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)

Anodised Aluminum Coatings

For SEM analysis of anodised aluminum oxide coatings (typically 2-150 µm thick, often with structural pores 10-400 nm in diameter), edge retention of the thin, brittle oxide layer is critical:

• Wrap the sample in aluminum foil prior to cold mounting in castable epoxy under vacuum. The foil supports the anodic film during grinding and polishing, preventing edge chipping.
• Use the standard preparation procedure that follows; the anodic film polishes well at 1 µm diamond and 0.05 µm alumina.
• For SEM imaging, use backscattered electron (BSE) mode; the coating/substrate boundary shows clear compositional contrast that secondary electron imaging does not reveal on a perfectly flat polished surface. Alternative: chemical etching before sputter-coating to create topographic contrast.
• The sample must be conductive for SEM; either use a conductive mounting resin or sputter-coat with a thin layer of iridium, carbon, or Au-Pd.

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. 600 grit (P1200) 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 series and the features you want to reveal. Keller's reagent is the standard first choice for most aluminum alloys; specialized etchants (Graff & Sargent's, Barker's, Weck's color etch) cover specific use cases.

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 for most wrought and cast Al alloys; reveals grain boundaries and constituent particles. The standard first choice.
  • Composition: 95 mL DI water + 2.5 mL HNO₃ + 1.5 mL HCl + 1.0 mL HF
  • Use: Immerse 10-20 seconds. Rinse in warm water. Use only fresh.
• Graff & Sargent's Reagent: wrought aluminum alloys, particularly 2xxx and 7xxx age-hardened series.
  • Composition: 84 mL water + 15.5 mL HNO₃ + 0.5 mL HF + 3 g CrO₃
  • Use: Immerse 20-60 seconds with mild agitation.
• Tucker's Reagent: general-purpose for Al-Si cast alloys (A356, A380, etc.) and high-Si wrought alloys; more aggressive than Keller's.
  • Composition: 45 mL HCl + 15 mL HNO₃ + 15 mL HF + 25 mL water
  • Use: Swab 10-15 seconds. Rinse immediately. Use only fresh.
• NaOH / NaF Etch: Al-Cu alloys (2xxx series); grain contrast and grain boundary lines.
  • Composition: 2 g NaOH + 5 g NaF + 93 mL DI water
  • Use: Immerse 2-3 minutes.
• Barker's Reagent: electrolytic anodizing for grain visualization under polarized light. The standard technique for measuring grain size in wrought aluminum.
  • Composition: 4-5 mL HBF₄ (48%) in 200 mL DI water
  • Use: Sample as anode, stainless steel cathode, 20-30 V DC, 1-3 minutes. View under crossed polarizers; grains appear in color.
• Weck's Reagent for Aluminum: color (tint) etch revealing grain structure, dendritic segregation, and orientation contrast in brightfield.
  • Composition: 100 mL DI water + 4 g KMnO₄ + 1 g NaOH
  • Use: Immerse until surface develops color (typically 20-60 seconds).
• 1% NaOH (aqueous): quick macro-etch for surface defect inspection, weld evaluation, and general orientation.
  • Use: Swab or immerse 30-60 seconds; rinse with concentrated HNO₃ to remove the resulting black smut.

Cast Al-Si Alloys: Special Considerations

Cast Al-Si alloys (A356, A380, AlSi7Mg, etc.) are typically modified with small additions of strontium (~0.01-0.015 wt%) to transform the brittle acicular eutectic silicon into a fine fibrous morphology. Metallographic quality control of foundry castings centers on confirming this morphology change and on detecting Mg₂Si precipitates that indicate incorrect heat treatment.

• As-polished: Examine first under brightfield and DIC. Eutectic Si morphology (acicular vs. rounded/fibrous) is often visible without etching. Microporosity is also best assessed in the as-polished condition.
• Keller's reagent: Reveals Al matrix grain structure and outlines eutectic Si.
• Tucker's reagent: More aggressive; useful for revealing Si phase morphology in heavily modified castings.
• Color etching: For QC of strontium modification and T6 heat treatment, color (tint) etching with Klemm's-style reagents or specialty dyes (e.g., phthalocyanine blue in nitrocellulose lacquer applied after a deep NaOH etch) provides excellent contrast between the α-Al matrix, eutectic Si, Mg₂Si precipitates, and microporosity. Capillary action draws the dye into porosity, outlining pores in unstained relief: a powerful QC tool for casting defect detection.

Etching Procedure

1. Ensure sample is clean and dry; residual polishing debris will give mottled etch results.
2. Apply etchant by swab for Keller's and similar; immerse for Barker's electrolytic and color etches.
3. Etch for the recommended time (5-30 seconds for most chemical etches; 1-3 minutes for Barker's electrolytic).
4. Rinse immediately with water, then alcohol.
5. Dry with compressed air. Do not wipe.

Tip: Start with the short end of the time range and increase if needed. Over-etching obscures fine details and creates pitting. Most HF-containing etchants lose activity within 30-60 minutes of mixing; make small batches fresh.

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.

Additional Reading

• Zipperian, D.C. Metallographic Handbook, Section 11.1.1 (Aluminum and Aluminum Alloys). PACE Technologies, Tucson, AZ. The house reference. The grinding, polishing, and etchant recipes in this guide come directly from this source.
• García, L., Dietz, C., Criado, A.J., Martínez, J.A. "Colour metallography of cast aluminium alloys." Practical Metallography 51 (2014) 7, 514-529. Source for the phthalocyanine blue / NaOH deep-etch color metallography technique for cast Al-Si quality control.
• Long, J., Borissova, A., Wilson, A.D., Avelar-Batista Wilson, J.C. "Sample preparation of anodised aluminium oxide coatings for scanning electron microscopy." Micron 101 (2017), 87-94. Comparison of mechanical fracturing, cryogenic fracturing, and metallography for anodised Al; source for the Al-foil-wrap edge-retention technique.
• ASM Handbook, Vol. 9: Metallography and Microstructures. ASM International. Comprehensive aluminum alloy preparation and etching reference.
• Vander Voort, G.F., Asensio-Lozano, J., Suárez-Peña, B. "Color Metallography and Electron Microscopy Techniques Applied to the Characterization of 413.0 Aluminum Alloys." Microscopy and Microanalysis 19 (2013), 1019-1026. For cast Al-Si color etching.
• Petzow, G. Metallographic Etching, 2nd ed. ASM International. Comprehensive reference for Keller's, Graff & Sargent's, Tucker's, Barker's, and other aluminum etchant formulations.
• ASTM E407 — Standard Practice for Microetching Metals and Alloys (aluminum etchants section).
• ASTM E3 — Standard Guide for Preparation of Metallographic Specimens.
• ASTM E112 — Standard Test Methods for Determining Average Grain Size (relevant for Barker's anodizing applications).
• EN 1706 — Aluminium and aluminium alloys — Castings — Chemical composition and mechanical properties.