According to the introduction above, the hydrodynamic deep drawing process utilizing a combined floating and static cavity can be divided into two stages: the first stage is the hydrodynamic deep drawing subprocess using the floating die cavity, and the second stage can be regard as the hydrodynamic deep drawing subprocess utilizing the static die cavity. In this method, the essence of the first stage of deep drawing is hydrodynamic deep drawing based on the floating die, and the corresponding liquid cavity pressure is determined by the contact state between sheet metal and floating liquid die cavity. Therefore, the two-step pressure loading path corresponding to the two-stage forming process is adopted in both the simulation and experiment, as shown in Fig.10. During the first stage, the liquid pressure starts from 0MPa and linearly rises to the maximum value while the die stroke reaches the preset value, and then maintains this value to the end of the first stage. After the first stage finished, the floating die starts to move down with the punch, and the corresponding seal formed by the contact between the plate material and the floating die fillet will lose effect. The liquid pressure will be reduced to the level caused by resistance overflowing from the gap between the static die cavity and the plate, because an effective seal has not yet formed at the fillet of the static die cavity, and the corresponding maximum pressure will be determined by the contact force between the plate and the static die cavity. As illustrated in Fig.10, the two stage loading route which characteristic is that the pressure fluctuates in the conversion process of the first stage and the second stage will be adopted for the modified process, which is agreement with the working condition of hydrodynamic deep drawing based on a combined floating and static die cavity.
Finite element simulation and preliminary experiment will be carried out to evaluate the effect of the novel method of hydrodynamic deep drawing utilizing a combined floating and static die cavity. In order to make the results obtained from two methods comparable, the set up used in the common hydrodynamic deep drawing is obtained by removing the floating die cavity from the equipment used in the novel method based on the combined floating and static die cavity, and the pressure loading path is completely consistent. As illustrated in Fig.10, the pressure loading route adopts the two stage loading route. During the first stage, the liquid pressure starts from 0MPa and linearly rises to the maximum value of 16MPa when the die stroke reaches 28mm, and then maintains this value to the die stroke 50mm. In the second stage, the pressure rises linearly from 2MPa and up to 20MPa when the die stroke reaches 75mm, and then remains this value until the end of the process.
Adopted the pressure loading curve shown in Fig.11, the conical cups illustrated in Fig.6 are formed by using both the common hydrodynamic deep drawing process and the modified method using a combined floating and static die cavity respectively. It is clear that the forming parts from both the simulation and experiment for the ordinary hydrodynamic deep drawing process have obvious wrinkle in the upper of the cone wall, as shown in Fig.12(a) and Fig.12(b). In contrast, the cone wall of the conical parts formed from both the simulation and experiment by the improved hydro mechanical drawing method utilizing a combined floating and static die cavity are smooth and wrinkle free, which is illustrated in Fig.12(c) and Fig.12(d). One thing is very clear that the conical wall of the conical cup will inevitably appear wrinkles when the traditional hydraulic deep drawing is used to form the conical cup, while when the hydrodynamic deep drawing process utilizing a combined floating and static die cavity is used to form the part, no wrinkles appear. According to the above analysis, it can be concluded that the modified process can effectively depress the wrinkling in the forming process of conical cups and improve the surface quality of the formed parts.
The fracture usually occurs at the small end of the conical part during the conical part hydrodynamic deep drawing process, and the forming limit of the conical part is determined by both of the rupture and wrinkling. The thinning ratio of wall thickness is one of the simple and effective criteria for fracture judgment. Fig.13 is a comparative diagram of the wall thickness distribution of the parts from the simulations for both the dynamic deep drawing using a combined floating and static liquid pool and the common hydrodynamic deep drawing. Under the same technological conditions, the minimum wall thickness of the conical cup obtained by hydrodynamic deep drawing using a combined floating and static die cavity is 0.5674mm, while that obtained from the ordinary hydrodynamic deep drawing is 0.5669mm. That is to say, the floating die has no effect on the thinnest point of wall thickness distribution, which means that the floating die will not deteriorate the thinning thickness of the thinner point, and has no effect on the rupture limit determined by the rupture.
From the above analysis, it can be seen that the floating die can improve the wrinkling of the conical part in the process of hydraulic deep drawing, and improve the quality of the part. Moreover, besides improving the wrinkling, it has no effect on the forming limit determined by the small end fracture.
4.1. The effect of pressure loading path of the first stage of hydroforming process
4.1.1 Influence of maximum pressure in the first stage
The pressure-loading path is one of the key parameters during the HDDC, and has important effects on the forming process. The process of hydrodynamic deep drawing utilizing a combined static and floating die cavity can be divided into two stages, and then the corresponding pressure loading path is adopted two sage stepwise loading. As shown in Fig.14, the loading pressure path during the first stage of HDDC is similar to that in conventional hydromechanical deep drawing, in which a linear loading pressure path of one inflection point is implemented, during the second stage, the cavity pressure starts from a pre-set value and rises lineally to the maximum value. Keeping the pressure loading path of the second stage unchanged, the influence of the maximum pressure in the first stage on dimension and surface quality the forming part is studied by using the four pressure loading paths in the first stage as shown in the figure 14.
It can be seen from the figure that the change of the maximum pressure in the first stage affects the forming dimensional accuracy and surface quality of the conical wall. In the first stage, when the maximum liquid pressure changes within 12~18Mpa, the roundness of the cross section of the conical part is lied in the mouth of the conical part, and the maximum value is 0.11mm. The wrinkling height of different height sections of conical parts is shown in Fig.15. It can be seen from Fig.15 that the wrinkling height of the parts is less than 0.09mm under the above process conditions, the dimension precision and surface quality of the upper half part of the conical cups are lower than that of the bottom half part. It can be seen from Fig.16 that the dimensional accuracy and surface quality of the parts obtained under the maximum pressure of 16MPa are higher. The maximum value of the first stage and 16MPa is a best choice for the maximum pressure during the first stage.
4.1.2 The reaching moment of the maximum pressure of the first stage
In order to study the influence of the moment when the maximum liquid pressure arrives on the forming accuracy in the first stage of liquid pressure loading path, four loading paths are designed as shown in Fig.17. All of the maximum pressure in the first stage among the four pressure loading paths is 16MPa. The difference is that the maximum pressure is reached when the punch stroke is 20 mm, 24 mm, 28 mm and 32 mm respectively. The pressure in the second stage starts to load linearly from 2MPa, and reaches the maximum liquid pressure of 20MPa when the punch stroke is 75mm, and the maximum liquid pressure is maintained until the punch stroke is 80mm.
It can be seen from Fig.18 that the maximum liquid pressure reaching moment in the first stage affects the dimensional accuracy and surface quality of the part. When the maximum liquid pool pressure in the first stage is 20MPa, the maximum value is reached when the die stroke is 28mm, and the dimensional accuracy and surface quality of the parts are relatively high. In the first stage, under the premise that the floating punch plays a role, the influence of the liquid pool pressure on the forming accuracy and the maximum wrinkle height is small.
4.2. The effect of the loading path of Stage 2
4.2.1. Influence of the maximum pressure in the second stage of HDDC process
The liquid pressure is a key parameter during sheet hydroforming process, and the pressure loading path during HDDC process is divided into two stages as illustrated above. In order to investigate the effect of the maximum pressure of the second stage on the forming process, a series liquid loading paths illustrated in Fig.20 is adopted in the following investigations.
It can be seen from Fig.21 that the change of the maximum pressure in the second stage has little influence on the dimensional accuracy of the part. However, the change of the maximum pressure in the Stage2 has a great influence on the surface quality of the conical wall obtained in the second stage of liquid filling drawing. It can be seen from the Fig.22 that when the pressure in the second stage is 20MPa, the wrinkle at the mouth of the conical part is the weakest, and the surface quality is relatively average.
4.2.2. Influence of maximum pressure arriving moment in the second stage
In order to study the influence of the maximum liquids pressure arriving moment in the second stage on the dimensional accuracy of the forming part, four loading paths of different four maximum pressure arriving moments in the second stage are designed respectively as shown in Fig.23. As shown in Fig.23, with the punch stroke increasing, the liquid pressure rises linearly from 0MPa to the maximum of 16MPa when the punch stroke arrives at 28mm, and then maintains this value until to the punch stroke of 50mm while the first stage of hydraulic deep drawing is completed. In the second stage, all the four loading paths increase linearly from 2MPa to the maximum pressure of 20MPa, but there are differences in the time when the maximum pressure reaches and the maximum pressure arrives at 60mm, 65mm, 70mm and 75mm respectively.
Fig.24 shows the influence of the maximum force arriving moment on the roundness and wrinkling height of the formed part in the second stage of pressure loading. As shown in Fig.24, the moment when the maximum liquid pressure reaches in the second stage has a significant impact on the precision of the formed part. As shown in Fig.25, the moment when the maximum liquid pressure arrives in the second stage has a significant effect on the roundness and wrinkle height of the upper part of the conical wall of the forming part. The later the maximum pressure arrives in the second stage, the higher the forming accuracy of the forming part. Among the four designed loading paths, the one while the pressure in the second stage reaches the maximum at the stroke of 75mm, the dimension accuracy and surface quality of the forming parts are relatively highest.
4.2.3. Influence of initial pressure in the second stage
During the hydrodynamic deep drawing utilizing a combined floating and static die cavity, the beginning of the second stage of the process is very important because the contact state of the suspended sheet between the static die and the punch will change caused by the floating die moving along with the punch. In order to study the influence of the initial pressure of the second stage on the accuracy of the forming parts, four loading paths are designed as shown in Figure 26. As shown in Fig.26, in the first stage, the liquid pressure rises linearly from 0MPa and reaches to the maximum value of 16MPa at the punch stroke of 28mm, and then maintains the value until the punch stroke of 50mm, which means the first stage of liquid filling drawing is completed. Accordingly, in the second stage, the pressure linearly rises from different values to the maximum of 20 MPa when the punch stroke is 70 mm, and maintains this value until the process completely finished. The four loading paths are mainly different from the initial pressure, which are set as 0, 4, 6 and 10MPa respectively.
Fig. 27 shows the maximum winkle height at different heights of the conical parts obtained from the simulations. It can be seen from the figure that the initial pressure has a significant effect on the precision of the formed part in the second stage of pressure loading. Moreover, the smaller the initial pressure is, the better the precision of the formed part is. It can be concluded that the path of linear loading from zero after the loading pressure of the first stage completed is most beneficial to the dimensional accuracy and surface quality of the formed parts.
4.3. Influence of blank holder force on forming accuracy
As illustrated in Fig.10, a two stages pressure loading strategy for the blank holder force (BHF) is used in hydrodynamic deep drawing process utilizing a combined static and floating die cavity. Three kinds of BHF of the first stage, 30t, 40t and 50t, are designed to study the influence of the BHF of the first stage on the forming accuracy of the part under the condition that the blank holder force of the second stage is kept constant value of 40t. The simulation results are shown in the Fig.28 and Fig.29. It can be seen from the figures that under the condition of ensuring the minimum blank holder force without wrinkling, increasing the blank holder force in the first stage of the deep drawing cannot significantly improve the forming accuracy of the part, and the change of the blank holder force in the first stage has little effect on the roundness and wrinkling height of the part.
In the case of the blank holder force for the first stage keeping 40t, the influence of the blank holder force of the second stage on the forming dimensional accuracy of the formed part is studied by adjusting the second stage blank holder force, and four kinds of second stage blank holder forces of 30, 40, 50 and 60t are set respectively. The result of forming precision is shown in the Fig.30 and 31. It can be seen from the figures that, compared with the influence of blank holder force in the first stage, the blank holder force in the second stage has a significant influence on the forming accuracy of the part. The forming accuracy of the part with larger blank holder force in the second stage is higher, and it mainly affects the forming roundness and surface ripple height of the upper part of the part. It can also be seen from the figure that the forming precision is the highest when the blank holder force is 50t.