Internal Flow-Turning – An innovative process for the manufacture of semi-finished steel products for lightweight design with a varying wall thickness and mechanical properties

Funding organisation:

Forschungsvereinigung Stahlanwendung e. V. (FOSTA)


Internal Flow-Forming is an innovative process for the manufacture of tailored steel tubes with a variable wall thickness contour over the longitudinal axis. The wall thickness distribution depends solely on the internal diameter that is created by the roller tool. The outer diameter of the manufactured tube remains constant, which greatly simplifies further processing, such as by hydroforming. For this reason, internal flow-turned parts offer beneficial characteristics when used as semi-finished parts and are favourable for downstream processing. A wall thickness reduction of up to 70% is feasible with internal flow-turning, which is greater than the current limit for internal-working manufacturing processes. In addition, there is a significant reduction in the roughness of the processed surface. The cold forming process increases  the strength and hardness of the work piece by up to 70% of the initial state. Given the analogies with the flow forming process, internal flow-turning is likely to maintain tighter tolerances by increasing the dimensional and geometrical accuracy of the part.


The required wall thickness is generated by the process-controlled gap adjustment between the roller tool and the die on the outside wall of the tube. This allows rapid adjustment of local thinning, ensuring a low economic batch quantity and hence cost-efficient manufacture in small numbers (for prototypes, etc.). In addition, the self-compensating force path permits a compact design for the manufacturing unit, which also has an impact on cost-efficiency.

Studies carried out to date within the scope of the "Internal flow-turning" project have shown that it is possible to produce hollow steel parts with geometries that have so far been difficult to manufacture and which hold a high potential for lightweight design. Further investigations should reveal the influence of the process parameters on the component properties and the process limits, for purposes of optimizing the process control. In addition to this, new tool concepts will be developed in order to further extend the forming and geometrical limits.

Further information :

Contact persons

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Dr.-Ing. Eugen Wiens

Umformende und Spanende Fertigungstechnik

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