In the field of light weight frame structures the assurance of dimensional accuracy and the prediction of structural properties especially during and after welding processes are of great importance. The problem in this regard mostly arises from the used welding technique which is characterised by complex interactions of various parameters. A simulative approach is useful in order to predict the structural behaviour and to improve the geometrical quality of joined light weight components after welding. As such, it contributes to reduce process adjustments in the early stage of the product life cycle, and therefore helps to save time and costs. In this paper an approach for modelling the innovative joining process of composite extruded profiles by friction stir welding is presented.