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a grain size of 50-100 µm, the corresponding primary par- ticle size is slightly larger than the grain size while for the alloys with a grain size larger than ~ 150 µm, the primary particle size is obviously smaller than the grain size.


As discussed above, for the alloy refined with 1% MgCO3 (grain size = 53 µm), the original equiaxed dendrites in the as-


cast microstructure experience three stages, the initial coars- ening, structure separation and spheroidization, to evolve into spheroidal particles in the semisolid microstructure during partial remelting. Based on this mechanism, it can be easily concluded that the finer as-cast microstructure must result in the semisolid microstructure with smaller and more spheroidal primary particles. However, it is certain that the smaller the initial grain size, the larger the grain boundary interfacial en- ergy of the solid microstructure and the larger the solid/liquid interfacial energy of the resulting semisolid microstructure. To decrease these two kinds of interfacial energy, intensive coalescence of the neighboring dendrites or primary particles must occur during partial remelting. The authors’ investiga- tions of ZA27 alloy or pre-deformed AZ91D alloy indicated that the separation and coalescence of the primary particles were in strong competition condition during the structure separation and subsequent spheroidization stages.23,28


So, it


can be suggested that when the initial as-cast microstructure was refined to a given degree, the primary particle size in the resulting semisolid microstructure (when the heating time is given) might reach a given minimum value and could not be further decreased due to the coalescence. In addition, because of the effect of surface curvature of a solid particle on its melt- ing point, some of the small-sized particles may completely melt during partial remelting.24


It is due to the coalescence and


melting that the resulting primary particle size is slightly larg- er than the corresponding initial grain size for the alloys with relatively small grains (50-100µm).


As shown in Fig. 8, the two neighbor- ing dendrites A and B merge during the initial coarsening (#1) and structure separation stages (#2), and they are not separated by liquid phase and finally evolve into a large-sized spheroidal particle through the spheroidization process (#3). That is to say that this spheroidal primary particle in the semi- solid microstructure originates from the two dendrites A and B in the ini- tial as-cast microstructure. At the same time, dendrite C does not merge with its neighboring dendrites and evolves into a small-sized polygonal-shaped par- ticle through the structure separation process. Finally, it completely melts during the spheroidization stage due to its relatively low melting point. It can be expected that the dendrites which can merge must have the same crystal


orientation or very little crystal misorientation. But in the solid alloy, the number of the dendrites that match this requirement should be very few. However, in the semisolid alloy, there is liquid phase around the primary particles and the particles can adjust their orientations through rotation.21


So the coalescence


as-cast grain size is 53 µm, but the primary particle size of the semisolid microstructure (when the structure separation has just completed, i.e., the specimen is heated for 10 min) is increased to 82 µm (Fig. 4), which implies that intensive coalescence has occurred during the structure separation stage.


should occur more during the late period of structure separa- tion stage and the whole spheroidization stage than the initial coarsening stages. For the alloy refined with 1% MgCO3


, its


Figure 7. Variations of primary particle size and shape factor in semisolid microstructure and grain size in as-


cast microstructure with addition amount of MgCO3 particles.


(1)


(2)


(3)


Figure 8. Illustration of microstructural evolution during partial remelting of the AZ91D alloy with small-sized grains.


International Journal of Metalcasting/Winter 2012 49


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