Team Scorpion 7475 Aluminum Alloy Design Contest

Team Scorpion (left to right): Cade Casto, Serina Deus, Ross Duplain, Lauren Nguyen, Sidney Brisson, Max Yanes Diehl

Team Scorpion was challenged in a contest to create an aluminum alloy with maximized yield strength, elongation, and electrical conductivity. The alloy had to contain at least 90 wt% aluminum, and the team used Ansys Granta software to find a preexisting alloy that used only permitted alloying components and had appropriate properties. The team selected the AA7475 T651 alloy, with predicted yield strength of 67-74.1 ksi, elongation of 9-10.5% strain, and electrical conductivity of 45.4-47.9 %IACS.

1. Alloying elements: Al, Si, Mg, Fe, Ni, Cu, Zn, Ti, Cr, Mn
2. Book molds
3. Furnace
4. Crucible
5. Rolling mill
6. Rolling mill furnace
7. Rockwell hardness tester
8. Electrical conductivity tester
9. Optical microscope

Selecting the Aluminum Alloy

For this project, we chose a modified Al 7475 alloy due to its high strength, hardness, and excellent electrical conductivity compared to other aluminum alloys.

Creating Two Alloy Variants

We will produce two variations of the Al 7475 alloy:

1. Standard Al 7475
2. Modified Alloy: 0.1 wt.% of magnesium will be replaced with manganese.

Key Properties to Focus On

The primary properties we aim to maximize in these alloys are:

1. Yield strength
2. Elongation
3. Ductility

Why These Modifications Matter

1. Adding Manganese: Increases overall strength and elongation while maintaining ductility.
2. Reducing Magnesium: Enhances total elongation with minimal impact on yield strength.

By making these adjustments, we enhance specific properties tailored for optimal performance without compromising on other critical attributes.

Procedures:

1. Alloying elements weighed for the two alloy castings.
2. Crucibles filled with 500g and 750g of aluminum and heated until aluminum melted
3. Zn and Mg were wrapped in aluminum foil to ensure complete dissolution into the liquid metal. Zn was wrapped in 20g bundles.
4. Al, Cu, Fe, Mn, Cr, Si, and Ti were added to the crucible and mixed with graphite stir-rod until completely dissolved. For all alloying component additions, the induction melter was turned off before the crucible was manipulated in any way.
5. Zn was added in 20g increments and held underneath the surface until completely dissolved.
6. Two to three minutes before casting, foil-wrapped Mg was added to the crucible and mixed.
7. The crucible was removed from the furnace and the molten metal was poured into the preheated molds. Molds were overfilled until a cap formed to reduce thermal shrinkage. Excess alloy was discarded.
8. The metal was allowed to solidify.

Observations

Casting Attempt 1

The 1000g alloy was melted and poured into mold without any issues, but was scrapped as only 1 g Zn was added, instead of the required 57 g

The 750g casting was aborted as the melt emitted green flames when stirred. This was caused by alloying components reacting with air. The melt's alloying component composition was approximately 27 wt% as excess aluminum was not added.

Casting Attempt 2

Both the 750g and 1000g alloys were melted and poured into molds without issue and correct alloying compositions were achieved.

Procedures:

1. The 750g and 1000g as-cast alloys were mounted in phenolic resin at 330°F and 4100 psi. Mounts were heated for three minutes and cooled for six minutes.
2. Mounts were ground using 240, 320, 400, and 600 grit silicon carbide abrasive paper and polished with 6-μm and-3 μm diamond pastes and 0.05-μm colloidal silica
3. Mounts were examined under an Olympus GX53 inverted microscope and micrograph images were taken at 100x, 200x, 500x, and 1000x magnifications

Procedures:

1. Both aluminum alloys were homogenized at 475°C for 48 hours under normal atmospheric conditions. Alloys were placed into the furnace at approximately 11:20 am on Monday, Oct. 1, and removed at approximately 11:20 on Wednesday, Oct. 3.
2. Alloys were allowed to cool to room temperature before further processing and testing.

Procedures:

1. A metallographic sample from the 1000g alloy was cut, mounted, and polished. Micrographs were taken at 100x, 200x, 500x, and 1000x magnifications.
2. Billets were ground to remove unwanted oxide from their surfaces and allow electrical conductivity testing. Fifteen electrical conductivity readings were measured from each billet using %IACS units. Electrical conductivity values were taken at separate regions and averaged.
3. Five hardness measurements were taken from each billet at different locations. Hardness values were measured using the HRB scale.

Procedures:

1. Both alloys were preheated to 475°C for 5-10 minutes before rolling began.
2. Rollers were heated to 475°C and coated with boron nitride to reduce friction
3. Alloys were heated to 475°C in the rolling mill's preheating furnace for 2-3 minutes before each rolling pass.
4. Thicknesses reduced from 12-13 mm to 2-3 mm. Billets were cut in half midway through rolling to ensure the entire rolled billet fit within the mill furnace’s heating zone.

Observations:

As the alloys reduced in thickness, they cooled much faster after removal from the heating zone. Some of the thinner alloys split longitudinally during rolling.

Procedures:

1. Alloys were heat-treated at 471°C for three hours under normal atmospheric conditions. The hot-rolled alloys were placed in furnace at 12:00 pm on Monday, Oct. 14, but the furnace did not reach the target temperature until approximately 12:15 pm
2. Alloys were removed from furnace at 3:15 pm and immediately quenched to room temperature with water.

Procedures:

1. Both the 750g and 1000g alloys underwent 15%-reduction cold-rolling.
2. Alloys were reduced from approximately 3 mm to 2.5 mm to introduce strain hardening.

Procedures:

1. Both the 750g and 1000g alloys were artificially aged at 121°C for 25 hours to achieve a T6-temper.
2. Alloys were placed in the furnace around 10:00 am on Tuesday, Oct. 15, immediately after cold rolling. Alloys were removed around 11:00 am on Wednesday, Oct. 16.
3. Alloys were cooled to room temperature after aging.

Procedures:

1. Metallographic samples for the fully processed 750g and 1000g alloys were mounted, ground, polished, and etched. Etching was performed using the Papageorge two-step technique. Micrographs were taken of both mounts at 100x, 200x, 500x, and 1000x magnifications.
2. Both the 750g and 1000g alloys underwent final electrical conductivity and hardness testing.
3. Tensile bars were cut from each processed billet. Bars cut closer to the edges underwent preliminary tensile testing.

Results/Observations:

1. Electrical conductivity readings for the 1000g alloy were slightly higher than the 750g alloy, averaging around 2 %IACS difference
2. Hardness readings for the 750g alloy averaged around 8-9 HRB higher than the 1000g alloy
3. The yield strength for the 1000g alloy was higher than the 750g alloy, resulting in slightly over 400 MPa while the 750g alloy ranged from around 375 to slightly less than 400 MPa
4. The elongation for the 1000g alloy resulted in over 1% higher than the 750g alloy, measuring at 4.8% for the 1000g alloy and 3.0%-3.6% for the 750g alloy
5. Significant precipitates were found in the micrographs, which would serve as crack propagation sites, decreasing the elongation from the predicted amount

Based on these differences, the team decided to use the 1000g alloy for the final testing.

During the competition, one tensile sample was chosen from the 1000g alloy for electrical conductivity testing, as well as tensile testing for yield strength and elongation.

Results:

1. The yield strength was measured to be 427.1 MPa, increasing 5% from the final analysis testing
2. The percent elongation only reached 4.0%, decreasing 16% from the final analysis testing
3. The electrical conductivity was measured at 36.06 %IACS, increasing by 0.5% from the final analysis testing

The team placed fifth out of eight teams, winning the yield strength category, but falling short in the electrical conductivity and the elongation category, therefore not receiving a high enough overall score to win.