Economical and Environmental Factors in Light Alloys Automotive Applications
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A vehicle[1,2] designed to maximize the use of aluminum alloys may be up to 36 pct lighter than a conventional steel car, with a concomitant fuel economy improvement, and hence reduced emissions, of over 20 pct. Magnesium is less dense than aluminum, and extensive[3–10] use of magnesium alloys may lead to even greater mass reduction and increased fuel efficiency. Carpenter et al.[11,12] pointed out that mass reduction may also prove crucial to the performance of vehicles with hybrid and fuel cell powertrains, which are expected to be heavier than internal combustion powertrains. Aluminum alloys, currently with over 100 kg average use per car, have a clear lead over magnesium alloys at only about 6 kg.[3,5,13,14] In part, this is so because despite a continuously decreasing cost over the past few years, Mg alloys are still relatively expensive in comparison with Al alloys and polymers.[8,11,15,16] Indeed, the relatively high cost of both Al and Mg alloys in relation to steel is normally identified as the single most important deterrent to a wider use in motor cars,[1,3,12,15,17–21] especially when the liability arising from CAFE (Corporate Average Fuel Economy[22]) standards is considered. From the environmental point of view, for metals like Al and Mg, which are ‘‘dirty’’ products to make, but are ‘‘clean’’ to use, initial emissions will be higher than those of a metal such as
CARLOS H. CACERES, Reader in Casting Technology, is with the CAST Co-operative Research Centre and ARC-Centre of Excellence Design in Light Metals, Materials Engineering, School of Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted October 11, 2006. Article published online July 3, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe, which is clean to make but dirty in use.[23] The difference between the two emissions impact totals must be fully offset over a time shorter than the vehicle’s expected lifespan to make any substitutions environmentally feasible.[1,4,6,18,21,23–28] In this article, the cost and environmental viability substitutions of ferrous components by Mg and Al alloy components are discussed by using M.F. Ashby and coworkers’ method of analysis, which involves material indices, penalty functions, and exchange constants.[29,30,31] The viability of substituting Mg components for existing Al components is also examined. Sivertsen et al.[6] reviewed a number of recent publications dealing with the lifecycle assessment (LCA) of automotive light alloys, pointing out some of the difficulties inherent to this sort of analyses, which ultimately limit their practical value, namely, lack of reliable data, lack of an accepted method for weighing the environmental impact, the methodologies used being too comprehensive and rigorous (i.e., too time and resource consuming) for widespread application, difficult translation of conclusions into information useful for decision making, and lack of transparency due to the incorporation of weight factors within the ana
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