Casting and Processing a 7xxx Series Aluminum Alloy | Aluminum Alloy Design Contest 2024

The Aluminum Alloy Design Contest is a materials engineering competition that challenges teams to design, cast and thermomechanically process an aluminum alloy from scratch. The properties of the final product - particularly yield strength, ductility and electrical conductivity - are then put to the test.

Rules of the Game

Teams had the liberty of making any aluminum alloy between 1xxx series and 7xxx series, with any tempering procedure that available equipment allows for. The design limitations are as follows:

1. The alloy must be at least 90% aluminum
2. The material must be rolled to a final thickness of 2-3 mm so it can be machined into tensile bars

The scoring categories are strength, ductility and electrical conductivity; each category is worth up to 100 points. Final scores will the product of the points in each category and normalized amongst teams. It is best to aim for a balance in all three categories to achieve the most points.

We're The Honey Badgers, and we chose to make an alloy similar to commercial AA7085-T7452. This Instructable will present the research, design and production process of our alloy.

Base metals/alloys

1. 99.9% Cu
2. 99.9% Zn
3. 50% Si – 50% Al
4. 50% Mg – 50% Al
5. 10% Fe – 90% Al
6. 6% Ti – 94% Al
7. 20% Cr – 80% Al
8. 60% Mn – 40% Al
9. Aluminum foil

Casting

1. Induction furnace
2. Permanent rectangular billet mold

Thermomechanical Processing

1. IRM 2-high rolling mill
2. IRM in-line push thru pre-heating furnace
3. Electric box furnace

Metallography

1. Buehler SimpliMet 4000 Mounting System – for mounting sample
2. Bakelite (phenolic thermosetting powder) - mounting sample
3. Beuhler AutoMet 2000 Power Head - automatic grinding and polishing.
4. Sandpaper and Pan-W polishing cloth
5. LECO microid diamond compound extender - polishing
6. LECO 0.05 μm colloidal silica suspension – final polishing steps

Microscopy

1. Olympus GX53 bright field optical microscope
2. PAXcam microscope camera and imaging software

Testing

1. Wilson Hardness Rockwell 574 Twin - hardness testing
2. Electrical conductivity meter

Standards and Procedures

1. Mounting, grinding and polishing – ASTM E3-11 (2017): Standard Guide for Preparation of Metallographic Specimens
2. Etching – Two-step etching procedure by Wayne Papageorge
3. Hardness testing – ASTM E18-22: Standard Test Methods for Rockwell Hardness of Metallic Materials
4. Tensile testing - ASTM B557-15: Standard Test Methods for Tension Testing Wrought and Cast Aluminum- and Magnesium Alloy Products

We began by researching the properties of various commercial aluminum alloys to determine which one to base our custom alloy on.

We used Ansys Granta, a massive materials database, to generate graphical comparisons between the properties of various aluminum alloy series.

7xxx series alloys (highlighted purple in the graphs above) stood out because they are generally higher in yield strength than the 1xxx-6xxx series alloys (highlighted grey). We scoured this group to find one alloy that appeared well-balanced in terms of strength, electrical conductivity and ductility – which, for us, was 7085-T7452 (highlighted red).

Composition decisions:

In selecting the composition, we followed AA7085-T7452 closely (see commercial alloy content table above), but reduced the Cu and Mg compositions to be 1% each to ensure we were above 90% Al content and curb concerns of embrittlement.

We also added small-fraction alloying components, including Fe, Mn, Cr and Ti, upon the speculation that they would contribute to an important precipitate phase. Ironically, as we will see, this led to undesirable, embrittling effects...

Next, we developed a plan for thermomechanical processing (TMP). The following is a list of the general processing steps we wanted to utilize:

Homogenization: we planned to leave our cast alloy in the oven for 96 hours to ensure sufficient time for diffusion to provide a uniform structure.

Hot rolling: We aimed to reduce the thickness of our material to competition requirements. We did not want to risk cracking in our material, as it is 7xxx series with a high alloy content, so no cold rolling was added to the plan.

Solution heat treatment (SHT): Solution heat treatment increases the vacancy concentration in the alloy and quenching locks those vacancies into place. Without the quenching step the vacancies would annihilate; near instant cooling removes the ability for them to diffuse. Vacancies are an important precursor to precipitation hardening.

Eventually, we decided to try two different solution heat treatment temperatures - so we ended up with samples of two different temper types (475 °C for 1 hr, 500 °C for 1 hr) to test out, as indicated in the flowchart.

Precipitation hardening/artificial aging: This facilitates the diffusion of second-phase particles into precipitates in the microstructure, which increase the hardness and yield strength of the material.

The temper designation of our model alloy is a T7, indicating over-aging. This is aging long enough to surpass the point of peak hardness and re-introduce ductility into the material.

Preparing alloy components

The desired alloying components for AA7085 were not available to us in pure form, but instead alloyed with Al. The mass of aluminum in the base alloys, along with any aluminum foil used, must be accounted for in the desired total mass of aluminum.

Each base alloy was weighed to achieve the desired elemental composition (target masses are outlined in the above table). We determined the mass of Al from the base alloys and subtracted this from the total mass of aluminum needed to reach the desired Al mass. The remaining mass would be fulfilled by (roughly) pure Al shot or foil.

We aimed for: 1000g mass of all components

Actual total Al weight = 908.14 g

Actual alloying element weight = 91.84 g

Total mass available to cast = 999.98 g

Note: Zn and Mg samples must be wrapped tightly in aluminum foil to prevent direct contact with liquid aluminum, as both elements react strongly with oxygen at high temperatures. Proceed with caution when alloying with reactive alloys.

We heated a crucible in the induction furnace, and preheat the billet bold in an electric furnace to reduce shrinkage and defects in the casting.

The base alloys were added to the crucible, with the Mg and Zn being the last components to stir into the melt. The molten metal was then poured into the billet mold until overflowing. Excess was added to an ingot mold.

Always wear heat-protective PPE and a face shield while casting.

Casting results

After the casting is cooled down and removed from the mold, it is a good idea to inspect your casting for defects. We had some Zn segregation at the surface, cracking near the top of the billet, and insignificant shrinkage. Areas with cracks are not suitable for producing tensile bars, so these regions were sawed off before proceeding with TMP.

We saved a piece of our cast billet to use for microscopy. Metallography and microscopy procedures are outlined in a later step.

The billet was placed in an electric box furnace for 96 hours at 475 °C for homogenization, then air-cooled at room temperature. This helps to attain uniformity of grain size and composition.

The sample was hot rolled at 475°C to a thickness of 3 mm using an IRM two-high rolling mill.

1. Initial roller height was set just low enough so that the billet could not pass through the line.
2. Roller height was adjusted (lowered) by 1 mm after each pass until the desired thickness was achieved.

Some large cracks appeared on the edge and longitudinal direction of the sample. Although the visible cracks in the casting were cut off, there could have been some cracks still present in the sample that expanded and propagated during rolling.

Cracked sections of the rolled samples were sawed off - this resulted in two defective test pieces. Four rolled plates remained that were suitable for machining into tensile bars.

Using an electric box furnace, two different solution heat treatments were completed:

1. Three plates at 475°C for 1 hour
2. Three plates at 500°C for 1 hour

(one plate of each of the above tempers was a test plate).

After heating, the samples were removed from the furnace with tongs and immediately submerged in a large bin of room-temperature water. SHT resulted in bubbles on the surface of each sample, probably due to gas expansion inside of unclosed pores from the original casting.

When aging our samples, we referred to the aluminum alloy heat treatment guidelines published by ASM, that is, for 7050-T7 aluminum alloy plates, samples were aged at 107°C for 6 hours and then at 163°C for another 24 hours. This procedure was chosen because the alloy series, temper designation and sample geometry were quite similar to those of our alloy.

As we processed our alloy, we periodically tested its electrical conductivity (%IACS) and hardness (HRB).

Electrical conductivity testing was performed using a metal electrical conductivity meter, which functions simply by contact between a handheld probe and the surface of the material being tested. A Wilson Hardness tester was used to collect hardness data in accordance with ASTM E18-22.

Electrical conductivity improved steadily as we proceeded through the TMP schedule, reaching an average of roughly 41%IACS in both sample types post-aging. In fact, we didn't see any significant difference in properties between samples of different SHT types.

The hardness decreased significantly between homogenization and hot rolling, even dipping below the lower limit of the HRB range. Hardness increased to about 68 HRB in both samples after SHT.

Cuttings from the as-cast sample and the two final tempered samples were mounted in Bakelite powder using the Buehler SimpliMet mounting press.

The Buehler AutoMet was used to take samples through the grinding and initial polishing procedure in accordance with ASTM E3-11. The final polishing step was completed on a polishing wheel with a 0.05 μm colloidal silica suspension.

After polishing, the two samples in their final tempered condition were etched using the Papageorge two-step etching procedure for AA7075-T6, requiring Keller's reagent and Weck's reagent. Etching is a form of controlled corrosion that acts at the grain boundaries and darkens them for microscopy. Always wear gloves and safety goggles when working with etchants, as they are very skin-corrosive!

An Olympus bright field optical microscope was used in combination with PAXcam camera and software to view/capture micrographs of the alloy in its as-cast and final tempered conditions.

The above image shows the final samples' microstructure under 100x and 500x magnification. Both temper conditions exhibited long black lines of precipitates along the rolling direction, visible in the In 100x magnification image (Figure 1a) as well as the 500x magnification image (Figure 1b).

Micrographs of the as-cast condition were also captured. In 100x and 1000x micrographs, dark, spatially dispersed streaks are visible between light dendrites.

Analysis of Microstructure

Since Fe and Mn have low solubility in aluminum alloy systems, they are most likely the components of the dark second phase. Regions of this phase got stretched out during hot rolling, but because they are so insoluble in aluminum, remain in place even at recrystallization temperature. They are essentially pinned in place, reducing the strength and % elongation of the material.

Otherwise, there are some regions of nice, large, fairly equiaxed grains that will be much more ductile.

Four sample plates were used to cut tensile bars - two from the SHT at 475°C, and the other two from the SHT at 500°C. We performed preliminary tensile tests on some tensile bars to determine which would be ideal for competition day.

Each plate was labeled with its SHT temperature and a unique symbol or shape. The tensile bars were labeled with this information after being cut to keep track of which plates they were from.

Bars underwent testing on a MTS Criterion 43 electromechanical tensile testing system in accordance with ASTM B557-15. Paired with an extensometer, this system allowed us to collect preliminary yield strength and elongation data. The average yield strength of the seven bars tested was 278.1 MPa. The average ductility was 6.4%.

The fracture surfaces of most bars had large layered, lateral cracks, which may have reduced yield strength and ductility. A tensile bar cut from the sample labeled "475 Circle" was chosen for the final testing, as the tested bars from that sample showed high yield strength and moderate percent elongation compared to the other samples.

Our tensile bar was the last to be tested in the competition. The tensile bar selected recorded an electrical conductivity of 41.87%IACS, a yield strength of 270.8 MPa, and an elongation at fracture of 7.1%.

Compared to the best team samples in each parameter, our sample scored 91 on electrical conductivity, 63 on yield strength, and 57 on ductility. We were thrilled by the performance of our bar, particularly its excellent conductivity. It also exceeded our expectations for ductility.

The fracture surface of the competition bar had fewer voids and cracks. This is a possible reason for the nicer ductile fracture in this bar compared to our preliminary bars.

We made second place with a grand total score of 330,518. The first-place winners had a score of 373,765, and in third place, they scored 324,046.