubes into the shapes we want. The PID fuzzy logic controller is implemented to control the proportional direction valve and pressure reducing valve of this dual-cylinder electro-hydraulic system so that the loading path can follow the optimal forming curve of axial-feeding and pressure prescribed. From the experimental results, it is clear that the near-optimal PIDFLC controller designed via modified genetic algorithms can make the loading path follow the prescribed curve, and effective for reducing system uncertainty caused by the varying loads and system unstability resulting from the nonlinear characteristics of the hydraulic system.

Weight reduction is a hot topic in automotive industry. Both the applications of tube hydroforming technique and magnesium alloys offer a large potential for reducing the weight of automotive components. In this research, the relationship between forming pressure and bulge height, the maximum forming pressure and the forming limit during the tube hydraulic bulging process are first analysed. A self-designed warm hydraulic bulge forming equipment and the seamlessly extruted magnesium alloy AZ61 tubes, are used for carrying out a series of warm hydraulic bulge tests, and discussing the formalibility of the magnesium tubes at various temperatures. Furthermore, the flow stress curves are determined by the mathematical model in this paper with the bulge forming test results. Then the validity of the analytical model is verified by comparing the forming pressure and bulge height between analytical and experimental values.

In order to effectively and efficiently hydroform new automotive components, the formability of new tubular steels must be evaluated. Standard forming limit diagrams have been used for decades to evaluate and predict the formability of sheet steel formed along linear strain paths. However, tube hydroforming can present a problem since the pre-bending stage used in many hydroforming operations causes multiple non-linear strain paths. This thesis has modified a formability test method that deforms small-scale sheet steel samples in a single plane. The sample geometries were designed such that the strain paths achieved at the center of the samples were very near the plane strain condition. The four steels chosen for this study were: a deep drawing quality (DDQ), a high strength low alloy (HSLA) and two dual phase steels (DP600 and DP780). The plane strain formability for each of the four steels was tested in both the rolling and transverse directions. Three objective criteria were employed to evaluate and directly compare the formability of the four steels tested: difference in strain, difference in strain rate and local necking. The DDQ steel showed the highest formability followed in order by the HSLA, DP600 and DP780 steels. The repeatability in determining the forming limit strains using the difference in strain, the difference in strain rate and the local necking criteria for a 95% confidence interval was ± 1.5%, ± 1.2% and ± 3.2% engineering strain, respectively. The forming limit data collected for this thesis has been compared to results from full-scale tube hydroforming operations and free expansion tube burst tests carried out by researchers at the University of Waterloo on the same four materials. It was found that local necking results could be used to predict failure of hydroformed HSLA steel tubes with low levels of end-feed. However, this same method could only predict the failure of hydroformed DP600 steel tubes at higher levels of end-feed. The three objective criteria were not found to be suitable for predicting failure of free expansion tube burst tests. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2007-09-27 15:00:35.873