Flow stress measuring by use of cylindrical compression test and special application to metal forming processes

Abstract

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...

Flow Curve Determination for Metal under Dynamic Recrystallization Using Inverse Analysis

“…Temperature and plastic deformation tend to exhibit inhomogeneous distribution, because there exist friction between the tool and the workpiece, heat conduction of the workpiece and the tool, internal heat generation of the workpiece and interface heat dissipation due to friction. The interface friction, which causes inhomogeneous plastic deformation, can be reduced by using a lubrication film 1) or Raskatiev testpiece, [2][3][4][5][6] which has a lubrication pit at the interface. Many experimental facilities were developed with aims to obtain an accurate flow curve of metal under hot forming.…”

Multistage and High-speed Compression Testing Machine for Obtaining Flow Stress and Microstructure Evolution in Hot Forming of Steels

“…To resolve b), three methods have already been developed, The first is to use a wedge-type stopper for the main ram, which has been adopted to Servotest testing machine, 4,10) and second is to use two different actuator moving in opposite directions, when the main ram (forming ram) reaches the final strain, the opposite ram moves in synchrony, so that the distance between the main ram and opposite ram remains constant in the final strain, or deformation can be stopped. 11) Third is the ram motion is forced to stop by a stop plate, whenever that plate engages against the cross stop at same position, and the reduction of the workpiece is adjusted a hydro/wedge system placed on the opposite side of the main ram. 9) We adopt third method, the main ram is force to stop the end of the cylinder of actuator and that the reduction of the workpiece is adjusted the sub-jack driven by trapezoidal thread, in stead of the hydro/wedge system, installed on the opposite side of the main ram.…”

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Development of Forgeable Ni-Base Alloys for USC Steam Turbine Applications by use of Microstructure Simulation and Formability Testing

“…Additionally, microstructure coupled simulation could be used for rating the materials according to their performance during the process. Uniaxial cylinder compression tests with Rastegaev specimen were carried out to measure the flow curves [1]. Test temperatures covered a range from 900°C to 1100°C for Inconel 706 and Waspaloy, and from 950°C to 1150°C for Inconel 617 in steps of 50°C.…”