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

搅拌摩擦焊（FSW）（包括电辅助搅拌摩擦焊- EAFSW）的实际应用表明，可能会形成内部缺陷，如空洞。它们对焊接接头的力学性能有很大的影响，其形成原因尚不清楚，需要对搅拌摩擦焊塑性区的形成特点进行详细的研究，并对相应的应力状态进行分析。进行了最新的研究，根据实验和物理-数学模型的现象在塑性区，没有提出任何解释和标准的原因，空隙形成过程。其中一个主要因素是FSW条件下塑性区中塑化金属的变形特性未知。基于标准实验程序（热压缩、扭转、霍普金森杆试验等）的不同材料本构定律，不让他们的决心（例如，应变率，动态粘度）。该项目的目标是在对塑性区内应力的详细研究和应力条件假设的基础上，制定FSW（EAFSW）过程中塑性区空洞形成过程起源的一般准则。为了实现这一目标，建立了FSW（EAFSW）过程的复杂热力学模型，该模型将考虑搅拌摩擦焊（EAFSW）的真实的操作参数、在搅拌摩擦焊条件下金属的温度相关的热物理和变形特性以及由焊接工具引起的塑性应变，以及热产生，该模型所需的材料（工具、工件、背板）的温度相关热物理性质将取自本地ISF数据库和文献。在类似FSW的试验过程中，通过计算程序处理接收到的实验数据，将获得塑性区金属的机械性能。经过校准和验证，FSW的热-机械耦合模型（EAFSW）将允许计算材料粘性流的主要特性：流动速度的分布、应变率、内压的分布以及塑性区金属的应力张量。基于所提出的应力状态假设，并详细分析所获得的内部压力和应力的分布，它将是可能的，提出了一般的标准，为空洞的起源。

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.