Axial hydro-extrusion: A novel stress-transformation-based method to synchronously reduce multi-dimensional deviations of profiled hollow members

To address multi-dimensional geometric deviations in extruded profiled thin-walled hollow members, which are challenging to correct using traditional processes, this study proposes an axial hydro-extrusion method based on a novel stress-transformation mechanism for synchronous control of multi-dimensional deviations. Unlike traditional tensile-based forming, this process induces global plastic extrusion deformation through axial feeding under internal pressure, transforming non-uniform axial, circumferential, and shear stresses into a nearly uniform and predominantly compressive stress state. This shift eliminates stress gradients, thereby controlling deviations in different dimensions and enabling synchronous outer contour correction of the hollow member. First, the effect of stiffeners on wrinkling and shape accuracy of the outer contour of the hollow member in axial hydro-extrusion is analysed, establishing a critical instability internal pressure model. Next, stress states generated by different geometric deviations during fitting die deformation and their changes during axial extrusion are investigated through theoretical and simulation methods, clarifying the reasons for deviation changes in different dimensions. Additionally, a critical extrusion displacement model incorporating work hardening is developed to quantify the stress transformation process and analyse the effects of initial deviations on critical extrusion displacements. Finally, a specialised axial hydro-extrusion forming platform is designed for single-cavity and multi-cavity hollow members with various deviations, achieving over 95% reduction in dimensional deviations. This study provides a new strategy for solving the problem of multi-dimensional geometric deviation synchronisation control in complex thin-walled hollow members.