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Porosity characterization


The average volume porosity versus section thickness is presented in Figure 7(a). It is evident from the diagram that Melt 1 has the lowest volume porosity but there is a ten- dency for higher porosity in the thicker section. Melts 2 and 3 show a different behaviour with a peak volume porosity in the 15 mm (1.5 cm) section which is more pronounced in Melt 3. However, the volume porosity decreases slightly in the direction of thicker sections. This parameter was derived from Archimedes’ principle whereas the following param- eters were extracted from the image analysis.


The pore number density and equivalent pore diameter are illustrated in Figure 7 (b) and (c). Both melts with high scrap ratios show considerably higher pore number densities. Melt 1 has fewer pores in all sections and shows a clear trend to less but larger pores in the thicker sections. In Melts 2 and 3 the number of pores decreases with increasing section thick- ness. They possess a second maximum value in the 20 mm


(2 cm) thick section before their number density decreases to a low level comparable to the less contaminated Melt 1. In contrary, the pore diameter increases steadily but the pore size can be regarded as small as compared to Melt 1. Shape factors (SFs) of the pores in the three melts are all in the same range. Figure 7(d) shows the results. The mean SF is in the range of 0.3 up to 0.4 which refers to elliptical, elongated or irregular, interdendritic shaped pores.


The typical pore morphologies are illustrated by the micro- graphs in Figure 8. Although, a variety of shapes were found, irregular interdendritic and round partially elongated pores were predominant. It is obvious that the castings from Melt 1, (Figs. 8a–c) possess larger and rounder pores in comparison with the other melts. The pore size distribution of each melt is shown in Figure 9. The first graph illustrates the trend in Melt 1, which clearly tends towards bigger pores at the expense of small pores with rising section thickness. Melts 2 and 3 show an opposite trend, up to the 20 mm (2 cm) section, primarily the fraction of small pores increases significantly.


(a)


(b)


(c)


(d)


Figure 7. Porosity characterization (a) using the Archimedes’ principle (volume porosity); (b-d) using the image analysis technique (equivalent diameter, number density and shape factor) vs. section thickness.


International Journal of Metalcasting/Spring 2012 45


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