We continue to review some of the most important materials in heat treatment and metallurgy.

Magnesium (chemical symbol: Mg)

Prior to 1755, magnesium and calcium were thought to be the same substance until Scottish chemist Joseph Black showed by experiment that the two were different. Black, a friend of both scientist James Watt and economist Adam Smith, also discovered carbon dioxide and made progress in our understanding of temperature and latent heat. The name magnesium originates from the Greek word for a district in Thessaly called Magnesia, where a precursor to magnesium (magnesium carbonate) was found. In 1808 in London, Sir Humphrey Davy isolated magnesium by passing electricity through salts. He also discovered barium, calcium and strontium in this manner.

Today, magnesium is the third-most prevalent structural metal after steel and aluminum. It is most commonly employed as an alloying element in aluminum. Magnesium alloys are the lightest of the structural metals used for die casting. They are used in automotive applications for cam covers, steering columns, steering wheels, brake and clutch pedals, clutch housings, seat frames and dashboard supports, among others. Their strength-to-weight ratio also makes them useful in power tools, garden tools, portable electronics and recreational products such as snowmobile components, archery bows and spotting scopes.

One of the most interesting properties of magnesium is its flammability. It burns with an intense, white light at a temperature of 3100°C (5600°F), as discovered by Wright Aeronautical during World War II. Wright used a magnesium crankcase in their Duplex Cyclone aviation engine (Fig. 1). This design was extremely light, but in early designs the crankcase was susceptible to catastrophic engine fires, which jeopardized the Boeing B-29 heavy bomber in which it resided. Due to this property of magnesium, special precautions are necessary when machining, grinding and heat treating magnesium die castings. Magnesium's bright light when burning makes it widely used in photography, flares and pyrotechnics.

Magnesium plays an important role in biological processes, and it is essential to both plant and animal life. Chlorophyll, the chemical that allows plants to convert sunlight into energy using photosynthesis, is a magnesium-centered porphyrin complex, with a single atom of magnesium at its center. In humans, magnesium is essential to the working of hundreds of enzymes. Referred to as a macro-mineral, magnesium is needed by the body in much larger quantities than trace elements. Calcium, sodium and potassium are also macro-minerals. The average human body contains about 25 grams of magnesium, roughly equivalent to five or six glass marbles, mostly in the bones. Magnesium is one of the six essential minerals that must be consumed in the diet, and humans eat about 250-350 milligrams each day.

In 1618, a farmer at Epsom in England tried to give his cows water from a local well, but the cows wouldn't drink the water due to its bitter taste. The farmer noticed the water helped heal rashes and scrapes. The substance became known as Epsom salts. It was eventually recognized as hydrated magnesium sulfate and used for hair and skin care. Today, magnesium compounds such as milk of magnesia, chloride and citrate are used for medicinal purposes.

Here are a few scientific and engineering facts about magnesium.

  • Atomic number: 12
  • Classification: Metallic
  • Density of solid: 1738 kg m-3
  • Molar volume: 14.00 cm3
  • Thermal conductivity of the chemical elements on a miniature periodic table spark table Thermal conductivity: 160 W m1 K1
  • Melting point: 650°C (1202°F)
  • Boiling point: 1090°C (1994°F)
  • Enthalpy of fusion: 8.7 kJ mol-1