This article was first published in 2001.
If you've looked carefully under a late model car you may have seen an intricately curved engine subframe that is quite unlike previous pressed and welded designs. Or, you may have marvelled at the superb bending of the exhaust header tubes on a BMW M3. Or looked with puzzlement at the inner bumper steel pressing on a Ford Mondeo. How have they formed the steel so seamlessly into such an intricate shapes? The answer is - hydroforming.
Hydroforming is the expansion of tubular metallic tubes by internal water pressure in a closed die. In general the die is opened, closed and maintained in its closed position by a hydraulic press.
1. Close die
The starting tube - a straight or pre-bent tubular section - is placed into a die which has the contour of the component to be produced.
2. Fill with water
The die is closed and held closed by means of a hydraulic press. Hydraulically actuated sealing punches close off the tube ends and fill the tube with the fluid medium, thus building up internal pressure
3. Start hydraulic press, regulate water pressure
In the actual forming process the material is expanded by the internal water pressure. At the same time the sealing punches compress the tube ends. This causes the material to flow into the contour of the die. Precise control of the internal pressure and of the axial cylinder movement enables the material to take on the form of the internal die's contour.
4. Open press, unload component
Hydroforming completed, the hydraulic press opens the die and the component can be unloaded.
Starting material consists of pre-bent or otherwise pre-formed metallic hollow sections. By the means of hydroforming, hollow bodies of complex geometries can be achieved, which with other processing methods could only be produced in multiple parts. The special features of hydroformed components are consistent high tensile strength and rigidity, optimised weight and accurate geometry.
Cycle times of hydroform processes typically range between approximately 14 to 40 seconds. The essential factor of a hydroform process control is the co-ordination of the individual process parameters with each other. Too high an internal pressure may cause bursting. If, on the other hand, the internal pressure is too low, the part will form wrinkles.
Hydroforming equipment, in general, consists of the following components:
- Hydraulic hydroforming press,
- Hydroforming unit with hydroforming water system (tank, pumps, valve units, filters for the forming fluid) and pressure intensifiers,
- Control and regulating techniques consisting of elements to operate the press, the hydroform water system and to control the forming process.
In principle, all materials suitable for other cold forming processes, eg deep drawing, can be used for hydroforming. These include:
- Construction and auto body steels
- Aluminium and its alloys
- Heat resistant and stainless steels
- Copper and its alloys
- Titanium and its alloys
Next week - hydroforming may allow automotive spaceframes to make a comeback.
For the new Volvo 2.8 litre V6 motor, Schuler Hydroforming and the Volvo Car Corporation have developed fresh air suction pipes and turbo high pressure pipes using hydroforming. The special features of these components are their low weight and the reduced number of parts. The alternative to hydroforming was to use the traditional mix of rubber, silicon and aluminium parts linked with hose clips. However, taking this approach gave higher internal pressure losses - up to 22 kPa. Another approach could have been to use sand-cast tubes, however this gave higher weight and required the expensive cleaning of components The internal pressure loss of the hydroformed components was only 10 kPa.