Monika Grasser

Columnar to Equiaxed Transition During Ingot Casting Using Ternary Alloy Composition

The present paper deals with the formation of macrosegregation in a benchmark ingot using (Fe-C-Cr) ternary alloy composition. The numerical investigation of complex multiphase phenomena is a difficult study, because the thermophysical properties depend strongly on the temperature, concentration, pressure and chemical composition as well. For the numerical modeling of solidification and melting processes different phases (e.g. liquid, equiaxed crystals and columnar dendrite trunks) have been considered. The mass, momentum, energy conservation and species conservation equations for each phase have been solved. The Eulerian-Eulerian model equations have been implemented in the commercial Finite Volume Method based FLUENT-ANSYS v6.3 CFD software using User-Defined Functions (UDF). The mass transfer has been modelled by diffusion controlled crystal growth by applying an advanced tip tracking algorithm for columnar solidification. The modeling of the grain density transport has been improved. The derivatives of the mass fraction quantities for each component appear in the nucleation rate term. It means that we obtain a new term of the right hand side of the grain density transport equation for using ternary alloy composition. This paper focuses on both the process and simulation parameters and their influence on the macrosegregation formation. The results show that the macrosegregation pattern does not change significantly above a well-chosen number of grid cells, and the computational time could be decreased, when the time step size has been increased.

In Situ Observation of Solidification in an Organic Peritectic Alloy System

Up to date very few organic substances have been reported that show a non-faceted/non-faceted (nf/nf) peritectic phase diagram in a temperature range suitable for direct observation in a micro Bridgman furnace setup. Sturz et al. [1] and Barrio et al. [2] studied the peritectic phase diagram for the organic model alloy TRIS (Tris(hydroxymenthyl)aminomethane) - NPG (Neopentylglycol). The phase diagram is based on thermal analysis by means of DSC measurements [1, 2] and evaluation of lattice parameters measured with x-ray diffractometry [2]. In the current work we present investigations on the system TRIS – NPG that have been obtained by optical investigations of directional solidification in a micro Bridgman-furnace with various initial alloy concentrations and pulling rates in a fixed temperature gradient. The phase diagram [1, 2] was confirmed by direct comparison of DSC measurements and optical investigations. Furthermore we present in situ observations of solidification in the peritectic region. They show a solidification behavior that was clearly distinguishable from the solidification in hyper- and hypoperitectic regions of the phase diagram.

Numerical investigation of grid influence on formation of macrosegregation

Abstract The investigation of grid influence on numerical prediction of the formation of macrosegregation is an important issue in the point of view of numerical modelling. The estimation of numerical accuracy for the simulation of complex multiphase phenomena is a difficult modelling process, since the thermophysical properties depend on the temperature and concentration as well. The numerical stability and accuracy of the modelling also depend on the chosen time step and grid size. This paper focuses on the grid influence and modelling questions on macrosegregation in a benchmark ingot using Fe-0·34 wt-%C steel. The FLUENT-ANSYS v6·3 commercial software does not have built-in multiphase solidification and melting module for simulating columnar to equiaxed transition. Therefore, a multiphase model was implemented using User-Defined Functions. The number of grid cells has been increased from 180 to 4300 to define an optimal grid size, to prove the reliability of the model implementation based on solution accuracy. The results show, the macrosegregation pattern does not change significantly above a well-chosen number of grid cells.

A 3-phase model for mixed columnar-equiaxed solidification in DC casting of bronze

A three-phase Eulerian approach is used to model the columnar-to-equiaxed transition (CET) during solidification in DC casting of technical bronze. The three phases are the melt, the solidifying columnar dendrites and the equiaxed grains. They are considered as spatially interpenetrating and interacting continua by solving the conservation equations of mass, momentum, species and enthalpy for all three phases. The so defined solidification model is applied to a binary CuSn6 DC casting process as a benchmark to demonstrate the model potentials. Two cases are studied: one considering only feeding flow and one including both feeding flow and equiaxed sedimentation. The simulated results of mixed columnar and equiaxed solidification are presented and discussed including the occurrence of CET, phase distribution, feeding flow, equiaxed sedimentation and their influence on macrosegregation.

Numerical Study and Experimental Investigation of Solidification Phenomena in Semi-Continuous Casting of Bronze

In semi condinuous casting of technincal bronze alloys homogenuous microstructure is very important for the assurance of material properties. The improvement of the knowledge about both, thermodynamics of the ternary system Cu-Sn-P and the solidification process is of main interest for the involved industry. To describe solidification of these alloys, the ternary system Cu-Sn-P in the Cu-rich corner is experimentally investigated. DSC measurements, diffusion and annealing experiments have been performed and compared with computational thermodynamics based on the Calphad approach. The so defined thermodynamic information is coupled with solidification simulation. For this, CFD calculations are done with a two phase solidification model including mass, momentum, energy and concentration transfer and applied to semi-continuous casting of technical Bronze alloys. The predicted macrosegregation pattern is in good qualitative agreement with experimentally observed result.

Numerical Study of the Thermosolutal Convection Induced Macrosegregation during Columnar Solidification

As a response to “call for contribution to a numerical problem for 2D columnar solidification of binary alloys” [Bellet et al., Int. J. Therm. Sci., Vol. 48(11)(2009), p. 2013], the macrosegregation in a Pb-18wt.%Sn benchmark casting is numerically studied with a two-phase columnar solidification model developed by the current authors. The studies were done with 2D calculations in response to the call, and a 3D calculation was performed to confirm the consistency with the 2D case. A grid-sensitivity study was done to ensure the reliability and accuracy of the present results. The segregation mechanism due to thermosolutal convection was analyzed and the uncertainties resulting from the inaccurate thermophysical properties, modelling and process parameters are discussed. The numerical model was evaluated by comparison with experiments.

Thermodynamic description of the system Cu-Sn-P experimental and numerical investigation

Abstract The aim of the presented experimental and numerical investigation is to verify the ternary phase diagram Cu–Sn–P in the Cu-rich corner, especially the presence and position of the ternary eutectic transition point. Annealing experiments within a concentration gradient have been performed and compared to computational thermodynamics. The identification of phases and phase regions is based on differential thermal analysis, scanning electron and light microscopy. The discussed ternary eutectic transition in the Cu-rich corner is confirmed, phases and phase regions, as proposed by computational thermodynamics, are verified. In addition, the existence of the γ phase was confirmed but the ∊ phase was not found in the as-cast microstructures which decreases its importance for technical bronze alloys with Sn up to a mass fraction of 0.2.