All of these variations can be more clearly seen by the quanti- tative examination results shown in Fig. 12. The reasons why the particle size and morphology change in such manners can be found in the above section. The present results indicate that, to obtain a semisolid ingot with uniform microstructure, it should be a priority to purchase ingot with a uniform as-cast microstructure. This is especially important for large-sized in- gots. Figure 12 also shows that the particle size is larger than the corresponding grain size for the alloys with a grain size of 50-100 µm while the former size is smaller than the latter size for the alloys having grains larger than 150 µm.
structural evolution during partial remelting at 580C (1076F) can be divided into four stages: • • • •
Conclusions For the AZ91D alloy refined with 1% MgCO3
the initial coarsening structure separation spheroidization final coarsening
(a) , its micro-
A semisolid microstructure with small and spheroidal pri- mary particles can be obtained after it progresses through the previous three stages.
The initial coarsening results from the dissolution of eutectic between the dendrite arms and the resulting coalescence of the dendrite arms. The structure separation is attributed to the melting of the residual eutectic and subsequent penetra- tion of the first formed liquid along the original grain bound- aries. The spheroidization is ascribed to the melting of the polygonal particles’ edges and corners. The final coarsening is due to the coalescence of the spheroidal particles and sub- sequent Ostwald ripening.
The initial as-cast microstructure of AZ91D alloy has large effect on its semisolid microstructure: the finer the as-cast microstructure, the smaller and more spheroidal the primary particles in the semisolid microstructure.
For the alloys with a grain size of 50-100 µm, due to the co- alescence of neighboring dendrites or primary particles and
(b)
(c)
(d)
(e)
(f)
Figure 11. Semisolid microstructures of different positions of the AZ91D alloy rods with different diameters. (a) and (b) 16 mm, (c) and (d) 45 mm, (e) and (f) 70 mm.
52 International Journal of Metalcasting/Winter 2012
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