# Experimental Study on the Current Pretreatment-Assisted Free Bulging of 304 Stainless Steel Sheets
## 3 Results and Discussion
### Influence of Mold Size on 304 Stainless Sheet Bulging  
Under the pressure condition of 0.5 Mpa and holding time of 1.5 s, the 304 stainless steel sheet was subjected to 25 A current treatment, and the stainless steel sheet was subjected to bulging experiments using molds with different convex and concave mold sizes (slot width W of 1.2 mm, 1.6 mm, and 2 mm). The specific mold parameters are shown in Table 4. The single-channel bulging parts under different mold sizes are shown in Figure 12. The depth-width ratio of the bulging parts under different mold sizes is shown in Figure 13.  
Figure 11: Distribution of the single-channel wall thickness and thinning rate. (**a**) Wall thickness distribution. (**b**) Wall thickness thinning rate. It can be seen in Figure 13 that the single-channel aspect ratio of the bulging part increases as the mold size increases. When the groove width W of the convex and concave mold is 1.2 mm and the groove depth H is 0.6 mm, the aspect ratio (W/H) of the bulging part is 0.32. When the groove width W of the convex and concave mold is 2 mm and the groove depth H is 1 mm, the aspect ratio (W/H) increases from 0.32 to 0.4, which results in a better filling effect. This is mainly because the mold cavity size in the bulging process of the material has a certain constraint. The plastic deformation of the sheet is limited to the fixed deformation zone of the convex and concave molds. During the forming process, the sheet on the periphery of the forming area is tightly attached to the surface of the mold under the action of the pressing force \(Fc\), and it does not enter the deformation zone from the outside and the material inside the forming area does not transfer to the outside of the deformation zone. Under the action of the punch pressure load FP and the inertial force, the material in the deformation zone is stretched and thinned because of bidirectional tensile stress (radial stress \(\sigma_{1}\) and tangential stress \(\sigma_{2}\)), and it starts to flow from the rounded corner part to the inside and gradually fills the entire mold cavity, forming a single-channel micro-groove. The bulging principle is shown in Figure 14. Under the same load, when the mold size is smaller, the material inside the convex-concave model cavity is more constrained during the filling process, and flow and plastic deformation are more difficult. Therefore, the depth-to-width ratio of the entire bulging member is small. As the size of the convex-concave model cavity increases, the material flows more easily into the cavity, thereby better completing plastic deformation, and the aspect ratio gradually increases. In addition, during  
Figure 12: Real picture of the 304 stainless steel sheet formed under different die sizes. (**a**) Slot width of 1.2 mm; (**b**) slot width of 1.6 mm; and (**c**) slot width of 2 mm. Figure 13: Depth-to-width ratio of the 304 stainless steel sheet for a single channel under different mold sizes. the forming process, the load on the material per unit area is the same. As the size of the convex and concave molds increases, more materials participate in plastic deformation, which can obtain greater deformation energy and increase the deformation amount of the sheet. The thickness of the stainless steel sheet bulging parts under different mold sizes was measured at different positions, and the wall thickness distribution diagram shown in Figure 15 was obtained. It can be seen in the figure that as the mold size increases, the overall wall thickness distribution of the bulging part becomes more uniform. When the mold size has a groove width W of 2 mm and groove depth H of 1 mm, the thinning rate at fillet position 9 improves significantly and drops to about 19.4%. This is because as the mold cavity size increases, the fillet of the convex and concave molds also increases from the original 0.12 mm to 0.2 mm.