A380 Cast Aluminum

A380 is a soft (80 HB) die-cast aluminum alloy with Al-8.5Si-3.5Cu composition. Similar preparation to A356, with the additional copper-bearing Al2Cu intermetallic phase. The primary challenges are the same hardness contrast between soft aluminum matrix and hard silicon particles, plus the preservation of porosity typical of die castings. Gas porosity and shrinkage voids are important microstructural features that must not be smeared shut or filled with grinding debris during preparation.

Use an abrasive cut-off wheel designed for non-ferrous materials (SiC bonded) or a low-speed diamond saw. Cutting speed: 200-300 RPM for cut-off saws. Apply moderate feed pressure with continuous coolant. For porosity studies, take care not to smear pores closed during sectioning. Position cuts to avoid areas of interest where possible. Die castings often have complex shapes; select a section plane that captures the skin zone, interior, and any features of interest (gates, cold shuts, etc.). Leave 2-3 mm allowance for grinding.

Cold mounting with castable epoxy is recommended, particularly for porosity evaluation. For general microstructural examination, hot compression mounting at 150-180 degrees C is acceptable. When porosity preservation is critical, mount without vacuum impregnation so pores remain visible. For interface studies (die cast surface against a mating component), edge-retaining compounds are essential. If porosity support is needed to prevent smearing during grinding, vacuum impregnation with a fluorescent dye-loaded epoxy allows pores to be identified under UV illumination while providing mechanical support.

Similar to A356 preparation. The soft aluminum matrix (80 HB overall) with hard silicon and Al2Cu intermetallics requires controlled grinding. SiC papers or diamond grinding discs both work. Use moderate pressure (20-30 N per 30 mm sample). Disc speed: 200-300 RPM. Porosity must not be smeared; avoid excessive pressure.

Grinding sequence:

• 240 grit: Remove sectioning damage (20-40 seconds). Moderate pressure. Watch for pore smearing.
• 320 grit: Remove previous scratches (20-40 seconds). Rotate specimen 90 degrees.
• 400 grit: Further refinement (20-30 seconds).
• 600 grit: Final grinding step (20-30 seconds). Verify porosity is still open and visible.

Rotate specimen 90 degrees between steps. Complementary rotation. Abundant water lubrication to flush debris from pores.

Use low-nap to napless cloths to minimize relief between aluminum matrix and intermetallic phases, and to prevent debris from filling pores.

Diamond polishing sequence:

• 9 micrometer diamond: 3-5 minutes on a medium-hard synthetic pad with moderate pressure (20-30 N per 30 mm sample). Remove grinding damage.
• 3 micrometer diamond: 2-4 minutes on a medium-hard synthetic pad (20-25 N). Monitor for differential polishing around Si and Al2Cu phases.
• 1 micrometer diamond: 2-3 minutes on a napless pad (15-20 N).

Final polishing:

• 0.05 micrometer colloidal silica: 1-3 minutes on a soft final polishing pad with light pressure. Colloidal silica provides chemical-mechanical action that enhances phase contrast. The Al2Cu phase becomes readily visible as a distinct phase without etching.

After final polishing, inspect pores under the microscope. If pores appear to contain polishing debris, brief ultrasonic cleaning in ethanol can dislodge loose material.

A380 responds to the same etchants as A356, with Al2Cu providing additional microstructural features.

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

• Composition: 2 ml HF (48%), 3 ml HCl (concentrated), 5 ml HNO3 (concentrated), 190 ml water
• Application: Immerse for 8-15 seconds.
• Reveals: Dendritic structure, Al-Si eutectic, Al2Cu (theta) phase (appears as bright blocky or rosette shapes), iron-bearing intermetallics, and grain structure. Excellent for overall microstructural assessment of die castings.
• Rinse: Water, then ethanol. Dry with warm air.

0.5% HF Solution (Chemical Etching) - For gentler etching:

• Composition: 0.5 ml HF (48%), 99.5 ml water
• Application: Immerse for 10-30 seconds.
• Reveals: Primary aluminum dendrites. Preferred for DAS measurement and skin zone vs. interior comparison.

Safety: HF is extremely dangerous. Calcium gluconate gel must be available. Full PPE including HF-rated gloves required. Work in a fume hood.