Issue 49

M. Abbadeni et alii, Frattura ed Integrità Strutturale, 49 (2019) 282-290; DOI: 10.3221/IGF-ESIS.49.28 283 There are many advantages for this process, over conventional metal forming processes, such as better surface quality, increased drawing ratio, forming of complex shapes, cost-effective parts and lower tool costs [2]. HDD process, illustrated in Fig. 1(b), is one type of sheet hydroforming. Comparing the HDD process with conventional deep drawing process (CDD) (Fig. 1(a)) it can be noticed some differences in tools, contact conditions and producing technology. The female die in the CDD process is replaced by a cavity filled with a fluid in the HDD process. The final form of the part is determined by the punch. In this process, when the punch penetrates in the die cavity, the liquid is pressurized and pushes the sheet onto the punch surface (Fig. 2). The fluid pressure in the cavity can be controlled by a valve. The use of the fluid makes possible to reduce the friction and to prevent metal-to-metal contact at the blank-die interface which improve the possibilities of obtaining a better geometry of the final products [3-6]. Warm pressurized fluid can be used to increase the formability of lightweight alloys [7,8]. The study of the HDD process and the effects of the fluid pressure were the subject of several research works. Singh and Kumar [9] studied the HDD process to show the influence of the fluid pressure in the cavity on the thickness distribution and the surface quality of the final products. They found that thinning of the sheet at the corner zone which exists in traditional sheet forming processes was reduced in the HDD process. They also noticed that the thickness distribution became more uniform with the increase in the maximum fluid pressure in the cavity. Modanloo et al. [10] studied the effects of forming media on the HDD process experimentally and by simulation using a typical pressure path. Water and oil were used as a forming media. They reported that by using oil, thinning decreases at the punch corner radius. In another work [11] they investigated, by experiments and simulation, the effects of forming parameters (fluid pressure, punch velocity, friction coefficients, punch and die corner radius…) on the thinning ratio of the deformed part. They found that a higher corner radius of the punch and the die lead to an increase in thickness reduction. Moreover, an optimization method was presented to improve thinning ratio. In order to improve formability of the parts in HDD process, Salahshoor et al. [12] investigated the effects of geometrical and process parameters on thickness distribution of cylindrical parts. The results showed that the pressure path has a great influence on the formability. By increasing the maximum fluid pressure, a reduction in thinning tendency is observed. Furthermore, by decreasing the friction in the blank-blank holder contact region, a decrease in thinning and maximum punch force was observed. The effect of the initial pressurization during the HDD process was studied by Lang et al. [13]. The obtained results showed that the use of an initial pressurization has a significant influence on the first stages of forming. The rise of the pressure increases the surface of contact between the part and the punch surface which avoid the rupture in the contact area. However, for excessive values of the pressure, the rupture will take place in the critical zones of the part. In another work [14], they studied the influence of the fluid pressure in the cavity on the deformation of the part. In this work, a uniform pressure was applied on the lower surface of the sheet. They found that a high pressure in the cavity can reduce the thickness of the formed part. Hama et al. [5] established an experiment on the HDD process. The fluid pressure was measured in various positions (in the cavity, in the die corner and between the sheet and the die). From this experiment study, they showed that the drawing ratio in the HDD process is better compared to CDD process. Figure 1 : Conventional and hydromechanical deep drawing processes. Blank (Sheet) Die Blank holder Fluid cavity Punch Relief valve (a) Conventional deep drawing (b) Hydromechanical deep drawing