Study on Forming Mechanism of Bimetal-Pipe by External Hydraulic Contraction

It is widely known that a bimetal-pipe can be produced by internal hydraulic expansion. An inevitable limitation of which is that the elastic recovery of the outer pipe should be greater than that of the inner pipe when the forming pressure is released. And the existing techniques usually result in plastic deformation of the inner pipe. Considering the circumstance that elastic recovery of the inner pipe is greater than that of the outer pipe, or only elastic deformation in the inner pipe is allowed, an analytical model was developed to investigate the forming process of bimetal-pipes by external hydraulic contraction. The deformation compatibility condition of the bimetal-pipe was obtained according to the deformation rule. The formula relating the applied external hydraulic pressure and the residual contact pressure between the inner and the outer pipes was derived, and the lower and upper limits of the external hydraulic forming pressures were studied. A two-dimensional finite element analysis of the hydraulic contraction forming process of the bimetal-pipe was conducted. The comparison between the theoretical results and the finite element solutions shows that they are in good agreement. The presented methodology can be used in design and manufacturing of bimetal-pipes where the forming pressure should be applied on the outside surface.