Magnesium is an attractive metal for use in cars - primarily because of its light weight. Per unit volume it is 36 per cent lighter than aluminium and a substantial 78 per cent lighter than iron. And as well as being light, it can also be very strong - when alloyed with other metals, it has a very high strength-to-weight ratio. Magnesium is abundant - in fact it is the eighth most common element in the Earth's crust. It can be derived from seawater - which contains about 0.14 per cent magnesium - but processing it from magnesite (28.8 per cent magnesium) is a better bet. The metal is recyclable, and past corrosion problems have now been largely overcome with the development of new alloys.
However, there is a key factor that has held back its more widespread utilisation in automotive applications - cost. The cost of magnesium has been both high and unstable, with this history related more to a lack of competition in the industry than production costs. It is likely though that this will soon change, with some very large magnesium production plants currently being built, including a $A1.2 billion magnesium metal project at Stanwell near Rockhampton, in Australia. When commissioned, this will make Australia the world's largest producer of magnesium.
Mention magnesium and many will think of the incandescent brightness of a burning magnesium flare, however in automotive applications, the flammability of the metal is not a major issue. In fact, components greater than 3mm in thickness will cease burning once heat is removed. During machining, though, appropriate safety precautions need to be observed, which may increase the cost of machining magnesium castings as opposed to those made of aluminium.
The potential areas in which magnesium can be used in vehicles are widespread, as this table shows:
Potential Magnesium Components
| Body |
Powertrain |
Chassis |
- unibody and closures
- glass
- hardware
- exterior and interior trim
- body electricals
- seats
- passenger restraints
- instruments and controls
- climate control |
- engine and accessories
- engine electricals
- engine controls
- engine cooling system
- transmission or transaxle
- clutch (if manual)
- drive line (rear-wheel drive)
- differential
- transfer case |
- suspension steering system
- bumper system
- brake system
- subframes
- fuel storage system
- chassis electricals
- exhaust system
- wheels and tyres |
In fact, almost all current cars have at least some magnesium components - seat frames, steering wheel cores and rocker covers are parts now frequently made from magnesium. A typical current car is said to have about 3.6kg of magnesium within it.
While magnesium is available in sheet and bar form and can be welded, glued and riveted, the major production process being used in automotive applications is casting. Magnesium can be cast with very thin walls, reducing weight still further and allowing the design to be optimised. In a dashboard support casting, for example, the thin walls allow more room for the tightly-packed components inside the dashboard. Additionally, magnesium castings have good sound and vibration damping properties, making their use in applications such as valve rocker covers advantageous.
The use of magnesium in areas such as steering wheels has hastened the investigation of the metal's yield and impact absorption characteristics. In the US, Ford originally used a magnesium alloy dubbed 'AZ91D' for a steering column lower bracket and mating bearing retainer in the Ford Mystique and Contour. Developed in the late '80s, the column met the safety regulations then applying. However, when these regulations were changed, a different magnesium alloy - AM50A - had to be specifically developed for the application. Its deformation and yield characteristics, together with a different casting process, gave the required new crash properties.
Magnesium is also being used in the automotive industry as a 'fix-it' when a design is becoming over-weight. A bracket holding an ABS unit on a Chrysler min-van was initially fabricated by a supplier from hot-rolled steel. However, it failed during testing and had to be beefed up. But this added to the part's weight - soon it was weighing-in at 1.1kg. And that was just too heavy. So the company turned to a magnesium casting instead, with the final bracket weighing just 360 grams - and it passed all testing!
Seat frames are also now often being made of magnesium rather than stamped and welded steel. The table below summarises the benefits of die-casting magnesium seat frames:
| Why Die Casting? |
- reduced total part count
- reduced seat assembly operations
- improved dimensional accuracy and repeatability
- reduced squeaks and rattles |
| Why Magnesium? |
- lower specific weight than other options
- better elongation than the other die casting metals
- 20% - 30% shorter cycle times than aluminium die casting
- longer die life (about double) than aluminium die casting
- ability to produce thinner walls than aluminium die casting |
However, magnesium does have disadvantages.
1. Creep
The material is prone to creep, especially at elevated temperatures. This has the potential to allow bolts to loosen and tolerances to be lost. One magnesium alloy has been shown to creep under ambient temperatures with an applied stress only 39 per cent of its yield stress. A total loss of clamping force may occur after 20,000 hours at 150 degrees C - a temperature at which many engine components work. However, the study also pointed out that 20,000 hours is beyond the expected life of most automotive components. This creep characteristic does not rule out the use of the metal to form blocks and heads, though much development still needs to be made in this area. (The VW Beetle used a magnesium engine block until the 1970's, however the design would not meet current required standards.)
2. Corrosion Resistance
Despite manufacturers of magnesium suggesting that new alloys have overcome this problem, others disagree. Phosphate coatings, powder coating and other more complex coatings have been used, but long-term corrosion resistance in adverse environments is still to be proved in automotive applications.
3. Recycling
While magnesium is recyclable, the process is not the same as with, say, aluminium. Magnesium has a higher oxygen potential than most metals and so the oxidation of molten magnesium does not naturally stop (as occurs with aluminium) but instead accelerates. Melting must therefore be carried out under a controlled atmosphere. Additionally, impurities in magnesium do not naturally float to the surface, so non-metallic inclusions must be mechanically removed by techniques that include passing the molten material through stainless steel filters.
Conclusion
The use of magnesium in cars is growing rapidly. While currently mostly confined to parts located inside the cabin and so not susceptible to temperature or corrosion extremes, as development of new alloys continues, it can be expected that many components currently made from aluminium - and to a lesser extent, steel - will be replaced with the light and strong material.
Some resources:
http://www.litemetals.com/index2.htm
http://www.transportation.anl.gov/ttrdc/publications/papers/magnesium.html#N_1_
http://www.uscar.org/techno/magnesium.htm
http://www.tech.plym.ac.uk/sme/mech330/magcast.htm
http://me.mit.edu/2.01/Taxonomy/Characteristics/Magnesium.html
http://my.noranda.com/Noranda/magnesium/Magnesium+Overview/Magnesium+Overview.htm
http://www.austmg.com
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Magnesium Facts
- The lightest of all structural elements.
- The eighth most common element in the earth's crust (2.7%).
- Easily malleable, easily alloyed.
- Does not exist in nature in its metallic form.
- Found in varying concentrations in seawater, The Great Salt Lake, brine, and in various magnesium ores.
- Named after ancient Greek city Magnesia, where magnesium was first found.
- First recognized as an element by Joseph Black (1755).
- Discovered in oxide state by Sir Humphrey Davy (1808).
- First isolated by Antoine-Alexander Bussy (1828).
- Originally used in wire or powder form for photographic purposes.
- Magnesium is also an essential constituent in chlorophyll (C55H72MgN4O5).
- In 1999, over 375,000 tons of magnesium were produced worldwide
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Some Current Automotive Magnesium Components
Rear Transfer Case
This rear transfer case is made from AZ91D, an alloy produced by Noranda Magnesium. The casting has a mass of 2.7kg and replaces a part that was previously manufactured from aluminium. The surface shown here is 'as cast'.
Instrument Panel Support
This product is used in General Motors Buick and Cadillac models. A die-cast component, it is 1372mm long and about 320mm high and deep. It weighs 5.9kg and replaced an assembly formerly fabricated from steel.
Seat Support
Produced from Noranda Magnesium's AM60B alloy, this seat is made by die castor Meridian for Fiat and Jaguar models. The seat back and base are both about 600 x 800mm, with a wall thickness of 2.5mm. The complete seat support weighs 2.6kg.
Steering Wheel Core
Produced by Spartan Light Metal Products for the US Toyota Camry, this steering wheel frame has a mass of just 750 grams. It replaces a part previously die cast from aluminium.
Cam Cover
This cam cover is used on the Ford Zetec engine. Produced again by Spartan Light Metal Products, it is formed from AZ91 magnesium alloy. It replaces a product formerly made from injection moulded plastic and weighs just 900 grams! Design requirements included rigidity in addition to light weight.
Intake Manifold
The Cadillac Northstar engine uses this inlet manifold produced by the Lunt Manufacturing Co from AZ91D magnesium alloy. This part of the inlet manifold weighs 2.9kg, while the upper half weighs only 1.6kg. Design requirements included an as-cast flatness of 0.5mm over the entire surface and being leak-proof to 150cc/minute at 1 Bar pressure differential.
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