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tensile strength increased from 218 to 267 MPa, without a significant decrease in elongation (Fig. 6). Te cooling rate during dendrite formation increased by about 100% at locations near the helium supply. Te average SDAS in the helium-assisted castings at a flow rate of 4 L/minute was 46 µm com- pared to 60 µm in the baseline castings (Fig. 7). Surface finish was not affected. According to Saleem and Makhlouf,


Fig. 5. Porosity present in the plate castings was greatest in the helium-assisted cross-flow casting in the partially encapsulated mold, but the helium-assisted parallel-flow castings exhibited less porosity than the baseline casting.


the test results showed that beyond a certain helium flow rate, the benefits began to show diminishing returns, so they defined a “performance index” as a ratio of the increase in yield strength to the cost penalty of using helium (Table 1). Tey found that a 2.6 L/minute flow rate (parallel to each face) was the opti- mum helium flow rate that maximizes the performance index. During the testing and analysis,


Fig. 6. Helium-assisted sand castings produced with parallel flow exhibited increased tensile properties over the baseline castings and the helium-assisted cross-flow modes.


the researchers recorded a thermal gradient in the parts cast via helium assisted sand casting with parallel flow, which resulted in a small gradi- ent in SDAS and tensile proper- ties. Te gradient is insignificant due to the small length of the plate but might be more significant in larger parts. Saleem and Makhlouf recommend introducing helium at more than one location, which can be determined with casting process modeling, to mitigate the gradient. Or, helium could be used for local- ized cooling or to induce directional solidification, replacing chills to simplify the mold design. According to the authors of


“Helium-Assisted Sand Casting,” a cost analysis not discussed in the paper showed that helium-assisted sand cast- ing in the parallel-flow mode compares favorably to traditional sand casting up to a flow rate of 4 L/minute.


Tis article is based on the paper “Helium-Assisted Sand Casting,” which first appeared in the International Journal of Metalcasting.


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Fig. 7. Secondary dendrite arm spacing decreased with helium-assisted sand casting via the parallel-flow mode.


Visit www.moderncasting.com to read the paper on which this article was based.


December 2012 MODERN CASTING | 43


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