As a typical stiffened component, cylinders with ribs are widely used in space vehicles to meet the requirements of light weight and high performance [1]. According to the geometric characteristics of ribs, cylinders with ribs can be divided into single directional rib structure or cross rib structure. Oblique cross inner rib structure has good bearing performance against axial compression, bending, torsion and internal pressure, and it is one of the most lightweight structures used in spacecraft. For example, the fuel tank of launch vehicle uses cylinder with 45° cross inner ribs. With the increasing demand of integral forming, flow forming has become an advanced method to form stiffened cylinders [2]. Usually, flow forming of cylinders with ribs is based on traditional flow forming process, but in which the mandrel with the grooves is adapted. Under the action of multi-roller, the deformed material either flows into the grooves to form inner ribs or along the axial direction. At present, how to improve the filling efficiency of ribs is one of the challenges of flow forming technology on manufacturing cylinder with cross inner ribs.
Due to the advantages of high production efficiency, good dimensional accuracy and smooth surface, multi-roller has been widely used in the flow forming of cylinders. In the multi-roller group, not only the axial and radial offset amount of each roller exists, but also the profile radius of roller between them can be different. Xu et al. [3] investigated the influence of the number and arrangement of rollers on the flow forming of cylinder. It was found that multi rollers are beneficial to balance the flow forming force, but the change of roller number affects the size of the contact zone and material flow. Xue et al. [4] and Shan et al. [5] studied the effect of feed ratio and thinning rate on wall thickness deviation through multi-roller flow forming experiments. Lin et al. [6] analyzed the effects of roller profile radius, feed ratio and thinning rate on wall thickness accuracy and inner diameter deviation of cylinder through simulation and experiment. The results showed that a smaller reduction rate can improve wall thickness accuracy and reduce inner diameter deviation, while the changes of feed ratio and roller fillet radius had the opposite trend. Shinde et al. [7] reported that the response surface algorithm can be carried out to obtain good forming quality of production, and the change of parameters of the first roller has the greatest influence on material flow.
Due to the existence of ribs, the flow forming mechanism and the height of ribs of stiffened cylinder have attracted much attention of scholars. The integral forming of cylinder with longitudinal inner ribs is similar to flow forming of inner gears. Xia et al. [8] found that the unsymmetrical extrusion force was produced by rollers on the entry side and exit side of the gear tooth, which led to the uneven filling. What’s more, they found that compared with the non-staggered flow forming, the staggered flow forming can improve the saturation degree of inner tooth. Xia et al. [9] summarized the influence of process parameters on the forming quality of spun inner gears, and pointed out that the total thinning rate is a primary factor affecting the filling height of gear teeth, followed by the blank thickness, while the feed ratio and blank diameter have no obvious influence. Xu et al. [10] adopted multi-roller flow forming to form complex inner gear, and found that increasing the total thinning rate, feed ratio and sheet metal thickness are helpful to improve the height of inner tooth. The structure of cylinder with cross ribs is more complex than the inner gear, and in recent years, flow forming technology has been developed. An aluminum cylinder with transverse and longitudinal inner ribs was flow-formed by Zeng et al. [11], and the results show that the height of longitudinal rib was lower than that of transverse rib, meanwhile, it was found that the filling height of rib increases with the increase of the thinning rate and feed ratio. Lyu et al. [12–13] revealed the asymmetric deformation phenomena during spiral inner rib flow forming based on numerical simulation, meanwhile, parameterized analysis shows that the thinning rate and feed ratio has influence obviously on the rib height, followed by the profile radius and attack angle of the roller.
The above studies mainly focus on the smooth-walled cylinder and traditional parameters. However, the effect of different flow forming process parameters in the same roller group on the material flow is not considered. Meanwhile, how roller number affects the ribs filling of cylinder with ribs has not been developed yet. Therefore, it is necessary to reveal the effect of multi-roller flow forming process parameters including the number of roller, the roller profile radius and offset amount arrangement of each roller in same roller group on saturation degree of ribs of cylinder with cross inner ribs.
In this study, a simulation model for multi-roller flow forming of cylinder with 45° cross inner ribs was established to study the influence of the ribs height by the roller number, radial offset distribution and roller profile radius distribution. At the same time, experiment of forming aluminum alloy cylinders with cross inner ribs was completed by multi-roller flow forming, and the flow forming quality and mechanical properties of the spun cylinders were analyzed.