A Case for Low Pressure Sand Casting of Aluminum
Less common than gravity sand casting and low pressure permanent mold casting, low pressure sand molding holds a distinct combination of advantages for large aluminum castings.
Franco Chiesa, Centre de Métallurgie du Québec, Trois-Rivières, Québec, Canada, and Jocelyn Baril, Technology Magnesium & Aluminium, Trois-Rivières, Québec, Canada
(Click here to see the story as it appears in the January/February issue of Metal Casting Design & Purchasing.)
A majority of aluminum castings are produced via sand or permanent mold casting, but for large precision components, another viable option is low pressure sand casting, which uses principles from both low pressure permanent mold (LPPM) and gravity pour sand casting.
Low pressure sand casting marries the use of bottom pouring for tranquil filling of the mold (which avoids metal oxidation) with the flexibility to make larger parts. The capable process can be ideal when producing large “top quality” aluminum castings. The process also can be considered when walls are too thin (such as 0.1 in. [2.5 mm]) to be obtained by gravity casting.
LPPM casting is a common process producing high quality castings due to tranquil filling of the mold and the application of pressure to fill the mold efficiently and cleanly.
The two main characteristics of the LPPM process are:
1. The filling from the bottom of the mold is perfectly controlled compared to the turbulent flow associated with gravity casting. Also, the liquid metal is drawn from under the melt surface, preventing dross entrainment into the mold cavity.
2. Efficient feeding from the bottom injection pipe occurs through pressure applied to the melt during solidification, eliminating the need for risers. The resulting yield is high: typically 80-90% versus 50-60% for gravity permanent mold casting. However, not all casting geometries are amenable to the LPPM process.
In low pressure sand casting, the sand mold rests on top of a pressurized enclosure as shown on Figures 1 and 2. The similarity between LPPM and low pressure sand casting is in the controlled tranquil filling of the mold with a dross-free melt. Both processes also share the ability to produce thinner walls than gravity pouring would.
However, in contrast with the LPPM process, in low pressure sand casting no excess pressure is applied at the end of filling. Feeding from the bottom is interrupted early and long before the casting is fully solidified, so risers are necessary, just as in gravity casting.
Low pressure sand casting eliminates liquid metal handling, so the process is also advantageous over gravity sand casting when pouring large parts.
Size, quality and wall thickness will be the primary considerations when deciding between LPSM and gravity sand casting.
Compared to gravity sand casting, the low pressure sand molding process simplifies the filling of the mold. A single operator can repeatedly fill the mold for a 600-lb. casting at the push of a button, compared to the manpower necessary to fill the mold by gravity through multiple sprues. The filling metal is also cleaner.
Solidification times are typically five times longer in sand casting than in permanent mold. This is why the low pressure sand molding is no match to LPPM when castings are small enough to be produced on a LPPM press. Since the majority of aluminum castings are relatively small, the LPPM process is much more widely used than low pressure sand casting. But when the dimensions are too large for LPPM, low pressure sand casting is a viable option. Dozens of casting facilities are capable of this process and often cater to the aerospace industry.
A good candidate is illustrated through the following case study of a cast A356 aluminum mold used to make plastic parts for the food container industry.
The overall dimensions of the casting are 32 x 18 x 66 in. (800 x 460 x 1700 mm). Its inner surface will be polished to a 60 grit finish, so the as-cast surface roughness must be less than 250 RMS. For the same reason, subsurface porosities greater than 150 µm are not acceptable.
In Figure 3, the quiescent filling is illustrated by showing the melt temperature at three seconds, 10 seconds, 20 seconds and 40 seconds after the start of filling. This rate of filling was obtained by applying a rise in pressure 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 casting) 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].)
The 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. The results, shown in Table 1, indicate that 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 minutes. 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.
In Figure 5, the three green dots indicate the locations of three thermocouples which were inserted inside the mold cavity during molding.
The responses of the thermocouples are shown in Figure 6. The arrival time of the liquid metal and the start and finish of solidification are listed in Table 2.
The measured solidification times are reasonably close to the predicted values shown in Figure 7. It is predicted the solidification 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. This ensures a directionally solidified shrink-free casting.
Because the casting will be submitted 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 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. The porosity level was related to the local solidification time and temperature gradient. Because these thermal parameters were readily available from solidification modeling, it was possible 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 porosity in the gate was 0.8%, in reasonable 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 metallographic 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 surface of molten aluminum for higher metallurgical quality, thin wall capability, easy metal handling, and cost efficiency. ■