Hydroforming

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External high pressure forming

Hydroforming (IHU short) is the hydroforming (mostly used synonymously) attributable means and the forming of metal tubes or hollow body in the closed mold by means of internal pressure. It is similar to deep drawing with active media . It is standardized in DIN 8584 and counted as part of tensile compression forming .

From other active media-based methods for forming metal sheets, e.g. B. external high pressure forming (see picture on the right, sheet hydroforming ), hydraulic deep drawing or hydromechanical deep drawing , the process differs in that the workpiece itself largely forms the sealing envelope of the pressure.

The pressure is z. B. introduced into the hollow body by a water-oil emulsion . During the forming process, the openings are sealed by sealing punches that are driven by hydraulic cylinders. The main process parameters are the internal pressure, which can be up to 30,000 bar, and, in the case of pipes, the pushing in of material or upsetting from the component ends with the help of the sealing stamp. There are a large number of process variations (e.g. preform operations) and additions (e.g. integrated punching operations). The processes take place at room temperature.

history

The process principle of hydroforming was already known in the 19th century. The method was based on the fact that when the material is pushed in from the component ends, there is a change or reduction in volume inside, which builds up the forming pressure. The process could not be regulated and was therefore difficult to control. It only gained importance again in the second half of the 20th century, when the high pressures in the active medium could be generated.

The classic component that was manufactured using hydroforming and is still used today is the copper T-piece. B. is used in water pipes.

In the course of the 1990s, the process gained increasing importance as hydroformed components were increasingly used in the automotive industry. The first mass-produced vehicle that had larger hydroforming parts built into the body was the Audi A8 (since 1994).

As a special process, hydroforming occupies an important place in the field of metal forming.

process technology

Plant engineering

The molding tools for the production of hydroforming components are typically operated in hydraulic presses, which are supplemented by a corresponding water hydraulic unit and the associated control. The hydraulic press serves as a locking device for the tool. Due to the variable pressure profiles in the tool that want to push the tool halves apart, a controlled locking device is essential in most cases.

Due to the high pressures in the tool, presses with large locking forces are usually required. The tool design must also take the high pressures into account in order to guarantee the appropriate service life.

Process design and feasibility limits

Since hydroforming is a complex process with many adjustable parameters, a high level of effort is often required to design the process. In many cases, feasibility studies and preliminary process designs can be carried out using FEM simulations. Nevertheless, prototyping is often advisable to confirm feasibility or a complex commissioning process can be expected.

The feasibility depends essentially on the maximum expansion / expansion required to manufacture a specific component. Normally, this must not exceed the uniform expansion of the material used. This can only be partially exceeded in the so-called feed area (area in which material can be pushed in with the help of the sealing stamp). If this is not sufficient, an intermediate annealing process may be necessary.
Materials with high elasticity (e.g. stainless steel, copper) are better suited for the hydroforming process than those with low elasticity (e.g. high-strength steels, aluminum). Further developments of the hydroforming process also made it possible to safely process materials with low elasticity. Deformation limit curves (see Forming Limit Curve ) of the material used are indispensable for feasibility studies, FEM simulations and process design.

Another feasibility limit is the maximum available internal pressure and the maximum possible shape of the component (especially in case corners and with small radii). The maximum internal pressure is in turn limited by the economically feasible system technology.

Advantages and disadvantages

The main disadvantages of the hydroforming process are the relatively long cycle times (compared, for example, with conventional deep drawing) and the high investment costs (for hydroforming / tube forming tools).

The advantages, on the other hand, are extensive design freedom, saving of individual parts (by combining individual components, for example with the previously used half-shell construction), thus eliminating assembly and / or welding operations, strain hardening of the material during the forming process and weight savings as a result of the aforementioned points. The high precision and repeatability of the components also speak for the process.

Areas of application

Hydroforming is preferably used for pipes with a variable thickness and for T-pieces such as fittings or water taps . In bicycle production, too, tube sets with more complex shapes are produced using hydroforming. This process allows the wall thickness to be adapted and shaped as required; this can save weight and also costs.

In the body shop, hollow sheet metal parts are produced by first welding 2 sheets together in certain seam shapes and then building up pressure between them. Quilted seams, some with openings, ensure z. B. for compliance with specific load requirements. In chassis construction, parts production is sometimes drastically simplified.

research

Selected research directions in the field of internal pressure forming are:

Tailored hybrid tubes made of steel and aluminum

The aim was to develop improved lightweight components for automotive engineering. For this purpose, lightweight construction on the material side using mixed steel-aluminum connections was combined with lightweight construction using pipes. The pipe structures were produced using laser brazing. The hydroforming process was used to manufacture the closed hollow profiles. In this way, steel-aluminum pipes with a deformable joining zone were developed.

titanium

A multi-stage process chain for the production of hydroforming components from titanium was developed. For this purpose, a suitable material model of the alloy titanium grade 2 and an annealing treatment were designed. Due to the low maximum degree of deformation, titanium can normally only be deformed with great effort. Thanks to the annealing treatment, the material of the formed component could be returned to the state it was in before the forming process and thus further formed. A two-stage forming process could be developed and hydroforming parts made of titanium for the first time.

literature

Individual evidence

  1. ^ Peter Freytag: Process balance sheet . Technical advances improve hydroforming potential. In: BLECH inForm. 05/2006, Carl Hanser Verlag, Munich
  2. ↑ https://www.schulergroup.com/technologien/produkte/grundlagen_innenhochdruckumformung_bauteile_fahrwerk/index.html hydroforming production of a chassis part from a tube
  3. ↑ Researchers succeed in forming hybrid tubes made of aluminum and steel. IPH, accessed May 14, 2018 .
  4. ↑ Manufacture sophisticated lightweight components in one step - by hydroforming tubes and profiles. (PDF) Retrieved May 14, 2018 .

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