- Practice · 6 marks
Classification of aluminium alloys
Short answer
Aluminium is light (2.7 g/cm³), conductive, corrosion-resistant due to natural oxide film. Wrought alloys: non-heat-treatable (1xxx,3xxx,5xxx) and heat-treatable (2xxx,6xxx,7xxx); cast alloys include Al-Si. Duralumin (3.5–4.5% Cu,0.5% Mg, Mn) age-hardened for aircraft. Y-alloy (4% Cu,2% Ni,1.5% Mg) retains strength at high temp., used for pistons.
Write short notes on the classification of aluminium alloys. State the properties of pure aluminium that make it important, and give the composition and uses of duralumin and Y-alloy.
Properties of aluminium
- Low density (2.7 g/cm, about one third of steel) and FCC structure (ductile at all temperatures).
- Good electrical and thermal conductivity, high reflectivity.
- A natural AlO film gives good corrosion resistance.
- Pure Al is weak (tensile strength about 90 MPa), so it is alloyed (Cu, Mg, Si, Zn, Mn) and strengthened by cold work or precipitation hardening.
Classification
Aluminium alloys
|-- Wrought (rolled, extruded, forged)
| |-- Non-heat-treatable : 1xxx, 3xxx (Mn), 5xxx (Mg)
| `-- Heat-treatable : 2xxx (Cu), 6xxx (Mg-Si), 7xxx (Zn)
`-- Cast
|-- Non-heat-treatable : Al-Si (LM6)
`-- Heat-treatable : Al-Cu, Al-Si-Mg
| Series | Main element | Features |
|---|---|---|
| 1xxx | 99%+ Al | Conductors, chemical equipment |
| 2xxx | Cu | High strength, aircraft |
| 3xxx | Mn | Moderate strength, cans, utensils |
| 5xxx | Mg | Good corrosion resistance, marine |
| 6xxx | Mg and Si | Extrusions, architectural frames |
| 7xxx | Zn | Highest strength, aircraft |
Special alloys
- Duralumin: 3.5-4.5% Cu, 0.5% Mg, 0.5-1% Mn, rest Al. Age hardened; used in aircraft structures and automobile parts.
- Y-alloy: 4% Cu, 2% Ni, 1.5% Mg, rest Al. Retains its strength at high temperature; used for pistons, cylinder heads and aero-engine parts.
- Al-Si (silumin): 10-13% Si; excellent castability, used for engine castings and housings.
✓ Answer checked by Claude Opus 5.5 (high effort)
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