---
title: "Classification of aluminium alloys"
subject: "Material Science"
chapter: 9
key: "semester_3/material_science:classification-and-designation-of-aluminium-alloys"
source: https://www.bctengineer.com/q/semester_3/material_science/classification-and-designation-of-aluminium-alloys
site: BCT Engineering
---

# Classification of aluminium alloys

Material Science · Chapter 9: Non-ferrous Alloys

- Asked in: year not given, 6 marks
- Video: [Classification of Aluminium Alloys || Metallurgy](https://www.youtube.com/watch?v=MjhQ_JK1jlc) — Metallurgical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/classification-and-designation-of-aluminium-alloys
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-9

## Question

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.

## 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.

## Answer

#### Properties of aluminium
- Low density (2.7 g/cm$^3$, about one third of steel) and FCC structure (ductile at all temperatures).
- Good electrical and thermal conductivity, high reflectivity.
- A natural Al$_2$O$_3$ 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

```text
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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which series of wrought aluminium alloys are heat-treatable?
   A. 1xxx, 3xxx and 5xxx
   B. 1xxx and 5xxx only
   C. 2xxx, 6xxx and 7xxx
   D. 3xxx only
   Answer: C. Cu (2xxx), Mg-Si (6xxx) and Zn (7xxx) alloys can be precipitation hardened; 1xxx, 3xxx and 5xxx are strengthened by cold work. (Key point: Heat-treatable series)

2. What is the main alloying element of duralumin, and how is it strengthened?
   A. Si, by casting
   B. Zn, by cold work only
   C. Cu (3.5-4.5%), by age hardening
   D. Ni, by quenching only
   Answer: C. Duralumin is Al with 3.5-4.5% Cu, 0.5% Mg, 0.5-1% Mn, age hardened for aircraft parts. (Key point: Duralumin)

3. Why is Y-alloy (4% Cu, 2% Ni, 1.5% Mg) used for pistons?
   A. It is very cheap
   B. It is non-magnetic
   C. It retains strength at high temperature
   D. It has no copper
   Answer: C. Y-alloy keeps strength when hot, suiting pistons, cylinder heads and aero engine parts. (Key point: Y-alloy)

## Related in Material Science

- [Aluminium alloys compared with steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aaluminium-versus-steel-applications) — Material Science, chapter 9

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