Quasi-in-situ reconstruction and regulating mechanism of plastic flow in steel friction stir welding

Resolving the mystery of plastic flow in steel friction stir welding (FSW) is critical for process. However, constrained by limitations in flow field techniques and insufficient understanding of the underlying physics, a holistic understanding of plastic flow and its regulating mechanism remains largely empirical. In this study, the material response to the mechanical processing of the FSW tool is reconstructed through a quasi-continuous observation technique. The mechanism of cavity filling, the effective range of tool-workpiece contact states, and the real-time boundary of the shear layer are analyzed. At finer scales, multiple independent vertical components are identified, inducing either unstable periodic flow or mass-balancing effects. These components are characterised as vortex structures. Accordingly, a dynamic model is proposed to specifically elucidate the formation of local vortex structures. The model uses tool–workpiece interaction as the basis for a qualitative description to assess the location of vortex activation, a process that can be semi-quantitatively represented through finite element simulations. The dynamic evolution of the vortex is attributed to the constraining effect of solid-state boundaries on the flow field. The real-time boundary of the shear layer is considered as one form of solid-state boundary, whose constraining effect promotes localised vortex formation. Specifically, the formation of captured vortexes is defined based on the assumption of tool-workpiece interaction and the delineation of shear layer boundaries. Model adaptability is preliminarily verified, and a low-cost method is proposed for capturing previously hidden plastic flows. Across a wide range of process parameters, this model effectively explains plastic flow behaviour. These analyses not only advance a comprehensive knowledge of flow dynamics and associated shear behaviour in steel FSW, but also demonstrate that the proposed dynamic model deepens the fundamental understanding of the complex physical mechanisms during the process. Therefore, this study lays a foundation for optimising welding parameters and supports future academic investigations focused on plastic flow or shear behaviour control.