Zamak 5

Zamak 5 is a soft (91 HB, ~95 HV) zinc-aluminum-copper die-casting alloy. Very similar preparation to Zamak 3, with the additional consideration of Cu-rich intermetallic phases that are harder than the zinc matrix. The copper addition makes Zamak 5 slightly harder than Zamak 3, but the same careful soft-metal preparation techniques apply.

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. Cutting speed: 150-250 RPM. Apply light to moderate pressure. Die-cast parts may have porosity near the surface. Leave 1-2 mm allowance for grinding away sectioning damage.

Cold mounting with epoxy resin is preferred to avoid heat effects on the soft material and cast structure. Use a low-shrinkage epoxy for best edge retention. The mount should be slightly harder than the sample for better edge retention.

Hot compression mounting is acceptable since the melting point (386°C) is well above mounting temperatures, but cold mounting is recommended for the soft material. For die-cast parts, ensure the mounting material fills any porosity.

The softness (91 HB) requires careful grinding to avoid smearing. Use standard SiC grinding papers with abundant water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (20-30 N per 30 mm sample). The Cu-rich intermetallic phases are harder than the zinc matrix and may cause relief if pressure is excessive.

Grinding sequence:

• 240 grit: Remove sectioning damage (20-40 seconds).
• 320 grit: Remove previous scratches (20-40 seconds).
• 400 grit: Further refinement (20-40 seconds).
• 600 grit: Final grinding step (20-40 seconds).

Rotate specimen 90° between steps. Use complementary rotation. Abundant water lubrication is critical.

Use napless or low-nap cloths. Monitor for relief around Cu-rich and Al-rich intermetallic phases.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad with light pressure (20-25 N).
• 3μm diamond: 2-4 minutes on a medium-hard pad (15-20 N).
• 1μm diamond: 2-3 minutes on a soft pad (12-18 N).

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad. Monitor for relief around intermetallic phases.

Avoid over-polishing. Check surface frequently for smearing and orange peel.

Zamak 5 responds to the same etchants as Zamak 3, with the Cu-rich phases providing additional microstructural features.

1% Nital (Chemical Etching) - Primary choice:

• Composition: 1 ml HNO₃, 99 ml ethanol
• Application: Immerse for 5-15 seconds.
• Reveals: Grain boundaries, zinc matrix, Al-rich and Cu-rich phases.
• Rinse: Water, then ethanol. Dry with warm air.

Chromic Acid Solution (Chemical Etching) - For phase identification:

• Composition: 200 g CrO₃, 15 g Na₂SO₄, 1000 ml H₂O
• Application: Immerse for 10-30 seconds.
• Reveals: Cu-rich and Al-rich intermetallic phases with excellent contrast.
• Note: Chromic acid is highly toxic and carcinogenic. Handle with extreme care.