7475 Aluminum

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 soft material requires gentle handling. Cutting speed: 150-250 RPM for most cut-off saws. Apply light to moderate pressure - the soft material requires gentle handling to avoid excessive deformation and work hardening. Avoid forcing the cut which can cause sample damage, work hardening, and significant deformation. 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 microstructure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity.

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. Ensure proper cooling under pressure to minimize shrinkage. The material requires careful handling during mounting to avoid deformation. For critical applications, ensure the mounting material is compatible with the intended use environment.

The medium hardness (150 HB) of 7475 Aluminum requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (25-35 N per 30 mm sample) - avoid excessive force that could cause work hardening. Use sharp, fresh grinding papers to minimize deformation. Over-grinding can affect grain boundary revelation during etching. The work hardening behavior means strain-hardened material may show different grinding characteristics than annealed material.

Grinding sequence:

• 240 grit: Remove sectioning damage (30-60 seconds). Use moderate pressure to remove heat-affected zone.
• 320 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.

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 hardness requires careful polishing. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent deformation. The work hardening behavior means strain-hardened material may show different polishing characteristics than annealed material.

Diamond polishing sequence:

• 9μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (25-35 N per 30 mm sample). Start with 9μm to minimize damage. ensure complete scratch removal at each step.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (20-30 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a medium-hard synthetic pad with lighter pressure (20-30 N). These pads provide gentle material removal.
• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with very light pressure. 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 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 or excessive relief.

7475 Aluminum responds well to standard etchants for aluminum alloy materials. The alpha aluminum with zn-mg-cu precipitates will reveal clearly with appropriate etchants. 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.
• Reveals: Grain boundaries and grain structure clearly. Excellent for general microstructure examination.
• 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 (Chemical Etching) - Standard etchant for this material:

• Application: Follow standard procedures for 0.5% HF.
• Reveals: Grain boundaries and microstructure clearly.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Use appropriate safety measures. Consult material-specific guidelines.

Etching Strategy:

• Start with Keller's Reagent for general microstructure examination
• 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 details

Safety: All etchants require proper PPE (gloves, safety glasses, lab coat), proper fume hood, and appropriate safety measures. Handle with care.