Role of fragmentation in as-cast structure: numerical study and experimental validation

A volume average solidification model is extended to incorporate fragmentation as the source of equiaxed crystals during mixed columnar-equiaxed solidification. This study is to use this model to analyze the role of fragmentation in the formation of as-cast structure. Test simulations are made for the solidification of a model alloy(Sn-10wt.%Pb) with two different geometries. The first one is a 2D rectangular domain(50 × 60 mm~2) as cooled from the top boundary. Solidification starts unidirectionally as columnar structure from the top. The solute(Pb) enriched interdendritic melt is heavier than the bulk melt, and sinks downwards, hence leads to solutal convection. Fragmentation phenomenon occurs near the columnar tip front. The fragments are transported out of the columnar region, and they continue to grow and sink, and finally settle down and pile up at the bottom. The growing columnar structure from the top and pile-up of equiaxed crystals from the bottom finally lead to a mixed columnar-equiaxed structure, in turn leading to a columnar-to-equiaxed transition(CET). The second geometry is a 3D plate, 100 × 60 ×10 mm~3, as cooled laterally from one side. It was cast experimentally and analyzed for the as-cast structure. The equiaxed fragments are produced in the solidification front and transported into the bulk melt, leading to a special pattern of as-cast structure: columnar structure in the cool wall side and equiaxed structure in the upper left corner near the hot wall side, extending downwards to the middle bottom region. Numerically calculated as-cast structures agree with the experiment results.     

Modeling of Multiscale and Multiphase Phenomena in Materials Processing

In order to demonstrate how CFD can help scientists and engineers to better understand the fundamentals of engineering processes, a number of examples are shown and discussed. The paper covers (i) special aspects of continuous casting of steel including turbulence, motion and entrapment of non-metallic inclusions, and impact of soft reduction; (ii) multiple flow phenomena and multiscale aspects during casting of large ingots including flow-induced columnar-to-equiaxed transition and 3D formation of channel segregation; (iii) multiphase magneto-hydrodynamics during electro-slag remelting; and (iv) melt flow and solidification of thin but large centrifugal castings.     

On the Importance of Electric Currents Flowing directly into the Mould during an ESR Process

In the present paper a numerical model is developed to predict the exact electric current paths in the slag region of an Electro‐Slag‐Remelting (ESR) process. The model solves the momentum and energy equations. The solidification of the slag at the mould is modelled with an enthalpy‐porosity approach. The magnitude of the Joule heating and the Lorentz force are derived from the computed electric current lines. The localization where the Joule heating occurs controls the temperature distribution. The electric current distribution is in turn influenced by the temperature field through the temperature dependant electric conductivity. With this numerical tool the electric current paths are exactly computed by choosing the less resistive way to the liquid pool, or to the mould. For a given electric intensity the model predicts the power generated by the system and the solidified slag thickness at the mould. The model is validated by comparing its results with experiments on a small scale ESR process with high current density.     

A Numerical Investigation on the Electrochemical Behavior of CaO and Al2O3 in the ESR Slags

Metallurgical and Materials Transactions B - Field structures including electromagnetic, concentration of ions, and flow fields in an ESR-like process composed of a graphite crucible containing a...     

Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments

Metallurgical and Materials Transactions A - It is well known that the growth and motion of equiaxed crystals govern important microstructural features, especially in larger castings such as heavy...     

A Parametric Study of the Vacuum Arc Remelting (VAR) Process: Effects of Arc Radius,Side-Arcing,and Gas Cooling

Metallurgical and Materials Transactions B - Main modeling challenges for vacuum arc remelting (VAR) are briefly highlighted concerning various involving phenomena during the process such as...