Aluminum Sample Preparation

Introduction

Aluminum and its alloys are relatively soft materials with a wide range of alloy applications — from soft pure 1xxx grades to high-strength precipitation-hardened 7075-T6. Common analytical targets include 6061 (the most widely used aluminum alloy), 7075 (high-strength aerospace), and cast Al-Si alloys such as A356 and A380.

The primary preparation challenge for aluminum is preserving its oxide inclusions. Aluminum's microstructure routinely contains aluminum-oxide particles introduced during casting or processing — and those particles ARE the analysis target in many quality-control jobs. Proper preparation retains them in place. Improper preparation (too much force, too coarse an initial grit, embedding-prone abrasives) pulls them out, drags them across the surface, and produces an image that misrepresents the real microstructure.

6061 aluminum alloy microstructure, properly prepared and etched. Correct preparation reveals the true grain structure without introducing artifacts.

The recipe in this guide is PACE's house method for aluminum: aluminum-oxide (ALO) paper for plane grinding to avoid SiC embedding, light force throughout (5-10 lb / 22-45 N) to preserve oxide inclusions, a single 1 µm DIAMAT diamond rough polish, and a final 0.05 µm Nanometer alumina polish that preserves second-phase particles at their true position.

Sectioning

When sectioning aluminum samples, use a low cutting speed to minimize heat generation and deformation. Softer alloys require particularly careful handling, while precipitation-hardened alloys can tolerate slightly more aggressive parameters.

MAX-C abrasive cut-off blades designed for soft non-ferrous materials like aluminum. Thin blades (0.5-1.0 mm) minimize heat generation and deformation.

• Use a MAXCUT abrasive blade (MAX-C or MAX-I series) — designed for soft and medium-soft non-ferrous metals
• Use a thin abrasive cut-off wheel (0.5-1.0 mm thickness) to minimize kerf and heat input
• Apply steady, light pressure — allow the wheel to do the cutting
• Use adequate coolant to prevent overheating and smearing
• Use a slower feed rate than for harder materials; controlling sectioning damage upstream makes the rest of the recipe work

For more information on sectioning blades, visit our Abrasive Blades collection.

Mounting

Mounting provides edge retention and easier handling. The PACE default for aluminum is compression mounting with phenolic, epoxy, or diallyl phthalate (DAP) resins.

Exception — heat-sensitive precipitation-hardened tempers: for 2024-T6, 6061-T6, 7075-T6, A356-T6, and similar age-hardened alloys, use castable epoxy instead. Compression mounting cycles run at 150-180°C, which overlaps with the ageing temperatures these tempers were originally heat-treated at. A 150°C mount cycle is effectively a small further age and can shift the precipitate distribution being analyzed. Castable epoxy cures at room temperature and leaves the microstructure intact.

Compression Mounting (default)

1. Clean the sample thoroughly to remove cutting fluid and debris
2. Place sample in mounting press with phenolic, epoxy, or DAP resin
3. Apply pressure: 3000-4000 psi for phenolic, 2000-3000 psi for epoxy
4. Heat to 150-180°C and hold for 5-8 minutes
5. Cool under pressure to room temperature

Castable Mounting (for heat-sensitive tempers)

1. Clean and dry the sample
2. Place in mounting cup with epoxy resin
3. Allow to cure at room temperature (typically 4-8 hours)

For more information on mounting equipment, visit our Compression Mounting and Castable Mounting equipment pages.

Grinding

Grinding flattens the sample and removes sectioning damage while keeping oxide inclusions intact and the deformation layer shallow. The PACE method starts on aluminum-oxide (ALO) paper rather than silicon carbide for the plane step. ALO does not embed into aluminum the way SiC does, which protects surface integrity for the polish steps that follow. SiC papers are used afterward, at very fine grits where the embedding risk is much lower.

Progressive grinding for aluminum: aluminum-oxide (ALO) paper for plane grinding, then fine SiC papers at P2400 and P4000. Rotate the sample 90° between steps.

Grinding Sequence

1. 1200 grit ALO paper: Water lubricant, light force (5-10 lb / 22-45 N), 100/100 rpm head/base, grind until the sample is plane.
2. 800 grit (P2400) SiC paper: Water lubricant, 5-10 lb, 100/100 rpm.
3. 1200 grit (P4000) SiC paper: Water lubricant, 5-10 lb, 100/100 rpm, 1 minute.

Important: Rotate the sample 90° between each grit to confirm complete scratch removal from the previous step. Keep force in the 5-10 lb range; the goal is to remove sectioning damage and prepare a flat surface, not to remove material aggressively. Aluminum tolerates very little force before subsurface deformation begins to compromise the polish.

For more information on grinding supplies, visit our Abrasive Grinding Papers collection.

Polishing

The polishing path for aluminum is short by design. A single 1 µm diamond rough polish followed by a 0.05 µm alumina final polish removes the residual SiC scratches from grinding without introducing additional deformation, and preserves oxide inclusions and second-phase particles at their true position. Final-polishing on alumina (rather than colloidal silica) avoids the slight chemical attack that can lift oxide inclusions out of the matrix.

DIAMAT polycrystalline diamond suspension provides consistent cutting on the ATLANTIS polishing pad with DIALUBE Purple Extender lubricant.

ATLANTIS pad for the 1 µm DIAMAT rough polish; NAPPAD for the final 0.05 µm Nanometer alumina polish.

Rough Polishing

1. 1 µm DIAMAT diamond on ATLANTIS polishing pad: DIALUBE Purple Extender lubricant, light force (5-10 lb / 22-45 N), 100/100 rpm head/base, 2 minutes.

Final Polishing

1. 0.05 µm Nanometer alumina on NAPPAD polishing pad: Light force (5-10 lb), 100/100 rpm, 1 minute.
2. Rinse thoroughly with water and dry with compressed air.

For high-strength heat-treated alloys (2024-T6, 6061-T6, 7075-T6) and cast Al-Si alloys (A356, A380): insert an intermediate 3 µm DIAMAT diamond on ATLANTIS pad step between the SiC grinding and the 1 µm DIAMAT rough polish (DIALUBE Purple Extender, 5-10 lb, 100/100 rpm, 2-3 minutes). These harder alloys leave residual SiC scratches that 1 µm diamond alone won't fully clear; the 3 µm step removes them efficiently.

Important: Keep force light (5-10 lb) and check the surface between steps. Over-polishing introduces relief around second-phase particles and inclusions, and can begin to round the very oxide particles you are trying to preserve.

Compressed air only — no heated drying: Aluminum can recrystallize at modest elevated temperatures, especially after the cold-work that grinding and polishing introduce in the near-surface layer. Hot-air dryers can alter the very grain structure you are trying to image. Use room-temperature compressed air after every rinse.

For more information on polishing supplies, visit our Diamond Abrasives and Polishing Pads collections.

Etching

Etching reveals the microstructure by selectively attacking grain boundaries and phases. The choice of etchant depends on the alloy and the features you want to reveal. Keller's reagent is typically the first choice for most aluminum alloys, with Weck's reagent and electrolytic methods available for more specialized analysis.

Aluminum-silicon alloy etched with Keller's reagent, 400X magnification. Proper etching reveals grain boundaries and phase structure without over-etching artifacts.

Common Etchants for Aluminum

• Keller's Reagent: General purpose, reveals grain boundaries and second phases.
• Weck's Reagent: Tint etchant for grain structure, dendritic segregation, and orientation contrast under bright-field.
• Barker's Reagent: Electrolytic anodization at 20-30V DC for grain structure examination under polarized light.
• Graff & Sargent's Reagent: For age-hardened alloys (2xxx and 7xxx wrought series).

Etching solutions and reagents for aluminum. Common etchants include Keller's Reagent, Weck's Reagent, and electrolytic solutions. Etching time typically ranges from 5-30 seconds depending on the etchant and alloy.

Etching Procedure

1. Ensure sample is clean and dry
2. Apply etchant with cotton swab or immerse sample
3. Etch for 5-30 seconds (time varies by etchant and alloy)
4. Immediately rinse with water, then alcohol
5. Dry with compressed air

Tip: Start with shorter etching times (5-10 seconds) and increase if needed. Over-etching can obscure fine details and create pitting. For electrolytic etching, use appropriate voltage and time settings.

For more information on etchants, visit our Etchants collection.

Troubleshooting

Common Issues and Solutions

• Oxide inclusions pulled out (appear as black voids on the surface): Force was too high during grinding, or the grit was too coarse to start. Drop pressure to the 5-10 lb range, confirm the plane step is on ALO paper (not SiC), and re-section if the sample also shows deep cutoff damage. Pulled-out oxides cannot be recovered — only avoided.
• Embedded abrasive particles (bright spots that don't move on rotation): If they are dark/grey under bright-field they are likely SiC pressed in from the fine-grind steps. Re-run the 1 µm DIAMAT polish on a fresh ATLANTIS pad for a longer time, then re-check. The 0.05 µm alumina final will lift most residual particles; if they remain, the SiC paper was glazed or over-loaded — replace it next time.
• Scratches remaining: Insufficient time at one of the grinding steps, or skipped grit. For heat-treated and cast alloys, add the optional 3 µm DIAMAT step described in the Polishing section.
• Smearing: Too much pressure, or the polishing pad is too soft for the alloy. Use lighter pressure and ensure the ATLANTIS pad is the right hardness for the rough polish step.
• Relief around second phases: Over-polishing or excessive force. Reduce the final polish time and confirm 5-10 lb force.
• Contamination: Clean between steps, use fresh abrasives and lubricants, rinse the sample thoroughly before moving to the next pad.
• Deformation (smeared "orange peel" surface after etching): Too much pressure during grinding or polishing — cold-work was driven deep enough that the etchant reveals the deformation layer instead of the true microstructure. Re-prep at lighter force.
• Over-etching: Reduce etching time or dilute the etchant. Start with shorter times (5-10 seconds) and work up.
• Pitting after etching: Etchant too strong or etching time too long. Dilute the etchant or reduce time.