The influence of length–diameter ratio in forming area on viscous outer pressure forming and limit diameter reduction

This paper introduces a forming method on viscous outer pressure of thin-walled tubes, and the forming process under constraint at tube-blank end is analyzed. In this work, the influences of stress and strain of tube blank and length–diameter ratio in forming area on the deformation mode and limit diameter reduction are obtained, and are also validated by experiment and numerical simulation methods. The results show that under the tube-blank end constraint, the deformation can be divided into three areas, namely the area where the tube-blank surface area decreases continuously (area A), the area where the tube-blank surface area decreases first and then increase (area B) and the area where the tube-blank surface area increase (area C). In area B and C, the diameter reduction can be remarkably improved if the viscous pressure restrains the wrinkles growing within the critical forming amount and changes the deformation mode from compression type to extension type.