Materials Science Forum Vols. 539-543

Abstract: The kinetics and topology of grain growth in three dimensions were simulated using a phase-field model with anisotropic grain-boundary mobilities. In order to perform large scale calculations we applied both modifications of algorithms and parallel coding techniques to the Fan and Chen's phase-field algorithm. Kinetics of abnormal grain growth is presented. It is observed that the grains of a minor component which are at the beginning surrounded preferentially by boundaries of high mobility grow faster than the grains of a major component until the texture reverses completely. Additionally, topological results of grain structures, such as grain size distributions and grain face distributions, are discussed

Abstract: The kinetics of phase transformations in medium-carbon forging steels (MCFS) have been modeled based on CALPHAD multicomponent thermodynamics and the classical nucleation-growth theory. New treatments include the time dependency of parabolic growth rate of proeutectoid ferrite (α) , which account for the soft impingement effect by carbon enrichment in austenite (γ). And a potential transition of γ/α interface equilibrium has also been considered depending on temperatures and velocity of the moving interface. To make a realistic prediction of the onset of pearlite (P) transformation, a normal distribution of γ grain size has been assumed and successive α→P transformation kinetics in each grain size have been summated. The developed program coupled with thermodynamic solver, 'ThermoCalc', calculated the isothermal kinetics of MCFS and has been found to predict well the effect of minor difference of chemical composition / holding temperatures.

Abstract: During the last decade Genetic Programming (GP) has emerged as an efficient methodology for teaching computers how to program themselves. This paper presents research work which utilizes GP for developing mathematical equations for the response surfaces that have been generated using hybrid modelling techniques for predicting the properties of materials under hot deformation. Collected data from the literature and experimental work on aluminium are utilized as the initial training data for the GP to develop the mathematical models under different deformation conditions and compositions.

Abstract: Dynamic recrystallisation (DRX) is an important aspect for industrial applications in hot metal working. Although DRX has been known for more than thirty years, its mechanisms have never been precisely investigated, in part because it was not readily possible to make local texture measurements. In the present work, the material behaviour during DRX is investigated and modelled based on the microstructure of 316L stainless steel. The developed model is based on a constitutive equation Modelling technique which incorporates the strain, strain rate and instantaneous temperature for predicting the flow stress of material being deformed under hot conditions.

Abstract: Oxide scale behaviour in thermomechanical processing has been the subject of intensive research for several years that allowed developing a finite element (FE) based model to simulate a range of events of relevance to the process and the surface quality of the hot rolled product. A range of experimental techniques have also been developed, each providing a partial insight. An overview of this research is presented in the sequence of rolling and finishing with descaling. The model has been extended to provide the basis for detailed numerical investigations of the roll/stock interface behaviour during multi-pass hot rolling operations. The modelling techniques have been used for providing design criteria for AISI430 ferritic stainless steel scale failure during bending. Modelling of near surface deformation during rolling of aluminium alloys is under consideration.

Abstract: This work highlights and solves problems with the prediction of the compressive strength, limited by local instabilities, of sandwich material compounds based on honeycomb cores and very thin facesheets. Analytical methods in conjunction with periodic finite element unit cell models are utilized for this task. The finite element models are found to be well suited for all kinds of buckling predictions. Different uni- and bi-axial loadings are considered as well as influences of core height, core material, core geometry, and facesheet thickness are investigated. Finally, a new analytical approach is introduced for the treatment of the rather unexpected core cell wall buckling under in-plane compression of the sandwich, which predicts the critical load very accurately.

Abstract: The kinetics of the solid-state reactive diffusion between Au and Sn was experimentally observed using Sn/Au/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed at a temperature of T = 453 K. Due to annealing, AuSn, AuSn2 and AuSn4 compound layers are formed at the interface in the diffusion couple. The experimental results were used to evaluate quantitatively the effect of Ni on the growth of the Au–Sn compounds. The evaluation indicates that the addition of Ni into Sn between 1 and 5 mass% accelerates the growth of the Au–Sn compounds at T = 433–473 K.

Abstract: To model the microstuctural and mechanical responses of quenched metallic components, the evolution of the thermal field must be known precisely; the latter, in turn, depends on accurate values of the thermal boundary conditions. In this work, the heat transfer boundary conditions on both sides of a stainless steel disk, held horizontally while a water column impinged on its lower surface to cool it from 850°C to room temperature, were characterized as heat flux histories which are functions of the radial coordinate. Thermal responses, measured with embedded thermocouples and a computer-controlled data acquisition system, were used to estimate the heat flux histories by solving the corresponding inverse heat conduction problem (IHCP), considering radial symmetry. The optimization problem also included the estimation of sub-areas associated with different heat extraction rates on both the lower and upper surfaces of the disk. The fluctuating interaction between the water column and the cooling disk was captured in the estimated heat flux histories. The estimated thermal boundary conditions were validated by computing the thermal response at the thermocouple locations by solving the direct heat conduction problem (DHCP) with a computer program based on the finite-element method. A good agreement between experimentally determined and computed thermal responses was observed, thus verifying the methodology employed.

Abstract: The most influential injection-molding factors were chosen and the Taguchi method of experiment planning was used to determine the number of the experiments needed. The test specimens in the form of a standardized tensile specimen were injection-molded under six different conditions. The analysis of variance was used to determine the parameters influencing the dimensions of a green part. Under the same conditions the test specimens were then subjected to debinding and sintering, and the influences of injection on the mass, mechanical properties and dimensions were analyzed once again.

Abstract: In the paper the problem of casting and mould thermophysical parameters identification is discussed. So, it is assumed that in the mathematical model describing the thermal processes in the system considered the selected parameter (or parameters) is unknown. On the basis of additional information concerning the cooling (heating) curves at the selected set of points the unknown parameter can be found. The inverse problem is solved using the least squares criterion in which the sensitivity coefficients are applied. On the stage of numerical simulation the boundary element method is used. In the final part of the paper the examples of computations are shown.