Casting Protocols for the Production of Open Cell Aluminum Foams by the Replication Technique and the Effect on Porosity

The overall goal of this procedure is to produce a metal foam by the infiltration method. This is accomplished by first loading a mold with fine sodium chloride at the bottom, infiltration, sodium chloride in the middle, and an aluminum bar on top. Then placing the lid on the chamber.

The second step is to attach the mold to a valve system and place it in a preheated furnace to melt the aluminum. Next argon gas pressure is applied to the molten aluminum, forcing it into the free spaces between the infiltration sodium chloride grains. The final steps are to remove the mold from the furnace and retrieve the foam sample.

Then cut the sample to size and remove the sodium chloride in a water bath. Ultimately, the weight and volume of the sample will indicate its density and inform modifications in the protocol to obtain foams of different characteristics. Generally, individuals new to this method will struggle because slight variations in the procedure have great effect on the outcome.

As a first step, set the furnace to increase 20 degrees Celsius per minute to a final temperature of 740 degrees Celsius as it is reaching its final temperature, begin preparation of the preform, the preform will be placed in a mold. The mold in this experiment is cylindrical 150 millimeters tall and has an inner diameter of 51 millimeters. Prior to this experiment, a commercial purity aluminum ingot has been cast for use in the mold.

The aluminum is formed into a cylindrical bar with a 50 millimeter diameter. The bar is cut into four equal parts sufficient for four experiments. Next, begin collecting the sodium chloride to be used for infiltration.

Select a sieve with openings about the size of the smallest acceptable diameter, and another with openings about the size of the largest pore diameter. Place a container to catch filtered sodium chloride on the bench. On top of that, place the sieve with the smallest opening.

Place the sieve with the larger openings above it at this point, ready, either high purity or supermarket sodium chloride. Put approximately 500 grams of sodium chloride into a container and pour it into the stacked sieves. Then agitate the sieves for a minute After the agitation, discard the contents of the top sieve.

Also discard the sodium chloride from the bottom container. The sodium chloride in the middle sieve will be used for infiltration after it is weighed. The experiment in this video uses both an infiltration, sodium chloride and a fine sodium chloride.

Use the sieve technique once to collect 150 grams of infiltration, sodium chloride, and a second time to collect 100 grams of fine sodium chloride. After collecting the sodium chloride, proceed by working with the infiltration mold. It should be clean and free of any impurities from previous use.

Use a boron nitride aerosol spray to create a thin coating inside the cylinder. After the cylinder has dried, use fine sandpaper to remove boron nitride residue from the ends of the cylinder with the mold prepared. Work with the mold base.

Place a graphite gasket on the groove of the base. Then place the bottom of the mold cylinder into the groove on top of the gasket with a mallet, tap the cylinder top to secure it in the base before continuing. Now get the 100 grams of vine sodium chloride and pour it into the mold cylinder.

Place an uncut aluminum bar on the top and tap it lightly with a mallet to increase the packing density and make the surface of the sodium chloride flat. Remove the uncut bar and proceed by pouring the sodium chloride to be infiltrated into the mold. Hold the top of the cylinder in place and pick up the base to shake the assembly slightly until the sodium chloride forms a flat surface at the top.

Next, place the prepared aluminum bar on top of the sodium chloride preform. Place the mold lid with gasket on top of the mold cylinder and secure it. Transport the mold to where it will be attached to a gas supply and vacuum pump.

Lace the mold under the tubing for the valve system. Then properly attach the tubing to the top of the lid. The valves immediately above the mold control isolation of the mold argon gas flow, and access to the vacuum pump.

Initially, all valves should be closed. To test the mold seal, open the valves to the mold and the vacuum pump. Pump to the lowest possible pressure before stopping and checking the loss rate.

After testing the mold seal, return the system to ambient pressure using the pump's relief valve. Continue by closing the valve, leading to the vacuum pump. With the mold still attached to the valve system, move the mold to the preheated furnace.

Let it remain there for one hour. After one hour, start the infiltration process By closing the valve to the mold, all valves in the system should be closed. Next, open the valves to the Argonne gas cylinder.

To provide the infiltration pressure, prepare to open the mold valve. Quickly open the mold valve and wait for one minute. Remove the mold from the furnace and place it on a cooling surface here, a copper block.

For the next five minutes, monitor the pressure on the Argonne regulator and adjust Argonne to maintain infiltration pressure. When the mold is cool enough, take appropriate steps to detach it from the valve system. When this is done, transport the mold to a workbench where the sample can be extracted.

At a workbench equipped with a vice, secure the base of the mold in the vice grip and unscrew the hardware holding the lid to the top of the cylinder. To remove the top with the top removed, use a mallet to lightly tap the mold cylinder perpendicular to the vice grip in order to loosen the mold cylinder from the base groove. Next, release the vice grip to remove the mold base.

Once removed, put the mold base aside and prepare to place the mold cylinder in the vice. Secure the mold cylinder in the vice grip so that the remaining aluminum at the top of the sample is accessible with the mallet. Tap the remaining aluminum at the top of the sample to push it out of the mold cylinder.

The recovered material will have a foam sample, will surplus aluminum samples of a desired height can be cut using a band saw. After samples of a desired height to cut, the sodium chloride must be removed from the preform. To do this, have a beaker filled with sufficient water to cover the foam and a magnetic stir bar on a stirring hot plate.

Start the stirring and add the foam. Set the temperature to 60 degrees Celsius and be prepared to change the water every 10 minutes until there is no sodium chloride left in the foam. The protocol was followed three times for different size sodium chloride grains.

The foam pore sizes range from one millimeter to about 1.2 millimeters, 1.4 millimeters to 1.7 millimeters, and two millimeters to about 2.4 millimeters. The average porosity of the samples is 70%Each sample is made with 99.95%aluminum and measures 51 millimeters in diameter and about 25 millimeters in height Once mastered, this technique can be done in two hours if it is performed properly.