Incremental sheet forming (ISF) has garnered significant scientific attention during the last decade. The process is highly flexible and provides a new agile production route for the short series production of sheet metal parts. The primary proposed drawbacks for the process are the production rate and the obtainable part accuracy, while high formability and flexibility are considered the main advantages. The evolution of the material properties inflicted by the forming process itself has received less attention. The introduction of ISF as an alternate production process for wider use requires knowledge of this evolution. In this Ph.D. thesis, a forming cell based on an industrial robot is used to study the ISF process and its effect on material property evolution. Additionally, the effects of the changing material properties on the process itself are considered. Finally, a method to control the material properties is presented. The forming parameters have a major impact both on the formability and on the forming process itself. By monitoring the forces during the forming process, valuable information about part and accuracy failure prediction was obtained. Sheet thickness has a pronounced effect on the forming forces due to the presence of bending in the process. The near quadratic forming force-to-thickness dependence accounts for the smaller geometrical deviations with higher sheet thicknesses. At higher forming angles the radial in-plane force becomes dominant and accounts for geometrical deviations. Incrementally formed materials exhibit lower than expected strain hardening. The material property evolution can be analysed using mathematical expressions. FCC structured materials exhibit higher than expected strain hardening during ISF at low strains. A relative strengthening plot is presented as a mean to help assess the need for tailored blanks.A good Johnson-Avrami-Mehl type of sigmoid dependence can be found for strain induced martensite formation during ISF. The effect of martensite transformation can be observed in the required forming force. Temperature remains the most effective way to control martensite formation. A novel method for the control of martensite formation in ISF is also presented. The stability of the stainless steel affects its transformation tendency. Stable grades enable a higher degree of accuracy but smaller fractions of transformation. The comparison of post-forming properties between shear spinning and ISF indicates that the idealised shear spinning theory is inapplicable to the shear spinning of thin sheets.
|Translated title of the contribution||Materiaali- ja prosessikäyttäytymisestä numeerisessa painomuovauksessa|
|Publication status||Published - 2014|
|MoE publication type||G5 Doctoral dissertation (article)|
- incremental sheet forming
- control of material properties