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Schikorra,M.; Kleiner, M.:
In: Proceedings of the Nafems World Conference,17.05.-20.05. 2005, Malta
Extrusion is a forming process in lightweight construction commonly used for the production of a wide range of profile geometries made of aluminium. Since the history of extrusion simulation is still young, no standardised method has been established until now. The different investigations in this field deal with completely different formulations, material descriptions, and physical boundary conditions. This paper gives a short overview of extrusion simulation itself and provides a benchmark regarding the usability of the applied commercial software systems by comparison of two Finite-Element models (one based on the Lagrangian and one on the Eulerian formulation) with results of experimental investigations. In addition to conventional extrusion, the potential of a composite extrusion process for the production of continuously reinforced profiles using standard AA6060 aluminium billets as base material and a reinforcement made of steel for modern lightweight constructions will be shown. Here, reinforcement leads to an increase in profile strength as well as in profile stiffness, and thus increases the lightweight potential. The occurring problems of experimentally ascertained stress cracking of the reinforcement depending on the die geometry and boundary conditions will be discussed and explained by numerical analysis of the material flow. It will be shown that the insertion of a reinforcement leads to a significant local perturbation of the material flow inside the forming tool: while the velocity of the base material increases due to the increasing press ratio, the velocity of the reinforcement remains constant at the profiles exit velocity. This effect leads to an induction of high tensile stresses into the reinforcement in the interface zone, which will result in failures like cracking during the extrusion process. By use of a coupled thermo-mechanical Finite-Element simulation with the commercial FE-codes Superform from MSC and HyperXtrude from Altair the velocity fields of a simplified extrusion process with and without reinforcement were analysed and the resulting stress components were compared. Based on these results, a process optimisation to reduce stresses on the reinforcement and to improve the material flow has been carried out, for example by a modification of the friction or die geometry. The numerical results went along with experimental investigations to verify the calculated failures and the predicted optimisation of the process.