externally solidified cells (escs)
Selected samples containing ESCs were placed in the SEM for EDS analysis. The results of this analysis determined that the ESCs are essentially Mg and contained little or no Al.
Using the metallographic samples, the ESCs (Figure 6[c]) were examined and quantified using image analysis. The data from this analysis showed that a wide distribution in size of the ESCs was present. Although the average ESC di- ameter was 27 µm, there were a small but significant number of much larger (~60 µm) ESCs present as well.
However, it should be noted that the ESC measurements did not correlate with location. The ESC data from a specific lo- cation in a casting was different in absolute value compared to data obtained from the same location in a different cast- ing. For example, the ESC data from Location #3 in Castings #1 and #7 were statistically different.
The distribution of the ESCs was also quantified. The re- sults from this analysis found the general trend that the ESCs follow a generally normal-shaped distribution. As Figure 8 illustrates, the concentration of ESCs increased as the dis- tance towards the center of the casting decreased. Addition- ally, the majority of the larger ESCs were grouped toward the middle of the casting as well.
Porosity type
Because porosity is a key microstructural feature in many cast materials, the porosity was examined in detail using the SEM.
In all of the samples, both shrinkage porosity (Figure 9) and entrapped air porosity (Figure 10) were observed. As these two figures detail, the two types of porosity can be distin- guished by their appearance when examined at high magni- fication using an SEM. Low magnification optical observa-
Figure 8. The distribution of ESCs as a function of sample location and distance through the tensile specimen coss-section for the AM50 HPDC casting. (Location as shown in Figure 1)
tions often cannot distinguish between the two types, espe- cially when the porosity is small in size. A typical shrinkage pore (like the two in Figure 9) is irregularly-shaped, often taking the shape of the dendrites surrounding it. Addition- ally, the interior of the pore has a distinctive dendritic struc- ture. In contrast, a pore created by the entrapment of air dur- ing the diecasting process is more hemispherical in shape and tends to have a smoother interior with very few, small- scale features present. Further, energy-dispersive x-ray anal- ysis (EDS) has found that shrinkage pores tend to have very small levels of oxides present on the interior surfaces of the pore. This is in contrast to pores arising from entrapped air, where a significant amount of oxide is present on the interior surfaces.
It was notable that most of the porosity observed was shrinkage porosity (Figure 9); only a small number of en- trapped air pores (Figure 10) were present. This observa- tion was somewhat surprising; conventional thought is that
Figure 9. Typical shrinkage pore. 22
Figure 10. Typical entrapped air pore. International Journal of Metalcasting/Winter 2012
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