Tomorrow’s cars will handle better, offer improved acceleration, braking and cornering, be lighter and more fuel efficient, and cause less pollution. But light weight at any price is not the sole objective. Safety and style, technical feasibility, environmental impact and affordability, are vital factors.
Audi’s launch of the aluminium A8 and its £47m joint venture manufacturing facility with Alcoa, have further highlighted the automotive industry’s increasing interest in alternative materials for the ‘body-in-white’. These, and similar, developments threaten the dominant position of steel as the quintessential automotive material. Or do they?
Material Trends in the Automotive Industry
Aluminium alloys, plastics and composites are the ‘new materials for the automotive industry’ (some of them have existed, in various forms, for two or three decades) and their publicity has eclipsed that for similar advances in steel production and technology. Today’s average European car contains 70 kg of aluminium and up to 120 kg of plastics (both almost double the 1980 figures). Sheet steel contributes 400 kg, and all steels in total account for 55-60% of typical vehicle weight (as they have for 14 years, even after considerable weight reductions). Despite, or because of, steel’s dominance in high volume car production, it is increasingly seen as ‘yesterday’s material’, and the fact that over 50% of modem automotive steels have been introduced since the mid 1980s is frequently overlooked by the motoring media and public eager for more attractive alternatives.
The automotive steels of the 1990s have better formability, greater capacity for localised/necking elongation, higher forming limits, smaller bend radii limits, less springback in pressing operations, and lower sensitivity to galling and surface damage than alternative materials. The new hot rolled, high-strength carbon-manganese and carbon-manganese-silicon sheet steels combine yield and tensile strength with cold formability and they can now increase component strength, or achieve equivalent strength for less weight (useful in underbody applications). Bake hardenable steels, in which strength and dent resistance increase after stowing in the paint shop, are particularly useful in panels exposed to minor knocks and scrapes, such as doors and boot lids.
The Push for Alternative Materials
The arguments for alternative materials concentrate on weight reduction and fuel efficiency, better performance, tooling cost, and environmental friendliness.
In North America legislative pressure to reduce fuel consumption has sparked the search for a lighter car. Aluminium and plastics can indeed produce vehicles that are lighter than current steel models. And these lighter vehicles also have other benefits, such as fewer parts, by using space frame construction though steel could also do this. An aluminium panel weighs about half as much as a steel panel of equivalent strength, and using more aluminium could, it is claimed, also meet other criteria, although the past 15 years have seen considerable savings (albeit offset by luxury fittings and safety features) achieved through rationalisation of car body structures and the use of lighter gauge, higher strength steels.
At present, alternative materials are most competitive in low volume production where tooling, rather than materials, most affects unit cost. Aluminium could reduce body weight by up to 40%, but new steel technologies promise reductions of up to 35%, leaving aluminium only just ahead.
Weight reduction also improves overall performance and handling. A 10% weight loss can reduce acceleration time from 0 to 60mph by about 8%. Research using finite element and design sensitivity analysis shows that a 20% or greater reduction in body weight can be achieved by combining new steels and manufacturing technologies (such as adhesives and weld bonding). Work involving the American Iron and Steel Institute (AISI), Ford and Porsche Engineering Services, has shown that structures 15% stiffer and nearly 20% lighter than existing base saloon cars, could give savings of about 140lbs per vehicle. As steel is used almost universally in the automotive industry, reductions could be introduced into existing facilities almost immediately, increasing the cost advantages of weight reduction using steels rather than other materials.
In contrast, potential savings with plastics are less clear. Lower densities, relative to steel, are offset by the need for thicker panels to achieve equivalent stiffness, inherent problems with consistent panel quality in high volume manufacture, and a tendency to crack on impact. New manufacturing equipment, such as injection moulding tools, represent a significant price barrier to a plastic ‘body-in-white’, especially for manufacturers with substantial investment in stamping and pressing equipment. Although high corrosion resistance, shape flexibility, and dent and stone chipping resistance, make plastics useful for vulnerable parts, such as bumpers, they are never cheaper than steels of equivalent strength and sometimes cost four times as much.
The weight, size and design (including materials) of a car body all contribute to its behaviour in an accident (crashworthiness) and the safety of the passengers. Aluminium structures are capable of absorbing energy equivalent to those using steels. But the proven performance of steel-bodied cars in a wide range of countless real life crashes cannot be reproduced easily. Aluminium costs five times as much as mild steel, however, and while bare aluminium is undoubtedly more resistant to atmospheric corrosion than bare steel, new coatings and galvanised steels mean that corrosion is no longer the primary determinant of a car's lifespan.
The largest, and most immediate problem in high-volume production with alternative materials is the expense of a new infrastructure to handle design, manufacture and repair. Much of that used for steel cars is either unsuitable or incompatible. For example, aluminium and plastics cannot use presses with magnetic handling so new handling and post stamping facilities would be needed. Plastics pose problems with fixing and painting and, like aluminium, are difficult to integrate with monocoque steel body design, so new joining techniques would be needed.
Aluminium welding poses special problems. It requires more welding spots to compensate for lower fatigue strength. Fusion welding is difficult because of oxide formation, frequently making MIG/TIG welding necessary. And aluminium's higher surface reflectivity makes laser welding more difficult. Steels require lower welding currents and lower contact pressures, while electrode life is longer, and energy consumption three times less.
The constraints on manufacturing and cost have led to non-traditional body structures, such as the spaceframe which is made up of aluminium extrusions capable of being bent or formed, with castings for connection points and aluminium sheet panels. This uses half as many parts and fewer joints than a sheet metal body and has led to claims of a 35% reduction in primary body structure weight and a 50% cut in tooling cost, compared with traditional methods.
Disadvantages of Aluminium and Spaceframes
Yet for all their ability to be readily extruded, compared with autobody steels, aluminium alloys have a lower formability and a greater tendency to springback in press forming. They are also more prone to handling damage by denting and scraping, and require prelubricated strip or protection during pressing. This has an impact on repair, as well as manufacturing processes and a specialised panel and frame repair system, including a network of dedicated repair shops, would be needed. Shaping and straightening aluminium parts requires greater temperature control and special paints to detect overheating or microcracks, and separate tools and equipment to prevent iron deposits from corrupting aluminium welds. Spaceframes may require even more sophisticated facilities, particularly to correct body misalignment, and complex repairs may be impossible, leading to automatic replacement of damaged parts and higher insurance premiums.
Environmental issues increasingly influence automotive design, particularly noise, recycling and life-cycle pollution. In theory, an aluminium structure’s resistance to vibration is one third that of an identical steel structure. This means 10 db more noise, while steel's higher sound-damping capacity reduces road noise. Aluminium cars can compete by alternative designs or insulation, but the penalty is extra cost in materials and more weight. Passenger comfort may also be reduced: aluminium's thermal conductivity is five times that of steel. This presents problems with external and internal temperatures on hot days so air conditioning may become a necessary extra.
Legislation in Germany and North America appears to be heading towards the 100% recyclable car. Marketing campaigns have presented aluminium as a highly recyclable material. Pound for pound it has ten times the value of steel sheet scrap. Aluminium, the material, is potentially recyclable with an appropriate infrastructure and a limited mix of alloys.
But recycling aluminium cars is highly complex; the different products and alloys likely to be used are incompatible for recycling into wrought products for car bodies, and it will take 10-15 years to create an adequate stockpile of scrap, by which time the alloys in use today may be obsolete.
But steel is the world’s most recycled material and recycling is as old as steel production itself. Over 400m tons are recycled worldwide each year. This is about ten times the combined total of all other automotive body materials. Secondary aluminium production in Europe in 1991 was only 1.6m tonnes. These 400m tons include 20m tons of automotive steel scrap in the USA and Europe using existing recycling networks. Plastics recycling is minimal and automotive plastics are causing problems with landfill space. Steel can be separated magnetically, while plastic resins or aluminium alloys must be segregated with 100% accuracy, because of their low tolerance to impurities. Different molten aluminium alloys could be mixed, but only at the expense of downgrading.
Some 80% of today’s car is recycled (including over 95% of the steel), requiring much less energy than refining from ore. The energy required for primary aluminium production is five times that of steel, and producing a part from aluminium, rather than steel, needs almost three times the energy. Although the excess energy in vehicle parts manufacture is ‘paid back’ during its life, there is an overall deficit because of increased emission of greenhouse gases in primary aluminium production, and other problems, such as the caustic ‘red mud’ produced in alumina extraction, and disposal of toxic potliners containing cyanides and fluorides.
Car manufacturers are examining future options, but have already decided the way ahead for the near future. Very few of the future models are known to include significant amounts of aluminium or plastic body panels. Meanwhile, Chrysler is replacing the plastic wing on its LHS with steel, and problems with the Viper's plastic bumper led to 66% of 1993 production being lost.
Radical new designs, such as spaceframes, could make aluminium and plastic panels viable, but are still at the experimental stage. Monocoque bodies - the most economical construction - are designed for steel panels and substituting alternatives would lead to problems. Few manufacturers have worked with aluminium in mass production, and introducing new product forms and working practices will take time and money. While aluminium alloys and plastics undoubtedly offer some benefits, car manufacturers are nothing if not realistic and the bottom line is: alternative materials may exist, but are they cost-effective in producing safer, ‘greener’ cars? With today's steels, an annual run of just 25,000 units could save US$140 per car (IISI study Competition between steel and aluminium for the passenger car), or US$525 on a run of 200,000 units.
By the time the claims made for aluminium, plastic and composite exteriors have been examined and proven (or not) steel will probably have evolved still further, maintaining, or even improving, its current advantage and preventing mass penetration of the all-important high volume markets.