Flexible joining of hollow frame structures with hydroforming – HydroFrame


Hollow frame structures, which consist of several formed sub-parts, are widely-used components in fields of leisure (cycleframes, hometrainer), furniture (chairs frames, table frames), automobile (chassis, space frames), aviation (helicopter skid) or architecture (banister).

Joints [Figure 1] are characteristic for the construction of these frame structures. These joints are subject to complex, partly unforeseeable use situations. They must therefore possess a highly specific strength as well as a sufficient amount of energy absorption capacity in case of misuse. High-quality frames in visible areas and components with a sophisticated design must furthermore satisfy aesthetical aspects in the form of organic transitions and free-form surfaces as well as very narrowly tolerated join patches. The manufacturing process of the structures is a complex procedure in which welding is mostly used. The length of the process chain is a significant expense factor.


Aim of the project is to make use of the advantages of hydroforming in an intelligent way by forming hollow work pieces and joining them into frame structures simultaneously. Functioning prototypes of pre-elected component demonstrators shall be available at the end of the project. For their production, a prototype of a forming machine suitable for SMEs is being developed and built. In addition, corresponding solder-/ adhesive-/ lubricant systems are being adapted. The consideration of individualization trends in the form of flexible forms, geometries, material combinations and strength will be the focus of a later method application.

The degree of novelty and the functional advantage of the method lie in the clear reduction of labour-intensive manual work implied in process integration when compared to conventional production methods of high-quality frame structures. Both the process of forming and the process of joining (without welding) are carried out within one tool and in a single process step. By doing so it is possible to provide joint patches which satisfy aesthetic aspects and simultaneously strength requirements.

Developement status

The joining of angularly arranged hollow sections by using hydroforming was developed within the last 4 years at PtU. [Figure 2] shows manufactured joints consisting of diverse materials which have been joined using hydroforming.

The strength of the connection is based on friction. Furthermore, form-locking connections can be produced integrated by appropriately varying the process and the geometry parameters.

The joining is integrated in the forming process [Figure 3]. Therefore two tubular hollow sections [Figure 3, (1)] are placed into a hydroforming tool [Figure 3, (2)], formed and joined [Figure 3, (3)]. Finally the joined connection is lifted out of the tool [Figure 3, (4)].

The flexibility of this method with respect to the adjustment of different geometries can be seen in further preliminary works. At present the chair armrest of project partner Kusch & Co represented in [Figure 4] is being produced with an aluminium pressure die-casting method in a one-piece style. The joined geometries are tubular steels with an outside diameter of 20 mm and wall thicknesses ranging from 0.8 mm to 2.0 mm. The parts are placed in an angle of 100° to each other. This example illustrates clearly the flexibility and the functional advantage of this joining method. Different lengths, angles and wall thicknesses are realisable in one forming step. The calibration of a desired strength is integrated in the process and carried out via the adequate choice of process, system and geometry parameters.

In the context of this project the steps from the principle feasibility under laboratory conditions to an industrial batch production shall be prepared.