Aluminum Alloy Design Competition Team 6

Two samples of a 2036-T4 aluminum alloy were cast and thermo-mechanically processed for a university class competition where the goal was to make an alloy with the highest combination of yield strength, percent elongation, and electrical conductivity.

Raw Materials

1. 99.9% Commercially pure aluminum pieces and shots
2. 99.9% Cu shots
3. 50% Mg - 50% Al magnesium-aluminum alloy
4. 60% Mn - 40% Al manganese-aluminum alloy

Machinery

1. Custom billet steel cast
2. Ceramic crucible
3. Induction furnace
4. IMR furnaces
5. Custom rolling mill
6. Buehler Simplimet 4000 Hot Mounting Press
7. Bakelite mounting powder
8. Belt sanders
9. Polishing wheels
10. 6 μm diamond paste
11. 0.05 μm colloidal silica with water as lubricant.
12. Olympus inverted metallographic microscopes with Pax-cam 3 cameras
13. Rockwell Hardness testing machine
14. Tensile testing machine
15. Digital caliper
16. Digital scale

Programs

1. Granta EduPack
2. Microsoft Excel

For this competition, the goal was to design an aluminum alloy with the highest combination of yield strength, percent elongation, and electrical conductivity - scored multiplicatively by relative percent in each category to the class's maximum score in that category. To clarify, if an alloy has the highest percent elongation in the class, it would score 100% in that category. If the alloy has a yield strength of 200 MPa and the highest in the class is 500 MPa, it would score a 40% there.

Using the Granta EduPack metals database, dozens of aluminum alloys were compared. A 2036 alloy with a T4 treatment process was chosen for its excellent elongation and high conductivity and relative minimal yield strength loss. The alloy's minimal additive elements will ease difficulties and risks with cracking and related defects during casting and processing.

Alloy selection process focused on balancing the yield strength, elongation and electrical conductivity properties. In the selection, 7xxx and 6xxx series alloys were considered, but casting issues and poor strength performance disqualified them. The final alloy chosen was 2036-T4 due to its ability to proficiently balance the desired properties. According to the Granta analysis, this alloy lacked in yield strength, which pushed the group to utilize thermomechanical processes to bring performance up to standard.

T4 (solution heat treating and natural aging) thermomechanical processing is especially helpful to our alloy, as it helps increase strength while preserving elongation and electrical conductivity performance. After casting, the group would move forward with homogenization, natural aging, hot rolling, and solution heat treatments.

Two 2036 Al alloy mixtures, one with 1000 grams and the other 750 grams were prepared to be cast into a billet. Each mixture's compositions were kept primarily consistent, as seen in the tables above. The second mixture changed the Mg weight percentage from 0.5% to 0.58% taken from the Al percentage. The Al, Cu, Mg, and Mn were measured out with the highly reactive Mg wrapped tightly in aluminum foil. The pure aluminum was melted in a ceramic crucible placed in an induction furnace, then the additive elements were added while the molten solution was consistently stirred. When both mixtures were created some additive elements were spilled. The additives were recovered as best as possible, but the real compositions of each mixture were likely altered. The molten solutions were poured into steel billet casts which had been preheated in a furnace. Once the casted alloys were sufficiently air cooled they were removed from the casts and inspected for defects. Both mixtures fully filled the molds; however, the first cast had a prominent vertical line down its center that resulted from the molten metal solidifying on the mold wall and feeding from the center of the plate. Both casts also exhibited hairline cracks from hot or cold tearing seen in the image above.

The two castings were homogenized before further processing. The homogenization step takes place at an elevated temperature to increase diffusion and make the alloying elements redistribute evenly throughout the material. This provides a homogeneous material for further processing and consistent samples. The two castings were homogenized at 460°C for the longest possible time, 144 hours, to make the most homogenous material possible. This long homogenization time reduced the dendritic structure from casting, redistributed precipitates, and relieved internal strain.

The castings were reduced to their final thickness with hot rolling. Cold rolling was considered but rejected because the added dislocations would provide nucleation points that would result in an inhomogeneous dispersion of precipitates during the aging process. Hot rolling was done at 450°C and the castings were reduced by 1 mm per pass to the target thickness of 2.5 mm. The castings were cut in half during rolling due to their size, doubling the number of samples.

2036 has the best properties for the competition with a T4 temper. The T4 temper uses solution heat treating and natural aging. The solution heat treatment step dissolves the alloying elements and locks them in place with rapid quenching. Five samples from each casting were made and used. Solution heat treatment was done at 500C for 1:30, 1:45, 2:00, 2:15, and 2:30. A range of times was selected because, while optimal properties appear at 2:00 according to industry standards, differences in composition from the standard 2036 alloy might change this time. During natural aging, elements dissolved during solution heat treatment precipitate out. These precipitates increase the strength of the alloy. The samples were naturally aged for the longest possible time for the maximum effect.

Each sample was was tested for hardness and electrical conductivity following homogenization and T4 processing. Hardness testing used the Rockwell Hardness B scale and indicated which sample would have the highest yield strength. Electrical conductivity was one of the properties being maximized and also indicated the relative ductility of the samples. The testing served to analyze each sample's performance by solution heat-treatment time length. Electrical conductivity was very consistent between the samples and two stood out with the highest hardness measurements. These were tensile tested which verified the hardness testing results. Sample 2.2.1 was selected for the competition because it had the highest yield strength.

Metallography and microscopy were performed after casting, homogenization, and T4 processing to examine how the microstructure changed. Standard metallographic procedures were used to grind, polish, and etch samples of the alloy. Microscopic images of the castings were taken after casting and before homogenization or processing at 100x, 200x, 500x, and 1000x magnifications. More images were taken of the casting after homogenization at 200x and 1000x magnification. A final set of images was taken after all processing at 200x and 1000x magnification.

A tensile bar from the 2.2.1 Sample which had been solution heat treated for 2 hours was chosen for the competition due to its highest results in elongation, electrical conduction, and yield strength. These were all lower than the listed property ranges in the Granta software. The elongation was especially lower with the highest measured at 14.7% EL almost half the listed 21.2%-24.8% EL range due to oxide bifilms.

The alloy placed 4th overall in the 8-group competition. All of the relevant properties scored below expectations from previous tensile testing. The alloy's percent elongation was the highest total at 12.4% EL, scoring 100% in that category. Electrical conductivity remained relatively high at 80% of the maximum conductivity achieved. Yield strength underperformed, scoring 37% and largely limiting overall performance in the competition.

After completing the competition, the sample results were not what was initially expected when determining the composition of the alloy. As seen in the image, the microstructure exhibits bifilms and microporosity. These two characteristics can lead to detrimental defects in an alloy's properties. For example, this alloy's strength was lower than intended due to the internal cracks and small voids introduced by the bifilms and microporosity. The alloy's ductility also suffered due to microporosity. It's also worth noting that the variability in the data from the test competition sample and the actual competition sample was due to an increase in variability from the bifilms. Overall, the alloy's properties were slightly lower than what was initially tested, and the mechanical properties were lacking compared to what was initially intended.

To avoid these defects and improve the alloy, more care should have been taken during the pouring and casting process. Decreasing the turbulence during the pour would minimize the chance of trapping air, which creates bifilms. Additionally, using a proper/better mold design could provide adequate venting and gating systems, decreasing the chance of air entrapment and bifilms. There were inconsistencies in controlled temperature during the melting process due to a furnace malfunction. This could have led to unintended cooling and solidification, which can also trap air and create bifilms. By maintaining the proper melting temperature, decreasing turbulence during the pour, and using a better mold design, the alloy's properties could have been significantly improved.