Aluminium-silicon alloys

Thursday, September 27, 2012

Tags :  advanced-materials

Although cast iron has enjoyed something of a resurgence since the advent of compacted graphite iron (CGI) in passenger cars, in racing it is an anachronism, a throw-back to much earlier times. It is usually used for large structural castings only where regulation mandates it. Otherwise, aluminium is the material of choice.

Aluminium combines low mass and reasonable strength, and it finds use for pistons and all sorts of other components. The specific modulus or specific stiffness of aluminium is typical for metals. If we divide the elastic modulus of a metal by its density, for most common metals that we use in a race engine we will normally arrive at a similar number. Steel, titanium, aluminium and magnesium all generally fall within the range 24-28 GPa/(g/cm3) range.

If we want to use something with a greater specific modulus, we generally have to look at something fairly exotic, such as a metal-matrix composite (MMC), where particulate ceramic reinforcement is introduced in order to enhance certain properties.

However, there are a number of commercially available aluminium alloys that differ substantially from the 'usual' properties we would expect of aluminium. Silicon is widely used as an alloying element, and can commonly be found in a number of wrought and cast aluminium materials used in race engines. Piston materials with around 12% aluminium are commonly used for bespoke forged racing pistons, and higher percentages of silicon are used for some cast racing pistons.

With increasing silicon percentage comes decreased ductility, and materials with greater than 20% silicon are felt to lack the elongation required for a piston, hence we find that cast high-silicon pistons are rarely more than 19% silicon. However, one advantage of the higher silicon alloys is their lower thermal expansion coefficient. This allows the engine designer to run tighter cold clearances between the piston and cylinder bore.

Use is made of the relationship between silicon and decreasing thermal expansion coefficient in the manufacture of controlled thermal expansion alloys, and with very high proportions of silicon, these can match steel and even titanium in terms of their low thermal expansion coefficient. There are many applications outside of racing where this very low thermal expansion in a lightweight alloy is of interest, but it is possibly just of academic curiosity to us.

However, these alloys possess other properties that might be of real interest to us. They are much stiffer than conventional aluminium alloys and are also less dense. In fact, this combination of properties would make some aluminium-silicon alloys fall foul of the FIA Formula One limit of 40 GPa/(g/cm3) limit for metallic engine materials, even though they are not an MMC.

In racing applications, the limiting factor for the use of such materials is likely to be lack of ductility. While we generally don't design engine or transmission components to by cycled in the plastic region of the stress-strain curve, there is often some very local plastic deformation, especially on component edges and in areas of contact against stiff adjacent components. It is from such areas that fatigue cracks can initiate.

Written by Wayne Ward

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The high silicon (hypereutectic) aluminum alloys can be forged or permanent mold cast.  But they cannot be sand cast.  The high silicon aluminum alloys also have good hot strength properties, and that's why they make good piston materials.

If the rules allowed fiber-reinforced aluminum, this material would make an excellent piston:

http://www.tritonsys.com/products/bearingliners.html
"...felt to lack the elongation required for a piston."  I'm not convinced, yet, that pistons require alloys with much elongation.  But, then, I'm not quite sure of how one defines "elongation."  Is it the same thing as modulus of elasticity?  

The stress cracking risk referred to could be determined for alloys of any stiffness.  It is quite conceivable that an alloy could be very stiff and yet not incur cracking.  The determinant would be the specific characteristics of a given alloy or metal matrix.