Materials Science Forum Vol. 508

Abstract: Multi-crystalline silicon ingot casting using directional crystallisation is the most costeffective technique for the production of Si for the photovoltaic industry. Non-uniform cooling conditions and a non-planarity of the solidification front result, however, in the build-up of stresses and viscoplastic deformation. Known defects, such as dislocations and residual stresses, can then occur and reduce the quality of the produced material. Numerical simulation, combined with experimental investigation, is therefore a key tool for understanding the crystallisation process, and optimizing it. The purpose of the present work is to present an experimental furnace for directional crystallisation of silicon, and its analysis by means of numerical simulation. The complete casting procedure, i.e., including both the crystallisation phase and the subsequent ingot cooling, is simulated. The thermal field has been computed by a CFD tool, taking into account important phenomena such as radiation and convection in the melt. The transient thermal field is used as input for a thermo-elasto-viscoplastic model for the analysis of stress build-up and viscoplastic deformation during the process. Numerical analysis is employed to identify process phases where further optimisation is needed in order to reduce generated defects.

Abstract: Multi-crystalline silicon ingot casting using directional crystallisation is the most costeffective technique for the production of Si for the photovoltaic industry. Non-uniform cooling conditions and a non-planarity of the solidification front result, however, in the build-up of stresses and viscoplastic deformation. Known defects, such as dislocations and residual stresses, can then occur and reduce the quality of the produced material. Numerical simulation, combined with experimental investigation, is therefore a key tool for understanding the crystallisation process, and optimizing it. The purpose of the present work is to present an experimental furnace for directional crystallisation of silicon, and its analysis by means of numerical simulation. The complete casting procedure, i.e., including both the crystallisation phase and the subsequent ingot cooling, is simulated. The thermal field has been computed by a CFD tool, taking into account important phenomena such as radiation and convection in the melt. The transient thermal field is used as input for a thermo-elasto-viscoplastic model for the analysis of stress build-up and viscoplastic deformation during the process. Numerical analysis is employed to identify process phases where further optimisation is needed in order to reduce generated defects.

Abstract: Including detailed information of equilibrium phase diagrams (EPhD) is is one of the necessary features in phase transformation simulations. It is important for a simulation to provide a simple but accurate EPhD calculation method which is based on the knowledge of the phase boundary lines in the EPhDs as an mathematical function. A simple, fast and iteration free approximate calculation method has been developed earlier (ESTPHAD – Estimated Phase Diagrams). The input data may be either experimental values, or a data field calculated by CALPHAD, or digitized data from the graphical representation of measured and published EPhDs . By this method important parameters such as the beginning and ending temperatures of solidification or solid state transformations can be determined for each phase. The partition ratios as a function of the concentration can be calculated as well. In this study ESTPHAD is used to estimate the EPhD of binary Fe-Ni system.

Abstract: The atoms site preference in the Al₃Ti-Zn system has been studied using H. Rietveld method. The L12 Ti(Al, Zn)₃ particles evolve in a ZnAl25 melt from the L1₂TiZn₃ particles in the ZnTi4 master alloy. It is found that Zn is replaced by Al during the transformation TiZn₃ → Ti(Al, Zn)₃. In the evolving fcc L12 Ti(Al, Zn)₃ phase Ti occupies (0, 0, 0) position while Al and Zn occupy (0, 0.5, 0.5) position, similarly to Al and X in the Al₃Ti-X systems, where X = Ni, Cr, Mn, Cu, Ag, Pd [1, 2].

Abstract: The solidification and solid-state phase equilibria of four Al-Mg-Si alloys containing 30- 70%Mg and 0.5-3.5%Si, selected on the basis of an isothermal section of the Al-Mg-Si system calculated at 300 °C, have been investigated. Solidification paths of Mg-rich Al-Mg-Si alloys finish on ternary eutectics and the temperatures of two of these eutectic reactions, i.e. L↔(Al)+β+ Mg2Si and L↔(Mg)+γ+ Mg2Si, have been determined to be at ~ 448 °C and ~ 436 °C respectively by DTA. The characteristic temperatures recorded on the DTA curves are analysed and a linear relationship is found between the peak temperature and the square root of the scanning rate.

Abstract: This paper presents a method aimed at controlling free surface flow and stirring melt via a magnetic field induced by the permanent magnets. The rotating magnetic field (RMF) can realize the free surface shape control and the melt stirring simultaneously. Numerical model was built to analyse the magnetic field distribution. Two drivers that have the same structure were analysed and optimised. Quasi-steady-state free surface was obtained by regulating the rotating velocity of the magnetic drivers, which is proportional to the magnetic force. Solidification experiment was preformed on a platform of a mini-continuous caster. The solidifying front was observed via addition of a small quantity of Sn-wt.43%Pb into the continuous casting alloy Sn-wt.3.5%Pb, it was found that the solidifying shell grows uniformly under the condition of a proper imposition of the double-permanent-magnet-driver.

Abstract: A simple but thermodinamically well-founded method is described for the recalculation of liquidus surfaces and partition coefficients of multicomponent equilibrium phase diagrams used a data file of the liquidus temperature and partition coefficients calculated by CALPHAD method. The applicability of the method is shown in case of Al-Mg-Si ternary alloy systems.