Fig. 4. Ultimate tensile strength and elongation to failure showed significant increases when solidified under pressure at a pouring temperature of 1,300F.


pressure was not reached before solidification began. Decreased pouring temperature and increased pressure


during solidification also resulted in improved ultimate tensile strength (UTS) and elongation (Fig. 4 and 5).Based on porosity reduction alone, it would be expected the castings poured at 1,400F would show a greater improvement in properties, as the change in the amount of porosity was greater when solidified under pressure, but this was not the case. Increased elongation typically correlates with increased ultimate tensile strength. In previously published research, A356 castings poured at 1,400F and solidified under pressure showed increased elongation but no significant UTS effects. While the recent study on wedge nobake castings showed this same trend at a 1,400F pouring temperature, the expected improvement in UTS as elongation increased occurred at 1,300F.


Exploring Section Thickness


As expected, bulk density increased as pouring temperature decreased and with increasing solidification pressure. For castings poured at 1,300F, little difference was recorded in bulk density; the average density was 2.658+/-0.008 gm/cm3 when solidified under 1 atm and 2.67+/-0.004 gm/cm3 when solidified under 10 atm. For castings poured at 1,400F, the difference in bulk density was larger but still less than 1%. Te castings’ mechanical properties showed only a mild


correlation with density, which indicates that additional vari- ables, such as casting defects and dendrite arm spacing, were


March 2011 MODERN CASTING | 31


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