Papers by Author: Walter Reimers

Abstract: Today, innovative lightweight constructions increasingly demand for profiles with higher strength and stiffness. In this investigation an axially moveable stepped mandrel allowed the manufacturing of load adapted (tailored) aluminum tubes with axial variable wall thicknesses by the extrusion process. Thus, on the one hand it is possible to produce thick-walled sections for highly stressed areas and on the other hand save profile weight by applying reduced wall thicknesses in areas with lower loads. Varying the extrusion ratio along tube direction affects the product velocity as well as the profile exit temperature und thus the microstructure in different tube sections. At high temperatures and high strain rates the microstructures revealed very large grains due to static recrystallization. However at low temperatures and low strain rates dynamic recovery lead to a microstructure dominated by fibrous grains. Small equiaxed grains were also found indicating geometric dynamic recrystallization.

Abstract: In order to investigate the multi-variable friction phenomenon during aluminium and magnesium extrusion, tribological experiments with the alloys AA6060 and AZ31 against hot working steel 1.2344 were carried out. Using a new axial friction test, the effects of the normalized normal stress (1.5-6), the temperature (300 °C - 500 °C) and the relative sliding speed (0.1 - 50 mm/s) were investigated. The influence of each parameter on the friction behavior is analyzed, the friction results are depicted and a modified Tresca friction model is developed.

Abstract: In this work the extrusion of tailored aluminum tubes is investigated. Therefore, a stepped mandrel was applied and moved in axial direction in order to vary the tube’s wall thickness. Since surface irregularities were observed on the tube’s surface in transition areas between the different wall thicknesses, the material flow was analyzed by FEM in order to clarify the origin of the surface defects. It was revealed that the inwardly directed material flow in combination with the lack of inward resistance when the mandrel step is moving in the region of the die bearing causes the geometric inhomogenities.

Abstract: A new axial friction set up for high speed friction was successfully developed and tested at the Extrusion Research and Development Center of TU Berlin. The friction behavior between the Aluminium alloy EN-AW 6060 and the hot-working steel 1.2344 at 300 °C and 400 °C was studied. The experiments were carried out with a relative friction speed from 0.1 to 50 mm/s, and a normalized contact/normal pressure of σ_n/k_f ~ 0 and 1.5. The friction stress curves were depicted and compared with the Tresca friction model. A similar tendency between them was found, however all the theoretical results were around 15 and 30 % higher than the experimental results. In general, higher friction forces were found at 300 °C. Moreover, the effect of normal stress on the friction stress could be studied. By increasing the normalized normal stress from 0 to 1.5 k_f the friction stress increases around 30 % at 300 °C and 10 % at 400 °C. A strong adhesion effect was observed especially at 400 °C, which represents around the 90 % of the total friction stress. The friction results can be reproduced using a friction factor of m= 0.85 at 300 °C and m= 0.7 at 400 °C.

Abstract: Due to their low density and high specific strengths Mg-alloys provide an excellent potential to be used for light weight constructions. However, their equilibrium potential is very low resulting in relatively low corrosion resistance, especially in contact with other, more noble metals. In order to separate Mg from a corrosive environment hybrid billets with Al-alloy coating and AZ31-core were coextruded. Thus, the extrusion and coating of Mg-profiles can be done in a single production step, resulting in aluminum coated Mg-profiles. The influence of the extrusion ratio as well as of different die angles on the formation of diffusion layers at the interface was investigated. Furthermore the phases formed in the diffusion zones were analyzed using EDS and synchrotron XRD. Additionally, FEM-simulations were conducted in order to reveal the material flow of core and shell material during the forming process and to identify differences in using different die angles. The FEM-results were verified by comparison with the real extrusion experiments. Finally, the shear strengths of the produced compounds were evaluated in push-out tests.

Abstract: An algorithm is presented for characterization of the grain resolved (type II) stress states in a polycrystalline sample based on monochromatic X-ray diffraction data. The algorithm is a robust 12-parameter-per-grain fit of the centre-of-mass grain positions, orientations and stress tensors including error estimation and outlier rejection. As examples of use results from two experiments – one on interstitial free (IF) steel and one on copper – will be presented. In the first experiment 96 grains in one layer of IF steel were monitored during elastic loading and unloading. Very consistent results were obtained, with resolutions for each grain of approximately 10 μm in position, 0.05˚ in orientation and 80 μstrain. When averaging over all grains a resolution of 10 μstrain was obtained. In the second experiment it was demonstrated that the strain states of more than 1000 grains in a plastically deformed Cu specimen could be determined to an accuracy of 100 μstrain.

Abstract: For investigating the twinning and detwinning behaviour under cyclic loading conditions in-situ stress measurements with different applied strain amplitudes were performed at the beamline 7T-MPW at BESSY II. Intensity measurements of the (10-10), (11-20) and (0002) reflections served for the determination of the twinned volume fraction. The measurements of the (hkil) dependent strains and stresses gave information on the load partitioning between different grain orientations. The intensity measurements performed in the compression regime showed that the evolution of the (0002) reflection intensity depends on the strain amplitude. For strain amplitudes ≥ 0.75 % an increase in intensity is observed whereas for smaller strain amplitudes the (0002) intensity decreases with increasing cycle numbers. In the tensile regime an increase in the (0002) reflection intensity is found for the whole range of applied strain amplitudes.

Abstract: Due to the increasing demand of deep drawing applications for magnesium alloys in the future magnesium sheets with good mechanical and forming properties are required. These properties depend on the processing route of the sheet material. The deformation behavior of magnesium alloys is strongly influenced by the texture. Extruded magnesium sheets exhibit a different texture than rolled magnesium sheets. Therefore, the forming properties of the extruded magnesium sheets are supposed to be different compared to rolled sheets. Thin extrusion of the magnesium alloy AZ31 with a thickness of 1.5 and 2 mm were performed. Adjacent the extruded sheets were tested for their microstructure, texture and mechanical properties. The texture stability and evolution after the rolling of extruded magnesium sheets were investigated. Thus some of the 1.5 mm sheets were rolled to 1.0 mm and analyzed by OIM, X-Ray and mechanical testing. Concluding the results were compared to the properties of the just extruded 1.5 mm sheet and conventionally rolled sheet of 1 mm thickness.

Abstract: At the HARWI II beamline at the GKSS outstation at DESY a new experiment for position sensitive diffractometry and tomography called DITO was built and commissioned this year. Due to the available high energy synchrotron radiation with photon energies up to 100 keV it is possible to investigate the bulk of metallic samples of a few mm thickness with both methods. The diffractometry detector allows the investigation of the phase composition as well as phase sensitive determination of residual stresses with a spatial resolution of 6 μm while the tomography detector can either measure a whole tomogram in high resolution mode with a spatial resolution of 2 μm within 3 to 4 hours or in high speed mode recording a whole tomogram within 15 seconds with a spatial resolution of 40 μm.

Abstract: In the course of the increasing discussions about a reduction of the CO2 emissions magnesium has gained importance since it is the lightest metal for structural applications. Currently magnesium alloys are almost exclusively used as cast parts in the automotive industry because due to their microstructure extruded magnesium profiles exhibit a strong asymmetry in the mechanical properties under tensile and compressive loading (strength differential effect). In order to improve the mechanical properties a detailed knowledge about the influence of the different extrusion parameters on the microstructure of the extrudates is necessary. Therefore, the parameters extrusion method, billet temperature, product speed, extrusion ratio and cooling condition were varied for the extrusion of the magnesium alloys AZ31, AZ61 and AZ80. Subsequently the microstructure was analyzed and the mechanical properties determined. With an additional analysis of the deformation modes of the extruded and cold deformed products it could be discovered that an improvement of the mechanical properties can be achieved by a modification of the extrusion process. Since the strength differential effect in caused by twinning which due to the texture of the extrudates is only active under a compressive loading along the extrusion direction the modification of the extrusion process aims at a suppression of this twinning. Because on the one hand compared to that for dislocation glide the Hall-Petch-Constant for twinning is bigger a grain refinement of the extruded products could be achieved by a predeformation using ECAE similar processes. On the other hand a process has been developed where the profiles are extruded into a hydrostatic counter pressure in order to alter the texture during the extrusion. Thereby the twinning is already activated during the extrusion. Both modifications of the extrusion process result in an increase of the critical resolved shear stress for twinning during the subsequent cold deformation and thus in improved mechanical properties.