Investigation of the voids formation process in friction stir welding by numerical modeling and experiment on the basis of a stress-condition-hypothesis

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The practical usage of friction stir welding (FSW) (including electrically assisted friction stir welding - EAFSW) showed that internal defects such as voids may form. They have a significant influence on the mechanical properties of the welded joints.The reason of the formation of these voids is unclear and requires a detailed investigation into peculiarities of building the plastic zone in FSW and an analysis of the corresponding stressed condition. Conducted up-to-date studies, based on experiment and physical-mathematical modeling of the phenomena in the plastic zone, have not presented any explanations and criteria on the reasons of the voids formation process. One of the main factors here is that the deformation properties of the plasticized metal in the plastic zone under the conditions of FSW are unknown. Different constitutive material laws, based on the standard experimental procedures (hot compression, torsion, Hopkinson bar tests, etc.), do not allow their determination (for example, strain rate, dynamic viscosity).The goal of the project is to formulate general criteria for the origin of the voids formation process in the plastic zone during FSW (EAFSW) on the basis of both a detailed investigation into internal stresses in the plastic zone and a stress-condition-hypothesis.To reach this goal a complex thermo-mechanical model of the FSW (EAFSW) process, describing thermal and mechanical phenomena, namely heat generation, heat propagation, plastic deformation, viscous flow will be developed.The model will consider the real operational parameters of FSW (EAFSW), the temperature dependent thermo-physical and deformation properties of the metal in the conditions of FSW and produce plastic strain induced by the welding tool, as well as heat generation, heat and mass transfer in the workpiece.The temperature dependent thermo-physical properties of the materials (tool, workpiece, backing plate), needed for the model, will be taken from a local ISF-database and literature. The mechanical properties of the metal in the plastic zone will be obtained during FSW-alike trials with processing the received experimental data by a calculation procedure.After calibration and verification, the coupled thermo-mechanical model of FSW (EAFSW) will allow the calculation of the main characteristics of the material viscous flow: distribution of the flow velocities, strain rate, distribution of the internal pressure and stress tensor of the metal in the plastic zone. Based on the proposed stress-condition hypothesis and analyzing the obtained distributions of the internal pressure and stresses in details, it will be possible to propose general criteria for the voids origination.