7075 Aluminum

7075 Aluminum is a medium-hard (150 HB) high-strength heat-treatable aluminum-zinc-copper-magnesium alloy (Al-5.6Zn-2.5Mg-1.6Cu-0.23Cr) with an alpha aluminum matrix containing Zn-Mg-Cu precipitates. One of the highest strength aluminum alloys, commonly used in aerospace, military, and high-performance applications. The medium hardness allows for slightly more aggressive preparation than softer aluminum alloys, but still requires careful attention to avoid smearing and excessive relief. Use moderate pressure throughout grinding and polishing. The precipitation-hardened microstructure (T6 temper) contains fine Zn-Mg-Cu precipitates that should be preserved during preparation. The higher alloy content compared to 6061 means it can tolerate slightly more pressure, but still monitor for smearing and relief. Commonly used in aircraft structures, bicycle frames, and golf clubs due to excellent fatigue resistance.

Use slow-speed diamond saw or abrasive cut-off wheel designed for non-ferrous materials. Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating and minimize deformation. The medium-hard material can tolerate slightly more aggressive cutting than softer aluminum. Cutting speed: 150-250 RPM for most cut-off saws. Apply moderate pressure - the medium-hard material allows for reasonable feed rates but still requires careful handling. Avoid forcing the cut which can cause sample damage and work hardening. Leave adequate allowance (~1-2 mm) for grinding away the heat-affected zone and any deformation from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the precipitation structure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity. The mount should be slightly harder than the sample for better edge retention.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. The medium-hard material requires careful handling during mounting to avoid deformation. Ensure proper cooling under pressure to minimize shrinkage. For aerospace and high-performance applications, ensure the mounting material is compatible with the intended use environment.

The medium hardness (150 HB) of 7075 Aluminum allows for slightly more aggressive grinding than softer aluminum alloys. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply moderate pressure (25-35 N per 30 mm sample) - the medium-hard material can tolerate more pressure than soft aluminum, but still monitor for smearing. Use sharp, fresh grinding papers to minimize deformation. The precipitation-hardened microstructure (T6 temper) contains fine Zn-Mg-Cu precipitates that should be preserved during grinding.

Grinding sequence:

• 240 grit: Remove sectioning damage (20-40 seconds). Use moderate pressure to remove heat-affected zone. The medium-hard material can tolerate slightly more pressure than soft aluminum.
• 320 grit: Remove previous scratches (20-40 seconds). Monitor for smearing, though less likely than with softer aluminum.
• 400 grit: Further refinement (20-40 seconds). Ensure complete scratch removal.
• 600 grit: Final grinding step (20-40 seconds). Prepare surface for polishing. Ensure all scratches are removed before proceeding.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Adequate water lubrication is critical - avoid drying during grinding which can cause smearing. The medium-hard material means grinding times may be slightly longer than for softer aluminum.

The medium hardness allows for slightly more aggressive polishing than softer aluminum alloys. Use diamond polishing with appropriate polishing pads for each stage. Apply moderate pressure throughout to prevent deformation. The fine Zn-Mg-Cu precipitates in T6 temper should be preserved during polishing.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with moderate pressure (25-30 N per 30 mm sample). Start with 6μm rather than 9μm to minimize damage. The medium-hard material can tolerate slightly more pressure than soft aluminum. Monitor for smearing and reduce pressure if needed.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with moderate pressure (20-25 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a soft synthetic pad with lighter pressure (15-20 N). These pads provide gentle material removal suitable for medium-hard aluminum. Monitor for orange peel or smearing.

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad (e.g., MICROPAD) with light pressure. Soft pads are recommended for colloidal silica and produce a mirror finish. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The medium-hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing, orange peel, or excessive relief. The fine Zn-Mg-Cu precipitates in T6 temper should be preserved and visible after proper polishing.

7075 Aluminum responds well to standard aluminum etchants. The high zinc (5.6%), magnesium (2.5%), and copper (1.6%) content form Zn-Mg-Cu precipitates in T6 temper, which will be visible after proper etching. The alpha aluminum matrix will reveal grain boundaries well with standard etchants. The precipitation-hardened microstructure means T6 temper will show different etching characteristics than solution-treated or over-aged material. Two primary etchants are recommended:

Keller's Reagent (Chemical Etching) - Primary choice for general microstructure:

• Composition: 2ml HF, 3ml HCl, 5ml HNO₃, 190ml H₂O
• Preparation: Add acids to water slowly with stirring. Prepare fresh for best results. Store in plastic container (HF attacks glass).
• Application: Immerse sample or swab for 10-20 seconds. Classic general-purpose micro-etchant for Al alloys. The high alloy content means etching behavior is similar to other high-strength aluminum alloys. The precipitation-hardened structure (T6 temper) will reveal grain boundaries and fine precipitates clearly.
• Reveals: Grain boundaries, grain structure, and fine Zn-Mg-Cu precipitates clearly. Excellent for general microstructure examination. Good contrast for alpha aluminum matrix. The fine precipitates in T6 temper will appear as small particles in the matrix.
• Rinse: Immediately with water, then alcohol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several weeks if stored properly. Use in fume hood.

0.5% HF (Light Etching) - For sensitive microstructures or when Keller's is too aggressive:

• Composition: 0.5ml HF (48%) in 100ml distilled water
• Preparation: Add HF to water slowly with stirring. Solution is stable for several months if stored in plastic container.
• Application: Immerse sample or swab for 15-30 seconds. Less aggressive than Keller's reagent, good for revealing fine grain structure and precipitates without over-etching. Particularly useful for revealing fine precipitation structure in T6 temper.
• Reveals: Grain boundaries, fine grain structure, and fine Zn-Mg-Cu precipitates. Less contrast than Keller's but gentler on the microstructure. Good for revealing fine precipitation details.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Ensure good ventilation. HF is highly toxic - use proper PPE and fume hood.

Etching Strategy:

• Start with Keller's reagent for general microstructure examination - it provides good contrast and reveals grain boundaries and precipitates clearly
• Use 0.5% HF if Keller's is too aggressive or for revealing fine precipitation details, especially for T6 temper
• Etching time may vary with heat treatment condition - T6 temper (solution-treated and aged) will show fine precipitates, while solution-treated material may show different structure
• Over-aged material may show coarser precipitates - adjust etching time accordingly
• Always clean and degrease before etching
• Use short initial etch times (a few seconds), check under the microscope, repeat if needed
• Check etching progress frequently - over-etching can obscure fine precipitation details
• For aerospace applications, ensure proper surface preparation to reveal any service-related changes, fatigue damage, or stress-corrosion cracking
• For high-performance applications, ensure proper surface preparation to reveal any microstructural changes from service or heat treatment
• The fine Zn-Mg-Cu precipitates in T6 temper should be visible as small particles in the matrix after proper etching
• Grain size may vary depending on heat treatment - solution-treated and aged material typically has fine grains
• The high alloy content provides excellent fatigue resistance - ensure proper etching to reveal any fatigue-related microstructural changes

Safety: All etchants contain HF or fluoride salts – full PPE (gloves, safety glasses, lab coat), proper fume hood, and HF-specific safety measures are mandatory. HF can cause severe burns and is toxic if ingested or absorbed through skin.