samples produced at 82 m/s, although the relative differences in position parameter are much less such that the curves nearly overlap (Fig. 9[b]). One feature that is interesting to note is the differences in Weibull modulus for tensile strength between the two different melt velocities, for the various alloys. In Alloy 3, m is more than doubled when going from the lower melt veloc- ity to the higher one, whereas for Alloy 1, the increase is less.
Therefore although all alloys clearly benefit from the higher melt velocity at the gate, Alloy 3, which contained higher Zn levels, benefits most from this change. Additionally, despite the almost identical results of m for Alloys 1 and 2 at 26 m/s, the differences between these two when produced at 82 m/s are significant. Differences are also pronounced in comparisons of the Weibull distribution for elongation at the different melt ve-
Table 4. Data for Alloy 3 Samples Produced at 26m/s or 82 m/s
Figure 6. Fracture surface of a sample of Alloy 2 showing a combination of an oxide film plus a large foam-like shrinkage defect. Views (a) secondary electron mode, (b), backscattered electron (BSE) mode show the oxide and defect cluster, (c & d)) BSE at higher magnification. Note also in (d) there are significant quantities of Fe-bearing particles present on the fracture surface, the white phase (0.2% proof stress, 184 MPa, UTS 296 MPa, 1.7% Ef
). International Journal of Metalcasting/Summer 2011 45
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