nonuniform compared with that of the alloys containing small-sized grains and more β phase distributes between some of the neighboring secondary arms (in the regions circled by the dotted lines in Fig. 9[a]). So more β phase is left between them (in the regions circled by the dotted lines in Fig. 9[b]) after the initial coarsening stage and arm A do not merge with the neighboring arms (Fig. 9[b]). Dur- ing the subsequent structure separation stage, the residual eutectic melts to lead the coarsened arm A to be surrounded by liquid pools (the liquid pools along the dotted line in Fig. 9[c]). Then the coarsened arm A is gradually separated from its parent dendrite B through penetration of the first formed liquid phase along the original boundaries (along the dotted line in Fig. 9[d]) and one dendrite separates into two individual irregular particles (Fig. 9[e]). Particle A originates from two arms of the parent dendrite B, so its size is always obviously smaller than that of the parent particle B. Namely, the size difference in the resulting par- ticles is quite large. The particle shapes become spheroidal to some degree through spheroidization, but they are still very irregular and their size differential is still large (Fig. 9(f), the small-sized particles marked by arrows may origi- nate from the separated arms). That is to say that one origi- nal dendrite evolves into two or more particles for the large grained alloys. So the resulting particle size is smaller than the original grain size. Therefore, the evolution process for the large-grained alloys can be schematically expressed in Fig. 10, with numbers denoting the three stages, the initial coarsening, structure separation and spheroidization, re- spectively, (a-f) correspond to the typical microstructures shown by Fig. 9(a-f), respectively.
Based on the above discussion, it can be concluded that the size of a dendrite is a main factor that deter- mines if it separates during the struc- ture separation stage. In addition, the dendrite morphology should be an- other factor. If the size of a dendrite is within 50-100 µm, but its morphol- ogy is not equiaxed, the long den- drite arms may separate from its par- ent. On the contrary, if the size of a dendrite is relatively large (over 150 µm), but its shape is quite equiaxed, it possibly evolves into one large- sized spheroidal particle without separation. However, it should be noted that, generally, the more devel- oped the dendrite and thus the more irregular, the higher the possibility of separation. In addition, there is a value for grain size within 100-150 µm, one dendrite should evolve into one particle without coalescence and separation. This theory will be clari- fied in future work.
(a)
Therefore, it can be concluded that the initial as-cast mi- crostructure has a large effect on the semisolid micro- structure: the finer the as-cast microstructure, the smaller and more spheroidal the primary particles in the semi- solid microstructure. Due to the coalescence of dendrites or primary particles and the melting of small-sized par- ticles during partial remelting, the resulting primary par- ticle size is slightly larger than the initial grain size for the alloys with a grain size of 50-100 µm. But for the alloys with a grain size larger than ~150 µm, the former size is always smaller than the latter size because of the separation of one original dendrite into several primary particles.
Microstructure Uniformity of Semisolid Rods with Different Diameters
Figure 11 shows the semisolid microstructures at the cen- ter and edge regions of the rods with varying diameters. It shows that the primary particles in the edge are smaller than those in the center for each rod and the particle size in the same region increases as the diameter increases. But the variation of particle size in the edge region is very small once the diameter exceeds 45 mm (comparing Figs. 11d and f). In addition, the particle size difference between these two regions also increases with increasing diameter. Consider- ing the as-cast microstructures,20
it can be found that these
changes are also consistent with that of the as-cast grain size: the finer the as-cast grains, the smaller and more spheroidal the primary particles.
(b) (c)
(1) (d) (e) (f)
(2)
(3)
Figure 10. Illustration of microstructural evolution during partial remelting of the AZ91D alloys with large-sized grains.
International Journal of Metalcasting/Winter 2012 51
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