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The geometric accuracy of an incrementally formed part is dependent on the underlying deformation mechanism and the residual stresses induced in the material during forming. This thesis presents new insights into the deformation mechanism and residual stresses of the Single Point Incremental Forming (SPIF) variant of the ISF process. In addition, strategies to control, suppress and adjust the deformation mechanism and residual stresses to increase the geometric accuracy and improve part properties are presented.
The deformation mechanism of the ISF process is investigated. A novel methodology is developed to split the plastic energy dissipation during the SPIF process as a contributions of energies from dominant deformation modes. It is found that the dominant deformation mode can be bending, shear or membrane stretching, depending on the selected values of the process parameters. By controlling the contribution of each deformation mechanism, the outcome of the SPIF process can be designed for maximum geometric accuracy within the constraints of process time and formability.
The build-up of the residual stresses in the SPIF process is investigated and a relation between the geometric accuracy and the residual stresses under changing process parameters is developed. The wall angle parameter has the highest influence on residual stresses. Moreover, the intensity and the magnitude of the residual stresses can be controlled by adjusting the process parameters.
A direct approach based on post-forming Stress Relief Annealing (SRA) is developed to suppress residual stresses and increase geometric accuracy. In this regard, a modular tooling set-up is designed to perform the SRA under partial constraint. For maximum efficiency, optimal SRA parameters are determined. Two parts with practical significance are incrementally formed and subsequently stress relief annealed. A considerable increase in the geometric accuracy is observed with SRA in comparison to the parts without SRA.
In the last part of this thesis, the very first approach is presented to generate a target distribution of high magnitude residual stresses in the ISF process for improving the mechanical properties. The desired residual stresses are generated in the conventionally formed disc springs in the surface treatment approach. In an integrated forming approach, the forming of the disc springs and the desired residual stresses are generated in a single step. Mechanical properties of the disc springs are better for both approaches in comparison to conventional disc springs. Hence, a targeted generation and distribution of the residual stresses to improve the mechanical properties is possible.