ALLOY DESIGN

For the MATSCEN 3331 Aluminum Alloy Design Competition, our team engineered a custom alloy inspired by 7040 (7xxx series) aluminum. We chose a precipitation-hardened Al–Zn–Mg–Cu alloy because this combination provides some of the highest strengths available in aluminum, while also allowing for decent ductility and electrical conductivity through heat treatment.

1. 90.5 wt% Al - maximizes electrical conductivity and provides a lightweight, ductile matrix
2. 5.7 wt% Zn - the most important strengthening element in 7xxx alloys
3. 1.7 wt% Mg - forms MgZn₂ strengthening precipitates with Zn; boosts hardness and yield strength
4. 2.1 wt% Cu - improves conductivity in overaged conditions and increases strength & toughness

We chose a 7xxx series alloy for its high precipitation-hardening potential, and the T7 temper for improved ductility and conductivity compared to peak-aged T6.

The following materials, equipment, and software were used to cast, process, and test our custom 7040-based aluminum alloy.

Materials Available for Casting

1. 750 g of commercially pure aluminum (foil, pieces, and pellets)
2. 99.9% Zinc shot (added to reach ~5.7 wt% Zn)
3. 50% Magnesium – 50% Aluminum master alloy (added to reach ~1.7 wt% Mg)
4. 99.9% Copper shot (added to reach ~2.1 wt% Cu)
5. Aluminum foil (used to wrap Mg and Zn additions to limit oxidation)
6. Aluminum oxide or steel stir rod
7. Ceramic or graphite crucible
8. Preheated steel billet mold

Thermomechanical Processing Methods Available

1. Rolling mill (for hot and cold rolling)
2. Furnace for homogenization (~465 °C)
3. Furnace for hot rolling reheats (~450 °C)
4. Furnace for solution heat treatment (~480 °C)
5. Furnace for T7 artificial aging (~160 °C)
6. Calipers for thickness measurements

Testing Methods Available

1. Tensile testing machine with extensometer
2. Electrical conductivity tester (%IACS meter)
3. Rockwell hardness tester
4. Optical microscope for microstructure observations

Programs Used

1. ANSYS Granta EduPack (for alloy family comparison and property selection)

We chose a 7xxx alloy because it gave us the best chance at ranking well in all three competition categories. These alloys naturally offer very high yield strength because of the η and η′ precipitates that form during aging, and they also allow ductility to be adjusted simply by changing the heat-treatment schedule. When overaged, 7xxx alloys also regain a decent amount of electrical conductivity, which is important for scoring well in this competition.

We verified all of this using ANSYS Granta EduPack. The three property charts (yield strength vs. elongation, yield strength vs. conductivity, and conductivity vs. elongation) showed that the 7xxx family consistently sits in the best region when all three properties are considered together. The plotted points for 7040-type alloys were especially promising, which confirmed that a Zn-Mg-Cu system was the right direction for our design.

We specifically chose the T7 temper over T6 because T6 only gives peak strength, while T7 creates a better overall balance. T6 has very low elongation and noticeably lower conductivity. T7 sacrifices a bit of strength, but it significantly improves ductility, uniform strain distribution during deformation, electrical conductivity, and even stress-corrosion resistance. Since this competition scores all three properties, T7 provides a much stronger combined performance than T6.

For casting, we started by weighing out our alloying elements of approximately 1.7 wt% Mg, 5.7 wt% Zn, and 2.1 wt% Cu on a basis of 750 total grams for the billet. The Mg shot used had a composition of 50% aluminum that needed to be accounted for in total concentration calculations. Also, the Zn and Mg shot required to be wrapped in aluminum foil in order to assist in adding the alloys to the crucible. The aluminum content of the aluminum foil was also taken into account and then the remaining needed aluminum was measured out. Next, the aluminum was added to the crucible and placed in an induction furnace where it was heated until the aluminum completely melted. We then added each of our alloying elements one at a time to the crucible, ensuring that the alloy was submerged into the molten aluminum, and allowed time for the alloys to melt and mix with the aluminum through occasional stirring. A billet mold was then heated with a blow torch in order to reduce thermal shock during the casting process. Once the mold was set, the crucible was lifted out of the furnace and the molten alloy was poured at a constant controlled rate into the mold. The alloy was poured until the mold was full and a mushroom head was poured over the opening to account for shrinkage during cooling. Once cooled, the mold was taken apart and the solid billet was removed. The billet was then allowed to cool and inspected for any cast defects, where none were found. Finally, any extra metal on the billet from the spout of the mold was cut off resulting in a rectangular billet.

Our billet was homogenized before any further processing. This step takes place in an oven at an elevated temperature to increase diffusion of the alloying elements as well as to develop a more equiaxed grain structure. In our case, we homogenized for approximately 16 hours at 465 degrees Celsius. We then allowed our billet to slowly cool within the oven for approximately three hours. From there the billet was removed from the oven and allowed to air-cool to room temperature.

The goal of mechanical processing was to improve the mechanical properties of our alloy while also reducing the thickness to the desired value. We primarily used hot rolling to make reduction passes through the mill. This method typically increases the material's ductility, reduces internal stresses, and allows recovery when operated above the recrystallization temperature. While hot-rolling reduces dislocation density and therefore yield strength, we were confident that the inherent strength of 7xxx series alloys would be sufficient.

To compare the effects of different processing techniques, a last-minute decision was made to cold work one of our samples following heat treating and aging. Cold-working increased strength through strain hardening, so it was assumed that this alloy would outperform other samples in terms of yield strength. Testing confirmed this assumption: cold working improved yield strength while having a negligible effect on elongation compared to the other samples.

Overall, mechanical processing enabled our team to produce the strongest alloy in the competition, with elongation at fracture consistent with typical values for 7xxx series metals.

When choosing which heat treatment to conduct on our alloy, we decided to look online at research for 7040 alloys in hopes of finding what time range and temperature would be best for achieving high strength, ductility, and electrical conductivity. After many attempts to find how long to heat treat for, Pete Fallon helped us during our lab section to narrow down the best time. We followed his recommendations of 30 minutes per quarter-inch thickness of metal and heat-treated at 485 °C for roughly 10.5 minutes. Once the heat treatment was complete, the metal was quickly pulled from the furnace using appropriate PPE and quenched immediately in room-temperature water.

The purpose of solution heat treating is to dissolve all of the coarse alloying element precipitates back into the matrix to form a supersaturated alpha-rich Al phase. The reason why a quick and cold quench is most effective is that if too much time, creating a slow undercooling, is given to the metal after leaving the oven, those precipitates that we worked to dissolve will come back out of the matrix. However, those precipitates that come out will be coarse and non-coherent, which is not ideal for the properties that we were looking to hit.

From one of our group members' knowledge of heat treatment tempers from a recent internship, we decided to go for a T7 temper, which includes solution heat treating + artificial over aging. When compared to T6, a popular temper for 7xxx series alloys which creates peak strength, the T7 temper is known for boosting electrical conductivity as well as ductility, but dropping the alloy's strength, an exchange we were aware of and willing to risk.

For the aging part of our competition, we referred to the aluminum alloy heat treatment guidelines published by ASM, which helped us to choose a two-step aging process. Although this guide helped us narrow down options on how long to age for and at what temperature, since 7040 was not explicitly tested in this source, we made estimated guesses on time and temperature to appropriately fit a T7 temper. We chose T7 tempers for 2 of our 3 samples, one being a "low-end" T7, where we experimented with aging at the same temperature as the "high-end" but for a shorter period of time. For our third sample, we decided to cold roll a 3.5% reduction and artificially age for 24 hours at 120 °C, a heat treatment procedure resembling a T6 temper for peak strength but reduced ductility and electrical conductivity. A full schedule of our aging procedures for all three samples is shown above in this section, accompanied by a video of our quench as well as a micrograph of our low-end T7 sample at 500x magnification.

The purpose of aging is to slowly bring out those alloying element precipitates, but in a controlled manner, alloying for fine and more coherent theta/theta prime/GP zones to form, giving the metal its high strength. When Aluminum is overaged, those fine coherent precipitates turn into coarser and less coherent precipitates. This decreases strength because dislocations can now more easily move by these particles, making deformation easier (higher ductility), and allows electrons to move by more more easily or with less scattering (higher EC) because there are fewer "obstacles" in their way that could deflect them off path.

Testing was one of the most important steps throughout the competition because it provided our team with an idea of how each alloy performs under the given conditions. The alloy was tested in a tensile tester to ensure the yield strength and % elongation were as predicted. Sample 1 (High-End T7) had an average yield strength of 195.6 MPa and 7.4% elongation. Sample 2 (Cold Worked) had an average yield strength of 448.7 MPa and 6.5% elongation. Sample 3 (Low-End T7) had an average yield strength of 361.2 MPa but only 5% elongation. These results revealed that the cold-worked sample was significantly stronger than the T7 ones and had the second-best elongation. From this observation, we determined that the second sample would perform the best in the competition mechanically and give us the best chance at winning.

After the Testing concluded, our group scored highest in the strength category with a yield strength of 427.5 MPA, shown in Figure 1. Our biggest weakness was our elongation electrical conductivity, where we scored 6th in Electrical Conductivity (33.3% IACS) and 8th in Elongation (7%).

Corrosion testing was implemented for the alloy, spending two weeks in a 0.7M NaCl solution with a hydroperoxide corrosion agent added to kickstart the process. After the two weeks, the alloy was tested again, and micrography revealed that pitting corrosion was the leading cause of corrosion in the alloy. The table displays the effects of the two-week submersion in terms of mass loss and changes in dimensions.

The 7040 alloy has a very small window for achieving strong EC% as well as high ductility, a risk we acknowledged but decided to pursue. When using GRANTA, 7040 was placed in the middle of the field compared to other 7xxx alloys, which was a safe choice for us, given the small margin for error in the other two categories.

Getting high EC% and Ductility came down to over-aging at the appropriate temperature and time. We predicted that our T7 samples (SHT + Overage) would perform best, as they would have increased ductility by softening the metal through hot rolling, and increased EC% through the long overage process. We understood that over-aging would drastically reduce strength, but since our 7xxx series was known for having high strength, we were okay with giving up some strength for the other two categories, as the competition treats each category equally. Because we did not overage for long enough with an accurate temperature, we lost out on these properties, making the two T7 samples we made not competition-worthy.

Our biggest weakness with our competition preparation came down to the heat-treatment we chose. We did not have a good basis for how long to overage, which left our metal losing out on EC%, as well as ductility. The competition sample we ended up choosing was cold-worked following a T6 recipe. In the end, this choice gave us peak strength at the expense of poor ductility and EC%. Because we did not have proper over-aging recipes, we had to rely on our mechanical processing, which we knew more about going into this competition.

Moving forward, we recommend researching more about heat treatment and overage recipes for our given alloy, in hopes that we can properly achieve a T7 temper rather than having to rely solely on our knowledge of mechanical processes like hot and cold rolling.