Regulation mechanism of corner filling in hydro-pressing of TA18 tubes

Tube hydro-pressing enables the manufacture of high-strength hollow components with small corner radii under low internal pressure. However, the dimensional accuracy of the formed components is often compromised by friction-induced corner filling nonuniformity. To clarify the underlying mechanism, a mechanical analysis of the corner filling process was conducted, supported by parametric studies on pressing force, corner radius evolution, and wall thickness variation. The analysis reveals that friction leads to an imbalance of internal forces between the upper and lower corners, resulting in nonuniform material flow and corner geometry. A two-step loading strategy was proposed to regulate the corner uniformity by actively rebalancing the circumferential internal forces through adjustment of the relative direction between material flow and friction across the two forming steps. Finite element simulations were performed to validate the proposed mechanical model and reveal the origin of localized thickening in the corner region. Furthermore, tube hydro-pressing experiments on TA18 titanium alloy tubes were conducted to investigate the effects of friction and internal pressure on corner uniformity and to validate the proposed strategy. The results indicate that reducing the internal pressure and applying lubrication alleviate corner nonuniformity but cannot fully eliminate the effect of friction. Using single-step loading, the corner radius nonuniformity rate reaches 9.8% at an internal pressure of 7 MPa without lubrication and decreases to 2.9% at 3 MPa with MoS₂ lubrication. The optimal pressing ratios for the two-step hydro-pressing were theoretically determined and experimentally verified. Compared with single-step forming at 3 MPa without lubrication, the proposed two-step strategy reduces the corner nonuniformity rate from 6.3% to 0.8%. These results are of significance for clarifying the corner filling mechanism and improving corner uniformity in the hydro-pressing forming process.