Aluminium Matrix Composites in Automotive Applications

Automotive

Aluminium matrix composites in automotive applications Md. AI Mehedi, Bangladesh University of Engineering and Technology (BUET)

Aluminium matrix composites (AMCs) have become a very high-grade material for the automotive world nowadays. AMCs are highly used in producing cylinder boxes, brake rotors and drums, driveshafts, camshafts, crankshafts and other parts of cars. Steel is used as the main manufacturing alloy in the automotive industry. But the use of aluminium and magnesium is increasing in the automotive sector due to their light weight, which contributes toward reducing the fuel consumption. In fact, a weight reduction of 10% can lead to 5% reduction in fuel consumption (1). Aluminium is also useful in having a high strength-to-weight ratio and better stiffness. AMCs are generally reinforced with ceramic particles and fibres to obtain improved mechanical and wear properties. The reinforcement improves the fatigue resistance and lowers the thermal expansion. For example, the addition of 60 vol.-% of alumina (Al2O3) fibres to aluminium increases its elastic modu-

ALUMINIUM · 7-8/2011

lus from 70 GPa to 240 GPa, and reduces the coefficient of expansion from 24 to 7 ppm/°C. The wear resistance of Al9Si reinforced with 20 vol.-% of SiC increases by varying degrees, up to values better than or equivalent to very hard grey cast iron (2). Car manufacturers are finding ways to use AMCs for several components, depending on the model. Recently the Germany-based automotive supplier KS Aluminium Technologie AG has developed a MMC cylinder lining for Porsche cars, for better wear resistance (2). Toyota launched a new engine built with TiAl-V alloy reinforced with TiB2 particles, a costeffective way to achieve very high wear resistance as well as tensile and fatigue strength (3). Why aluminium is replacing steel With the growing concern about greenhouse effects, the use of aluminium is overtaking that of steel. As aluminium has one third the density of steel, its lightness is used by technologists to reduce the weight of vehicles so as to reduce fuel consumption. Beside this the characteristic properties of aluminium, good formability, good corrosion resistance, high strength and stiffness-to-weight ratio and re-

before. This is because the vehicle consumes less fuel and less capacity is therefore needed for the fuel tank. It is estimated that a vehicle weight reduction of 10% results in 8 to 10% fuel economy improvement (6). For chassis applications, aluminium castings are used for about 40% of wheels and for brackets, brake components, suspension (control arms, supports), steering components (air bag supports, steering shafts, knuckles, housings, wheels) and instrument panels. Aluminium matrix composites Metal matrix composites have become a regularly used component in the automotive world nowadays. When a metal matrix is mixed with particles or fibres of one or more other materials, this is termed a Metal Matrix Composite (MMC) where the materials are insoluble in each other and have superior desired properties. Ceramic particles or fibres such as SiC, Al2O3, TiB2, carbon, etc. are mainly used as the added element in aluminium matrix. Metal matrix composites have better mechanical and wear properties than the matrix metal. These materials posses higher strength, stiffness, corrosion resistance and wear resist-

Source & Diagram: GDA

The increasing use of Aluminium Matrix Composites (AMCs) has become an economic issue in the modern automotive world. Although AMCs are quite costly compared with traditional materials, they have potential to replace traditional materials like steel or cast iron or even Al/ Mg alloys, because they provide higher strength, stiffness, wear resistance, corrosion resistance, high fatigue limit and lower thermal expansion. These properties can lead to higher value addition such as long life of engines, wheels and brakes as well as better service from them. The market for aluminium matrix composites was projected to exceed USD68 million by 2010. The main use of AMCs in the automotive industry includes brake discs and drums, cylinder blocks, cylinder liners, pistons, crankshafts, camshafts, valves, pushrods, connecting rods, brake callipers, turbo exchangers etc. According to a report by Global Industry Analyst, Inc. the use of AMCs in ground transportation amounts to 57% of the total uses of metal matrix composites, with an 8% Compound Annual Growth Rate (CAGR) in the 2001 to 2010 time period. In the near future AMCs instead of steel can be introduced into the body panel and structure of cars.

Fig. 1: Average use of aluminium in passenger cars produced in Europe and differentiated by application (4)

cyclability, make it an ideal metal to replace heavier steel. The use of aluminium in the automobile sector has almost doubled in the last ten years (see Fig. 1). Total use of aluminium per vehicle is predicted to rise to 250 kg or 340 kg, with or without taking body panel or structure into account (5). It is reported that the use of aluminium enables car manufacturers to use smaller engine blocks for the same vehicle performance as

ance. In recent years the use of aluminium in the automobile sector has been increasing substantially. However, aluminium has very poor wear resistance on its own, which makes it incompatible for use in many inner and outer parts of a car. If ceramic particles or fibres are added, or even carbon, the wear resistance is increased to very high value. This character­istic helps to increase the use of aluminium metal matrix (AMC) in the automobile sector.

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Automotive

Use of AMC in various applications The use of aluminium matrix composite was initiated by Toyota in 1983. The company launched a piston for a diesel engine by selective reinforcement of an aluminium matrix with chopped fibres preformed in the ring groove area of pistons. This AMC improved the fatigue and wear resistance of the piston. It also helped to lower the coefficient of thermal expansion, and so reduced the tendency to cracking and distortion. The engine is the hottest spot in a car. Thus the material inside the engine needs high temperature strength and good fatigue strength as pistons produce cyclic loads inside the chamber and on the surface of the cylinder block, and also on the piston itself. To avoid wear loss of pistons, cylinders and other engine components it is highly recommended to use AMCs. The applications of AMCs in different engine parts and beyond are as follows: Pistons: The piston is one of the most vital parts of an automobile. Pistons are generally exposed to a dynamic thermal and mechanical environment. Thus they need good high temperature strength and fatigue strength, low thermal expansion and good wear resistance. To ensure these properties, the car manufacturers are now concentrating on the use of AMCs. The selectively reinforced pistons have better wear resistance and a lower coefficient of thermal expansion. They also have better heat exchanging properties and high tolerance to cracking. Although AMCs are more expensive than traditional materials, mass production involves only a single step and this optimises the cost with value-added service (7). Due to the low CTE of AMCs, the clearance between cylinder wall and piston can be reduced, which results in less noise and less wear of both surfaces. Cylinder blocks: The cylinder block is a high-temperature chamber where ignition takes place. The cylinder block accommodates continuous impacts from the pistons, and must therefore have very good wear resistance, fatigue strength and creep resistance. The use of AMCs in cylinder blocks provides these properties. Al-Mg or Al-Si alloys with ~30% SiC or Al2O3 are mainly used in cylinder blocks. The Porsche 911, Boxster, Toyota Celica and Honda Prelude are some examples of models where AMCs are used in the cylinder block. Cylinder liners: Cylinder liners are provided to reduce the friction between the piston and the cylinder wall. Cylinder liners contribute mainly toward increasing the wear resistance. AMCs have better wear resistance than

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traditionally used liners made of cast iron. In addition, AMC liners can contribute a weight reduction of almost 20%. AMC liners have better thermal conductivity which helps the engine to operate at lower temperatures. This phenomenon increases the engine life quite remarkably (8). Honda was the first company to start using an AMC by adding a hybrid mixture of 9% graphite and Al2O3 to the aluminium matrix. In recent years Honda, Toyota and Porsche have been using hybrid composite liners in some of their models. Crankshafts and camshafts: The crankshaft is an associated part of the cylinder block, which is attached to the cylinders and transfers energy by reciprocal movement. Camshafts are associated with tappets to open or close valves. Crankshafts and camshafts have to have very good fatigue strengths and wear resistance for a long lifetime. High thermal coefficiency is also an issue while selecting materials for crankshafts and cam shafts. AMCs are a very good option for providing the desired properties for these parts. Driveshafts: The driveshaft is a mechanical device associated with the torque or rotation transmission to the gearbox. In large vehicles generally two driveshafts are used for length restriction. With AMCs this problem can be resolved, because the use of AMCs enables the use of driveshafts of a certain length with a small diameter or a certain diameter with a smaller length. The critical speed is well maintained with this system, along with a minimum weight saving of 9 kg. Aluminium wrought alloy 6061 with 10% Al2O3 powder particles is widely used in driveshafts nowadays. The GM Corvette, GM trucks, Ford Crown police cars and a number of racing cars use driveshafts of this type. Brake discs and drums: In brake discs and drums, AMCs have brought a new era of fuel economy and service life. In traditional cars cast iron is used as the manufacturing material of the brake system. But AMCs are more beneficial due to their light weight, better wear resistance and thermal conductivity. The use of AMCs also contributes to the increase of acceleration and reduces the braking distance; it also provides less noise and wear in the system. Mainly Al-Mg or Al-Si-Mg alloys with 20 to 30% SiC are used in the brake system. The use of these materials was initiated by the ‘Lotus Elise’ in 1996 when four disc rotors of directly reinforced aluminium (DRA) were introduced. The temperature in the AMC brake rotors never exceeds 380ºC, which leads to less heating in brake discs and drums. Various car manufactures are using AMCs in some of their models. Some significant

names are VW Lupo, Toyota RAV-4EV, Plymouth Prowler, GM EV-1 and various racing cars. Brake callipers and pads: The use of AMCs in brake callipers enables very rapid stopping of wheel rotation compared with cast iron. Formula 1 racing cars use 2124/SiC/25p which supports the cars with great acceleration and quick stopping. Currently, apart from ceramic matrix composites AMCs are also used in brake pads. Pushrods: Pushrods are components which transfer motion from the camshaft to valves. In high-speed cars, it is necessary to have very high rpm (revolutions per minute) to have great speed and acceleration, which is unobtainable with traditional steel used in cars. AMCs are the best materials for obtaining higher rpm due to their good stiffness along with very good wear resistance and fatigue strength. 3M Corporation initiated the use of AMCs in pushrods with 30% Al2O3 in an aluminium matrix instead of 4340 steel. The new material provided almost 25% higher stiffness and twice the damping capacity compared with the traditionally used 4340 steel. This AMC increases the engine speed to 250 to 400 rpm. 3M expects further development of AMC pushrods with the optimisation of camshafts. Connecting rods: Connecting rods connect the pistons and crank shafts in reciprocating engines. The rods have to be relatively lightweight to avoid any secondary vibration forces. In fact lower reciprocating loads can lead to lower friction loads and can thus increase fuel efficiency. Some prototype connecting rods have been produced by using AMCs, but bulk production is not yet due because of the very high cost involved. Further research may lead to a new era of using AMCs in connecting rods. Body panels and structure: Using metal matrix composites in the car body panel is one of the most widely discussed issues today. AMCs provide good mechanical properties to be used instead of steel but their wear resistance is slightly lower than that of steel. If the AMC sheets can replace steel sheets and the joining technique is modified, then there will be no hindrance in using AMCs in the car chassis or body structure. Others: AMCs are used in energy transmitting devices, wheels, valves, etc. In the gearbox there are certain uses of AMCs due to their low CTE and other thermal properties. Economic impact of AMCs In the last decade the use of AMC in the transport sector has increased remarkably. A

ALUMINIUM · 7-8/2011

Automotive

research report prepared by Global Industry Analysts, Inc. states: “The aluminium matrix composites market is projected to exceed USD68 million by 2010. Among the various end-use industries for metal matrix composites, the ground transport market, with a share estimated at 57% for 2008, forms the largest, while the aerospace market is expected to emerge as the fastest-growing, with a CAGR of 7% over 2001-2010.” References 1) G. Cole, A. Glove, R. Jeryan, G. Davies, s.l.: Steel

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World, Vol. 2 (1) (1997), pp. 75–83. 2) Surappa, M. K. Feb/April 2003, s.l.: S¯adhan¯a, pp. 319–334, Vols. Vol. 28, Parts 1 & 2. 3) Cost effective Titanium Matrix Composite. s.l.: 2002, Toyota Central R&D Lab, Inc. 4) GDA Gesamtverband der Aluminiumindustrie (German Aluminium Association) 5) Automotive Engineering: Strategic Overview 2 (1). Sears, K. 6) Aluminium alloys for automobile applications, in: A. Morita. 7) Automotive Applications of Metal-Matrix Composites. Hunt, W. H., Miracle, D.B., s.l.: OH: ASM International, 2001. pp. 1029-1032. 8) Kevorkijan, V. M. (No. 11), s.l.: JOM, Vol. 51.

9) Aluminium alloys for automobile applications. A. Morita. 2010, Wear, pp. 124-126.

Author Md. A.I. Mehedi is an undergraduate student of the Bangladesh University of Engineering and Technology (BUET). He is doing his B.Sc. on Materials and Metallurgical Engineering. His primary interest lies in metal matrix composites, renewable energy materials and failure analysis of materials. He is on the Dean’s list of the university and has been awarded the university merit scholarship for extraordinary academic performance. Contact: aminulmehedi_ [email protected], Tel: +88-01726294358.

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