Designing and Testing a 7055 Aluminum Alloy

To make an aluminum alloy with maximum strength, ductility, and conductivity, it is important to consider which materials will be best suited for the application. To narrow down the options, Granta EduPack software was utilized and an ashby plot was constructed to evaluate the desired properties of various aluminum alloys. While finding a metal having its properties excel in all 3 categories is ideal, it was unrealistic and more feasible to optimize 2 properties instead of 3. Since a 7000 aluminum series had high strength and elongation, we decided to make a 7055 aluminum alloy.

Supplies

Alloy selection:

• Master alloys for additive elements 
• Weigh boats 
• Digital scale 
• Aluminum foil 

Casting 

• Billet mold 
• Furnace 
• Tongs 
• Heat-protective PPE 

Processing 

• Rolling mill
• Furnace

Microscopy 

• PAXCam software 

Metallography

• Metallography mounting press 
• Bakelite resin powder 
• Hand grinder w/ 240, 320, 400, and 600 SiC grit paper 
• Polishing wheel 
• 6 um diamond paste with diamond extender 
• 3 um diamond paste with diamond extender 
• 1 um diamond paste with diamond extender 

Etching 

• Modified Keller’s reagent 
• Modified Weck’s reagent 

Microscopy

• Olympus inverted microscope 
• PAXCam software 

Cutting 

• Abrasive saw 
• Water jet 

Conductivity testing 

• Electrical conductivity meter 

Tensile testing 

• Tensile testing machine 
• Extensometer 

Step 1: Smelting

Use the mass percentage chart for the alloy to calculate the amount of each master alloy needed to reach the desired percentage of each elemental additive. Gather the correct amounts of each additive element and record their mass. Wrap all zinc samples in two layers of aluminum foil and all magnesium samples in a single layer of aluminum foil. After finding the amount of aluminum added from each of the additive master alloys, subtract that amount from the total aluminum needed to find the mass of pure aluminum needed to reach the desired mass percentage. Heat a crucible in an induction furnace and place materials for the alloy to melt. After completely liquified, pour in a cast within a container of sand to cool down and dissipate the heat. Examine for any cracks and trim the edge once at room temperature.

Step 2: Metallography

Cut off a small portion, about half an inch, from the very top of the cast alloy. From that portion, cut a cross-section from the center of the alloy, and another from the edge. Label the samples C and E, respectively, and draw an arrow in the direction of rolling, to avoid any discontinuity.

Weigh 18 grams of Bakelite, a phenolic mounting powder, into a beaker. On two different mounting presses, adjust the parameters to 330°F, and a pressure of 4100 psi, with a heating and cooling cycle of nine minutes. Place the two samples with the rolling direction facing up onto the mounting press stage. Lower the stage until the whole sample is below the opening, and pour in the Bakelite only until the sample is completely covered. Then lower the stage all the way down, and pour in the rest of the Bakelite. Seal the press, and hit 'Start'.

Once the press is finished, extract the sample and use a grinding machine to smooth down the edges. Inscribe each sample with their respective sample name, C and E.

Set up a hand grinder machine with four types of grinding paper: 240, 320, 400, and 600 grit. Each paper should have silicon carbide as the abrasive material. After each step, rinse the sample with water, and rotate 90°.

After grinding, polish each sample manually. The first polish utilizes a 6 um diamond paste with diamond extender compound. Over the course of three minutes, polish the sample in a counter-clockwise motion using relatively heavy pressure by holding the sample onto the plate with one hand, and pressing down on the top of it with the other. Use a slow speed of rotation in this part. Every thirty seconds, rotate the sample 90°. After 3 minutes, rinse the sample immediately under water and then with alcohol. Air dry the sample and then visual inspect under a standard microscope and make sure the visibility of grains are improving before moving on. The second polish utilizes a 3 um diamond paste with diamond extender. The same methodology is used in this step. The final polish utilizes a 1 um diamond paste with diamond extender. For this step, polish for 1 minute using a relatively light pressure and a fast rotation speed. Inspect the sample under the microscope and polish further as needed.

Step 3: Pre-Processing Optical Microscopy

Using an Olympus inverted microscope connected to a PAXcam software, place one sample upside down on the stage and focus on the surface at 10x magnification. Once focused and appropriately white balanced, capture two images at 20x, 50x, and 100x magnification, for a total of six images per sample. Repeat this process for the other sample. Save the images under file names that include which sample it is and what magnification it is.

Step 4: Thermomechanical Processing

After casting, cut the billet into two halves, and label them T (top) and B (bottom). Then, homogenize the samples for 1 week at 520°C. After, hot roll the samples to a final thickness of 3.5mm. Prior to rolling, hold the samples at 450°C, then roll at 400°F with the roller furnace at 450°C. Then cut the samples further, to yield 4 total. Label T1, T2, B1, and B2 respectively. Carry out the appropriate steps for each sample as seen in the diagram above. Once finished, cut the samples into 5 tensile bars each using a water jet.

Step 5: Metallography of Mechanically Processed Alloys

Repeat the procedure for metallography from Step 3, but this time with four samples from T1, T2, B1, and B2. Place a different colored clip on each alloy and record which one it was so that you keep track of the samples.

After completing the polishing, it is time to do the etching. For this, you need Modified Keller's reagent and Modified Weck's reagent. Wearing protective gloves and eyewear, place each reagent into a small dish. Once you have your set up complete, take a pair of metal tongs and pick up one of the samples. Making sure to keep a firm grip on the sample, submerge it in the Keller's regent for 10 seconds, not letting go off the sample. Once the 10 seconds are up, remove the sample and immediately rinse it under water. After rinsing, submerge the sample in the Weck's reagent for 10 seconds. Once the time is up, remove the sample and rinse it under water. Repeat these steps for the remaining three samples.

Once all samples have been etched, repeat the procedure for optical microscopy from Step 4, taking images at 10x, 50x, and 100x magnification.

Step 6: Conductivity/Tensile Test

Clean the samples before measuring conductivity. Place the probe of the electrical conductivity meter on the surface of the alloy sample being tested. Record the %IACS on the meter readout and move the probe to a different cleaned area of the sample. Continue until ten readings have been measured and calculate the average %IACS for the sample. Repeat for the remaining alloy samples so that the average %IACS is calculated for all samples. 

Measure the width and length of the tensile bar being tested using calipers. When prompted by the testing software, enter the values and label the sample accordingly. Place the tensile bar upright into the vices of the tensile tester and tighten it to keep it in place. Attach the extensometer to the middle of the tensile bar and begin the testing. Record the values of percent elongation and 0.2% YS. Removed the broken tensile bar, reset the testing machine back to zero, and continue testing all necessary tensile bars while recording necessary values. 

Step 7: Testing Results

Based on the testing results, the tensile bar chosen for the competition was B1-4. This sample underwent homogenization at 520°C for one week, followed by hot rolling to 3.5mm, then cold rolled to 2.5mm prior to solution heat treatment at 500°C for 2 hours. Then it was quenched, followed by our 1-step aging plan, 120°C for 18 hours. The B1 and T2 samples, both aged with the 1-step plan, had higher strength but lower conductivity (~33% IACS) when compared to the 3-step samples. Our other aging plan, for samples T1 and B2, had 3-step aging for 8 hours each at various temperatures. These samples had lower strength but higher conductivity (~38 IACS). We believe this was because these samples were overaged, meaning that they were past the peak strength, but as we continued aging, the electrical conductivity increased. Our B1 samples had the best overall elongation (~7%), this being due to the equiaxed grains, as a result of cold rolling prior to solution heat treatment. The T2 samples had the highest overall yield strength (~490 MPa), but had the lowest elongation (~4%, with one sample testing at >1%). We also were getting better results with tensile bars cut from the outer portion of our samples, which influenced the decision to chose the bar to the right of the middle (4) rather than the exact middle (3). This could be due to having defects in the middle, which is the last region to solidify. Our projected results for our chosen sample are:

YS offset 0.2% = 415MPa

Total Elongation = 7%

Electrical Conductivity = 33 %IACS

Results for the tested B1-4 tensile bar were less than expected in comparison to other B1 bars. It exhibited decreased elongation and yield strength in comparison to 2/3 other bars tested. It only excelled when compared to B1-2 in elongation. When compared to the other teams competing, the Pyjama Sharks had the alloy sample with the greatest yield strength (393 MPa) and was tied with two other teams for the greatest elongation (6.8%). The conductivity was very low compared to other alloys but this was expected as we focused on maximizing yield strength and elongation. The final choice the team made was deciding whether to test B1-3 or B1-4. There wasn't a clear pattern location-wise with the tensile bar cut-out. The collected data didn't skew towards either the edges or the middle of the bar where the tensile bars were cut out. The team went with B1-4, but B1-3 was tested post-competition and was found to have the same yield strength but a higher elongation, similar to B1-1. If there was a pattern on which tensile bar would have yielded the best results it was not discerned and thus the bar with the best overall results was not included in the final test.