- What Are They?
- Design Notes
- Typical Uses
- Competing Materials
- The Environment
- Technical Notes
- Attributes of Magnesium Alloys
- Technical Attributes
- Aesthetic Attributes
- Features (Relative to Other Metals)
What Are They?
Magnesium is the second of the light metal trio (with partners
The push for compact, light, electronics (laptops, mobile phones), and lightweight vehicles (wheels, in-cabin metal parts) has prompted designers to look harder at magnesium alloys than ever before, and has stimulated production and driven prices down. What do they offer? Magnesium has a low density, good mechanical damping, much better thermal conductivity than steel, less good electrical conductivity than copper and aluminum but still good. It survives well in the protected environment of a house or office, but it corrodes badly in salt water and acids; even sweat is enough to tarnish it. Magnesium is flammable, but this is a problem only when its in the form of powder or very thin sheet. It costs more than aluminum but nothing like as much as titanium. It is easy to machine, but because of its low stiffness, parts must be firmly clamped while doing so. Magnesium alloys are designed for specific forming purposes. Some (like AZ31B) are good for extrusions. Others (az63, az92, and am100) have been formulated for investment casting; the az91 range are used for die-casting. Most magnesium alloys can be welded using tig or mig methods; and soldering and adhesive bonding are both feasible. Spot and seam welds are possible but only in low stress applications; riveting is better, provided aluminum rivets are used to avoid galvanic corrosion.
Aerospace; automotive; sports goods; nuclear fuel cans; vibration damping and shielding of machine tools; engine case castings; automotive wheels; ladders; housings for electronic equipment, office equipment, and lawnmowers.
Magnesium is the fifth most abundant metal in the earth’s crust, and the third in its oceans – and it can be extracted economically from both (the Dead Sea, thick with dissolved salts – is the best source of all). But its extraction is very energy-intensive, consuming three times more per unit weight than commodity polymers and nearly twice as much as aluminum. It can be recycled, and doing this uses barely one-fifth as much energy.
The classification system of the American Society for Testing Materials (ASTM) is the most widely used. In this system, the first two letters indicate the principal alloying elements. The letter corresponding to the element present in the greatest quantity is used first; if they are equal, they are listed alphabetically. The letters are followed by numbers which represent the nominal compositions of the principal alloying elements in weight percent rounded to the nearest whole number; thus AZ91 means the alloy 90% Mg, 9% Al, and 1% Zn.
Attributes of Magnesium Alloys
Price, $/kg: 2.60–11.40 Density, mg/m3: 1.73–1.95
El. modulus, gpa: 40–47 Elongation, %: 1.5–20 Fr. toughness, mpa·m1⁄2: 12–70 Vickers hardness, hv: 35–135 Yld. strength, mpa: 65–435 Service temp., C: −40–300 Specific heat, j/kg·k: 950–1060 Th. conduct., w/m·k: 50–156
Energy content, mj/kg: 300–500 Carbon footprint, CO2 (kg/kg): 35
Recycle potential: High
Low (0), High Pitch (10): 8–9 Muffled (0), Ringing (10): 3–6 Soft (0), Hard (10) 8 Warm (0), Cool (10): 8–9 Reflectivity, % 68
Features (Relative to Other Metals)
Exceptionally lightweight Easy to die-cast Adequate strength