Circumferential twist in flow forming of tubular parts: Characterization, understanding and control

Circumferential deformation in flow forming of tubular parts affects the axial and circumferential mechanical properties of the deformed workpiece. In this study, by capturing the deflection of weld line in flow forming of welded tube, the characteristic and mechanism of circumferential twist in flow forming are investigated. Simulation and experiment show that the weld line is deflected to a helix with the helix orientation being contrary to that of the helical trajectory of roller on workpiece, which suggests that the circumferential distortion occurs in the form of twist. The circumferential twist is a combination of the shear deformations in θ-z and r-θ planes (θ-circumferential, r-radial, z-axial). The shear deformation in θ-z plane is nearly the simple shear mode, which is the basis of circumferential twist. The shear strain ε_θz is almost equal to the helix angle of the deflected weld line, which is defined as the twist angle for evaluation of the twist degree. The shear deformation in r-θ plane (ε_rθ) results in a little difference of the circumferential twist distance between the inner and outer surfaces of tube, but makes little difference on the twist angles of two surfaces. Reduction (ψ_t) and feed rate (f) are the first and second significant factors for twist angle, respectively. Twist angle is increased with ψ_t and f. A convenient method is proposed to control the circumferential twist by applying a circumferential constraint on workpiece. The tailor-developed flow forming with the applied constraint is more efficient and general than cross flow forming.