Fig. 9. Solidification modeling plots the predicted porosity distribution in the casting (% volume).
Fig. 10. Micrographs of aluminum A356 in the gate (left) and inside the feeding tube is shown.
casting, down to the feeding gates maintained under pressure until their complete solidification (10 minutes), and from the mid-height up toward the top risers in the other direction. This ensures a direction- ally solidified shrink-free casting. Because the casting will be sub- mitted to service temperatures up to 482 F (250 C), any hardening via heat treatment would be lost after a few hours of operation. Conse- quently, the mold will be used in the as-cast condition. The inner surface was polished to a pit-free finish shown in Figure 8. To ensure the lowest porosity level, the melt was degassed to a Reduce Pressure Test sample density of 2.63. Te porosity level was related to the local solidification time and tempera- ture gradient. Because these thermal parameters were readily available from solidification modeling, it was pos- sible to predict the distribution of the porosity (Fig. 9). Samples were cut out at two
locations where thermocouples had been inserted, i.e. in the feeder tube and in a gate. The measured poros- ity in the gate was 0.8%, in reason- able agreement with the predicted results. The actual porosity level of the aluminum solidified inside the steel tube was 0.4%, much lower than predicted. The long solidification time (19.2 minutes) of the quiescent liquid melt inside the steel tube is believed to have allowed natural degassing to take place. Figure 10 shows the metal- lographic aspect of both samples at low magnification. Due to the longer solidification time inside the tube, the secondary dendrite arm spacing was larger than in the gate (90µm vs 71µm). While low pressure sand casting is not as common as gravity sand casting or LPPM, it holds a distinct combination of advantages when pouring large castings, including tranquil filling, obtaining metal from underneath the oxidized sur- face of molten aluminum for higher metallurgical quality, thin wall capability, easy metal handling, and cost efficiency. ■
40 | METAL CASTING DESIGN & PURCHASING | Jan/Feb 2016
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