In contrast, the X-ray images of samples taken from Loca- tions #2 and #4, showed almost no defects discernible in the gage length; as Figure 5 illustrates, only a minor amount of small-scale porosity was observable, with Location #4 hav- ing a slightly larger quantity relative to Location #2. Con- sequently, it was not possible to correlate the failure loca- tion with any feature that might have initiated failure. It was also noted that Locations #2 and #4 showed the highest and second-highest averages of elongation-to-failure when com- pared to the other locations.
It was more difficult to determine the effect of microstructural features on failure samples taken from Locations #3 and #9. This was a result of the ribbing that was present on the surface of the tensile samples. X-ray and visual examination of all of the Location #3 samples revealed that all of the samples failed along a rib. The samples from Location #3 were all very clean, however, and no defects were visible from the X-rays, so it is apparent that the ribs were creating stress risers along the gage length and the samples were failing there preferentially. This was somewhat expected because, in this location, the samples were machined with the ribs transverse to the tensile direction.
A somewhat different result was observed in the samples taken from Location #9. At this location, it was observed that 7 of 17 samples also failed at a rib; however, the remain- ing 10 samples failed in locations that did not intersect a rib. In these 10 samples, several failed at defects identified in the X-rays; these defects were either pores or knit lines. Only 5 samples in this set presented clean surfaces.
The results from this X-ray examination were also consistent with those reported in a previous study by Coultes et al.,7
who also examined locations in a com-
plex casting. They found that the individual variation in the elongation-to-failure data correlated moderate- ly with the location on the test casting. Coultes et al. found that the areas that were the last to fill, the last to solidify, or were prone to knit lines tended to have a lower-than-average elongation-to-failure compared to the other areas. They also found that areas that were close to the ingates tended to have a higher-than-aver- age elongation-to-failure.
Microstructure
In general, the microstructure of the AM50 alloy consist- ed of a matrix containing primary α-Mg dendritic cells surrounded by a divorced eutectic structure consisting of α-Mg and β-Mg17
α-Mg and porosity were present as well (Figure 6). Al12
An interesting feature of these samples was the lack of any discernible “skin” or transition region. This may be due to the 2.5 mm cross-sectional thickness of the tensile sample. As discussed previously in the section on the first study, skin and transition regions were only observed in plates having a thickness of 4 mm. Addi- tionally, Cao and Wessen26
only observed the presence
of a skin layer in samples having a thickness greater than 3 mm.
(Figure 6). Additionally, ESCs of
Figure 5. X-Ray images of the AM50 HPDC tensile samples. (Note: Sample ID = “Casting #” – “Location” (as shown in Figure 1); Red circles indicate locations of Flow/Knit Lines; Light-colored lines on samples 15-3 and 11-9 are ribbing on external side of tensile sample)
20 International Journal of Metalcasting/Winter 2012
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