Fig. 6. The recordings of the thermocouples are indicated for the tube (gray), gate (orange) and riser (blue).
start of filling. Tis rate of filling was obtained by applying a rise in pres- sure of 10 mB per second inside the crucible enclosure. Figure 4 presents a map of the
metal front temperature anywhere in the casting. It indicates no risk of cold shuts (seams in the cast-
Fig. 7. Illustrated is the predicted solidification time inside the casting (in minutes).
ing) exists because the liquid metal front temperature never drops below 1,159 F (626 C). (Alloy A356 begins to solidify at 1,135 F [613 C].) Te molten aluminum is fed from
the furnace to the runners by thin gage 1.5-in. (38-mm) diameter steel tubes. Given the great propensity
of aluminum to dissolve iron, the composition of the A356 alloy after a run was measured in a runner and in the steel tube and then compared to that of the furnace melt. Te results, shown in Table 1, indicate only a small amount of iron (up to ~0.02%) was picked up when the melt remained fully liquid and still inside the steel tube for several min- utes. Because the transit time of the aluminum in the tube during filling is of the order of one second, the iron pick-up is negligible.
Thermal Analysis In Figure 5, the three green dots
indicate the locations of three ther- mocouples that were inserted into the mold cavity during molding. Te responses of the thermocouples
are shown in Figure 6. Te arrival time of the liquid metal and the start and finish of solidification are listed in Table 2.
Te measured solidification times Fig. 8. The inner surface of the casting is finely polished.
Table 1. Composition of the Alloy in the Furnace, Inside the Steel Tube and in the Runner Si
Fe
Furnace Fill tube Runner
7.0 6.9 6.8
0.14 0.16 0.14
Cu
0.082 0.075 0.097
Mn
0.017 0.016 0.021
Mg 0.39 0.38 0.38
Zn
0.019 0.016 0.018
Table 2. Time of First Liquid Metal Contact, Beginning and End of Solidification TC
Riser Gate Tube
Line Color Blue
Orange Gray
Contact at 85s
66.5s 64.0s
42 | MODERN CASTING February 2016
Solidification Starts 112s 129s 243s
Ti
0.14 0.15 0.14
Sr
0.013 0.014 0.013
are reasonably close to the predicted values shown in Figure 7. Te solidifi- cation progresses from the mid-height of the 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. Tis ensures a directionally solidified, shrink-free casting. Because the casting will be submit-
Solidification Ends 689s 564s
1150s
ted to service temperatures up to 482 F (250 C), any hardening via heat treatment would be lost after a few hours of operation. Consequently, the mold will be used in the as-cast condi- tion. Te inner surface was polished to a pit-free finish shown in Figure 8. To ensure the lowest porosity level,
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