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Hydroforming is a plastic forming technique that uses a liquid or a die to shape a workpiece; it is also known as hydraulic forming. Depending on the liquid medium used, it is generally divided into hydroforming with water and hydroforming with oil. This technique requires only a female die or a male die, with the liquid medium serving as the male die or the female die accordingly. Since the liquid acts directly on the part, sealing is difficult, and the manufacturing process is much more complex than that using steel or rubber molds. As a result, it is not yet widely used in mass production; at present it is mainly applied in small-batch production. Features of internal high-pressure forming technology: The main advantage of internal high-pressure forming technology is its ability to form hollow parts with complex cross-sections, whose axis has a two-dimensional or three-dimensional curved shape. Starting from a circular cross-section, it is possible to create rectangular, trapezoidal, oval, or other irregular closed cross-sections, as shown in the figure. Traditional manufacturing processes generally involve first stamping two or more half-shells and then welding them together to form a whole. To reduce welding deformation, spot welding is typically used, resulting in a cross-section that is not closed. Furthermore, the cross-sectional shape of stamped parts is relatively simple, making it difficult to meet the requirements of structural design. The internal high-pressure forming process is mainly divided into three stages: (1) Initial filling stage – the tube blank is placed in the mold cavity and the molds are closed; axial punches at both ends move forward horizontally to create a seal. Expel the air from the tube by priming with liquid. (2) Forming stage: While the tube blank is pressurized and expanded, the punch moves inward to fill in the material according to a predetermined loading curve; under the combined effect of internal pressure and axial filling, the tube blank is brought into close contact with the mold. At this stage, most of the area except for the transition R corner has been formed. (3) Shaping stage: The internal pressure is increased to ensure that the transition R-corner fits perfectly against the mold cavity, completing the shaping of the workpiece. The main technical parameters of internal high-pressure forming include initial yield pressure, cracking pressure, forming pressure, axial feeding force, clamping force, and material feeding amount. (1) Initial yield pressure: The pressure required for the pipe billet to undergo plastic deformation. (2) Cracking pressure: The pressure at which the pipe billet cracks. (3) Shaping pressure: The pressure required during the later shaping stage to ensure complete formation of the part. (4) Axial feeding force: The basis for selecting the axial propulsion cylinder is determined by three factors, namely the force required to ensure plastic deformation of the tube blank, the high-pressure reaction force from the punch, and frictional forces. (5) Clamping force: The force required to close the mold during the forming process; it is the main factor in selecting a hydraulic press. (6) Feed amount: An important parameter for determining the stroke of the horizontal cylinder. Due to the effects of the loading path and friction, the amount of material supplied cannot be delivered entirely to the forming zone; as a result, the wall thickness in that zone decreases. The actual amount of material supplied is usually 60% to 80% of the ideal amount. Compared to traditional sheet metal stamping and welding processes, internal high-pressure forming uses tubes as the material to be processed, and it has the following advantages: 1) This process can reduce development and manufacturing costs, lower the weight of the vehicle body, and improve material utilization efficiency. Internal high-pressure forming usually requires only one set of dies, whereas sheet metal stamping often entails three or more processing steps, which increases the costs associated with tool development and subsequent manufacturing. As the number of processing steps decreases, the process waste also decreases accordingly. While meeting the requirements for part performance, internally pressurized formed hollow parts can achieve a weight reduction of 20%–30% compared to stamping-welded assemblies, with a material utilization rate increase of 30%–50%. 2) The internal high-pressure forming process can improve the machining accuracy of parts and the safety performance of the vehicle body. For parts with complex shapes, internal high-pressure forming enables one-step molding, avoiding the cumulative errors that occur during multiple processing steps and thereby improving the precision of the parts. Internal high-pressure forming is a cold working process; through work hardening during the deformation process, it is possible to **increase the strength of the parts. Moreover, the original tube blank retains good integrity, which ensures adequate overall stiffness. As a result, its use in the load-bearing structural components of automobile bodies can enhance the safety of those bodies. 3) Since high pressures are required for internal high-pressure forming, the tonnage of the clamping press needed is also high, usually exceeding 3500 tons. The sources for generating such high pressures as well as the electrical control systems are relatively complex, resulting in high manufacturing costs for the equipment. Furthermore, since the quality of part formation and wall thickness distribution are closely related to the loading path, the costs associated with their development and prototyping are high. To some extent, these factors have restricted the development and widespread application of the internal high-pressure forming process. Application areas: Internal high-pressure forming is an advanced manufacturing technology that has been developed to meet the need for lighter structures in vehicles such as cars and airplanes. There are two main approaches to structural lightweighting: one is the material approach, which involves using lightweight materials such as aluminum alloys, magnesium alloys, titanium alloys, and composite materials ； The second is the structural approach, which employs structures such as hollow variable-cross-section and variable-thickness thin-walled shells, as well as monolithic structures. According to statistics, for a certain weight reduction goal in the aerospace industry, the contribution of using lightweight materials to achieving weight reduction is approximately 2/3, while the contribution of structural weight reduction is about 1/3 ； In the automotive industry, on the contrary, weight reduction through structural changes is primarily employed. When the material is fixed, the main method to reduce weight is to design a lightweight structure. For structures subjected primarily to bending and torsional loads, the use of hollow variable-section members allows for both weight reduction and optimal utilization of the material’s strength. In recent years, high-pressure forming technology for tubular parts has been rapidly adopted in the automotive industry due to its process and cost advantages. To further promote lightweight vehicle design, more hollow components will be developed for the vehicle body. As internally pressurized formed parts are increasingly used in various parts of the vehicle body, they may revolutionize people’s understanding of car design and body structures.