Because the mechanical property data indicated that there were no discernible differences between different processing conditions for a given sampling location, the data for each processing condition was pooled. Figures 2-4 plot the pooled property data as a function of sample location. It should be noted that only samples that failed in the gage length were used in these plots; thus, each location does not necessarily represent the same number of tests.
The results detailed in Figure 2 show that there was a sta- tistical difference in the elongation-to-failure based upon the location in the casting. Additionally, the results for each given location show a great deal of variation as well. These results are consistent with the previous results from much of the published literature reviewed earlier in this report. When the yield strength results (Figure 3) were examined, it was observed that there was significantly less variation in the data based upon the location of the samples. Additionally, within a given location, the ma- jority of the yield strength results were within +10% of the average value. This amount of variation is also con- sistent with the majority of the published studies on cast AM50/60 magnesium alloys.
Figure 4 presents the ultimate tensile strengths obtained from the HPDC casting. As this figure shows, the variation in the ultimate tensile strength data based upon location is intermediate between that of the yield strength and elonga- tion. Further, the amount of variation within a location is also intermediate between that for the yield strength and elongation. These results are consistent with published re- sults; the results also make sense, since the ultimate tensile strength is affected by those factors that affect the yield strength as well as those that affect the elongation.
X-ray examination
This examination began with a comparison of the location of the tensile fracture surface with the features observed in the X-ray images taken prior to testing. This comparison found that some of the observed mechanical behavior could be ex- plained by the structure in each sample.
Figure 2. A Box and Whisker Plot of the elongation-to- failure as a function of sample location in the AM50 HPDC castings.
As Figure 5 details, Location #1 showed a strong tendency to fail at locations on or close to knit lines or flow lines in the sample gage length. Many of the samples had such large knit line defects that the samples had little or no ductility at all and the samples failed to even reach their yield points. This is the primary reason for the low ductility values obtained; it is also a reason for the large overall variability in the strain to failure observed in Location #1.
Overall, only five of the twenty tested Location #1 samples had “clean” gage lengths without knit lines, flow fronts, or large- scale voids. These “clean” samples were evenly distributed through the DOE and did not appear to be influenced by the processing parameter changes in this study. “Clean” samples exhibited failure strains on the average of 3.4 +/- 1% versus failure strains of less than 1 % in the samples with knit lines.
Similarly, the X-ray images for Location #6 showed that the majority of the samples had flow/knit lines that correlated well with the failure location (Figure 5.) Similar to Location #1, Lo- cation #6 had only five samples that showed show “clean” gage lengths and, consequently, higher elongation to failure values (2.5–4 % range). Again, there was no correlation with the DOE.
Figure 3. A Box and Whisker Plot of the 0.2% yield strength as a function of sample location in the AM50 HPDC Castings.
International Journal of Metalcasting/Winter 2012
Figure 4. A Box and Whisker Plot of the ultimate tensile strength as a function of sample location in the AM50 HPDC castings.
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