Structure equiaxed zone

Figure 22.5 Classic grain structure of casting (or ingot) containing chill zone, columnar zone, and equiaxed zone. From Bower and Flemings [5].

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. 

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 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. 

Fig, 6.2 Possible casting structures (a) wholly columnar except for chill zone (b) partially columnar, partially equiaxed (c) wholly equiaxed Heine et al. ). 

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