Magnesium has a special place in the hearts of VW enthusiasts. We all know (or SHOULD know) that the engine case of the Type I and 3 air-cooled motor is made of an alloy of it, as is the gearbox casing. We might also know that it burns. If we're good at trivia, we could say that Volkswagen once used 38,000 tonnes per annum, nearly one sixth of the entire world's output.
In actual fact, one Beetle contains about 22 kg of magnesium castings, because the air-cooled engine design was ideally suited for ultra-light alloy. The trend to front-mounted water-cooled engines has now reduced VW's magnesium consumption considerably, although it still finds uses mainly in gearboxes and other transmission components. The pressure diecasting plants operated by Volkswagen remain prime examples of highly developed technology.
Magnesium is known as the lightweight champion of the metals. It is the lightest of all metals used in general engineering, with a relative density only 1.7 times that of water! Aluminium is more than half as heavy again, iron and steel are four times heavier, and copper and nickel are five times heavier. Perhaps not unexpectedly though, magnesium is also relatively weak in the pure state, with a tensile strength of only 110 newtons per square millimetre. However, when alloyed with various other metals it can be strengthened two or even three times over, yet still remain very light. Aluminium, zinc and thorium are common alloying metals.
Public knowledge of magnesium is sparse, perhaps because we don't have much to do with it. Yet did you know it is the fourth most abundant metal, found in the sea as well as on land? Magnesium was isolated for the first time in 1808, earlier than aluminium, and efforts were made to produce it commercially throughout the nineteenth century. However, development was slow partly due to technical and partly economic problems.
Magnesium is expensive to produce because it's difficult to extract it from its mineral ores. Electrolysis must be used in a process similar to that used for extracting aluminium. Metallurgists also found that magnesium has a great affinity for oxygen, meaning that it burns with an intensely hot flame. In fact, magnesium was used as the main constituent of incendiary bombs during WW2, and is still used today for the production of intense light, such as fireworks flashlights and flares. It may be difficult to believe that so flammable a metal can be used for any structural purpose, let alone things like cooking pots and frying pans! Of course, the explanation here is that it must first be melted or powdered, and be in contact with ample supplies of heat and air before it will ignite.
The Germans pioneered the special methods needed for smelting and refining magnesium, casting the alloys, and later fabricating it into various finished forms. They also discovered some of the effects of alloying with the metals I've already mentioned, although the Brits also did much work, particularly during WW2. In fact, world magnesium production increased ELEVEN-FOLD between 1937 and 1943, thanks to the wartime needs of the aircraft industries.
It was discovered that small amounts of manganese added to magnesium would improve corrosion-resistance. Zirconium acts as a grain refiner. "Rare earth" metals such as thorium and zirconium give a further increase in strength, particularly at high temperatures. This became very useful when development of the jet engine began in both Britain and Germany.
Magnesium alloys can be subjected to all the usual metallurgical treatments, including casting, diecasting, rolling, forging, extruding, and pressing. In spite of its flammability in the molten state, magnesium can be cast with ease and safety if precautions are taken against the metal over-heating.
No metal can be cut, filed, drilled or shaped as easily or as quickly as magnesium. This also reduces the cost of the finished product. Magnesium also lends itself easily to welding and riveting. The strength of cast and wrought magnesium alloys in relation to their weight can be very high indeed, cast alloys showing strengths of 300 newtons per mm2 and wrought alloys up to 380 newtons. The strength to weight ratio of such material resembles that of high tensile steel!
Naturally, magnesium alloys find great application where lightness and strength is required. During the war retractable undercarriage parts were cast in magnesium, pilots' seats were made of welded magnesium tubes, petrol tanks of magnesium sheet, while engine parts such as supercharger blower and reduction gear casings were made of magnesium alloys. Today, apart from automotive uses already mentioned, you'll find magnesium alloys in everything from wheels on racing cars (BRMs!) to textile machinery parts, portable tools, cameras, office equipment and many pieces of military hardware.
Much of the present output comes from seawater, of which each cubic kilometre contains a million tonnes of magnesium in the form of magnesium chloride. Recent developments have centred on the use of natural brine, as they allow for magnesium production and the additional sale of chlorine gas produced as a by-product. New plants using this process are already operating in the USA.
You might not realise that by far the most widespread use of magnesium, requiring nearly half the world's tonnage, is in the alloying with aluminium. This includes the VW components we know - and also the 4~/2 per cent of the world's magnesium used for the easy-open tops of drink cans!