Aluminum Alloy Design Competition: the Black Vultures

Introduction

The purpose of this project is to create an aluminum alloy that meets the following criteria. We will need to develop our own procedures for thermomechanical processing in order to achieve an alloy with the best possible combination of yield strength, total elongation, and electrical conductivity. The team with the highest overall score will win the competition.

Criteria:

1. The alloy must contain at least 90% Aluminum.
2. The goal is to maximize to following;
3. yield strength (YS)
4. total elongation (% EL)
5. electrical conductivity
6. The sample must be 2 - 3 mm thick.

Competition Scoring:

The team with the highest scores in yield strength, % elongation, and electrical conductivity will receive 100 points. The remaining scores will be adjusted relative to that highest score. The winner will be determined by multiplying the scores from these three categories.

Alloy of Choice:

1. 6022-T62 Aluminum alloy

Instruments/Materials Needed:

1. Hot Rolling Mill
2. Heating Furnace
3. Induction Furnace (for crucible)
4. Water Bucket (with water)
5. Gloves
6. Heat-Resistant Suit
7. Microscope
8. Blowtorch
9. Rockwell Hardness Tester
10. Mounting Press
11. Etchant
12. Sample Clip
13. Alloying Materials (refer to the image and table below)
14. Tongs
15. Tabletop Bandsaw

First, start by selecting an alloy. Use Granta to identify an alloy that offers an exceptional balance of characteristics, including the highest yield strength, percentage elongation, and electrical conductivity.

The chosen alloy is 6022-T62, which has a predicted yield strength of 300 MPa, an expected elongation of 5.1% EL, and an electrical conductivity of 43.3 S/m.

The alloying constituents are detailed in the table above.

Make sure that the user performing the casting process is wearing the appropriate safety gear and is using the correct tools. Start by weighing your alloying constituents and pouring them into the crucible. Place the crucible into the induction furnace and wait until the constituents have completely melted. Next, use a blowtorch to heat up the molds. Upon satisfaction, pour the contents of the crucible into the molds. Allow the mold and ingot to cool. The ingot is now ready for homogenization.

Homogenization is the process in which heat is used to promote diffusion, allowing even distribution of constituents throughout the sample. This results in a more a more homogeneous and equiaxed grain structure. Appropriate levels of homogenization can improve yield strength, percentage elongation, and electrical conductivity. In our case, we chose to homogenize for approximately 64 hours at temperature of 540 degrees Celsius.

After homogenization, we decided to hot-roll our sample to achieve the desired thickness. It is important to ensure that anyone performing the casting process is wearing appropriate safety gear and using the correct tools.

First, the sample was cut in to four parts using a tabletop band saw. The sample was then heated to 500 degrees Celsius in a furnace and subsequently placed into a pre-hot rolling furnace until it was deemed suitable for hot rolling. A long rod was required to push the sample into the rolling region. The rolling mill itself was heated to 200 degrees Celsius.

Each individual sample was then rolled until it reached a thickness of 2 to 3 mm. This process was repeated for all four samples. After the hot rolling was complete, samples were left to cool on bricks.

The aluminum sheets should be placed in an oven and heated to 560°C for 2 hours. After this process, remove the sheets and quench them in water to achieve solution heat treatment. Following this, place the aluminum sheets in an oven set to 175°C and hold them at this temperature for different durations: 2 hours, 6 hours, 8 hours, and 24 hours. Allow the sheets to air cool after removal. Note that times and temperatures can be adjusted to suit the specific alloy being used.

After the solution heat treatment and artificial aging, multiple tensile bars should be cut from each plate. The scraps generated from this process can be used to test hardness and conductivity. Tensile testing indicated that hardness and elongation peaked at the six-hour aging mark, suggesting that the T62 temper was achieved at this duration. The results from the 2-hour and 8-hour tests were promising; however, they were inconsistent. For replicating results effectively, it is essential to test tensile bars from multiple treatment times to confirm that the T62 temper has been achieved and to select the bar with the desired quality.

The competition results indicated that the predicted electrical conductivity of the 6-hour 6023-T62 alloy sample closely matched the measured value of 43.34 S/m. However, the yield strength and elongation values were slightly lower than expected, measuring 291.4 MPa and 4.3% elongation, respectively. This performance placed us in 5th place in the competition.

Summary:

The Aluminum Alloy Design Competition mandates a minimum aluminum content of 90% and a material thickness of 2-3 mm to optimize yield strength, total elongation, and electrical conductivity. For this purpose, the Granta 6022-T62 alloy was selected using Ansys.

The casting was nearly flawless, with only minor surface cracks present.

The thermomechanical processes commenced with a homogenization period lasting four days at 540°C. This process included solution heat treatment followed by artificial aging of the alloy. Tensile testing results demonstrated that the sample aged for six hours exhibited the most consistent tensile strength and elongation.

The alloy showed an electrical conductivity of 43.34 S/m, an elongation of 4.3%, and a yield strength of 291.4 MPa.

Recommendations:

In order to avoid the porosity issues we encountered while testing our tensile bars, it is crucial to ensure that the pouring process is slow and controlled. Pour the metal steadily and consistently to avoid introducing air during solidification. Additionally, heat the mold adequately using a blowtorch or furnace to prevent thermal shock, which can cause rapid solidification and increase porosity. A higher temperature and extended homogenization may yield better samples that fracture in a more predictable manner. Additionally, it may be important to try longer aging times, particularly with overaging.