Papers by Christof Sommitsch
Optimized rod rolling to achieve desired mechanical properties
STEELSIM 2005, 1st International Conference on Simulation and Modelling of Metalurgical Processes in Steelmaking, Oct 25, 2005
Das Kompetenznetzwerk für Fügetechnik JOIN4
A process model developed by the authors for the calculation of the microstructure evolution of n... more A process model developed by the authors for the calculation of the microstructure evolution of nickel-base alloys during the hot forming process in a rolling mill helps to optimise the rolling process and aids in achieving the desired microstructure. The simulation of the particle development during and after a hot forming process is based on the numerical Kampmann-Wagner model. The chemical driving forces for precipitation are obtained from the thermodynamic equilibrium module of the computer program 'MatCalc'. The multicomponent diffusion coefficients are evaluated from the atomic mobilities that are compiled in the mobility-database of the DICTRA software.
The estimation of the contact interface temperature during resistance spot welding of zinc coated steels using numerical technique
Materialwissenschaft und Werkstofftechnik, 2010
In this paper the unit area electrical contact resistance curves for galvanized DP600 steel at th... more In this paper the unit area electrical contact resistance curves for galvanized DP600 steel at the electrode‐sheet and sheet‐sheet interface were estimated using experimental resistance data and the finite element modeling technique. A modified experimental resistance spot welding procedure was employed to analyze the total dynamic resistance as a function of welding current at the peak in the first half wave. The resulting experimental data was used in a numerical procedure with the finite element modeling program SYSWELD to predict unit area electrical contact resistance curves. The numerical procedure enabled the separation of the bulk material resistance contribution as well as the prediction of the contact interface temperature at each current input in the simulation. The new contact resistance curves were used in a resistance spot welding simulation and the resulting shape and radius of the molten zone and heat affected zone were compared to experimental weld cross sections.Es...
Case Studies in Engineering Failure Analysis, 2016
Cracks were analysed at the root of the third blade row of low-pressure steam turbine blades of d... more Cracks were analysed at the root of the third blade row of low-pressure steam turbine blades of different natural frequencies. The root cause of the fatigue crack initiation was pitting corrosion of the forged ferritic/martensitic X20Cr13 material. Metallographic investigations, finite element analysis and fracture mechanics analysis combined with experimental data from the literature are used to evaluate crack propagating stresses to discuss the operating conditions. The calculations show that corrosion pits at the root of the turbine blade increase the local stresses above yield strength. Excitation of natural frequencies by changing the rotor speed is not responsible for the crack propagation. The centrifugal load and superimposed bending load caused by unsteady steam forces are responsible for the crack propagation.
Interaction of Recrystallization and Precipitation during Hot Forming of Alloy 80A
Materials Science Forum, Mar 15, 2007
In the high temperature deformation window of the nickel base Alloy 80A the lower temperature reg... more In the high temperature deformation window of the nickel base Alloy 80A the lower temperature region during open die forging was examined with regard to the materials formability. For that purpose, hot compression samples were investigated by means of EBSD and TEM in order to look at recrystallization, precipitations and ductile damage as well as their reciprocal effects. Further a microstructure model was used, which calculates the materials strengthening, softening and the particle kinetics. A micro mechanical damage model of the effective stresses was coupled with the grain structure development in order to describe a retarded damage rate due to the ongoing recrystallization.
Quantitative Metallography
Imaging & Microscopy, Mar 1, 2006
ABSTRACT
Grain-subgrain structure analyses of hot deformed superalloy Allvac 718plusTM by EBSD
Dynamische Rekristallisation von Nickelbasis-Legierungen, Bildanalytische Methoden und EBSD
Systematische Bestimmung des rekristallisierten Gefügeanteiles einer Nickelbasislegierung in Abhängigkeit vom Verformungsgrad
Characterisation of the subgrain structure of the aluminium alloy AA6082 after homogenization and hot forming by EBSD
EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany
The properties of a hot rolled aluminium alloy are strongly dependent on the homogenization and t... more The properties of a hot rolled aluminium alloy are strongly dependent on the homogenization and the forming conditions [1,2], because the microstructure of the specimen will be changed by the various processing steps [3]. A powerful tool to investigate the obtained grain — subgrain structure is the backscatter electron diffraction (EBSD) technique [4]. The evolution of the microstructure of the
Studies on Ductile Damage and Flow Instabilities during Hot Deformation of a Multiphase γ-TiAl Alloy
Key Engineering Materials, May 1, 2014
Gamma titanium aluminides are promising alloys for low-pressure turbine blades. A significant dis... more Gamma titanium aluminides are promising alloys for low-pressure turbine blades. A significant disadvantage of such intermetallic alloys is failure induced during forming processes due to ductile damage and flow instabilities. Previous investigations on a gamma titanium aluminide alloy (TNM), have shown ductile damage due to tensile stress components and instabilities such as shear bands, pores and micro-cracks at low temperatures and high strain rates. The main part of the current work is to delineate damage and unstable regions in the low temperature region. Hot deformation experiments are conducted on a Gleeble®3800 thermomechanical treatment simulator to obtain flow curves to be implemented in a finite element method (FEM) code. Instabilities in the material are described by existing instability criteria as proposed by Semiatin and Jonas and implemented into FEM code DEFORMTM 2D. Predictions of ductile damage models and the instability parameter are validated through detailed microstructural studies of deformed specimens analysed by light optical- and scanning electron microscopy.
Determination of isothermal flow curves and the dynamic recrystallization kinetic of the nickel-basis superalloy Allvac 718 PlusTM by means of rastegaev specimens
Friction welding and friction stir welding – an overview
An EBSD-study of recrystallisation behaviour of a Nickel base alloy caused by hot compression
On meta-dynamic and static recrystallisation of the nickel based alloy 80a
Friction Stir Welding of dissimilar material combinations
On the determination of the recrystallised fraction during hot forming Recrystallisation during h... more On the determination of the recrystallised fraction during hot forming Recrystallisation during hot forming processes is a decisive process for formability and control of the microstructure. Forming processes can be optimized by the simulation of grain structure development. The relevant computational models need material parameters, which are essentially determined by laboratory experiments. This includes determination of recrystallised fractions and grain size development. Direct and indirect methods exist for the analysis of the grain structure. First tries to determine the softening by relaxation measurements or by the comparison from multi-step sequential deformations. The latter consists of the metallographical investigation of a two-dimensional cross-section by methods of light microscopy and automatic image analy sis or with the help of electron microscopy and grain orientation analysis. In consequence different methods are compared by means of the nickel base alloy Alloy 80A. 
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Papers by Christof Sommitsch