Height inhomogeneity in flow forming of thin-walled tubes with helical grid-stiffened ribs: characterization, mechanism and control method

Height inhomogeneity between the opposing helical ribs (HIOHR) is a key problem restricting the high-quality manufacturing in the flow forming of thin-walled tubes with helical grid-stiffened ribs (TTHGRs). In this study, the characteristic and mechanism of the HIOHR were investigated, and a method for controlling this HIOHR was developed. It was found that the HIOHR is induced by the different radial flow tendencies and accumulation of material in the deformation zone. Moreover, it was observed that the extent of this HIOHR increases almost linearly during the forming process. On these bases, an improved flow forming method with a cyclic positive–negative helical loading trajectory in one single forming pass, which can cyclically exchange the filling behavior of two types of helical rib, was developed to control the HIOHR. Finally, the proposed cyclic positive–negative loading flow forming method was validated.