Aluminum Alloy Design Contest: Cookie Cutter Sharks

Competition Introduction

The Cookie Cutter Sharks are participating in a competition to design an aluminum alloy with the highest yield strength, elongation, and electrical conductivity properties. This is a start to finish project where the team chooses their own set of thermomechanical processes to improve these properties. At the end of the competition, the final chosen created sample competes with 5 other teams to determine the best alloy and set of processes.

Competition Criteria

1. The chosen alloy must be more than around 88% aluminum
2. The only properties of interest are yield strength, percent elongation, and electrical conductivity
3. The final thickness of the test sample must be between 2 and 3 millimeters
4. Only 1 sample from the team is tested and results are based on that

Competition Scoring

The highest yield strength, % elongation, and conductivity will be assigned 100 points and all other teams' scores will be normalized based on the 100 point sample. The product of all three categories' scores will determine the winner.

Selected Competition Alloy

AA 7255

Materials

The materials involved in casting AA7255 are commercially pure aluminum and mixture stock of Copper, Magnesium, Zinc, Silicon, and Manganese.

Tools

Completing the process from casting to age hardening requires specialized tools. These include a furnace with ceramic crucible, high temperature ovens, a high load roll mill compatible with different temperatures, a pre-rolling furnace, a mounting machine, a sink for safe sample etching, an optical microscope, a conductivity meter, high precision/high pressure water jet cutting machine, and tensile bar tester.

PPE

Protective clothes and materials must be worn in varying degrees at all steps of the process to avoid injuries. These items include safety glasses, fire suit, nitrile gloves, a face shield, and long non-flammable pants.

Selection Methods and Process

Team Cookie Cutter Sharks used Granta EduPack, a database of material properties, to choose a wrought aluminum alloy that maximized yield strength, percent elongation, and electrical conductivity.

Selected Competition Alloy

AA 7255

Estimated Properties of Interest:

Yield Strength: ~670 MPa

Max Percent Elongation: ~12%

Electrical Conductivity: ~40.2 % International Anneal Copper Standard (IACS)

Due to limitations of raw materials, the original ratio was not used.

Added Elements and their elemental percentage:

Aluminum (Al): 88.5%

Copper (Cu): 2.1%

Magnesium (Mg): 1.50%

Manganese (Mn): 0.07%

Silicon (Si): 0.85%

Zinc (Zn): 0.98%

Material Preparation

Each element that goes into the 7255 alloy must be precisely weighed out and set aside so that it can be added into the furnace. The alloying elements are not pure substances, so the mass of each must take into account the amount of aluminum that is already mixed in with the additive. The relative purity of each alloying element used in creating this AA7255 sample is shown below.

The cookie cutter first melted the aluminum as it was the bulk amount. Once liquified, the copper, silicon, and manganese elements are added and mixed in with a stir rod.

Both Zinc and Magnesium cannot be added directly to the liquid aluminum, as the high temperatures and presence of oxygen in the atmosphere cause a violent chemical reaction. To prevent this, both elements must be first wrapped in layers of aluminum foil, and once added, pushed and held to the bottom of the liquid solution to ensure proper melting before exposure to the atmosphere.

Caution!

Once thoroughly mixed, and before pouring, its important to take precautions and put on a full fire suit. The liquid aluminum is over 600 degrees Celsius and will cause severe injuries if skin contact is made. The casting molds should also be preheated in an oven and/or with a blowtorch in order to evaporate any water inside. If water is not evaporated, an explosion could occur when the liquid aluminum is poured.

While wearing full protection, the crucible needs to be picked up with tongs, maneuvered over the steel mold, and poured so that the mold is overflowing with the liquid alloy. Allow sufficient time to cool before removing the solidified alloy and before handling.

Finally, the sample should be cut down to avoid working with any cracks or defects, as well as to make more individual pieces for processing.

Homogenization is the process of heating the alloy up to a high temperature for an extended period of time so as to allow for mass transport and diffusion to happen, which makes the alloy more uniform and closer to solutionized.

For this alloy, the Cookie Cutters chose a temperature of 500 degrees Celsius and a cook time of approximately 1 week. This time and temperature is up to the experimenters discretion, and is not an exact process.

After the homogenization reached the desired runtime, the ovens were turned off and the samples were allowed to cool to room temperature along with the oven.

The now homogenized samples are ready to be rolled down to an appropriate thickness, which for this competition is between 2 and 3 mm.

Caution! From this point on in this step, all samples will be very hot and should not be handled directly as contact will cause severe burn injuries. Appropriate protective gloves and safety glasses are required to work through this step.

Already preheated samples are placed in the furnace pre-rollers, and given time to reach 400 degrees Celsius. This isn't hot enough to melt the sample, but it does make it ductile enough to work without excessively high loads. The wait time for the sample to heat up isn't exact, and there isn't a change in color of the sample due to heat to indicate its ready, so a judgment call has to be made.

Once the sample is believed to be hot enough, a long pushrod is used to move the sample out the far end of the furnace, and into contact with the rollers, which grab it and pull it through. The sample is quickly retrieved from the outlet side and returned to the pre-roller furnace to achieve temperature again. The rollers then need to be lowered so as to continue making reductions. This waiting, pushing, rolling, retrieving, replacing, and lowering process needs to be repeated until the sample is the right thickness.

After the final roll, samples are to be set aside on a brick to cool.

The hot rolled samples must now be heat treated and quenched to harden the worked alloy.

The heat treatment is achieved by heating up the samples in an oven to a high temperature, in this case 520 degrees Celsius, and holding for 1 hour.

Quenching immediately follows this cook time, whereby the samples are removed from the oven using tongs, and plunged into a bath of water to cool rapidly. A few seconds after the samples are dunked, they are completely cool and can be handled safely.

Following the heat treatment and quenching, some of the alloy samples were subjected to more work hardening via cold rolling. The same roll mill and overall process is used as with hot rolling, but there is no preheating of samples before they are flattened.

For the samples set aside for rolling, a very small overall reduction was made of between 0.2 and 0.1 mm. This rolling process was not intended as a method by which to achieve a thinner sample, but instead to elongate and add direction to the grains within the sample. These grains, both with higher surface area and greater in number, help with precipitate growth during artificial age hardening, the next step in this process.

Precipitation hardening is a process by which keeping an alloy sample at a high temperature for an extended period of time promotes the growth of precipitates within the sample. The process has two goals. First it removes precipitates from the matrix of the alloy, decreasing obstructions for electron flow and increasing electrical conductivity. Secondly the creating of small precipitates, specifically on grain boundaries, inhibits dislocation motion within the alloy, which increases strength properties. Since this competition scores both both of these properties, this is an ideal step to perform.

Both the time and temperature for this process require consideration, and most be chosen as a balancing act to get the desired effect. Higher temperature dictates the speed at which microscopic events will occur and move within the alloy, and time is the window in which it will happen.

The parameters for this process should be researched and chosen for each unique alloy. For the Cookie Cutter Sharks AA7255 alloy, 2 different aging processes were used. The first process involved 2 stages; first hold the sample at 105 degrees Celsius for 8 hours, and then 175 degrees Celsius for 24 hours. The second process involved only 1 step, which was to hold the sample at 140 degrees Celsius for 16 hours.

Following both aging process, the respective samples were set out to cool at room temperature.

Next, the samples need to be evaluated to predict how they will perform in the competition. The samples were tested via three methods: metallography, electrical conductivity testing, and tensile testing.

Metallography is the process of mounting a metal sample, grinding it on varying degrees of grit paper, and polishing with diamond paste or colloidal silica. To make a mount, the sample needs to be sectioned to a size where it will fit in 1.25 inch mount. This can be done with a precision cut or power cut machine. From there, around 19-20 grams of Bakelite powder should be measured out and put into a mounting press along with the metal sample. Once the mounting press is finished, samples should be labeled with a vibratory pen (ex. M1, M2, etc.). The mount then needs to be ground along 240, 320, 400, and 600 grit paper with running water. Next, the mount needs to be polished on a wheel with 6 micron and 3 micron diamond paste before a final polishing with colloidal silica. Remember to rinse the mount with cold water between steps and spray it with ethanol before drying. Safety glasses should be worn in the metallography lab at all times.

After the mount is polished, it should be etched so that the microstructure of the sample can be observed. AA 7255 does not have a specific etch proven to work best, so Keller's etch was used since it works well on other aluminum alloys. The mount was etched by rubbing a cotton ball soaked in the etch on the face of the mount for 8-9 seconds before being rinsed with water and placed in a bowl of dye for another 8-9 seconds. After rinsing the dye off with water and spraying with ethanol, the mount can be imaged once dry. Images should be taken on an inverted microscope in brightfield. For best practice, take overview images at around 100x before moving onto higher magnifications. To best see grain boundaries and precipitates, a few images should be taken at 500x and 1000x.

For electrical conductivity testing, a larger surface area is needed, so use a bigger sample for this test. Clean the surface of the sample by rubbing grit paper on it. For best results, the sample can be ground on a belt grinder as well. Then the sample can have its conductivity read with an electrical conductivity meter.

Finally, a tensile test is done on the sample to measure its yield strength and % elongation. The Cookie Cutter Sharks sent a few samples to the Center for Design and Manufacturing Excellence (CDME) to cut tensile bars via water jetting. Once samples were received by the Cookie Cutter Sharks, each bar had its thickness and width measured. Using a tensile tester and an extensometer, two bars from each sample should be tested one at a time. It is important to not use the computer controls while another person is taking a bar in or out of the machine as there is a risk for fingers to get caught in the machine.

Testing results were as follows:

Average % Electrical Conductivity

Sample 1 - 35.23%

Sample 2 - 38.63%

Sample 3 - 35.83%

Sample 4 - 36.29%

Average Ultimate Yield Strength (MPa)

Sample 1 - 327.965

Sample 2 - 327.669

Sample 3 - 327.560

Sample 4 - 339.361

Average % Elongation

Sample 1 - 3.367%

Sample 2 - 1.6%

Sample 3 - 4.55%

Sample 4 - 5.6%

The Cookie Cutters decided to enter Sample 4 into the competition due to it having the highest average yield strength and % elongation. Sample 4 also had the second high electrical conductivity. The Cookie Cutters predicted the following results:

An electrical conductivity of 36.29%, an ultimate yield strength of 339.361 MPa, and 5.60% elongation.

After all process and initial testing was completed, a single sample was chosen for the competition, and went head to head with other groups;' alloys. During competition testing, the properties of Cookie Cutter Shark's alloy are as follows. Yield Strength: 320 MPa. Percent Elongation: 5.6%. Electrical Conductivity: 36.3%.

Both the percent elongation and yield strength followed earlier testing, but the yield strength was unexpectedly low. This is believed to be a symptom of non-uniformity within the samples, and the chosen test bar having a unique defect which lowered its overall strength.

A review of the process, data, and research suggest more insights and recommendations. It is believed that the initial homogenization temperature of 500 degrees Celsius was not high enough. As a result this process did not maximize material properties, and specifically the non-inclusion of copper during this early process led to low strength performance.

The process by which the final samples were made could also have been improved to maximize the material's potential. Specifically during the age hardening process, it would be advantageous to have multiple samples be heated at the same temperature, but for varying amounts of time, and for each one to be tested for best qualities. This is a more experimental methodology which would require more time and more samples to work with.

Hopefully this has been an informative and interesting look into the casting and thermomechanical processing of a unique aluminum alloy. Please leave any comments or feedback on this outline, and we hope to continue investigating material properties and possibilities!