Material
Aluminum Al
The default structural metal of aerospace: a third the density of steel, cheap by comparison to titanium, easy to machine and form, and available in a wider range of tempers and product forms than any other light alloy.
Overview
Aluminum carries most of the world's airframe structure because it is light, inexpensive relative to titanium, readily formed and welded in some alloys, and machines fast enough that cutting a part from solid plate is often cheaper than tooling for a forging. Its limits are temperature, where it gives out around 150 degrees Celsius, and fatigue, where unlike steel it has no true endurance limit. In defense specifically it dominates ground vehicle armor structure, missile bodies, and any airframe where titanium's cost cannot be justified. Domestic primary smelting capacity has contracted sharply, so most US aluminum mill product now originates from imported or recycled metal.
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Defense applications
Airframe structure
Skins, stringers, frames, spars and machined bulkheads across almost every military aircraft. The 2000 and 7000 series alloys were developed for exactly this, trading corrosion resistance for strength.
Wing skins and spars, Fuselage frames, Machined bulkheads, Floor beams
Ground vehicle armor and hull structure
5083 and 7039 aluminum armor plate carries the structural hull of light armored vehicles, where the armor is the structure rather than an appliqué layer bolted onto a steel frame.
Bradley hull structure, AAV and amphibious hulls, Applique armor panels, Hatches and doors
Missiles and launch structure
Airframe bodies, fins, launch canisters and interstage structure where weight directly buys range, and where the service temperature stays inside aluminum's envelope.
Missile body sections, Launch canisters, Interstage adapters, Fin assemblies
Naval superstructure
5xxx series marine alloys are used above the waterline to keep topside weight and therefore the metacentric height under control, though fire performance and fatigue cracking have driven some programs back toward steel.
Deckhouse structure, Superstructure panels, Masts, Fast ferry and LCS hull forms
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| 2024 | 2xxx series, aluminum-copper | The classic damage-tolerant airframe alloy. Good fatigue crack growth resistance, which is why it goes where cracks are expected and must propagate slowly. | Sheet, Plate, Bar and Rod, Extrusions | |
| 6061 | 6xxx series, aluminum-magnesium-silicon | The general purpose structural alloy. Weldable, corrosion resistant, extrudable and available everywhere, at moderate strength. | Plate, Sheet, Bar and Rod, Extrusions, Tube and Pipe, Forgings | |
| 7075 | 7xxx series, aluminum-zinc | Very high strength, approaching some steels on a strength-to-weight basis. Poor weldability and susceptible to stress corrosion cracking in the T6 temper, which the T73 overage temper addresses at some cost in strength. | Plate, Bar and Rod, Extrusions, Forgings | |
| 7050 | 7xxx series, aluminum-zinc | Developed for thick section aerospace plate, holding strength and toughness deeper into heavy sections than 7075 and with better stress corrosion resistance. | Plate, Forgings, Extrusions | |
| 5083 | 5xxx series, aluminum-magnesium | Marine and armor alloy. Not heat treatable, excellent seawater corrosion resistance and weldability, and the basis of aluminum armor plate specifications. | Plate, Sheet, Structural Shapes | |
| 7039 | 7xxx series, aluminum-zinc | A weldable high strength armor alloy used where 5083 lacks the ballistic performance required. | Plate | |
| A356 | 3xx.x casting alloy | The workhorse aluminum casting alloy. Good castability and mechanical properties after T6 heat treatment, widely used for structural castings. | Castings |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 2.70 | g/cm3 | 0.098 lb/in3; roughly one third that of steel |
| Melting point | 660 | degrees C | 1220 degrees F for pure aluminum; alloys melt over a range |
| Elastic modulus | 69 to 72 | GPa | About one third that of steel, so stiffness-driven designs need more section |
| Tensile strength, 6061-T6 | 290 minimum | MPa | 42 ksi; the general purpose structural workhorse |
| Tensile strength, 7075-T6 | 525 typical | MPa | 76 ksi; among the strongest conventional aluminum alloys |
| Thermal conductivity | 120 to 200 | W/m-K | High, which is why it machines cool and makes good heat sinks |
| Coefficient of thermal expansion | 23.6 | micrometre/m-K | Roughly triple that of titanium; a problem at composite and titanium joints |
| Maximum service temperature | approximately 150 | degrees C | Conventional alloys lose strength rapidly above this |
| Fatigue behavior | No endurance limit | Unlike steel, aluminum will eventually fail at any stress amplitude given enough cycles | |
| Galvanic behavior | Anodic | Corrodes preferentially against carbon fiber composite and titanium; requires isolation |
Advantages
- About one third the density of steel at a fraction of titanium's cost
- Machines fast, which often makes cutting from solid cheaper than tooling for a forging
- Very wide range of alloys, tempers and product forms, all readily available
- High thermal and electrical conductivity
- Readily extruded into complex profiles at low tooling cost
- Fully and economically recyclable, which supports a large domestic secondary supply
Limitations
- Loses useful strength above roughly 150 degrees Celsius
- No true fatigue endurance limit, so life is always finite
- Elastic modulus one third that of steel, so stiffness-driven parts gain little
- Galvanically anodic against carbon composite and titanium, requiring isolation and sealant
- High strength 2000 and 7000 series alloys are difficult to fusion weld and prone to stress corrosion cracking
- US primary smelting capacity has contracted sharply, so most domestic mill product starts from imported or recycled metal
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| AMS 4027 | SAE International | 6061 sheet and plate |
| AMS 4045 | SAE International | 7075 sheet and plate |
| AMS 4050 | SAE International | 7050 plate |
| ASTM B209 | ASTM International | Aluminum and aluminum alloy sheet and plate |
| ASTM B221 | ASTM International | Aluminum and aluminum alloy extruded bar, rod, wire, profiles and tube |
| MIL-DTL-46027 | U.S. Department of Defense | Armor plate, aluminum alloy, weldable 5083 and 5456 |
| MIL-DTL-46063 | U.S. Department of Defense | Armor plate, aluminum alloy, 7039 |
| AMS 2772 | SAE International | Heat treatment of aluminum alloy raw materials |