Hydroforming

In the classical sense is meant by internal high -pressure forming (short IHU ) or hydroforming (often used synonymously ) the forming of metallic pipes in a closed mold by means of internal pressure which is introduced by a water -oil emulsion in the tool or pipe ( in the English tube hydroforming ). The pipe ends are during the forming process by sealing ram, driven by hydraulic cylinders, sealed. Significant process parameters are the internal pressure, which may be in series production usually up to 3000 bar, and the subsequent pushing of material or compressing of the component ends here with the help of the sealing die. There are a number of process variations (such as pre- surgery ) and supplements (e.g., built- hole operations). The processes take place at room temperature.

In a broader sense also more media-based method for forming of metal sheets, the internal high -pressure forming / hydroforming are often attributed ( in English sheet hydroforming ). It is variously but used in specialist publications, the notion of external high pressure forming. These methods include in particular the hydraulic deep drawing and the hydromechanical deep drawing, and a variant of the method of the Active Hydromechanical deep drawing.

History

The process principle of the hydroforming was known already in the 19th century. The method based on the fact that a change in volume or reduction in the subsequent pushing of the material results from the component ends thereof inside, whereby the forming pressure has been established. Thus, the process could not be controlled and was therefore difficult to control. He won only in the second half of the 20th century in importance as the modern control and instrumentation technology has opened up new possibilities.

The classic component that is manufactured using hydroforming and is still used today, is the copper T- piece which is inserted into the water lines almost every building. During the 1990s, the process has become increasingly important, as increasingly shaped hydroformed components are used in the automotive industry. The hydroforming takes today as a special method an important place in the field of metal forming.

The first mass-produced car, which had standard equipment larger hydroformed parts in the body, was the Audi A8 ( since 1994).

Process engineering

Plant technology

The molds for the production of hydroformed components are typically operated in hydraulic presses, which are supplemented by a corresponding water hydraulic unit and the associated control. The hydraulic press is used as a retaining device for the tool. Due to the variable pressure gradients in the tool who want to push apart the mold halves, a controlled locking device in most cases is essential.

Due to the high pressures in the mold presses with large clamping forces are usually required. The mold design must take into account the high pressures to ensure appropriate times.

Process design and feasibility limits

Since this is a complex process with many adjustable parameters in hydroforming, a high effort for process design is often required. In many cases, the feasibility studies and Verfahrensvorauslegungen can be performed by means of finite element simulations. Nevertheless, a prototyping is often advisable to confirm the feasibility or to expect a complex commissioning process.

The feasibility depends mainly on the required for the production of a specific component maximum expansion / elongation from. This must not exceed the uniform elongation of the material used in the normal case. Only in the so-called Nachschiebebereich (range where with the help of the temple sealing material can be pushed ), this can be partially exceeded. This is not sufficient, possibly a Zwischenglühprozess is required.

Materials with high extensibility (for example, stainless steel, copper ) are so far better suited for the hydroforming process than those with low extensibility (for example, high-strength steels, aluminum). But also the effective processing of materials with low extensibility made ​​possible by advancements of the hydroforming process. FLC curves of the material used are essential for professional feasibility studies, FEM simulations and methods of interpretation.

Another feasibility limit is the maximum available internal pressure and the resulting maximum possible shape of the component (in particular in case corners and at small radii ). The maximum internal pressure is in turn limited by the economically viable systems engineering.

Pros and Cons

Significant disadvantages of the hydroforming process are the relatively long cycle times ( compared for example with conventional deep drawing ) and the high investment costs ( at IHU / Rohrumformwerkzeugen ).

The advantages, however, are broad design freedom, saving individual parts ( by combining individual components, for example, previously used semi- monocoque construction ), by elimination of assembly and / or welding operations, work hardening of the material during the forming process and weight savings as a result of the aforementioned items. Also, the high precision and repeatability of the components features of the process.

Areas of application

IHU is used preferably for pipes with a variable thickness and for fittings such as tees or taps. Even in the modern bicycle construction tube set with complex shapes are produced by hydroforming. By this method, one can adapt and shaping the wall thickness as required; this can save a lot of weight and cost.