The viscous backpressure and loading path are the key factors affecting blank forming performance and forming quality. In order to that the high-quality hemispherical parts can be formed in one step, the effects of viscous backpressure and loading path on the forming process of hemispherical parts are analyzed by using the finite element software ANSYS / LS-DYNA. By the comparison of numerical simulation and forming limit diagram of LF2, feasibility of the proposed method and its optimum process parameters can be determined. The finite element analysis model is shown in Fig. 6, composed of mediun bin, viscous medium, blank, blank holder, punch. The plunger, blank, blank holder and punch are meshed with SHELL163 thin-shell elements, and viscous medium are meshed with SOLID164 solid elements; The friction coefficient between the blank and the blank holder and the media bin is 0.125; A pressure is set on the plunger, which can be transmitted to viscous medium as backpressure; A constant blank-holder gap, 0.01mm, is used.
3.1 Finite element analysis scheme
According to the mechanical properties of LF2 and Eq. (2), if the part is formed by a forward flexible die, the required pressure is 7.4MPa. Considering the characteristics of viscous backpressure forming method, the viscous backpressure is selected as 0, 6, 9, 12, 16 MPa respectively during finite element analysis. When the viscous backpressure is 0 MPa, it is equivalent to drawing with rigid punch. Meanwhile, three different loading path is used as shown in Fig. 7. Path I is to load the viscous backpressure after the rigid punch contacts the blank. Path II is to load the viscous backpressure after the punch stroke reaches 10mm. Path II is to load the viscous backpressure after the punch stroke reaches 20mm. Through the above method, appropriate viscous backpressure and loading path, which can improve the quality of the hemispherical part, is determined.
3.2 Finite element analysis results
3.2.1 Deformation process analysis
Fig 8 shows the central cross-sectional shape of the part under different CVP and loading path conditions when the punch stroke is 5, 15, and 25mm, respectively. It can be seen from Fig. 8 (a) that when the punch stroke is 5mm and the viscous backpressure is loaded under path I, because the suspended area between the punch and the blank holder is large and the viscous backpressure is directly loaded at the early stage of the deformation process, so a “inverse deep drawing” phenomenon appears due to viscous backpressure, namely a “pre-inverse forming”. And the greater the viscous backpressure, the more obvious the “pre-inverse forming” effect. However, for loading paths Ⅱ and Ⅲ (Fig. 8 (b), Fig. 8 (c)), the viscous backpressure is loaded during the deformation process. Due to the work hardening of the material and the suspended area decreases, the “pre-inverse forming” area is smaller. Also, the loading the viscous backpressure at a larger punch stroke (path Ⅲ) could lead to the decrease of the “pre-inverse forming” area. It can also be seen from Fig. 8 that as the stroke of the punch increases, the space between the punch and the blank holder gradually decreases. As a result, the “pre-inverse forming” effect gradually disappears and gradually changes from “inverse deep drawing” to “forward drawing” until the blank fits the mold completely.
3.2.2 Stress-strain analysis
Fig. 9 shows the maximum equivalent stress distribution of the formed parts under different viscous backpressure and loading path conditions. It can be seen that the maximum equivalent stress of the part after the pressure of the viscous backpressure is reduced compared with that without the viscous backpressure, and the greater the pressure of the viscous backpressure, the greater the reduction of the equivalent stress. Under the three paths, compared with the case of path Ⅱ and path Ⅲ, the equivalent stress decreases more in the case of path Ⅰ. This shows that the earlier the viscous backpressure is loaded, the better the “inverse pre-forming” effect, and the more conducive to the forming of hemispherical parts. Fig. 10 shows the minimum wall thickness distribution of formed parts under different viscous backpressure and loading paths. The minimum wall thickness under the three paths with viscous backpressure is higher than that of 0.56 mm without viscous backpressure. And with the increase of the viscous backpressure, the minimum wall thickness of the part continues to increase. At the same time, due to the viscous backpressure, the position of the minimum wall thickness of the part has also changed. Compared with the position of the minimum wall thickness at the top of the part when there is no viscous backpressure, the position is transferred to the fillet area in the condition with backpressure, which is consistent with the equivalent stress distribution result (see Fig. 9).
The above analysis results show that, on the one hand, a “pre-inverse forming” effect appears under the combined effect of the viscous backpressure and the rigid punch, which changes the deformation law of the blank. At the same time, due to the change of the geometry of the blank in the deformation process, the stress and strain state has also changed greatly, including the maximum equivalent stress and its location, the maximum wall thickness reduction and its location, resulting in the improvement of the uniformity during deformation. On the other hand, the blank clings to the punch under a higher backpressure, which increases the contact area between the blank and the punch. At the same time, there will be a large viscous friction between the viscous medium and the blank. Under the influence of “double friction” between rigid die and viscous medium, it can not only effectively improve the bearing capacity of force transfer zone, but also prevent the occurrence of “inner wrinkling” in the forming process. Therefore, the earlier the viscous backpressure is loaded, and the higher the pressure, the more conducive to the forming of high-quality parts.
Fig. 11 shows the major and minor strain in forming limit diagram under different viscous backpressure and loading paths. It can be seen that the forming limit of the part is in the rupture zone during forming without the viscous backpressure, which means that the part cannot be formed in one step. Only in the condition of path I with a viscous backpressure ≧ 12MPa,, the forming limit of the part is in the safe zone, and the hemispherical part can be formed in one step.