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markedly less than that in the microstructure heated for 60 minutes. All of these results imply that the coarsening after 60 minutes is mainly attributed to Ostwald ripening.


From the above discussion, it can be concluded that the microstructural evolution during partial remelting of the AZ91D alloy can be divided into four stages: • • • •


the initial coarsening structure separation spheroidization final coarsening


The initial coarsening of the dendrites is attributed to the dissolution of eutectic β phase especially in the inter-arm regions. The structure separation is ascribed to the melting of residual eutectic β phase and subsequent penetration of the first formed liquid along the coarsened dendrite bound- aries. The spheroidization results from the partial melting of polygonal particles. The final coarsening is first due to the coalescence and then due to Ostwald ripening.


Effect of Initial As-Cast Microstructure on Semisolid Microstructure


Figure 6 shows the typical semisolid microstructures of the AZ91D alloys refined by different amounts of MgCO3


Together with Fig. 3(b), it indicates that the solid phase changes from the large and irregular structures to the small and spheroidal particles as the MgCO3


. amount increases


from 0% to 1% and then the particles gradually become large and irregular again when the amount exceeds 1%. A previous investigation indicated that the grain size of as-


cast AZ91D alloy significantly decreased as the MgCO3 amount increases within 1% and then slowly increased when the amount exceeds 1%.20


That is to say that there is


a good corresponding relationship between these two-state microstructures: the finer the as-cast microstructure, the smaller and more spheroidal the primary particles in semi- solid microstructure. The detailed relations between them can be more clearly seen by the quantitative examination results shown in Fig. 7. It also indicates that for alloys with


(a)


(b)


(c)


Figure 6. Microstructures of the AZ91D alloys refined by different amounts of MgCO3 at 580°C (1076ºF) and then water quenched. (a) 0%, (b) 0.4%, (c) 0.8% and (d) 1.2%.


48


(d) after being heated for 20 minutes


International Journal of Metalcasting/Winter 2012


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