Showing papers on "Hydroforming published in 2007"

Mechanism of improvement of formability in pulsating hydroforming of tubes

Abstract: The mechanism of improvement of formability by the oscillation of internal pressure in a pulsating hydroforming process of tubes was examined. Free bulging hydroforming experiments of mild steel tubes under oscillating and constant inner pressures were performed. For a high constant pressure, a round bulge with local thinning was observed, whereas wrinkling occurred for a low constant pressure. The occurrence of these defects was prevented by oscillating the internal pressure in the pulsating hydroforming. In the pulsating hydroforming, a uniform expansion in the bulging region was obtained, and thus the formability was improved by preventing the local thinning. It was found from an observation of deformation behaviour, using a video camera, that the tube is uniformly expanded by repeating the appearance and disappearance of small wrinkling. The cause of the uniform expansion for the pulsating hydroforming was also interpreted from the variation of stress components. In addition, a similar deference in deformation behaviour between the oscillating and constant inner pressures was also obtained in finite element simulation.

Structural optimization of an automotive door using the kriging interpolation method

Abstract: Weight reduction of automobile components has been sought to achieve fuel efficiency and energy conservation. There are two approaches for weight reduction: use of materials lighter than steel and structural redesign. The latter has been performed by adopting hydroforming, tailor-welded blank (TWB) technology, optimization, etc. In this research, the kriging interpolation method and a simulated annealing algorithm are applied in the design of a front door made by TWB technology. The design variables were the thicknesses of the parts and the positions of the parting lines. A thickness set, considered as a candidate design of each part, is not arbitrarily determined, but selected from the thicknesses of standard products, and so it is a discrete set. This research presents the discrete and continuous structural optimization of an automotive door design.

Prediction of Necking in Tubular Hydroforming Using an Extended Stress-Based Forming Limit Curve

Abstract: This paper presents an extended stress-based forming limit curve (XSFLC) that can be used to predict the onset of necking in sheet metal loaded under non-proportional load paths, as well as under three-dimensional stress states. The conventional strain-based eFLC is transformed into the stress-based FLC advanced by Stoughton (1999, Int. J. Mech. Sci., 42, pp. 1-27). This, in turn, is converted into the XSFLC, which is characterized by the two invariants, mean stress and equivalent stress. Assuming that the stress states at the onset of necking under plane stress loading are equivalent to those under three-dimensional loading, the XSFLC is used in conjunction with finite element computations to predict the onset of necking during tubular hydroforming. Hydroforming of straight and pre-bent tubes of EN-AW 5018 aluminum alloy and DP 600 steel are considered. Experiments carried out with these geometries and alloys are described and modeled using finite element computations. These computations, in conjunction with the XSFLC, allow quantitative predictions of necking pressures; and these predictions are found to agree to within 10% of the experimentally obtained necking pressures. The computations also provide a prediction of final failure location with remarkable accuracy. In some cases, the predictions using the XSFLC show some discrepancies when compared with the experimental results, and this paper addresses potential causes for these discrepancies. Potential improvements to the framework of the XSFLC are also discussed.

A study on the analytical modeling for warm hydro-mechanical deep drawing of lightweight materials

Abstract: The warm hydroforming process has become an emerging technology in recent years to reduce the weight of automotive body structure and minimize the number of process steps. In this study, analytical models were developed to investigate the effects of process conditions such as temperature, hydraulic pressure, blank holder force and forming speed. The analytical model under hydro-mechanical deep drawing (HMD) condition was developed based on experimental results in the literatures. FE models were also developed to validate the analytic models. Then, the analytic model was validated through comparisons with both existing experimental results and FE results. The analytical model provided rapid and reasonably accurate results for the design of warm hydroforming process. Based on this analytic model, several parametric studies were performed regarding to the temperature, hydraulic pressure, blank holder force, and punch speed conditions. It was demonstrated that the process windows for a successful part forming could be rapidly predicted with a reasonable accuracy by the analytic model compared to lengthy and costly thermo-mechanical FEA or experimental trial and error.

3-D Finite Element Simulation of Pulsating T-Shape Hydroforming of Tubes

Abstract: The effect of oscillation of internal pressure on the formability and shape accuracy of the products in a pulsating hydroforming process of T-shaped parts was examined by finite element simulation. The local thinning was prevented by oscillating the internal pressure. The filling ratio of the die cavity and the symmetrical degree of the filling was increased by the oscillation of pressure. The calculated deforming shape and the wall thickness are in good agreement with the experimental ones. It was found that pulsating hydroforming is useful in improving the formability and shape accuracy in the T-shape hydroforming operation.

A study of techniques in the evaluation of springback and residual stress in hydroforming

Abstract: The increasing need for high strength complex hollow bodies for automotive application is pushing towards the use of sheet hydroforming techniques in conjunction with high strength steels. Unfortunately high strength steels are characterised by high springback levels. In this paper the springback and residual stresses have been analysed by means of laboratory trials carried out using the double sheet hydroforming technique. The attention has been focused on the upper blank of TRIP800 steel. The analysis has been performed using different approaches: i) characterisation of sample accuracy by means of a 3D coordinate measuring machine using a new proposed method based on the standard deviation calculation; ii) FE-analysis of both hydroforming and springback stages using an implicit FEM code; iii) residual stress evaluation by means of X-ray diffraction and laser cutting techniques. The effect of pressure, die insert geometry and friction at the blank holder on springback and residual stresses have been analysed and discussed in detail.

Improvement of Filling of Die Corners in Box-Shaped Tube Hydroforming by Control of Wrinkling

Abstract: The filling of the die corner in hydroforming of a tube with a box die was improved by controlling wrinkling under oscillation of internal pressure. In this process, a small wrinkle occurs near the die corner in the former stage, and then the wrinkle is eliminated in the latter stage because the flat bulge appears in the former stage due to the oscillation of internal pressure. A hydroforming process of steel tubes with a box die was performed in both three dimensional finite element simulation and experiment. The filling of the die corner for the mean linear pressure was not sufficient due to large wrinkles appearing in the former stage, whereas bursting occurs for the peak linear pressure due to round bulging. On the other hand, the uniformity of wall thickness of the formed tube was improved by the pulsating pressure.

Tube expansion by gas detonation

Abstract: The expansion of tubes by direct application of gas detonation waves is an alternative forming method for hollow section workpieces In particular the process can be used for typical hydroforming parts, for example car body or exhaust elements in automotive industry The gas charge of oxygen and hydrogen is both pressure medium and energy source and has the potential to cause high forming velocities The introduced process belongs to the category of high speed forming methods and provides typical advantages such as higher achievable strains compared to quasi-static methods using high water pressure Another advantage of this process is the avoidance of high press forces by application of an “inertia-locked tool” system due to the extremely short process time To develop a controllable process, good knowledge of the interdependencies in the system “medium, workpiece and tool” is essential This can be achieved using simulations in combination with experimental investigations The results are topic of this paper, also including special investigations on the material behavior at high strain rates and temperature gradients

Equal wall stator

Abstract: A method for making a stator assembly including the steps of providing a generally cylindrical stator casing, hydroforming the stator casing into a generally helical shape, and positioning a stator liner having a generally helical shape inside the stator casing.

Optimization using finite element analysis, neural network, and experiment in tube hydroforming of aluminium T joints

Abstract: In tube hydroforming (THF) of T joints, loading conditions (internal pressure and axial feeding) should be determined in such a way that the tube does not wrinkle or burst and is fully calibrated. In the current study THF of an aluminium T joint is simulated with the finite element method (FEM) using a commercial code. An explicit method is used to overcome convergence problems that are encountered in an implicit method. Internal pressure and axial feeding are two variables in the optimization problem and the loading path is optimized. The objective function is the clamping force, and the constraints of wrinkling, minimum thickness, and calibration should be achieved. The objective and constraint functions are obtained by training a neural network and the objective function is minimized using several optimization methods including hill-climbing search, simulated annealing, and complex method. The axial feeding and internal pressure obtained by optimization methods are used to conduct an experiment...

Method and device for producing a cutout or aperture in the wall of a component formed according to the hydroforming process

Abstract: The invention relates to a method of producing a cutout or aperture in the wall of a component formed according to the hydroforming process, for example a hollow body which is located in the forming tool (1), by means of a punch (8, 10) which is displaceable from outside to inside or from inside to outside and which is pressed into the wall of the hollow body. In order to carry out a cutting process in the tool, it is proposed that the wall of the initial workpiece or workpiece mounted in the forming tool (1) and fixed and positioned preferably with sealing plungers (3, 4) first of all be precut abruptly by means of the punch (8, 10) except for a residual wall thickness. The forming process is combined with processes of chipless forming for processing the apertures and cutouts and with finishing, which take place in the tool. Furthermore, a corresponding device is proposed.