Papers by Author: Martin Grüner

Abstract: Aluminium alloys show a great potential for lightweight constructions due to their high strength and low density but the production of this material is very energy consuming. Also the recycling of aluminium alloys, e.g. chips from the milling process, shows different challenges. Beside contamination by cooling lubricant and oxidation of the surface of the chips the melting and rolling process for new semi finish products needs a high amount of energy. TEKKAYA shows a new approach for recycling of aluminium alloy chips by an extrusion process at elevated temperatures producing different kinds of profiles. A new idea is the production of components directly out of chips using severe plastic deformation for joining of the chips similar to the accumulative roll bonding process in sheet metal forming. In a first approach aluminium alloy chips out of a milling process were uniaxial compressed with different loads inside an axisymmetric tool installed in a universal testing machine. The compressed chip disks subsequently were tested with two experiments to gain information on their stability. First experiment is a disk compression test with the disk standing on its cylindrical surface, giving information on the stability perpendicular to the compression direction. Second experiment is a stacked disk compression test with three disks to investigate the stability parallel to compression direction. During all three tests force and displacement values are recorded by the universal testing machine. These data are also processed to calculate or identify input parameters for the numerical investigations. For numerical simulation ABAQUS in conjunction with the Drucker-Prager-Cap material model, which is often used for sintering processes, seems to be a good choice. By numerical simulation of the experiments and comparison with the experiments input parameters for the material model can be identified showing good accordance. This material model will be used in future numerical investigations of an extrusion process to identify tool geometries leading to high strains inside the material and by this to an increased stability of the parts.

Abstract: The use of high and ultra high strength steels in modern bodies in white raises steadily since the 1980’s. This trend is caused by the consumers’ wish of low fuel consuming cars with an increased passenger’s safety. The processing of these steels brings new challenges e.g. high flow stresses and a low formability at room temperature or high tool loads. These challenges can be resolved by warm forming at temperatures up to 600 °C reducing the flow stresses and increasing formability. For the production of complex parts that can not be produced by deep drawing hydroforming is an appropriate technology which can also help to reduce the number of parts and thus the weight of the body in white. Nowadays typical fluids used for hydroforming are only temperature stable up to about 330 °C so that it is not possible to combine the benefits of warm forming and hydroforming. Media like gases and fluids tend to leakage during the process which can only be avoided by a sealing or high blank holder forces. A new approach is the use of ceramic beads as medium for hydroforming at elevated temperatures. Building up a heatable tool for hydroforming with granular material used as medium makes it necessary to consider thermal conductivity so that there is a need of thick insulation plates. These insulation plates show high elastic deformations affecting the blank holder forces during the forming process. Measurements of the compressibility of these plates and implementation in numerical simulation allow a significant increase of the prediction accuracy of the model. A comparison of real part geometry and numerical results from models with and without consideration of elastic deformation will be given.

Abstract: Increasing demands regarding security aspects and light weight construction lead to the application of advanced high strength steels (AHSS) and ultra high strength steels (UHSS) in the automotive sector. Due to high process forces and the reduced formability of these steel grades within cold forming new manufacturing technologies like the hot stamping process are required. Furthermore, crash-performance plays an important role in the automotive industry. Therefore functional optimized components are necessary. Hence, actual research work within the community is focused on manufacturing components with local adjusted mechanical properties. One of the strategies to realize the contradictorily requirements regarding energy absorption and structural integrity is the Tailored tempering process where the cooling rates are adjusted by controlled heating or cooling of different tool zones within the hot stamping process. Thereby knowledge concerning the influence of the different heated tool parts on the heat transfer and the resulting mechanical properties is necessary. Furthermore, the applicability and the accuracy of the calculation approaches used for characteristic values like the heat transfer coefficient in the FE-based simulation have to be analyzed and evaluated. Due to this experiments with a tool which exhibits a heated and a cooled zone were performed according to the Tailored tempering process. During the experiments contact pressures and tool temperatures in the heated tool part were varied and analyzed regarding the influence on the heat transfer. Furthermore, the heat transfer coefficients were calculated and verified by a numerical model built according to the experimental setup and the accuracy of the model was evaluated by the comparison of characteristic values calculated from the experimental and numerical process data.

Abstract: The need of light weight construction for high efficient vehicles leads to the use of new materials like aluminium and magnesium alloys or high strength and ultra high strength steels. At elevated temperatures the formability of steel increases as the flow stresses decrease. Forming high complex geometries like chassis components or components of the exhaust system of vehicles can be done by hydroforming. The hydroforming process by oils is limited to temperatures of approximately 300 °C and brings disadvantages of possible leakage and fouling. Using granular material like small ceramic beads as medium could be an approach for hydroforming of ultra high strength steels like MS W1200 and CP W800 at temperatures up to 600 °C. The material properties of granular material are in some points similar to solid bodies, in other points similar to liquids. For understanding and simulation of the behaviour of the medium a basic characterisation of ceramic beads with different ball diameters is necessary. Powder mechanics and soil engineering give ideas for experimental setups. For the conversion of these approaches on the one hand the behaviour of the ceramic beads itself has to be characterized, on the other hand the contact between a blank and the beads have to be investigated. For the tests three different kinds of spheres with a diameter between 63 microns and 850 microns are used. In unidirectional compression test compressibility, pressure distribution in compression direction and transversal compression direction and the effect of bead fracture are investigated. The tests are carried out at different compression velocities and for multiple compressions. For determination of friction coefficients between blank and beads and determination of shear stress in bulk under compression a modified Jenike-Shear-Cell for use in universal testing machines with the possibility of hydraulic compression of the beads is built up. The gained data can be used for material modelling in ABAQUS using Mohr-Coulomb or Drucker-Prager model.