Rib filling behavior in flow forming of thin-walled tube with helical grid-stiffened ribs

Flow forming is a promising metal forming technology to manufacture thin-walled tube with helical grid-stiffened ribs (TTHGRs). High quality of grid-stiffened ribs is the primary concern in this forming technology. Thus, the rib filling behavior during this process was investigated in this study. It is found that there exist two kinds of asymmetric filling behavior during flow forming of TTHGRs—temporary asymmetric filling behavior of helical rib and permanent asymmetric filling behavior of cross region. The former is caused by the combination of spiral distribution of helical rib and local loading pattern of flow forming. It introduces an asymmetry of rib shape at early stage during rib-filling. The latter is induced by the decreasing of extrusion force of roller on material when the roller rotates across the cross region. It introduces an asymmetry of rib shape in the whole forming process. Moreover, the temporary asymmetric filling behavior of helical rib introduces height difference between the two helical ribs, and the permanent asymmetric filling behavior of cross region leads to a dramatic variation of rib height across the cross region. These together introduce an uneven distribution of rib height. Therefore, the rib height and its distribution inhomogeneity were quantitatively evaluated to analyze the rib forming qualities at different processing parameters. Furthermore, an integrated quality index evaluating rib height and inhomogeneity was proposed and correlated with these parameters using the response surface method. On these bases, the rib forming quality was improved by optimizing the processing parameters in flow forming.