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Metallographic techniques for preparing Al-Mg materials

Mounting

Mount in epoxy using molds with 1.25 in. diameter.

Rough grinding

Step 1

• Carbide paper: 120 grit
• Machine: Buehler DATAMET
• Lubricant: water
• Speed: 150 rpm
• Force: approximately 5to 8 lbs/sample
• Time: from 60 to 180 seconds

Step 2

• Carbide paper: 320 grit
• Machine: Buehler DATAMET
• Lubricant: water
• Speed: 150 rpm
• Force: approximately 5 to 8 lbs/sample
• Time: from 45 to 90 seconds

Step 3

• Carbide paper: 600 grit
• Machine: Buehler DATAMET
• Lubricant: water
• Speed: 150 rpm
• Force: approximately 4 to 6 lbs/sample
• Time: from 60 to 120 seconds

Polishing

Step 4

• Abrasive: 15 µm diamond paste
• Lubricant: oil
• Cloth: MEG-068 nylon polishing cloth
• Machine: LECO by hand
• Speed: 200 to 300 rpm
• Force: approximately 5 lbs
• Time: 1 to 3 min.

Step 5

• Abrasive: 6 µm diamond paste
• Lubricant: oil
• Cloth: MEG-068 nylon polishing cloth
• Machine: LECO by hand
• Speed: 200 to 300 rpm
• Force: approximately 5 lbs
• Time: 1 to 3 min.

Step 6

• Abrasive: 1 µm diamond suspension spray
• Lubricant: deionized water
• Cloth: MEG-218 final polishing cloth
• Machine: LECO by hand or mechanical
• Speed: 300 rpm
• Force: approximately 2 lbs/sample
• Time: 3 to 8 min.

Final polish

Step 7

• Abrasive: colloidal silica solution (LECO no. 812-110-300)
• Lubricant: deionized water
• Cloth: MEG-218 final polishing cloth
• Machine: LECO or Buehler VIBRAMET (preferred)
• Speed: mid-range
• Force: no added weights necessary
• Time: 3 to 8 min. on LECO and 10 to 30 min. on VIBRAMET

Notes

Between each step, the samples are cleaned with Simple Green detergent and deionized water then dried. Ultrasonic cleaning followed by rinsing in deionized water is used before and after each polishing step using diamond abrasives. Before polishing with colloidal silica, the samples are always washed in the ultrasonic bath and rinsed in deionized water.

Etching

• Etching solution: Barker’s reagent (1 part tetrafluoroboric acid in 40 parts deionized water)
• Voltage: 20 Volts
• Etching time : from 20 to 70 seconds, depending on alloy

View using an optical microscope with crossed polarizing filters.

Polishing of overetched samples

Prepare for repolishing

• Abrasive: 45 µm diamond paste (alternatively can use 600-grit carbide paper)
• Lubricant: oil
• Cloth: MEG-068 nylon polishing cloth
• Machine: LECO by hand
• Speed: 200 to 300 rpm
• Force: approximately 5 lbs
• Time: 1 to 3 min.

Repolishing

Then 15 µm, 6 µm, 1 µm, and colloidal silica, as before.