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inserted into the mold cavity—four near the risers and one at the geomet- ric center—to monitor the change in temperature of the plate during cast- ing solidifi cation (Fig. 4). A contact profi lometer characterized the surface roughness of the top surface of the cast plate. In each case, 20 measurements were taken and recorded. Saleem and Makhlouf examined eight specimens from each casting experiment to determine the room temperature tensile properties. T ey also performed microstructure characterization on the samples in their as-cast condition. The 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 circular intercept method, and SDAS was quantified from micrographs of etched samples taken at a magnification of 100


“ 3


The castings produced via the parallel-fl ow mode showed fewer pores than those in the cross-fl ow helium-assisted castings and baseline castings.”


Results and Conclusions


Micrographs from the helium-assisted castings made using the cross-


fl ow mode showed more pores than the baseline sand castings, indicating an increased average area of poros- ity as well as an increase in the size of the average pore (Fig. 5). Saleem and Makhlouf deduced that helium fl owing in the cross-fl ow mode passes through


the molten alloy, causing more pores. T e castings produced via the


parallel-fl ow mode, however, showed fewer pores than those in the cross-fl ow helium-assisted castings as well as the baseline castings. At a 4-L/minute helium fl ow rate


parallel to each face of the plate, the yield strength increased over the base- line casting from 179 to 241 MPa and the ultimate tensile strength increased from 218 to 267 MPa, without a sig-


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


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


28 | METAL CASTING DESIGN & PURCHASING | Jan/Feb 2013


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