Copyright 2003, The AISE Steel Foundation, Pittsburgh, PA. All rights reserved.111.1 Solidification Structure11.1.1 Crystal MorphologyMany researchers developed the basic principles of solidification quite some time ago. Among them, Chalmers (1964),1Flemings (1974),2and Kurz and Fisher (1984)3have published three very famous books.Solidification structure is first determined by crystal morphology, which is the result of the stabil- ity of the solid/liquid interface. Stability depends on the velocity of the interface and the temper- ature gradient. Kurz and Fisher show the complete set of morphology as a function of the above two parameters, as shown in Fig. 11.1.3The morphology of crystals changes from planar to cellu- lar to dendritic as velocity and temperature gradient increase. Further, Trivedi and Kurz have pub- lished a comprehensive review on dendritic growth.4In recent years computer simulation techniques have greatly advanced. Rapaz made a review with a good survey on evolution of microstructure models.5The phenomena are now fairly well under- stood in general, but some problems have yet to be solved that are very interesting from an acad- emic point of view.11.1.2 Solidification StructureThe reason solidification structure is important in the casting process is simply that it may affect the quality and performance of the product. Poor structure often causes many troubles and defects. Which is good or which is bad depends solely on the customers. Customers demands are often diverse, and so it is not wise to simply choose dendrites or equiaxed crystals as the best structure without taking other factors into consideration. Because many factors affect the quality of casting, we need to properly understand the phenomena occurring in a continuous casting process from an industrial point of view.Fig. 11.2 shows a typical example of the solidification structure appearing at the cross-section of an ingot.2Three zones are visible in the figure: (1) the small free crystal grain zone near the wall (called the chill crystal zone), (2) the columnar crystal grain zone midway to the center and (3) the equiaxed crystal grain zone in the center. When a liquid is poured into a cooled mold, manyChapter 11Structural Control of CastingShozo Mizoguchi, D.Eng., PhD, DIC, Professor, Inst. Advanced Materials Processing, Tohoku UniversityCasting Volume2Copyright 2003, The AISE Steel Foundation, Pittsburgh, PA. All rights reserved.crystals are nucleated at various nucleation sites. The most common site is the mold wall. This is the typical example of the heterogeneous nucleation of crystals.The crystal of iron or steel is oriented for preferential growth in a certain direction. For example, the 110 axis of -ferrite is a preferential growth direction. The small crystals nucleated on the mold wall have ran- dom crystal orientation, and among them the crystal grain having the preferential orientation parallel to the heat flow can grow faster than other grains. Heat flow is normally perpendicular to the mold wall. Thus, crystal grains having the preferential growth orientation perpendicular to the mold wall will be dominant. These grains with the preferred orientation form the columnar structure midway to the center of the casting. However, in due course of the growth of columnar grains, the sudden change in morphology may take place when the growth condition for equiaxed crystal grains becomes dominant.11.1.3 Transition from Dendritic to Equiaxed Crystal Grain StructuresIn order to understand the change in the growth mechanism from a dendritic to an equiaxed crystal, one must understand constitutional undercooling. Based on the classical theory of crystal growth, Kurz and Fisher schematically show in Fig. 11.3 the concentration and temperature profiles at the solid/liquid interface together with the two-component equilibrium phase diagram.3First, the solute concentration is C0/k at the interface as a result of solute element partition according to the phase diagram on the right. Second, the concentration (CL) gradually decreases by the diffusion of the solute element into the liquid, as shown in the top graph. The liquidus temperature (TL) is then defined by the above-mentioned concentration profile and equilibrium phase diagram, as shown in the bottom graph. In fact, the actual temperature in front of the interface is shown as the line T under the condition of one-dimensional heat flow from liquid to solid. The gap between TLand T means that the ambient liquid is undercooled below the liquidus temperature. This difference (TL T) is known as “constitutional undercool- ing.”Two major factors affect the degree of under- cooling. First, diffusivity of the solute element affects the concentration profile of solute ele- ments in front of the dendrites. Second, thermal diffusivity also affects the temperature gradients in front of the dendrites. Therefore, speed of the advancing interface and cooling rate are major condit