MAGNESIUM

GERALD D. COLE, LIGHTWEIGHTSTRATEGIES

Magnesium is the eighth most common element and the sixth most abundant metal, making up about 2.5% of Earth's surface; seawater contains 0.14% magnesium. The element is concentrated in the minerals carnallite (MgCl6H₂O), magnesite (MgCO₃), and dolomite (MgCO₃CaCO₃).

Magnesium is the lightest industrial metal, with a density that is about the same as engineered plastics, two-thirds the density of aluminum, and one-quarter that of steel. It is a little weaker (except in fatigue) and a bit softer than aluminum; conventional alloys creep at temperatures above 125 °C, but newer alloys almost equal aluminum. Most properties are significantly better than engineered polymers. Magnesium's main limitations are its low formability at room temperature, but this is much improved at temperatures greater than 200 °C. Molten magnesium alloys have outstanding fluidity and can be cast into very large (1 m x 2 m), ultrathin (1–2 mm), high-surface-detail shapes.

It was the influence of war that changed magnesium's stature from a curiosity to an industrial material. And it was in Germany where the body of knowledge originated on how easily magnesium could be alloyed (with aluminum, zinc, and manganese) and formed (cast, forged, extruded, welded, and machined). In fact, magnesium was referred to as "the German metal" well into the 20th century.

FLYING HIGH A Convair B-36A nuclear bomber. The plane contains about 10,000 kg of Mg. COURTESY OF BRIAN LOCKETT, GOLETA AIR & SPACE MUSEUM

Magnesium's first uses in flares, tracer bullets, incendiary bombs, fireworks, and flash photography were based on its significant chemical reactivity. In fact, most of us learned about magnesium from our high school teachers as we watched a lit ribbon produce a bright flare. And it is this fiery perception that remains to this day. It was only in the late 1990s that magnesium was reclassified from a reactive to an engineering metal by the Minerals, Metals & Materials Society, the dominant North American metals society.

A quick search on Google reveals over 1.5 million citations for "magnesium." More than 1.4 million are connected to nutritional and medicinal uses, since it is an important element in both plant and animal life. Magnesium has valuable chemical uses: in reducing uranium and titanium from their ores, in eliminating sulfur from steel, and in producing high-strength (spheroidal graphite) cast iron. Magnesium oxides are also central to high-temperature refractories. Up until recently, magnesium's main use was as a 1–3% alloy, with 20 million tons of aluminum alloy produced annually.

The recent history of magnesium is marked by two events. First, 70,000 World War II Messerschmitt and Stuka dive bombers contained magnesium fuselages, engine parts, and wheels. (U.S. Convair B-36 bombers contained 10,000 kg.) There is no magnesium in today's aircraft, but there is in some jet engine parts, rockets, missiles, and helicopter rotor housings (machined from high-temperature alloy castings). Second, the 1939–2003 Volkswagen Beetle showed the world that magnesium has significant automotive applications, with almost 25 kg of magnesium castings in its transmission housing and air-cooled engine.

Five years ago, magnesium's cost was twice that of aluminum (about $4.00 per kg). This was based on 40,000–60,000-ton expensive, continuous, chemical plants that reduced the combined magnesium in magnesite, carnellite, and seawater to MgCl₂; smelters (with their expensive electricity) reduced the chloride to metal. Using thousands of small, low-cost, coal-fired kilns, Chinese producers now directly reduce dolomite in low-vacuum steel tubes and condense magnesium "crowns" in 60-kg batches at one-fortieth the cost per ton of capacity compared with Western producers. From nothing six years ago, China now produces one-half the world's supply and has forced magnesium prices down to nearly those of aluminum ($1.60 per kg).

Magnesium production is one-fortieth that of aluminum, but it is growing five times faster, at 5–10% annually, as its valuable properties are increasingly exploited to reduce weight, increase fuel efficiency, and reduce emissions. The automotive industry is the largest user. Light, rugged, "handheld everything" that requires electronic shielding can be made from ultra-thin (about 1 mm) high-pressure die castings, injection-molded semisolids (Thixomolding), and stamped sheets. Tens of millions of laptop computers, PDAs, cell phones, camera bodies, and projection equipment are produced annually this way. There is a growing use for magnesium in power tools; kitchen appliances; and lawn, garden, and sporting equipment where magnesium's excellent vibration absorption adds consumer value. Unlike most polymeric materials, magnesium alloys have outstanding impact and dent resistance--and they are 100% recyclable.

As prices decrease, new alloys will be invented and new uses found to make magnesium the wonder metal of the 21st century.

Gerald D. Cole is the principal in LightWeightStrategies LLC and is a senior technical adviser for the Australian Magnesium Corp. He was a Ford Motor Co. global lightweighting expert for 35 years until his retirement in 2001.

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