Flow-coupled thermo-mechanical analysis of frictional behaviors at the tool-workpiece interface during friction stir welding

Friction stir welding (FSW) has been applied to many fields including aerospace manufacturing and the railway industry. However, the interfacial friction behavior remains unclear due to the lack of reliable thermo-mechanical-flow-coupled analysis. In this study, the frictional behaviors, including the interfacial temperature, friction-induced heat generation, interfacial stick-slip state, and mass transfer during the FSW process of AA2024-T4 are elucidated based on experimental investigation and a novel three-dimensional computational fluid dynamics (CFD) analysis. It is found that the temperature is heterogeneously distributed at the tool-workpiece interface which ranges from 380 to 431 °C. The heat generation contributed by the slipping friction is less than that contributed by deformation heating. This stems from the fact that friction-induced interfacial stick is presented over a large area of the tool-workpiece interface, while a significant slip occurs at the shoulder periphery and the middle of the pin end. As a result of the interfacial stick, a highly-localized high-velocity zone (HVZ) is formed within the vicinity of the tool-workpiece interface with nonnegligible vertical velocity. Fast mass transfer occurs within the HVZ in a maelstrom pattern; while relative slow mass transfer occurs in a straight-through pattern outside the HVZ. Moreover, the friction-induced temperature field and material flow field in the vicinity of the tool-workpiece interface during the FSW process are predicted by using numerical simulation and validated by the measured temperature and the observed weld macrograph. By using the state-of-the-art simulation approaches, this study provides a quantitative analysis of the friction behavior during FSW, which benefits an in-depth understanding of FSW and thus brings new design concepts.