Aluminum Alloy Design Project

Team Leaf-Nosed Bats competed in an aluminum alloy design competition with their classmates. The goal was to create an aluminum alloy that met specific requirements as well as held the highest yield strength, elongation, and electrical conductivity compared to the competition. The requirements set before the competition began included a final thickness between 2-3 millimeters, and the alloy must contain 90% aluminum content. Once a specific alloy was picked based on the teams personal goals, the alloy was casted, homogenized, and went through a variety of thermo-mechanical processing methods before metallography was performed on the sample and testing was done. Unfortunately, the team did not win the competition but they gained much from this experience.

Elements allowed for use in alloy creation included aluminum, silicon, manganese, iron, nickel, copper, zinc, titanium, manganese, and chromium. No other elements were permitted.

The team met up and using the competition guidelines and the Granta software, deliberated on possible alloys to move forward with. After some research and debate, alloy 2618-T61 was chosen for its good balance between properties and predicable nature.

The team followed an industrial recipe for the alloy we chose and mixed all the alloying elements in with the aluminum. Due to smaller crucible size, our recipe had to be adjusted to account for a 750g total mass instead of the initial planned upon 1000g. Once the alloying elements were weighed out and added to the aluminum, the entire mixture was stirred and heated at 660 degrees Celsius for 25 minutes. After the mixture was fully melted, it was poured into a mold to be used for the project.

The sample was homogenized at 475°C for 48 hours. The team shared a furnace with another team so they could not homogenize at their desired temperature (480°C). They were reassured that the slight temperature difference would not make that big of a difference in the mechanical processes. After the homoginization process was completed, their was no visible deforminites or cracks on the surface. During the homogenization process, the team recieved news that the furnace was turned off or unplugged. They did not know the cause of this or how long the furnace was off. To try and counter any of the effects this might have caused, the team decided to leave their sample in the furnace for an additional hour and a half.

In order to provide a foundation for greater success of our heat treating and aging processes, the team decided to hot roll the sample. Any benefits that would have been gained cold rolling would have most likely been undone by the heating processes that followed, so hot rolling was the ideal choice. The sample was rolled to between 2 and 3 mm thickness, with the original two bars being cut in half due to the length of the sample. This step was ended with 4 bars, labeled T and B according to their position in the oven during homogenization.

The team solution heat treated our sample at 510 degrees Celsius for 6 hours before quenching in boiling water. The goal of this step was to increase the ductility and electroconductivity of our sample.

The team aged the sample for 20 hours at 185 degrees Celsius. The goal of this step was to improve the strength of our sample.

The team did various testing throughout the process of making the alloy, including metallography, Rockwell Hardness testing, and tensile testing to ensure that the alloy was meeting expectations. The electroconductivity was a bit lower than expected, and the elongation was much lower than the expected values. However, our yield strength was within our expected range.

An additional step to the testing process was metallography of the sample. Small sections of the sample were cut and mounted before being ground and polished down to a smooth finish. The samples were then etched to make grain boundaries and precipitates more visible before micrographs were taken (pictured above). Large deposits of Titanium were identified as well as numerous cooper-magnesium precipitates along the grain boundaries of the alloy.

The team scored 7th overall during the competition, with a yield strength of 301.9 MPa, an elongation of 2.7%, and an electroconductivity of 36.28%. The actual competition results showed that the values were pretty close to what was predicted from the teams test samples. The test sample averages do contain outliers which could be why their values are drastically different than those given by Granta.

After assessing performance and speaking to Dr. Beach, we have concluded that most of the cause of our poor performance stems from two sources. One of these was our solution heat treating time. It was long enough that some of the copper formed extremely rigid precipitates within the microstructure of our alloy, reducing the elongation drastically. The other main problem we had was the titanium present in our sample. It did not melt down and homogenize very well, so the sample had large chunks of titanium in it, making it very brittle. Doing this in the future, we would want to reduce our solution heat treating time and add no extra titanium to our alloy. We also would like to try quenching after our aging process like much of the literature we found recommended. We would also like to attempt a multi stage homogenization process to try to get a more uniform structure across our sample.