Material
Magnesium Mg
The lightest structural metal in production use, about a third lighter than aluminum. The United States currently has no primary magnesium production at all, which makes this the most import-exposed material in the atlas.
Overview
Magnesium is chosen when weight dominates every other consideration: helicopter and aircraft gearbox housings, missile and munition components, and portable electronics and equipment cases. It die casts beautifully, machines faster than any other structural metal, and damps vibration better than aluminum. Against that, it is galvanically the most active engineering metal, corrodes readily unless coated and isolated, and burns in fine chip or powder form, which imposes real handling requirements on any shop that machines it. Its position in the US industrial base is precarious in a way none of the other materials here match.
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Defense applications
Rotorcraft and aircraft transmissions
Magnesium gearbox and transmission housings are long established in rotorcraft, where the weight saved on a housing directly buys payload or range. Requalifying these parts in aluminum is expensive, so the legacy demand persists.
Helicopter main transmission housings, Accessory gearbox cases, Intermediate and tail gearboxes
Missiles and munitions
Airframe sections, fin assemblies and structural housings where the flight duration is short enough that long-term corrosion is not the governing concern.
Missile body sections, Fin and canard assemblies, Guidance section housings
Portable and dismounted equipment
Cases and chassis for radios, optics, sensors and man-portable equipment, where every ounce is carried by a soldier and the vibration damping is a secondary benefit.
Radio and electronics housings, Optics and sight bodies, Portable generator components
Pyrotechnics and countermeasures
Magnesium's combustion behavior, a liability in machining, is the entire point in flares, incendiaries and infrared countermeasures.
Infrared decoy flares, Illumination rounds, Incendiary compositions
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| AZ91D | Magnesium-aluminum-zinc casting alloy | The dominant die casting alloy, covering the large majority of magnesium castings. The high purity D variant substantially improves corrosion resistance over earlier AZ91 grades. | Castings | |
| AZ31B | Magnesium-aluminum-zinc wrought alloy | The standard wrought magnesium alloy, available as sheet, plate and extrusion, and the one most likely to be specified when a design needs wrought rather than cast magnesium. | Sheet, Plate, Extrusions, Bar and Rod | |
| ZE41A | Magnesium-zinc-rare earth casting alloy | A rare earth bearing sand casting alloy with better elevated temperature properties and pressure tightness than AZ91, long used for aerospace gearbox housings. | Castings | |
| WE43 | Magnesium-yttrium-rare earth alloy | A high performance alloy holding strength and creep resistance to substantially higher temperature than conventional magnesium, with markedly better corrosion behavior. Used where magnesium would otherwise be disqualified. | Castings, Extrusions, Bar and Rod, Plate | |
| ZK60A | Magnesium-zinc-zirconium wrought alloy | The highest strength conventional wrought magnesium alloy, zirconium grain refined, used for extrusions and forgings where AZ31B is not strong enough. | Extrusions, Forgings, Bar and Rod |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 1.74 | g/cm3 | 0.063 lb/in3; about 36 percent lighter than aluminum and the lightest structural metal in use |
| Melting point | 650 | degrees C | 1202 degrees F |
| Elastic modulus | 45 | GPa | About two thirds that of aluminum, so sections must be thicker for equal stiffness |
| Tensile strength, AZ91D cast | 230 typical | MPa | 33 ksi as die cast |
| Tensile strength, AZ31B sheet | 260 typical | MPa | 38 ksi in the H24 temper |
| Thermal conductivity | 70 to 160 | W/m-K | Varies substantially with alloy content |
| Coefficient of thermal expansion | 26 | micrometre/m-K | Higher than aluminum, which complicates mixed-metal assemblies |
| Galvanic behavior | Most active engineering metal | Corrodes sacrificially against essentially every other structural metal; isolation is mandatory | |
| Damping capacity | High | Substantially better vibration damping than aluminum, useful in housings and cases | |
| Ignition behavior | Chips and fines are flammable | Bulk magnesium is difficult to ignite; machining swarf and dust are a genuine fire hazard requiring Class D suppression |
Advantages
- The lightest structural metal available, about 36 percent lighter than aluminum
- Outstanding die castability, supporting thin walls and complex geometry
- Machines faster and with lower cutting forces than any other structural metal
- Superior vibration damping compared with aluminum
- Good electromagnetic shielding for electronics housings
- Excellent strength-to-weight in compression-dominated housing applications
Limitations
- No primary magnesium production currently operating in the United States
- Galvanically the most active engineering metal, requiring coating and isolation everywhere it touches other metals
- Machining chips and dust are flammable and require Class D fire suppression and dedicated handling
- Elastic modulus about two thirds that of aluminum, so stiffness-driven parts gain less than density suggests
- Limited high temperature capability, with creep becoming a problem well below aluminum's limit in common alloys
- Poor weldability compared with aluminum, and generally requires inert atmosphere
- Thin domestic supplier base for wrought product, with sheet and extrusion particularly limited
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| ASTM B90 | ASTM International | Magnesium alloy sheet and plate |
| ASTM B107 | ASTM International | Magnesium alloy extruded bars, rods, profiles, tubes and wire |
| ASTM B94 | ASTM International | Magnesium alloy die castings |
| ASTM B80 | ASTM International | Magnesium alloy sand castings |
| AMS 4427 | SAE International | WE43 magnesium alloy castings |
| AMS 4439 | SAE International | ZE41A magnesium alloy sand castings |
| MIL-M-3171 | U.S. Department of Defense | Magnesium alloy, processes for pretreatment and prevention of corrosion on |