Papers by Author: Julian Knigge

Abstract: A promising approach to handling low ductile aluminium alloys in a forming process is forming under superimposed hydrostatic pressure. The influence of superimposed hydrostatic pressure on the flow stress as well as on the formability for various hydrostatic pressures and temperatures was analysed [15, 3, and 7]. By increasing the formability of the workpiece, larger local plastic strains could be achieved. The results reveal highly increased formability at superimposed pressure of 85 MPa for workpieces from thermosetting alloy AlSi1MgMn (EN AW 6082) in comparison to those from self-hardening alloy AlMg4.5Mn0.7 (EN AW 5083). As a general tendency, the self-hardening alloys show a lower increase in formability when forged under superimposed pressure. But additionally, a charge-dependent influence of macro- and micro defects on the crack resistance was detected for alloy AlMg4.5Mn0.7. By evaluating damage models in finite element models the damage occurring in cold forming processes under superimposed hydrostatic pressure was predicted.

Abstract: The emphasis, in respect of content regarding the here presented project, lies within the production of localized reinforcements, by means of transformation-induced α’ martensite formation in solid and sheet metal components. During the forming process of metastable austenitic steels, high-strength martensite areas, next to ductile austenitic regions, are to be adjusted to enable the production of load-adapted components. To this end, extensive basic analyses are also necessary in order to determine the description of the mechanical behavior of α’-martensite structures, as well as to determine the extension of the numerical simulation as regards the structural change. The results achieved within the area of steel forming include the development of a temperable deep-drawing die (T = -35 °C until T = 100 °C) that carefully facilitates structural conversion at a constant forming-degree. Moreover the crash performance, based on transformation-induced martensite structures is improved. So-called Forming Curves (FCs) were developed as a new approach towards the material characterization of structured steel. In bulge forming components, comprised of chrome and nickel steels as well as manganous hard steel, martensite was specifically generated under the use of differing forming parameters. The tool design was aided by Finite Element Analysis (FEA). Moreover, fundamental simulations were carried out in order to calculate the structural change. The modification and extension of a semi-analytical model of the material followed so that the martensite content could be calculated in the previously examined sheet components, as in the massive forming.

Abstract: The industrial application of stainless steels is of high importance because of their high corrosion resistance and forming behaviour. The evolution of martensite during the deep drawing processes leads to an increasing strain hardening of the material. In the collaborative research centre 675 “Erzeugung hochfester metallischer Strukturen und Verbindungen durch gezieltes Einstellen lokaler Eigenschaften” (Creation of high strength metallic structures and joints by setting up scaled local material properties), metal forming processes is being researched. Emphasis on this part of the project is the stress-induced formation of martensite in sheet metal and bulk metal components in metastable austenitic steel. The aim of the investigations is to develop partial structure fields of martensite in sheet metal components in order to construct a lightweight structure. Therefore, components are divided into stretched and non-stretched parts. This leads to a defined buckling of components, for example in case of a crash. Furthermore, the effect of the transformation induced formation of martensite in metastable austenitic steel should be utilised on bulk metal forming components. Thereby special load adapted components with locally optimized properties are producible, like austenitic ductile regions and martensitic high-strength areas.