Experiments

With no lab equipment with us in quarantine, we had to get creative with our experimentation.

One easy material characteristic to test at home is hardness. The technique is to bounce a ball bearing off a specimen, making sure that it is secured down. The ball bearing contains a certain amount of kinetic energy which is dependent on the height it was dropped from. When the ball bearing hits a hard surface, a large amount of its energy is conserved in its motion, and it bounces high. When the ball bearing hits a less-hard surface, energy is absorbed into the surface, and the ball bearing does not bounces as high.

On the left, a ball bearing dropped from 48 inches onto a steel cylinder reaches 21 inches. On the right, the same ball bearing dropped from the same height onto a cast aluminum disk reaches 5 inches. As we suspected, aluminum is much softer than steel.

The next test was based of our reading findings that increasing the cooling rate of cast aluminum shrinks its grain size, which increases its hardness. We were going to crudely vary the cooling rate by casting the liquid aluminum in sand (slowest cooling rate), in a metal tin (middle cooling rate), and in a metal tin + a dip in cold water (fastest cooling rate).

Unfortunately, the foundry failed before the sand and water tests could be carried out, leaving us an aluminum-encrusted crucible and the unanswered question in our minds of whether increasing cooling rate truly does increase hardness.

Casting Defects

After successfully casting aluminum in various shapes, we wanted to look to see if there were any evident defects in the results. We knew that, since we were using basic equipment to cast metal, the chances of defects were very high. The most obvious of the defects that we found tended to be in the form of pores. Some pores were simply small air bubbles (left) whereas some were cracks due to uneven cooling (right).

Next, we were able to see some grain size defects. The center of the ingots pictured here have obvious dips in the center, which leads us to believe that uneven cooling. This results in more shrinkage in the center, causing the dip in the center.

Lastly, we could see some other defects that seemed to be the result on non-aluminum materials making it into the casts. While the alloys we used were mostly aluminum, there are other metals that were contained in the metal. Not only that but we melted everyday objects like aluminum cans which have more components that aren't entirely accounted for. So it is almost certain that some of these non-aluminum materials made it into the results. Examples of such defects are shown below.

Through looking at these defects, we found the most optimal conditions in which to let the metal cool. For some casts, we submerged them in water as soon as we could to reduce the time needed for them to cool. The majority of the casting defects that were pores or grain size related seemed to be due to this rapid cooling. The casts that looked the best were allowed to sit and cool on their own for more extended periods of time before being put in water. This is most likely due to the fact that casting defects occur from uneven cooling, which almost certainly happens in the rapid cooling scenario. If the metal is given an extra couple of minutes to actually solidify before trying to cool with water, then these defects will occur less often.