Showing papers by "Gerhard Hirt published in 2010"

Homogenization strategy and material characterization of high-manganese TRIP and TWIP steels

Abstract: In this work a hot forming strategy, consisting of forging and hot rolling, to homogenize casted blocks of high-manganese steels with 0.3 % carbon and 22 % manganese is introduced. The resulting distribution of carbon and manganese is evaluated by microprobe scans. The micro-segregation of manganese could be reduced from 7 weight percent to 2. To create the obtained hot forming strategy hot compression tests have been carried out. The deformation behavior has been characterized for two steels with 22 % manganese and between 0.3 and 0.7 % carbon content in the temperature range between 700 and 1200°C and strain rates between 0.1 and 10 s-1.

Phenomenological microstructure simulation of incremental bulk metal forming using a multi mesh method

Abstract: For the process design of incremental forming processes like ring rolling or stretch forging there is need for fast and accurate simulation techniques. For many applications it would be necessary to predict the microstructural evolution during the process. For this reason the FEM‐software Larstran/Shape which is used for the plastomechanical simulation can be coupled with the microstructure simulation module Strucsim. This software uses phenomenological equations for the calculation of recrystallization (dynamic and static) and grain size evolution [7].To accelerate the simulation a multi mesh method has been developed. This method uses an adapted simulation mesh with fine elements only in the locally limited contact and forming zone to achieve a reduction of the number of elements. Due to the relative movement of the tool and workpiece the adapted FE‐mesh has to be remeshed regularly according to the position of the tool. To avoid loss of information caused by the use of coarse elements the multi mesh me...

Grain size prediction during open die forging processes

Abstract: One of the most important target parameters during open die forging is the microstructure, respectively the grain size. This paper details different semi-empiric models that ultimately help to predict the microstructure properties of a forged block. As a first step, trials in industrial scale were performed by Buderus Edelstahl GmbH and attended by SMS Meer GmbH. The collected process data was used by the Institute of Metal Forming (IBF) for the numerical analysis of the open die forging process and to validate the microstructure prediction module STRUCSIM. The numerical prediction of the grain size shows a good agreement with the results obtained from the metallography. In a second step models for the core fibre of a forged block were developed at the IBF. The models use data from the online process measurement and simplified plastomechanical interrelations for the calculation of equivalent strain and the temperature in the core of the forged part during the process. With their results the microstructure in the core fibre of the workpiece can be predicted online. The models are still in development and the most recent results will be presented in this paper.

Physical and Numerical Simulation of Cold Rolling of an AlFeSi Alloy in Consideration of Static Recovery

Abstract: The influence of static recovery on the yield stress of AA8079 was investigated in lab-scale cold rolling experiments. The yield stress of AA8079 in the cold rolling process is affected by static recovery, but the softening caused by static recovery is completely compensated in the subsequent cold rolling pass. Thus, the effect of static recovery on the yield stress of the final product is of minor importance. For the TPM, the kinetics of static recovery of the AlFeSi alloy AA8079 were determined for different temperatures and strain rates. The measured softening kinetics were then implemented in the physically based flow stress model 3IVM+. This flow stress model was extended with an empirical approach for static recovery to enable the through-process modeling of cold-rolled aluminum in consideration of static recovery. Future work will focus on physically based modeling of static recovery without using empirical approaches.

Semi-solid forming of non axis-symmetric parts from steel grade X210CrW12 with PVD coated tools

Abstract: Semi-solid forming of steels has the potential to produce components with good mechanical properties and complicated geometry from high strength steels due to the advantages of forging and casting processes combination. Due to the high process temperatures thixoforging of steels has to meet high demands on process parameters accuracy and control. The thixoforging process parameters like billet temperature, temperature distribution after the reheating, Argon gas pressure, transportation time and forging load were experimentally investigated on thixoforging of a non axis-symmetric 4 kg demonstrator part of steel grade X210CrW12. Due to the high process temperatures, thermal and mechanical loads the development of a suitable tool concept is necessary. Hardened hot working steel X38CrMoV5-1 as a tool bulk material with protecting thin film crystalline (Ti,Al)N/γ-Al2O3 deposited by means of Physical Vapour Deposition (PVD) showed good results in first experimental series. The Scanning Electron Microscope images show mechanical damage of the tool radii edge while the rest tool surface look intact after 170 thixoforging cycles.

Derivation of Recovery Kinetics From Stress Relaxation Tests

Abstract: The recovery behavior of a commercial aluminum alloy 3103 was investigated by the means of two alternative experimental methods: stress relaxation (SR) and double tension tests (DT). In case of SR, the stress-time evolution after deformation was recorded, and for DT the yield stress after several recovery times were measured. The DT tests were further sub-divided into tests with and without external load during recovery. The results revealed that the recovery kinetics is clearly accelerated by the external stress during the SR. However, the difference between the DT and SR stresses is much larger. It is caused by continued dislocation glide after the deformation, which causes continued plastic elongation of the specimens. This is demonstrated quantitatively by appropriate evaluation models for both experiments. In contrast to DT, the SR evaluation accounts for the elastic SR due to plastic elongation, but the recovery parameters are the same ones as for DT. This makes it possible to replace DT by SR experiments, which are materially less laborious.

Schnelle Modelle zur Simulation der Gefügeevolution beim Ringwalzen

Abstract: Kurzfassung Das thermomechanische Behandlungsverfahren (TMB), welches durch den Verzicht auf eine separate Warmebehandlung hohe Potenziale zur Einsparung von Energie- und Prozesskosten bietet, hat sich im Bereich der Ringherstellung bislang kaum etablieren konnen. Die Ursache fur den geringen Entwicklungsstand lasst sich auf das fur das Ringwalzen typische, stark instationare und damit auserst komplexe Prozessverhalten zuruckfuhren. Diese Besonderheit bringt selbst moderne, auf Basis der Finite-Elemente-Methode (FEM) arbeitende Simulatoren an ihre Leistungsgrenzen. Durch die inakzeptabel langen Rechenzeiten scheidet die FEM daher zur Simulation der Gefugeevolution als Grundlage zur Entwicklung thermomechanischer Behandlungsstrategien fur den Ringherstellprozess aus. Im Rahmen des vorgestellten Forschungsvorhabens wurde deshalb ein weitestgehend analytisch und somit schnell rechnendes Simulationsmodell entwickelt. Zur globalen Simulationssteuerung wurde ein verbreiteter, kommerzieller Prozesssimulator in d...

Size effects on miniaturised rolling processes

Abstract: Experimental and theoretical research on the wire flat rolling process as well as plane strain compression tests have proven that the rolling of small and thin strip out of wire is affected by size...

Investigations on Semi-solid Joined Steel Components and their Bonding Quality

Abstract: To characterise the bonding quality of semi-solid joined components, fundamental investigations were carried out by producing composite test specimens. By using steel as a semi-solid base matrix and different steel or copper alloys as inserted materials, an interesting field of material combinations was realised. Two different steel grades, the cold forming tool steel X210CrW12 and the bearing steel 100Cr6, were chosen as the material of the base matrix. Geometry variations were tested along with the variation of materials mentioned. The results supply information about the influence of the inserts shape on an early cooling of the base material during form filling or remelting of the insert caused by overheating. Qualitative and quantitative analyses of the mechanical characteristics of the bonding were carried out using mechanical tests. Metallographic analyses complete the description of the bonding mechanisms at the contact zone between the solidified matrix and the insert. The investigations on basic geometries show a good joining quality, which is mainly based on a strong force and form closure caused by contraction of the solidifying and cooling base matrix. In addition, material closure occurs in areas that are strongly heated during the joining process. These results provide key information for the design of prospective components of a new generation.

Hot Forging of C45 using PVD (Ti,Al)N/γ‐Al2O3 Coated Dies

Abstract: The process of hot forging with permanent moulds is a challenge in respect to the very high thermal, mechanical and tribological loads on tools. Ensuring sufficient lifetime application of protective films can be beneficial. Initial screening experiments using PVD coated compression plates show that one of the metastable phases of alumina, the γ-phase, exhibits high strength and toughness and fulfils the requirements for a protective coating. The next important step in the development towards an industrial application is the implementation on complex tool shapes and verification in real forming experiments. After coating deposition using an industrial coating unit, coated dies were tested in forming experiments under industrial conditions. The forming experiments show an improvement of the wear resistance after 1000 forming cycles for the coated dies compared to the uncoated dies.

Numerical prediction of microstructure in high-strength ductile forging parts

Abstract: Governmental, environmental and economic demands call for lighter, stiffer and at the same time cheaper products in the vehicle industry. Especially safety relevant parts have to be stiff and at the same time ductile. The strategy of this project was to improve the mechanical properties of forging steel alloys by employing a high-strength and ductile bainitic microstructure in the parts while maintaining cost effective process chains to reach these goals for high stressed forged parts. Therefore, a new steel alloy combined with an optimized process chain has been developed. To optimize the process chain with a minimum of expensive experiments, a numerical approach was developed to predict the microstructure of the steel alloy after the process chain based on FEM simulations of the forging and cooling combined with deformation-time-temperature-transformation-diagrams.

Numerical Analysis of Stake Mark Induced Failure of a Rotor Shaft

Abstract: In this paper, the stake mark induced failure of a generator rotor forging is analysed using finite element analyses in combination with a Cockroft & Latham failure model. Evidence is found from damage analysis that the rotor failed due to micro cracks and residual stresses induced by a double stake mark, which was located at a position subject to concentrated rotating-bending stress amplitudes and torsional mean stress. The analysis presented here verifies the results of the damage analysis by showing that the stake mark configuration of the failed rotor was the most detrimental among a number of similar configurations. According to the finite element analysis, this stake mark configuration combines a high probability of inducing propagating sub-surface micro-cracks with considerable tensile residual stresses.

Prediction of Microstructure in High‐Strength Ductile Forging Parts

Abstract: Governmental, environmental and economic demands call for lighter, stiffer and at the same time cheaper products in the vehicle industry. Especially safety relevant parts have to be stiff and at the same time ductile. The strategy of this project was to improve the mechanical properties of forging steel alloys by employing a high‐strength and ductile bainitic microstructure in the parts while maintaining cost effective process chains to reach these goals for high stressed forged parts. Therefore, a new steel alloy combined with an optimized process chain has been developed. To optimize the process chain with a minimum of expensive experiments, a numerical approach was developed to predict the microstructure of the steel alloy after the process chain based on FEM simulations of the forging and cooling combined with deformation‐time‐temperature‐transformation‐diagrams.

An Incremental Modelling Approach For TheComputation Of Elastic Tool Effects During MetalForming Simulations

Abstract: The elastic behaviour of the forming tools has a significant influence on the quality of the end components. For example, in the case of the flat rolling process, the strip quality criteria, i.e., strip flatness and thickness profile are highly influenced by the elastic deformation of the working rolls. The Finite Element (FE) Method is the standard approach for the computation of such forming processes. Considering the elastic effects of the tolls in a single FE model using a traditional approach (standard in the case of commercial FE programs) will lead to large FE models (as very fine descretization of work piece as well as tool at the contact regions is necessary). As a result, the computation requires tremendous amount of time and resources. This paper presents a recently developed concept, which meets the above mentioned demands very efficiently. Within this concept, the computation of the tool elastic effects is separated from the process simulation. The advantages of this concept include, easy handling of the contact situations, reduced FE model size as the tool is modelled as the rigid body and good convergence of the computation. On the other hand, for the validation of the concept, optical measuring techniques were used to validate the numerical simulations. The experimental results of the flat rolling process were compared. To show the portability of the developed concept, qualitative results of the forging process were also given.