Team Molten Monsters Aluminium Alloy Design Competition - Al 7005-T6

A group of 6 Material Science students at the Ohio State University were tasked with designing an aluminum alloy sample for their 3331 Material Science and Engineering Lab 1.

The competition had one major requirement: the aluminum alloy sample had to consist of at least 90 percent aluminum. After doing some research using Granta software to find alloys that had the best combination of yield strength, elongation percentage, and conductivity, the Team ended up producing a billet of an aluminum alloy sample with a similar blend to Al 7005.

We designed a custom aluminum alloy for the MSE 3331 Aluminum Alloy Design Competition at The Ohio State University. Our team, the "Molten Monsters," aimed to create a material that maximized three competing properties: elongation, electrical conductivity, and yield strength.

We had access to all machines and supplies located in Fontana Lab 1135 to aid in the creation of our alloy.

The Design Philosophy:

We initially selected 7020-T5 as our baseline because it offers a consistently high balance of our target properties compared to other standard aluminum alloys. 7xxx series alloys (Al-Zn-Mg) are known for high strength, but we needed to carefully balance this with ductility. Our specific design constraints included maintaining an aluminum concentration above 90 wt% and avoiding the use of Zirconium (Zr). Titanium (Ti) was briefly considered, but we were recommended away from its use due to sample quality.

While our design began as an approximation of Al 7020 intended for a T5 temper (cooled from shaping + artificially aged), our final composition and processing realities aligned more closely with Al 7005 treated to a T6 temper (solution heat treated + quenched + artificially aged). This alloy is renowned for its weldability and stress corrosion resistance without requiring the extreme quench sensitivity of alloys like 7075. It also maintains the relatively high yield strength, characteristic of 7xxx alloys compared to other series, while maintaining a higher ductility than other 7xxx alloys.

Our Recipe:

To achieve precipitate hardening while maintaining conductivity and corrosion resistance, we utilized:

1. Zinc (Zn) & Magnesium (Mg): The primary strengthening precipitates.
2. Manganese (Mn): For yield strength and corrosion resistance.
3. Copper (Cu): A small addition (0.05 wt%) to promote precipitate hardening.
4. Chromium (Cr): Controlled (0.1 wt%) to manage grain size without destroying electrical conductivity.

Processing Route:

Casting - Homogenization - Hot Rolling - Solution Heat Treatment - Water Quench - Two-Stage Artificial Aging (T6).

Recipe Recap:

To achieve our target composition of 94.5% Aluminum, 4% Zinc, and 1.1% Magnesium, we calculated a charge using the following master alloys:

1. Base: Commercially Pure (CP) Aluminum shot/pieces
2. Solute Sources:
3. 99.9% Pure Zn shot
4. 99.9% Pure Cu shot
5. 50% Al - 50% Si master alloy
6. 50% Mg - 50% Al master alloy
7. 20% Cr - Al master alloy
8. 60% Mn - 40% Al master alloy

Starting Microstructure:

Upon solidification, the alloy exhibits a classic "as-cast" structure characterized by:

1. Dendritic Structure: Tree-like arms of aluminum growing from the mold walls.
2. Microsegregation (Coring): The alloying elements (Zn, Mg, Cu) are not evenly spread; they are concentrated in the interdendritic regions (the spaces between the "branches").
3. Non-Equilibrium Eutectic Phases: Brittle phases formed at grain boundaries where the solute concentration is highest.

Why This Matters:

The as-cast structure is chemically inconsistent. The center of a grain has different properties than the edge. To achieve the high performance of a T6 temper, we need a uniform distribution of atoms. If we heat treated this material immediately, the segregated regions might melt (incipient melting) or fail to harden correctly.

Process Parameters:

1. Temperature: 450 C [842 F]
2. Time: 12 Hours
3. Cooling: Furnace cool

Metallurgical Changes:

The "as-cast" structure is full of chemical peaks and valleys. At 450 C, we provide enough thermal energy for the Zinc and Magnesium atoms clustered between the dendrites to diffuse back into the aluminum matrix.

1. Diffusion: Solute atoms move from high-concentration areas (grain boundaries) to low-concentration areas (dendrite cores).
2. Dissolution: Brittle eutectic phases formed during solidification are dissolved.

Why This Matters:

Homogenization "flattens" the chemical landscape, as the process is named "homogenizing" the Solid State Solution. Without this step, the subsequent heat treatment would be uneven—some areas would be weak while others might melt. This step prepares for the T6 temper.

Process Overview:

1. Temperature: Approximately 420 C
2. Reduction: Rolled down to final gauge thickness [2–3 mm]

Metallurgical Changes:

1. Grain Refinement: The heavy deformation breaks up the coarse as-cast grains.
2. Defect Creation: Rolling introduces dislocations (line defects) into the crystal lattice. While many of these are annealed-out during the subsequent solution heat treat, they help break up any remaining constituent particles.

Why This Matters:

It closes porosity and sets the final geometry, without introducing additional internal stress.

Process Parameters:

1. Temperature: 475 C [887 F]
2. Quenchant: Room temperature water
3. Transfer Time: Rapid (<15 seconds)

Metallurgical Changes:

We heat the alloy high enough to dissolve all the Zinc and Magnesium atoms into a single, uniform solid phase (Solid Solution). By quenching rapidly in water, we trap these atoms in place, creating a Supersaturated Solid Solution. The atoms want to precipitate out, but they are "frozen" in the unstable matrix.

Why This Matters:

This establishes the potential energy required for strengthening. The faster the quench, the more solute we trap in solution, and the higher the potential strength after aging.

In an attempt to further boost Yield Strength, we isolated one sample and subjected it to a 20% Cold Rolling reduction at room temperature. This was an investigation into strain hardening (work hardening)—effectively trying to achieve a condition similar to a T8 temper (Solution Heat Treated - Cold Worked - Artificially Aged) or simply adding strain to the T6 condition.

Process Parameters:

1. Method: Room temperature rolling
2. Reduction: 20% reduction in thickness (from 2.8mm to ~2.2mm)
3. State: Performed after quenching but before artificial aging.

Metallurgical Changes:

Cold rolling introduces dislocations (line defects) into the aluminum crystals.

1. Dislocation Multiplication: The deformation forces crystal planes to slip over one another, generating billions of new dislocations.
2. Entanglement: These dislocations tangle around each other and around the precipitates that make further deformation very difficult.
3. Precipitation Sites: In a T8 scenario, these dislocations can act as nucleation sites for precipitates, potentially refining the structure further.

Why We Excluded It:

While this process almost certainly increased the Yield Strength (by making it harder for the material to deform), it was deemed sub-optimal for our specific multi-variable goals (Strength + Elongation + Conductivity) for two reasons:

1. Elongation: Cold working dramatically exhausts the material's ductility. Since we needed a balance of properties, sacrificing 20–50% of our elongation for a moderate gain in strength was a poor trade.
2. Conductivity Drop: The increased density of crystal defects (dislocations) acts as scattering centers for electrons, slightly lowering the electrical conductivity.

The sample became too brittle for the competition's objectives, sacrificing too much ductility for some additional yield strength (conductivity remained largely unchanged, despite common knowledge suggesting otherwise). We decided to rely solely on the T6 precipitation hardening mechanism rather than combining it with work hardening.

Process Parameters (7005-T6 Convention):

1. Stage 1: 6 hours at 93 C [200 F]
2. Stage 2: 4 hours at 160 C [320 F]
3. Cooling: Air cool

Metallurgical Changes:

We used a classic two-step aging practice common for 7005 alloys:

1. Nucleation (93 C): The lower temperature encourages the formation of high-density GP zones (Guinier-Preston zones). These are tiny clusters of Zn and Mg atoms that act as anchors for future precipitates.
2. Growth (160 C): The higher temperature provides energy for these zones to grow into semi-coherent precipitates. These are the main strengthening phase.

Why This Matters:

Artificial aging involves heating the supersaturated alloy to a specific intermediate temperature to provide the thermal energy needed for solute atoms (Zn, Mg in this case) to diffuse and cluster rapidly, a process that would take years at room temperature. If we went straight to 160 C, typical nucleation would be sparse, resulting in fewer, larger precipitates and lower strength. The two-stage approach gives us the best of both worlds: a high density of fine precipitates.

Preparation for Testing:

To make our metal testable, we turned the rolled sheets of our alloy into multiple tensile bars. This was done using a pneumatic floor shear to cut our sheet into bars of the correct length and width before using a Tensilkut I precision mill to machine it to the ASTM tensile test standard.

Process and Results:

The tensile bars were tested using a standard material tester to measure and record their yield strength and total elongation before fracture. We only tested four of our eight total tensile bars, leaving some T6 and T8 processed bars for the contest.

As Cast Alloy (Image 1):

1. Minimal shrinkage, implying good quality of the cast
2. Atypical for 7xxx series aluminum alloys
3. Tree-like dendritic segregation of alloying elements with lamella structures
4. Formed during cooling
5. Non-uniform -> requires homogenization

Homogenized + Hot-Rolled Alloy (Image 2):

1. Uniform microstructure achieved by homogenization
2. Reduced and elongated microstructure along the rolling direction
3. Enhances the strength of the alloy
4. Visible crack (approx 688 microns) along the rolling direction
5. Likely caused by
6. Excessive rolling force
7. Thermal stress from uneven heating

Finalized Contest-Ready Alloy (Image 3):

1. Number of precipitates in the microstructure remains mostly the same compared to Sample 2
2. Precipitates are coarse, also similar to Sample 2
3. Indicates a non-ideal heat treatment process
4. Insufficient temperature or heat treatment duration
5. The large, pith-like structure (approx 434 microns) is likely to be iron
6. Non-ideal for the applications of our aluminum alloy design
7. Reduce strength
8. Hinder recrystallization
9. Prone to corrosion

Corroded Alloy (Image 4):

1. Visible crack on the edge of the sample's surface
2. Signs of exfoliation corrosion
3. A form of Intergranular Corrosion (IGC) specific to aluminum alloys
4. Confirmed when compared to a micrograph of corroded commercial Al7075 (Image 5)

Process Description:

We weighted and measured the width and thickness of each of the commercial alloys and our team alloys before the corrosion testing. For the corrosion testing we placed the tensile bars in 0.7 mol saltwater and 25 mL of hydrogen peroxide in a glass sealed container for two weeks. Wooden rods were used to keep the tensile bars from touching one another. After the two week period we weighted and measured each tensile bar again. Comparing the weight after corrosion to the before, we determined that T6-4 corroded the least with a weight difference of 0.0177 grams. We included tables of our testing results from before and after corrosion testing along with a graph we created to display the tensile test data for our Aluminum 7005 T6 sample.

We tested a sample (T6-3) which provided a baseline alongside a second sample that failed prematurely due to an internal casting defect.

Microstructural Observation:

Upon examining the final microstructure under an optical microscope, the aluminum matrix appeared remarkably "clean." There were very few visible precipitates or dark spots at the grain boundaries.

Interpreting the Results:

Our yield strength (~107 MPa) was lower than typical commercial 7005-T6 (~290 MPa), yet our conductivity was exceptionally high (40.7% IACS). Combined with the "clean" microstructure, we have developed three hypotheses to explain the performance:

1. Where we fell short

1. The Theory: In 7xxx series metallurgy, the primary strengthening precipitates are nanometric (typically 5–10 nm). They are far too small to be resolved by standard optical microscopy.
2. The Evidence: The lack of visible coarse particles suggests we successfully avoided "overaging." If we had overaged the material, the precipitates would have clumped together into large, visible masses, killing ductility.
3. Conclusion: The high conductivity (40.7 %) confirms that the Zinc and Magnesium did not stay trapped in the aluminum lattice (which would lower conductivity). Instead, they precipitated out. The matrix is clean because the solute successfully transformed into a fine, invisible mist of particles as intended. The failure was due to random happen-stance, and the drop in yield strength in our testing samples compared to the commercial alloy was due to unforeseen and unknown ramifications in our alterations to the 7005 recipe.

2. Casting Defect

1. The Theory: The submitted sample failed at a very low stress (99 MPa) despite having high ductility (12.5 %).
2. The Evidence: Post-failure analysis revealed an internal defect running through the gauge length of the sample, corroborated by an uneven non-sheer failure surface not located in the middle of the tensile sample.
3. Conclusion: This acts as a massive stress concentrator. In more brittle materials (i.e. ceramics), this would cause brittle fracture. In our ductile aluminum, it simply reduced the effective cross-sectional area, causing the math to show a much lower "Engineering Stress" than the material might have actually been capable of handling locally. This was supported by testing done on other tensile samples prepared for preliminary testing, but does not address that even in our best result why yield strength was almost 100 MPa lower than expected.

3. Compositional Drift

1. The Theory: While the T6 heat treatment schedule was valid (indicated by the clean matrix and high conductivity), the overall strength was limited by the actual amount of Zinc available.
2. The Evidence: Zinc has a low boiling point and completely leave solution during casting, forming a powder on the surface of the ingot. While this was observed, the exact amount lost is unknown. The combination of Low Yield Strength + High Conductivity makes the metal casting and consequent processing delicate.
3. Conclusion: We likely achieved a successful T6 temper on a dilute alloy. We got the structure right, but we may have simply had less Zinc than calculated, leading to a softer, highly conductive, and very ductile final metal.

In all likelihood, it was a combination of all three. While our submitted sample failed due to a defect (2), we did have a sample that performed without obvious signs of being effected by a defect. So too much Zinc leaving solution (3) and unforeseen mechanisms due to the design liberties we took when determining the composition (1) were very likely driving factors in our resulting properties. This disparity is evident in the results of our corrosion tests, where a corroded tensile bar performed significantly better in property tests than an as-fabricated tensile bar.