Effect of anisotropy and prebulging on hydromechanical deep drawing of mild steel cups
Introduction
The hydromechanical deep drawing (HDD) process is usually used to form hollow sheet metal parts with high drawing ratios or complicated shapes [1]. Compared with the conventional deep drawing process, the limit drawing ratio can be increased from 1.8 to 2.8 using the HDD process, the tool costs can be reduced remarkably as only one tool half (the punch) is used, the female die is replaced with the chamber fluid, only the punch needs to be varied when drawing parts with different shapes and dimensions [2], [3]. The formed parts have a more uniform thickness and a better surface quality. Compared with the conventional hydroforming process, the rubber diaphragm is dispensed with, the production efficiency is remarkably increased, it is therefore suitable for batch production.
The quality of the formed parts can be influenced by the material properties. Anisotropy has more influence on the part shape and the thickness distribution than in the conventional deep drawing process as the drawing ratio in the HDD process is usually very high [4].
The HDD process and the quality of the formed parts can be influenced by many process variables, such as counter pressure, blankholder force, prebulging pressure, friction and punch speed [5], [6], [7]. Prebulging is believed to be able to repress wrinkles and to improve thickness distribution, thus the limit drawing ratio can be increased. When prebulging is used, the unsupported portion of the blank between the punch and the blankholder is forced to bulge upwards (in the direction opposite to that of the punch travel). As a result, tensile stresses are produced in the circumferential direction of the extended portion so that the compressive stresses due to the drawing deformation are released. The tension forces in the radial direction are also increased by the prebulging pressure, the body wrinkles are thus repressed. If the prebulging pressure is suitable, thinning is effected with the result that the thickness can be more uniform than without prebulging. It is therefore necessary to investigate the influence of prebulging on wrinkling and thickness distribution so that suitable prebulging pressure values and loading curves may be determined.
Finite element methods have been used in the research of the HDD processes in recent years [8], [9], [10], [11], [12]. It has been proved possible to analyze the HDD processes and to predict the possible process defects with finite element methods [13].
In this paper, the HDD processes of mild steel cups were experimented with and simulated by the explicit finite element method with different prebulging pressures. The influences of the anisotropy and the prebulging pressure are discussed in detail. The numerical results were discussed and compared with the experimental results.
Section snippets
Experiments
Mild steel cups were formed with the HDD process, the experimental system is shown in Fig. 1.
The material properties and the process parameters are shown in Table 1. The experiments were carried out on a 200 t hydraulic press. In the process four specially designed spacers were used to limit the gap between the blankholder and the die, which was assumed a constant value and was measured to be 1.11 mm during the experiments on steel cups when a constant blankholder pressure of 20.25 MPa was used.
Finite element models
The explicit finite element code DYNA3D was used to simulate the HDD processes of the mild steel cups in the present paper. Mattiasson et al. [14] discussed the explicit FE method and its applications in sheet deep drawing. Whirley et al. [15] introduced the techniques used in simulations with the DYNA3D code. Galbraith et al. [16] introduced the applications of the various material models, including Barlat and Lian’s three-parameter material model.
The FEM models are shown in Fig. 4 in an
Effects of the anisotropy on the shape and thickness variations
Fig. 5 shows the deformed profile of specimen SF3, which shows apparent earning due to the anisotropy.
Fig. 6(a) shows the measured shape variations of the cup wall along the radial direction, in rolling direction (0°), in the direction perpendicular to the rolling direction (90°) and in the direction between them (45°) separately. It is seen that the shapes in the 0° and 90° direction are very close, while the shape in the 45° direction is quite different from them. This is due to the effects
Conclusions
The HDD processes have been analyzed experimentally and numerically. Mild steel cups with the drawing ratio of 2.5 were formed with the fixed gap method and the conventional method successfully. The processes are also analyzed with the explicit finite element method successfully. The anisotropy of the sheet material has a strong influence on the shape variation of the cup wall and the thickness distribution of the cup. The thickness variation and the shape variation increase with the increase
Acknowledgements
The present work is performed under the Danish Materials Development Programme financed by the Danish Agency for Development of Trade and Industry, the Danish Natural Science Foundation and the Danish Technical Research Council. The work is also supported by the Natural Science Foundation of China.
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