Plus software). Higher magnification imaging (Fig. 1[c & d]) showed that this is a region where dispersed shrinkage po- rosity was present, because there was evidence of an almost foam-like, random dendritic structure. There was also a small oxide film present in the exact centre of Fig. 1(d) which helps highlight the wide range of features that may be present in a complex defect cluster, formed during diecasting.
Another important result was that the tensile properties of the actual specimen photographed in Fig. 1 were very close to the statistical mean (Table 2). An implication of this ob- servation is that either a) the properties of specimens are not particularly affected by the presence of such large defects, or b) all of the 25 specimens tested were of equivalently (low) quality. Since images of the fracture surface of an average specimen produced at a higher melt velocity of 82 m/s (Fig. 2) show less evidence of large defects, and the specimen has higher tensile properties, it may be assumed that explanation a) is likely to be incorrect. As a result of the microstructural differences between Figs. 1 and 2, it is a reasonable assump- tion that strain localization during plastic strain is far more severe in the sample produced at 26 m/s and that this differ- ence will arise primarily from the defect distribution which is observed on the fracture surface.
The Weibull distribution was used to further analyze the data presented in Table 2. In Fig. 3, the Weibull distribu- tions for tensile strength (UTS) and elongation at failure (Ef
)
are shown and the differences between specimens produced at the two melt velocities are significant. For example, Fig. 3(b) shows that at a stress level of 320 MPa, the probability of failure in the 26 m/s samples is 95%, whereas for those produced at 82 m/s, the probability of failure is only 1.1%. Conversely, 1.1% probability of failure in the 26 m/s sam- ples occurs at close to 260 MPa, or a difference of 60 MPa. As shown by Fig. 3(a), the Weibull modulus was 27.3 for the samples produced at 26 m/s, and 42.3 for samples pro- duced at 82 m/s. This increased Weibull modulus for the higher velocity material indicates that since the spread of data for the tensile strength is reduced, the flaw size distribu- tion must also be reduced. (It is also important to note that such a lower value of Weibull modulus as seen in the 26 m/s samples will produce a more pronounced size and bend ef- fect, as described by Eqns 8-11). As may be appreciated, the raw data of tensile results may also be used to examine the spread of elongation at failure, Ef to examine the strain term Ef
. Although it is not as usual using the Weibull distribution,
the elongation to fracture is a more sensitive indicator of casting quality2
so both tensile strength and elongation are
Figure 1. Fracture surface of an average sample produced at 26 m/s at different magnifications. International Journal of Metalcasting/Summer 2011 41
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