Papers by Author: Jan Bohlen

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Authors: Kseniya Illková, František Chmelík, Patrik Dobron, Jan Bohlen, Dietmar Letzig, Karl Ulrich Kainer
Abstract: This paper focuses on the analysis of the deformation behaviour with respect to the acoustic emission response and mechanical properties of differently extruded magnesium Mg-Mn alloys. Six types of Mg-Mn alloys (with Al, Ca, Ce, Y, Nd additions) were processed by indirect extrusion with two different speeds. Alloys with addition of Ca, Al and Nd were selected and presented in this manuscript due to their prospective mechanical properties. Additions of Ca and Al had significant influence on the microstructure refinement and texture of the Mg-Mn alloy, whereas addition of Nd led to the absence of the tension-compression asymmetry. Measurements of the acoustic emission were done during the uniaxial tension and compression tests. The results are discussed with respect to the influence of alloying elements on the deformation behaviour in terms of the activation of dislocation glide and twinning.
Authors: Karl Ulrich Kainer, Roland Hoppe, Jan Bohlen, Gerrit Kurz, Sang Bong Yi, Dietmar Letzig
Abstract: Magnesium as the lightest structural metal offers significant potential weight saving compared with steel and aluminium. Cast magnesium components are widely used, e.g. as engine blocks or gear box housings. To open more opportunities for weight saving, it is necessary to widen the application fields to wrought products, such as large thin walled components for which sheets as semi-finished products are needed. The production and processing of magnesium sheets material is currently hampered by the limited formability of magnesium and high costs. The use of the twin-roll casting (TRC) as a cost-effective manufacturing process and the development of new calcium-containing alloys are leading to a decisive reduction of those obstacles.
Authors: Dirk Steglich, Jan Bohlen, Xiao Wei Tian, Stefan Riekehr, Nikolai Kashaev, Swantje Bargmann, Dietmar Letzig, Karl Ulrich Kainer, Norbert Huber
Abstract: A hollow rectangular profile, as an example of a typical structural component made of magnesium alloy sheets has been built, tested and evaluated in order to assess its behaviour during axial crushing. The profiles were joined from plane sheets of AZ31 and ZE10, respectively, by laser beam welding and were then tested in compression. Numerical simulations have been conducted to understand the complex interplay between hardening characteristics of the materials under investigation, profile cross-section variation and energy absorption. The results from the compression testing of the profiles show that the welds are not the source of damage initiation and failure. The performance of the magnesium profiles in terms of dissipated specific energy is confirmed for small and intermediate displacements to be comparable to that of aluminium profiles. For large displacements, however, the shear-type failure mode of magnesium causes a sharp drop of the crushing force and thus limits the energy absorption. These findings demonstrate the requirement for an alloy and wrought magnesium process development specifically for crash applications which aims at progressive hardening along with high ductility for improving the bending and shear behaviour.
Authors: Dietmar Letzig, Lennart Stutz, Jan Bohlen, Karl Ulrich Kainer
Abstract: Sheet metal forming experiments have been carried out on AZ31 and ZE10 sheets produced by rolling conventionally DC cast slabs as well as twin roll cast (TRC) strips. Nakajima tests were performed on the various sheet materials over the temperature range from RT to 200 °C using Hasek type samples of specified geometries to generate various strain paths. The strain path data were used to derive the forming limit curves as plotted in forming limit diagrams for the two alloys. The temperature dependence of the sheet formability is discussed in terms of the operating deformation mechanisms and the roles of alloy composition, initial texture and processing history.
Authors: Ligia Nascimento, Sang Bong Yi, Jan Bohlen, Dietmar Letzig, Karl Ulrich Kainer
Abstract: The influence of crystallographic texture on high cycle fatigue behaviour has been studied using an extruded rectangular profile of the AZ31 (Mg-Al-3wt%-Zn-wt1%) alloy. The fatigue samples, cut at 0, 45 and 90° to the extrusion direction correspond to different initial textures. Besides high cycle fatigue tests, quasi-static tensile and compression tests were performed to assess the tension-compression asymmetries as a function of the initial texture. The micro-mechanisms of fatigue crack initiation were investigated using scanning electron microscopy and electron backscatter diffraction. Differences in the mechanical properties and the endurance limit for the different sample directions are related to the initial texture and, subsequently, the easiness or difficulty of slip/twinning.
Authors: Gerrit Kurz, Jan Bohlen, Dietmar Letzig, Karl Ulrich Kainer
Abstract: Reducing vehicle weight and emissions by lightweight design is a major goal of the automotive industry. Magnesium as the lightest structural metal offers a significant weight saving potential compared to steel and aluminium. Cast magnesium components are widely used, e.g. as engine blocks or gear box housings. The application of magnesium sheets is currently hampered by the low formability of magnesium which means that a large number of rolling passes is required to roll a DC cast slab to final gauge sheet. This large number of rolling steps is the main reason for the high cost of magnesium sheets. Twin-roll casting (TRC) is an alternative, economic production process for the generation of fine-grained feedstock materials that subsequently can be warm rolled to thin sheets. It therefore receives attention in actual research and development projects for the application of magnesium alloys as prospective light metal solutions. This production process for thin strips combines solidification and rolling into one single production step and therefore saves a number of rolling and annealing passes in comparison to the conventional rolling process. The main goal of the activities at the Magnesium Innovation Centre MagIC of the Helmholtz-Centre Geesthacht (HZG) is the development of wrought magnesium alloys and their introduction into industrial, structural applications. The current focus of the research work is on alloy design and their processing for magnesium sheets produced by twin roll casting. In order to understand the influence of process parameters on the microstructure and texture the first twin roll casting experiments were performed with the alloy AZ31 (Mg-3Al-1Zn-Mn) as benchmark. As an example, the influence of melt temperature on the microstructure of the strip is presented and discussed with respect to arising material properties. Optimisation of process parameters of twin roll casting and the subsequent rolling of the sheets, offers the possibility to produce high quality sheet material.
Authors: Lennart Stutz, Jan Bohlen, Gerrit Kurz, Dietmar Letzig, Karl Ulrich Kainer
Abstract: The substitution of conventional materials such as aluminium alloys and steels with the lightest structural metal magnesium and its alloys can yield significant weight saving in the transportation industry and hence, reduce vehicle weight and greenhouse gas emissions. Producing magnesium sheets by conventional hot rolling is expensive due to the large number of rolling passes to final gauge and annealing steps at elevated temperatures between the rolling passes. Twin roll casting is a well established processing route for aluminium sheets which can reduce the necessary rolling passes to a bare minimum to reduce the production costs. This process is receiving increasing attention for the production of magnesium sheets. This study reveals first hand results of sheet metal forming experiments on magnesium sheets rolled from twin roll cast strip as well as conventional DC cast slabs. Two different alloys, AZ31 (Mg-3Al-1Zn-Mn) and rare earth element containing ZE10 (Mg-1Zn-RE) were investigated. It is known that these alloys show significant differences in the microstructure development during conventional rolling as a result of recrystallisation. For hot rolled AZ31, distinct textures are formed with the majority of basal planes oriented in the sheet plane and hence, unfavourably for basal slip. Conventionally rolled ZE10 commonly shows a much weaker texture. Forming limit diagrams are presented and discussed with respect to the initial texture of the sheets. Strain response to various strain paths and plastic anisotropy are evaluated. Results of twin roll cast sheets are compared with conventionally hot rolled sheet of the same alloys. Competitive formability can be achieved at 200°C for all tested sheets. While conventionally rolled sheets show a generally higher formability than their twin roll cast counterparts, ZE10 outperforms AZ31 for both processing routes.
Authors: Jose Victoria-Hernandez, David Hernandez-Silva, Jan Bohlen, Sang Bong Yi, Dietmar Letzig
Abstract: In this work, the superplastic behavior of AZ31, AZ61 and AZ80 magnesium alloys was investigated. The alloys were hydrostatically extruded at only 150 °C to get fine grained microstructures (−2, 10−3 and 10−4 s−1. It was found that all alloys exhibited superplasticity at 200 °C, 175 °C and 225 °C for AZ31, AZ61 and AZ80 alloys, respectively. Low temperature dynamic recrystallisation played an important role for generating a finer and homogeneous microstructure during testing which enhances the deformation behavior of the alloys at these temperatures.
Authors: Sang Bong Yi, Jan Bohlen, Stefanie Sandlöbes, Stefan Zaefferer, Dietmar Letzig, Karl Ulrich Kainer
Abstract: Microstructural evolution during the annealing of cold rolled Mg, Mg-1.5Nd and Mg-3Y sheets has been examined. The experimental results show a significant difference in recrystallization kinetics and grain growth between pure Mg and Mg-RE alloy sheets. Pure Mg sheet shows rapid recrystallization and grain growth, whereas recrystallization is considerably retarded in the Mg-RE alloys. Although recrystallized grains which are triggered at shear bands in the cold rolled pure Mg sheet show a relatively weak texture with a basal pole split into the sheet rolling direction, rapid grain growth is accompanied by re-strengthening of the basal-type texture. In contrast, a weak texture appears in the early recrystallization stage in Mg-RE alloys and is retained during annealing due to retarded recrystallization and grain growth.
Authors: M. Nebebe, Jan Bohlen, Dirk Steglich, Jörn Mosler
Abstract: This work is concerned with numerical analyses of the forming behavior of magnesium at elevated temperature. For that purpose, a thermodynamically consistent, rate-dependent, finite-strain elasto-plastic constitutive model is presented. This model captures the stress differential effect as well as the anisotropy of magnesium. Furthermore, the change in shape of the yield locus (distortional hardening) is also taken into account. This constitutive law, together with its parameter calibration based on uni-axial tensile tests, is finally combined with the localization criterion originally proposed by Marciniak and Kuczynski and applied to the simulation of forming limit test. Comparisons to experiments show the excellent predictive capabilities of the model.
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