General
The above appraisal of designs shows how the choice of material is paramount to achieving today’s objectives and how this choice is widening. Having minimized the weight of a specific design and assuming the best materials have been specified, the next consideration is to optimize material with regard to each link in the chain of processing operations necessary to produce a functional part, and as will be described in Chapter 4 each of these elements can strongly influence the selection. For instance, complex parts require maximum formability which requires a compromise with strength, realistically placing a maximum at around 300 N/mm2 proof stress, although for simple sections such as door reinforcement beams levels of 1200 N/mm2 may be specified. The constraints imposed by local steelmakers may obviate certain grades
where for instance a bake-hardening or isotropic steel is required, and a restricted choice of coating types may be available. However, despite these minor restrictions apart from obvious exceptions most manufacturers are maintaining a conservative steel grade policy, requiring only minimal changes in processes, and as has been seen above the use of predominantly aluminium structures is only evident by one or two
of the more adventurous companies who can absorb the extra supply and manufacturing costs. The majority would still prefer the more cautious approach employing the advantages of aluminium for closure or ‘bolt-on’ parts and using the accompanying weight savings to satisfy legislative weight-band requirements or added sports car performance. Many manufacturers are, however, gaining valuable manufacturing experience by building low volume sports models in aluminium, e.g. NSX or BMW Z8 or specific parts Peugeot 607. Once the different disciplines demanded by this less robust material are fully understood and a way is found of absorbing the extra cost it may then find a wider usage. Plastics as referred to later in this section require
much development in an engineering context and only very expensive derivatives fulfil impact and other functional requirements. Until the market price falls then use will be limited to exterior cladding and trim items. Thus, for the main body structure the increasing use of high strength steel will continue to develop and the trend for a typically progressive European car manufacturer such as BMW is shown in Fig. 2.14 – a weight saving of 10–15 per cent being achieved for selective parts via thickness reduction. Magnesium is now starting to find favour as the quest for lower density materials intensifies, but has been used occasionally in the past, e.g. Austin Maestro gear box covers. The latest interest is for vehicle cross-beams and similar aluminium products as described in the following sections.
Materials for Automobile Bodies
Geoff Davies F.I.M., M.Sc. (Oxon)
AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD
PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO
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