The most abundant metal in the Earth's crust, aluminium, has a long way to go if it is to replace steel as the default material for car body construction. However, for some applications engineers believe that the benefits of a density one third that of steel more than outweighs the disadvantages of higher raw material cost and relatively low strength, stiffness and formability. Aluminium has low density in its favour, but its elastic modulus of 69 GPa typically is only a third that of steel. Taking a simple case, the linear stiffness of an aluminium part can be made the same by raising material thickness by half. It will still offer a weight saving of almost as much. Cost is another matter. But nor can cost be ignored when considering high strength steel (HSS): they are more expensive to buy and to process than ordinary forming steels. Elevated yield strengths can reduce the suitability of the material for complex deep drawn shapes, one of the criticisms commonly levelled at aluminium. There are issues too with joining different steel grades, weld quality the most important, because of dissimilar carbon content. Jaguar continues to review new manufacturing techniques for lightweight structures, and also new materials and production methods. Aluminium tailored blanks are not used by Jaguar because of concerns about the integrity and performance of the welds. Instead, it is investigating tailor-rolled blanks to increased strength locally for regions of greater stress. The process uses profiled rollers to produce varying section thickness across a sheet of homogeneous material. It has also looked at a multialloy technology from Novelis, formed last year as a spin-off from Alcan. Novelis, which supplies 150kg of 5000 and 6000 grade sheet for every XK, has developed a new method of casting ingots. Dubbed fusion technology, it allows several grades of material to be cast simultaneously to form a single ingot: a big improvement over the cladding method used for the past 60 years. Complex process control prevents surface oxidation of the different grades, resulting in better intermolecular bonding at interfaces. The ingot is then rolled into sheet, including gauges commonly used for automotive. As-rolled thickness of the outer layers can be 5-15 per cent of gauge. The result is a material where core strength can be adjusted to offer increased formability while retaining high strength at the outer fibres together with a quality surface finish. Sheet made this way will cost more, but Novelis believes that the automotive industry will be a big market for fusion. Fusion adds value throughout the supply chain: downgauging, robustness at high volume and ease of forming are all key attributes.
Mixed messages
Automotive Engineer ; Sep. ; 36-38
2006
3 Seiten, 3 Bilder
Article (Journal)
English
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