Modern materials and their incorporation into vehicle design

Rob Hutchinson, BSc, MSc, MRIC, CChem, MIM, CEng

The aim of this chapter is to:

• Introduce the broad range of materials that designers can draw upon;

• Introduce the properties of materials that are required for vehicle design;

• Demonstrate particular uses of material properties by case studies;

• Demonstrate the material selection process and its interactivity with design.

2.1 Introduction

The main theme of this chapter will be the study of the various inter-relationships between the structure of engineering materials, the methods of component manufacture and their ultimate designed behaviour in service. The four major groups of engineering materials are metals and alloys; ceramics and glasses; plastics and polymers and modern composites, such as silicon carbide reinforced aluminium alloys. Illustrative case studies will make up a significant section of this chapter.

The full range of these engineering materials is used in the construction of motor vehicles. It is a common myth that the aerospace, defence and nuclear industries lead the way in the use of materials for aggressive environments and loading regimes. The automotive industry has its own agenda with the added criteria of consumer demands of acceptable costs as well as critical environmental issues. Engineers, in general, are familiar with metals since they have the all-round properties, which are required for load bearing and other applications. This situation is helped economically by the fact that of the hundred or so elements within the earth’s crust, the majority are metals. This means that whilst some are more difficult to extract than others, a wide range of metals is available to supplement iron, aluminium, copper and their wide-ranging alloys. Metals have adequate strength, stiffness and ductility under both static and dynamic conditions. Other physical properties are also acceptable such as fracture toughness, density, expansion coefficient, electrical conductivity and corrosion and environmental stability. A wide range of forming and manufacturing processes have been developed as well as an extensive database of design properties (Timmings and May, 1990). There is also a well-established scrap and recycling business.

Only when extreme properties such as low density, low thermal and electrical conductivity, high transparency or high temperature and chemical resistance are required, and where ease of manufacture and perhaps low cost are important, do engineers consider fundamentally different materials, such as polymers and ceramics. These two groups of materials have alternative engineering limitations such as low strength or brittleness. Consequently, combinations of these three materials groups have been used to form the fourth group of engineering materials known as composites, of which the major tonnage group is glass reinforced polymers. Ceramic reinforced metals also form a significant technical group of composite materials (Sheldon, 1982). All four groups of these materials have an essential part to play in the design, construction and service use in vehicle engineering.

In addition to the direct engineering issues, the vehicle designer needs to consider the political issues such as pollution and recycling due to the vast quantities of materials used in automotive manufacture. In Western Europe, the EC politicians now expect vehicles to be clean, safe, energy efficient, affordable and also ‘intelligent’, which means that they should be able to anticipate the actions of the driver and other road users. This has lead to significant research funding which, in the materials area, has involved work in the areas of combustion engine materials, batteries and fuel cells, wear resistant materials and light weight vehicle body materials. Such work is expected to continue. However, whilst engineering, environmental and safety issues will be of general concern, the manufacturer will continue to be motivated by profit whilst the driver will still expect personal freedom. On this issue government is caught between the environmental lobbies and the car industry, which makes a considerable contribution to gross domestic product. Thus, road usage is likely to continue to increase, so that some form of overall traffic management may well become essential as road building programmes are scaled down due to economic and environmental pressures.

2.2 Structure and manufacturing technology of automotive materials

Engineering materials are evolving rapidly, enabling new vehicle component designs, for load bearing structures and bodywork, engines, fuel supply, exhaust systems, electrical and electronic devices and manufacturing systems. Modern materials include fibre composites, technical ceramics, engineering polymers and high temperature metal alloys (Ashby et al., 1985). The vehicle designer must be aware of these developments and be able to select the correct material for a given application, balancing properties with processing, using a basic understanding of the structural inter-relationships.

2.2.1 Metals and alloys

Many metals are not abundant and so can only be used for specialist applications such as in catalytic converters and powerful permanent magnets. In contrast, iron, copper and aluminium are very abundant and more easily obtained and so are widely used in both pure and alloy forms (Cottrell, 1985).

Iron-based or ferrous metals are the cheapest and the most widely used at present. For low load applications, such as bodywork and wheels, mild or low carbon steel is sufficiently strong with yield strengths varying between 220 and 300 MPa. It is also easy to cut, bend, machine and weld. For drive shafts and gear wheels, the higher loads require medium carbon, high carbon or alloy steels, which have yield strengths of about 400 MPa. Higher strength and wear resistance are needed for bearing surfaces. Medium and high carbon steels can be hardened by heat treatment and quenching to increase the yield strengths to about 1000 MPa. Unfortunately, these hardened steels become brittle following this heat treatment, so that a further mild re-heating, called tempering, is required. This reduces the brittleness whilst maintaining most of the strength and hardness. Stainless steels are alloys with a variety of forms, Austenitic, Ferritic, Martensitic and the newer Duplex steels. A common composition contains 18% chromium and 8% nickel, as shown in BS 970, 1991. Their corrosion resistance and creep resistance are superior to plain carbon steels, particularly at high temperatures. However, higher material and manufacturing costs limit their use in vehicle engineering to specialist applications such as longer life exhaust systems. Cast irons have 2 to 4% carbon, in contrast to the 1% or less for other ferrous metals mentioned above. This makes it brittle, with poor impact properties, unless heat-treated to produce ductile iron. It is more readily cast than steel, since the higher carbon content reduces the melting point, making pouring into complex shaped moulds much easier. In addition, the carbon in the form of graphite makes an ideal boundary lubricant, so that cylinders and pistons have good wear characteristics, for use in diesel engines. However, it is now largely replaced by the much lighter aluminium alloys for these applications in petrol engines.

Copper and its alloys form a second group of vehicle engineering metals, including copper itself, brass, bronze and the cupro-nickels. Copper is more expensive than steel, but is ductile and easily shaped. It also has high thermal conductivity, giving good heat-transfer for radiators, although more recently replaced by the lighter aluminium in this application. Its high electrical conductivity is made use of in wiring and cabling systems. Brass is a copper alloy, commonly with 35% zinc, which makes it easier to machine yet stronger than pure copper. Thus, complex shapes can be produced for electrical fittings. However, such alloys suffer from a long term problem, known as ‘dezincification’, in water. Corrosion can be minimized by using the more expensive copper alloy, bronze, where tin is the alloying element, although this material may be harder to machine. Copper-nickel alloys have good creep resistance at high temperatures where they are also corrosion resistant. The latter property is made use of in brake fluid pipe-work.

Aluminium and its alloys have a major advantage over steels and copper alloys, as vehicle engineering materials. Their much lower densities lead to lower weight components and consequent fuel energy savings. Whilst aluminium ores are abundant, the extraction of pure aluminium is very energy intensive, being electro-chemical in nature rather than the purely chemical process used for steels. Copper occupies an intermediate position on this point. Thus, pure aluminium is more expensive than iron and copper and has lower inherent strength and stiffness. However, it does have corrosion resistance with good thermal and electrical conductivity. A wide range of alloys is now available with various heat treatments and manufacturing opportunities. These materials have now replaced steels and copper alloys in many vehicle component applications, where their higher materials costs can be designed out, see Figure 2.1. Nevertheless, materials developments are such that aluminium alloys are themselves in competition with polymers and...