Aluminium is soft enough to embed the abrasive that is preparing it and ductile enough to smear over the result. The published route starts on paper rather than a diamond disc and keeps the loads low throughout.

Why aluminium alloys are difficult

It embeds the abrasive

Loose grit pressed into a soft matrix stays there. Embedded particles read as second phase, wreck an image analysis and cannot be polished out without removing more material than the sample can spare. The low loads in this route are the defence.

It smears, and the smear looks like a finish

Aluminium moves ahead of the abrasive rather than being cut by it. The resulting layer is bright and featureless, which is exactly what a finished surface is supposed to look like — so the failure is invisible until the sample is etched or viewed under polarised light.

Silicon in a cast alloy behaves nothing like the matrix

A hypereutectic Al–Si alloy contains particles several times harder than what surrounds them. They will stand proud or be plucked out, and both outcomes are preparation artefacts that get reported as microstructure.

The published route

Akasel publishes this sequence for aluminium alloys. The figures are theirs. Times are quoted for a 300 mm preparation system and forces for a single 40 mm diameter sample.

Step	Surface	Abrasive or suspension	Speed	Force	Time
1 — plane grinding	Rhaco Grit	Water	300 rpm	20 N	Until plane
2 — fine grinding	Aka-Largan 9	DiaUltra, 9 µm	150 rpm	30 N	3:30 min
3 — diamond polishing	Aka-Moran-U	DiaUltra, 3 µm	150 rpm	25 N	2:30 min
4 — oxide polishing	Aka-Chemal	Colloidal Silica Suspension 50 nm Alkaline	150 rpm	15 N	2:00 min

Source: Akasel Aka-Brief #4, Aluminium Alloys, 2022 edition, page 1. Every figure in this table is printed on that page; none of them is ours.

Where the method and this catalogue differ

We say what the method specifies and what we list, and we do not put the nearest thing we sell in its place.

Water — no consumable — the disc runs under water.

Reading the sequence

The shape of the route says more than any single number in it. This part is our reading, not a manufacturer statement.

Step 1 is paper, not a diamond disc. Unlike the steel routes, plane grinding here runs on SiC paper at 20 N. A fixed diamond disc removes aluminium far faster than the sample can tolerate; paper is gentler and, being consumed, presents fresh grit rather than loading up with metal.

Loads are low everywhere and fall to 15 N at the end. Every number in this route is below the equivalent step for steel. Force is what drives embedding and smear in a soft metal, so the route trades speed for surface quality at every stage.

It finishes on an oxide suspension. Colloidal silica gives a chemical-mechanical action that lifts the deformed layer instead of pressing on it, which is the only reliable way to get a smear-free aluminium surface. Flush the cloth at the end of the step; dried silica on a soft matrix is worse here than anywhere else.

The same sample, after each step

Akasel photographs the same specimen after every stage of this route and publishes the images on page 2 of Aka-Brief #4. We have not reproduced them here. The titanium guide shows what that sequence looks like for a material we do hold the images for.

Scaling it to your machine

Smaller platen, longer steps. The maker’s guidance is to add roughly a third to the times on a 250 mm system and to double them on a 200 mm one. The abrasive does the same work over less swept area.
Bigger samples, more force; smaller samples, less. The quoted loads are per 40 mm mount. Scale with mounted area rather than guessing, and if your machine cannot deliver the figure, extend the step instead of exceeding it.
Head speed is fixed. 150 rpm on the sample holder throughout, constant across every published method in this series.

What the route needs

6 consumables, and the sequence does not work without them:

Where these figures come from

The preparation route is Akasel’s, published in Akasel Aka-Brief #4, Aluminium Alloys, 2022 edition, page 1. The reading of it — why the sequence is shaped this way, what goes wrong, what to watch — is ours, and it is an interpretation rather than a manufacturer statement. We have not run this method in our own laboratory and we do not claim to have. Where the method calls for something we do not list, the guide says so above rather than substituting the nearest thing we sell.

Sources

Akasel Aka-Brief #4, Aluminium Alloys, 2022 edition, page 1 — every figure in the step table.
Akasel product pages and data sheets, cited row by row on each linked product page.
The reading of the method, and everything under “Why” and “Reading the sequence”: P2 Industrial.

Preparing aluminium alloys for metallography