# Experimental Study on the Current Pretreatment-Assisted Free Bulging of 304 Stainless Steel Sheets
## 2 Experimental Procedure and Mold Design  
In this experiment, a cold-rolled 304 stainless steel sheet with dimensions of 150 \(\times\) 20 mm \(\times\) 0.1 mm was selected as the current pretreatment-assisted free bulging material,and TC4 titanium alloy was used as the comparison material. Its chemical composition is shown in Table 1. Before conducting the bulging experiment, a pulse current pretreatment experimental platform was independently built. The current pretreatment platform is shown in Figure 1. The platform consists of a pulsed power supply and a treatment device. The treatment device includes a metal sheet, a baseplate, pressing plates, clamps, and electrodes. The baseplate is made of high-temperature resistant asbestos board, and the pressing plates are made of ceramic fiber material, which can withstand high temperatures and play an insulating role. The clamps are used to hold the sheet. A T2 copper sheet with a thickness of 3 mm was used as electrodes to achieve rapid current transmission. The 304 stainless steel sheet was pretreated using the current pretreatment platform. The pulse current parameter information is shown in Table 2. In order to prevent the metal sheet from oxidizing, a layer of boron nitride paint was coated on its surface. At the same time, an infrared thermometer gun was used to monitor the temperature changes in the sheet in real time. Finally, the pretreated material was cut into 20 \(\times\) 20 mm square blanks for micro-bulging, as shown in Figure 2. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Material** & \multicolumn{6}{c}{**Element Content**} \\ \hline
304 stainless steel & C & Si & Mn & P & S & Ni & Cr & Fe \\  & 0.08 & 0.75 & 2 & 0.045 & 0.03 & 8\(\sim\)10.5 & 18–20 & Bal \\ TC4 titanium alloy & Al & V & Fe & C & H & O & Ti & \\  & 5.917 & 4.614 & 0.115 & 0.005 & 0.008 & 0.071 & Bal & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of 304 stainless steel and TC4 titanium alloy (wt%). Figure 1: Current pretreatment platform. (**a**) The pulse power supply used to provide current. (**b**) The treatment device used for current pretreatment. The micro-bulging experimental device consists of a pressure-loading device and a bulging mold. The pressure-loading device uses an SK-3T three-plate and four-column pneumatic press. The various process parameters are shown in Table 3.  
Since the pneumatic press relies on gas to transmit pressure, an external air source device was required. An air compressor capable of supplying 0.7 MPa was selected based on the requirements of the suitable air source range. The whole pressure-loading device is shown in Figure 3.  
The bulging mold is the core of micro-forming. On the one hand, it must be accurately connected to the press, and on the other hand, it must ensure the mold closing accuracy of the upper and lower molds. This article uses traditional rigid mold micro-bulging. The mold is shown in Figure 4, which mainly consists of the upper and lower templates, convex and concave molds, pillars, washers, and several parts. The entire mold adopts an inlaid design, and the mold base is fixed. The convex and concave molds are replaced with different sizes to bulge parts of different sizes. The forming principle relies on the press to provide pressure to complete the closing of the convex and concave molds, causing the material to deform plastically in the mold to form micro parts. The experimentally designed single-channel convex and concave mold is shown in Figure 5. The main parameters of the single channel include groove width, groove depth, and fillet. Three sets of convex and concave molds of different sizes are used in this article. The ratio of the groove width to groove depth satisfies 2:1 to analyze the influence of mold size on bulging. The geometric parameters are shown in Table 4.  
Figure 3: Pressure-loading device. (**a**) Pneumatic press. (**b**) Pressure-loading device. Figure 2: Square blanks.