Wayne Ward Examinesrepublished Theonline Current State of the Art and Future Prospects in Metals Technology for Motorsports
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MEDIA. POWER Nitriding steels remain the most popular materials HIGH for race crankshafts (Courtesy of Arrow Precision) OF REPUBLISHEDONLINE BE OR TO PROPERTY PRINT NOT IN FOCUS : ADVANCED METALS MEDIA. StaunchPOWER HIGH alloysOF Wayne Ward examinesREPUBLISHED theONLINE current state of the art and future prospects in metals technology for motorsports he subject of metallic materials – their proper selection, of the two. This doesn’t just apply to durability (as judged by fatigue metallurgy and properties – alreadyBE fills countless textbooks, performance); equally we could consider corrosion resistance, friction, so it would be fruitless and impractical to try to ORcover these temperature resistance and so on. topics in any depth in this article or even in an entire issue Companies who make bespoke engines – particularly the few who ofT this magazine. What we can do though is give an appraisal of the make a large proportion of the parts themselves – need to know which present state of metals development and possible future developments, materials will best suit a given application if they are to produce a based on input from motorsportTO and metals industry experts. light, powerful, durable and economical engine. They also need a While anyone upgrading an existing production engine will need good knowledge of the function of the component to which they will at least some materials knowledge, it is the companies who supply apply their chosen material choice, as well as the critical quantity or bespoke race engines who need to be up to date not only on the quantities that affect the design. materialsPROPERTY but also their application and new developments in the For example, say we would like to make the lightest possible test technology. Although an improved PRINTmaterial specification can only piece that will withstand a 10,000 N tensile load. If we imagine that rarely make upNOT for some deficiency in the design or manufacture of we are limited to using steel, we need to choose a material with a particular part, it is also true that an improved specification will, the greatest tensile strength (assuming that every type has the same for example, offer the opportunityIN to make a part lighter for the same density). We may therefore find one with a tensile strength of 2500 2 durability, or more durable given the same mass, or a combination N/mm and we have our answer. t 29 FOCUS : ADVANCED METALS material for a given application. The materials suppliers take it that your knowledge of your application is good enough to make the correct material choice given the information they supply to you. It is a good idea to involve material suppliers at an early MEDIA.stage, not only to gauge what is available but also to listen to what knowledge they may have of similar applications. Without access to some complex FEA, and a method of validating the results, we have to make a lot of assumptions about what we require for a given part in terms of materials, and the knowledge of a good materials supplier or expert Fig. 1 – Norton was using magnesium for major engine components more than 50 years ago, and the material is now widely used in automotive engines POWER can be invaluable here. There are a few motorsport If we then find another material with a strength of 1500 N/mm2 companies who are rich enough to venture into the world of material strength but with half the density of steel, however, then we have development individually, but the rest of the industry is well served a better candidate. The critical quantity, or property in this case, is in that many of the materials we use are also used by the aerospace specific ultimate tensile strength – that is, ultimate tensile strength industry. It is mainly the aerospace industry, and its constant striving divided by density. We can take our pile of materials property books for improvement, that we have to thank for better materials being and do the calculations to see which suits this simple application.HIGH developed that suit our needs. However, most real-world applications are not so easy to work out. Take the case of a con rod. If we look at the stress field within Magnesium alloys a con rod, it is complex and consequently much of the rod is barely The regulation proscribing the use of magnesium alloys in Formula stressed. We could optimise the rod, based on an even stress field One and NASCAR engines is puzzling. In the days when the flow of t (most sensibly by matching the fatigue strengthOF of the material) to arrive at a very light rod that would cause the engine to fail. REPUBLISHED The reason is that the form of a con rod is dictated ONLINE largely by a stiffness requirement, which is controlled by the geometry of the part and the elastic modulus of the material. If we are presented with a material that is 10% stronger and 3% more dense, butBE has an elastic modulus equal to the existing rod material, OR would it make sense to change? We need to know the volume of material limited by strength and compare this to the amount of material required to fulfil the stiffness function. If the whole TOlength of the rod beam was limited by strength in the given design, then it would make sense to change material in this case, but if the only material limited by strength is a small regionPROPERTY around the little-end oil drilling then the answer is that we should use the existingPRINT material. As can be seenNOT from this example, the issue of correct material choice depends only partly on knowledge of the best materials. Ashby’s book (1) Fig. 2 – Aluminium is commonly used for major structural engine castings, IN as seen here in this V10 endurance engine (Courtesy of Engine Developments) is an excellent reference on how to select the best 30 FOCUS : ADVANCED METALS and 10 mm deep, for the same mass we can use a beam of 50 mm wide and 15 mm deep in magnesium. The stiffness of the beam is proportional to the product of the modulus of elasticity and the moment of inertia of the section. The modulus, E, of aluminium is 72 GPa and that of magnesium is 44 GPa. MEDIA. Doing the calculation for stiffness gives magnesium a 106% stiffness advantage in this example. If we were happy with the stiffness of the aluminium part, we could match the beam bending stiffness using a section depth of 11.78 mm, and the mass of the magnesium part would be 21% less than that of the equivalent aluminium part. This example shows that neither modulus, nor even specific modulus (modulus divided by density), represent a good way to select the best material here. Where there is no imperative for strength, as in an unstressed cover, magnesium offers an advantage over cast aluminium as it can be cast POWERin the same thickness, hence its widespread use for this purpose in roadcars. Such is the advantage it offers that BMW uses magnesium to cast cylinder blocks. There is no doubt though that magnesium has some disadvantages Fig. 3 – This forged race piston is made from ‘the old favourite’ 2618, or RR58. It remains the mainstay for race pistons (Courtesy of Omega Pistons) compared to aluminium. It has poor corrosion behaviour, and low strength and stiffness, which is why people are likely to get money into the sport was even more limited than it is now, magnesium unsatisfactory results when simply trying to substitute aluminium with wasn’t beyond the wallet of even small engine manufacturers.HIGH The magnesium. However, if we take magnesium’s mechanical properties NASCAR engine regulations use the word ‘magnesium’ twice, on into account and design around them, it is an attractive material and both occasions stating, “Magnesium and other exotic materials will found success when used for structural castings such as sumps in not be permitted”. Frankly, magnesium is about as exotic as a slice Formula One before it was banned. of bread and is, as we will show, simply a sensible choice in certain There have been several attempts to use magnesium for more applications. OF highly stressed engine components, notably pistons. The fact is that, Magnesium has been commonplace in race engines for many in some respects, magnesium is not an ideal piston material. In terms decades and it has also been used quite widely in road-goingREPUBLISHED engines of physical properties for piston materials, high thermal conductivity for cars and motorcycles for more than 20 years now, even in ONLINEis considered important, and that of magnesium is much less than that reasonably low-budget vehicles. Fig. 1 shows a 1951 Norton model of aluminium. However, when people are looking to thermal-barrier 40M, which has magnesium crankcases. coatings on piston crowns, the low thermal conductivity of magnesium Magnesium has a valuable role to play in making a lightweight alloys for use as a piston material may no longer be the obstacle it was engine, which is important for roadcars becauseBE the supporting once considered to be. structure can therefore be made lighter – and a lighter car ORcan There are a number of high-strength wrought magnesium alloys that then have a smaller, lighter engine, giving better economy. Just as could be considered as candidates where aluminium components are motor racing must look to embrace technologies which roadcar currently used. manufacturers value, it should not outlaw materials which the car The use of magnesium can in some cases be limited by its corrosion manufacturers commonly use.