# Experimental Study on the Current Pretreatment-Assisted Free Bulging of 304 Stainless Steel Sheets
## 3 Results and Discussion
### Influence of Pulse Current on 304 Stainless Steel Sheet Bulging  
The 304 stainless steel plate was treated with different current sizes (0, 10, 15, 20, 25 A), and the convex and concave molds with a slot width of 1.6 mm and a slot depth of 0.8 mm were used to carry out the free bulging experiments under the conditions of a pressure of 0.5 Mpa and a holding time of 1.5 s. The formed bulging parts are shown in Figure 8.  
Under different current sizes, the bulging depth of the stainless steel sheet is shown in Figure 9, and it can be seen from the figure that the bulging depth of the rolled sheet without current application is only 464 \(\upmu\)m. After the application of current treatment, the bulging depth shows a gradual increase in the trend of the bulging depth. When the current is 10 A, the bulging depth is about 476 \(\upmu\)m, which is an increase of about 2.6% compared with the rolled state. When the current is 15 A, the bulging depth is about 482 \(\upmu\)m, which is an increase of about 3.9% compared with the rolled state. When the current is 20 A and 25 A, the bulging depth Is 491 \(\upmu\)m and 503 \(\upmu\)m, which increases by 5.9% and 8.4%, respectively. According to the research of [PERSON] [20], during the plastic deformation process of the sheet, its flow stress and dislocation satisfy the following relationship:  
\[\sigma=\sigma_{0}+\alpha\mathrm{Gb}\sqrt{\mathrm{p}} \tag{2}\]  
where \(\sigma_{0}\) is the flow stress without work hardening, \(\alpha\) is a constant, G is the shear modulus, \(b\) is the Berthold vector, and \(p\) is the dislocation density. Since the rolled stainless steel sheet undergoes large strain deformation, there are a large number of dislocation entanglements and martensite transformation inside it, which seriously hinders the slip and climb of dislocations and causes the flow stress to increase during the bulging process of the sheet. At the same time, the degree of plastic deformation is small, and the bulging depth is low. Compared with the rolled sheet, after the stainless steel sheet is treated with the appropriate pulse current, under the coupling effect of the Joule heating effect of the current, and the electron wind force, the dislocation density inside the sheet is significantly reduced and the martensite undergoes dynamic recovery, which reduces the flow stress. During bulging, large plastic deformation can occur, and the bulging depth increases. In addition, the inertial effect also has a certain impact on the bulging depth. The inertial effect means  
Figure 8: Physical diagram of the bulging of 304 stainless steel sheets with different current sizes. (**a**) Rolling state; (**b**) 10 A; (**c**) 15 A; (**d**) 20 A; and (**e**) 25 A.  
that when the external load on the material disappears, it will continue to deform and move. Research by [PERSON] et al. [21] shows that microscopic cracks inside the material will cause the inertia effect to weaken. However, under the action of pulse current, the material will generate thermal compressive stress at the microcracks, which promotes the healing of microcracks and micropores [22]. With the reduction in microcracks, the inertial effect of the material is enhanced, thereby improving the plasticity and increasing the depth of bulging. Since the formed single channel is a symmetrical shape, 10 positions of half of the formed section are taken for wall thickness measurement, and the wall thickness measurement positions are shown in Figure 10. Each position was measured five times, and the average value was taken. The thickness change in the forming section is expressed by using the thinning rate, and the formula of the thinning rate can be calculated by the following formula:  
\[\mathrm{T}=\frac{\mathrm{t_{0}-t_{i}}}{\mathrm{t_{0}}}\times 100\% \tag{3}\]  
where \(\mathrm{t_{0}}\) is the thickness of the sheet before forming and \(\mathrm{t_{i}}\) is the thickness at the measurement point after forming.