Targeted creation of residual stresses by forming of local structures in sheet thickness direction via an embossing process to improve component performance

• Applicants: Professor Dr.-Ing. Thomas Böhlke; Dr.-Ing. Jens Gibmeier; Professor Dr.-Ing. Mathias Liewald
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Mechanics ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 374768210

The general objective of the research project is to develop a forming process route for the targeted generation of residual stresses (RS) into thin-walled sheet metal components. The forming-induced RS is intended to improve the fatigue strength of structural components made of sheet metal that are predominantly subjected to dynamic loads. Within the scope of the project, it was demonstrated during the I. and II. funding period that by combining a form-ing and embossing process, compressive residual stresses can be generated in a thin-walled sheet metal component, which sustainably increase the cyclic operational strength of the component. A particular focus of this research project will be on the forming of coarse multiphase materi-als (duplex steel X2CrNiN-23-4). In particular, the contribution of phase-specific micro-RS will be systematically evaluated. In this context, on the one hand, measurement and evaluation strategies for local RS analyses are developed in the field of materials analysis. On the other hand, the mechanism-based modeling of RS of the I., II. and III. kind by means of numerically efficient mean-field approaches offers the possibility of two-scale modeling of RS in two-phase microstructures. The findings from these experimental and numerical investigations finally lead to the development of a combined manufacturing process for a thin-walled struc-tural component, into which compression RS are induced in a targeted manner by combining a forming operation with simultaneous embossing and subsequent reforming. These local RS counteract the tensile load stresses that act when the components are in operation.The aim of funding period III is to transfer the knowledge gained in the previous two funding periods with regard to the generation, analysis, stability and simulation of RS onto a cyclically loaded real component, in particular an abstracted carrier plate. This includes the holistic nu-merical design of the tool concept and its manufacturing implementation. The measurement, evaluation and simulation strategies developed in funding periods I and II are considered with regard to the most exact possible determination of the induced RS in the carrier plate on all scales (I. to III. kind). For the simulation and experimental analysis of RS on all scales, the focus in the upcoming funding period will be on crystallographic texture. In addition to consid-ering the texture gradient across sheet thickness, the influence of deformation-driven texture evolution will be investigated numerically and experimentally. The methods for simulation and analysis of RS developed in the project will be validated on all scales and additionally applied within cross-project benchmarks.The research project is carried out as a joint project between the IFU, University of Stuttgart, the IAM-WK and the ITM-KM of the Karlsruhe Institute of Technology (KIT) within the frame-work of the call for proposals of the Priority Program 2013.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2013: The utilization of residual stresses induced by metal forming