Improved forming accuracy through controlling localized sheet metal deformation in the friction-assisted stretch bending process

Abstract

Tension-dominated stress state in the stretch bending process probably leads to localized tensile stress, especially in the manufacture of thin-walled, high-strength, or complex-shaped components. That results in nonuniform residual stress distribution and inferior shape accuracy. In this paper, a friction-assisted stretch bending (FASB) process is proposed to deal with this problem. A rubber sheet is attached with sheet metal to generate tangential friction at rubber/metal interface. The tangential friction assists in modifying stress distribution in the loading process. Finite element analysis was conducted to determine the effects of process parameters on deformation behaviors of the sheet metal. The results show that a rubber sheet with hardness ranging from Shore 57D to 70D and a rubber/metal thickness ratio (ξ) value around 1.0 can effectively reduce sheet metal thinning and decrease springback in the stretching and bending stress states. By taking the simulated process parameters as a reference, fuselage profiles with variable bending curvature and sections were successfully manufactured. Maximum thinning ratio of the formed part was 6.1%, and the section deviation after unloading was reduced by 52.9%, on average, compared with conventional stretch bending. The improved forming accuracy can be attributed to the relatively uniform distribution of longitudinal stress. That results in uniform elastic recovery in longitudinal direction. Further industrial application of the FASB process for the manufacture of large-sized and complex-shaped parts will be expectable.

Influence of Contact Interface Friction on Plastic Deformation of Stretch-Bend Forming

“…The specimen is kept on bending contact with the mold under pre-tension. (5) The displacement control method is used for tensioning with a loading speed of 3 mm/min and a strain rate of 0.115 s −1 . Figure 9 shows the equivalent stretch-bend experiment of 30° contact mold.…”

“…In the process of stretch-bend forming, the friction of the contact interface between the stretch-bend mold and the profile is inevitable. With the increase of the contact mold angle, the contact interface changes continuously, and the frictional shear stress distribution is not uniform [5]. The tensile stress along the thickness and length of the specimen is distributed in a gradient, resulting in uneven plastic deformation of the stretch-bend specimen.…”

“…The spring-back decreased by 21% in comparison with the crank motion simple bending by using ordinary metal dies. Xiang et al [5] stated that tangential friction occurred at the rubber/metal interface during friction-assisted stretch-bend. Tangential friction assists in modifying stress distribution in the loading process.…”