Investigation of formability and deformation behavior for forming concave-convex parts in single point incremental hydraulic forming

Single point incremental forming (SPIF) is a promising, dieless, rapid forming technology with great potential for forming complex thin-walled parts. However, in SPIF without turning sheets, the forming of convex features is limited by the features of die-less forming, especially the forming of complex concave-convex parts. To address this issue, a new process integrating the SPIF with hydraulic forming was proposed. Concave-convex feature parts are obtained by plastic deformation and elastic deformation of SPIF, hydraulic bulging, and hydraulic support. Then, three forming strategies are proposed based on the forming sequence of SPIF, hydraulic bulging, and hydraulic support. The geometric accuracy, thickness distribution, strain distribution and forming force of different forming strategies are studied separately. In addition, a new theoretical model is proposed to predict the thickness of concave-convex parts. Both FE simulation and experimental results show that concave-convex parts can be successfully formed either by adopting the strategy of bulging first and then SPIF or by bulging first and then hydrostatic support SPIF. Hydrostatic support with appropriate pressure is more conducive to improving the forming accuracy, and the theoretical model can accurately predict the forming thickness of complex parts. The new process is expected to manufacture complex concave-convex parts that are difficult to form in traditional SPIF at low cost, high efficiency, and high quality in one clamping.