Aluminium Casting vs Cast Iron: Properties, Cost and When to Use Each
Aluminium and cast iron are two of the most widely used materials in metal casting, but they have very different properties, costs, and areas of application. Choosing between them is rarely obvious as both have genuine strengths, and the right choice depends on the specific demands of your component. This guide compares the two materials directly so you can make an informed decision.
Overview of Aluminium Casting
Aluminium casting alloys are lightweight, corrosion-resistant, and easy to cast using die casting, permanent mould casting, or investment casting processes. Aluminium melts at around 660°C, which is significantly lower than iron, making it cheaper to melt and less demanding on tooling. Density: 2.7 g/cm³, approximately one third the weight of cast iron. Typical tensile strength: 150 to 310 MPa depending on alloy and heat treatment. Melting point: approximately 660°C. Common alloys: A356, A357 for investment and permanent mould casting; ADC12 and A380 for die casting.
- Produce complex internal and external geometry in a single operation, eliminating joints and welds that are potential failure points
- Cast difficult high-performance alloys such as nickel superalloys, titanium, and cobalt chrome that are hard or impossible to machine from solid billet economically
- Achieve near-net-shape parts that minimise material removal and reduce the buy-to-fly ratio
- Deliver consistent, repeatable dimensions across production runs with full traceability
Overview of Cast Iron
Cast iron is an iron-carbon alloy with carbon content above 2%. It has high compressive strength, excellent wear resistance, good vibration damping, and very good machinability. It is one of the most cost-effective structural metals per kilogram. Density: 7.2 g/cm³, approximately three times the weight of aluminium. Typical tensile strength: 150 to 400 MPa depending on grade. Melting point: approximately 1,200°C. Common grades: grey cast iron (EN-GJL), ductile iron (EN-GJS), white iron for wear resistance.
Direct Comparison
| Property | Aluminium Casting | Cast Iron |
|---|---|---|
| Density | 2.7 g/cm³ | 7.2 g/cm³ |
| Tensile strength | 150 to 310 MPa | 150 to 400 MPa (grade dependent) |
| Weight | Lightweight, approx. 1/3 of cast iron | Heavy |
| Corrosion resistance | Good, natural oxide layer | Poor, requires coating or painting |
| Thermal conductivity | High, approx. 205 W/mK | Moderate, approx. 50 W/mK |
| Thermal mass | Low | High, good vibration damping |
| Machinability | Excellent | Good (grey iron) to moderate (ductile) |
| Wear resistance | Moderate | Excellent (especially white iron) |
| Cost per kg | Higher | Lower |
| Casting processes | Die casting, investment casting, permanent mould | Sand casting, investment casting |
| Typical applications | Automotive, aerospace, marine, food | Machine tools, engines, pipes, cookware |
Weight
This is the most significant difference for most applications. Aluminium is approximately one third the density of cast iron, which translates directly to lighter components. In applications where weight affects performance, such as automotive, aerospace, and portable equipment, aluminium’s weight advantage is often decisive. Cast iron’s higher density is occasionally an advantage. In machine tool bases, engine blocks, and flywheel applications, mass provides vibration damping and thermal stability. Where weight is a design benefit rather than a penalty, cast iron may be the correct choice.
Strength and Structural Performance
The comparison depends on which grade of each material is used. Grey cast iron has lower tensile strength and is brittle, meaning it will fracture under impact loading rather than deforming. Ductile iron has significantly better tensile strength and toughness, comparable to mild steel. Aluminium casting alloys have lower absolute tensile strength than ductile iron, but because aluminium components are lighter for equivalent geometry, the strength-to-weight ratio is competitive. For applications where stiffness per unit weight matters, aluminium is often the better choice.
Corrosion Resistance
Aluminium forms a natural oxide layer that provides good corrosion resistance in most environments without any surface treatment. In marine, food processing, and chemical environments where cast iron would require painting or coating, aluminium’s natural corrosion resistance reduces maintenance requirements and lifecycle cost. Cast iron corrodes in the presence of moisture and requires painting, coating, or regular maintenance to prevent rust. In dry indoor environments this is manageable. In outdoor, marine, or wash-down environments, cast iron’s corrosion susceptibility is a significant disadvantage.
Corrosion Resistance
Aluminium forms a natural oxide layer that provides good corrosion resistance in most environments without any surface treatment. In marine, food processing, and chemical environments where cast iron would require painting or coating, aluminium’s natural corrosion resistance reduces maintenance requirements and lifecycle cost. Cast iron corrodes in the presence of moisture and requires painting, coating, or regular maintenance to prevent rust. In dry indoor environments this is manageable. In outdoor, marine, or wash-down environments, cast iron’s corrosion susceptibility is a significant disadvantage.
Thermal Properties
Cast iron has higher thermal mass and retains heat longer than aluminium. This is an advantage in applications like cookware, engine blocks, and industrial heating equipment where thermal stability is required. Aluminium conducts heat much more efficiently than cast iron, with thermal conductivity of approximately 205 W/mK compared to 50 W/mK for grey iron. In heat exchangers, electronics housings, and applications where heat dissipation is important, aluminium is the better choice.
Cost
Cast iron is cheaper per kilogram than aluminium, and the melting and casting process requires less energy. For large, heavy components where material cost dominates the total part cost, cast iron has a clear cost advantage. For weight-sensitive applications, the comparison changes when cost per unit volume is used rather than cost per kilogram. Aluminium’s lower density means you need less of it by volume to fill a given part geometry, which partially offsets the higher price per kilogram.
Machinability
Both materials machine well, but for different reasons. Grey cast iron machines easily due to its graphite content acting as a lubricant, though it produces abrasive dust. Aluminium machines very easily and at high speeds, produces manageable chips, and is less abrasive on cutting tools. For high-volume machined components, aluminium’s machinability advantage translates to lower machining cost and faster cycle times.
When to Choose Aluminium Casting
- Weight is a primary design consideration
- Corrosion resistance is required without surface coating
- Heat dissipation is important
- The component is in a food, medical, or marine environment
- You are using die casting or investment casting processes
- Fast machining cycle times are important
When to Choose Cast Iron
- Maximum wear resistance is required (white or chilled iron)
- Vibration damping is a design requirement
- Thermal mass and stability are needed
- The component is large and heavy where weight is not a concern
- Cost per kilogram is the primary driver and weight is irrelevant
- The application involves compressive loading where cast iron’s compressive strength excels
