alloys with different as-cast microstructures are discussed. In addition, the uniformity of semisolid microstructures of cast rods with a larger diameter (45 mm and 70 mm) has been examined.
Experimental Procedure
ed with the molten alloy (i.e., there was no gas to release), the melt was held for 10 min, then cooled to 705°C (1301°F) and poured into a steel mould with a 16 mm varying diam- eter cavity. Repeating the above experimental procedure, the cast rods refined by different amounts of MgCO3
This study used a commercial AZ91D alloy with a composi- tion of Mg-9.04Al-0.6Zn-0.31Mn (Note: the percentages in this paper all refer to weight percentage). The AZ91D alloy was first remelted in a resistance furnace and MgCO3 added at 790C (1454F). After the MgCO3
completely react- ranging
mm in height) were cut from the rods. The specimens refined with 1% MgCO3
MgCO3
from 0-1.4% were performed. The microstructures varied with the added MgCO3
had the finest grains.20
amount and the rods refined by 1% Some small specimens (10
were heated at 580C (1076F) for different
diameters of 45 mm and 70 mm were prepared respectively. Two specimens (16 mm diameter and 10 mm height) were machined from the center and edge regions of each rod and also heated at 580C (1076F) for 20 minutes.
durations to investigate the microstructural evolution during partial remelting. One specimen of each rod was heated at 580C (1076F) for 20 minutes to study the effect of the initial as-cast microstructure on the semisolid microstructure. In addition, to examine the uniformity of semisolid microstruc- tures of cast rods with large diameters, repeating the above melting procedures, two rods (treated with 1% MgCO3
) with Microstructural Evolution during Partial Remelting
a microstructure containing fine and uniform equiaxed den- drites.20
In the center region of Fig. 1(a), the whole mor-
phology of an equiaxed dendrite can be seen: it is com- posed of six secondary arms with almost equal length and the arms are separated by eutectic structures. The eutectic β phase (Mg17
All of the heated specimens were quickly water-quenched, finished and polished according to standard metallographic techniques. Specimens were then etched using an aqueous solution containing glycerol, nitric acid, hydrochloric acid
drites and in small particle form between the arms. The eu- tectic structures belong to the divorced eutectic (Fig. 1[b]). It can be expected that the residual liquid phase for eutectic solidification is very little for AZ91D alloy and the eutec- tic α phase preferentially grows on the existing primary
Al12 ) exists in skeleton form between den-
To verify the microstructural evolution during partial re- melting, the initial as-cast microstructure should be first clarified. The AZ91D alloy refined with 1% MgCO3
yields
and acetic acid and observed using an optical microscope (OM). The typical microstructure of the cast rod (16 mm diameter) refined using 1% MgCO3
was also observed using
a scanning electron microscope (SEM). To quantitatively examine the primary particle sizes and shape factors in the resulting semisolid microstructures, the obtained OM micro-
graphs (at 200X magnification) were analyzed. The area Ai and perimeter Pi
was /π)1/2 ]/N of each primary particle were obtained and
the average particle size D was calculated from the formula: D = [∑2(Ai
Where: N is the total particle number. The shape factor, F, was calculated from the formula:
F = (∑Pi 2 /4πAi )/N
If the particles are perfectly spherical, the shape factor has a value of 1; it increases for less spheroidal particles.21
Results and Discussion
(a) Figure 1. (a) OM and (b) SEM micrographs of the ZA91D alloy refined with 1.0% MgCO3 44 . International Journal of Metalcasting/Winter 2012
(b)
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