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Subject on This Issue:
* Steels & Properties
* Heat Treatment


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Polishing Annealed Aluminum Sample

Hand polishing of annealed aluminum sample for metallographic examination is a skillful operation, often accompanying with scratches and embedding of abrasive particles on the polished surface due to its soft characteristic. This makes polishing of annealed aluminum sample very time-consuming. An experiment was carried out to improve the quality of the polished aluminium specimens to a satisfactory level by eliminating the presence of abrasive particles on the polished surface.

Experimental

Alumina slurry is normally used for the polishing process. This alumina slurry comes in many different grades of abrasion. 1.0 micro and 0.5 micro alumina suspensions were used for this study. For this experiment, a finer polishing base was also provided - 0.1-micron alumina slurry. Since this base is made up of finer particles, a better surface finish can be expected.

Six aluminum specimens have been prepared for this experiment (Specimen 1, 2, 3, 4, 5 and 6). Each ground using standard abrasive papers from grit size 120, 240, 600, 800 to 1200 accordingly. This procedure provides a relatively smooth and flat specimen surface before polishing.

Every specimen was polished continuously for 120 sec. Therefore, odd numbered specimens were each rotated 120 revolutions and even numbered specimens were each rotated 240 revolutions due to the different specimen rotation speeds.

Once polished, the specimen surfaces were inspected under the microscope for defects (scratches and inclusions). Subsequently, they were each etched using an aluminium etching solution (95ml distilled water, 1.5ml HCL, 2.5ml HNO3 and 0.5ml HF 48%) for approximately 4 - 5 min. After etching, the solution is washed off with warm water and dried. Their microstructures were then inspected under the microscope. Photos of the surface and microstructure from the best-polished specimen were taken.

Results

Results have shown positive improvement in the quality of the polished surface. Photos of the polished surface and microstructure are taken. As there are few photos available for each polished specimen, only two photos indicating good quality of a sample are displayed in Figs.1 and 2.

Polishing with 1.0 micro slurry could achieve smooth surfaces, with very few scratches. However, the inclusions on the surface are relatively large and vast. On the other hand, specimens polished using 0.5 micro slurry possess clean surfaces, whereby the inclusions are small and sparse. But numerous deep scratches are also found on the surface.

Specimens polished using 1.0 micro slurry followed by 0.1 micro slurry possess a good surface finish with very few scratches and inclusions. This is because the smaller particles are unable to penetrate deep into the specimen and scratches. They are only able to work on the very surface of the specimen. It would also take a much longer time of polishing in order for it to penetrate to the scratches. With this in mind, both grades of alumina slurry could be well incorporated together to produce a very good surface finish. Specimens could be polished by means of the 1.0-micron alumina slurry first to remove scratches and 0.1-micron alumina slurry then follows. This is done to clean and remove inclusions from the surface.

al1
Fig.1 Polished surface using 0.5 micro slurry

al2
Fig.2 Etched sample (Microstructure)

 
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