With the increasing demand for high-speed trains, large aircraft, and large carrier rockets requiring integral, high-precision, and lightweight structural parts, the sheet parts are developing rapidly. There is a need for producing the parts of larger size, thinner walls, deeper cavities, and more complex surfaces, all while using difficult-to-deform materials. The application of high-performance lightweight alloys for manufacturing large integral components is the primary technical direction for increasing the part bearing capacity limit in both the aviation and aerospace. For example, the aluminum alloy has several advantages, including the low density, high strength, and corrosion resistance; as such, it has been widely used in the aerospace field.
Large-scale ellipsoid parts are the key components of rocket tank; the desired shape is mainly achieved by applying the mechanical force and liquid pressure to drive part deformation. Currently, there are two main methods for manufacturing the large rocket tank: (1) decomposing the part into smaller parts, which are then welded together after forming or (2) using heavy equipment to achieve the integral manufacturing of large parts.
For the former (1), there are several examples. Feng et al.  designed a melon petal dies through numerical simulation and experimental research; the results were then applied to successfully manufacture the smooth surface and wrinkle-free melon flap parts (the deep drawing was used). However, the drawing forming needs multiple mold tests, increasing the mold manufacturing cost. Yang et al.  found pre-deformation introduced in creep age forming can reduce springback and improve mechanical performances. Xu et al.  found the creep deformation is improved and the springback is reduced in the non-isothermal creep ageing process, in compared with the isothermal creep ageing process. Yang et al.  established the finite element model of creep aging forming to manufacture the vehicle fuel tank melon flap. By analyzing the creep strain, equivalent stress, and yield strength, the melon flap parts of a carrier rocket fuel tank were successfully manufactured. However, the creep aging forming has several limitations, including the long production cycle and high cost - both the forming molds and hot pressing tanks are expensive. Additionally, the accurate springback prediction is another problem encountered when using creep aging [5–6]. For this reason, the traditional rocket tank manufacturing process by using melon petal forming and welding generally has low manufacturing efficiency and poor performance.
Regarding the latter, the use of heavy equipment to produce the large parts (2), China's largest vertical spinning machine with (1000 kN capacity) has the following dimensions: it is 30 m long, 18 m wide, and stands 13 m tall . However, it can process the parts with maximum diameter up to 2 m. Similarly, Yuan et al.  have built the double-acting sheet metal hydraulic forming equipment with the world's largest tonnage – 150 MN. The machine is 19.5 m high and has a 4.5 m × 4.5 m working table. It is evident that the huge structure and high price inherent to large equipment pose a great challenge to the equipment manufacturing industry. Therefore, enabling the use of smaller equipment to precisely manufacture the large, thin-walled, and curved aluminum alloy parts is among the critical scientific and technological problems.
The electromagnetic forming (EMF) is a type of special processing method with both high energy rate and high speed. Compared with the traditional quasi-static forming method, EMF technology has a higher speed and the advantage of being non-contact method. This allowed it to greatly improve the material forming limit, simplify the mold manufacturing, reduce residual stress, enhance forming accuracy, and easily control the energy, improving the production automation. For example, Cui et al. [9–10] simulated the electromagnetically-assisted forming of V-shaped and U-shaped parts. Cui et al.  proposed a novel reverse-bending method using EMF. The plastic-strain increases and stress decreases were found in the sheet-bending region following the electromagnetic forming. It should be noted that all the studies presented above show that EMF substantially reduces the springback.
Currently, there are two main strategies for manufacturing the large size parts using EMF:
Using high-energy equipment and a large coil structure – Lai et al.  established an electromagnetic forming device without the assistance of traditional stamping. Combined with the electromagnetic blanking and inertial constraints, 5083-O aluminum alloy hemisphere part with about 1000 mm diameter and 225 mm deep was successfully developed. However, two sets of electromagnetic forming devices over 800 kJ discharge energy and thecoil diameters over 800 mm were required.
A low-energy equipment and a small coil structure – adopted to achieve the integral forming by moving the coil discharge. Cui et al.  proposed electromagnetic incremental forming (EMIF). In this method, the working coil moves step-by-step along the mold profile, with each step being discharged twice in each position. The first discharge reduces the distance between the sheet and the mold profile, while the second one makes the sheet and mold fit completely.
Tan et al.  used EMIF to successfully develop a double-curvature integral wainboard. The influence of technological parameters such as discharge voltage, capacitance, coil height, discharge path, and coil and mold fit overlap ratio was studied experimentally. Furthermore, Li et al.  analyzed the forming mechanism and defect law in large-sized curved surfaces made of aluminum alloy using the EMIF process. They used numerical simulation and experimental verification, finding that the wrinkling was primarily caused by the circumferential compressive stress, which was a consequence of stress wave propagation caused by electric discharge. Finally, Cui et al.  proposed the electromagnetic partitioning forming to achieve precise manufacturing and control the springback when producing curved parts.
Cui et al.  combined the traditional drawing process and the EMIF aiming to manufacture large curved thin-walled parts made of aluminum alloy. After 36 discharges, curved surface parts with a 580 mm diameter and a height of nearly 75 mm were obtained. When the blank holder radius is set to 390 mm, the deformed sheet surface is smooth, and there are no wrinkles, as shown in Fig. 1(a). Inside the dotted red line, the sheet regions deformed using die are shown. Furthermore, to obtain a larger ellipsoid part, the blank holder radius is enlarged to 475 mm. When the forming height is 60 mm, the notable wrinkles appear on the sheet surface (see Fig. 1(b)); therefore, the wrinkling problem must be solved when forming large parts.
In this paper, we applied the electromagnetic incremental forming with a variable blank holder structure, aiming to manufacture larger parts. The thickness-to-diameter ratio of sheet metal is approximately 0.1%, which causes severe wrinkling on sheet in traditional drawing processes. The numerical simulation and the experiment were carried out to analyze the forming process and wrinkling in large parts, both in traditional drawing and EMIF process.