Hydroforming is a kind of shaping processing technology that uses liquid or die to shape the workpiece, also known as hydraulic forming. It is a method of plastic processing of sheet metal with liquid pressure instead of rigid punch or die, such as bench hydraulic press, deep drawing press, bulging of flat blank, bulging of space blank shape, etc. It is achieved by all kinds of hydraulic press.
Compared with the traditional forming technologies such as stamping and welding, hydroforming is a new metal forming technology. It can form various complex parts, and has the advantages of good surface quality, reduced process, simplified die and no special stamping equipment.
At the beginning, hydraulic machinery was mainly used in aerospace, automobile manufacturing and other fields. With the continuous development and maturity of technology, it is now gradually applied in civil manufacturing.
According to the different liquid media used, hydroforming can be divided intohydraulic press oil or water. The medium used in hydroforming is pure water or an emulsion composed of water and a certain proportion of emulsified oil; The medium used for hydraulic forming is hydraulic transmission oil or engine oil.
According to different blanks, hydroforming can be divided into three types: tube hydroforming, sheet hydroforming and shell hydroforming. The pressure used in tube hydroforming is relatively high, which is also called internal high-pressure forming.
1. Tube hydroforming (internal high-pressure forming)
Tube hydroforming technology is a forming technology that uses tubes as raw materials, applies liquid pressure to the tube cavity and applies load in the axial direction to make it plastic deformation in the given mold cavity, and the tube wall fits with the inner surface of the mold, so as to obtain parts with the required shape. It is an advanced manufacturing technology developed to adapt to the lightweight structure of vehicles, aircraft and other means of transport.
The feature is that hollow parts with special-shaped section whose axis is two-dimensional or three-dimensional curve can be formed as a whole, and the initial round section of the pipe can be formed into rectangular, trapezoidal, elliptical or other special-shaped closed sections

Advantages:
① Light weight and material saving (frame and beam can be reduced by 20% – 40%, hollow shaft can be reduced by 40% – 50%);
② Reduce the weight of parts and molds, and reduce mold costs;
③ It can reduce the amount of subsequent machining and assembly welding, and improve the production efficiency;
④ Improve strength and stiffness, especially fatigue strength;
⑤ High material utilization rate (up to 90% – 95%);
⑥ Reduce production costs.
Disadvantages:
① Due to high internal pressure, large tonnage hydraulic press is required as the die closing press;
② High voltage source and closed-loop real-time control system are complex and costly;
③ The cost of parts R&D and trial production is high.
Its forming process can be roughly divided into three stages:
In the first stage, the filling stage, the pipe is placed in the lower mold, and then the upper mold is closed to fill the pipe with liquid (and remove gas), and the two ends of the pipe are sealed with a horizontal punch;
In the second stage, forming stage, the punch at both ends of the tube is pushed inward according to the set loading curve while the liquid inside the tube is pressurized and bulged, and the tube is basically close to the die under the combined action of internal pressure and axial feeding;
The third stage, shaping stage, is to increase the pressure to make the fillet in the transition area fully close to the die and form the required workpiece. There is basically no material supplement in this stage, and the pipe can be changed into rectangular, ladder, elliptical or other special-shaped sections from the perspective of section.
Its process parameters mainly include:
① Initial yield pressure
Internal pressure required when plastic deformation of pipe starts
② Shaping pressure
It is used to form the transition fillet of the section and ensure the dimensional accuracy
③ Axial feed force
Achieving axial feeding
④ Clamping force
Make the mold closed without gaps
⑤ Replenishment amount
Reduce the wall thickness of the forming area and increase the expansion rate
2. Sheet metal hydroforming
After 20 years of development, sheet metal hydroforming technology has received widespread attention in various fields. It has been widely used in aviation, aerospace, automobile and household appliance manufacturing in developed industrial countries abroad.
Sheet metal hydroforming is a sheet metal forming method that uses liquid as a force transfer medium to transfer the load, so that the sheet metal can be formed on a single side die. Its advantages mainly include:
① Under the pressure of friction holding effect, a friction holding effect is formed between the sheet metal and the punch, which can enhance the bearing capacity of the sheet metal in the punch fillet area, improve the forming limit, and reduce the forming times.
② Fluid lubrication effect The liquid pressure in the liquid chamber makes the sheet close to the punch, and the liquid forms fluid lubrication between the upper surface of the die and the lower surface of the sheet, which can reduce the scratches on the surface of the part, so that the part has good quality, high dimensional accuracy, and even wall thickness distribution.
③ The wrinkle of curved parts is restrained. Due to the reverse hydraulic pressure under the formed sheet metal, the suspended area of curved parts in the die hole is eliminated, making the blank close to the punch, forming a “boss”, reducing the “suspended section” during the drawing of complex parts such as hemispherical and conical parts, and effectively controlling the occurrence of defects such as material internal wrinkle.
④ Under the condition of reducing die and no die, it can process the automobile sheet metal forming workpiece with complex curved surface, change the multiple stretch forming process of the traditional rigid forming process into one-time flexible forming, improve the surface accuracy and internal strength of the formed part, and save a large amount of manpower, material resources and time for die design, manufacturing, and debugging, especially in the production of large sheet metal forming with multiple varieties and small batches, Can overcome the cost and time constraints, so that the product updates faster and faster.
It can be further divided into liquid filled drawing (using liquid medium to replace the female die) and liquid punch drawing (using liquid medium as the male die) according to the liquid medium to replace the female die or the male die.
Liquid filled deep drawing: hot liquid filled deep drawing, which combines the thermal properties of materials with the technical advantages of liquid filled deep drawing, can improve the formability of light body materials with poor formability, such as aluminum alloys and magnesium alloys, and promote their application in the aerospace field.
Liquid filled deep drawing technology
(a) Liquid filling stage (b) Pressure application stage
(c) Forming stage (d) End of forming

The advantages of the liquid filled deep drawing technology are to improve the forming limit and reduce the forming passes, while the disadvantages are that it takes time to fill the liquid, the production efficiency is low, and the tonnage of the forming equipment is large, which makes it inconvenient to move and install.
Liquid punch deep drawing: liquid punch deep drawing uses liquid medium to transfer load instead of punch. Hydraulic pressure acts as the main driving force to deform the blank, and the blank gradually flows into the female die. Finally, under the action of high pressure, the blank is close to the cavity of the female die, and the shape and size of parts are guaranteed by the female die. This forming method can make the blank stretch bulge forming by reasonably controlling the blank holder force, and strain hardening can improve the rigidity, bending resistance and impact resistance of curved shell parts. Therefore, it is very suitable for forming parts with complex shapes (especially small fillets in some parts) and shallow depths of light alloy sheets such as aluminum alloy and high-strength steel.

The advantage of liquid punch drawing technology is that it can form complex surface parts with large depth, while its disadvantage is similar to that of liquid filled drawing technology, that is, it takes time to fill liquid and has large tonnage.
3. Shell hydroforming
The shell hydroforming is to use a closed polyhedral shell of a certain shape as a preform. After the closed polyhedral shell is filled with liquid, it is pressurized in the closed polyhedral shell by liquid medium. Under the action of internal pressure, the shell produces plastic deformation and gradually tends to the final shell shape. The final shell shape can be spherical, ellipsoidal, annular and other shaped shells.
Dieless hydroforming of spherical vessels
(a) Blanking (b) Bending (c) Assembly and welding (d) Hydraulic bulging

Its forming process is to first form a closed polyhedron shell by welding the flat plate, then fill the closed polyhedron with liquid medium (generally water), and apply pressure through a certain pressure system. Under the action of internal pressure, the shell produces plastic deformation and gradually tends to spherical shell.
The main advantages are no need for molds and presses; It is easy to change the shell wall thickness and diameter; High product precision; Reduce cost and shorten manufacturing cycle.
Disadvantages:
① As the technology is “welding before forming”, the welding seam of the closed polyhedral shell bears certain plastic deformation during the forming process. If there is a problem with the quality of the welding seam, it will cause cracking and cause the whole shell to be scrapped. This problem is more serious for thick plates and low alloy steels. Therefore, the key is to control the welding quality.
② For large shells, the support foundation in the forming process is difficult and expensive. For example, the spherical shell with a diameter of 12.3m and a volume of 1000m3 needs to solve the problem of supporting 1000t water and the self weight of the shell.
As with ordinary deep drawing, the pressure is too large, and it is easy to form and blast at the junction of convex ridge and straight wall.
The basic process of the process is to assemble and weld the flat plate or single curvature shell plate through roll bending to form a closed polyhedral shell, and then the closed polyhedral shell is filled with liquid medium (water), and internal pressure is applied to the closed polyhedral shell through a pressurizing system. Under the effect of internal pressure, the shell produces plastic deformation and gradually tends to spherical shell.
For single curvature shell, the main process of this process is: blanking – bending – assembly welding – hydroforming.
In the long run, the shell hydroforming will use light transmission medium, because water as the current transmission medium has the advantages of low cost and cleanliness. However, for large shells, the quality of water is very high, which limits the development and application of this technology. Therefore, the development of medium with density less than water is a major development direction of shell hydroforming technology.
In the welding link, high energy beam welding technology and automatic process will be used to weld the closed shell. At present, manual arc welding is mostly used for closed shells, which is easy to cause quality problems of welded joints and lead to cracks during forming.
In terms of materials, lightweight materials such as aluminum alloy are also a direction of spherical shell hydroforming.