The Spotted Wobbegong’s Aluminum Alloy Design Competition

This contest required making an Aluminum alloy with 90% Aluminum while maximizing electrical conductivity, yield strength, and elongation. The available materials to include in the 90% aluminum alloy were aluminum, chromium, copper, iron, magnesium, manganese, silicon, titanium, zinc, and nickel. The available thermomechanical processes included homogenization, hot rolling, annealing, quenching, cold rolling, and aging.

These are the elements used and their composition.

Aluminum (Al) - 89.865%

Silicon (Si) - .05%

Magnesium (Mg) - 2.25%

Iron (Fe) - .06%

Copper (Cu) - 1.8%

Zinc (Zn) - 5.7%

Titanium (Ti) - .03%

Chromium (Cr) - .215

Manganese (Mn) - .03%

The Granta software was used to compare a variety of alloys based on elongation and yield strength. This narrowed the search down to the 7000 series alloys. Electrical conductivity was then incorporated to find the most well-rounded alloy with low variability, which was Aluminum alloy 7475.

It was calculated how much of each element needed to be added to the 90% aluminum alloy. The elements were weighed out and melted, starting with aluminum. The other elements were stirred to ensure proper mixing before being poured into a preheated mold.

Two samples were homogenized in a high temperature furnace at different temperatures with different time periods. The first was at 510°C for 168 hours and the second was at 470°C for 24 hours.

The samples were hot rolled at 450°C to achieve a roughly 80% reduction. One sample was reduced from 12.5 mm to 2.5 mm. The other sample was reduced from 12.5 mm to 2.765 mm. A problem encountered during hot rolling was that the samples became too long for the furnace, so they were cut in half to ensure proper heating before rolling. Another problem faced was that the samples became curved as they reached their final thickness; however, the bars became flatter with more passes. 

The samples were heat treated at 510°C for 3 hours, except for one, which was heat treated at 510°C for 96 hours. The samples were quenched for 15 seconds in cold water.

The samples were cold rolled to reduce each by 0.1 mm while introducing internal stresses and dislocations that would serve as precipitate nucleation sites and increase the strength of the sample.

The samples were aged with a T761 temper. This a two-step aging process where the samples are aged at 121°C for 3 hours followed by 163°C for 10 hours. The samples were slow-cooled in the aging oven by turning it off and allowing the oven to naturally cool.

The samples were prepared for metallography according to ASTM E3-11 standards. The samples were then etched using the Papageorge 2-step.

The microstructures of the samples were examined with a microscope according to ASTM 883-17 standards. This included the microstructure of an as casted 7475 alloy, a sample homogenized at 510 for a week and annealed for 3 hours, a sample homogenized at 470 for 24 hours and annealed for 3 hours, and a sample homogenized at 510 for a week and annealed for 96 hours, and. Before examination, each sample was etched according to ASTM E407-07 standards. They were examined at 500x and 200x.

Each sample's electrical conductivity was measured 10 times and an average was taken. The sample homogenized at 510 ºC for 168 hours and annealed for 96 hours had an average electrical conductivity of 40.282 %IACS. The sample homogenized at 510 ºC for 168 hours and annealed for 3 hours had an average electrical conductivity of 39.94 %IACS. The sample homogenized at 470 ºC for 24 hours and annealed for 3 hours had an average electrical conductivity of 36.4074 %IACS.

Each sample was made into tensile bars and were measured for their width and thickness. Each sample had three tensile bars tested for their elongation and yield strength. These values were averaged and compared. The average yield strength and elongation of the tensile bars homogenized at 470 ºC for 24 hours and annealed for 3 hours were 411.61 MPa and 4.5 % respectively. The average yield strength and elongation of the tensile bars homogenized at 510 ºC for 168 hours and annealed for 96 hours were 357.63 MPa and 3.45% respectively. Finally, the average yield strength and elongation of the tensile bars homogenized at 510 ºC for 168 hours and annealed for 3 hours were 386.86 MPa and 4.2%.

With background knowledge from optical microscopy and the results of initial testing, the sample that was homogenized at 510 ºC for 168 hours and annealed for 3 hours was selected for the competition.

The competition sample produced an electrical conductivity of 39% IACS, a yield strength of 389.6 MPa, and an elongation of 6.1%, giving a total score of 686,165.