Experimental Study on the Current Pretreatment-Assisted Free Bulging of 304 Stainless Steel Sheets

In order to further improve the microplastic deformation ability and forming performance of metal sheets, a pulse current pretreatment-assisted micro-forming method was proposed. Firstly, a 304 stainless steel sheet was pretreated with current (0–25 A), and the microstructure changes in the sheet under the action of current were analyzed. Then, a current-assisted bulging experiment was carried out from three aspects as follows: current size, mold structure size, and material properties, to explore the influence of different process parameters on the micro-bulging of the sheet. Finally, the forming quality was analyzed and evaluated from the two perspectives of bulging depth and wall thickness uniformity. The research results show that when the current intensity increases from 0 to 25 A, the fibrous distribution in the thickness direction of the sheet is alleviated, the structure is more uniform, the bulging depth shows an increasing trend, and the thinning rate and wall thickness uniformity are improved. When the current intensity reaches 25 A, the bulging depth increases from the original 463 μm to 503 μm, and the thinning rate drops from the most serious 48.52% to 19.4%. At the same time, as the mold size increases, the single-channel aspect ratio (W/H) also increases accordingly. When the mold groove width (W) is 2 mm, the ratio reaches 0.4, the sheet deforms significantly, and the filling effect is better. In addition, the larger the roundness of the convex and concave molds, the more uniform the wall thickness distribution of the bulging parts. Under the same experimental conditions, the bulging depth of the 304 stainless steel sheet is higher than that of TC4 titanium alloy, and it is less prone to springback and is more conducive to plastic deformation.