How to Whip Up a Fresh Batch of 6022-T6 Aluminum

Bored and have a bunch of money laying around you need to burn? Want to learn first-hand the processes involved with creating an aluminum alloy? We have the project for you! This Instructable details the full process in creating AA 6022-T6 (aluminum alloy, 6000 series, T6 temper). It also provides insight on how to see the materials science working behind the scenes, from metallography to information on why certain processes and alloying elements are chosen. Hope you enjoy!

-Rolling Mill 

-Sand 

-Casting Die 

-Furnace 

-Induction Melter 

-Crucible and Tongs 

-melt scraper/spatula 

-analytical balance 

-Calipers 

-Metallography Lab 

-PPE 

-Various Base Alloys (Listed in Step 1) 

-Aluminum foil 

-Blow Torch 

-Boron Nitride Spray 

-Tensile Tester

-Macro Hardness Tester

-Electrical Conductivity Tester

To begin, look at the table to determine the percentage of each element to create the alloy. Each alloy has certain properties that effect yield strength, elongation, and electrical conductivity as follows:

Si: increases hardness and elongation

Cu: increases strength and hardness

Mg: increase strength

Mn: increases yield and Ultimate tensile strength

Cr: Improves ductility when mixed with Mn

Fe: Keep low- too much drastically decreases ductility and strength, as well as defects in the cast

Ti: increases strength

Zn: Enhanced properties when mixed with Mg, increased strength in small amounts

Safety is extremely important when working with molten metal! Aluminum melts at 660 °C, which is very hot. If it splashes on your skin, you will be gravely injured. For your protection, you will need a fireproof aluminized coat, gloves, and shoe coverings, as well as a face shield, as shown in the picture (missing gloves). These should be worn at all times when you are standing near the melter or handling the crucible.

Before you start casting, weigh out the elements to add. Some of them will vaporize slightly in the melt, so don’t worry about being extremely precise. Within a percent is good enough.

Special care must be taken when adding the zinc and magnesium. These elements are very reactive, and they will catch fire on contact with air if they get too hot. You are going to wrap them in aluminum foil and push them under the liquid to make sure they are completely submerged before they start melting. After you have weighed out the zinc and magnesium, tear off and weigh 6 sheets of aluminum foil, each about the size of printer paper. Wrap the zinc and magnesium in three sheets of foil, being sure to squeeze them tight so there aren’t any air pockets. Be sure to account for the weight of the aluminum foil in the total mass of aluminum you add to the crucible.

Place the aluminum in the induction melter. Once it begins to melt, add the other alloying elements, except for zinc and magnesium. You can do this safely by putting the elements in a cup and pouring it into the crucible.

Wait for the entire crucible to melt, and then add the zinc and magnesium. One at a time, add the foil-wrapped bundles and push them under the melt. Hold them there until they are completely melted. After everything is added and melted, stir the melt with the spatula.

Place the mold in a bucket full of sand. Before you pour, preheat the mold by blasting it with the blowtorch for about 30 seconds. This is to make sure there is no water vapor condensed on the side, which would rapidly boil on contact with the hot aluminum.

Now you’re ready to pour. Using the proper PPE, pick up the crucible with the tongs. Be sure not to squeeze the crucible, as it is fragile, just lift it by the rim. Use your dominant hand to support the weight and your off hand to grip the back of the tongs. Carefully pour the liquid into the mold until it is completely full – it’s ok if it overflows some. When the mold is full, dump the remaining melt into the sand.

Wait 10 minutes for the aluminum to cool, then remove it from the mold. At this point it is solid but still very hot, so you can put it in water to cool it further. Use a saw to cut off any bits that are sticking out, like places where the mold overflowed.

i. Homogenization: Place the samples into a furnace heated to 510°C for 16 hours. Allow the samples to furnace cool. During casting, precipitates will bias to group together, so homogenization will make these precipitates diffuse more evenly throughout the alloy. The chosen temperature and time are to allow precipitates enough time to thoroughly diffuse (Fick’s Law). Save a small homogenized sample for a further step.

ii.     Hot Rolling: Prepare a rolling mill with an inline furnace at 500°C. Spray the rollers with the boron nitride spray to prevent the sample from sticking during rolling. Hot rolling allows for large reductions, with the growth of new equiaxed grains through recrystallization, with the tradeoff being surface finish and reduction accuracy. The target thickness is 2-3 mm, so consider how much cold rolling (increases yield strength, decreases elongation through formation of directional grains and large amounts of dislocations that are high energy) you want to do to your samples in the next step. We did 4 different hot reductions: 50%, 60% 66% and 75% (relative to the original 12 mm sample). You can do around 1.7 mm per pass. Save small hot rolled samples for a future step. Be sure to label samples to keep track of them.

iii.     Solution Heat Treating: In order to achieve a T6 temper (peak temper), heat a furnace up to 535°C, and prepare a large, metal bucket of room temperature water. The amount of time the samples are to be left in the furnace depends on their thickness. Though, your samples are most likely around 3-12 mm thick, so 50 minutes to an hour is an appropriate amount of time. Once they have been in for a sufficient amount of time, remove the samples wearing gloves and using tongs one at a time, quickly quenching them in the water upon removal. Be sure to swirl the samples around to achieve quicker and equal cooling. This causes the alloy to form a solid solution, and then quickly “trapping” it in the quench. This is built upon by the next part. Ensure the samples are still well labeled.

iv.     Artificial Aging: The 2^nd step in achieving a T6 temper involves artificial aging. Heat a furnace to 175°C, and place the samples in the furnace for 8 hours. Allow them to furnace cool. Thanks to the solution heat treating, this strengthens the alloy by formation of hardening precipitates, in which the size of them effects the mechanical properties. This is tuned by the temper designation, in which T6 is the hardest. Note that any time between this step and the previous is called natural aging, though small amounts of time will not have immense impacts on the alloy. Save a small sample for a future step. Ensure the samples are still well labeled.

v.   Cold Rolling: Prepare a rolling mill, and spray the rollers with the boron nitride spray to prevent the sample from sticking during rolling. Unlike hot rolling, cold rolling requires small reductions, in which it forms directional grains with high dislocation density. These high energy defects increase the yield strength at the cost of elongation. At this point, you will need to roll the samples to somewhere between 2-3 mm doing about 0.1 mm per pass. Apply more boron nitride as needed, and ensure your samples are still labeled well. Save a small sample for a later step, and set aside sections for tensile bars (about 4.125 in long sections) and sections for hardness/electrical testing

Metallography

A fully stocked metallography lab is required for this step along with proper training in the use of its equipment.

Mounting:

Section the remaining samples from initial casting, homogenization, and thermomechanical processing.

Divide, label, and mount the sectioned samples in preparation for metallography (Table 1 details the mounting parameters to be used). Label the mounts with a carbide tipped engraver before chamfering the grips to aid with later polishing and grind the surface of the mount planar using a 120 grit SiC metallographic belt sander.

Grinding:

Rough grind the samples by hand, rinsing and rotating 90 degrees for each successive grit paper (Table 2 details the grinding method to be used).

Polishing:

Polish the mounts by hand, rotating 90 degrees every 30 seconds for 6- and 3-micron steps (Table 3 details the polishing method to be used). Rinse the samples thoroughly with water then ethanol (use a cotton ball for colloidal silica) before drying under a blow dryer. Inspect each sample before moving on, the scratch patterns should be uniform in size when viewed at 50/100X magnification for the e6 and 3-micron steps. Scratches should be mostly absent from the sample after final polishing when viewed at 100/200X magnification.

Etching:

SAFETY NOTICE: Etching aluminum requires Hydrofluoric Acid. Only proceed if you have been properly trained to handle HF and always etch with a partner, proper PPE, and a fume hood.

The finished samples (set aside the as-cast samples as they do not require etching to see their grain structure) should be etched using the Papageorge 2-step if possible. Keller’s reagent also works but not nearly as well. Please contact Papageorge.2@osu.edu for assistance with this etching technique as it is not a standardized ASTM etching procedure yet.

Optical Microscopy:

Image samples using an optical microscope at 100X, 200X, and 500X magnification with scale bars. Save micrographs as TIF’s.

Generally, more cold rolling will lead to an increase in hardness but a decrease in conductivity. Using these tests is a good benchmark to see if you did the Thermomechanical Processing step correctly. 

• Carefully cut a small piece out of each bar that needs to be tested.   
• Load each piece of aluminum into the hardness tester and test using the HRB scale.
• For electrical conductivity, first clean the surface of each piece using some sandpaper and alcohol. Then apply the electrode of the electrical conductivity tester to the area of the piece that was sanded and wait for a stable value.  

After all thermomechanical processes were performed, sections of the samples were cut and submitted for tensile bar machining. There should be 3 tensile bars machined and their widths and thicknesses measured. For each bar, place the bar in the tensile tester and input the specific width and thickness measurements. Tighten the vices around the bar and attach the elongation sensor to the center section of the bar. The test will run automatically and produce measurements for yield strength and elongation.  

Using the Granta software, expected values for the different properties were found as; 

• Yield Strength of ~275 – 303 MPa 
• Elongation 10-12% 
• Electrical Conductivity 53 – 55% IACS 

Due to thermomechanical processing, it was expected that the elongation and electrical conductivity would be lower than the original values and that yield strength would increase substantially.