In recent years the finite-element-method (FEM) simulation has become applicable for the development, design and optimisation of metal forming processes. To obtain accurate results within the simulation the exact description of the flow stress kr during the process is essential. The implementation of the flow curve into FE-packages can be realised through different methods. Using the measured data in tabular form seems to be the easiest way, but special interpolation methods are necessary and extrapolation is strictly forbidden. Hence the exact flow stress measurement up to highest strain rates (between 0.0001 and 300/s) to simulate industrial processes like rolling or extrusion is essential. This paper presents actual studies in the field of high-speed testing by use of a servohydraulic testing system in a temperature range between room temperature and 1300 DC. Furthermore the use of material models to describe the material behaviour is getting more and more popular. Therefore hot compression tests must be carried out to determine the necessary parameters for reliable prediction. Specifically for aluminium alloys, where homogeneous deformation in the temperature range above 300 DC is not possible at present, new strategies for the determination of the material models have been developed. The compression tests are deliberately performed under sticking conditions to obtain strong gradients of the forming and microstructure parameters within a sample. The model equations are then adapted by consideration of the local parameters. Finally an optimisation coupled with an FEM program is applied to the models. FlieBkurvenaufnahme im Zylinderstauchversuch und spezielle Anwendungen in der Umformtechnik. In den letzten Jahren bekam die Simulation mit Hilfe der Finiten-Elemente-Methode eine immer greBere Bedeutung in der Anwendung fur die Entwicklung, Auslegung und Optimierung von Umformprozessen. Urn hinreichend genaue Ergebnisse in der Simulation zu erhalten, ist die exakte Beschreibung der FlieBspannung kr wah rend des Umformprozesses notwendig. Die Implementierung der FlieBspannungsbeschreibung in ein FEM-Programmpaket kann auf verschiedene Weise erfolgen. Einerseits konnen die Daten direkt wahrend der Rechnung aus einer Tabelle eingelesen werden, andererseits konnen uber Regressionsverfahren verschiedenste Gleichungen mittels Interpolation angepaBt werden, die die FlieBspannung in Abhangigkeit von den Umformparametern beschreiben. Extrapolation so lite keinesfalls angewendet werden. Foiglich ist die genaue FlieBspannungsmessung im Stauchversuch bis zu hohen Umformgeschwindigkeiten (0.0001 bis 300/s) notwendig, urn Prozesse -wie z.B. das Warmwalzen oder Strangpressen -simulieren zu konnen, In dieser Veroftentlichunq werden unter anderem aktuelle Arbeiten im Bereich der Hochgeschwindigkeitsversuche auf einer servohydraulischen Prutmaschine in einem Temperaturbereich von RT bis 1300 DC erlautert, Weiterhin kommt der Ermittlung von Werkstoffmodellen zur Beschreibung des Werkstoffverhaltens in der Simulation immer greBere B...
To obtain higher accuracy in FEM simulations the incorporation of microstructure evolution models becomes more and more important. From the point of view of metal physics it is well known that effects like recrystallization and deformation texture have a big influence on the material properties, especially the mechanical ones. The present article will give an overview about parts of the research activities in the Collaborative Research Centre (SFB 370) of the Deutsche Forschungsgemeinschaft (DFG). Three different types of microstructure models have been developed at the IMM and were coupled at the IBF to an implicit FEM code. The so-called flow-stress model is based on dislocation density evolution to describe the flow curve of metals, mainly at high temperatures. The Taylor-type model is able to describe deformation texture during metal forming. The third model is a modified cellular automaton to predict grain size and microstructure evolution during static recrystallization. The simulation of a rolling trial of the Al-alloy AA3104 including the named three models has been made and the results will be validated with experimental findings.
The development of a new steam turbine generation for use in advanced coal fired power plants with prospective operating temperatures beyond 700 °C and a projected thermodynamic efficiency of about 55 % requires, amongst other innovations, the partial substitution of ferritic steels by wrought Ni‐base superalloys. Although Ni‐base alloys are already widely used in the aerospace industry, they are faced with demands regarding component size and operation temperature, which by far exceed current aero‐engine requirements. In this article, the potential of selected alloys for 700 °C steam turbine applications is discussed with respect to their manufacturability and mechanical performance. Hereby, the focus is on the steam turbine rotor, which probably is the most critical component. It is concluded that material solutions are available for operation conditions around 600 °C but not for temperatures of 700 °C and above. Based on these results, alloy development strategies are suggested in order to close this gap and two new alloys, DT 706 and DT 750, are introduced.
Knowledge of correct flow stress curves of Ni-based alloys at high temperatures is of essential importance for reliable plastomechanical simulations in materials processing and for an effective planning and designing of industrial hot forming schedules like hot rolling or forging. The experiments are performed on a computer controlled servohydraulic testing machine at IBF. To avoid an inhomogeneous deformation due to the influence of friction and initial microstructure, a suitable specimen geometry and lubricant is used and a thermal treatment before testing has to provide a microstructure, similar to the structure of the material in the real process. The compression tests are performed within a furnace, which keeps sample, tools, and surrounding atmosphere on the defined forming temperature. The uniaxial compressions were carried out in the range of strain rates between 0.001 and 50s−1 and temperatures between 950 and 1280°C. Furthermore, two-stage step tests are carried out to derive the work hardening and softening behavior as well as the recrystallization kinetics of the selected Ni-based alloys. At the end of this work a material model is adapted by the previously determined material data. This model is integrated into the Finite Element program LARSTRAN/SHAPE to calculate a forging process of the material Alloy 617.
For the industrial production of large safety components such as turbine discs and turbine shafts, FEM coupled microstructure modeling offers a possibility to describe the microstructural changes such as recrystallization and grain growth during forming. Therefore, the material performance during the production process can be judged by understanding of the microstructure information. In this project, the microstructure evolution and the formability during the industrial processes were investigated with the help of FEM simulations. For numerical investigation of a forging process, compression tests, stress relaxation tests and annealing tests were carried out. After the analysis of the experimental results and the metallographic research on the immediately quenched specimens, microstructure models for simulating recrystalization and grain growth on the basis of empirical-phenomenological equations were developed for each alloy. The models were verified and fitted by means of 3-step compression tests. Finally, a sequence of upsetting and hammer forging operations were simulated via FEM coupled microstructure simulation. To determine and compare the formability of the investigated Ni-alloys, compression tests were performed using specimens with flange geometry. The research was firstly focused on the commercial alloys Inconel 706, Inconel 617 and Waspaloy, to identify the best material candidate for the industrial processing and application of stationary steam turbine components. Based on these results, two novel alloy variants, "DT 706" and "DT 750", were developed and studied again with the above described approaches.
Knowledge of correct flow stress curves of Ni-based alloys at high temperatures is of essential importance for reliable plasto-mechanical simulations in materials processing and for an effective planning and designing of industrial hot forming schedules like hot rolling or forging. The experiments are performed on a computer controlled servo-hydraulic testing machine at IBF (Institute of Metal Forming). To avoid an inhomogeneous deformation due to the influence of friction and initial microstructure, a suitable specimen geometry and lubricant is used and a thermal treatment before testing has to provide a microstructure, similar to the structure of the material in the real process. The compression tests are performed within a furnace, which keeps sample, tools and surrounding atmosphere at the defined forming temperature. The uniaxial compressions were carried out in the range of strain rates between 0.001 and 50 s−1 and temperatures between 950 and 1280°C. Furthermore two-stage step tests are carried out to derive the work hardening and softening behaviour as well as the recrystallisation kinetics of the selected Ni-based alloys. At the end of this work a material model is adapted by the previously determined material data. This model is integrated into the Finite Element program LARSTRAN/SHAPE to calculate a forging process of the material Alloy 617.
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