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Lightweight champion
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01/03/2005
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Bumper crossbeams and crash boxes are key elements in passive passenger protection, and aluminium components offer lower weight with similar or even enhanced protective efficiency. Klaus Vollrath reports on their production
Aluminium solutions offer a number of interesting advantages in crash management applications,” says Ola Ivar Moen, director marketing and sales automotive structures at Hydro Aluminium in Raufoss, Norway. In modern cars, crash behaviour is controlled by key components specifically designed for the purpose. These are mainly the front and rear bumper crossbeams as well the ‘crash boxes’ in the front section. In the event of a collision, these absorb impact energy by plastic deformation.
During this process, force levels and thus energy absorption should be as uniform as possible during the whole deformation path. Additionally, these components are required to be as lightweight as possible since weight reduction continues to rank as a high priority. Because plastic materials do not exhibit the required mechanical properties, such applications are largely the domain of two metals with good deformability: steel, the dominant material in car building for a century, and aluminium, which still ranks as a relative newcomer.
“When assessing the suitability of materials for such applications, one should not rely on abstract strength properties deduced from laboratory procedures such as the uniaxial tensile test,” says Christian Eide Lodgaard, head of product development structures at Hydro Aluminium in Oslo, Norway. In purely abstract terms, one might be tempted to favour an austenitic stainless steel with a specific energy absorption capacity of some 50kJ/kg over the aluminium alloy EN 7108 with its mere 20kJ/kg. But in the real world of a crash component, other aspects prevail. For such a part, the main prerequisite is that deformation and failure should not be restricted to a small portion of the component, but involve as high a proportion of its whole volume as possible. The reason for this can be easily understood when considering the example of a slim rod subjected to lengthwise compressive loading: if the rod is too slim, it will tend to kink under the load. Once this failure mode has developed, subsequent deformation will involve only the small volume in the kink area.
For the same reason, the designer of a bumper crossbeam has to make sure this component—intended to serve as a supporting beam—does not dent prematurely. In this context, geometrical factors—the key is structural strength—play a major role, in addition to the purely material-related characteristics. In the case of a box-section beam, the decisive factor is the ratio of wall thickness to cross-sectional dimensions; if this ratio exceeds a critical value of about 1:24, the beam becomes too slim and will be susceptible to premature buckling. This would jeopardise its intended function. Since the cross-sectional dimensions of the part cannot be changed due to its other functions within the package, the designer is prevented from taking advantage of the use of a high-strength material.
“The challenge in designing a beam lies in achieving a uniform distribution of stresses,” says Lodgaard. In the case of a beam subjected to three-point bending, the stress level rises from outside to centre. To achieve optimal material use, the designer thus has to adapt the geometry of the part in such a way that he achieves a roughly uniform stress distribution for every cross section. This is achieved by varying part geometry as well as its wall thickness. This task is greatly facilitated if the designer can rely on extruded profiles instead of sheet parts as raw material. With a profile he has a high degree of freedom to shape it initially according to the requirements of the part, for example by varying its wall thickness or by adding extra flanges. Additionally, he can adapt the shape in line with the requirements of the subsequent forming process. Such variations are hardly possible when using sheets or tube parts as raw material.
If all these advantages are thoroughly used, crash components made of aluminium can absorb more impact energy than comparable steel parts despite their lower weight. Benchmark tests comparing aluminium alternatives to real steel parts reveal that for a VW Golf, aluminium crash parts absorb 35 per cent more energy despite their 32 per cent lower weight.
“From a technician’s viewpoint the advantages of aluminium are almost always convincing. The real battle is mostly raging on the price front,” says Moen. Aluminium’s base price is considerably higher than steel. Nevertheless, this handicap can be significantly offset by the advantages inherent in the extrusion process. For example, the additional degrees of freedom in part design make it possible to integrate additional functions and components, leading to cost reductions in downstream operations.
Despite this, the initial price disparity often cannot be fully compensated for. Thus the OEM must decide whether to accept a certain cost difference in exchange for the weight reduction. In such decision processes, absolute weight reduction is not the only factor—distribution of masses may also play a role. “A poorly balanced vehicle will not achieve excellent driving dynamics,” says Moen. Measures to improve the driving dynamics by reducing weight are more effective the further away from the car centre this reduction is achieved.
“As a result of our price disadvantage, production costs play a key role. We have to achieve cost reductions wherever possible,” says Moen. The company has invested heavily in high-yield state-of-the-art production technology. At the Raufoss site, it operates Europe’s largest extrusion press with a force of 65MN (6,500 tons), suitable for processing the high-strength 7000 grade aluminium alloys used for crash components. Further processing—heat treatment, forming, machining and the heat treatment necessary to optimise structure characteristics—are mainly performed using fully automated transfer press lines and machining centres. Countless robots perform the intermediate handling and final stacking of the finished parts on pallets. The transfer press lines are equipped with quick-exchange cassettes for the complex stamping and forming tools in order to achieve short retooling downtimes.
Among the highlights of the machining equipment is a twin cell with ultra-fast tripod milling spindles able to carry out all complex milling and boring operations on the parts handed in by robots extremely quickly. The cost advantages achieved by using such highly automated equipment have made it possible for the company to compete in new market segments. “Meanwhile, we have been successful in many smaller car applications despite the significantly stronger role costs play in this segment,” says Moen.
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Author
Klaus Vollrath
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