The Chill Zone

Solidification. When the ingot or casting solidifies, there are three main possible microstructures that form (see Figure 7.5). We will describe here only the final structures the thermodynamics of the liquid-solid phase transformation have been described previously in Chapter 2. The outside layer of the ingot is called the chill zone and consists of a thin layer of equiaxed crystals with random orientation. 

T.F. Bower and M.C. Flemings. Formation of the chill zone in ingot solidification. Trans. TMS-AIME, 239 216-219, 1967. 

The position of the transition was located by visual observation and optical microscopy. The distance from the chill zone of the sample was measured with a ruler. It is noted in Figure 1 that the CET is not sharp, showing an area where some equiaxed grains co-exist with columnar grains. As was reported before, the size of the transition area is in the order of up to 10 mm (Ares et al., 2007, 2010). The grain structure was inspected by visual observation under Arcano optical microscopy. 

A hot-chamber diecast AZ91D thin plate with a die chill skin on its surface was severely corroded in 5 wt% chloride solution 1600pA/cm ), whereas a plate with a die skin layer etched in an HF/H2SO4 aqueous solution to remove interdendritic phases had a substantially lower corrosion rate (3 to 16 tA/cm ). The die-chill skin was composed of a thin layer of chill zone and a thick layer composed of interdendritic Al-rich a-Mg/Alx2Mgx7 P phase particle/a-Mg grain composite. The chill zone (4+1 pm in thickness) had fine columnar and equiaxed grains and contained a distribution of submicron Mg-Al-Zn intermetallic particles. The removal of the primary P-phase from the diecast sample surface did not improve the corrosion performance of the specimen (Uan et al., 2008). 

Probably the most important factor in determining subsequent properties is the relative proportions of the columnar and equiaxed zones. The chill zone is normally only a small number of grains thick and has a very limited influence. 

The chill crystals nucleate on or near the mould wall. Observations of undercoolings make it clear that they are the result of heterogeneous nucleation. The formation of the chill zone was initially explained " in terms of the copious nucleation considered to occur in the thermally undercooling region adjacent to the mould wall. The extent of nucleation was largely determined by the thermal conditions at the mould wall, but also by the efficiency of the mould wall as a substrate for heterogeneous nucleation and by the existence of effective nucleants in the chilled liauid layer. The existence of localised constitutional supercooling can also play a p art. Overall, the numbers of crystals in the chill zone depend on the superheat of the liquid, the temperature of the mould, the thermal properties of the metal and the mould, as well as on the nucleation potency of the mould wall or of particles in the liquid. In extreme cases it would be possible for chill crystals formed initially to subsequently remelt. 

This nucleation theory has been modified to allow for crystal multiplication by the fragmentation of the initial nuclei. Crystal multiplication ly the remelting of dendrite arms resulting from growth fluctuations (produced, for instance, ty convection) was observed ty Jackson et al. This is shown in Fig. 6.4 and was used as a basis for a theory of equiaxed zone development (see Section 6.5). Biloni ° has established that both mechanisms are operable in the generation of the chill zone, with their relative importance depending on the casting conditions. The crystals in the chill zone are normally equiaxed and of random orientation. 

The columnar crystals develop predominantly from the chill zone and show a strong preferred ciystallographic orientation which corresponds with the preferred crystallographic directions of dendritic growth. " Some few grains appear in the columnar zone which... 

The crystals in the equiaxed zone usually have a grain size larger than that of the chill zone and their orientation is effectively random. As shown in Figs. 6.7 and 6.8, the formation of an extensive equiaxed... 

The result is a hard, abrasion-resistant surface, important in many appHcations of cast kon. The depth of the chill may be controlled by regulating the amount of tellurium added. The casting shows a sharp demarcation line between the chilled and unchilled regions there is no intermediate or motded zone. Yet, the chilled portion shows excellent resistance to spalling from thermal or mechanical shock. Tellurium-treated kon is more resistant to sulfuric and hydrochloric acids than is untreated, unchilled gray kon. The amount added ranges from 0.005 to 0.1% ca 60% is lost by volatilization. Excessive addition causes porosity in the castings. 

The throw of downward-projected heated jets or upward-projected chilled jets can be derived from Eqs. (7.85) and (7.88) for K equal to some value, e.g., 0.1. Helander and Jakowatz, in their work on heated jets projected downward, have called attention to some of the differences between the actual conditions and those assumed for analysis. One of these is the radial escape of warm air in the terminal zone of a hot stteam projected downward. This escaping warm air then rises and causes a change in ambient conditions for the upper part of the jet. The terminal zone and the edges of the jet are zones of marked instability, with definite surges and fluctuations, so that the jet envelope is very difficult to define or to determine experimentally. In the closure to the paper presented by Knaak, Dr. Helander suggested that from the point of view of practical application, the distance to the beginning of the unstable, tet-minal zone of the jet is about 80% of the jet throw. 

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