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the porosity in high-pressure die-castings is a result of air being entrapped by the injected molten metal. There have been a large number of entrapped air-reducing corrective techniques and process modifications that have been devel- oped over time; in general, no one technique/process has eliminated porosity in HPDC components. However, if the majority of the porosity in a Mg die-casting is shrinkage porosity, then the root cause of porosity is related to the solidification behavior of the alloy and not to the amount of air present. This may explain the limited success that the entrapped air-reducing techniques and processes have had on reducing porosity in die castings.


Porosity Quantification


Using image analysis techniques, the porosity in the tensile samples was quantified. The average pore diameter (for all examined samples) was found to be 12.8 mm +/- 1.6 µm. In general, the porosity in these samples had a bimodal-type distribution. Each sample was similar, having a small num- ber of large pores (averaging 100 µm) and a large number of very small pores (averaging 5 µm). Interestingly, the aspect ratio of the pores was a fairly constant 2.1 for all the samples.


The distribution of porosity was also quantified. The results of this quantification found no correlation between the distri- bution of porosity and the location of the porosity within the cross-section of a sample (Figure 11); similar results were found by Gokhale et al.28,29


While the area percent of poros-


ity within a sample followed a general Gaussian distribution for some of the examined samples, there were others that followed no distribution. Additionally, examining the distri- bution of porosity for similar locations in different samples produced results that did not follow any pattern.


Thus, the measured porosity data did not indicate a trend that could explain the observed elongation-to-failure dif-


ferences (such as in Locations # 2 and 6 having similar porosity levels but different elongation-to-failure values). Therefore, it appears that the distribution of porosity within the “Ladder” castings follows only a random pattern.


Bulk Porosity Measurements


Figure 12 details the results of the bulk porosity quanti- fication; this figure plots the elongation-to-failure results against the bulk porosity for each of the 51 sections that were examined. This figure shows that high levels of bulk porosity can impact the elongation-to-failure in some of these samples. For those samples with a bulk porosity greater than 1.5% and that did not fracture at a macroscopic feature (knit/flow line, rib, etc.), the general trend was that as porosity increased, total ductility decreased.


However, no general trend was observed in samples where the bulk porosity was less than 1.5%. In some cases, as po- rosity increased, total ductility decreased, while in others the ductility increased as the porosity increased; this was particularly true for the samples from Location #9 as well as some of the samples from Location #6. These results seem to suggest a threshold level for bulk porosity. Above a certain level, the presence of the porosity dominates the mechanical properties; below that level, the mechanical properties become increasingly insensitive to the presence of the pores. Similar results were observed by Bowles et al.,22


who reported that below a certain level of porosity


(<1.5%), there was little correlation between bulk porosity and failure strain.


fracture surface features


The fracture surfaces from three tensile samples were ex- amined using the SEM; the purpose was to look for fea- tures on the fracture surfaces that could point to a failure


Figure 11. Area percent porosity as a function of sample location and distance through the tensile specimen cross-section for the AM50 HPDC casting. (Location as shown in Figure 1)


International Journal of Metalcasting/Winter 2012


Figure 12. A comparison of the elongation-to-failure and the bulk porosity for selected AM50 HPDC samples. (Locations as shown in Figure 1)


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