The Equiaxed Zone

Figure 4.5 Section through a solidification ingot showing the various solidification zones. The equiaxed zone forms near the mold walls. In the interior is the columnar zone, where crystal growth is in the direction of heat flow. (After Lalena and Cleary, 2005. Copyright John Wiley Sons, Inc. Reproduced with permission.)...

Explanations of the extent and nature of the eolumnar zone begin, as described above, with the development from the chill crystals. The main controlling factor is the appearance of the equiaxed zone. 

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... 

I liner bands of V-segregatesalso occur which consist of fairly uniform Ncgregalc distributions on the surfaces of cones with their points downwards. These occur at the ingot centre in the equiaxed 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. 

Pig, 7.7 Optical micrographs of the stir zone for friction stir welding on Ti-6Al-4V. (a) Center of stir zone showing equiaxed grains with grain-boundary a phase and fine acicular a phase In a (3 matrix, (b) Near top surface, showing finer prior-p grain size than (a)... 

Microstructures near the TMAZ were also examined. A region near the shr zone/TMAZ interface, called the near-stir zone by the authors, exhibited a distinchve microstructural feature in welds made on both starhng microstructures. Small, equiaxed grains of a, approximately 1 pm in size, were reported. Again, their similar structure appeared to indicate that formation of the local microstructure was dependent on the thermomechanical cycle and not on the starhng microstructure. 

The base metal had equiaxed grains with an average diameter of 25 pm. Welds were produced with no apparent defects. Optical microscopy revealed that the microstructure of the stir zone was characterized by a high density of deformation twins within the grains. The twin density varied with position relative to the position of the tool shoulder, with denser twins found near the upper part of the weld. The... 

The TEM results indicated that the base metal had an equiaxed grain structure with a low twin content. The TEM characterization of the stir zone showed a large amount of twin embedded... 

In Fig. 12.23(a), the boxed areas indicate where the grain structure samples are taken to show the details of the stir zone and TMAZ. A close-up view of the stir zone in Fig. 12.23(b) shows very fine equiaxed grains. This is due to stirring and recrystallization. A close-up view of the TMAZ in Fig. 12.23(c) shows fine grains. For comparison, a close-up view of the base metal in Fig. 12.23(d) shows coarse grains. 

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 view of the transition zone (Fig. 5.38D) shows round spherulites in a fine textured matrix. The equiaxed spherulites, especially adjacent to the skin, indicate there is little or no orientation. PBT skin thickness has been shown to increase with decreasing melt and mold temperatures, resulting in increased impact properties in bars with molded-in notches [165]. 

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). 

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