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As recent trends in manufacturing engineering disciplines show a clear development in the sustainable as well as economically efficient design of forming processes, monitoring techniques have been gaining in relevance. In terms of monitoring of product properties, most processes are currently open-loop controlled, entailing that the microstructure evolution, which determines the final product properties, is not considered. However, a closed-loop control that can adjust and manipulate the process actuators according to the required product properties of the component will lead to a considerable increase in efficiency of the processes regarding resources and will decrease postproduction of the component. For most forming processes, one set of component dimensions will result in a certain set of product properties. However, to successfully establish closed-loop property controls for the processes, a systematic understanding of the reciprocity of the dimensions after forming and final product properties must be established. This work investigates the evolution of dimensional accuracy as well as product properties for a series of forming processes that utilize different degrees of freedom for process control.

The superordinate goal of this study is to develop a closed-loop control for a property-controlled freeform bending process based on the prediction of the residual stresses and further mechanical properties by a soft sensor. Therefore, it is hypothesized that different combinations of freeform bending process parameters lead to matching geometry pairs with different residual stresses that can be influenced by the bending die position. A novel correlation between the bending die position, residual stresses and hardness provides the basis for the soft sensor and for closed-loop control design.This paper focuses on freeform bending with movable die, for which a novel bending strategy is developed that results in a set geometry while also providing various residual stress states in the tube. A numerical simulation model for freeform bending with movable die successfully models the variation in degrees of freedom of the machine and provides high qualitative and quantitative results regarding the identification of geometry and residual stresses. Real freeform bent validation experiments and HV 10 Vickers hardness measurements were carried out. HV 10 Vickers hardness tests were identified as a suitable parameter for preliminary system identification and validation, hence making it an appropriate parameter for soft sensor development.