---
title: "Aluminium alloys compared with steel"
subject: "Material Science"
chapter: 9
key: "semester_3/material_science:aluminium-versus-steel-applications"
source: https://www.bctengineer.com/q/semester_3/material_science/aluminium-versus-steel-applications
site: BCT Engineering
---

# Aluminium alloys compared with steel

Material Science · Chapter 9: Non-ferrous Alloys

- Asked in: year not given, 5 marks
- Video: [Steel vs Aluminum - Debunking 4 Common Misconceptions](https://www.youtube.com/watch?v=9P7bd4MJnRA) — The Engineering Hub
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/aluminium-versus-steel-applications
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-9

## Question

Compare aluminium alloys with steel for structural use. State the advantages and limitations of aluminium alloys and give examples of applications.

## Short answer

Aluminium alloys have low density (2.7 g/cm³) and high specific strength, good corrosion resistance, but lower modulus (~70 GPa) and higher cost than steel. Advantges: light weight, extrudability, cryogenic toughnes. Limitations: lower stiffness, strength loss above ~200°C, galvanic corrosion with steel. Applications: aircraft frames (2024,7075), transmision lines, automotive panels.

## Answer

Aluminium alloys are used where low weight, corrosion resistance and good conductivity matter more than the highest stiffness or lowest cost.

| Property | Aluminium alloys | Structural steel |
|---|---|---|
| Density | about 2.7 g/cm$^3$ | about 7.85 g/cm$^3$ |
| Young's modulus | about 70 GPa | about 200 GPa |
| Yield strength | 30-500 MPa | 250-1500 MPa |
| Specific strength (strength/density) | High (heat-treated alloys) | Moderate |
| Corrosion resistance | Good (oxide film) | Needs coating |
| Thermal/electrical conductivity | High | Low |
| Weldability | Needs special process (TIG/MIG) | Good (low-carbon steels) |
| Fatigue limit | None (no endurance limit) | Distinct endurance limit |
| Cost per kg | Higher | Lower |

#### Advantages of aluminium alloys
- Light weight and high strength-to-weight ratio.
- Good corrosion resistance, easy to extrude and cast in complex shapes.
- Recyclable using only about 5% of the energy needed for primary production.
- Non-magnetic and non-sparking; retains toughness at low temperature (FCC).

#### Limitations
- Low modulus gives larger deflection for same section.
- Loses strength above about 150-200 °C; low melting point (660 °C).
- Galvanic corrosion when in contact with steel or copper.

#### Applications
Aircraft skin and frames (2024, 7075), engine pistons and blocks (Al-Si), transmission lines (1350), beverage cans (3004, 5182), window frames (6063), and automotive body panels.

## Check yourself

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

1. Why does an aluminium section deflect more than the same steel section?
   A. Its density is higher
   B. Its Young's modulus is about 70 GPa against about 200 GPa
   C. It has a distinct endurance limit
   D. It is not ductile
   Answer: B. Lower modulus means larger elastic deflection for the same load and section. (Key point: Stiffness limitation)

2. What is the main reason aluminium alloys are used for aircraft structures?
   A. Lower cost per kg than steel
   B. Higher modulus than steel
   C. High strength-to-weight ratio (specific strength)
   D. Higher melting point
   Answer: C. Density is about one third of steel's, so heat-treated alloys have high specific strength. (Key point: Why aluminium is chosen)

3. Which is a limitation of aluminium alloys listed in the answer?
   A. Poor corrosion resistance
   B. Loses strength above about 150-200 °C and has galvanic corrosion with steel or copper
   C. Magnetic and sparking
   D. Brittle at low temperature
   Answer: B. Strength drops at moderate temperatures (melting point 660 °C), and contact with steel or copper causes galvanic corrosion. (Key point: Limitations)

## Related in Material Science

- [Classification of aluminium alloys](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aclassification-and-designation-of-aluminium-alloys) — Material Science, chapter 9

## Related in other subjects

- [Compare cast iron, steel, aluminium](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acompare-cast-iron-steel-aluminium) — Civil Engineering Materials, chapter 8
- [Short note: alloys of steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aalloys-of-steel-note) — Civil Engineering Materials, chapter 8
- [Steel-aluminium bar with temperature rise: stresses](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Asteel-aluminium-bar-temperature-supports) — Strength of Materials, chapter 3
- [Rigid bar ABC: aluminium wire, steel bar](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Arigid-bar-abc-aluminium-steel-wire) — Strength of Materials, chapter 3