Manufacturing of three-dimensional parts by means of deep drawing is done from plane sheets, which in turn are cut from continuous strip. In the established manufacturing process over continuous casting, hot and cold rolling, the material properties in the strip are affected by microstructural effects (e.g. anisotropy, residual stresses, work hardening, voids, lattice defects etc.). These changes, especially voids and lattice defects, additionally influence part properties in a subsequent forming process (in this case deep drawing), specifically in the form of damage accumulation and evolution. Depending on the process route, different load paths can develop, leading to different damage evolution scenarios. While damage dependent on the load path of each material point can be mapped with different criteria and models, the results are uncertain both with regard to the site of maximum damage and to the amount of damage. Furthermore, the interdependencies between the (pre-)damage induced by the forming history and the damage evolution induced in the downstream deep drawing process are unknown.

The main goal of this project is therefore the quantitative description and prediction of interdependencies between the (pre-)damage induced by the forming history and the deep drawing process parameters as well as control of further damage evolution through tailored process routes. The research project is based on the working hypothesis that different load paths in the material can be induced by using a multi-step process route while maintaining initial and final part geometry from the single-step process, thus enabling a damage controlled forming process. The approach is based on three steps:

In the first step, the influence of the conventional deep drawing process parameters on the load path is analyzed. Characteristic material locations in the lower wall area, in the middle wall area, in the flange area, and in the transition areas between flange and wall as well as between wall and bottom are investigated. These locations indicate areas with critical load paths whose interdependencies with previous (pre-)damage need to be understood. In this regard, in the second step, the influence of multi-step process routes (by means of intermittent deep drawing, multi-step deep drawing, and reverse drawing) as well as deep drawing with counter-pressure on load paths in these characteristic material locations through changed stress states is investigated. In the third step, the influence of different forming histories on the load path is investigated. The experimental investigations are supported by numerical analyzes to determine and evaluate the corresponding load paths more exactly. Methods of the research project are experiments and numerical investigations based on the Finite Element Method (FEM). In the experiments, fundamental and model tests are done with Nakajima specimens and rotationally symmetric cups of a DP800 high strength steel. Additionally, metallographic investigations at characteristic material locations are performed using scanning electron microscopy (SEM), light microscopy (LM), micro-computed tomography (mCT), x-ray diffraction (XRD), and micro hardness tests (mHV). The numerical investigation of the fundamental and model tests using FEM furthermore allows the quantitative description of the influence of the process parameters on the stress-strain state, the strain rate, and the temperature at the defined material locations as well as the evaluation of existing damage models.

The result of the project is a descriptive-numerical model of the interdependencies between (pre-)damage and process parameters in deep drawing and minimization of damage evolution through a tailored multi-step process route. In particular, cooperation is done with project A04 with regard to material properties induced during rolling and their interdependencies with deep drawing parameters as well as with project A05 with regard to reference experiments and the influence of bending on the load path. Further cooperation exists with projects B02 and B03 for the microstructural characterization of specimens and with projects C02, C04 and C05 for damage modeling.

Using the description model from the first funding period, the findings are transferred to more complex, asymmetric drawing geometries and processes (e.g. stretch-deep drawing or sheet hydroforming) and the results are investigated. The result is a defined forming process with tailored load paths, leading to parts with identical true strain but reduced damage.
In the third funding period the thermal influence (e.g. local external laser irradiation) for the healing of material inhomogeneities and the service properties of the drawn part are specifically considered. This allows the production not only of a damage minimized part, but of a part that meets service requirements and uses the underlying mechanisms.

Project leader
Dr.-Ing. Patrick Mattfeld
Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University

Prof. Dr.-Ing. Thomas Bergs
Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University

Project coordinator
Matthias Nick M. Sc.
Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University