Thread Content
Based on automotive lightweighting technologies, Sheet hydroforming was studied. The principles of the hydroforming process, its characteristics, as well as the structural design of hydroforming dies were investigated. Simulation experiments and physical tests were conducted on engine cover stamping parts using hydroforming; the results of these experiments and tests were consistent, with stamping parts of qualified quality being produced in both cases. The application of sheet metal hydroforming technology has effectively promoted the development of automotive lightweighting technologies. Sheet hydroforming is a special type of flexible forming process in which liquid is used as a medium to replace rigid punches or dies for transmitting forces. Under the pressure of this force-transmitting medium, the sheet material is pressed against the punch or die, thereby enabling the formation of metal sheet or tube components. The components that can be formed using this method include cylindrical parts, box-shaped parts, components with complex curved surfaces, and hollow tubes. Sheet hydroforming can also be referred to as hydraulic forming. Hydroforming technology is primarily used in the production of a variety of complex parts in industries such as automotive, aerospace, and electrical instrumentation, in order to meet the strict quality requirements imposed by the specific operating conditions. This paper mainly focuses on the application of hydroforming technology in the lightweighting of automobile bodies. Figure 1 shows a typical part manufactured using hydroforming technology. Figure 1: Stamped parts produced by hydroforming technology I. Principle of the hydroforming process: 1. Process principle The hydroforming processes include hydraulic bulging, rubber diaphragm hydraulic bulging, fluid-filled drawing forming, dieless hydraulic bulging, and viscous medium hydraulic bulging, among others. The principle of the hydroforming process is shown in Figure 2; Figure a illustrates the use of a liquid to replace the die, while Figure b shows the use of a liquid to replace the punch. Figure 2 Principle of hydraulic forming process 2. Principle of hydraulic bulging process for sheets The fluid medium acts as the punch; under the pressure of the liquid, the sheet is pressed against the die cavity to form the part. Hydraulic bulging technology is characterized by the use of liquid pressure to shape sheet metal under mold constraints; it is even possible to achieve shaping without any molds, allowing for the production of complex hollow parts, as shown in Figure 3. Figure 3 Principle of hydraulic bulging process 3. Principle of hydraulic bulging process using rubber bladder dies In the early stages of hydraulic forming of sheets, rubber bladder hydraulic forming technology was employed. This technology uses a rubber bladder as an elastic female or male die, with hydraulic oil serving as the medium for transmitting pressure in order to shape sheet metal parts. During the forming process, a rubber diaphragm separates the liquid medium from the sheet metal; thus, the need for a female die is eliminated. Since the rubber remains in close contact with the part throughout the forming process, there are no scratches on the surface of the part. Moreover, under high pressure and friction, the plasticity of the material can be fully utilized, resulting in minimal springback, excellent adhesion of the coating, relatively uniform thickness distribution, and reduced internal damage to the material. This approach significantly improves the quality of part formation and the reliability of the structure, making it suitable for stamping complex automotive body panels, as shown in Figure 4. Figure 4 Principle of hydraulic bulging process for rubber bladder sheets 4. Principle of sheet metal hydro-punching drawing process The sheet metal hydro-punching drawing process mainly includes two types: radial drawing and radial hydro-punching reverse drawing. The drawing process consists of a flange, a die, a hydro-fluid chamber, and a punch, as shown in Figure 5. Figure 5 Principle of the sheet metal hydro-drawing process 5. Principle of the hydraulic forming process with a movable die The hydraulic forming technology using a movable die for sheet metals involves the use of a combined die consisting of a fixed part and a movable part, to achieve combined forming processes of drawing and bulging. By utilizing the hydraulic forming technique with movable concave dies, the thinning of the sheet metal can be significantly reduced, and its forming limits are greatly improved. This new technology is suitable for the stamping of sheet metal parts with complex shapes, as well as of low-plasticity lightweight materials such as aluminum alloys and magnesium alloys, as shown in Figure 6. Figure 6 Principle of hydraulic forming process with a movable die II. Characteristics of the hydraulic forming process: Compared with traditional stamping processes, the hydraulic forming process has the following main characteristics: (1) For sheet metal hydraulic forming, only a die or punch is required; the other component is replaced by a liquid medium, which reduces the cost of die manufacturing. The die cost can generally be reduced by more than 30%, thereby lowering the production cost of the parts. At the same time, hydraulic forming molds can be manufactured from inexpensive materials, and parts of different thicknesses and made from various materials can be produced using the same set of molds. (2) Sheet hydraulic forming can improve the quality of products and significantly enhance their performance. Parts formed by this method have advantages such as light weight, high strength, high hardness, and good quality; they also feature high material utilization, high dimensional accuracy, low springback, and low residual stress. (3) By changing the shape of the die or punch, it is possible to form parts of various shapes. Hydraulic forming allows for the creation of some complex-shaped parts in a single stamping step, whereas traditional stamping requires multiple steps to achieve the same result, thus **optimizing the stamping process**. (4) Sheet hydraulic forming is particularly suitable for materials with poor formability or high strength; it can also be used to form composite materials, which helps to reduce the weight of components and contributes to the development of lightweighting technologies in automobiles. (5) The time required to replace the mold and carry out molding is relatively short, **improving production efficiency. III. Structural design of the hydraulic forming tool and die: As shown in Figure 7, a represents the structural diagram of the hydraulic forming tool and die, while b represents its sealing system. Figure 7: Mold structure diagram. IV. Results and analysis of hydroforming experiments: To verify the application of hydroforming technology in sheet metal forming, hydroforming simulation experiments and physical tests were conducted on the engine cover of a certain vehicle model. This sheet metal part has a relatively complex spatial geometric shape; when formed using conventional molds, defects such as rounded corners and cracked side walls, as well as wrinkling and overlapping issues, often occur. These defects in sheet metal forming cannot be completely eliminated by adjusting the stamping process parameters. However, through the application of hydroforming technology, the aforementioned defects such as cracking and wrinkling are essentially eliminated. As shown in Figure 8, which depicts the forming limit diagram for the engine cover stamping parts, it can be seen from the forming limit FLD diagram that the products fall mainly within the green safe zone, with no defects such as cracking, wrinkling, or overlapping on their surfaces. Figure 9 shows the physically verified stamping parts. As can be seen from the figure, the product quality meets all technical requirements; the results of the simulation experiments are consistent with those of the physical tests, indicating that hydraulic forming technology yields qualified stamping parts. Figure 8: Hydroforming limit diagram of the engine cover. Figure 9: Physical prototype for the hydroforming of the engine cover. V. Conclusions: (1) The results of research on hydroforming technology show that it can improve product quality and address issues related to the forming of complex stamping parts. Additionally, the use of hydroforming technology helps to reduce costs associated with mold development. (2) The application of hydroforming technology can **optimize the stamping process and reduce the number of stamping steps. (3) The application of hydroforming technology has effectively promoted the rapid development of automotive lightweighting technologies. Author: Zhu Meiyun Source: Forging Equipment and Manufacturing Technology