Advanced Metals: Magnesium
In previous articles on advanced metals, we have looked at a number of materials currently used in motorsport and some which are just starting to be used. In this article, we will look at a material which has, at times, been held up as a 'wonder-material' and at other times almost completely neglected when we consider engine design. Despite this it has been widely used in racing engines for structural and reciprocating parts, and is commonly found on many road vehicles. It is Magnesium to which I refer, and in this article we will look at the various applications for which it has been considered in the past, where it stands presently, and where we might see it used in the future.
Magnesium enjoys a very low density and so is naturally attractive to the designer of a bespoke engine. With a density of 1800kg per cubic metre, it is less than twice as dense as water and is over 30% less dense than a typical aluminium alloy. However, in keeping with most metals, its stiffness or, more accurately, elastic modulus is roughly in proportion to its density and this is typically around 45GPa. However, providing that we have enough space, and that the part isn't stressed purely in tension or compression, we can take advantage of the low density to employ the same mass, intelligently placed, to provide greater stiffness than the equivalent mass of aluminium or steel. The fatigue strength of magnesium can though compare favourably to aluminium and work is being done to develop new alloys with improved tensile and fatigue properties.
So, where have we seen magnesium used in engines? Well the obvious application is in castings. Every major casting of a racing engine has been made at one time or another from magnesium. There have been experimental Formula One cylinder blocks (at a time when this common material used widely on series production engines was legal in Formula One), cylinder heads, crankcases, cam carriers and covers, not to mention the many non-structural covers and castings around the engine. So, apart from nonsense rules, why is it not more widely used today? Maintenance and corrosion are two good reasons. Common complaints about magnesium parts are that they are prone to threads being pulled out (people tend to make a simple material substitution and forget that threads will pull with maybe 30% less applied stress) and that the parts corrode very quickly. People who manage to keep magnesium and moisture apart are generally much happier to use the material. It is among the least noble metals and is prone to general corrosion and particularly susceptible to galvanic corrosion. This is why magnesium is most commonly seen with some sort of surface treatment applied. It has been used as a piston material and owing to good high temperature mechanical properties it might appear a good candidate at first examination. However, it has some other properties which render it less attractive as a piston material. Among these is low thermal conductivity. It has been used in valvetrain applications with success too. Anywhere where there is little or no moisture, and an oil film seem to be relatively trouble-free applications.
BMW has recently introduced a series production magnesium composite cylinder block, where the outer part is magnesium and the inner part, which is in contact with the water circuit is made from aluminium. This is one of the major reasons for the large decrease in mass compared to the previous equivalent engine from the same manufacturer. Magnesium has been popular for none-structural covers on motorcycle engines for decades and in the push for smaller lighter engines as we seek to improve fuel economy, it will become a more common material for series-production motor vehicles. The accompanying picture shows a 1952 Norton motorcycle engine which uses magnesium crankcases. Almost 60 years later Formula One rule makers disallow this material, whilst it becomes common on road cars. In racing, we must be careful not to become an irrelevant backwater of old materials technology.
Fig. 1 - 1952 Norton engine.
Written by Wayne Ward