Indirect Fabrication of Lattice Metals with Thin Sections Using Centrifugal Casting

The overall goal of this video is to demonstrate the process of filling a cellular microcavity with molten metal for the fabrication of lattice metals by centrifugal casting. This message can help answer key questions such as, how the process parameters, material properties of molten metal, and geometry of lattice mold cavity are correlated with the flow and solidification in rapid microcasting. This technique supplements direct 3D printing massive of metals, in manufacturing lattice metals, and expands the material selection in the design space of lattice metals.

Today, I will be demonstrating the procedure with doctoral student Jiwon Mun. In preparation, draw a sacrificial pattern of an octet truss structure with a sprue system using computer-aided design software. This integrated pattern of an octet truss structure with a sprue system gets melted during casting.

Using the CAD software, record the volume of the sacrificial pattern to calculate the required metal mass. After printing the acrylic model, proceed with the casting process, which is detailed in the text protocol. The next step is the melting out of the support material.

Following this, the residue of support material is further removed. Then, the pattern is assembled. Following its assembly, an investment mixing process is performed.

Be sure to remove all lumps in the investment water mixture. Next, is the burnout process. To begin, set the time on the furnace according to the heating and cooling schedule.

For a flask of this size, the burnout will take six hours. Now, turn on the furnace and start the burnout schedule. During the burnout, the sacrificial pattern melts and dissociates into a gas.

Now, continue with the casting process described in the next two sections. Before beginning, ensure that the centrifugal casting machine's arm spins with an angular velocity of 425 RPM. Find the setting needed for this speed using a tachometer.

Select a ceramic crucible to melt each alloy. Using a metal cutter, chop the alloys into 10 millimeter to 20 millimeter pieces. Chop up around 150 grams of each alloy, which in this case are aluminum and copper.

Now, weigh out the precise mass of each alloy needed for the lattice design. The pattern cavity must be filled precisely to capacity with molten alloy. Now, get a bucket of water at room temperature ready.

30 liters is sufficient. Next, put on flame retardant clothes and put on gloves and goggles. Now, take the plaster mold temporarily out from the furnace.

Place it into the flask cradle of the centrifugal casting machine, and balance the arm of the centrifuge. Then, return the plaster mold back into the furnace, and start heating the machine to 482 degrees Celsius. Meanwhile, place the crucible in the crucible holder, and load the chopped alloy into the crucible.

Next, set up the acetylene torch. Open the valve of the connected oxygen tank, and maintain the gas pressure near 96.5 kilopascals or 14 PSI. Then, carefully ignite the torch and control the intensity of the flame with the gas pressure.

Be very careful. The flame gets up to 1, 200 degrees Celsius. Now, carefully melt the chopped alloy in the crucible until it is completely liquified.

Use a carbon rod to stir the alloy and melt it with the neutral part of the torch's flame. The proper melting of the metal is critical, because if the metal is too cool, it will not fill the entire form. But, if it's too hot, the increasing porosity will cause structural issues.

Now, take the preheated plaster mold from the furnace, and load it into the flask cradle holding arm next to the crucible with the molten alloy. Close the cover of the machine and let the arm spin for at least three minutes. The machine starts spinning automatically when the cover is closed.

About 420 RPM's creates an inlet velocity at the mold cavity of 8.03 meters per second, as calculated by the macroscopic particle dynamics and the angular velocity. After three minutes, cut the power to the centrifugal casting machine. Then, open the cover, and using tongs, remove the plaster mold from the flask cradle.

Let the mold cool at room temperature for 15 to 20 minutes, until the color of the alloy returns to that of its solid phase. Then, using tongs, quench the plaster mold in the water bucket for about five minutes. When the mold is near room temperature, it is completely quenched.

Now, to obtain the lattice metal inside the mold, just dissolve the mold in water. The Jibsen mold will easily dissolve within two minutes. Post-processing is now possible, such as cleaning by sandblasting, which is described in the text protocol.

Be sure to use the described sandblasting pressure as the molds are somewhat fragile, especially the aluminum octet truss which has thin walls. Using the indirect additive manufacturing described in the protocol section, aluminum and copper alloys were used to manufacture octet truss metals. The burnout process was carried out for six hours to melt out the sacrificial pattern.

After the centrifugal casting process, the final products were octet truss metals made with aluminum and copper alloys. The molten aluminum alloy fully filled the entire lattice mold cavity without a misrun. On the other hand, the molten copper alloy appeared to have a casting defect caused by premature solidification at the early stage of injection of molten metal at the inlet.

After watching this video, you should have a good understanding of the effect of process parameters on filling of molten metal into cellular microcavity, doing indirect fabrication of lattice metals using centrifugal casting. After its development, this technique paved the way for researchers in the field of lattice metals to explore indirect fabrication, by combining 3D printing of polymers with centrifugal casting.