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heat chemistry. The powder was poured into the steel stream as it entered the downsprue of the mold. A target pouring temperature of 1620ºC was used for pouring the molds.


O3


The test casting consisted of a plate casting, 2.51 cm thick, 12.7 cm wide, and 25 cm long, with a large riser (Figure 1). Green sand molds were made from AFS GFN 65 silica sand with a 1.7% calcium bentonite, 1.8% sodium bentonite clay content and 2.4% water content. A jolt/squeeze machine pre- pared the molds. After pouring, the molds cooled for one hour before shakeout. This resulted in a casting temperature below 535ºC at shakeout. Then, the casting continued to air cool to room tempera- ture after shakeout.


Each plate was sectioned into 2.25 cm square, 12.7 cm long samples. One sam- ple was metallographically examined and used for op- tical emission spectroscopy to verify the plate’s chemi- cal analysis. The remaining four bars were turned into 1.27 cm diameter tensile bars according to ASTM standard E8-04. Machin- ing and tensile testing were completed by a local found- ry with a servo-hydraulic mechanical testing frame with an extensometer.


The metallographic sam- ples were prepared using standard metallographic techniques. A semi-auto- matic polisher ground and polished the samples. Pol- ishing was accomplished with 6 µm and 1 µm poly- crystalline diamond and a final polish of 0.05 µm


54


added. The melt was tapped into a ladle at 1720ºC. The final aluminum deoxidation and RE additions were added. Rare earth additions were accomplished by either the addition of misch metal or RE silicide. The additions were added to the melt by placing them into the tapping stream when the ladle was approximately one-third full. Table 1 lists the chemical analysis of these two master alloys. For each RE addition type, two different target RE contents were used, 0.1% and 0.2%. Table 2 lists the chemistries for each heat. The heats listed with MM in the name had a misch metal addition; the heats with RS in the name had a RE silicide addition. To assist with heterogeneous nucleation -325 mesh chemically pure La2


powder was added to one mold poured from each


alumina. A 3% nitol etch was employed. Electron micros- copy was conducted on the metallographic and tensile bar fracture surfaces with an energy dispersive spectrometer (EDS) equipped scanning electron microscope (SEM). An accelerating voltage of 15 kV was used.


Results and Discussion Gating System


Computer simulations of the gating system were carried out prior to casting production. The simulations exam- ined the fluid flow and solidification of the test casting. No macroshrinkage or microshrinkage porosity was predicted within the region where tensile bars would be removed. The relatively large riser size assisted in creating directional solidification within the test section of the part. Fluid flow simulation results predicted low fluid flow velocities within the casting and the in-gate. A velocity profile line had been inserted into the simulation geometry to determine the ve- locity profile across the in-gate where it entered the casting at each simulation time step. Velocity peaked at 0.7943 sec- onds into filling and decreased after that. The highest veloc- ity observed in the simulation results was 0.518 m/s (Figure


Figure 1. Illustration of test casting used.


Figure 2. In-gate velocity predictions at 0.7943 seconds. International Journal of Metalcasting/Spring 2012


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