Development of an experimentally supported Friction Stir Welding model for predicting the strength of dissimilar joints based on the example of aluminum-steel joints

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Friction stir welding is a special case of friction welding where no relative movement between the joining partners is used. A rotating welding tool plunges into the joining partners and is moved along the joint line. The rotation of the tool causes the material to heat, plastify and move behind the tool creating the weld seam. The strength of the weld seam is particularly dependent on its geometry and the thermomechanical treatment of the materials during the friction stir welding process. For joints between similar materials there are a number of numerical process models.However, there is no process model for the friction stir welding process of dissimilar joints.Therefore, the strength properties of dissimilar joints can merely be determined by experiments.The aim of this project is the development of a simulation model to predict the strength properties of friction stir welded joints of different metals depending on the welding parameters. The model is developed at the example of the technically relevant material combination aluminum-steel. To accomplish this aim, a three divided procedure is applied. To validate the results of the single steps reference welds are used.In the first step, the so called process model, the friction stir welding process is continuum mechanically simulated using strain, strain-rate and temperature dependent material models. In order to deal with the very large deformations and to treat the interaction of different materials as a multiphase problem, complex computation methods have to be used. The results of the process model - weld seam geometry as well as the time dependent temperature and strain fields of both materials - are used as input for the following two steps. The results of the process model are validated by micrographs and temperature measurements.In the subsequent step a damage mechanics model is adapted, to describe the relation between thermomechanical treatment of the materials during friction stir welding, the resulting changes in microstructure, the yield strength and the damage mechanical parameters. Based on the results of the process model areas of similar thermomechanical treatment are identified in the reference weld seam. From these areas samples are taken and tested for calibrating the damage mechanics model.In the last step, the results of both preceding steps are merged into a strength model of the weld seam. The failure behavior of the weld seam resulting from the process model is determined by the damage mechanics model. The numerically determined strength properties of the joint are validated by experimental tensile tests.With the assembly of the three steps, it is possible to predict the geometry and the strength properties of dissimilar joints starting from the process input variables of the friction stir welding process.