The metal bellows is a tubular part with a continuous corrugated profiled section and is an important component of the bellows assembly. Due to the special structure, it has better functions than connecting straight pipe fittings or metal hoses in connection, sealing and shock absorption. It is also because of this special shape of the pipe wall that more demands are placed on its forming technology and forming equipment. This paper focuses on the high-pressure forming technology of metal bellows and the internal high-pressure forming equipment, which are widely used in the industry. .

The metal bellows hydroforming（internal high pressure forming） technology is a typical application of modern internal high pressure forming in pipe forming. The internal high-pressure forming metal bellows has fewer forming processes, high production efficiency, good product quality, high degree of automation of forming equipment, and simple operation.

The inner high pressure forming corrugated pipe is mainly used for processing stainless steel, low carbon steel, copper alloy and other materials with good plasticity at room temperature, and the thickness of the machineable wall is in the range of 0.08–4 mm. Generally, the machineable wave depth coefficient k is not more than 2 (the wave depth coefficient is the ratio of the outer diameter to the inner diameter of the bellows, which is one of the key parameters determining the geometry of the bellows), and the shape of the tube wall corrugation has a U shape, V-shaped, square, S-shaped, etc., which are easily removed from the mold.

From the analysis of the forming process, the continuous hydroforming（internal high pressure forming） requires a step of positioning, fixing, sealing and the like for the pipe, and the forming efficiency and the forming quality are slightly lower than the overall hydroforming（internal high pressure forming）. Moreover, the continuous internal high pressure forming has many repeated steps, and the rubber seal and the rubber core mold are more likely to be worn and the service life is shorter. The inner diameter of the corrugated tube processed by the whole inner high pressure forming is smaller, because only the side cylinder pusher is required to inject the high pressure emulsion into the inside of the tube blank during the forming process, and the mandrel protruding into the inside of the tube blank is omitted. The mold used in the overall hydroforming（internal high pressure forming） has a high degree of specialization. Generally, one product corresponds to one mold core, and one mold core can only produce one product. Compared to continuous hydroforming（internal high pressure forming）, the degree of generalization of the mold is not high.

Application of metal bellows The metal bellows hydroforming（internal high pressure forming） technology is widely used in practical industrial production. The forming range covers a wide variety of bellows, and has fewer forming processes, high production efficiency, and good product quality. Metal bellows hydraulic forming equipment is popular with bellows manufacturers due to its high degree of automation, stable operation and simple operation. As shown in Fig. 5, some representative metal bellows products processed by hydroforming technology.

The long-distance bellows is a typical large-diameter reducer pipe with a diameter ranging from 240mm to 500mm and a material of 5A03 aluminum alloy. Since the distance between the corrugations is relatively long, generally greater than 100 mm, the existing techniques for manufacturing bellows cannot manufacture such long-distance bellows. The current manufacturing process is manufactured by the "roller-welding" technical route, with low qualification rate and poor weld reliability. The difficulty of forming the whole inner high pressure of the long-distance bellows is as follows: the large diameter is large, and the axial thrust is up to 8000 kN. The displacement accuracy is difficult to control during axial feeding, which is easy to cause wrinkling or cracking; when the whole shape is formed, the number of waves is more than 10, The middle corrugation cannot be replenished with materials, and the thinning rate is large, which cannot meet the design requirements. For the bellows, if all the corrugations are simultaneously formed, since the middle corrugation cannot obtain material replenishment, the maximum thinning rate is as high as 24%, which does not satisfy the design requirement that the corrugation reduction ratio is less than 15%. Moreover, due to the poor plasticity of the 5A03 aluminum alloy pipe, severe thinning is also likely to cause cracking. Long-distance bellows integrally formed wall thickness variation and fracture defects (a) thinning rate distribution; (b) cracking defects. In order to solve the problem of the overall forming of long-distance bellows, a self-feeding partial forming method was invented, which used the internal pressure of the corrugated area to generate the axial pulling force, and pulled the pipe material into the corrugated area to realize self-feeding and improve the wall thickness. Sex. Since each corrugated zone applies internal pressure only at the account portion, the required clamping force is only 300 t, which is reduced to 1/10 of the overall forming. And since the circumferential hydraulic reaction force acts only on a very narrow annular region, the axial thrust is only 30t, which is reduced to 1/20 of the overall shape. The self-feeding partial forming method was used to obtain the long-distance bellows test piece and its wall thickness compared with the wall thickness of the integrally formed test piece. It can be seen that the wall thickness reduction of the corrugated area of ​​the partially-formed test piece is significantly smaller than the wall thickness of the integrally formed test piece. After the end of the forming, the total length of the pipe is shortened by 107 mm, and the average feeding amount of each corrugation is 10.7 mm. The amount of corrugated feed is slightly different, resulting in slightly different thinning rates, but less than 10%, in line with design requirements. Long-distance bellows and wall thickness variation of corrugated area (a) long-distance bellows; (b) wall thickness distribution of corrugated areas. After forming, the minimum wall thickness appears at the top of the corrugation, the corrugation deformation is uniform, the wall thickness difference of the bellows is 3.4%, and the wall thickness uniformity is better.