cast, five samples had gas pores ranging in size from 375 µm to 1000 µm in diameter, and two samples had crack- like defects with lengths ranging from 2200 and 2500 µm. For LPPM, only one sample does not have sponge shrink- age defects. For T-Mag and ablation, more than half of the samples do not have apparent defects. According to the de- fect rating summary, 92% samples are L1 or less for the T- Mag and ablation processes. However, only 58% and 33% samples meet this criterion for squeeze cast and LPPM, re- spectively. Consequently, according to the defect analysis, the best two among the four casting processes are T-Mag and ablation.
The details of fracture surfaces of FPB tests for the speci- mens from each casting process are shown in Figure 9. Dif- ferent casting defects were found in the fracture surfaces. In the squeeze cast and LPPM samples (Figures 9a and 9b), the casting defects were mainly shrinkage porosity that is pres- ent as a type of ‘cluster.’ In the T-Mag and ablation casting processes (Figures 9c and 9d), no apparent defects but some small gas porosity can be found on the fracture surfaces, which further confirms the better quality castings produced via these two processes. Figures 10 and 11 shows typical oxide film defects from the fracture surfaces of the FPB sam- ples taken from the control arms produced by squeeze cast
and LPPM. They were observed to have a film-like struc- ture, where a folded and wrinkled appearance was found in the fracture surfaces. No such features were observed for the T-Mag and ablation samples in this study.
The UBS for the FPB tests, which were obtained from the measured load and sample dimensions, with the data ar- ranged in an ascending order for each casting process, is shown in Table 3. The Weibull plots obtained from these data are shown in Figure 12, and the Weibull parameters have been summarized in Table 4. Since none of the abla- tion samples were broken during the FPB tests, the ablation process is not included in the Weibull plots. The Weibull moduli for the UBS are 7.93, 11.54, and 17.29 for squeeze cast, LPPM, and T-Mag, respectively. Dispersed shrink- age porosity and oxide film defects were observed in the squeeze cast samples, which in turn results in more scatter in FPB test results than in LPPM and T-Mag samples. The mean values of the UBS are 295.80MPa, 334.93MPa, and 352.84MPa, for squeeze cast, LPPM, and T-Mag, respec- tively. For the ablation process, an average UBS level of 536.35MPa was reached before the sample bottomed out. These results indicate that the T-Mag and ablation cast- ings have better mechanical properties than squeeze cast and LPPM castings.
Table 2. Defect Analysis for Each Casting Process
42
International Journal of Metalcasting/Fall 2011
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