Improvement of rib-grid structure of thin-walled tube with helical grid-stiffened ribs based on the multi-mode filling behaviors in flow forming

•Rib-filling behavior in flow forming of thin-walled tube with helical grid-stiffened ribs is revealed.•Material accumulation in front of roller and primary material flow direction are two key factors determining the behavior.•Four kinds of rib-filling modes exist in the forming process, depending on the degree of influence of the two factors.•Mechanism of the height difference between two ribs with opposite helical directions is revealed.•A new hexagonal rib-grid structure is proposed to decrease the height difference between the two ribs. Flow forming process is an attractive technology for forming thin-walled tube with helical grid-stiffened ribs. The helical rib-grid structure and helical loading trajectory may lead to various rib-filling behaviors, and then produce large difference in rib-filling height. To this end, the rib-filling behavior and height during the process were investigated, and then to improve rib-filling quality a new hexagonal rib-grid structure was designed. It is found that the rib-filling behavior is mainly determined by two factors. One is the material accumulation in front of roller, which is related to the axial and circumferential distances between one rib and its prior rib. The other is the primary material flow direction, which depends on the relative helical direction between rib and roller trajectory. Due to the geometrical feature of helical rib-grid, there exist two ribs with the opposite helical directions and significant variation of axial and circumferential distances between two adjacent ribs. Their coordinated variation will produce different combinations of the above two factors, thus form four kinds of rib-filling modes during the forming process. Moreover, these rib-filling modes vary along the axial direction due to the variation of axial and circumferential distances between one rib and its prior rib, which leads to the variation in rib height and large height difference between the two ribs with opposite helical directions. To decrease this height difference, a new hexagonal rib-grid structure is proposed through introducing a transverse rib to reduce the variation range of axial and circumferential distances.