Comparison of modeling results for the superplastic forming of two types of three-layered hollow structures

The development of science and technology in the aviation industry requires not only the development of new manufacturing processes, but also improvement of the existing manufacturing processes of products of hard alloys, such as two-phase titanium alloys. One of such processes is the superplastic forming (SPF) combined with pressure welding (PW). The use of SPF / PW technique allows one to obtain parts and structures which cannot be manufactured by other methods, for example, multilayered hollow structures. Improvement of the technology of the multilayer structure SPF requires solving a number of problems. First, to obtain the most homogeneous deformation of the material when forming the structure, it is necessary to ensure the optimal temperature and strain rate to achieve the superplasticity (SP) conditions. Failure to comply with these conditions can lead to inhomogeneous deformation and, as a result, a substantial inhomogeneity of the thickness and even to fracture of the metal sheets. Secondly, the SP strain rate is very small and the corresponding time of SPF is very long and can last for hours. Each element of the structure is subject to different degrees of deformation and one needs to take care of the most deformable part of the structure, which are the ribs. Moreover, if the SPF is carried out at a constant pressure, the strain rate in the stiffeners may vary by several orders of magnitude, which leads to violation of the SP condition. Therefore, the pressure cycle must be designed so that the strain rate satisfies the optimal SP conditions and reduces the SPF time. In this work an attempt to improve the process of production of multilayer hollow structures made of VT6 titanium alloy through the SPF technology is made. Two types of structures are analyzed under the conditions of axial symmetry. Based on the results of numerical modeling, for both types of structures, the pressure cycle is offered to satisfy the deformation of the ribs under the superplasticity conditions while reducing the overall SPF time.