Aluminium

Thursday, May 16, 2013

Tags :  advanced-materials

One of the usual requirements when designing a race engine is low mass; low engine mass aids low car mass. Where there is no lower limit for overall vehicle mass, a light engine gives a light car and therefore the best possible acceleration for a given engine output. Where overall vehicle mass is limited though, a light engine means the mass can be used elsewhere on the chassis, usually to better effect. Having spare mass means weight distribution and chassis stiffness can be experimented with.

One material that engine and transmission design engineers choose in order to achieve this aim is aluminium. There is a bewildering array of aluminium alloys, with many available commercially and with several choices of heat treatment. Naturally, with larger budgets and in aiming for the very best performance from every component, the choice available is less limited by finances. We are often able to choose the strongest, most reliable and least dense or stiffest alloys. As with so many things, motorsport feeds after the aerospace and defence industries have eaten; we have access to materials that have been developed by them. It should come as little surprise then that many of the aluminium stockholders supplying motorsport also have a lot of aerospace and defence business.

Still widely favoured for race engine pistons, among other things, 2618 is an alloy originally used many decades ago for aero engine pistons. Other traditional 2000-series alloys enjoy widespread use in motorsport, as they maintain a useful percentage of room temperature strength to temperatures in excess of 150 C. Examples of widely used 2000-series alloys are 2014 and 2024; the former is widely used in Europe while the latter is more popular in the US.

A relatively new class of aluminium alloys are those containing lithium. Enjoying high strength and stiffness, combined with low density, these are a premium material, but one that can offer significant advantages in certain applications. They can offer up to 10% increase in elastic modulus, and up to 10% decrease in density, compared to conventional aluminium alloys. In terms of specific modulus – that is, elastic modulus divided by density – they offer a huge advantage compared to other aluminium alloys. Most such commercially available alloys are classified in the 2000 series – 2050, 2090 and 2099 are examples. Alloy 8090 is the main commercial Al-Li which is not in the 2000 series; it contains magnesium and is among the least dense aluminium alloys.

The high-strength 7000 series alloys remain popular for applications where temperature is not an overriding concern. Adequate up to moderate temperatures, such alloys have very high strength. 7075 is widely used for high-strength components, as it has an ultimate tensile strength of >500 MPa (72.5 ksi) in a number of commercially available tempers. 7068 is the strongest of the commercially available aluminium alloys, with a minimum tensile strength approaching 700 MPa (101 ksi). It is not yet thought however to have found widespread use for engine and transmission components.

Moving briefly from wrought aluminium alloys to cast products, aluminium is used widely for major structural components in race engines. The typical A35x alloys which have been used widely in motorsport and aerospace continue to be popular. There are though new, very high strength casting alloys under development for racing applications, having been used initially in aerospace. The A20x family will find increasing use in race engines.

Written by Wayne Ward

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