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Procedure Researchers investigated

three modes of supplying helium to the mold (Fig. 1-2):

• Cross flow in a partially encapsulated mold.

• Cross flow in an unencapsulated mold.

• Parallel flow in an unencapsulated mold. The mold design for cross flow in

a partially encapsulated mold con- sisted of a base plate through which helium is supplied to the drag, an encapsulation case that provided partial encapsulation, and a top sealing assembly to seal the periph- eral gap between the mold and the encapsulating case. The helium was supplied at the base of the mold and forced to pass through the mold in an upward direction under the pres- sure of the incoming helium supply. In the cross-flow method for an

unencapsulated mold, only the base of the device providing the cross flow of helium was required. Helium was supplied in the same way as the encap- sulated mold, but more system losses were expected. For parallel flow in an unencapsu- lated mold, two supply plates—one for the cope and one for the drag—were designed for supplying helium to the mold. Te plates were attached opposite the pouring basin and affixed to the mold so the point of helium supply was positioned at the middle of a groove cut in the mold to ensure par- allel flow. Te relatively shorter travel distance along the casting, the position of the groove (cut close to the casting surface), the pressure of the incoming gas, and peripheral sealing of the sup- ply plate ensured the helium traveled along the surfaces rather than leaking or moving to the sides (Fig. 3). Te effect of the helium flow rate,

flow direction and mold design on average as-cast grain size, SDAS and room temperature tensile properties were investigated and compared to castings produced in the typical sand casting process. Te cross-flow modes were inves- tigated with a 4-L/minute helium flow rate, while the parallel-flow mode

was investigated at 1-L/minute, 4-L/ minute and 8-L/minute flow rates. Helium was introduced to the mold after it was completely filled with molten aluminum alloy 319. For each casting, 40 lbs. of alloy

were melted in an induction furnace. Te average pouring temperature was 1,562F (850C). Five thermocouples were inserted into the mold cav- ity—four near the risers and one at the geometric center—to monitor the change in temperature of the plate during casting solidification (Fig. 4). A contact profilometer characterized the surface roughness of the top surface of the cast plate. In each case, 20 mea-

surements were taken and recorded. Saleem and Makhlouf examined eight specimens from each casting experiment to determine the room temperature tensile properties. Tey also performed microstructure characterization on the samples in their as-cast condition. Te researchers quantified the amount of porosity in each casting in terms of percent area of pores observed at a magnification of 50 times. Grain size was measured by the Hilliard circu- lar intercept method, and SDAS was quantified by the linear intercept method from micrographs of etched samples taken at a magnification of 100 times.

Fig. 1. This schematic represents the general apparatus for applying helium to the sand mold.

Fig. 2. Shown is the schematic for the cross-flow (left) and parallel-flow modes for helium- assisted sand casting.

Table 1. Increase in Yield Strength Relative to the Baseline and the Corresponding Performance Index for the Flow Rates

Helium flow rate (parallel to each face) % Increase in yield strength* Performance Index 1 4

8 *With regard to the baseline casting December 2012 MODERN CASTING | 41

0.085 0.344 0.264

0.085 0.086 0.033

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