Modelling of Solidification Microstructures. M. Rappaz. Laboratoire de Métallurgie Physique Ecole Polytechnique Fédérale de Lausanne, SWITZERLAND

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1 Modelling of Solidification Microstructures M. Rappaz Laboratoire de Métallurgie Physique Ecole Polytechnique Fédérale de Lausanne, SWITZERLAND Modelling of microstructure formation in solidification processes is an important step in the field of computational metallurgy. Indeed, as-cast microstructures determine to a large extent the properties of shape castings, but also fix the initial conditions for many subsequent treatments (homogenization, precipitation, etc.). In a way similar to the various techniques used for the characterization of as-cast components, adapted modelling tools have to be developed for the various scales of interest. At the scale of the process, standard finite difference/finite element methods are used to solve average conservation equations in order to predict global fields such as temperature, enthalpy, velocity, average concentration, etc. At the scale of the grains, cellular automata have been developed for the prediction of grain morphology and texture. Within a grain, the detailed microstructure, including the formation of various phases, can now be predicted using the phase-field or equivalent pseudo-front tracking methods, coupled with phase diagrams calculations. The present contribution will briefly review these various techniques and outline their strengths and weaknesses. Simulation of Casting, Homogenisation, and Hot Rolling: Consecutive Process and Microstructure Modelling for Aluminium Sheet Production A. Ludwig 1, P.R. Sahm 1, G. Gottstein 2, M. Schneider 2, R. Kopp 3, L. Neumann 3 1 Gießerei-Institut, RWTH Aachen, GERMANY 2 Institut für Metallkunde und Metallphysik, RWTH Aachen, GERMANY 3 Institut für Bildsame Formgebung, RWTH Aachen, GERMANY A production line of Aluminium sheet in general consists of encloses continuous casting, homogenisation and hot rolling. Since several years the numerical simulation of each production step has made great progress. Nevertheless, a consecutive process and microstructure modelling is rarely put forward as it requires outstanding expertise on different fields in metallurgy and metal physics. In the frame of the DFG research centre Integral Materials Modelling different modelling approaches have been combined to realise a numerical process route description from the melt to the final sheet. This comprises simulation of solidification including melt convection and grain movement, simulation of homogenisation including evolution of precipitations and dissolution of eutectic phases and simulation of hot rolling including deformation, texture evolution and hardening. The work focused on an ternary AlCu4Mg alloy. All numerical predictions were compared with experimental results.

2 Simulation of weld solidification microstructure coupled to the macroscopic heat and fluid flow modelling V. Pavlyk Institut für Schweißtechnische Fertigungsverfahren, RWTH Aachen, GERMANY The microstructure exerts a strong influence on the mechanical properties and on the integrity of welded joints. The prediction of the formation of the microstructure during welding and of other solidification processes may be an important and supporting factor for technology optimisation. Nowadays, the increasing computing powers allow the direct simulations of the dendritic and the cell morphology and also of the columnar grain in the molten zone on specific temperature conditions. Modelling is, on the one hand, carried out with the Finite Difference Cellular Automata and, on the other hand, with the phase field method. The determination of the solidification conditions during fusion welding (temperatur gradient, local solidification rate, weld pool shape) is now carried out with a numerical, macroscopic FEM calculation of the weld pool fluid flow and of the temperature distribution, as presented in this article. Aided by computers; the influence of the process parameters is investigated. As with the use of accurate physical models the simulations are carried out with a spatial resolution of the microstructure, many assumptions and restrictions from traditional, analytical or phenomenological attempts may be eliminated. The possibilities of the new numerical algorithms for the generation and visualisation of the microstructure formation during solidification are demonstrated. The spectres of application extend from welding and casting to processes with rapid solidifaction. In particular, the computer simulations of the solidification conditions and the formation of the dendritic morphology during the directional solidification in TIG welding are described. Moreover, the simulation results are compared with the experimental findings. Through Process Modelling of Texture Evolution during Production of Commercial Aluminium Alloys M. Crumbach 1, M. Goerdeler 1, G. Gottstein 1, L. Neumann 2, H. Aretz 2 and R. Kopp 2 1 Institut für Metallkunde und Metallphysik, RWTH Aachen, GERMANY 2 Institut für Bildsame Formgebung, RWTH Aachen, GERMANY In Aluminium sheet production hot rolling and cold rolling with subsequent annealing are the processing steps where significant texture evolution occurs and where the final texture can be influenced to achieve the desired material properties. Thus models to predict this texture evolution are important modules for physically based integrative modelling of the whole production process. In an example of multi-pass hot and cold rolling of an aluminium alloy the deformation texture model GIA (Taylor model considering grain interactions) is used to simulate the deformation texture evolution. The evolution of the recrystallisation texture of the hot band and during annealing of the cold band is calculated with a statistical analytical recrystallisation texture model, based on nucleation and growth. Nuclei are considered to consist of different nucleation spectra, dependent on the orientations before the deformation and on the texture evolution during the deformation. The orientation distributions of nuclei in Cube bands and of nuclei in grains with strong orientation gradients or transition bands are modelled with modifications of the GIA model, depending on deformation conditions and material properties. The stored energy in each grain is related to the accumulated slip in it during the whole deformation process thus is considered orientation- and strain path dependent. The required input data for these models the velocity gradient tensor during the deformation and work hardening behaviour is provided by FE-modelling (FE-code LARSTRAN/SHAPE) and a dislocation density based work hardening model (3IVM), respectively.

3 FE-based analysis for the prediction of microstructure evolution in hot rolling J. Yanagimoto Institute of Industrial Science, The University of Tokyo, JAPAN A numerical model for recrystallizations in hot rolling and phase transformation after hot forming is applied to the analysis of industrial hot rolling processes. This model is based on incremental formulation for recrystallizations and conventional nucleation and grain growth theory. Transient change in dislocation density is traced consistently from hot rolling to continuous cooling, taking dislocation density to describe the effect of hot forming to phase transformation. Upsetting experiments under different cooling rates, plastic strains and deformation temperatures are performed to validate the accuracy of the proposed model. The proposed model has been applied to simulate phase transformation and final microstructure after 4-pass bar rolling and other industrial rolling processes. The metallurgical modelling system TKS-StripCam for simulation of hot strip manufacture structure and application in research and production practice U. Lotter, H.-P. Schmitz, L. Zhang ThyssenKrupp Stahl, Duisburg, GERMANY During production of hot strip in a hot rolling mill a great variety of metallurgical processes is involved in transforming the microstructure of the cast slab into that of the coiled finished product. For the construction of a simulation system predicting the mechanical properties of hot strip two tasks have to be done. The first step is to set up models for all the metallurgical processes relevant for the development of microstructures during manufacture. The target herewith is to numerically simulate the evolution of microstructure in dependence on the production conditions, and finally to calculate relevant microstructural parameters of the finished product. The second step is then to establish algorithms for the conversion of the calculated microstructural parameters into mechanical properties. This could be done by metallurgically based modelling or by use of a neural net. At ThyssenKrupp Stahl such a metallurgically oriented simulation system has been developed and established under the name TKS-StripCam. It is based on empirical-physical models. It allows on the one hand to predict important mechanical properties as yield strength, tensile strength and elongation at fracture, with considerable precision from production parameters as rolling schedule, cooling condition etc. On the other hand by means of this simulation system course and kinetics of every metallurgical process included may be studied. It is evident that in a steel plant such a powerful tool finds a great variety of applications. In offline use TKS- StripCam is applied with metallurgical research and with the improvement or development of steels and production processes. Typical problems in this field are if-than scenarios, sensitivity analyses or the investigation of the interaction of metallurgical processes under given conditions. The online use of TKS-StripCam to predict the mechanical properties just during rolling is of high economical interest. To achieve this, TKS-StripCam is integrated into the data flow of the rolling mill and directly fed with all the necessary data. On the basis of the calculated values certificates may be established instead of taking values from materials testing. Thus the high costs for the latter may be partly saved. A very promising use of TKS-StripCam is the inline application. Here the metallurgically oriented simulation program intervenes into the process control via the process computer, in order to optimise the quality of the material referring to reproducibility and long-term stability of the production.

4 Some examples of microstructural based processing models B. Buchmayr Institut für Werkstoffkunde, Schweißtechnik und Spanlose Formgebungsverfahren TU Graz, AUSTRIA The principles and benefits of coupled thermomechanical modelling and simulation in order to improve the product performance or to optimize the processing technologies are illustrated by various practical case studies. Semiempirical models are used to describe the microstructural development during hot forming like strip/wire rolling or forging. The final mechanical properties can be predicted within a standard deviation of 20 MPa. By linkage of user-defined subroutines to commercial 2D/3D-bulk forming software, the final grain size distribution and local properties can be predicted. Further refinements can be obtained by more physical based models, taking into account thermodynamics, kinetics and particle-dislocations-interactions which have been used for hot forming of Nickelbase superalloys. In order to get the prediction right the first time, some areas of uncertainty will be discussed. Computer-aided design of injection moulded parts E. Schmachtenberg Institut für Kunststoffverarbeitung (IKV) RWTH-Aachen, GERMANY Compared to other construction materials such as metals and ceramics thermoplastics show a complex, non-linear viscoelastic material behaviour even at room temperature. The load situation (e. g. time, temperature and strain) as well as the internal properties (e. g. fibre orientation, degree of crystallisation) have an important influence on the mechanical material constants und so on the part behaviour. Because of the short time from the determination of a market requirement up to the product introduction into the market a plastics-oriented simulation of the part behaviour in the product development is absolutely necessary. Therefore different approaches for a computer-aided design of injection moulded parts will be discussed in the presentation. The injection moulding process simulation as well as the structure simulation will be regarded. It will be shown for selective examples how thermoplastics could be designed in a plastics-oriented way considering the particular load situation (crash load, short-term and long-term load).

5 Modelling of Materials Behaviour for the design of Aero engine turbine vanes, blades and casings E. E. Affeldt MTU Aero Engines München, GERMANY The design of turbine components is based on the thermal and mechanical loading conditions expected during service and on the experience of the typical damage mechanisms found during overhaul or testing. One of the most important damage mechanisms observed is the initiation and propagation of cracks under cyclic thermal and mechanical loads, which caused an increasing amount of testing being performed under thermo-mechanical fatigue (TMF). The TMF test results are thought to be a good reference for the deformation behaviour and the cyclic life under realistic (anisothermal) test conditions, with the advantage of a well-defined cyclic mechanical strain and temperature history with negegible gradients in the gauge length and the feasibility to measure the stresses evolved. The main objective of design (and testing for design) is obviously to guarantee life during service. To predict the life of turbine components the deformation behaviour and the number of cycles till failure are predicted in models which describe the deformation behaviour and the damage evolution and are compared to the test results. These models are used after the verification with test resuls - for simulation of the behaviour of engine components. A brief summary of TMF test results will be shown and the models used for simulation will be discussed with reference to the experimental data. Protective Coatings for Gas Turbine Components Degradation Parameters and Modelling Approaches R. Herzog Institut für Werkstoffe und Verfahren der Energietechnik 2 Forschungszentrum Jülich GmbH, GERMANY Thermal barrier coatings are exposed to complex thermo-mechanical loads during service leading to interaction between of different life limiting processes and parameters, such as bond coat oxidation, sintering of the ceramic top coat, cyclic strains, phase transformations, visco-plastic and relaxation properties, crack resistance and interface roughness. A survey of damage and failure modes under different loading conditions and examples of damage evolution will be given. The driving forces for initiation and propagation of cracks, which cause coating spallation, are influenced by time-at-temperature (bond coat oxidation, sintering) as well as fatigue controlled processes (thermal cycling, TMF). Therefore, life-prediction models for TBC spallation cannot be purely timebased or fatigue-based models. A life prediction model has been developed for plasma sprayed TBCs, which accounts for loading and material parameters such as thermally induced strains, mechanical strain range, oxidation and sinter kinetics, elastic and visco-plastic properties, interface roughness and crack resistance. The model provides the life time and number of cycles to failure. Damage evolution is interpreted as an accumulative two-stage process. The crack growth kinetics in the initial phase is assumed to be governed by the local crack stopping -effect, which is related to bond coat oxidation and interface roughness. An incubation time is introduced, which is a measure for the transition from the initial to the subsequent phase, in which crack propagation is treated as subcritical crack growth.

6 SFB nd International Symposium on "Integral Materials Modelling" September 11, 2002, 9:00 am - 5:00 pm RWTH-Aachen (former PH-Building) Ahornstr. 55 Room AH 5 Entrance: Mies-van-der-Rohe Str. Further information is available via Internet: Contact: M. Deilmann Institut für Metallkunde und Metallphysik RWTH Aachen D Aachen Germany sfb370@imm.rwth-aachen.de phone: fax:

Simulation of microstructure development and formation of mechanical properties in metal forming technology.

Simulation of microstructure development and formation of mechanical properties in metal forming technology. Dr. Nikolay Biba, QuantorForm Ltd. Abstract. The paper presents an approach that combines the