P 972 – FEA-assisted optimisation of blanking aimed at damage minimisation at cut edges of dual-phase and complex-phase steel sheets
The objective of the project was an optimised blanking process, which leads to higher cut edge formability of thin DP and CP steel sheets in comparison to the conventional cutting process. To achieve the objective, the damage in the cut edge area induced by cutting had to be suppressed. The damage was to be suppressed based on the basic principles of damage mechanics with the help of the following two effects: 1) work-hardening of a small material volume at the future cut edge under compressive stresses prior to shearing, 2) maintaining of compressive stresses at the future cut edge during shearing. The planned suppression of damage in the form of pores at ferrite-martensite interfaces, which is visible in the scanning electron microscope, was achieved with the help of the optimised pro-cess. However, the formability of the cut edge seems to be not exclusively defined by the degree of this damage. Even after the optimised cutting process with very low blank holder heel heights of only 50 μm, it is notably depleted and is approx. 18 % lower in comparison to the conventional cutting process despite the fact that the visible damage in the cutting edge area was suppressed. The clear reduction of scatter of the edge extension until fracture from the conventional cutting process (difference between the maximum and minimum engineering edge extension until fracture of approx. 12%) to the optimised cutting process (the same parameter ap-prox. 7%) can be considered as yet an advantage of the proposed optimised cutting processes.
In the future, it seems to be reasonable to investigate the influence of the punch velocity on pore initiation between ferrite and martensite at the cut edge, as in the frame of the project first indications were obtained, which hint to pore initiation in the dual-phase microstructure at high punch velocities only. Furthermore, it would be also reasonable to study the influence of material porosity in the cut edge area on the fatigue life of the cut edge. With that, another advantage of the optimised cutting process could be determined. In the area of material characterisation, the nature of damage accumulation at negative stress triaxialities could be looked at, which goes against the currently common assumption with its higher rate at low plastic strains and lower rate at higher plastic strains. Furthermore, the depletion of the material formability due to plastic deformation at various stress states should be more closely related to material changes on the microstructure level, for a better understanding of damage mechanisms be obtained, which is a prerequisite for their accurate mathematical modelling. For a more accurate numerical simulation of sheet metal cutting, an accurate determination of the strain rate and temperature dependence of material behaviour is required at approximately adiabatic conditions. With this data taken from the literature, the quality of the numerical analysis of cutting still remains insufficient. For fracture prediction in numerical simulations of multiphase materials on the microstructure level, improvements regarding computation stability at high plastic strains are necessary.
The research project (IGF-Nr. 17587 N) was carried out at Gottfried Wilhelm-Leibniz-Universität Hannover, Institut für Umformtechnik und Umformmaschinen. FOSTA has accompanied the research project work and has organized the project funding from the Federal Ministry of Economics and Technology through the AiF as part of the programme for promoting industrial cooperation research (IGF) in accordance with a resolution of the German parliament.
Only available in german language.
B.-A. Behrens, A. Bouguecha, I. Peshekhodov